Human Physiology | Module 3 Unit 2 | Calicut University First Semester BSc. Zoology Notes PDF |

 Human Physiology: Calicut University Free Study Material 

Hello everyone! In this study material, I’ve put together notes from Module 3, Unit 2 – Muscular System, from the First Semester B.Sc. Zoology. I’ve made sure the notes are well-organised, exam-oriented, and easy to revise, with simplified explanations of the major physiological concepts. Whether you’re preparing for your internals, semester exams, or just brushing up your basics.

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Summary of What’s Covered in This Study Material

🧬 Types of Muscle Tissue

In vertebrates, muscles are classified into three major types: skeletal (striated), smooth (non-striated), and cardiac. Each type is structurally distinct and functionally specialised.

🦴 Skeletal (Striated) Muscle

Skeletal muscles are voluntary muscles attached to bones by tendons and are responsible for body movements. They constitute nearly half of the total body weight. Under the microscope, they display characteristic striations due to the organised arrangement of contractile proteins.

Structurally, skeletal muscles are composed of bundles called fascicles. Each muscle fibre is surrounded by endomysium, each fascicle by perimysium, and the entire muscle by epimysium. These connective tissue layers provide structural support and facilitate force transmission.

Two types of skeletal muscle fibres are observed in birds and mammals: red (slow-twitch) and white (fast-twitch) fibres. Red fibres contain abundant myoglobin and mitochondria, making them resistant to fatigue and suited for prolonged activity. White fibres have fewer mitochondria and rely largely on anaerobic metabolism, enabling rapid but short-lived contractions.

🫁 Smooth (Non-Striated) Muscle

Smooth muscles are involuntary muscles found in the walls of visceral organs such as the stomach, intestine, urinary bladder, blood vessels, and respiratory tract. These spindle-shaped, uninucleate cells lack visible striations.

Functionally, smooth muscles are categorised into single-unit and multi-unit types. Single-unit smooth muscles contract as a coordinated mass, as seen in the intestine and uterus. Multi-unit smooth muscles function independently and are found in structures such as the iris of the eye.

Smooth muscles produce slow, sustained contractions essential for processes like peristalsis and regulation of blood flow.

❤️ Cardiac Muscle

Cardiac muscle is found exclusively in the heart and functions involuntarily. It shares features with both skeletal and smooth muscle but possesses unique characteristics.

Cardiac muscle fibres are short, branched, and interconnected. They contain centrally placed nuclei and are joined by intercalated discs, specialised junctions that include gap junctions and desmosomes. These structures ensure synchronised contraction and rapid electrical conduction across the heart.

Unlike skeletal muscle, cardiac muscle is highly resistant to fatigue, enabling continuous rhythmic contraction throughout life.

🔬 Ultra Structure of Skeletal Muscle

The skeletal muscle fibre is a long, multinucleated cell containing numerous myofibrils. Each myofibril consists of repeating structural units called sarcomeres, which are the functional units of contraction.

A sarcomere is bounded by two Z-lines. The dark A-band corresponds to thick filaments composed of myosin, while the light I-band contains thin filaments made primarily of actin. The H-zone represents the central region of the A-band where only myosin is present, and the M-line stabilizes the thick filaments.

The striated appearance of skeletal muscle results from the regular arrangement and overlap of actin and myosin filaments. This highly ordered structure enables efficient contraction.

⚙️ Sliding Filament Theory

The mechanism of muscle contraction is explained by the Sliding Filament Theory, proposed in 1954 by Hugh Huxley and Jean Hanson.

According to this theory, muscle contraction occurs when thin actin filaments slide over thick myosin filaments, shortening the sarcomere without altering filament length. This process is powered by ATP hydrolysis and regulated by calcium ions released from the sarcoplasmic reticulum.

When a nerve impulse reaches the neuromuscular junction, the neurotransmitter Acetylcholine is released. It binds to receptors on the sarcolemma, initiating depolarization. The action potential travels along the membrane and through T-tubules, triggering calcium release.

Calcium binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin. Myosin heads attach to actin, forming cross-bridges. The bending of myosin heads, known as the power stroke, pulls actin filaments inward. As a result, the I-band shortens and the H-zone disappears, while the A-band remains constant.

🔥 Physiological Changes During Contraction

Muscle contraction involves electrical, chemical, and thermal changes. Electrical changes begin with the action potential generated on the sarcolemma. Sodium influx causes depolarization, followed by potassium efflux during repolarization.

Chemically, ATP is hydrolyzed by myosin ATPase, releasing energy for cross-bridge cycling. For sustained contraction, ATP is regenerated through anaerobic glycolysis and aerobic respiration. When oxygen supply is insufficient, lactic acid accumulates, contributing to fatigue.

Heat production accompanies contraction. Initial heat is produced during ATP breakdown, while recovery heat appears during restoration of energy reserves. Continuous muscular activity plays a significant role in maintaining body temperature.

🔄 Muscular Relaxation

Relaxation begins when motor nerve stimulation ceases. Acetylcholine is broken down by acetylcholinesterase, stopping further depolarization. Calcium ions are actively pumped back into the sarcoplasmic reticulum by calcium ATPase pumps.

As calcium levels fall, tropomyosin again blocks the binding sites on actin, preventing cross-bridge formation. ATP binding causes detachment of myosin heads. Elastic elements within the muscle restore the sarcomere to its resting length.

🔗 Neuromuscular Junction

The neuromuscular junction is the site where a motor neuron communicates with a skeletal muscle fibre. It consists of the synaptic knob, synaptic cleft, and motor end plate.

The synaptic knob contains vesicles filled with acetylcholine. The motor end plate possesses numerous receptor sites that respond to this neurotransmitter. Transmission at this junction converts a neural signal into a muscular response.

⏱ Muscle Twitch and Tetanic Contraction

A muscle twitch is a brief contraction resulting from a single stimulus. It consists of three phases: latent period, contraction period, and relaxation period.

If a second stimulus arrives before complete relaxation, summation occurs, increasing contraction strength. Rapid successive stimulation produces tetanus, a sustained contraction.

Incomplete tetanus shows partial relaxation between stimuli, while complete tetanus results in a smooth, continuous contraction.

⚰️ Rigor Mortis

Rigor mortis is a post-mortem stiffening of muscles caused by the depletion of ATP. Without ATP, actin–myosin cross-bridges cannot detach, leading to rigidity. It typically begins within a few hours after death and gradually disappears as tissue decomposition occurs.

A downloadable PDF version of this module is available for structured revision and easy reference before exams.

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