The Muscular System

Structure of Skeletal Muscles

Skeletal muscles are composed of specialized cells called skeletal muscle fibers, which are long and thin. These fibers are surrounded by a thin layer of extracellular matrix known as the endomysium. Groups of 10–100 muscle fibers form a fascicle, surrounded by perimysium. All fascicles in a muscle are enclosed by epimysium, which is continuous with the fascia, the most superficial connective tissue sheath. Tendons attach muscles to bones or other structures, enabling movement. Skeletal muscles are richly supplied with blood vessels and nerves, and are voluntary, requiring nervous system stimulation to contract.

Fascicle Patterns and Muscle Shapes

Muscles are classified by the arrangement of their fascicles and overall shape:

• Parallel: Evenly spaced fascicles, tendon same width as muscle (e.g., Sartorius).

• Convergent: Broad at one end, tapers to a single tendon (e.g., Pectoralis Major).

• Circular: Encircles a structure, acts as a sphincter (e.g., Orbicularis Oculi).

• Fusiform: Thick in the middle, tapered at ends (e.g., Biceps Brachii).

• Pennate: Fascicles attach to tendon at an angle, feather-like appearance.

  • Unipennate: Fascicles on one side (e.g., Flexor Pollicis Longus).

  • Bipennate: Fascicles on both sides (e.g., Rectus Femoris).

  • Multipennate: Several tendons, fascicles attach like multiple feathers (e.g., Deltoid).

Naming Muscles

Muscles are named based on size, location, shape, appearance, position, function, and structural features:

• Size: Major, Minor, Longus, Brevis, Vastus

• Location: Superior, Inferior, Medial, Lateral, Anterior, Posterior, Profundus, Superficialis, Supra-, Infra-, External, Internal, Intercostal

• Action: Flexor, Extensor, Levator, Adductor, Abductor, Pronator, Supinator, Depressor, Erector

• Body Region: Abdominis, Brachii, Capitis, Carpi, Cervicis, Digitorum/Digiti, Femoris, Gluteal, Hallucis, Oculi, Oris, Pectoralis, Pollicis

• Fiber Orientation: Oblique, Orbicular, Rectus, Transversus

• Heads: Biceps (two), Triceps (three), Quadriceps (four)

• Shape: Deltoid (triangular), Maximus (largest), Minimus/Minimi (smallest), Minor (small), Quadratus (rectangle), Rhomboid (rhombus), Serratus (jagged), Trapezius (trapezoid)

Functions of Skeletal Muscles

The primary function of skeletal muscles is to contract and generate muscle tension, which produces body movements and actions. Muscle tension also generates heat, converting chemical energy (ATP) into mechanical energy, with heat as a byproduct (e.g., shivering).

Functional Groups of Muscles

• Agonists (Prime Movers): Provide most force for movement.

• Antagonists: Oppose and slow the action, usually on the opposite side of the joint.

• Synergists: Work with agonists to guide and smooth movement.

• Fixators: Hold bones in place, making movement efficient and reducing injury risk.

Muscle Origin and Insertion

• Origin (O): More fixed attachment point.

• Insertion (I): Attachment point that moves.

• Example: Biceps brachii attaches to scapula (origin) and radius (insertion).

• Origin and insertion can switch depending on movement (e.g., Iliopsoas).

Lever Systems in Body Movements

Body movements use lever systems with four components:

• Lever: Bone

• Load: Object or body part being moved

• Force: Muscle tension

• Fulcrum: Joint

Types of levers:

• First-Class Lever: Fulcrum in the middle (e.g., atlanto-occipital joint).

• Second-Class Lever: Fulcrum at one end, load in the middle (e.g., metatarsophalangeal joints).

• Third-Class Lever: Load at one end, force in the middle, fulcrum at the other end (e.g., elbow joint).

Mechanical Advantage and Disadvantage

• Mechanical Advantage: Small force moves a large load.

• Mechanical Disadvantage: Greater force required to move the load.

