Introduction to Muscle and Contraction

Animals have a lot of differences from the plants in their mode of living, ways of obtaining food, adaptations to their habitat, etc. One of the major differences between animals and plants is that of movement or locomotion. Animals can move from one place to another while plants spend their life at the same place.

The movement in animals is possible due to the development of skeletal muscles. They are the most abundant tissues found in vertebrates. The contraction of these skeletal muscles is responsible for the movement of different parts of an animal as well as the movement of the complete body.

In addition to skeletal muscles, two other types of muscles are also present in vertebrates i.e. smooth muscles and cardiac muscles. In this article, we will discuss the structure of different types of muscles, their mechanism of contraction, and the functions performed by them.

Types of Muscles

The muscular tissue in the human body is divided into three types;

• Skeletal Muscles: They are the most abundant muscular tissue in our body. They are present in association with the bones and are responsible for movements.
• Smooth Muscles: They are found associated with the visceral organs of the body and are responsible for movements of internal organs such as movements of the gut during digestion, etc.
• Cardiac Muscles: They are present in the heart and are responsible for the pumping of blood.

All these categories of muscles are discussed in the article below.

Skeletal Muscles

Skeletal muscles make up more than 40% of the total body weight of an average human. These are the voluntary muscles controlled by the somatic nervous system. They are responsible for all types of movements exhibited by a person.

Read more about Skeletal Muscles

Structure

Skeletal muscles consist of multinucleated cells that are arranged parallel to each other forming skeletal muscle fibres. Each fibre consists of hundreds of skeletal muscle cells called myofibrils.

Myofibrils are the multinucleated cells arranged parallel to one another in skeletal muscles. These cells are covered by a plasma membrane called sarcolemma. The cytoplasm of these cells is called sarcoplasm.

The sarcolemma has internal invaginations called T-tubules that are responsible for carrying action potential to the depth of muscle fibres. The sarcoplasm consists of abundant mitochondria to provide ATP for contraction. It also has abundant sarcoplasmic reticulum that stores and releases calcium for contraction.

Actin and myosin are the two contractile proteins present in the myofibrils. These proteins are arranged in such a way to form two distinct bands.

• Light bands (I bands) formed by actin
• Dark bands (A bands) formed by myosin as well myosin-actin overlap

These alternate light and dark bands give a striated appearance to the skeletal muscles.

Sarcomeres

These are the contractile structures present in myofibrils. Each sarcomere is an area between two successive Z-disks. Z-disks are the filamentous proteins to which actin filaments are attached.

Each sarcomere comprises of two I bands surrounding an A band in the centre.

The actin and myosin filaments interdigitate to cause contraction of the skeletal muscles. When the light and dark bands overlap, the sarcomeres shorten in length, and contraction of the muscle occurs. The shortening of sarcomeres causes the shortening of all the muscle fibres that undergo contraction.

Excitation

Skeletal muscles are excited by the motor neurons. The axons of motor neurons innervate the skeletal muscles by forming a motor endplate.

It is a junction between the axon of a motor neuron and a skeletal muscle fibre surrounded by a Schwann cell for insulation. It is a type of chemical synapse between an axon terminal and skeletal muscle fibre. 

When a nerve impulse reaches the axon terminal, acetylcholine molecules are released into the synaptic cleft. These neurotransmitters diffuse across the synapse and bind to acetylcholine receptors on the sarcolemma of muscle fibres.

The activation of acetylcholine receptors causes the opening of sodium channels. The sodium ions diffuse into the muscle fibres and cause depolarization, generating an action potential. This action potential immediately spreads to the complete length and depth of the muscle fibre.

Contraction

The contraction of skeletal muscles begins once they are excited by the motor neurons. The action potential is carried by the T-tubules to the sarcoplasmic reticulum, present in contact with the sarcolemma.

The depolarization of the plasma membrane causes activation of DHP receptors that are associated with the calcium channels. The calcium channels are opened and calcium ions diffuse to the cytoplasm of the muscle fibre.

Here, calcium ions facilitate the process of muscle contraction by binding to troponin, exposing the myosin-binding sites on actin filaments.

Once the myosin-binding sites are exposed, the actin-myosin bridges are formed and the contraction cycle begins. It involves the following steps;

• The hydrolysis of ATP charges the myosin heads so that they have enough energy to bind to actin filaments.
• These charged heads bind to the binding sites on actin filaments to form actin-myosin cross-bridges.
• The myosin heads bend towards the center, pulling the actin filaments to the center of the sarcomere. This results in decreasing the length of the sarcomere, thus contracting the muscle fiber. The ADP and inorganic phosphates bound to the charged myosin heads are also released.
• The cross-bridges break when another ATP molecule is attached to the myosin heads.
• The myosin head becomes charged by the hydrolysis of ATP and the cycle continues.

The contraction process needs large amounts of ATP that are provided by the abundant mitochondria present in the sarcoplasm of skeletal muscle fibres.

