Exercise Physiology 

Anatomy Drawing

Exercise physiology is a fundamental course in, and its knowledge base central to, the discipline of kinesiology. The word “exercise” evolved from the Latin term exercitius, which means to “drive forth.” Exercise, as defined by Bouchard and Shephard (1994), is physical activity usually performed on a repeated basis with a specific external objective such as the improvement of fitness, physical performance, or health. However, this definition seems most appropriate for defining the term “exercise training.” A superb definition of exercise is advanced in the textbook ACSMʼs Advanced Exercise Physiology by Tipton & Franklin (2006). They define exercise as “a displacement of the homeostasis and rest elicited by muscle contractions resulting in movement and increased energy expenditure.” The word “physiology” is derived from the Greek word physiologia, meaning “natural science” or the study (logia) of nature (physis). Today “physiology” is defined as the study of the biological functions of living organisms including the organic processe and chemical phenomena of an organism or any of its parts or bodily processes. Therefore, “exercise physiology” can be defined as the study of the functions and adaptations of living organisms, their bodily parts, and their organic and chemical processes as a result of increased energy demand due to muscle contraction.

Brief History of Exercise Physiology

 

Skeletal System

The Skeletal System Overview

The adult human skeletal system consists of 206 bones, as well as a system of  tendons, ligaments and cartilage that attaches them. The skeletal system performs essential functions that enable us to survive including; support, movement, protection, blood cell production, calcium and mineral storage and endocrine regulation.​ Human babies are born with approximately 270 bones, some of which fuse together as the body develops of time. The skeletons of adult males and females have some differences, primarily to adjust for childbirth. The female pelvis is flatter, more rounded and larger. Whereas, a male's pelvis is approximately 90 degrees or less of angle, while a female's is 100 degrees or more.

While bones are generally more brittle and rigid when outside of the body, internally the bones are very much alive and are nourished by a network of blood vessels from the circulatory system and nerves from the nervous system. The characteristic of a bone is one that has a dense and tough outer layer. Next is a deposit of spongy bone, which is lighter, slightly flexible and can absorb compression forces. In the centre of some bones is jelly-like bone marrow, where new cells are constantly being produced for blood.

The human teeth are often considered part of the skeletal system but they are not considered as bones. Teeth are made of dentin and enamel, which is strongest substance in the human body. Teeth also play a key role in the digestive system and help to breakdown food once when chewing the food. The skeletal system has two distinctive parts: the axial skeleton and the appendicular skeleton. 

The axial skeleton consists of a total of 80 bones, including the vertebral column, the rib cage and the skull. The axial skeleton spreads the weight from the head, trunk and the upper extremities down to the lower extremities at the hip joints, which help humans maintain the upright posture. The appendicular skeleton has a total of 126 bones, and is formed by the pectoral girdles, the upper limbs, the pelvic girdle and the lower limbs. The primary functions of the appendicular skeleton are to make walking, running and other movements possible and to provide protection to the major organs responsible for digestion, excretion and reproduction.

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Figure 1. Human Skeleton

 

The Vertebrae of the Spine

The human spine has 33 individual bones arranged one on top of the other. Ligaments and muscles connect these vertebrae bones together and keep them aligned. The spinal column provides structural support for your body, allowing you to stand upright, bend, and also twist. Protected deep inside the bones, the spinal cord joins your body to the brain, allowing movement of your arms and legs. Strong muscles and bones, flexible tendons and ligaments, and sensitive nerves contribute to a healthy spine. Keeping your spine healthy is vital if you want to live an active life without back pain.

Spinal curves

 

When observed from the side, an adult spine has a natural S-shaped curve. The neck (cervical) and low back (lumbar) regions have a small concave curve, and the thoracic and sacral regions have a moderate convex curve (Figure 1). The curves act like a coiled spring that can absorb shock, maintain balance, and allow range of motion throughout the spinal column.

Vertebrae

 

The vertebrae have 33 individual bones that interconnect together to form the spinal column. The vertebrae are numbered and divided into regions: cervical, thoracic, lumbar, sacrum, and coccyx (Figure 1). Only the top 24 bones are flexible and moveable; the vertebrae of the sacrum and coccyx are fused. The vertebrae in each region have distinctive features that allow them complete their primary functions.

 

Cervical (neck) - the primary function of the cervical spine is to support the weight of the head (about 4.5kg). The seven cervical vertebrae are numbered C1 to C7. The neck has the greatest range of motion because of two specialised vertebrae that connect to the skull. The first vertebra (C1) is the ring-shaped atlas that connects directly to the skull. This joint allows for the nodding or “yes” motion of the head. The second vertebra (C2) is the peg-shaped axis, which has a projection called the odontoid, that the atlas pivots around. This joint allows for the side-to-side or “no” motion of the head.

Thoracic (mid back) - the primary function of the thoracic spine is to clamp the rib cage and protect the heart and lungs. The twelve thoracic vertebrae are numbered T1 to T12. The range of motion in the thoracic spine is restricted.

 

Lumbar (low back) - the main function of the lumbar spine is to bear the weight of the body. The five lumbar vertebrae are numbered L1 to L5. These vertebrae are much larger in size to absorb the stress of lifting and carrying heavy objects.

 

Sacrum - the main function of the sacrum is to connect the spine to the hip bones (iliac). There are five sacral vertebrae, which are fused together. Together with the iliac bones, they form a ring called the pelvic girdle.

 

Coccyx region - the four fused bones of the coccyx or tailbone provide attachment for ligaments and muscles of the pelvic floor.

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While vertebrae have unique regional features, every vertebra has three functional parts:

• a drum-shaped body designed to bear weight and withstand compression 

• an arch-shaped bone that protects the spinal cord 

• star-shaped processes designed as outriggers for muscle attachment 

Intervertebral discs

 

Each vertebra in your spine is separated and cushioned by an intervertebral disc, keeping the bones from rubbing together. Discs are designed like a radial car tire. The outer ring, called the annulus, has criss-crossing fibrous bands, much like a tyre tread. These bands attach between the bodies of each vertebra. Inside the disc is a gel-filled centre called the nucleus, much like a tyre tube.

Discs function like coiled springs. The criss-crossing fibres of the annulus pull the vertebral bodies together against the elastic resistance of the gel-filled nucleus. The nucleus acts like a ball-bearing when you move, allowing the vertebral bodies to roll over the incompressible gel. The gel-filled nucleus is composed mostly of fluid. This fluid absorbed during the night as you lie down and is pushed out during the day as you move upright.

