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Q. Explain the basic Physiology of Muscle Contraction?

Answer 1

Ans) Muscle is contractile tissue and is derived from the mesodermal layer of embryonic germ cells. Its function is to produce force and cause motion, either locomotion or movement within internal organs.

- 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).

- When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments.

- 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 of the motor neurons axon and the muscle fibres it stimulates.

- When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. This reaction results in the start of a contraction and the sliding filament theory.

- The reaction, created from the arrival of an impulse stimulates the 'heads' on the myosin filament to reach forward, attach to the actin filament and pull actin towards the centre of the sarcomere. This process occurs simultaneously in all sarcomeres, the end process of which is the shortening of all sarcomeres.

- Troponin is a complex of three proteins that are integral to muscle contraction. Troponin is attached to the protein tropomyosin within the actin filaments, as seen in the image below. When the muscle is relaxed tropomyosin blocks the attachment sites for the myosin cross bridges (heads), thus preventing contraction.

- When the muscle is stimulated to contract by the nerve impulse, calcium channels open in the sarcoplasmic reticulum (which is effectively a storage house for calcium within the muscle) and release calcium into the sarcoplasm (fluid within the muscle cell). Some of this calcium attaches to troponin which causes a change in the muscle cell that moves tropomyosin out of the way so the cross bridges can attach and produce muscle contraction.

You are here: Home › Training Design › Anatomy and Physiology › The Physiology of Skeletal Muscle Contraction
The Physiology of Skeletal Muscle Contraction
In this page we look at the physiology behind muscular contraction and what causes a contraction to cease. Low and behold one simple mineral is really quite critical...

What is the Sliding Filament Theory of muscular contraction?
The sliding filament theory is the explanation for how muscles contract to produce force. As we have mentioned on previous pages, the actin and myosin filaments within the sarcomeres of muscle fibres bind to create cross-bridges and slide past one another, creating a contraction. The sliding filament theory explains how these cross-bridges are formed and the subsequent contraction of muscle.

The Sliding Filament Theory
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).

personal%20training%20motor%20unitNote 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 of the motor neurons axon and the muscle fibres it stimulates.

When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. This reaction results in the start of a contraction and the sliding filament theory.

The reaction, created from the arrival of an impulse stimulates the 'heads' on the myosin filament to reach forward, attach to the actin filament and pull actin towards the centre of the sarcomere. This process occurs simultaneously in all sarcomeres, the end process of which is the shortening of all sarcomeres.

Troponin is a complex of three proteins that are integral to muscle contraction. Troponin is attached to the protein tropomyosin within the actin filaments, as seen in the image below. When the muscle is relaxed tropomyosin blocks the attachment sites for the myosin cross bridges (heads), thus preventing contraction.

When the muscle is stimulated to contract by the nerve impulse, calcium channels open in the sarcoplasmic reticulum (which is effectively a storage house for calcium within the muscle) and release calcium into the sarcoplasm (fluid within the muscle cell). Some of this calcium attaches to troponin which causes a change in the muscle cell that moves tropomyosin out of the way so the cross bridges can attach and produce muscle contraction.

personal%2520training%2520cross%2520bridge

In summary the sliding filament theory of muscle contraction can be broken down into four distinct stages, these are;

1. Muscle activation: The motor nerve stimulates an action potential (impulse) to pass down a neuron to the neuromuscular junction. This stimulates the sarcoplasmic reticulum to release calcium into the muscle cell.

2. Muscle contraction: 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 – this is discussed in greater detail in the energy system folder here at ptdirect).

3. Recharging: ATP is re-synthesised (re-manufactured) allowing actin and myosin to maintain their strong binding state

You are here: Home › Training Design › Anatomy and Physiology › The Physiology of Skeletal Muscle Contraction
The Physiology of Skeletal Muscle Contraction
In this page we look at the physiology behind muscular contraction and what causes a contraction to cease. Low and behold one simple mineral is really quite critical...

What is the Sliding Filament Theory of muscular contraction?
The sliding filament theory is the explanation for how muscles contract to produce force. As we have mentioned on previous pages, the actin and myosin filaments within the sarcomeres of muscle fibres bind to create cross-bridges and slide past one another, creating a contraction. The sliding filament theory explains how these cross-bridges are formed and the subsequent contraction of muscle.

The Sliding Filament Theory
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).

personal%20training%20motor%20unitNote 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 of the motor neurons axon and the muscle fibres it stimulates.

When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. This reaction results in the start of a contraction and the sliding filament theory.

The reaction, created from the arrival of an impulse stimulates the 'heads' on the myosin filament to reach forward, attach to the actin filament and pull actin towards the centre of the sarcomere. This process occurs simultaneously in all sarcomeres, the end process of which is the shortening of all sarcomeres.

Troponin is a complex of three proteins that are integral to muscle contraction. Troponin is attached to the protein tropomyosin within the actin filaments, as seen in the image below. When the muscle is relaxed tropomyosin blocks the attachment sites for the myosin cross bridges (heads), thus preventing contraction.

- nWhen the muscle is stimulated to contract by the nerve impulse, calcium channels open in the sarcoplasmic reticulum (which is effectively a storage house for calcium within the muscle) and release calcium into the sarcoplasm (fluid within the muscle cell). Some of this calcium attaches to troponin which causes a change in the muscle cell that moves tropomyosin out of the way so the cross bridges can attach and produce muscle contraction.

- In summary the sliding filament theory of muscle contraction can be broken down into four distinct stages, these are;

1. Muscle activation: The motor nerve stimulates an action potential (impulse) to pass down a neuron to the neuromuscular junction. This stimulates the sarcoplasmic reticulum to release calcium into the muscle cell.

2. Muscle contraction: 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 – this is discussed in greater detail in the energy system folder here at ptdirect).

3. Recharging: ATP is re-synthesised (re-manufactured) allowing actin and myosin to maintain their strong binding state

4. Relaxation: Relaxation occurs when stimulation of the nerve stops. 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 order for a skeletal muscle contraction to occur;

1. There must be a neural stimulus
2. There must be calcium in the muscle cells
3. ATP must be available for energy