Muscular System: Structure, Function, and Physiology

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Muscular System

Introduction

The muscular system is essential for movement and stability in the human body. Muscular tissue enables the body and its parts to move by converting chemical energy from food into mechanical energy, allowing muscle cells (fibers) to contract and shorten. There are three major types of muscle tissue in the body, each with distinct structures and functions.

Muscle fibers are specialized cells capable of contraction.
Movement is achieved by the shortening of these fibers.
Muscle cells convert chemical energy (from food) into mechanical energy.

Types of Muscle Tissue

Skeletal Muscle Tissue

Skeletal muscle, also known as striated or voluntary muscle, is responsible for voluntary movements and comprises 40% to 50% of body weight. It is attached to bones and displays crosswise stripes (striations) under a microscope.

Voluntarily controlled contractions.
Major contributor to body mass.
Microscopically striated appearance.

Cardiac Muscle Tissue

Cardiac muscle forms the bulk of the heart and is characterized by branching cells and unique dark bands called intercalated disks. The interconnected nature of cardiac muscle cells allows the heart to contract efficiently as a unit.

Found only in the heart.
Cells branch frequently and are interconnected.
Presence of intercalated disks for synchronized contraction.

Smooth Muscle Tissue

Smooth muscle, also called nonstriated, involuntary, or visceral muscle, lacks striations and appears smooth under a microscope. It is found in the walls of hollow organs such as the digestive tract, blood vessels, and ureters, and its contractions are involuntary.

Located in hollow visceral structures.
Involuntary control.
Microscopically smooth appearance.

Skeletal Muscle Structure and Attachments

Major Structures

Each skeletal muscle is an organ composed mainly of skeletal muscle cells and connective tissue. Most skeletal muscles span from one bone across a joint to another bone.

Muscles are attached to bones via tendons.
Connective tissue provides support and structure.

Parts of a Skeletal Muscle

Origin: Attachment to the bone that remains stationary during movement.
Insertion: Attachment to the bone that moves when the muscle contracts.
Body: The main part of the muscle.

Attachments and Associated Structures

Tendons: Strong cords of fibrous connective tissue; some are enclosed in synovial-lined tubes called tendon sheaths, lubricated by synovial fluid.
Bursae: Small synovial-lined sacs containing synovial fluid, located between some tendons and underlying bones.

Microscopic Structure

Muscle fibers: Contractile cells grouped into bundles.
Myofilaments: Thick (myosin) and thin (actin) filaments within fibers.

Sarcomere and Sliding Filament Model

Sarcomere: The basic functional (contractile) unit of muscle, separated by Z lines.
Sliding filament model: Thick and thin myofilaments slide past each other during contraction.
Contraction requires calcium ions and ATP.

Equation:

(energy released for contraction)

Function of Skeletal Muscle

Movement

Muscles produce movement by pulling on bones as they contract.
The insertion bone moves closer to the origin bone.
Movement occurs at the joint between origin and insertion.

Muscle Groups in Movement

Prime mover: Main muscle responsible for a movement.
Synergist: Muscles that assist the prime mover.
Antagonist: Muscles that oppose the action of the prime mover.

Posture

Tonic contraction: Maintains body position without producing movement.
Only a few muscle fibers contract at a time.
Good posture favors optimal body functioning.
Skeletal muscle tone counteracts gravity.

Heat Production

Muscle contraction produces most of the heat required to maintain normal body temperature.
Fever: Elevated body temperature, often due to illness.
Hypothermia: Body temperature below normal.

Fatigue

Reduced strength of muscle contraction due to repeated stimulation without rest.
Depletion of ATP and nutrients leads to fatigue.
Contraction without adequate oxygen produces lactic acid, causing muscle burning.
Oxygen debt: The metabolic effort required to burn excess lactic acid after exercise.

Integration With Other Body Systems

Muscle function depends on the coordinated activity of the respiratory, circulatory, nervous, muscular, and skeletal systems. Pathological conditions in other systems (e.g., multiple sclerosis, brain hemorrhage, spinal cord injury) can affect movement.

Muscle Stimulation and Contraction

Motor Unit

Stimulation by a nerve impulse is required for muscle contraction.
Motor neuron: Nerve cell transmitting impulse to muscle.
Neuromuscular junction (NMJ): Point of contact between nerve ending and muscle fiber.
Motor unit: Combination of a motor neuron and the muscle cells it innervates.

Muscle Stimulus

A muscle contracts only if the stimulus reaches a threshold level (threshold stimulus).
Once stimulated, a muscle fiber contracts completely (all or none response).
Different motor units have different threshold levels, allowing graded force production.

Types of Muscle Contraction

Twitch contractions: Quick, jerky responses to a stimulus.
Tetanic contractions: Sustained, steady contractions from rapid stimuli.
Isotonic contractions: Muscle changes length and produces movement (includes concentric and eccentric contractions).
Isometric contractions: Muscle tension increases without changing length; no movement produced.

Effects of Exercise on Skeletal Muscles

Regular exercise improves muscle tone, posture, heart and lung function, and reduces fatigue.
Prolonged inactivity causes disuse atrophy (muscle wasting).
Regular exercise increases muscle size (hypertrophy).
Strength training: Increases myofilaments and muscle mass, not the number of fibers.
Endurance (aerobic) training: Increases ability to sustain exercise, improves oxygen and nutrient delivery, but does not cause hypertrophy.

Types of Movements Produced by Skeletal Muscle Contractions

Flexion: Decreases the angle between two bones at a joint (bending).
Extension: Increases the angle between two bones at a joint (straightening).
Abduction: Movement away from the midline.
Adduction: Movement toward the midline.
Rotation and circumduction: Movement around a longitudinal axis.
Supination: Palm faces anteriorly; Pronation: Palm faces posteriorly.
Dorsiflexion: Elevation of the top of the foot; Plantar flexion: Downward movement of the foot.

Summary Table: Types of Muscle Tissue

Type	Location	Control	Appearance
Skeletal	Attached to bones	Voluntary	Striated
Cardiac	Heart	Involuntary	Striated, intercalated disks
Smooth	Walls of hollow organs	Involuntary	Non-striated

Summary Table: Types of Muscle Contraction

Type	Description	Movement Produced
Twitch	Quick, jerky response	Brief movement
Tetanic	Sustained contraction	Steady movement
Isotonic	Muscle changes length	Movement at joint
Isometric	Muscle tension increases, no length change	No movement

Additional info:

ATP is the primary energy source for muscle contraction.
Calcium ions are essential for the interaction of actin and myosin during contraction.
Muscle fatigue is a protective mechanism to prevent damage from overuse.