March 29, 2024

Muscles can be referred to simply as the flesh mostly associated with bones to give shape to the bodies of animals.

 

The flesh may be red in the case of mammals.

 

General properties of muscular system

All muscular tissues are contractile. This means that the muscle pulls its insertion towards its origin thereby becoming shorter and thicker. In doing this, some muscles reduce to one-half or one-third of their resting length. A muscle is said to be at rest when it is not contracting visibly to bring about movement. This said “resting” is when it is in a slightly contracted state known as the tone or tonus of the muscle. The muscle is well supplied with nerve by nerve endings of the end-plate types.

If the nerve supply is severed, the muscle tone is lost and the muscle becomes flabby. The muscle is also well supplied with blood through numerous blood vessels and capillaries. The muscles account for about 40% of the total body weight of a mammal. It is 75% water and the remainder consisting of protein (actin and myosin), fat, glycogen, mineral salts, ATP and phosphagen. The muscle is made up of a number of elongated cells called fibres which are in turn made up fibrils and filaments. The Muscles are effectors, that is structures which respond directly or indirectly to stimulus.

 

Structure of muscle

A whole muscle, such as the biceps of the upper arm of man is made up of numerous fibres of various length ranging from 1 to 40 millimetres. At the two ends are strong bands of white fibrous tissues called tendon by which muscles are attached to the bones. The tendon is also contractile.

 

Muscles and their attachment to bones in upper arm of man

Each muscle fibre is a multinucleate cell (syncytium) tapering at both ends, filled with specialized cytoplasm called sarcoplasm.

The delimiting or bounding membrane is called sarcolemma.

Myofibrils or fibrils are seen to run parallel from one end to the other. These fibrils are very important in the contraction of the muscle. Short striations cross the elongated myofibrils breadth-wise.

 

Types of muscles

There are three types of muscles:

  • unstriped muscle.
  • striped muscle.
  • cardiac muscle.

 

A) Unstriped muscle (involuntary muscle)

The unstriped muscle is also called involuntary muscle, smooth muscle, unstriated muscle, or non-skeletal muscle. It is found in the wall of the gut, blood vessels, iris and ciliary body of the eye and in the dermis of the skin. They are also present in the urinary bladder, genital tracts and wall of bronchioles. Unstriped muscle is made up of long narrow spindle-shaped fibres. Each fibre has a central nucleus surrounded by cytoplasm which contains fine contractile threads (myofibrils) running longitudinally.

 

Unstriped muscle is not easily fatigued and is not under the control of the will, its action is involuntary. It is under involuntary control being supplied (innervated) by autonomic nervous system. The movements are therefore slow.

 

The functions of the unstriped muscle are:

  1. The closing and opening of cavities, tubes and ducts.
  2. In the alimentary canal, circular muscles narrow the lumen while the longitudinal ones open it.
  3. In the iris, circular muscles will narrow the pupil while the radial muscles dilate it.

 

B) Striped muscle

Voluntary muscle is under the control of the will. It receives its nerve supply from the central nervous system (brain). The voluntary muscles are striped or striated in structure. Voluntary muscles are also called skeletal muscle because they are attached to bones. They contract and relax at will to bring about movements of the parts to which they are attached. The point of attachment to the stationary bone is called origin of the muscle while that on the movable bone is called point of insertion. Voluntary muscles work in pairs, a flexor causing bending at a joint and an extensor causing straightening.

Biceps contract to bend the arm at the elbow while the triceps is relaxed. The triceps contracts to straighten the arm while the biceps is relaxed. Each muscle consists of numerous fibres which are arranged in large numbers to form bundles called fasciculi (singular fasciculus). A fasciculus is bounded by a membrane called perimysium. The entire muscle is bounded by a similar membrane called epimysium.

 

Each muscle fibre within a fasciculus is in turn made up of a number of myofibrils of similar structure.

 

Functions of voluntary muscles

  1. Voluntary muscles contract to move the body in the various activities we carry out such as walking, swinging of the arms, running, swimming, jumping etc.
  2. Lifting and carrying of weights.
  3. Pulling, pushing, dragging, rolling and throwing of objects.
  4. Opening and closing of the mouth, chewing of substances in the mouth, shouting etc.
  5. Working on the field, playing games, sporting, hunting, driving, riding etc.

 

C) Cardiac muscle

Cardiac muscle is also called heart muscle because it is found in the walls of the heart. Each fibre of cardiac muscle is a short cylindrical cell. The cells are arranged in column connected by short oblique cross connexions to form the tissue. Cardiac muscle are striated. They are intermediate between the skeletal muscles and involuntary muscles in structure. Like the voluntary muscles its action is rapid but it is not easily fatigued. It contracts and relaxes without stop throughout the life of the animal. It is innervated by both the cranial nerve and the vagus nerve of the autonomic nervous system.

 

The vague nerve of the parasympathetic nervous system slows down or deaccelerates the heart beat while the sympathetic nervous fibre accelerates or quickens the heartbeat. During muscular action, adrenaline is found to be on the increase in the blood and the heart beats faster as more blood carrying oxygen gets to the tissue. More glucose is oxidized to release energy for the muscular action and more carbon (IV) oxide is produced. Adrenalin speeds up the heart beat.

 

HOW REGULAR EXERCISE HELPS MUSCLES AND JOINTS

Regular exercise is very important for an efficient working of the body muscles in the following ways.

  1. Help to tone the muscles so that they are able to contract well.
  2. Well exercised muscles prevent aches.
  3. Blood supply to the muscles is improved, thus making the heart and lungs muscles to work efficiently.
  4. Body joints work efficiently and smoothly.

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