Muscle

Table of Contents
1. Introduction
2. Functions of Muscles
3. Types of Muscle Tissue
4. Microscopic Structure and Mechanism of Contraction
5. Types of Skeletal Muscle Fibres
6. Muscle Response to Exercise
7. Muscle Contraction Types
8. Gross Anatomy and Arrangements
9. Muscle Naming Principles
10. Muscle Groups of the Body (Gross Anatomy)
11. Muscle Growth, Repair and Ageing
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Introduction

Muscles are specialised soft tissues found in humans and other animals that produce force and generate movement. They are composed primarily of the contractile proteins actin and myosin, which interact to shorten the muscle cell and produce contraction. The human body has over 600 muscles, which together account for roughly 40-50% of total body mass in a healthy adult. Muscles are richly supplied with blood vessels and nerves and are connected to bones and organs via connective tissues such as tendons.

Functions of Muscles

• Movement: Produce voluntary and involuntary movements including walking, grasping and eye movements.
• Posture maintenance: Keep the body and individual joints in stable positions without conscious effort.
• Respiration: Muscles such as the diaphragm and intercostals move the rib cage to bring air into and out of the lungs.
• Heat generation: Contractions produce heat that helps maintain body temperature.
• Communication: Facial and laryngeal muscles enable facial expressions, speech and non-verbal signals.
• Propulsion of substances: Smooth muscle moves food through the digestive tract, urine through the urinary tract and helps regulate blood vessel diameter.
• Cardiac pumping: The heart muscle pumps blood to deliver oxygen and nutrients.
• Reproductive functions: Uterine contractions in childbirth and other reproductive system movements.
• Sensory-related functions: Small muscles within the ear and eye contribute to hearing and visual adjustments.

Basic Properties of Muscle Tissue

• Contractility: Ability to shorten actively and generate force.
• Excitability (Responsiveness): Ability to respond to stimuli from nerves or hormones.
• Extensibility: Ability to be stretched without damage by opposing muscle groups or external forces.
• Elasticity: Ability to return to resting length after stretching or contracting.
• Highly vascularised: Rich blood supply supplies oxygen and nutrients and removes waste.
• Microscopic appearance: Some muscles are striated (striped) when viewed under a microscope, others are smooth.

Types of Muscle Tissue

Skeletal Muscle

• Located mainly attached to bones; responsible for body movement and posture.
• Voluntary control: Contraction is generally under conscious control via the somatic nervous system.
• Striated: Repeating sarcomeres give a striped appearance under the microscope.
• Multinucleated fibres: Each fibre contains multiple nuclei near its periphery.
• Contains abundant mitochondria and glycogen granules for energy.
• Approximately 40% of body mass in an average adult.

Cardiac Muscle

• Found only in the heart; specialised for continuous rhythmic contraction.
• Involuntary (autorhythmic): Cells generate and conduct impulses that trigger contraction without conscious input.
• Striated: Sarcomeric arrangement gives a striped appearance.
• Cells are generally single-nucleated and shorter than skeletal fibres.
• Cells are connected by intercalated discs and desmosomes which allow robust mechanical and electrical coupling between cells.

Smooth Muscle

Located in walls of hollow organs (digestive tract, blood vessels, urinary bladder, airways, uterus).

• Involuntary: Controlled by the autonomic nervous system and local factors (hormones, stretch).
• Non-striated: Actin and myosin are present but arranged irregularly, so fibres appear smooth under microscope.
• Cells are spindle-shaped with a single central nucleus.

Microscopic Structure and Mechanism of Contraction

Muscle fibre organisation

• Muscle: Composed of bundles called fascicles, each surrounded by epimysium.
• Fascicle: Bundle of muscle fibres wrapped by perimysium.
• Muscle fibre (cell): Long cylindrical cell surrounded by the sarcolemma and encased by endomysium.
• Myofibrils: Each fibre contains many myofibrils made of repeating units called sarcomeres, the functional contractile units.

Sarcomere and sliding filament mechanism

• Sarcomere structure: Bounded by Z-lines; contains alternating thin filaments (actin) and thick filaments (myosin), producing A and I bands.
• Troponin-tropomyosin complex: Regulates actin-myosin interaction in skeletal and cardiac muscle; calcium binding to troponin exposes myosin-binding sites on actin.
• Sliding filament mechanism: Myosin heads attach to actin, pull thin filaments toward the sarcomere centre (power stroke), detach and reattach in a cyclic process that shortens the sarcomere and generates tension.
• ATP role: ATP is required for detachment of myosin from actin and for powering the myosin head. Lack of ATP causes rigidity (as in rigor mortis).

