Chapter 10: The Muscular System – Anatomy & Physiology Study Notes

The Muscular System

Overview

The muscular system is essential for movement, posture, and many physiological functions. It consists of approximately 600 skeletal muscles, which account for about half of the human body’s weight. Muscles are specialized to convert chemical energy from ATP into mechanical energy for motion.

• Structural and functional organization of muscles

• Muscles of the head and neck

• Muscles of the trunk

• Muscles acting on the shoulder and upper limb

• Muscles acting on the hip and lower limb

I. Organization of Muscles

Types of Muscle Tissue

• Skeletal muscle: Voluntary, striated muscle attached to bones for movement.

• Cardiac muscle: Involuntary, striated muscle found in the heart.

• Smooth muscle: Involuntary, non-striated muscle found in walls of hollow organs.

Myology is the study of the muscular system.

Connective Tissues of a Muscle

Muscles are supported and organized by several connective tissue layers:

• Endomysium: Thin sleeve of loose connective tissue surrounding each muscle fiber (cell).

• Perimysium: Slightly thicker layer that wraps bundles of muscle fibers into fascicles.

• Epimysium: Fibrous sheath surrounding the entire muscle.

• Fascia: Sheet of connective tissue that separates neighboring muscles or muscle groups from each other and the subcutaneous tissue.

Table: Connective Tissue Layers in Muscle

Layer	Location	Function
Endomysium	Surrounds individual muscle fibers	Provides insulation and support
Perimysium	Surrounds fascicles (bundles of fibers)	Organizes fibers into groups
Epimysium	Surrounds entire muscle	Protects and supports muscle
Fascia	Surrounds muscle groups	Separates muscles, reduces friction

Fascicle Orientation of Muscles

The strength and direction of a muscle’s pull are determined by the orientation of its fascicles. Muscles are classified by fascicle arrangement:

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

• Parallel: Fascicles run parallel to the long axis (e.g., rectus abdominis).

• Convergent (triangular): Broad origin, narrow insertion (e.g., pectoralis major).

• Pennate: Fascicles insert obliquely on a tendon (unipennate, bipennate, multipennate; e.g., deltoid).

• Circular (sphincters): Fascicles arranged concentrically around an opening (e.g., orbicularis oculi).

II. Muscle Terminology

A. Muscle Origins and Insertions

• Origin: Bony attachment at stationary end of muscle.

• Belly: Thicker, middle region between origin and insertion.

• Insertion: Bony attachment to mobile end of muscle.

B. Functional Groups of Muscles

• Action: The effect produced by a muscle (movement or stabilization).

• Prime mover (agonist): Main muscle responsible for a movement (e.g., biceps brachii for forearm flexion).

• Synergist: Assists the prime mover (e.g., brachialis assists biceps brachii).

• Antagonist: Opposes the prime mover (e.g., triceps brachii opposes biceps brachii).

• Fixator: Prevents movement of bone (e.g., rhomboids stabilize scapula).

Table: Muscle Actions Across the Elbow

Role	Muscle Example
Prime mover	Biceps brachii
Synergist	Brachialis
Antagonist	Triceps brachii
Fixator	Rhomboids

C. Intrinsic and Extrinsic Muscles

• Intrinsic muscles: Entirely contained within a region (e.g., hand muscles).

• Extrinsic muscles: Act on a region but originate elsewhere (e.g., forearm muscles that move the fingers).

D. Muscle Innervation

• Innervation: Identity of the nerve that stimulates a muscle; important for diagnosing nerve injuries.

• Spinal nerves: Arise from the spinal cord, innervate muscles below the neck, branch into posterior and anterior rami, and form plexuses.

• Cranial nerves: Arise from the base of the brain, innervate muscles of the head and neck, numbered I to XII.

III. Axial Musculature

A. Head Muscles

• Frontalis: Raises eyebrows.

• Occipitalis: Retracts scalp, fixes galea aponeurotica.

• Galea aponeurotica: Aponeurosis connecting frontalis and occipitalis.