With age, our discs increasingly lose the ability to reabsorb fluid and become brittle and flatter; this is why we get shorter as we grow older. Also diseases, such as osteoarthritis and osteoporosis, cause bone spurs (osteophytes) to grow. Injury and strain can cause discs to bulge or herniate, a condition in which the nucleus is pushed out through the annulus to compress the nerve roots causing back pain.

 

Vertebral arch & spinal canal

 

On the back of each vertebra are bony projections that form the vertebral arch. The arch is made of two supporting pedicles and two laminae. The hollow spinal canal contains the spinal cord, fat, ligaments, and blood vessels. Under each pedicle, a pair of spinal nerves exits the spinal cord and pass through the intervertebral foramen to branch out to your body. Surgeons often remove the lamina of the vertebral arch (laminectomy) to access and decompress the spinal cord and nerves to treat spinal stenosis, tumours, or herniated discs. Seven processes arise from the vertebral arch: the spinous process, two transverse processes, two superior facets, and two inferior facets.

Spinal cord

 

The spinal cord is about 18 inches long and is the thickness of your thumb. It runs within the protective spinal canal from the brainstem to the 1st lumbar vertebra. At the end of the spinal cord, the cord fibres separate into the cauda equina and continue down through the spinal canal to your tailbone before branching off to your legs and feet. The spinal cord serves as an information super-highway, relaying messages between the brain and the body. The brain sends motor messages to the limbs and body through the spinal cord allowing for movement. The limbs and body send sensory messages to the brain through the spinal cord about what we feel and touch. Sometimes the spinal cord can react without sending information to the brain. These special pathways, called spinal reflexes, are designed to immediately protect our body from harm.The nerve cells that make up your spinal cord itself are called upper motor neurons. The nerves that branch off your spinal cord down your back and neck are called lower motor neurons. These nerves exit between each of your vertebrae and go to all parts of your body.

 

Any damage to the spinal cord can result in a loss of sensory and motor function below the level of injury. For example, an injury to the thoracic or lumbar area may cause motor and sensory loss of the legs and trunk (called paraplegia). An injury to the cervical (neck) area may cause sensory and motor loss of the arms and legs (called tetraplegia, formerly known as quadriplegia).

 

Spinal nerves

 

Thirty-one pairs of spinal nerves branch off the spinal cord. The spinal nerves act as “telephone lines,” carrying messages back and forth between your body and spinal cord to control sensation and movement. Each spinal nerve has two roots (Fig. 8). The ventral (front) root carries motor impulses from the brain and the dorsal (back) root carries sensory impulses to the brain. The ventral and dorsal roots fuse together to form a spinal nerve, which travels down the spinal canal, alongside the cord, until it reaches its exit hole - the intervertebral foramen . Once the nerve passes through the intervertebral foramen, it branches; each branch has both motor and sensory fibres. The smaller branch (called the posterior primary ramus) turns posteriorly to supply the skin and muscles of the back of the body. The larger branch (called the anterior primary ramus) turns anteriorly to supply the skin and muscles of the front of the body and forms most of the major nerves. The spinal nerves are numbered according to the vertebrae above which it exits the spinal canal. The 8 cervical spinal nerves are C1 through C8, the 12 thoracic spinal nerves are T1 through T12, the 5 lumbar spinal nerves are L1 through L5, and the 5 sacral spinal nerves are S1 through S5. There is 1 coccygeal nerve. The spinal nerves innervate specific areas and form a striped pattern across the body called dermatomes . Doctors use this pattern to diagnose the location of a spinal problem based on the area of pain or muscle weakness. For example leg pain (sciatica) usually indicates a problem near the L4-S3 nerves.

Coverings & spaces

 

The spinal cord is covered with the same three membranes as the brain, called meninges. The inner membrane is the pia mater, which is intimately attached to the cord. The next membrane is the arachnoid mater. The outer membrane is the tough dura mater (Fig. 8). Between these membranes are spaces used in diagnostic and treatment procedures. The space between the pia and arachnoid mater is the wide subarachnoid space, which surrounds the spinal cord and contains cerebrospinal fluid (CSF). This space is most often accessed when performing a lumbar puncture to sample and test CSF or during a myelogram to inject contrast dye. The space between the dura mater and the bone is the epidural space. This space is most often accessed to deliver aesthetic numbing agents, commonly called an epidural, and to inject steroid medication.

Divisions of the skeleton

 

The adult human skeleton usually consists of 206 named bones. These bones can be grouped in two divisions: axial skeleton and appendicular skeleton. The 80 bones of the axial skeleton form the vertical axis of the body. They include the bones of the head, vertebral column, ribs and breastbone or sternum. The appendicular skeleton consists of 126 bones and includes the free appendages and their attachments to the axial skeleton. The free appendages are the upper and lower extremities, or limbs, and their attachments which are called girdles. The named bones of the body are listed below by category.

 

Axial & appendicular skeleton

The human skeletal system is composed of individual and attached bones, with the support of ligaments, muscles, tendons, and cartilage. The skeletal system has supportive, protective and locomotive functions. The primary component, the bones, is made up of crystallised calcium minerals arranged over and around a protein matrix, which helps to withstand stress and greater loads on the skeletal system. The classification of bones can be done according to the anatomy, the histology of the bones and functions of the bones. Here, we will discuss their functional classification, and how they are similar and dissimilar on various grounds.

The axial skeleton consists of about 80 bones along the central axis of the human body, and it includes the skull, which includes the cranium and the facial bones, the ossicles of the inner ear, hyoid bone, rib cage, and the vertebral column. These bones are supported by soft tissues like ligaments of the vertebral column, muscles of the face and the throat, cartilage of the ribs, and tendons of the muscles. These bones have functions of central weight bearing and protection and maintenance of posture. The skull and the ribcage protect the brain and the organs of the chest cavity respectively. The ossicles of the ear have the function of maintaining the balance of the human body. The hyoid bone is an anchor point for various muscles covering the throat as a protective function for the airways, gullet, major arteries and nerves. The vertebral column has functions in proper weight distribution, protection of the spinal cord and maintaining proper posture.

The appendicular skeleton

 

The appendicular skeleton consists of 126 bones in the body, which are arranged symmetrically on either side of the body, which include the bones of the upper and lower limbs, and their connections to the axial skeleton. They are mainly made up of long bones and other bones. The upper arm is connected to the axial skeleton by the shoulder girdle, and that is supported by a myriad of tendons, cartilages, muscles and ligaments. The thigh is connected to the axial skeleton by the pelvic girdle. The main bones of the upper limb include the humerus, radius, ulna, carpal, metacarpal, and phalanges. The main bones of the lower limb include the femur, tibia, fibula, tarsal, metatarsal, and phalanges. The functions of the appendicular bones include balance and stability, along with the main functions of locomotion and manipulation.