Excitation-contraction coupling and neuromuscular junction

• The motor neuron releases acetylcholine (ACh) at the neuromuscular junction, triggering a muscle action potential across the sarcolemma.
• The action potential travels down transverse (T) tubules and stimulates the sarcoplasmic reticulum to release calcium ions (Ca²⁺).
• Increased intracellular Ca²⁺ enables actin-myosin interaction and contraction.
• A motor unit consists of a single motor neuron and all the muscle fibres it innervates; recruitment of motor units controls force production and precision.

Types of Skeletal Muscle Fibres

• Slow-twitch (Type I): High endurance, many mitochondria and rich blood supply; used for sustained, aerobic activities (posture, long distance running).
• Fast-twitch (Type II): Produce rapid, powerful contractions; rely more on anaerobic metabolism. Type IIa are more fatigue-resistant than Type IIb/IIx.

Energy Supply and Fatigue

• ATP is produced by creatine phosphate, anaerobic glycolysis and aerobic respiration in mitochondria.
• During intense short bursts, anaerobic metabolism predominates and lactic acid accumulates, contributing to fatigue.
• With prolonged activity, aerobic metabolism and oxidative phosphorylation in mitochondria supply the majority of ATP.
• Muscle fatigue results from multiple factors including depletion of energy stores, accumulation of metabolic by-products, ionic imbalance and central nervous system factors.

Muscle Response to Exercise

Immediate and short-term responses

• Increased blood flow to active muscles and increased oxygen delivery.
• Greater ATP demand met by creatine phosphate and glycolysis initially.
• Post-exercise muscle soreness is often due to microtears in muscle fibres and connective tissue and the associated inflammatory response.

Long-term adaptations

• With consistent resistance training, muscle fibres undergo cycles of damage and repair leading to hypertrophy (increase in fibre size) and increased strength.
• Endurance training increases mitochondrial density, capillary supply and oxidative enzyme activity-improving fatigue resistance.
• Appropriate nutrition including adequate protein and rest supports repair and growth.

Muscle Contraction Types

• Isotonic contraction: Muscle changes length while moving a load; concentric (shortening) and eccentric (lengthening) actions.
• Isometric contraction: Muscle generates tension without changing length (e.g., holding a weight steady).
• Auxiliary terms: Tone (baseline tension in resting muscle) and reflex contractions (automatic responses to stimuli).

Gross Anatomy and Arrangements

Fascicle arrangement

• Parallel: Fascicles run parallel to long axis; produce long range of motion (e.g., sartorius).
• Fusiform: Spindle-shaped with an expanded belly; a type of parallel arrangement (e.g., biceps brachii).
• Pennate: Short fascicles attach obliquely to a tendon; unipennate, bipennate or multipennate arrangements increase force (e.g., rectus femoris is bipennate).
• Convergent (fan-shaped): Fascicles converge toward a single tendon (e.g., pectoralis major).
• Circular: Fascicles arranged in rings around openings; contract to close openings (e.g., orbicularis oris).

Connective tissue coverings

• Endomysium: Surrounds each muscle fibre.
• Perimysium: Surrounds each fascicle.
• Epimysium: Surrounds the entire muscle.

Muscle Naming Principles

• Direction of fibres: e.g., rectus (straight), oblique (slanted).
• Size: e.g., maximus, minimus, longus.
• Location: Named for associated bones or regions (e.g., temporalis, frontalis).
• Number of origins: e.g., biceps (two origins), triceps (three).
• Shape: e.g., deltoid (triangular).
• Action: e.g., flexor, extensor, adductor.
• Origin and insertion: Some names include both attachment points.

Muscle Groups of the Body (Gross Anatomy)

• Muscles of the head and neck
• Muscles of the trunk
• Muscles of the upper extremity
• Muscles of the lower extremity

Selected Muscles and Their Actions

• Facial muscles: e.g., frontalis (raises eyebrows, wrinkles forehead), orbicularis oculi (closes eyelids), orbicularis oris (closes and protrudes lips), zygomaticus (smiling), buccinator (compresses cheek).
• Chewing (masticatory) muscles: masseter (elevates mandible), temporalis (assists in jaw closure).
• Neck muscles: sternocleidomastoid (flexes and rotates neck), platysma (tenses neck skin, depresses mandible corners).

