Skeletal Muscle

Skeletal Muscles

Skeletal muscle is one of the three types of muscles in the human body- the others being visceral and cardiac muscles. In this lesson, skeletal muscles, its definition, structure, properties, functions, and types are explained in an easy but detailed way.

Skeletal Muscle is a muscle tissue that is attached to the bones and is involved in the functioning of different parts of the body. These muscles are also called voluntary muscles as they come under the control of the nervous system in the body.

Structure Of A Skeletal Muscle

The structural of Skeletal Muscle is as shown below-

Structure of Skeletal Muscle

This muscle is attached to the bones by an elastic tissue or collagen fibers called tendons. These tendons are comprised of connective tissues. The skeletal muscles consist of a bundle of muscle fibers naming fascicule. These fascicules are cylindrical in shape as shown in the figure. These muscle fibers are surrounded by blood vessels and a number of layers of other tissues enclosing it.

Each muscle fiber is line by plasma membrane naming sarcolemma reticulum. It encloses a cytoplasm called as sarcoplasm which has the endoplasmic reticulum. The muscle fibers consist of myofibrils which have two important proteins naming actin and myosin in it. The fascicule is enclosed by perimysium and the endomysium is the connective tissue that encloses the muscle fibers.


The skeletal muscles have the following properties:

  • Extensibility: It is the ability of the muscles to extend when it is stretched.
  • Elasticity: It is the ability of the muscles to return to its original structure when released.
  • Excitability: It is the ability of the muscle to respond to a stimulus.
  • Contractility: It is the ability of a muscle to contract when in contact with a stimulus.

Skeletal Muscle Functions

  • The skeletal muscles are responsible for body movements such as typing, breathing, extending the arm, writing, etc. The muscles contract which pulls the tendons on the bones and causes movement.
  • The body posture is maintained by the skeletal muscles. The gluteal muscle is responsible for the erect posture of the body. The sartorius muscles in thighs are responsible for body movement.
  • The skeletal muscles protect the internal organs and tissues from any injury and also provide support to these delicate organs and tissues.
  • These also support the entry and exit points of the body. The sphincter muscles are present around the anus, mouth and the urinary tract. These muscles contract which reduces the size of the openings and facilitates the swallowing of food, defecation, and urination.
  • The skeletal muscles also regulate body temperature. After a strenuous exercise, the body feels hot. This is due to the contraction of skeletal muscles which converts energy into heat.

Types Of Skeletal Muscles

There are two types of skeletal muscles named as red and white muscles-

  • Red Muscles

Red muscles are due to the red pigment called myoglobin which is in high amounts in the human body. These muscles are smaller in diameter and have a large number of mitochondria in it. The myoglobin stores the oxygen which is used by the mitochondria for the synthesis of ATP. Red muscles have a large number of blood capillaries in it.

  • White Muscles

Unlike the red muscles, the white muscles are bigger in diameter and have a small amount of myoglobin in it. They also have less number of mitochondria in it.

Smooth Muscle

Smooth muscle is a type of muscle tissue that is utilized by different systems for the application of pressure to organs and vessels. It is an involuntary muscle which shows no cross stripes even when examined under a microscope. It is composed of cells that are narrow and spindle-shaped with a single nucleus that is located centrally. The cells are made up of fibers of myosin and actin that run through the cells and are backed by a framework of various proteins, wherein the filaments are arranged in a stacked pattern across the cell. It exhibits a ‘staircase’ arrangement of the fibers-actin and myosin, which is typically different than the structure observed in cardiac and skeletal muscles.

Unlike the striated muscles, the smooth muscle tissues contract at a slower pace automatically. Most of the musculature of the digestive system and the internal organs are composed of the smooth muscles. Upon the release of ATP to be used by myosin, the smooth muscles contract under the influence of stimuli. This ATP that is released relies on the magnitude of the stimuli which enables the smooth muscles to possess a graded contraction in contrast to the ‘on-or-off’ contraction observed in the skeletal muscles. The actin filaments run from one side of the cell to the other which is connected at the cell membrane and the dense bodies.

Cardiac Muscle

One of the three types of muscles, the cardiac muscles is a muscles tissue found in the heart wherein it is performing and bringing about coordinated contractions which enable the heart to pump blood throughout the body through the circulatory system.

The cardiac muscle tissues functions to cause continuous pumping of the heart through involuntary movements. This is one of the striking features of cardiac muscles that sets it apart from the muscle tissues which is under one’s control. It is able to do so through specialized cells known as the pacemaker cells which govern the heart’s contractions. The nervous system┬átransmits signals to the specialized cells which indicates them to increase or decrease the heart rate. The pacemaker cells are linked to the cardiac muscle cells which enables them to transmit signals which cause a wave of contractions in the cardiac muscles that in turn generates a heartbeat.


The heart muscles are composed of the following:

  • Nucleus
  • Gap junctions
  • Intercalated discs
  • Desmosomes


Q.1. Can you think of one similarity between cardio muscles and skeletal muscles?

A.1. Both skeletal muscles and cardio muscles are striated muscles.

Q.2. Mention one difference between skeletal muscles and cardiac muscles.

A.2. Skeletal muscles are governed by the somatic nervous system while the cardiac muscles are involuntary in nature.


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