A) Movement, and includes the change in the position of certain organ in relation to other parts of the body

B) Transportation from one place to another.

C) Continuous movement of blood inside blood vessels, and to maintain normal blood pressure inside blood vessels through the contraction of the involuntary smooth muscles in the wall of blood vessels.

D) Maintain body posture either in setting or standing positions especially by the muscles of neck, trunk, and lower limbs.

Structure of the muscle:

The muscle consists of a large number of units called muscle cells or muscle fibres. Each muscle fibre consists of a group of myofibrils, 1000 to 2000 in number, arranged longitudinally and parallel to the longitudinal axis of the muscle. Each muscle fibre contains a large number of nuclei (multinucleated).

The structure of a muscle fibre:

Each muscle fibre consists of:

a) The living matter (the cytoplasm) which is called in case of muscle fibres sarcoplasm.

b) The cell membrane and is called sarcolemma.

c) The muscle fibres are collected in groups called muscle bundles, which are surrounded by a membrane called perimycium.

d) Each myofibril contains:

Alternating dark and light bands (or disks) which give the muscle fibre a striated appearance. The light band called I-band and bisected by a dark line called Z-line and formed of a thin protein filament called actin.

The dark band is called A-band and bisected by a light area called H-zone and formed of another thick protein filament called myosin.

3. The distance between each successive two Z- lines is called sarcomere.

This pattern of striation is present in the skeletal muscles and cardiac muscles, and hence they are called striated muscles. This pattern is not present in the smooth muscles, so they are called unstriated muscles.

Muscle contraction:

Three main systems coordinate together to perform proper different body movements which are:

1. The skeletal (bony) system: As it provides a suitable site of connection of muscles, and to support the moving limbs. Joints play an important role in the movement.

2. The nervous system: Which gives the orders for muscles (in the form of nerve impulse) to contract and relax.

3. The muscular system: This is responsible for movement. Most of the body muscles are under the control of will and called voluntary muscles (skeletal, striated muscles). Other muscles are not under the control of will and called involuntary muscles (cardiac muscles and smooth muscles).

How does the muscle contract?

What is the role of nerve impulse and the physiology of muscle response to nervous stimulation?

How all these parts coordinate together?

In skeletal muscles, the external surface of the muscle fibre membrane is positively charged in relation to the internal surface which is negatively charged. This potential difference is due to the difference in ionic concentrations between outside and inside the membrane.

The stimulus for muscle contraction is the arrival of nerve impulses through the motor nerve coming from the brain and spinal cord, the ends of which are in close to the muscle fibres forming a synapse.

3. The nerve endings contain vesicles filled with chemical substances called the neurotransmitters (acetylcholine).

4. On the arrival of a verve impulse to the nerve ending, it results in the release of the neurotransmitters acetylcholine from the vesicles and Calcium ions play an important role in the process of release (as in the synaptic transmission). The neurotransmitter crosses the synaptic cleft (the narrow space between the nerve fibre and the muscle fibre). When the neurotransmitter reaches the membrane of the muscle fibre it leads to loss of polarity and potential difference and then reverse of polarity making inside positive and outside negative and the membrane is said to be depolarized. These changes are due to the increase of permeability of the muscle fibre membrane to sodium ions. This leads to muscle contraction.

5. After a part of a second the muscle fibre membrane returns to normal (repolarization) due to the action of the enzyme cholinesterase which destroys acetylcholine to choline and acetate. The membrane permeability to ions returns to the resting state, it is now ready to be stimulated and respond again and so on.

Mechanism of muscle contraction (the theory of sliding filaments):

The most acceptable theory for muscle contraction is the theory of Huxely. This theory depends on the microscopic structure of the muscle fibre which consists of myofibrils and each myofibril consists of thin actin filaments and thick myosin filaments. Huxely compares a muscle fibre in a state of contraction with another fibre in a state of relaxation using electron microscope.

Huxely concluded that: The protein filaments slide over each other due to the presence of transverse links extended from the myosin filaments and attach to the actin filaments. In presence of Calcium ions and energy the transverse links act as hooks that pull the actin filaments from both sides towards each other leading to muscle contraction. This theory does not explain the contraction of unstriated smooth muscle although scientific reports suggest that the protein filaments in smooth muscle are almost similar to that in skeletal muscles.

The motor unit:

The motor unit is the unit of function of the skeletal muscle. The muscle contraction is the summation of all motor units forming the muscle. Each motor unit consists of a group of muscle fibres and the nerve fibre supplying them. As the nerve fibre enters the muscle it divides into a large number of branches which supply a group of muscle fibres (5 – 100). Each terminal branch attaches one muscle fibre at a place called the motor end plate and the point of connection is called the neuromuscular junction.

Muscle fatigue:

Repeated rapid contraction of a muscle leads to its fatigue as a result of the inability of the blood to supply the muscle rapidly with the Oxygen needed for respiration and energy production. Accordingly, the muscle converts Glycogen to Glucose which in turn is oxidized anaerobically (in the absence of Oxygen) to produce rapid energy to give the chance for the muscle to contract. Continuous anaerobic respiration leads to the accumulation of Lactic acid. Lactic acid causes muscle fatigue and accordingly the subject stops moving to give the chance for the muscle to be supplied by enough amount of Oxygen to perform aerobic cellular respiration which produces a large amount of energy compared with that of anaerobic respiration.