 

The differences between the axial & appendicular skeleton

 

Both axial and appendicular bones are made up of similar basic constituents of calcium and protein matrix. Both have functions of weight bearing at differing levels, as well as those of stability, balance, and protection of organs. But the main concern of axial skeleton is that of posture, stability and balance, whereas the appendicular skeleton is that of locomotion, digital manipulation leading to feeding, and reproduction. The axial skeleton is fused, whereas the appendicular skeleton is not fused.

 

The axial skeleton and appendicular skeleton contain evolutionary variations, which are important in their specified functional variation, and are equipped to fulfil the necessary function, which help to protect and promote the human species

 

The Axial Skeleton consists of the:

 

• Skull (cranial bones and facial bones)

• Hyoid bone - not shown opposite

• Auditory ossicles 

• Vertebral column

• (also called the "spine" or "backbone")

• Sternum bone

• Ribs (which, together with the sternum, form the “thorax")

The Appendicular Skeleton consists of the:

 

• Shoulder girdles, which include the scapulae (shoulder blades) and a clavicle on each side of the bone (also known as "collar bones")

• Upper Limbs = Arms (incl. wrists and hands). See arm bones and hand bones.

• Pelvic (hip) girdle, which includes the hip bones (= "coxal bones") called the ilium, ischium and pubis

• Lower Limbs = Legs (incl. ankles and feet).

 

It is easy to remember how the axial skeleton and appendicular skeleton are classified by learning and remembering the meaning of the word appendage (or appendages in the plural). In general biology the word "appendage" refers to a natural body part that protrudes from the centre of an animal's body. Examples of appendages include the limbs of vertebrates (animals that have backbones e.g. humans). So in the case of the human body arms and legs are appendages. The human appendicular skeleton consists mainly of the four appendages of the human body - plus, of course, the shoulder girdle and the pelvic girdle by which the limbs are inter-connected with the rest of the human body.

Functions of the skeleton

 

Bone, the material that makes vertebrates distinct from other animals, has evolved over several hundred million years to become a remarkable tissue. Bone is a material that has the same strength as cast iron, but achieves this while remaining as light as wood.

 

The front leg of a horse can withstand the loads generated while this 1500-pound animal travels at 30 miles per hour. The upper arm is able to keep birds aloft through entire migrations, sometimes over 10,000 miles without landing. The antlers of deer, used as weapons in territorial clashes with other deer, undergo tremendous impacts without fracturing, ready to fight another day.

 

At some point, unfortunately, forces of impact exceed even bone's ability to hold up. Falling on the ice, suffering a collision in a car or a tumble on the ski slopes can cause the bone to fail. While fractures are disastrous, bone - because it is a live tissue - almost instantly begins a healing process. Without question, bone is the ultimate biomaterial. It is light, strong, can adapt to its functional demands, and repair itself. 

 

Functions of the skeleton 

  

  1. Structural support for heart, lungs and marrow 

  2. Protection for brain, uterus, and other internal organs 

  3. Attachment sites for muscles allowing movement of limbs 

  4. Mineral reservoir for calcium and phosphorus 

  5. Defense against acidosis 

  6. Trap for some dangerous minerals such as lead 

 

Bone architecture

 

There are two major kinds of bone, trabecular (spongy) and cortical. Trabecular bone gives supporting strength to the ends of the weight-bearing bone. The cortical (solid) bone on the outside forms the shaft of the long bone. This X-ray of a femur shows the thick cortical bone, and the trabecular bone which is arranged to withstand the stresses from usual standing and walking. Compressive stresses are those of the body weight pushing the bone down, and tensile stresses are from the muscles, pulling the bone apart. Human skeletal system consists of bones. Bone is partly organic (cells and matrix) and partly inorganic (mineralised component). There are four types of bones: long, short, flat and irregular. Long bones, especially the femur and tibia, are subjected to most of the load during daily activities and they are crucial for skeletal mobility. There are two types of bone tissue: compact and spongy. The names imply that the two types differ in density, or how tightly the tissue is packed together. There are three types of cells that contribute to bone homeostasis. Osteoblasts are bone-forming cell, osteoclasts resorb or break down bone, and osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.

 

Compact bone

 

Compact bone consists of closely packed osteons or haversian systems. The osteon consists of a central canal called the osteonic (haversian) canal, which is surrounded by concentric rings (lamellae) of matrix. Between the rings of matrix, the bone cells (osteocytes) are located in spaces called lacunae. Small channels (canaliculi) radiate from the lacunae to the osteonic (haversian) canal to provide passageways through the hard matrix. In compact bone, the haversian systems are packed tightly together to form what appears to be a solid mass. The osteonic canals contain blood vessels that are parallel to the long axis of the bone. These blood vessels interconnect, by way of perforating canals, with vessels on the surface of the bone.

 
 
 
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Spongy (cancellous) bone

 

Spongy (cancellous) bone is lighter and less dense than compact bone. Spongy bone consists of plates (trabeculae) and bars of bone adjacent to small, irregular cavities that contain red bone marrow. The canaliculi connect to the adjacent cavities, instead of a central haversian canal, to receive their blood supply. It may appear that the trabeculae are arranged in a haphazard manner, but they are organised to provide maximum strength similar to braces that are used to support a building. The trabeculae of spongy bone follow the lines of stress and can realign if the direction of stress changes.

 

 

Long bones have four main functions. The first function is structural. Bones provide the shape for our bodies and host vital organs. Body locomotion is the second function of the bone. The complicated kinematic skeletal system enables movement, using muscles that control bone positions and orientations. Bones transmit loads and act as levers. Joints are the fulcrums about which bones move. Therefore, the prime qualities of bones are strength and rigidity. Once the load exceeds the capacity of the bone, the fracture occurs. The body then initiates bone repair and the reconstruction process in order to restore bone functional properties. Details of the healing types and stages are described in the next section of this website. The third function of the bone is blood cell formation. Blood cell precursor cells, hemocytoblasts, are found in red marrow. The red marrow is simply a loose connective tissue that contains these blood cell precursors and the cells that they are making. The fourth function of the bone is inorganic salt regulation and storage of calcium, phosphate, sodium and  potassium. Bone structure can be described based on its overall macroscopic shape and microscopic composition. The end region of the bone is called the epiphysis and the middle region is called the diaphysis or bone shaft. The region between is called the metaphysis. Between the metaphysis and epiphysis is the epiphyseal disk or plate, which is responsible for longitudinal bone growth in childhood. It is at the epiphysis where one bone contacts another in a joint to allow for movement. Each epiphysis is coated with an articular cartilage. The articular cartilage is simply a coating of hyaline cartilage, which reduces friction and absorbs shocks at freely moveable joints.