Studying Muscles: Superficial Muscles

Muscles are categorized by region: head, neck, vertebral column, trunk, pelvic floor, pectoral girdle, upper limb, hip, and lower limb. Familiarity with superficial muscles aids in understanding muscle locations and functions.

Muscles of Facial Expression

Overview

Facial muscles often insert into skin or connective tissue, producing shape changes in the face. Key muscles include:

• Epicranius: Frontalis and Occipitalis, connected by epicranial aponeurosis; elevates eyebrows and forehead.

• Orbicularis Oculi: Closes eyelids.

• Levator Palpebrae Superioris: Opens eyelids.

• Corrugator Supercilii: Squinting.

• Levator Labii Superioris: Raises upper lip.

• Zygomaticus Major/Minor: Smiling.

• Risorius: Smirk.

• Orbicularis Oris: Controls lips for speech, eating, whistling.

• Depressor Anguli Oris, Depressor Labii Inferioris, Mentalis: Frowning, pouting.

• Buccinator: Sucking, chewing, whistling.

• Platysma: Jaw-dropping, tightens neck skin.

Extrinsic Eye Muscles

Overview

Six muscles move each eye, most originating from the common tendinous ring at the posterior orbit:

• Medial Rectus: Moves eye medially.

• Lateral Rectus: Moves eye laterally.

• Superior Rectus: Moves eye superiorly and medially.

• Inferior Rectus: Moves eye inferiorly and medially.

• Superior Oblique: Rotates eye inferiorly and laterally via the trochlea.

• Inferior Oblique: Rotates eye superiorly and laterally.

Muscles of Mastication

Overview

Muscles involved in chewing:

• Masseter: Elevates mandible.

• Temporalis: Elevates mandible.

• Medial Pterygoid: Elevates mandible, synergist.

• Lateral Pterygoid: Depresses mandible, protraction, side-to-side movement.

Muscles of Swallowing

Pushing Food to the Posterior Oral Cavity

• Digastric, Stylohyoid, Mylohyoid, Geniohyoid: Elevate hyoid bone, raise tongue and floor of mouth.

• Genioglossus: Protrudes tongue.

• Hyoglossus: Depresses tongue.

• Styloglossus: Elevates and retracts tongue.

Pushing Food into the Esophagus

• Sternohyoid, Omohyoid, Sternothyroid, Thyrohyoid: Depress pharynx, push food into esophagus.

• Pharyngeal Constrictor: Contracts sequentially to push food into esophagus.

Muscles That Move the Head and Neck

Overview

• Sternocleidomastoid: Rotates head, flexes head.

• Scalene Muscles: Pull head laterally, elevate ribs in deep breathing.

• Trapezius: Superior part extends head.

• Splenius Capitus and Cervicis: Rotate and extend head and neck.

Muscles of the Vertebral Column

Erector Spinae Muscle Group

• Lateral Iliocostalis: Lumborum, thoracis, cervicis.

• Middle Longissimus: Thoracis, cervicis, capitis.

• Medial Spinalis: Thoracis, cervicis, capitis.

Transversospinal Group

• Semispinalis, Multifidus, Rotatores: Extend head, neck, vertebral column.

• Quadratus Lumborum: Extends, laterally flexes vertebral column.

Muscles of Ventilation

Overview

• Diaphragm: Contracts and flattens for inspiration; relaxes for expiration.

• External Intercostals: Raise and spread ribs for inspiration.

• Internal Intercostals: Depress rib cage for forced expiration.

• Sternocleidomastoid and Scalenes: Assist in forced inspiration.

Abdominal Muscles

Overview

• Rectus Abdominis: Flexes trunk, enclosed by rectus sheath, central tendon is linea alba.

• External/Internal Obliques: Rotate and flex trunk laterally.

• Transversus Abdominis: Compresses abdominal cavity, raises intra-abdominal pressure.

These muscles facilitate urination, defecation, childbirth, and forced expiration.

Muscles of the Pelvic Diaphragm, Urogenital Diaphragm, and Perineum

Pelvic Floor

• Levator Ani: Pubococcygeus, Iliococcygeus, Ischiococcygeus; supports pelvic floor, anal canal, genitourinary organs.

