The skeletal muscle
The muscle consists of bundles of myocytes containing actin and myosin molecules. These molecules integrate and form myofibrils which cause muscle contraction in the same way as myocardial muscle.
Myocyte/sarcomere is elongated surrounded by cell membrane called sarcolemma with nuclei beneath and oxygen binding substance (myoglobin that store oxygen).
Myocyte is surrounded by a connective tissue called endomysium, a group of muscle fibres (fascicle) is surrounded by another connective tissue called perimysium and the whole muscle is surrounded by another connective tissue called epimysium
The three connective tissue forms a tendon which attaches a muscle to the bone surface (enthesis).
A muscle fascia is a fibrous tissue that surrounds the epimysium and tendon
Physiology of a skeletal muscle
Sarcomere is the functional unit of muscle contraction
When a muscle contracts light (actin) filaments and dark (myosin) filaments in a muscle come closer i.e. they interact to shorten the length of the muscle cell.
Many knobs (heads) like projection in myosin form cross-bridges with actin stimulating the muscle to contract, the cross bridge move pulling the filaments pass each other
When the cross bridge has moved as far as it can, it releases the actin and return to original position. The cross bridge is attached to actin of another place
When ATP is absent a muscle is an able to contract continuously (muscle fatigue) leading to muscle cramp.
Control of muscle contraction
The muscle is connected to CNS by motor neuron. Impulses from this neuron contract the muscle.
The point of contact between the motor neuron and muscle is called neuromuscular junction. Vesicle at the axon terminal release acetylcholine an important neurotransmitter
Impulses cause release of calcium ions within the cell which affect regulatory protein that allow actin and myosin to interact and form cross-bridges. A muscle cell will remain in a state of contraction until acetylcholine production stops
Acetylcholinesterase enzyme produced at the neuromuscular junction to destroy acetylcholine that permits re-absorption of calcium into the muscle cell and terminates contraction
Intensity of contraction depends on what you are trying to accomplish. The frontal lobe of the cerebrum decide what and how many muscle cells need to contract
Bones
- These form the bodies frame work i.e shape and form of the body They give attachment of muscle and ligaments.
- They also protect vital organs, store calcium, form blood elements and allow movement of the body as a whole
The function unit of a bone is an osteocyte
The bony matrix is produced by osteoblasts and comprises of colloids, mineral, carbohydrates and proteins
Various hormones are needed in maintain bony tissue i.e. thyroid hormones, growth hormones, calcitonin, parathyroid hormone and adrenal hormones
Joints
This is where two or more bones met. Joints allow flexibility and movement of the skeleton.
Types of joints
There are three types of joints
Fibrous joints
These are joints in which articulating surfaces of bones are connected by a fibrous tissue. Examples include sutures, middle radioulnar joint and joints between teeth.
There are three types of fibrous joints:
(1) Sutures are nonmoving joints that connect bones of the skull. These joints have serrated edges that lock together with fibers of connective tissue.
(2) The fibrous articulations between the teeth and the mandible or maxilla are called gomphoses and are also immovable.
(3) A syndesmosis is a joint in which a ligament connects two bones, allowing for a little movement (amphiarthroses). The distal joint between the tibia and fibula is an example of a syndesmosis.
Cartilaginous joints
The bone ends are united by a cartilage for example intervertebral joints, pubic symphysis and costochondral joints.
There are two types of cartilaginous joints:
(1) A synchrondosis is an immovable cartilaginous joint. One example is the joint between the first pair of ribs and the sternum.
(2) A symphysis consists of a compressable fibrocartilaginous pad that connects two bones. This type of joint allows for some movement. The hip bones, connected by the pubic symphysis, and the vertebrae, connected by intervertebral discs, are two examples of symphyses.
Synovial joints
These joint articular bony ends are lined by synovial membrane with synovial fluid to permit free movement. These include the ball and socket joints e.g. Hip and the hinge joints e.g. interphalangeal, knee joint etc
Synovial joints are characterized by the presence of an articular capsule between the two joined bones. Bone surfaces at synovial joints are protected by a coating of articular cartilage. Synovial joints are often supported and reinforced by surrounding ligaments, which limit movement to prevent injury. There are six types of synovial joints:
(1) Gliding joints move against each other on a single plane. Major gliding joints include the intervertebral joints and the bones of the wrists and ankles.
(2) Hinge joints move on just one axis. These joints allow for flexion and extension. Major hinge joints include the elbow and finger joints.
(3) A pivot joint provides rotation. At the top of the spine, the atlas and axis form a pivot joint that allows for rotation of the head.
(4) A condyloid joint allows for circular motion, flexion, and extension. The wrist joint between the radius and the carpal bones is an example of a condyloid joint.
(5) A saddle joint allows for flexion, extension, and other movements, but no rotation. In the hand, the thumb’s saddle joint (between the first metacarpal and the trapezium) lets the thumb cross over the palm, making it opposable.
(6) The ball-and-socket joint is a freely moving joint that can rotate on any axis. The hip and shoulder joints are examples of ball and socket joints.
Hyaline cartilage
It forms the articular surface and is vascular. It relies on diffusion from synovial fluid for its nutrition. It is rich in type II collagen that forms a mesh work enclosing giant macro – molecular aggregates or proteoglycans
DISEASES AFFECTING SKELETAL MUSCLES
MYASTHENIA GRAVIS
This is another autoimmune disease predominantly affecting females. There is un usual fatigue due to lack of acetylcholine receptor at the myoneural junctions which impair muscle contraction.
Signs and symptoms
- The onset is gradual.
- Excessive fatigue particularly towards end of the day with drooping of eye lids
- Frequent falls
- Difficult in chewing and swallowing
- Involvement of respiratory muscles may lead to respiratory failure
- A weak cough reflex may lead to accumulation of secretions and infections
Treatment and management
- Short acting anticholine esterase drugs like edrophonium
- Long acting ant cholinesterase drugs e.g. neostigmine or pyrisostigmine
- Thymectomy and steroids also provide relief.
- Exercises
MYOSITIS (MYOPATHY)
Myositis or myopathy refers to a group of primary diseases of muscles; Myositis (inflammation of muscles) can be genetically diseases.
Progressive muscular atrophy is a group of hereditary disorder characterized by progressive delegation of muscles without involvement of bones.
The wasting and weakness of muscles is symmetrically without any sensory loss. The affected muscles are large, firm but weak.
The child walks with a waddling giant like that of the duck
When rising from a supine lying position, in bed, the child rolls on his face (prone position) and then uses his arms to push his body up (tripod sign). Death in second decade is usual due to involvement of respiratory muscles.
Fibrositis – rheumatism (fibrocystic)
Fibrositis and muscular Rheumatism are terms used to describe recurring pains, stiffness in the muscles or the back, various parts of the body being involved from time to time.
The disease does not progress and such vague symptoms are attributable to emotional stress.
Treatment
Is usually symptomatic, heat and massages may be helpful and aspirin or one of the NSAIDS can be prescribed.