 

All bones are covered by a thin membrane called a periosteum. The periosteum is made of two layers of a dense connective tissue. The outer fibrous layer consists of fibroblasts and collagen fibres. The inside, or osteogenic, layer contains osteoprogenitor cells. Long bones have a hollow region called the medullary cavity in the middle of the diaphysis. The perimeter of the medullary cavity is covered with an endosteum. The cavity itself is filled with marrow. Marrow can be either red or yellow depending on its function and composition. Red marrow is responsible for generation of blood cells and yellow marrow stores fat.

 

There are two types of bone tissue: spongy and compact, also known as dense. Spongy bone makes up most of the tissue of epiphyses. It consists of lamellae arranged in an irregular latticework of thin plates of bone called trabeculae. The spaces between trabeculae are filled with red bone marrow. Compact bone structure is based on Haversian systems. Haversian systems are located in the diaphysis. They also cover spongy bone in the epiphyses. The functions of Haversian systems are to protect, support, and resist stress.

The terms osteogenesis and ossification are often used synonymously to indicate the process of bone formation. Parts of the skeleton form during the first few weeks after conception. By the end of the eighth week after conception, the skeletal pattern is formed in cartilage and connective tissue membranes and ossification begins.

 

Bone development continues throughout adulthood. Even after adult stature is attained, bone development continues for repair of fractures and for remodelling to meet changing lifestyles. Osteoblasts, osteocytes and osteoclasts are the three cell types involved in the development, growth and remodelling of bones. Osteoblasts are bone-forming cells, osteocytes are mature bone cells and osteoclasts break down and reabsorb bone.

 

There are two types of ossification: intramembranous and endochondral.

 

Intramembranous

 

Intramembranous ossification involves the replacement of sheet-like connective tissue membranes with bony tissue. Bones formed in this manner are called intramembranous bones. They include certain flat bones of the skull and some of the irregular bones. The future bones are first formed as connective tissue membranes. Osteoblasts migrate to the membranes and deposit bony matrix around themselves. When the osteoblasts are surrounded by matrix they are called osteocytes.

 

Endochondral ossification

 

Endochondral ossification involves the replacement of hyaline cartilage with bony tissue. Most of the bones of the skeleton are formed in this manner. These bones are called endochondral bones. In this process, the future bones are first formed as hyaline cartilage models. During the third month after conception, the perichondrium that surrounds the hyaline cartilage "models" becomes infiltrated with blood vessels and osteoblasts and changes into a periosteum. The osteoblasts form a collar of compact bone around the diaphysis. At the same time, the cartilage in the centre of the diaphysis begins to disintegrate. Osteoblasts penetrate the disintegrating cartilage and replace it with spongy bone. This forms a primary ossification centre. Ossification continues from this centre toward the ends of the bones. After spongy bone is formed in the diaphysis, osteoclasts break down the newly formed bone to open up the medullary cavity.

The cartilage in the epiphyses continues to grow so the developing bone increases in length. Later, usually after birth, secondary ossification centers form in the epiphyses. Ossification in the epiphyses is similar to that in the diaphysis except that the spongy bone is retained instead of being broken down to form a medullary cavity. When secondary ossification is complete, the hyaline cartilage is totally replaced by bone except in two areas. A region of hyaline cartilage remains over the surface of the epiphysis as the articular cartilage and another area of cartilage remains between the epiphysis and diaphysis. This is the epiphyseal plate or growth region.

 

Bone growth

 

Bones grow in length at the epiphyseal plate by a process that is similar to endochondral ossification. The cartilage in the region of the epiphyseal plate next to the epiphysis continues to grow by mitosis. The chondrocytes, in the region next to the diaphysis, age and degenerate. Osteoblasts move in and ossify the matrix to form bone. This process continues throughout childhood and the adolescent years until the cartilage growth slows and finally stops. When cartilage growth ceases, usually in the early twenties, the epiphyseal plate completely ossifies so that only a thin epiphyseal line remains and the bones can no longer grow in length. Bone growth is under the influence of growth hormone from the anterior pituitary gland and sex hormones from the ovaries and testes. Even though bones stop growing in length in early adulthood, they can continue to increase in thickness or diameter throughout life in response to stress from increased muscle activity or to weight. The increase in diameter is called appositional growth. Osteoblasts in the periosteum form compact bone around the external bone surface. At the same time, osteoclasts in the endosteum break down bone on the internal bone surface, around the medullary cavity. These two processes together increase the diameter of the bone and, at the same time, keep the bone from becoming excessively heavy and bulky.

Long bones

 

The bones of the body come in a variety of sizes and shapes. The four principal types of bones are long, short, flat and irregular. Bones that are longer than they are wide are called long bones. They consist of a long shaft with two bulky ends or extremities. They are primarily compact bone but may have a large amount of spongy bone at the ends or extremities. Long bones include bones of the thigh, leg, arm, and forearm.

Short bones

 

Short bones are roughly cube shaped with vertical and horizontal dimensions approximately equal. They consist primarily of spongy bone, which is covered by a thin layer of compact bone. Short bones include the bones of the wrist and ankle.

 

Flat bones

 

Flat bones are thin, flattened, and usually curved. Most of the bones of the cranium are flat bones.

Irregular bones

 

Bones that are not in any of the above three categories are classified as irregular bones. They are primarily spongy bone that is covered with a thin layer of compact bone. The vertebrae and some of the bones in the skull are irregular bones. All bones have surface markings and characteristics that make a specific bone unique. There are holes, depressions, smooth facets, lines, projections and other markings. These usually represent passageways for vessels and nerves, points of articulation with other bones or points of attachment for tendons and ligaments.

Articulation

 

An articulation, or joint, is where two bones come together. In terms of the amount of movement they allow, there are three types of joints: immovable, slightly movable and freely movable.

 

Synarthroses

 

Synarthroses are immovable joints. The singular form is synarthrosis. In these joints, the bones come in very close contact and are separated only by a thin layer of fibrous connective tissue. The sutures in the skull are examples of immovable joints.

