Muscle Physiology

Like nerve fibers, muscles are also excitable tissue. The contract when stimulated.

Muscles are classified as follows:

1. Skeletal muscles
2. Smooth muscles
3. Cardiac muscles

Skeletal muscles:

• They are voluntary and are attached to skeletal system
• The muscle fibers show alternate light and dark bands called striations
• They are supplied by somatic nerves

Smooth muscles:

• They are involuntary muscles and have no striations.
• They are supplied by autonomic nervous system
• The smooth muscles are present in the walls of GIT and hollow internal organs
• They are also called as Visceral muscles.

Cardiac muscles:

• They too are striated muscles like the skeletal muscles but are involuntary
• They are regulated by the autonomic nervous system
• They are the muscles of the heart.

Other classifications:

(a) Based on striations

• Striated muscle
• Nonstriated muscle

(b) Based on control

• Voluntary
• Involuntary

Structure of a Muscle

The typical skeletal muscle is attached to the bone by a tendon.

• The bulk of the muscles consist of number of muscle bundles called the fascicles
• Each fascicle is made up of large number of muscle fibers
• Each muscle fiber is a single cell which is cylindrical in shape and multinucleated.
• The muscle fiber contains several thousand myofibrils.
• Each myofibril is in turn made up of around 1500 myosin and 3000 actin filaments. Actin and Myosin are large protein molecules which cause muscle contraction. 
• The cell membrane of the muscle fiber is called sarcolemma
• The cytoplasm of the muscle cell is called the sarcoplasm
• The endoplasmic reticulum is called the sarcoplasmic reticulum

Muscle

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Fascicle

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Muscle fiber (muscle cell)

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Myofibril

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Myofilaments

Sarcomere

• It is the structural and functional unit of a muscle
• It is bound by two Z lines
• The muscle fiber shows alternate light and dark bands
• The A band is formed by myosin filaments
• The I band is formed by thin filaments made of troponin, tropomyosin and actin
• In the centre of A band the thin and thick filaments do not overlap forming H zone
• The centre of H zone forms the M line

Muscle proteins

The various types of muscle proteins are as follows

1. Myosin
2. Actin
3. Tropoin (Troponin I, T & C)
4. Tropomyosin

Sarcotubular System

• It is the system of communication between the outer sarcolemma and interior of the cell
• The sarcolemma invigilates into the muscle to form T (Transverse) tubule
• On the either side of T tubules are L tubules which is the sarcoplasmic reticulum
• The enlarged terminal part of L tubule is called the terminal cistern.
• T and L tubules together form the sarcotubular system

Functions:

• The sarcotubular system helps in transmission of impulse from the surface of the muscle to its interior. It stores calcium ions which are released during muscle contraction.

Excitation-Contraction coupling

• It is the electrochemical changes occurring in the nerve, neuromuscular junction and muscle in response to a stimulus.
• During this process, the electrical activity is transformed into mechanical activity.
• A threshold stimulus produces an action potential at the site of stimulation.
• The impulse travels along the nerve fiber and crosses the neuromuscular junction.
• This activates the motor end plate and generates the necessary signals to produce mechanical shortening of the muscle.