Smooth Muscles

These are the involuntary muscles present in the visceral organs. The contraction of these muscles is under the control of the autonomic nervous system. Thus, we cannot control their contraction according to our will. The sympathetic system causes contraction while the parasympathetic system causes relaxation of these muscles.

Structure

Smooth muscles consist of elongated, spindle-shaped cells having a single nucleus. The smooth muscle fibres don’t have any striations.

Each cell in smooth muscle fibres is surrounded by a basal lamina and is connected to other cells by a network of reticular fibres called endomysium. The centre of the cell is broadest as it contains the nucleus while the ends are tapered with decreasing diameter.

Cells are packed closely with the tapering end of one cell lying over the broad part of another cell. These cells are connected via gap junctions. Certain invaginations are present on the plasma membrane of smooth muscle cells called caveolae.

Smooth muscles lack a widespread network of sarcoplasmic reticulum and T tubules.

The contractile proteins are actin and myosin. However, they are not arranged in the form of sarcomeres. In smooth muscle cells, actin and myosin proteins are arranged in a criss-cross manner. Myosin filaments have a less regular arrangement as compared to actin filaments.

These cells have specialized structures to which actin filaments are attached, called the dense bodies. Some dense bodies are attached to the plasma membrane while others are present in the cytoplasm. Dense bodies of the adjacent smooth muscle cells may also be bonded via specialized intercellular protein bridges.

Location

As stated earlier, smooth muscles are associated with visceral organs. They are found in the walls of the stomach, intestines, and other hollow organs. They are also present in the walls of blood vessels and lymphatic vessels. Smooth muscles also line the wall of the ureters, urinary bladder, and urethra.

Excitation

Smooth muscles lack motor endplates as they are not innervated by the motor neurons. Instead, axons of autonomic neurons innervated these cells. The axon terminals of autonomic neurons form synapses with the smooth muscle cells.

These synapses are present in close association with the sarcolemma of the cells. These muscles have some intrinsic activity, even in the absence of a nerve stimulus. The autonomic nervous system increases or decreases this intrinsic activity.

Smooth muscles are innervated by both adrenergic and cholinergic fibres. Both these innervations are antagonists to each other. Any of them can cause muscle contraction based on the location of smooth muscles.

Contraction

The contractile mechanism of smooth muscles is different from that of the skeletal muscles. When a stimulus is applied to the smooth muscle cells, the calcium channels in the sarcoplasmic reticulum as well as in the plasma membrane are opened.

The calcium ions diffuse to the inside of the cell and bind to calmodulin protein to form the calcium-calmodulin complex. This complex converts inactive myosin light-chain kinase (MLCK) to its activated form.

The activated MLCK phosphorylates light chains of myosin, causing cross-bridge formation between myosin heads and actin filaments. When the cross-bridges are formed, the contraction takes place.

Cardiac Muscles

These are the specialized muscles present in the myocardium of the heart. Cardiac muscles are also involuntary muscles and are under the control of the cardiac pacemaker. The autonomic innervation can increase or decrease the effect of a pacemaker on cardiac cells.

Structure

Cardiac cells are also derived from the mesodermal cells. But these cells, instead of fusing as in skeletal muscles to form multinucleated cells, mesodermal cells of the heart tube remain separated by complex junctions. The cells have a branched appearance with interdigitating processes.

In cardiac muscles, the cardiac myocytes are interwoven in such a fashion that cells from one fibre branch and join the cells in the adjacent fibres. Cardiac muscle cells also have a striated appearance just like skeletal muscle cells. However, they have only one nucleus per cell that is present in the centre of the cell.

Within the cardiac myocytes, the actin and myosin filaments are arranged to form myofibrils. Just like the skeletal muscle cells, sarcomeres are the contractile units. However, the myofibrils in cardiac muscles are less dense as compared to the skeletal muscles.

Intercalated Discs

These are the specialized structures joining two cardiac myocytes. These appear as the dark-staining transverse lines crossing the chains of cardiac cells. Each intercalated disk is like a step having has two regions; a transverse region and a longitudinal region.

The transverse regions of these discs have specialized anchoring junctions desmosomes. In addition, facia adherents (a type of adhering junctions) are also present. Both these types of cellular junctions serve to keep the cells tightly adherent to one another.

The longitudinal regions of intercalated discs have numerous gap junctions. These junctions serve to connect the cytoplasm of the adjacent cells. Certain ions can easily pass between the cells via these gap junctions. Thus, they act as electrical synapses among the cardiac myocytes.

Cardiac Muscle as a Syncitium

Recall that cardiac myocytes are connected via intercalated discs. The gap junctions in these discs allow rapid diffusion of ions across the cells. Thus, the cells in cardiac muscles are electrically connected to each other.