 

Amphiarthroses

 

Slightly movable joints are called amphiarthroses. The singular form is amphiarthrosis. In this type of joint, the bones are connected by hyaline cartilage or fibrocartilage. The ribs connected to the sternum by costal cartilages are slightly movable joints connected by hyaline cartilage. The symphysis pubis is a slightly movable joint in which there is a fibrocartilage pad between the two bones. The joints between the vertebrae and the intervertebral disks are also of this type.

Diarthroses

 

Most joints in the adult body are diarthroses, or freely movable joints. The singular form is diarthrosis. In this type of joint, the ends of the opposing bones are covered with hyaline cartilage, the articular cartilage, and they are separated by a space called the joint cavity. The components of the joints are enclosed in a dense fibrous joint capsule. The outer layer of the capsule consists of the ligaments that hold the bones together. The inner layer is the synovial membrane that secretes synovial fluid into the joint cavity for lubrication. Because all of these joints have a synovial membrane, they are sometimes called synovial joints.

 
 

Section 2- Neuro-Muscular System 

Senior Physiotherapy
 

The human brain

The human brain (Figure 4) contains billions of neurons that communicate with nearly 10,000 others within the body. The ultimately controls practically all aspects of the human body (motor performance to psychological factors). It is beyond the scope of my knowledge to completely discuss the brain, some essential areas need to be communicated to help develop an understanding of adaptations to training. Importantly the brain stem (consisting of the medulla oblongata, midbrain, pons, and reticular formation)  contains the neural centre that for example regulates heart rate, the force of contractions, blood pressure, breathing, vision and also consciousness. the thalamus, hypothalamus, and pineal body or epithalamus (collectively termed diencephalon) is the control centre for sleep and relay mechanism of the brain (pineal body and thalamus respectively).  The hypothalamus is the main link between the nervous and endocrine systems because it is an endocrine gland under neural control.

The hypothalamus releases numerous hormones that inhibit or release hormones from the anterior pituitary glands controlling most essential functions of the body (e.g. homeostasis, autonomic control, body temperature, emotions). The largest part of the brain called the cerebrum which contains 75% of the neurons within the nervous system and is located in the cerebral cortex. Critical areas include the premotor cortex (where a voluntary muscle contraction begins), the primary motor cortex (where voluntary muscle contraction is controlled) and the primary sensory area (where sensory information is integrated). The basal ganglia are involved with control and planning of muscle function, posture, and regulating undesirable movements. Lastly, the cerebellum integrates sensory information and coordinates muscle activity.
 

It is important to understand that the ability to increase neural output is initiated in the higher brain centres such as the motor cortex. This is especially true when individuals attempt to produce high levels of force and power. Evidence has suggested that neural activity increases in the primary motor cortex as force increases (Dettmers, et al., 1996). Motor learning results in the functional organisation of the cerebral cortex. This has been demonstrated through visualisation training with untrained subjects demonstrating significant strength increases (visualising lifting weights without actually lifting them). Therefore cerebral adaptations are essential for developing coordination, motor learning, skill acquisition, strength, power, and speed. 

Descending corticospinal tracts

 

The descending corticospinal tracts are a collection of axons linking the cerebral cortex to the spinal cord. The motor pathway is characterised by neurons in the brain (primarily in the motor areas) forming synapses with other nerves that ultimately proceed down the spinal cord to the anterior root of exit for innervation of skeletal muscle. A substantial proportion of potential neural changes take place in the spinal cord along the descending corticospinal tracts. Untrained individuals typically display restricted ability to maximally recruit all of their muscle fibres. A study by Adams et al., (1993) demonstrated that only 71% of muscle mass was activated during maximal effort in untrained individuals. Carrol and colleagues (2002)  reported that a limitation in subjects central drive reduces strength and power with much of the inhibition originating from the descending corticospinal tracts. However, Pucci et al.,  (2006) reported that training can greatly reduce this deficit, thereby demonstrating a greater potential to recruit a larger per cent of muscle mass with training.

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Figure  4. The brain. Key areas of the brain such as the medulla oblongata, cerebrum, cerebellum, cerebral cortex, diencephalon, primary motor area, and sensory area are identified.

 
 
 
 
 
 
 

The nervous system

 

The nervous system is critical for regulating acute exercise performance and ensuing training adaptations. The main lines of communication between the brain and the muscular system are via these nerve networks, ensuring that signals travel to the appropriate locations within the body. From an exercise training perspective, the magnitude of these signals is essential in regulating the final strength or power output. This neural drive is critical to the individual motivated to maximise athletic performance. The neural drive is believed to occur in agonist muscle recruitment, firing frequency, and the pattern of discharge during high-intensity muscular contractions. It is thought that a reduction in the inhibitory mechanisms also occurs, although limited understanding is available on how these mechanisms co-exist.   However, it is apparent that the neural adaptation is multifaceted and may precede changes in the muscle. 

Functional organisation of the nervous system 

The nervous system serves predominately as the foremost control mechanism of the body (excluding the endocrine system). This system receives various sensory information in the form of pressure, temperature, joint position, muscle length and also pain. It can process, integrate, and respond to every tissue, gland, and organ controlling all outputs or responses. Additionally, the nervous system also controls our emotions, personality, and other cerebral (brain) functions. The nervous is comprised of two major branches the central and peripheral nervous system (Figure 5). The central nervous system (CNS) consists of the brain and the spinal cord. The peripheral nervous system is separated with two segments namely the motor and sensory divisions.   There are 31 pairs of spinal nerves that exist and exit on the sensory (posterior) and motor (anterior) roots of the spinal cord. 

The sensory nervous system detects varies stimuli and relays this information afferently to the CNS. The motor nervous system consists of two divisions (i) the somatic and the autonomic nervous system (ANS). The somatic system conveys information from the CNS efferently (away from the CNS) to the muscles leading to muscle contractions. The ANS consists of nerves relaying efferent information to cardiac, tissues, glands, and muscles. The ANS comprises of the sympathetic and parasympathetic nervous system, both of which are necessary for preparing the body for the physical stress of exercise and returning the body to pre-exercise state.  It has been suggested that physical training and exercise may produce adaptations along the neuromuscular chain, instigating in the higher brain centres and cascading down to individual muscle fibres. Aerobic training imposes specific neural demands although the rate coding or pattern of neural activation appears less complex than high-intensity anaerobic training stimuli, where high levels of strength and power are required. 
 

