Muscle – Types, Structure, Development, Functions

What is Muscle?

Muscle is a specialized contractile tissue of the body. It is made up of muscle cells or muscle fibres. These cells have the ability to contract, shorten and produce pulling force. Due to this, movement of body and internal organs takes place.

Muscle cells are excitable in nature. They respond to electrical or chemical stimulus. The contraction of muscle mainly occurs by the interaction of actin and myosin proteins. During this process, these proteins slide over one another and shortening of muscle fibre takes place.

There are three types of muscle tissues in human body. These are skeletal muscle, cardiac muscle and smooth muscle.

Skeletal muscles are attached with bones. These muscles are responsible for voluntary movement such as walking and movement of body parts. It also helps in maintaining posture and production of body heat.

Cardiac muscle is present only in the wall of heart. It contracts rhythmically and involuntarily. Due to this contraction, blood is continuously pumped throughout the body.

Smooth muscle is present in the wall of hollow internal organs such as stomach, intestine and blood vessels. It is involuntary in nature. It helps in movement of food, air and blood through different organs.

Characteristics of Muscle

• Excitability (Irritability or Responsiveness) – It is the ability of muscle cell to receive and respond to electrical, chemical or mechanical stimulus. During this, an action potential is generated in the muscle cell.
• Contractility – It is the special ability of muscle to shorten after stimulation. Due to this shortening, pulling force is produced.
• Extensibility – It is the ability of muscle cells to stretch or extend without any damage. The muscle fibre can increase its length during this condition.
• Elasticity – It is the ability of muscle tissue to return back to its original resting length. This occurs after the muscle is stretched or deformed.

Location of Muscle

• Skeletal Muscle – It is mainly attached with bones by the tendons. It helps in movement of different body parts. Skeletal muscle is also present in the skin of face, tongue, diaphragm, eye socket and upper part of oesophagus. It is also found around the openings of mouth and anus.
• Cardiac Muscle – It is present only in the wall of heart. The muscular wall of heart is called myocardium.
• Smooth Muscle – It is present in the walls of hollow internal organs. These include stomach, intestine, urinary bladder, uterus and other reproductive tracts. Smooth muscle is also found in blood vessels, respiratory tract, skin and eyes. In skin, it forms the erector pili muscle, while in eyes it is present in iris and ciliary muscle.

Structure of Muscle

• Whole Muscle – The complete skeletal muscle is covered by an outer connective tissue layer called epimysium. It gives protection and support to the whole muscle.
• Fascicles – Inside the muscle, the muscle fibres are arranged in bundles called fascicles. Each fascicle is surrounded by a connective tissue layer called perimysium.
• Muscle Fibres (Myocytes) – Muscle fibres are long and cylindrical muscle cells present inside each fascicle. Each muscle fibre is covered by a thin connective tissue layer called endomysium.
  • Sarcolemma – It is the plasma membrane of muscle fibre. The sarcolemma forms deep tube-like inward extensions called T-tubules. These tubules carry electrical signal into the inner part of muscle fibre.
  • Sarcoplasm – It is the cytoplasm present inside the muscle fibre. It contains myofibrils, mitochondria, glycogen and other cellular materials.
  • Sarcoplasmic Reticulum (SR) – It is a specialized membranous network present around the myofibrils. It stores and releases calcium ions required for muscle contraction. After contraction, calcium ions are again taken back into it.
• Myofibrils – These are long rod-like structures present inside the muscle fibre. They extend through the whole length of fibre. Each myofibril is formed by repeated units called sarcomeres.
• Sarcomere – It is the basic structural and functional unit of muscle contraction. A sarcomere is present between two Z-discs or Z-lines. Many sarcomeres are arranged one after another to form a myofibril.
• Myofilaments – These are very thin protein filaments present inside each sarcomere. Their sliding causes shortening of sarcomere and muscle contraction.
  • Thick Filaments – Thick filaments are present mainly in the central region of sarcomere. These are made up of myosin protein.
  • Thin Filaments – Thin filaments are attached with the Z-discs. These are mainly made up of actin protein. The regulatory proteins troponin and tropomyosin are also present with actin and control muscle contraction.

Types of Muscle

There are three types of muscles in human body. These are skeletal muscle, cardiac muscle and smooth muscle.

1. Skeletal Muscle – It is also known as voluntary muscle. It is attached with bones and helps in movement of body. These muscles also maintain body posture. Muscle fibres are long, cylindrical and unbranched. Many nuclei are present at the peripheral region. Light and dark bands are present, therefore it is striated in appearance. The bands are formed by arrangement of actin and myosin.
2. Cardiac Muscle – It is found only in the wall of heart. These muscles are involuntary and contract rhythmically. By this contraction, blood is pumped to different body parts. The cells are short, branched and striated. Generally one nucleus is present in the central region. The cells are connected by intercalated discs. These discs help in the combined contraction of cardiac muscle cells.
3. Smooth Muscle – It is also called visceral muscle. It is present in the walls of stomach, intestine, urinary bladder and uterus. These are also present in blood vessels and respiratory tract. Smooth muscles are involuntary in nature. They help in movement of food, air and blood. The cells are spindle-shaped and have tapering ends. One nucleus is present at the centre. Striations are absent.