• Epicranius: Collective term for frontalis, galea aponeurotica, and occipitalis.

• Temporalis: Elevates mandible.

• Zygomaticus: Draws corner of mouth back and upward.

• Orbicularis oris: Closes lips, protrudes lips (kissing), aids in speech.

B. Eye Muscles

• Six oculomotor muscles control eye movement:

  • Inferior, medial, lateral, and superior rectus

  • Superior and inferior oblique

• Orbicularis oculi: Closes the eye.

C. Neck Muscles

• Masseter: Elevates mandible.

• Digastrics: Two bellies; retracts mandible, elevates and fixes hyoid.

• Mylohyoid: Forms floor of mouth, elevates hyoid.

• Sternohyoid: Elevates hyoid.

D. Muscles of the Spine

• Spinalis (with splenius): Tilts and rotates head, extends head.

• Longissimus

• Iliocostalis

E. Oblique and Rectus Muscles

• External intercostalis: Elevates ribs.

• Internal oblique: Flexes and rotates vertebral column.

• External oblique: Flexes and rotates vertebral column.

• Transversus abdominis: Compresses abdomen.

• Rectus abdominis: Depresses ribs, flexes vertebral column.

IV. The Appendicular Musculature

Overview

Appendicular muscles position and direct movement of the appendicular skeleton. They are divided into muscles of the shoulder/arms and pelvic girdle/legs.

A. Muscles of the Shoulder

• Trapezius: Elevates scapula.

• Rhomboideus (major and minor): Retracts, rotates, and elevates scapula.

• Levator scapulae: Elevates scapula.

• Sternocleidomastoid: Flexes neck, moves head.

• Serratus ventralis (anterior): Positions scapula, elevates ribs.

• Pectoralis minor: Protracts and depresses scapula.

B. Muscles Acting on Arm

• Deltoid: Rotates and abducts arm.

• Supraspinatus: Abducts humerus.

• Teres major: Adducts and medially rotates humerus.

• Infraspinatus: Extends and laterally rotates humerus.

• Latissimus dorsi: Extends and medially rotates humerus.

• Biceps brachii: Flexes forearm (elbow), abducts arm.

• Triceps brachii: Extends forearm.

• Brachialis: Flexes forearm.

• Flexor carpi radialis/ulnaris: Flexes palm.

• Extensor carpi radialis/ulnaris: Extends palm.

Supination and Pronation

• Supination: Supinator muscle; palm faces anteriorly or superiorly.

• Pronation: Pronator quadratus and pronator teres; palm faces posteriorly or inferiorly.

C. Muscles of the Leg

Muscles that Move the Thigh

• Gluteal maximus: Extends and laterally rotates thigh.

• Gluteus medius (minimus): Abducts and medially rotates femur, maintains balance.

• Tensor fasciae latae: Abducts and medially rotates femur.

• Adductor femoris (magnus): Adducts and laterally rotates femur.

• Adductor longus: Adducts and laterally rotates femur.

• Iliopsoas: Medially rotates femur.

• Pectinius: Adducts and medially rotates femur.

Muscles that Move the Lower Leg

• Biceps femoris: Flexes knee, extends hip.

• Semimembranosus: Flexes knee, extends hip, medially rotates tibia.

• Semitendinosus: Flexes knee, extends hip, medially rotates tibia.

• Gracilis: Flexes knee, adducts femur.

• Quadriceps (Vastus muscles): Extends knee.

• Rectus femoris: Flexes hip, crosses leg.

• Sartorius: Flexes hip, crosses leg.

Muscles that Move the Ankle, Foot, and Toes

• Gastrocnemius: Flexes knee and foot; inserts on calcaneal (Achilles) tendon.

• Soleus: Flexes foot.

• Plantaris: Flexes foot.

Additional info:

• Muscle actions are often coordinated by several muscles working together in groups, with each muscle playing a specific role (agonist, antagonist, synergist, fixator).

• Understanding muscle innervation is crucial for clinical diagnosis of nerve injuries.

• Muscle fascicle arrangement affects both the strength and range of motion of a muscle.