Skeletal Muscle

Introduction

Muscles are the body’s enablers of movement. It is by using muscles that our body performs various functions. From running fast to deep breathing, everything that requires body displacement involves muscles. There are mainly three types of muscles:

1.     Smooth muscles

2.     Cardiac muscles

3.     Skeletal muscles

For the purpose of this article, we will study more about skeletal muscles and how they perform movement.

Skeletal muscle

Of the three types of muscles in vertebrates, skeletal muscles are the most abundant. They are also commonly known as muscles and are attached to bones with the help of tendons. 

Skeletal muscles are voluntary. That is to say, they are movable and controlled through voluntary action. The brain controls all muscular efforts of contraction. The mirror neuron that innervates muscle fibres receives electrochemical signals from the brain through the nervous system. Some actions performed by skeletal muscles are also partly reflexive such as breathing or chewing. In the case of reflexes, the signals are often sent by the spinal cord through an information loop with grey matter.

Skeletal muscles attach to bones and move the body by contracting and relaxing. This contraction of the muscles is brought about by complex mechanisms involving muscle fibres composed of specialised proteins.

Skeletal muscles are long and cylindrical, and when viewed under a microscope, they look like bundles of striped or striated fibres. This assistance is caused because the fibres are made up of regularly arranged contractile proteins. These are the proteins that bring about the movement in muscles.

Skeletal Muscle Fibre Structure

The cells of skeletal muscles are its fibres and are incredibly large. Each muscle fibre is bound by a plasma membrane called the sarcolemma. This membrane is the place where action potential conduction happens, which triggers muscle contraction. 

Each muscle fibre is made up of long cylindrical structures bundled lengthwise inside the fibre. These are called myofibrils. These myofibrils are attached to the sarcolemma at their ends. This results in the contracting of the entire fibre when the myofibrils contract.

Structure of Myofibrils

1.     The myofibrils appear striated or striped because of the arrangement of the contractile proteins myosin (thick) and actin (thin)

2.     Each myofibril has light I bands and dark A bands.

a.     The dark A bands contain an overlapping of the thick and thin protein filaments.

b.     The light I bands contain just the thing protein filaments.

3.     The I bands are divided in the middle by a dense line called the Z disc or Z line.

4.     Between two Z lines lies a sarcomere.

5.     A sarcomere is the functional unit of a myofibril.

6.     It is made up of one dark A band which is bookended by two halves of I band.

7.     A myofibril is made up of several sarcomeres and as these units contact so does the myofibril.

Contractile Proteins 

The proteins that cause contraction in muscles are called contractile proteins.

Contractile Proteins Classification

The contractile protein classification can be divided into two parts

·       Thick

·       Thin

These are the thick myosin protein and the thin actin protein.

Contractile proteins and how they are arranged;

The tail of myosin molecules connect with other myosin molecules to form the central region of a thick filament. The other ends of the molecules or their heads lie where the thin filaments overlap. The thin filaments are the actin proteins. These actin proteins are helical in structure and appear like strands of entwined pearls. Two other components of the thin filaments are tropomyosin and troponin. These two regulate the interaction between myosin and actin and prevent contraction when the muscles are relaxed.

Muscle Contraction and Locomotion

The sliding filament theory of contraction was proposed by A. F. Huxley and R. Niedergerke (1954), and H. E. Huxley and J. Hanson (1954) explained the working of muscles in the human body .

·       In order to contact the muscle the sarcomere must contract

·       To contract the sarcomere the myosin and actin filaments slide over each other increasing the overlap

·       The protein filaments themselves do not contract

·       But since they increase the overlap the  sarcomere contracts

·       This increase in overlap happens because the cross bridges, the bulbous structures found in the myosin, facilitate the movement of the filaments

What happens when a muscle contracts?

A sarcomere is the distance between two Z lines. So when a muscle contracts:

·       The distance between the Z lines reduces

·       The centre of the dark A band, called the H zone, which contains only thick filaments, shortens

·       The light I bands also shortens

·       The dark A band does not shorten

·       The thin filaments are pulled towards the centre of the sarcomere

·       This causes the Z line to come closer to the thick filaments

·       The zone where the thick and thin filaments overlap increases

This is how muscle contraction takes place.

Uses of Skeletal Muscle

1.     Mobility

Through the contraction and relaxation of muscles, we can move and perform functions like walking, running, and swimming. Muscles also help us perform finer or smaller movements such as writing, speaking, controlling facial expressions.

2.     Stability and balance

Muscles attached to joints contribute greatly in keeping our body balanced and stable during rest or movement. The muscles attached to the knee joints and shoulder joints are essential for this task.

Moreover the muscles of the abdomen, pelvis, and the back stabilise the body and help in lifting weights.

3.     Posture 

Skeletal muscles keep the body in the correct position while at rest, or while sitting or standing.

Strong and flexible muscles maintain good posture. At the same time, weak and stuff muscles lead to bad posture.

Posture is important because bad posture overtime can lead to a deterioration of the health of our joints and muscles.

4.     Urination 

Urination involves both involuntary and skeletal muscles. The nervous system works in conjunction with the muscles in the release of urine. 

5.     Vision 

The movement of the eye is controlled by 6 skeletal muscles around it. It because of these skeletal muscles that the eye is able to 

·       Swivel in the socket

·       Track objects

·       Focus on objects

6.     Organ protection

The internal organs in the abdomen cavity are protected by the muscles in the abdomen and the back.

The bones and joints are also protected by the muscles because they absorb shock and reduce friction.

7.     Temperature regulation

Skeletal muscles generate nearly 85% of the body heat. The contraction of the muscles plays a significant part in maintaining body temperature. When the body temperature falls, muscles increase their activity to raise the temperature.

Conclusion 

As can be seen, much muscle movement is a complex activity involving machinations at a cellular level. And the way each of these elements work together is fascinating to study. 

The muscle system of our body is essential to our existence, and we must take care of it. Since skeletal muscles are a misfortune composed of proteins and most of their functions are also carried out by proteins, we must maintain our protein intake. Along with a balanced diet, exercise them.