In the human body, a band or bundle of fibrous tissue that has the flexibility to contract, producing motion in or sustaining the place of components of the physique. There are round about 640 muscles in the human body. This system is called muscular system. There are three main types of muscles in human body. Skeletal Muscle, Cardiac Muscle (myocardium) and Smooth muscle.
1. Skeletal Muscle
1. The nerve cells are covered by a lipid membrane called neurolemma sheath. Similarly, skeletal muscle fibers are covered by the sarcolemma sheath. Skéletal muscle fibers are very long, a single fiber stretching across the whole length of the muscle.
2. Skeletal muscle fibers contain microscopic filaments called sarcomeres.
3. When activated the contractile elements which contain the protein, actinomyosin, The energy for this contractiön is supplied by the high energy phosphate bonds of ATP (adenosine triphosphate) and CP (creatine phosphate).
4. ATP breaks down into ADP (adenosine diphosphate) and releases the energy of the high energy phosphate bond. When the contraction is over, ADP is reconverted to ATP, the energy for reconversion being supplied by metabolism of glucose.
5. When skeletal muscle contracts, the fibers shorten and cause movement of the bones at the joint.
6. A skeletal muscle after repeated or sustained contractions gets fatigued and some time has to elapse for the muscle to recover from fatigue.
7. If the nerve supplied to the skeletal muscle is cut, the muscle cannot contract by nerve stimulation. However, it can still contract if the muscle is directly stimulated by electrodes. Direct stimulation by electrodes is not
normal. Stimulation by nerves is the normal mechanism of contraction of skeletal muscle.
2. Neuromuscular Transmission
1. We have already stated that the skeletal muscle contracts when the nerve supplying it is stimulated. In a skeletal muscle, every individual muscle fiber is supplied by a branch of the nerve which supplies the whole muscle.
2. The junction of the nerve and the muscle fiber is called the neuromuscular junction. The nerve ending is called nerve terminal. it is expanded like a foot and is called the foot pad.
3. There is no direct contact between the nerve terminal and the muscle fiber. There is a small gap called neuromuscular gap. The impulse from nerve is transmitted across the neuromuscular gap by a chemical substance, acetylcholine.
4. In the resting state, the nerve terminal is polarized. There is no electric current.
5. When an impulse comes down the nerve, the nerve terminal becomes depolarized.
6. Small globules of acetylcholine present in the nerve terminal are liberated in the neuromuscular gap.
7. Acetylcholine is the chemical transmitter across the gap and reaches the motor end plate of the muscle. The liberated acetylcholine combines in the motor end plate of the muscle with special receptors called acetylcholine receptors. Na+ and Ca++ channels are opened causing depolariudon of the muscle.
8. An action potential is created and the muscle contracts.
9. Acetylcholine liberated at the neuromuscular junction to cause contraction of the muscle, is destroyed by the enzyme cholinesterase. The neuromuscular junction returns to the resting state, i.e. it becomes repolarized. The muscle relaxes. The whole event takes place in a fraction of a second.
10. Knowledge of the physiology of neuro muscular transmission has been exploited in the development of drugs which block neuro muscular transmission in skeletal muscles. These drugs are very useful in anesthesia and surgery.
3. Smooth Muscle
In all respects smooth muscles resemble skeletal muscles except for the following:
1. They are not attached to bones.
2. They usually form sheets which go to make up the wall of tube-like muscular structures in the body, e.g. urethra, bile duct, intestines and blood vessels.
3. Smooth muscles are not voluntary control. They are controlled by the autonomic or involuntary nervous system.
4. Contraction of smooth muscles is slow and prolonged. The fibers contract when they are stretched.
5. Smooth muscles are never totally relaxed. They are in a state of partial contraction or tone so that the tube like muscular structure they form is also never totally dilated but is always in a state of partial contraction.
6. Contraction of muscle is similar to skeletal muscle in that the energy is supplied by ATP and the contractile element is actinomyosin.
7. The most important ion concerned in smooth muscle contraction is the Ca++ ion. The muscle fibers need an influx of Ca++ from outside the cell to inside the cell before contraction, i.e. the Ca++ channel must become active. If the influx of Ca++ is blocked the muscle cannot contract.
8. This knowledge has led to the development of Ca++ channel blocking drugs. These drugs inhibit the tone of the smooth muscles of the blood vessels. As a result, the blood vessels dilate causing a reduction of blood pressure.
4. Cardiac Muscle (Myocardium)
The heart muscle (cardiac muscle) is a unique muscle. The muscle cells are individual cells but they are inter-connected to form a sheet and the whole sheet behaves like a single muscle fiber.
This woven sheet-like cardiac muscle is called syncytial muscle. The heart muscle has the property of rhythmicity. Even if the muscle is isolated from its nerve supply, it will continue to contract and relax alternately in a rhythmic fashion. Immediately after the contraction the muscle relaxes and will not contract for a short period of time. This is called refractory period. The energy supplied is from ATP and the most important event is the opening of the fast sodium channel which allows in Na+ ions. The Na+ ions produce the action potential and depolarization. This leads to contraction of the muscle.