Development of Muscle

The development of muscle takes place by the following steps-

1. Skeletal muscles mainly develop from the embryonic mesoderm. Most of the muscles arise from the paraxial mesoderm.
2. The paraxial mesoderm divides from head region towards tail region. Small segments called somitomeres are formed. Seven somitomeres form muscles of head and neck. The remaining region forms 35 pairs of somites in the trunk.
3. The somitomeres undergo epithelialization. During this process, groups of epithelial cells are formed.
4. The mesodermal cells are changed into myogenic cells. These cells divide by mitosis and form postmitotic myoblasts.
5. The myoblasts increase in number during the early stage. Each myoblast contains one nucleus. Hundreds of myoblasts come close and fuse with each other.
6. The fusion of myoblasts forms long and multinucleated structures called myotubes. These myotubes later develop into myocytes or muscle fibres.
7. The newly formed myotubes start synthesis of structural and contractile proteins. These include actin, myosin, troponin and tropomyosin.
8. The thick myosin filaments and thin actin filaments become arranged along the long axis of muscle cell. These filaments aggregate near the cell membrane in regular arrays.
9. The arranged thick and thin filaments combine to form myofibrils. This process is referred to as myogenesis. The myofibrils form the contractile structures inside the mature muscle fibre.

Functions of Muscle

• Movement and Locomotion – Muscles produce pulling force and move the bones. It helps in walking, running, grasping objects and movement of different body parts. Facial expressions are also formed by muscle movement.
• Maintenance of Posture – Skeletal muscles maintain the upright position of body against gravity. Small contractions continuously take place in these muscles. It also stabilizes the joints and prevents unwanted movement.
• Production of Heat – During muscle contraction, ATP is broken down and heat is released. This heat helps in maintaining normal body temperature. During shivering, rapid muscle contractions produce more heat.
• Pumping of Blood – Cardiac muscle contracts continuously and rhythmically. Due to this contraction, blood is pumped from heart to different body parts. It also helps in maintaining blood pressure.
• Control of Blood Flow – Smooth muscle is present in the walls of blood vessels. Its contraction causes vasoconstriction, while relaxation causes vasodilation. By this process, blood flow and blood pressure are regulated.
• Movement of Food and Other Materials – Smooth muscles move food through the digestive tract by wave-like contraction called peristalsis. It also helps in movement of air, secretions and other substances inside the body.
• Control of Body Openings – Muscles are present around the openings of different internal tracts. These muscles control swallowing, urination and defecation. Their action may be voluntary or involuntary.
• Protection of Internal Organs – Skeletal muscles form a supporting layer around internal organs. It supports the organs of abdominal and pelvic region. It also protects these organs from external injury.

Examples of Muscle

The following are some examples of muscles in human body-

Skeletal Muscles

• Biceps brachii – It is present at the front side of upper arm. It helps in flexion of elbow.
• Triceps brachii – It is present at the back side of upper arm. It helps in extension of elbow.
• Gastrocnemius and Soleus – These are calf muscles present in the lower leg. They help in movement of foot.
• Quadriceps femoris – It is present at the front region of thigh. It extends the leg at knee joint.
• Gluteus maximus – It is a large muscle present in the buttock region. It helps in movement of hip.
• Sartorius – It is a long muscle present in the thigh. It helps in movement of hip and knee.
• Latissimus dorsi – It is a broad muscle present in the lower back. It helps in movement of arm.
• Pectoralis major – It is present in the chest region. It helps in flexion and adduction of arm.
• Trapezius – It is present in upper back and neck. It raises and rotates the shoulder blade.
• Deltoid – It is present around the shoulder. It helps in moving the arm away from body.
• Sternocleidomastoid – It is present in the neck region. It bends the neck and rotates the head.
• External oblique – It is present in abdominal region. It helps in bending and rotation of vertebral column.
• Diaphragm – It is present below the lungs. It helps in voluntary and involuntary breathing.

Cardiac Muscle

• Myocardium – It is the muscular tissue present in the wall of heart. It contracts rhythmically and pumps blood throughout the body.

Smooth Muscles

• Muscles of stomach and intestine – These muscles are present in the wall of digestive tract. They move food by peristalsis.
• Muscles of blood vessels – These are present in the walls of arteries and veins. They regulate blood flow and blood pressure.
• Muscle of urinary bladder – It expands during storage of urine and contracts during urination.
• Muscles of bronchioles – These muscles are present in the respiratory tract. They control the passage of air into lungs.
• Muscle of uterus – It is present in the wall of uterus. It contracts strongly during childbirth and helps in movement of fetus outside the uterus.

Difference between Skeletal, Cardiac and Smooth Muscle

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