From a physiologic point of view, it means that action potential generated in any cardiac myocyte will rapidly spread to the adjacent cells without any delay. Thus, the cardiac muscles act as a syncitium so that when one cell is excited, the action potential immediately spreads to all the cells in the syncitium and they contract together.

The human heart is composed of two syncytia; an atrial syncitium and a ventricular syncitium. Both these syncytia are separated by fibrous tissue so that action potential cannot travel from one to the other. This is the reason why all the cells in the atria contract together and after some time, the ventricular contraction takes place.

Excitation

Cardiac muscles are excited by the action potential travelling in the conducting system of the heart.

The sinoatrial node is the pacemaker of the heart. It does not need any external stimulus to generate an action potential because of its automaticity. The SA node automatically generates action potentials at a specific rhythm that determines the heart rate. This rhythm can be influenced by autonomic innervation.

The action potential generated by the SA node immediately spread to the atrial muscles and atria contract.

In the meantime, this action potential is carried by the AV bundle to the AV node. After some delay, the action potential passes into the Purkinje fibres that innervate the ventricular muscles. The action immediately spreads to all the ventricular muscles and they contract as a syncitium.

The delay in conduction to the ventricles makes sure that atrial contraction is over, before the contraction of ventricles.

Contraction

Once the cardiac muscles are excited, they undergo contraction according to the sliding filament model. The contraction of cardiac muscles is similar to the skeletal muscles with a few differences.

The action potential of cardiac muscles has a plateau phase. During this phase, the calcium channels are opened and the calcium ions begin to diffuse into the cells. These calcium ions are important for the contraction of the muscles. This influx of calcium ions is not seen in the case of skeletal muscles.

When the cardiac myocytes are depolarized, the calcium channels in the sarcoplasmic reticulum are also kept open. Thus, the concentration of calcium ions greatly increases inside the cardiac myocytes.

These calcium ions expose the myosin-binding sites on the actin filaments via the troponin-tropomyosin interaction. Once the binding sites are exposed, contraction occurs by the same process as in the skeletal muscles.

When the action potential is over, the calcium ions are actively pumped out of the cell. The binding sites are again covered by the tropomyosin and the cardiac muscles relax.

Summary

Muscles are specialized tissue having abundant contractile proteins in their cells. They can increase or decrease their length depending on the stimulus. This contraction and relaxation of muscles serve several functions in the life of an animal.

Based on structure and functions, the muscular tissue is divided into three types;

• Skeletal muscles
• Smooth muscles
• Cardiac muscles

Skeletal muscles are associated with the skeleton of the animal and are responsible for all types of voluntary movements.

• They are made up of longitudinal multinucleated cells
• They are innervated by the motor neurons
• Their contraction is under the conscious control
• The contractile proteins are arranged in the form of filaments that together form myofibrils, running throughout the length
• Sarcomeres are the basic contractile units between two Z-disks
• They are excited by an action potential at the moto end plate
• Contraction takes place when the actin filaments are pulled by myosin filaments towards the center

Smooth muscles are associated with the visceral organs and are responsible for their movements.

• They are found in the gut, ureters, urinary bladder, blood vessels, etc.
• They are made up of spindle-shaped cells with one nucleus per cell
• The actin filaments are attached to dense bodies with myosin filaments lying in between them
• They are excited by the neurotransmitters released by the autonomic nerves
• Their contraction occurs when the myosin filaments pull actin filaments between the dense bodies

Cardiac muscles are present in the heart and are responsible for pumping blood through circulation.

• They are made up of branched cells with a single nucleus
• The cells are connected by intercalated discs having gap junctions
• Gap junctions allow rapid diffusion of ions making them a syncitium
• Calcium ions do diffuse into these cells during the action potential
• Contraction takes place by the same mechanism as in the skeletal muscles

Frequently Asked Questions

What causes a muscle contraction?

A muscle contraction happens when a muscle fibre is excited or stimulated by a nerve impulse at the motor end plate. This nerve impulse is transmitted to the muscles via motor neurons. 

What are the three types of muscles?

The three types of muscles include skeletal muscle, smooth muscles and cardiac muscle. Skeletal muscles are under voluntary control while smooth and cardiac muscles cannot be controlled voluntarily.

How cardiac muscles are stimulated?

Cardiac muscles are stimulated by the electrical signal travelling along the conducting fibres of the heart. 

What type of muscles is present in the gut?

Smooth muscles are present in the gut or the intestine of human beings. These are involuntary muscles under the control of the autonomic nervous system. Contraction of these smooth muscles causes forward propulsion of food or peristalsis.

References

1. McCracken, Thomas (1999). New Atlas of Human Anatomy. China: Metro Books. pp. 1–120. ISBN 1-5866-3097-0.
2. Muscle Physiology – Myofilament Structure
3. Marieb, E. N., Hoehn, K., & Hoehn, F. (2007). Human Anatomy & Physiology. (7th ed., pp. 284–87). San Francisco, California: Benjamin-Cummings Pub Co