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Figure  5. Divisions of the Nervous System

Nerve cells

The nervous tissues comprise of supporting cells and neurons (Figure 6). The supporting cells are essential and they ensure constancy throughout the CNS, with neurons (nerve cells) communicate with other nerves and tissues. Motor neurons tend to be multipolar whereas sensory neurons tend to be unipolar. The key structures of the motoneuron include dendrites which receive input from other nerve cells. The cell body contains the organelles which are responsible for energy metabolism, protein synthesis, and transport. These cell bodies play a key role in integrating the stimuli from other neurons within the CNS and determine how much stimuli will transfer to the muscles. Other components of the neuron comprise of the axons which are long processes that are responsible for communicating with target tissues. The axon hillock is the area where the action potential is initiated once the critical threshold is reached. There is fatty tissue which is wrapped around the axon called the myelin sheath, which significantly increases the speed of signal transmissions. 
 

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Figure  6. Key structures of the motor-neuron such as the dendrites, cell body, axon.

Neural communication

Communication occurs between nerves and tissues through the generation of an electrical signal (current) called the action potential (AP).  The AP consists of three key actions: (i) integration, (ii) propagation, and (iii) neurotransmitter release. Integration ensues within the cell body and determines whether or not the the Ap will be transmitted to the target tissue (Figure 7). The cell body integrates the charges from other neurons and if the threshold voltage is reached then the AP will travel in an all-or-none mode to the end of the nerve terminal. Propagation is then brought about by ion movement (sodium and potassium)  down the axon at the nodes of Ranvier through a process called saltatory conduction.

The electrical current is quickly driven down the axon to the terminal with the myelin sheath further accelerating this process. At the nerve terminal, neurotransmitters are released allowing communication to occur with the target tissue. This entire process occurs rapidly and enables several APs to be conducted in under a second. A further detailed breakdown of the processes is found below

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Cell membrane activity 


The main roles of the nervous system is sensation, integration  and response, depending on the functions of the underlying neurons underlying these pathways. It is important for sports coaches and trainers to understand how neurons communicate and how action potential occurs if specific training exercises or practices are developed.

 

Generally speaking the cells of the human body use charged ion particles to generate a charge across the cell membrane. For muscles to contract, excitation-coupling requires input from a neuron. The cell membrane, therefore, is responsible for the regulation and movement between ions from the extracellular fluid and the cytosol.  The cell membrane, therefore, is responsible for the regulation of what crosses the membrane via a phospholipid bilayer. This aids in the diffusion (passage) of only substances that pass through the hydrophobic core, with charged particles unable to pass through without support (Figure 8). Channel proteins (transmembrane) allow this to ensue, with numerous passive channels and active transport pumps generating a transmembrane potential and an action potential (AP). Carrier proteins (sodium/potassium pumps)  help to regulate the ion concentration on both sides by moving sodium ions (Na+) out of the cell and potassium ions (K+) into the cell.


For the sodium/potassium pumps to function, they require energy in the form of adenosine triphosphate (ATP) (also referred to as an ATPase). The Na+ concentration is higher outside than inside of the cell and the concentration of K+ is higher inside than outside. As a result, this pump is regulating ions against a concentration gradient for sodium and potassium which is why ATP energy is required. Ion channels are openings that allow charged particles to cross the membrane in response to a current concentration gradient. Proteins can span the cell membrane (including the hydrophobic core) and can interact with ions charge because of the various properties of the amino acids located within the regions of the protein channels. For example, hydrophobic amino acids are located in the areas that are interconnected to the hydrocarbon tails of the phospholipids. 


Hydrophilic amino acids are exposed to the fluid environments of the extracellular fluid and cytosol. Hydrophilic amino acids are exposed to extracellular fluid and cytosol. Furthermore, the ions will interact with hydrophilic amino acids, which will be selective for the charge of the ion. Channels for positive ions (cations) will have negatively charged side chains in the opening (pore). Whereas negative ions (anions) will have positively side chains in the pore. This is called electrochemical exclusion, meaning that the channel opening is charge specific. 
 

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Figure 8. Cell Membrane and Transmembrane Proteins. 

The diameter of the pore has a specific impact on the ion channels. The is because the distance between the amino acids is specific for the pore diametre when it detaches from the water module surrounding it. Due to the size of the surrounding water molecules, the larger pores are not suitable for smaller ions because the water molecules will interact (due to hydrogen bonds) more willingly than with the amino acid chains (termed size exclusion). 

Ion channels do not always permit diffusion of ions across the membrane. Some channels are gated (ligand-gated) and are primarily found in the cells of the muscle tissue, epithelial, and connective tissues. A ligand-gated channel opens because a signalling molecule binds to the extracellular region of the channel. This type of channel is sometimes known as an ionotropic receptor because when the ligand (sometimes termed a neurotransmitter) in the nervous system, binds to the protein, ions cross the membrane changing its charge.

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Figure 9. Ligand-Gated Channels. 

Types of gated channels

A mechanically gated channel (Figure 10) opens because of a physical alteration of the cell membrane. Channels coupled with the sense of touch (somatosensorial) are mechanically gated. For example, as pressure is applied to the skin, these channels open and permit ions to enter the cell. Similar to this type of channel would be the channel that opens due to temperature changes, as in testing the water in the shower.

 

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Figure 10. Mechanically-gated channels. 

A voltage-gated channel (Figure 11) is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Typically, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow ions to cross the membrane .

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Figure 11. Voltage-gated channels. 

A leakage channel (Figure 12) is randomly gated, meaning that it opens and closes at random, thus the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leakage channels contribute to the resting transmembrane voltage of the excitable membrane.

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Figure 12. Leakage channels. 

The membrane potential

The cell membrane electrical state can have some variants of (termed membrane potential). The membrane potential is the interference of charge across the cell membrane (measured in millivolts [mV]). Normally comparisons are made inside of the cell relative to the outside. Therefore, the membrane potential is a value compared to the charge on the intracellular side of the membrane based on the outside is zero. The concentration of ions in extracellular and intracellular fluids is mostly stable, with a net neutral charge. However, a small variation in charge occurs at the membrane surface, both internally and externally. It is this difference that has all the power in neurons to produce electrical signals, including action potentials (Figure 13).

However, before the electrical signals can be described the resting membrane state must be clarified. When the cell is at rest, and the ion channels are closed the ions are distributed across the membrane in a certain fashion. The concentration of Na+ outside the cell is considerably greater (10 times greater) than the concentration inside. Moreover, the concentration of K+ inside the cell is greater than outside. The cytosol contains a high concentration of anions, in the form of phosphate ions and negatively charged proteins. With the ions distributed across the membrane at these concentrations, the variance in charge is measured at -70 mV (the value commonly defined as the resting membrane potential). This voltage would essentially be lower except for the contributions of some important proteins in the membrane. Leakage channels allow Na+ to gradually move into the cell or K+ to slowly move out, and the Na+/K+ pump restores them. 

