BIO201 – Topic 5: The Muscular System
Course Code: BIO 201 | Course Name: Human Biology
© Lesuyai Organic Guides 2025
5.1 Introduction to the Muscular System
The muscular system comprises all the muscles in the human body that are responsible for
movement, posture, and heat production. It works in close coordination with the skeletal and
nervous systems to enable locomotion and other bodily functions. Muscles are composed of fibers
containing myofibrils made of actin and myosin filaments, which are essential for contraction.
5.2 Functions of Muscles
1. Body movement and locomotion through contraction and relaxation. 2. Maintenance of posture
and body position. 3. Stabilization of joints. 4. Heat production (thermogenesis). 5. Circulation and
internal organ function via smooth and cardiac muscles.
5.3 Types of Muscular Tissue
Skeletal Muscle: Voluntary, striated, and multinucleated, attached to bones for body movement.
Smooth Muscle: Involuntary, non-striated, found in walls of internal organs such as the stomach
and blood vessels.
Cardiac Muscle: Involuntary, striated, with branched cells found only in the heart wall.
5.4 Structure of Skeletal Muscle
Skeletal muscle is composed of bundles of muscle fibers called fascicles. Each fiber has a
membrane (sarcolemma), cytoplasm (sarcoplasm), and contractile myofibrils. The functional unit,
the sarcomere, is made of actin (thin) and myosin (thick) filaments separated by Z-lines. Connective
tissues — endomysium, perimysium, and epimysium — support and bind these structures.
5.5 Mechanism of Muscle Contraction
The sliding filament theory explains muscle contraction as the sliding of actin filaments over myosin
using ATP and calcium ions. Nerve impulses at the neuromuscular junction release acetylcholine,
triggering calcium release from the sarcoplasmic reticulum. ATP powers cross-bridge formation and
detachment, leading to contraction and relaxation.
5.6 Antagonistic Muscle Action
Antagonistic muscles work in pairs to create coordinated movement. When one contracts, the other
relaxes. For example, the biceps flex the arm while the triceps extend it; the quadriceps extend the
leg while hamstrings flex it.
5.7 Major Muscles in the Human Body
Head and neck: masseter, sternocleidomastoid
Trunk: pectoralis major, rectus abdominis, diaphragm
Course Code: BIO 201 | Course Name: Human Biology
© Lesuyai Organic Guides 2025
5.1 Introduction to the Muscular System
The muscular system comprises all the muscles in the human body that are responsible for
movement, posture, and heat production. It works in close coordination with the skeletal and
nervous systems to enable locomotion and other bodily functions. Muscles are composed of fibers
containing myofibrils made of actin and myosin filaments, which are essential for contraction.
5.2 Functions of Muscles
1. Body movement and locomotion through contraction and relaxation. 2. Maintenance of posture
and body position. 3. Stabilization of joints. 4. Heat production (thermogenesis). 5. Circulation and
internal organ function via smooth and cardiac muscles.
5.3 Types of Muscular Tissue
Skeletal Muscle: Voluntary, striated, and multinucleated, attached to bones for body movement.
Smooth Muscle: Involuntary, non-striated, found in walls of internal organs such as the stomach
and blood vessels.
Cardiac Muscle: Involuntary, striated, with branched cells found only in the heart wall.
5.4 Structure of Skeletal Muscle
Skeletal muscle is composed of bundles of muscle fibers called fascicles. Each fiber has a
membrane (sarcolemma), cytoplasm (sarcoplasm), and contractile myofibrils. The functional unit,
the sarcomere, is made of actin (thin) and myosin (thick) filaments separated by Z-lines. Connective
tissues — endomysium, perimysium, and epimysium — support and bind these structures.
5.5 Mechanism of Muscle Contraction
The sliding filament theory explains muscle contraction as the sliding of actin filaments over myosin
using ATP and calcium ions. Nerve impulses at the neuromuscular junction release acetylcholine,
triggering calcium release from the sarcoplasmic reticulum. ATP powers cross-bridge formation and
detachment, leading to contraction and relaxation.
5.6 Antagonistic Muscle Action
Antagonistic muscles work in pairs to create coordinated movement. When one contracts, the other
relaxes. For example, the biceps flex the arm while the triceps extend it; the quadriceps extend the
leg while hamstrings flex it.
5.7 Major Muscles in the Human Body
Head and neck: masseter, sternocleidomastoid
Trunk: pectoralis major, rectus abdominis, diaphragm
