Muscular System

Muscle Types

The human body contains three main types of muscle tissue, each with distinct structural and functional characteristics.

• Skeletal Muscle: Striated, multinucleated, long cylindrical fibers; under voluntary control via the somatic nervous system. Responsible for movement, posture, and joint stability. Attached to bones by tendons.

• Smooth Muscle: Non-striated, single nucleus, spindle-shaped cells; involuntary control via the autonomic nervous system. Moves substances through hollow organs (e.g., digestive tract, blood vessels).

• Cardiac Muscle: Striated, branched fibers, single nucleus per cell; contains intercalated discs for cell communication. Involuntary control; found only in the heart, responsible for pumping blood.

Functions of Skeletal Muscle

Skeletal muscles perform a variety of essential functions for the body.

• Produce Movement: Facilitate movement of bones at joints (ambulation), stop movement via reflexes, maintain posture, stabilize joints, and enable facial expression.

• Control Movement of Substances: Sphincters (skeletal or smooth) regulate openings (mouth, anus, urethra); voluntary muscles control swallowing and urination.

• Respiration: The diaphragm, a skeletal muscle, is essential for breathing.

• Protection: Muscles act as physical barriers or cushions for underlying organs.

• Homeostasis: Muscle contractions generate heat, maintaining core body temperature (e.g., shivering).

Structure of Skeletal Muscles

Skeletal muscles are organized into layers of connective tissue that provide support and facilitate function.

• Muscle Fiber: Long, cylindrical, multinucleated cells.

• Endomysium: Wraps each individual muscle fiber, providing insulation and support.

• Perimysium: Surrounds bundles of muscle fibers (fascicles), allowing passage of blood vessels and nerves.

• Epimysium: Outermost layer, surrounds the entire muscle, extends to form tendons.

Overview: Muscle fibers are wrapped by endomysium, grouped into fascicles (perimysium), and the whole muscle is wrapped by epimysium.

Blood and Nerve Supply

• Arteries, veins, and nerves run through muscle layers, providing oxygen, nutrients, waste removal, and nerve signals for voluntary movement.

Formation of Tendons

• Connective tissue layers merge at muscle ends to form tendons, which attach muscle to bone.

Fascicle Arrangement/Shapes

Muscle fascicles are arranged in various patterns, affecting muscle function and strength.

• Parallel: Fibers run parallel to the muscle’s long axis (e.g., sartorius).

• Convergent: Fibers fan out from a broad origin and converge to a single tendon (e.g., pectoralis major).

• Pennate: Fibers attach at an angle to a central tendon; increases power.

  • Unipennate: One side (e.g., extensor digitorum longus)

  • Bipennate: Both sides (e.g., rectus femoris)

  • Multipennate: Multiple branches (e.g., deltoid)

• Sphincters: Fibers arranged concentrically around an opening (e.g., orbicularis oculi).

• Spiral: Fibers wrap around another structure (e.g., supinator).

• Fusiform: Muscle belly is thicker than the ends (e.g., biceps brachii).

Naming of Muscles

Muscles are named based on various criteria:

• Shape: Orbicularis (circular), serratus (saw-toothed), trapezius (trapezoid), quadratus (four-sided).

• Size: Maximus (largest), medius (medium), minimus (smallest), major/minor, vastus.

• Length: Brevis (short), longus (long).

• Position: Lateralis (side), medialis (midline).

• Direction: Rectus (straight), oblique (angled), transverse (horizontal).

• Location: Frontalis (forehead), capitis (head), carpi (wrist), oculi (eye), oris (mouth).

• Attachments: Sternocleidomastoid (sternum, clavicle, mastoid process).

• Number of Origins: Biceps (2), triceps (3), quadriceps (4).

• Actions: Flexor, extensor, abductor, adductor, levator, erector.

Functional Groups

Muscles work in groups to produce coordinated movements.

• Agonist (Prime Mover): Main force for a movement (e.g., brachialis for elbow flexion).

• Antagonist: Opposes agonist, relaxes during movement (e.g., triceps brachii opposes biceps brachii).

• Synergist: Assists agonist, adds force or stabilizes joint (e.g., biceps brachii assists brachialis).

• Fixator: Stabilizes and anchors bone, prevents unwanted movement (e.g., supraspinatus stabilizes shoulder).

Origins and Insertions

• Origin: Fixed attachment, usually closer to the axial skeleton; does not move during contraction.

• Insertion: Moveable attachment, usually farther from the axial skeleton; pulled toward origin during contraction.

Muscles of Facial Expression

Facial muscles enable emotions and non-verbal communication.

• Originate from bone/flat tendons, insert onto skin/hypodermis.

• Allow movement of skin for expressions; innervated by facial nerve (CN VII).

• Platysma: Tenses neck skin, depresses lower lip, creates frown.

• Orbicularis Oris: Purses lips, compresses lips, shapes lip movements for speech.

• Buccinator: Pushes food against teeth, controls air pressure, taut cheeks.

• Orbicularis Oculi: Closes eyelids; orbital (forceful closure), palpebral (gentle closure), lacrimal (tear drainage).

Clinical Note: Facial paralysis can result from facial nerve damage, causing loss of expression.

Muscles of Mastication

Muscles responsible for chewing and biting.

• Temporalis: Elevation and retraction of mandible.

• Masseter: Elevation and protrusion of mandible.

