Muscle types

In the human body, there are three important muscle types: skeletal, smooth, and cardiac muscle.

Based on the different properties of muscles, we can classify muscles into three groups

• Based on location
• Based on striations
• Based on control

Based on location

Based on location, we can classify muscles into three types

• Skeletal Muscle
• Smooth Muscle
• Cardiac Muscle

Based on striations

The presence of dark bands (A bands) and light bands (I bands) in muscle cells (Myocytes) gives rise to striations. Based on the presence or absence of striations, muscles can be classified into two groups-

• Striated muscle: – Striated muscles possess numerous striations.
• Non-striated muscle: – Striations are absent in non-striated muscles.

Based on control

Based on control, muscles are of two types:

• Voluntary muscles: The movement of these muscles is controlled as per our desire. Example: Movement of skeletal muscles during walking, swimming, etc.
• Involuntary muscles: The movement of these muscles is not controlled as per our desire. Example: Peristalsis in the esophagus.

Skeletal Muscle

• Skeletal muscles are voluntary and striated muscles.
• Fibers of skeletal muscles are attached to bones with the help of tendons.
• Muscle tissue is composed of many muscle cells, also known as myocytes or muscle fibers.
• A muscle cell or muscle fiber, is a multinucleated cylindrical structure.
• The membrane that encloses the muscle fiber is called the sarcolemma, and the cytoplasm is known as the sarcoplasm.
• The sarcoplasm of a muscle fiber contains many organelles, similar to other cells, such as nuclei, mitochondria, Golgi bodies, sarcoplasmic reticulum, ribosomes, myofibrils, etc.
• Each muscle fiber is surrounded by a layer of connective tissue, located just above the sarcolemma, called the endomysium.
• In muscle tissue, muscle fibers are organized into many parallel bundles. Each bundle of muscle fibers is called a fasciculus (plural: fasciculi).
• Each fasciculus is surrounded by a layer of connective tissue called the perimysium.
• In this way, many fasciculi come together to form a single muscle, which is surrounded by a layer of connective tissue called the epimysium.
• Another thick layer, called the fascia, is situated just above the epimysium. It separates one muscle from another.

Contractile and regulatory proteins of muscle

The contractile proteins present in the muscle are:

• Myosin
• Actin
• Tropomyosin
• Troponin.

Myosin:

• Myosin is made up of two parts: head and tail.  
• The myosin head has two binding sites: one for ATP and the other for the actin molecule.

Actin

The actin molecule is made up of two subunits known as:

• F-actin,
• G-actin.

Tropomyosin

Tropomyosin is an important regulatory protein present in muscles. During normal relaxed conditions, tropomyosin covers all the active sites of F-actin, thereby preventing muscle contraction.

Troponin

Troponin is made up of three subunits:

• Troponin T (binds with tropomyosin),
• Troponin C (binds with calcium ions),
• Troponin I (binds with F-actin).

Neuromuscular Junction

The neuromuscular junction is the place where a nerve fiber and a muscle fiber connect.

Action Potential

When a muscle or nerve is stimulated, a series of electrical changes occur in the stimulated cells. These changes alter the membrane potential of the cell and generate an action potential. The action potential then spreads from the nerve to the muscle through the neuromuscular junction with the help of a neurotransmitter like acetylcholine. This action potential is responsible for muscle contraction.

Muscle contraction and Contractile proteins of muscle

• During relaxed conditions, the active sites of F-actin are covered by tropomyosin.
• During the excitation of a muscle fiber, the nerve passes an action potential to the muscle fiber through the neuromuscular junction. Consequently, a large number of calcium ions are released into the sarcoplasm of the muscle fiber from the cisternae, where calcium is stored.
• Then, calcium ions bind with troponin C.
• The binding of calcium with troponin C changes the position of the troponin molecule. This process triggers the release of F-actin molecules from tropomyosin, and tropomyosin now binds with troponin T. During relaxed conditions, tropomyosin typically binds with F-actin.
• Then, the myosin head binds to ATP and F-actin. This binding causes contraction in the muscle fiber.
• The myosin head releases from F-actin and then attaches to other F-actin molecules and ATP. This process continues until muscle contraction occurs.

Smooth Muscle

• Smooth muscles are involuntary, non-striated muscles.
• Smooth muscles are present in various organs, such as the digestive tract, lungs, ureter, kidneys, mammary glands, and more.
• The contractile proteins present in smooth muscles are fewer compared to those in skeletal muscles.

Cardiac Muscle

• Cardiac muscles are involuntary, striated muscles.
• Cardiac muscles are responsible for the pumping action.
• Cardiac muscles form the pacemaker and conductive system of the human heart.
• The SA node, which is a type of modified cardiac muscle, is the pacemaker of the human heart.
• The conductive system of the human heart comprises the AV node, the bundle of His, and Purkinje fibers.

Conclusion

In the human body, different types of muscles have different functions. All these muscles work and coordinate together to perform vital roles. For example, skeletal muscles maintain posture and stability, smooth muscles protect our visceral organs, and cardiac muscles pump blood. Thus, all these activities sustain our livelihood.

FAQs

Q. Where does myoglobin occur in the body?

Ans- Myoglobin is generally present in muscle tissues, especially in skeletal and cardiac muscles. It binds with oxygen and transports it to muscle fibers.

Q. What are the main chemical ions responsible for muscle contraction?

Ans- calcium ions (Ca²⁺), sodium (Na⁺) and potassium (K⁺)

Q. What does tropomyosin do in a resting muscle fiber?

Ans- In a resting muscle fiber, tropomyosin covers the active binding sites on actin filaments.

Q. Which contractile protein in skeletal muscle involves ATPase activity?

Ans- Myosin

Q. What is the basic unit of muscle contraction?

Ans- sarcomere