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Figure 13. Measuring Charge across a Membrane with a Voltmetre

Action Potential 

Resting membrane potential depicts the steady-state of the cell, which is equalised by ion leakage and ion pumping. Importantly, without any outside stimulus, it will not change. To initiate an electrical signal, the membrane potential has to change. This commences with a channel opening for Na+ in the membrane because the Na+ concentration is greater outside the cell than inside. Ions hurry into the cell due to the concentration gradient. Due to sodium being a positively charged ion, it will immediately alter the relative voltage inside the cell comparative to directly outside. The resting potential is the state of the membrane at a voltage of -70 mV, so the sodium cation entering the cell will initiate a reduction in mV (termed as depolarisation).

Due to the strength of Na+, it will continue to enter the cell even after the membrane potential has become zero, resulting in the voltage directly around the pore becomes positive. The electrical gradient also contributes to this as negative proteins below the membrane attract the sodium ion. As a result, the membrane potential will reach +30 mV by the time sodium has entered the cell. As the membrane potential reaches +30 mV, other voltage-gated channels are opening in the membrane. These channels are specific for the potassium ion. A concentration gradient also acts on K+. As K+ starts to exit the cell the membrane potential begins to move back toward its resting voltage (termed repolarization). 


Repolarization returns the membrane potential to the -70 mV value that denotes the resting potential but overshoots the -70 mV value.  Potassium ions reach equilibrium when the membrane voltage is below -70 mV, so a period of hyperpolarization occurs while the K+ channels are open. Those K+ channels are slightly delayed in closing, accounting for this short overshoot. The change in the membrane voltage from -70 mV at rest to +30 mV at the end of depolarization is a 100-mV change. (also written as 0.1-V change). 

The membrane potential will stay at the resting voltage until something changes. The description above just says that a Na+ channel opens. However, there are several different types of channels that allow Na+ to cross the membrane. A ligand-gated Na+ channel will open when a neurotransmitter binds to it and a mechanically gated Na+ channel will open when a physical stimulus affects a sensory receptor. Whether it is a neurotransmitter binding to its receptor protein or a sensory stimulus activating a sensory receptor cell, some stimulus initiates the process. Sodium starts to enter the cell and the membrane becomes less negative.


Another channel that is an essential part of depolarisation in the AP is the voltage-gated Na+ channel. The channels that start depolarizing the membrane because of a stimulus help the cell to depolarise from -70 mV to -55 mV. Once the membrane reaches that voltage, the voltage-gated Na+ channels open. This is what is known as the threshold. Any depolarization that does not change the membrane potential to -55 mV or higher will not reach the threshold and thus will not result in an AP. 

 

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Figure 14. Graph of Action Potential.

A stronger stimulus, which might depolarise the membrane past the threshold, will not create a greater AP. Action potentials are “all or none”, either the membrane reaches the threshold and everything occurs, or the membrane does not reach the threshold and nothing else happens. All APs peak at the same voltage (+30 mV), so one action potential is not bigger than another. Stronger stimuli will initiate multiple AP more rapidly, but the individual signals are not bigger. 

As described above, the depolarisation and repolarisation of an AP are dependent on the voltage-gated Na+ channel and the voltage-gated K+ channel. The voltage-gated Na+ channel has two gates. One is the activation gate, which opens when the membrane potential crosses -55 mV. The other gate is the inactivation gate, which closes after a specific period. When a cell is at rest, the activation gate is closed and the inactivation gate is open. However, when the threshold is reached, the activation gate opens, allowing Na+ to rapidly enter into the cell. Timed with the peak of depolarization, the inactivation gate closes. During repolarization, no more sodium can enter the cell. When the membrane potential passes -55 mV, the activation gate shuts. After that, the inactivation gate re-opens, making the channel ready to start the entire process again.

The voltage-gated K+ channel has one gate, which is sensitive to a membrane voltage of -50 mV. However, it does not open as rapidly as the voltage-gated Na+ channel. It takes only just under a millisecond for the channel to open once that voltage has been reached. This timing coincides precisely when the Na+ flow peaks, so voltage-gated K+ channels open just as the voltage-gated Na+ channels are being inactivated. As the membrane potential repolarises and the voltage passes -50 mV, the channel closes. Potassium continues to exit the cell and the membrane potential becomes more negative, resulting in the hyperpolarising overshoot. Then the channel closes again and the membrane can return to the resting potential because of the ongoing activity of the non-gated channels and the Na+/K+ pump. All of these processes take place within 2 milliseconds (Figure 15). 


While an AP is in progress, another one cannot be initiated, this is denoted as the refractory period. There are two phases of the refractory period: the absolute refractory period and the relative refractory period. During the absolute phase, another AP will not start, this is because of the inactivation gate of the voltage-gated Na+ channel. Once that channel is at its resting state (less than -55 mV), a new action potential may be started only if a stronger stimulus other than the one that initiated the current AP. This is due to the flow of K+ out of the cell. Because that ion is rapidly exiting, any Na+ that attempt to enter will not depolarise the cell, but will only keep the cell from hyperpolarizing.

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Figure 15. Stages of an Action Potential.

Propagation of the action potential 

The AP is initiated at the beginning of the axon (termed the initial segment). There is a high density of voltage-gated Na+ channels so that depolarization can occur here. Progressing down the length of the axon, the AP is propagated because more voltage-gated Na+ channels are opened as the depolarization extends. This dispersal occurs because Na+ enters through the channel and travels along the inside of the cell membrane. As the Na+  flows its positive charge depolarizes more of the cell membrane. As the depolarization spreads, new voltage-gated Na+ channels open and more ions enter into the cell, spreading the depolarization farther.

Since the voltage-gated Na+ channels are inactivated at the peak of the depolarization, they cannot be opened again for a brief time (the absolute refractory period). The AP must propagate toward the axon terminals; as a result, the polarity of the neuron is maintained. Propagation applies to unmyelinated axons. When myelination is present, the AP propagates differently. Sodium ions that enter the cell at the initial segment start to extend along the length of the axon segment, but there are no voltage-gated Na+ channels until the first node of Ranvier. The distance between nodes is the optimum distance to keep the membrane still depolarized above the threshold at the next node. As Na+ spreads along the inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any further down the axon, the depolarization would have dropped off too much for the voltage-gated Na+ channels to be activated at the next node of Ranvier. If the nodes were any closer together, the speed of propagation would be longer. 

