biologgy

Group of Muscle Fibers

2. Motor Nerve Fiber (which divides

into a large number of terminal

branches) supplying them with

stimulus.

For illustration

Nerve-Muscle

Interaction

• No. of muscle fibers = no. of neuromuscular junctions = no. of

terminal branches = No. of motor end plates

• The stimulus which causes the contraction of voluntary skeletal muscles is the

arrival of a nerve impulse through the motor nerve fibers which come from the

brain or the spinal cord and whose nerve endings are attached firmly to the

muscle fiber forming a “Synapse” (Neuromuscular Junction)

• The nerve endings in the synapse have vesicles containing chemical

neurotransmitters as “Acetylcholine”.

How the skeletal (voluntary) muscle contracts:

The skeletal muscle contracts under the effect of the nerve impulses and the

physiology of the muscle response to this nerve stimulus, and this is due to

the coordination among the skeletal, nervous and muscular systems.

How the nerve impulse is transmitted to the skeletal muscle?

Critical pointCritical point

For illustration

changes

For illustration

Repolarization

For illustration

• Repeated and rapid contraction of muscles leads to its fatigue due to the inability

of blood to supply muscles rapidly with oxygen needed for aerobic respiration and

production of energy.

• Accordingly, the muscle converts glycogen (animal starch) to glucose which is

oxidized anaerobically (due to shortage of oxygen) to produce energy (2 ATP) to

allow and give the muscle a chance to contract. Anaerobic oxidation of glucose

leads to the accumulation of lactic acid which causes muscle fatigue.

• Shortage of ATP leads to the prevention of transverse links separation from the

actin filaments. So, they remain attached to the actin filaments and the muscle

remains in the state of contraction and cannot relax causing painful muscle

spasms.

• By stopping of movement (at rest), the muscle becomes supplied by enough

amount of oxygen for aerobic cellular respiration which produces large amounts

of ATP (38 ATP) causing separation of the transverse links from the actin filaments

and the muscle relaxes, then it goes on alternation of contraction and relaxation.

• The excessive muscle spasm may lead to muscle tear causing bleeding.

• The proper energy storage in the muscle is glycogen.

• The direct energy storage in the muscle is ATP.

Critical pointCritical point

Muscle spasm causes

• ATP shortage .

• Incorrect nerve impulse from brain.

• Decrease in Parathormone hormone(to be discussed later in Ch.2).

• Absence of Cholinesterase enzyme.

Critical pointCritical point

Cardiac and smooth muscles don’t get exhausted while skeletal muscles get

exhausted by vigorous activities where fatigue may occur.

Group of Muscle Fibers

Muscle fibers are essential components of muscle tissue, organized into bundles known as fascicles. Each motor unit consists of a motor nerve fiber that splits into numerous terminal branches, each innervating muscle fibers and providing stimuli necessary for contraction.

Nerve-Muscle Interaction

The number of muscle fibers is equal to the number of neuromuscular junctions, terminal branches, and motor end plates.
The contraction of voluntary skeletal muscles is triggered by the arrival of a nerve impulse via motor nerve fibers originating from the brain or spinal cord. The nerve endings of these fibers are firmly attached to the muscle fiber, forming a synapse known as the neuromuscular junction (NMJ).
Within the synapse, nerve endings contain vesicles packed with the chemical neurotransmitter acetylcholine, which conveys the signal from the nerve to the muscle fiber, initiating contraction.

Mechanism of Skeletal Muscle Contraction

Skeletal muscle contracts as a direct response to nerve impulses due to intricate coordination among the skeletal, nervous, and muscular systems. The process involves various physiological responses triggered by the nerve stimulus.

Transmission of Nerve Impulse to Skeletal Muscle

Upon stimulation, acetylcholine is released into the synaptic cleft, binding to receptors on the muscle fiber and causing depolarization of the muscle cell membrane, leading to contraction.

Muscle Fatigue

Repeated and vigorous muscle contraction can lead to fatigue, primarily due to insufficient blood flow to provide oxygen for aerobic respiration, necessary for efficient energy production.
In scenarios of oxygen shortage, muscles convert glycogen (animal starch) to glucose, which is then oxidized anaerobically to yield energy, generating 2 ATP molecules. This anaerobic breakdown results in the accumulation of lactic acid, contributing to muscle fatigue.
A lack of ATP prevents the transverse links (myosin heads) from detaching from actin filaments, causing the muscle to remain contracted. This state can lead to painful muscle spasms and an inability to relax.
Prolonged muscle activity demands rest; during rest, muscles receive adequate oxygen levels for aerobic cellular respiration, resulting in considerable ATP production (up to 38 ATP per glucose molecule) that facilitates the detachment of myosin heads from actin, allowing muscle relaxation.
Severe muscle spasms might cause muscle tears and internal bleeding.

Energy Storage in Muscles

The primary energy source stored in muscle tissue is glycogen. In immediate energy use, adenosine triphosphate (ATP) serves as the direct energy source for muscle contraction.

Causes of Muscle Spasms

Muscle spasms may occur due to various factors:

Shortage of ATP, leading to sustained contraction.
Incorrect nerve impulses sent from the brain, disrupting normal control.
Decreased levels of parathormone hormone, influencing calcium homeostasis essential for muscle function.
Absence or deficiency of the enzyme acetylcholinesterase, which breaks down acetylcholine in the synapse, affecting muscle relaxation.

Comparison of Muscle Types

Unlike skeletal muscles, which can become exhausted from intense activity, cardiac and smooth muscles maintain their performance without fatigue, due to their unique physiological properties and metabolic requirements.