Trunk

• Anterior thorax: pectoralis major (adducts and flexes the arm), intercostals (assist in breathing; external for inhalation, internal for forced exhalation).
• Abdominal wall: rectus abdominis, external and internal obliques, transversus abdominis (support trunk, assist forced expiration and maintain intra-abdominal pressure).
• Posterior trunk: trapezius (moves scapula, extends head), latissimus dorsi (powerful arm adduction and extension), erector spinae (extend vertebral column), quadratus lumborum (lateral flexion of spine).

Upper limb
Muscles crossing the shoulder, elbow and forearm provide complex control for reaching and manipulation.

• Biceps brachii: Prime mover for elbow flexion and forearm supination.
• Brachialis: Deep to biceps; important for elbow flexion.
• Brachioradialis: Assists elbow flexion especially with mid-pronated forearm.
• Triceps brachii: Main elbow extensor; antagonist to biceps.

Lower limb

• Large, strong muscles specialised for posture, locomotion and balance.
• Hip joint movers: Gluteus maximus (powerful hip extensor), gluteus medius (hip abductor, stabilises pelvis), iliopsoas (prime hip flexor), adductor group (bring thighs together).
• Knee movers: Quadriceps group (powerful knee extensors), hamstrings (knee flexion and hip extension), sartorius (assists in crossing leg).
• Ankle and foot: Tibialis anterior (dorsiflexion), extensor digitorum longus (toe extension, dorsiflexion), fibularis/ peroneal muscles (plantarflexion and eversion), gastrocnemius and soleus (plantarflexion).
• The gluteus maximus is the largest skeletal muscle; the stapedius in the middle ear is the smallest skeletal muscle.

Interactions of Skeletal Muscles

• Prime mover (agonist): Muscle that produces the primary action.
• Antagonist: Muscle that opposes the prime mover to control or reverse movement.
• Synergists: Assist the prime mover and reduce unwanted motion.
• Fixators (stabilisers): Specialised synergists that stabilise the origin of the prime mover so it can act more efficiently.

Common Conditions and Disorders Affecting Muscles

• Neuromuscular disorders: Amyotrophic lateral sclerosis (ALS), myasthenia gravis (autoimmune impairment of neuromuscular transmission), myopathies leading to weakness.
• Muscular dystrophies: Genetic disorders (more than 30 types) that cause progressive muscle weakness.
• Inflammatory disorders: Polymyositis and dermatomyositis produce muscle inflammation and weakness.
• Infections: Certain bacterial and viral infections may damage muscle fibres (e.g., some complications of Lyme disease).
• Trauma and overuse: Strains, tears, cramps, spasms and pain from acute injury or chronic overuse.
• Cardiovascular disease: Coronary artery disease and heart attacks affect cardiac muscle function; vascular disease can impair oxygen delivery to skeletal muscle.
• Medication and toxins: Some medications (for example certain statins, chemotherapeutic agents) and toxins can cause muscle pain or weakness.

Special Movements and Joint Actions

• Origin: Attachment of a muscle to the less movable bone.
• Insertion: Attachment to the more movable bone.
• Flexion: Decrease in joint angle (bringing bones closer).
• Extension: Increase in joint angle (straightening).
• Abduction: Movement away from the midline.
• Adduction: Movement toward the midline.
• Rotation: Turning around a longitudinal axis.
• Circumduction: Circular movement combining flexion, extension, abduction and adduction.
• Dorsiflexion: Lifting the foot toward the shin.
• Plantarflexion: Pointing the foot downward.
• Inversion / Eversion: Turning the sole medially / laterally respectively.
• Supination / Pronation: Rotational movements of the forearm that turn the palm anteriorly / posteriorly respectively.
• Opposition: Movement of the thumb to touch other fingertips, enabled by the saddle joint at the base of the thumb.

Muscle Growth, Repair and Ageing

• Exercise causes microtears in muscle fibres; inflammation and satellite cell activity repair and add contractile proteins, leading to hypertrophy.
• Adequate protein intake, rest and progressive overload training enhance repair and growth.
• Sarcopenia (age-related loss of muscle mass and strength) occurs with older age but can be reduced by resistance exercise and nutrition.

Practical Points for Muscle Health

• Maintain a balanced diet with sufficient protein and energy for muscle repair and function.
• Regular exercise combining endurance and resistance training preserves strength, endurance and metabolic health.
• Adequate rest and recovery reduce risk of overuse injury and support muscle adaptation.
• Regular medical check-ups help detect neuromuscular or systemic conditions that may impair muscle function.

Summary