Propagation along an unmyelinated axon (termed as continuous conduction) travels along the length of a myelinated axon (termed as saltatory conduction). Continuous conduction is slow because there are always voltage-gated Na+ channels opening, and more Na+ entering into the cell. Saltatory conduction is faster because the AP ‘jumps’ from one node to the next (saltare = “to leap”), and the new influx of Na+ renews the depolarized membrane. Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. 


Potassium concentration

Glial cells, especially astrocytes, are accountable for maintaining the chemical environment of the CNS tissue. The concentrations of ions in the extracellular fluid are the basis for how the membrane potential is established and changes in electrochemical signalling. If the balance of ions is upset, severe outcomes are possible. Normally the concentration of K+ is higher inside the neuron than outside. After the repolarizing phase of the action potential, K+ leakage channels and the Na+/K+ pump ensure that the ions return to their original locations. Following a stroke or other ischemic event, extracellular K+ levels are elevated. The astrocytes in the area are equipped to clear excess K+ to aid the pump. But when the level is profoundly out of balance, the effects can be permanent. Astrocytes can become reactive in cases such as these, which impairs their ability to maintain the local chemical environment. The glial cells enlarge and their processes swell. They lose their K+ buffering ability and the function of the pump is affected or even reversed. 

Figure  7. Key structures of the motor-neuron such as the dendrites, cell body, axon.

 

Synaptic Transmission

Synapses link nerve cells to other nerve cells and sensory and effector cells). Electrical synapses are direct, ion-conducting cell-to-cell junctions through channels in the region of gap junctions. They are accountable for the conduction of impulses between neighbouring smooth or cardiac muscle fibres and also ensure communication between neighbouring epithelial or glial cells.

Chemical synapses use (neuro) transmitters for the conduction of information and provide not only 1-to-1 networks but also assist as ‘switching elements’ for the nervous system. They can enable or inhibit the neuronal transmission of information or process them with other neuronal input. At the chemical synapse, the onset of an AP in the axon initiates the release of the transmitter from the presynaptic axon terminals (Figure 16). The transmitter then diffuses across the narrow synaptic cleft to bind post-synaptically to receptors in the subsynaptic membrane of a neuron or a glandular or muscle cell. Depending on the type of transmitter and receptor involved, the effect on the postsynaptic membrane may either be excitatory or inhibitory.

Transmitters are released by controlled exocytosis of synaptic vesicles. Each vesicle contains a certain amount of neurotransmitters. In the motor end-plate, approximately 7000 molecules of acetylcholine (ACh) are released. Some of the vesicles are already anchored and ready to exocytose their contents. An inbound AP functions as the signal for transmitter release. The higher the AP frequency in the axon the more vesicles release their contents. An AP increases the open probability of voltage-gated Ca2+ channels in the presynaptic membrane, thereby leading to an increase in the cytosolic Ca2+ concentration. Extracellular Mg2+ inhibits this process. Ca2+ binds to synaptotagmin, which initiates the interaction of syntaxin and SNAP-25 on the presynaptic membrane with synaptobrevin on the vesicle membrane, thereby triggering exocytosis of already anchored vesicles. Equally, Ca2+ activates calcium- calmodulin-dependent protein kinase-II (CaM-kinase-II), which activates the enzyme synapsin at the presynaptic terminal. This results in vesicles dock again on the active zone.

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Figure 16. Chemical Synapse

Synaptic Potentiation

If an AP reaches the presynaptic terminal immediately after another AP, the cytosolic Ca2+ concentration will not decrease to the resting value, and residual Ca2+ will accrue. As a result, the new rise in [Ca2+]i forms on the former one. [Ca2+]i increases to a higher level after the second stimulus than after the first, and releases more transmitters. Consequently, the first stimulus assists in the response to the second stimulus. Muscle strength increases at high stimulus frequencies for similar reasons. Among the many substances that act as excitatory transmitters are acetylcholine (ACh) and glutamate (Glu). They are frequently released together with co-transmitters which modulate the transmission of a stimulus. If the transmitter’s receptor is an ion channel itself the channels open more often and allow a larger number of cations to enter and leave the cell. Other, metabotropic receptors affect the channel via G proteins that control channels themselves. Because of the high electrochemical Na+ gradient, the number of incoming Na+ ions is much larger than the number of exiting K+ ions. Ca2+ can also enter the cell. The net influx of cations leads to depolarization: excitatory postsynaptic potential (EPSP). The EPSP begins at approx. 0.5ms after the arrival of an AP at the presynaptic terminal. This synaptic delay is caused by the moderately slow release and diffusion of the transmitter.

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Figure 17. Chemical SynapseEffect of IPSP on postsynaptic stimulation

A single EPSP generally is not able to generate a postsynaptic (axonal) action potential (APA) but requires the activation of a large number of local depolarizations in the dendrites. Their depolarizations are transmitted electrotonically across the soma and summed on the axon hillock. Should the individual stimuli arrive at different times, the prior depolarization will not have dissipated before the next one arrives, and summation will make it easier to reach the threshold. This type of temporal summation, therefore, increases the excitability of the postsynaptic neuron. 

 

Inhibitory transmitters include substances as glycine, GABA (Gamma-aminobutyric acid), and acetylcholine. They increase the conductance, g, of the subsynaptic membrane only to K+. The membrane usually becomes hyperpolarized in the process. Increases in gK occur when Em approaches EK (Figure 17). However, the main effect of this inhibitory postsynaptic potential IPSP is not hyperpolarization–which works to counter EPSP-related depolarization. Instead, the IPSP related increase in membrane conductance short circuits the electrotonic currents of the EPSP. Since both EK and ECl are close to the resting potential, stabilization occurs, that is, the EPSP is cancelled out by the high K+ and Cl– short circuit currents. As a result, EPSP-related depolarization is reduced and the stimulation of postsynaptic neurons is inhibited.

Termination of synaptic transmission can occur due to inactivation of the cation channels due to a conformational change in the channel similar to the one that occurs during an action potential (Figure 18). This very rapid process called desensitization also functions in the presence of a transmitter. Other terminating pathways include the rapid enzymatic decay of the transmitter (e.g., acetylcholine) while still in the synaptic cleft, the re-uptake of the transmitter (e.g., noradrenaline) into the presynaptic terminal or uptake into extra neuronal cells (e.g., in glial cells of the CNS), endocytotic internalization of the receptor, and binding of the transmitter to a receptor on the presynaptic membrane. In the latter case, a rise in gK and a drop in gCa can occur, thus inhibiting transmitter release.

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Figure 18. Termination of transmitter action