Question

Explain the entire process of contraction in a skeletal muscle, starting with how the signal enters the muscle cell at the neuromuscular junction. Include all relevant ions, molecules, structures, and processes in your explanation. How does the muscle carry out a sustained contraction? How does a muscle relax? How does skeletal muscle differ from the other two muscle types?

Answer 1

Skeletal muscles contract and relax to mechanically move the body. Messages from the nervous system cause these muscle contractions. The whole process is called the mechanism of muscle contraction and it can be summarized in three steps:

(1) A message travels from the nervous system to the muscular system, triggering chemical reactions.

(2) The chemical reactions lead to the muscle fibers reorganizing themselves in a way that shortens the muscle , that’s the contraction.

(3) When the nervous system signal is no longer present, the chemical process reverses, and the muscle fibers rearrange again and the muscle relaxes.

For a contraction to occur there must first be a stimulation of the muscle in the form of an impulse (action potential) from a motor neuron (nerve that connects to muscle).That one motor neuron does not stimulate the entire muscle but only a number of muscle fibres within a muscle.

The individual motor neuron plus the muscle fibres it stimulates, is called a motor unit. The motor end plate (also known as the neuromuscular junction) is the junction where the motor neuron reaches a muscle cell. Skeletal muscle tissue is composed of cells called muscle fibers. When the nervous system signal reaches the neuromuscular junction a chemical message is released by the motor neuron. The chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fiber. That starts a chemical reaction within the muscle.

A multistep molecular process within the muscle fiber begins when acetylcholine binds to receptors on the muscle fiber membrane. When acetylcholine reaches receptors on the membranes of muscle fibers, membrane channels open and the process that contracts a relaxed muscle fibers begins;

• When the Ion channels opens and positively charged Sodium ions rush across the membrane. And this sodium influx release a message within the muscle fibre to trigger the release of stored Calcium Ion.
• The released Calcium ion diffuse in to the muscle fibre.The relationship between the chains of proteins within the muscle cells changes, leading to the contraction.
  • The action potential travels down the sarcolemma and enters the T-tubules. Calcium is released from the Sarcoplasmic reticulum.Calcium binds to troponin on the thin filament and it moves .Troponin is a complex of three proteins that are integral to muscle contraction. Troponin is attached to the protein tropomyosin within the actin filaments.Calcium floods into the muscle cell binding with troponin allowing actin and myosin to bind. The actin and myosin cross bridges bind and contract using ATP as energy (ATP is an energy compound that all cells use to fuel their activity ). ATP binds to myosin heads on a thick filament and the heads back and bind to the Actin molecules on the thin filament. Myosin heads and pull a thin filament towards M-line ,the middle of Sarcomere. ATP bind myosin heads and heads release the actin filaments.A cross-bridge forms between actin and the myosin heads triggering contraction.

A tetanic contraction (also called tetanized state,or physiologic tetanus, the latter to differentiate from the disease called tetanus) is a sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. During this state, a motor unit has been stimulated maximum by its motor neuron . This occurs when a muscle's motor unit is stimulated by multiple impulses at a sufficiently high frequency. Each stimulus causes a twitch. If stimuli are delivered slowly enough, the tension in the muscle will relax between successive twitches. If stimuli are delivered at high frequency, the twitches will overlap, resulting in tetanic contraction. A tetanic contraction can be either unfused (incomplete) or fused (complete).

• Muscle Fibers Relax When the Nervous System Signal Is No Longer Present.
• When the stimulation of the motor neuron providing the impulse to the muscle fibers stops, the chemical reaction that causes the rearrangement of the muscle fibers' proteins is stopped. This reverses the chemical processes in the muscle fibers and the muscle relaxes. Calcium is then pumped back into the sarcoplasmic reticulum breaking the link between actin and myosin. Actin and myosin return to their unbound state causing the muscle to relax. Alternatively relaxation (failure) will also occur when ATP is no longer available.

In the muscular system, muscle tissue is categorized into three distinct types ;

Skeletal, Cardiac, and Smooth.

• Each type of muscle tissue in the human body has a unique structure and a specific role.
• Skeletal muscle moves bones and other structures.  
• Cardiac and skeletal muscle are both striated in appearance, while smooth muscle is not.
• Both cardiac and smooth muscle are involuntary while skeletal muscle is voluntary.
• Unlike skeletal muscle, smooth muscle is not under conscious control.
• Cardiac Muscle Contracts in Response to Signals from the Cardiac Conduction System.
• The skeletal muscle is controlled by the somatic nervous system,
• Cardiac muscle contracts the heart to pump blood.
• Smooth muscles, which generate involuntary movement, form part of the walls of the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, and blood vessels, among other portions of the body.
• Cardiac muscle is involuntary and found only in the heart.
• Skeletal muscle is striated in regular, parallel bundles of sarcomeres.