Sternocleidomastoid

• Origin: Sternum and clavicle

• Insertion: Mastoid process

• Actions: Flexion and rotation of neck

Clinical Note: Torticollis (wry neck) is caused by spasms or shortening of SCM.

Muscles of the Abdominal Wall

Four main muscles form the anterolateral abdominal wall:

• External Abdominal Oblique: Outermost layer

• Internal Abdominal Oblique: Middle layer

• Rectus Abdominis: Vertical muscle (six-pack)

• Transverse Abdominis: Deepest layer

Clinical Note: Median incisions along the linea alba minimize bleeding and provide access to abdominal organs.

Erector Spinae Muscles

Maintain posture and extend the vertebral column.

• Iliocostalis: Outermost column

• Longissimus: Middle column

• Spinalis: Closest to spine

• Actions: Extension, lateral flexion, rotation of head and vertebral column

Latissimus Dorsi

Powerful muscle for swimming, climbing, and pulling.

• Origin: Inferior T-vertebrae, iliac crest, ribs

• Insertion: Intertubercular groove of humerus

• Actions: Extension, adduction, medial rotation of humerus

Serratus Anterior

Stabilizes scapula, important for pushing and punching.

• Origin: Lateral ribs 1-8

• Insertion: Medial border of scapula

• Actions: Protracts and fixes scapula

Clinical Note: Winged scapula results from paralysis of serratus anterior (long thoracic nerve injury).

Trapezius

Stabilizes and moves scapula, assists in raising arm.

• Origin: Occipital bone, spinous processes C7-T12

• Insertion: Clavicle, acromion, spine of scapula

• Actions: Elevates, retracts, depresses scapula

Clinical Note: Injury to spinal accessory nerve (CN XI) causes shoulder droop.

Deltoid

Main abductor of shoulder, common site for IM injection.

• Origin: Clavicle, acromion, spine of scapula

• Insertion: Deltoid tuberosity of humerus

• Actions: Abduction, flexion, medial/lateral rotation of humerus

Rotator Cuff Muscles (SITS)

Stabilize shoulder joint and allow rotation/abduction.

• Supraspinatus: Initiates abduction

• Infraspinatus: Lateral rotation

• Teres Minor: Lateral rotation

• Subscapularis: Medial rotation, adduction

Clinical Note: Rotator cuff injuries often affect supraspinatus due to repetitive overhead use.

Biceps Brachii

• Origin: Coracoid process (short head), supraglenoid tubercle (long head)

• Insertion: Radial tuberosity

• Actions: Flexion at elbow and shoulder, supination

Clinical Note: Biceps rupture can cause loss of forearm flexion and supination.

Brachioradialis

• Origin: Lateral supracondylar ridge of humerus

• Insertion: Distal radius

• Actions: Weak flexion, strong in mid-pronated position

Triceps Brachii

• Origin: Long, lateral, and medial heads

• Insertion: Olecranon process of ulna

• Actions: Extension at elbow and shoulder

Gluteal Muscles

• Gluteus Maximus: Chief extensor, lateral rotation

• Gluteus Medius & Minimus: Abduction, medial rotation, stabilize pelvis

Clinical Note: Trendelenburg sign indicates weakness of gluteus medius/minimus.

Thigh Compartments

• Anterior: Extensors (flex thigh, extend leg); femoral nerve; includes iliopsoas, pectineus, sartorius, quadriceps femoris

• Medial: Adductors (adduct, medially rotate thigh); includes adductor longus, brevis, magnus, gracilis

• Posterior: Flexors (flex leg, extend thigh); includes semitendinosus, semimembranosus, biceps femoris

Quadriceps Femoris

• Four heads: rectus femoris, vastus lateralis, vastus intermedius, vastus medialis

• Origin: AIIS (rectus femoris), linea aspera (vastus lateralis/medialis), anterior femur (vastus intermedius)

• Insertion: Tibial tuberosity

• Actions: Extend knee; rectus femoris also flexes thigh

Iliopsoas

• Origin: T12-L5 (psoas major), iliac fossa (iliacus)

• Insertion: Lesser trochanter

• Actions: Flex thigh at hip, stabilize joint

Leg Compartments

• Anterior: Dorsiflexion, extension of toes; includes tibialis anterior, extensor hallucis longus, extensor digitorum longus; deep peroneal nerve

• Lateral: Plantarflexion, eversion; includes peroneus longus, peroneus brevis

• Posterior: Plantarflexion, flexion of toes; superficial (gastrocnemius, soleus, plantaris) and deep compartments

Clinical Notes

• Anterior Compartment Syndrome: Increased pressure causes ischemia and nerve injury; can result in foot drop.

• Foot Drop: Inability to dorsiflex; caused by nerve or muscle damage.

• Avulsion Fracture: Sudden inversion can fracture base of 5th metatarsal (peroneus brevis tendon).

• Calcaneal (Achilles) Tendon Rupture: Traumatic injury, often in men 30-40 years; sudden forced plantarflexion or violent dorsiflexion.

Summary Table: Muscle Types

Summary Table: Fascicle Arrangements

Summary Table: Functional Muscle Groups

Additional info:

• Muscle contraction generates heat via ATP hydrolysis, contributing to thermoregulation.

• Muscle injuries and clinical syndromes often relate to nerve supply or repetitive use.

• Muscle compartments are separated by fascia; increased pressure can cause compartment syndrome.