• Anococcygeal Ligament: Insertion site for pelvic floor muscles.

Perineum

• Urogenital Diaphragm: External urethral sphincter (urination control), deep transverse perineal muscle (supports organs).

• External Anal Sphincter: Voluntary control of defecation.

• Superficial Transverse Perineal, Bulbospongiosus, Ischiocavernosus: Stabilize perineal body, aid erection, expel semen.

Tables: Muscles of the Pelvic Diaphragm, Urogenital Diaphragm, and Perineum

Urinary Incontinence and Kegel Exercises

• Weak pelvic diaphragm muscles can cause urinary incontinence, especially after childbirth.

• Kegel exercises strengthen the levator ani muscle, treating incontinence and preventing pelvic organ prolapse.

• Recommended for pregnant women and men with incontinence or sexual dysfunction.

Muscles That Move the Scapula at the Pectoral Girdle

Overview

• Serratus Anterior: Protracts scapula, rotates superiorly.

• Pectoralis Minor: Protracts and depresses scapula.

• Trapezius: Elevates, retracts, depresses, and rotates scapula.

• Levator Scapulae: Elevates scapula.

• Rhomboid Major/Minor: Retract scapula.

Muscles That Move the Arm at the Shoulder Joint

Overview

• Pectoralis Major: Flexes, adducts, and internally rotates arm.

• Coracobrachialis: Assists flexion and adduction.

• Deltoid: Flexion, abduction, extension of arm.

• Latissimus Dorsi: Extends, adducts, internally rotates arm.

• Teres Major: Assists latissimus dorsi.

• Rotator Cuff: Stabilizes shoulder joint (Teres Minor, Supraspinatus, Infraspinatus, Subscapularis).

Muscles That Move the Forearm and Hand

Forearm at the Elbow Joint

• Biceps Brachii: Flexes and supinates forearm.

• Brachialis: Agonist of elbow flexion.

• Brachioradialis: Assists flexion.

• Triceps Brachii: Extends forearm, adducts arm, stabilizes shoulder.

• Anconeus: Assists extension.

Hand and Fingers

• Flexors: Anterior/medial forearm; flex hand and fingers.

• Extensors: Posterior/lateral forearm; extend hand and fingers.

• Intrinsic Hand Muscles: Abductor pollicis brevis, flexor pollicis brevis, lumbricals, interossei; move fingers and thumb.

Muscles of the Hip, Thigh, Knee, and Leg

Anterior and Medial Muscles

• Iliopsoas: Thigh flexion.

• Pectineus: Assists flexion and adduction.

• Adductor Group: Magnus, Longus, Brevis; adduct thigh.

• Gracilis: Adducts thigh.

• Sartorius: Flexion, abduction, lateral rotation of thigh; flexion of leg.

• Quadriceps Femoris: Rectus femoris (flexion, extension), vastus lateralis/intermedius/medialis (extension).

Posterior Muscles

• Gluteal Group: Maximus (extension, abduction, lateral rotation), Medius/Minimus (abduction, medial rotation).

• Piriformis, Obturator Internus/Externus, Gemelli, Quadratus Femoris: Lateral rotation.

• Hamstrings: Semitendinosus, Semimembranosus, Biceps Femoris (extension).

Muscles of the Ankle, Foot, and Toes

Foot Movements

• Dorsiflexion: Tibialis anterior, extensor digitorum longus.

• Inversion: Tibialis anterior.

• Eversion: Fibularis longus/brevis.

• Plantarflexion: Gastrocnemius, soleus, plantaris, tibialis posterior, fibularis longus/brevis.

Toe Movements

• Flexor Hallucis Longus: Flexes hallux.

• Flexor Digitorum Longus: Flexes toes 2–5.

• Intrinsic Foot Muscles: Support arches, adapt to terrain, skilled movements.

The Big Picture of Muscle Movement

Additional info: All tables have been recreated in HTML format, and only directly relevant images are included. Academic context has been expanded for clarity and completeness. These notes are structured for exam preparation and self-study.