Study Guide: The Muscular System (Chapter 9) – Human Anatomy & Physiology

The Muscular System

Structure of Skeletal Muscles

Skeletal muscles are composed of long, thin cells called skeletal muscle fibers, which 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 are enclosed by epimysium, which is continuous with the fascia, the most superficial connective tissue sheath.

• Tendons attach muscles to bones or other structures.

• Skeletal muscles are voluntary and require nervous system stimulation to contract.

• Rich blood and nerve supply is essential for muscle function.

Fascicle Patterns and Muscle Shapes

Muscle shape and fascicle arrangement influence function and movement.

• Parallel: Evenly spaced fascicles, e.g., Sartorius.

• Convergent: Broad origin, tapers to a single tendon, e.g., Pectoralis Major.

• Circular: Encircles openings, acts as sphincters, e.g., Orbicularis Oculi.

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

• Pennate: Fascicles attach at an angle to the tendon, resembling a feather.

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

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

  • Multipennate: Multiple tendons, e.g., Deltoid.

Naming Muscles

Muscles are named based on size, location, shape, appearance, position, function, and number of heads.

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

• Location: Directional terms (e.g., Superior, Inferior, Medial, Lateral).

• Attachment: Named for structures they attach to (e.g., Sternocleidomastoid).

• Function: Flexors, Extensors, Levators, Adductors, Abductors.

Functions of Skeletal Muscles

The primary function of skeletal muscles is to contract and generate muscle tension, enabling movement and heat production.

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

• Antagonists: Oppose and slow movement.

• Synergists: Assist agonists, guide movement.

• Fixators: Stabilize bones, enhance efficiency.

Muscle Origin and Insertion

Muscle attachment points are classified as origin (fixed) and insertion (moveable). Movement typically brings insertion toward origin.

• Example: Biceps brachii origin is scapula, insertion is radius.

• Origin and insertion may switch depending on movement context.

Lever Systems in Body Movements

Muscles and bones form lever systems to facilitate movement. Four components: lever (bone), load (object/body part), force (muscle tension), fulcrum (joint).

• First-Class Lever: Fulcrum between load and force (e.g., atlanto-occipital joint).

• Second-Class Lever: Load between fulcrum and force (e.g., metatarsophalangeal joints).

• Third-Class Lever: Force between load and fulcrum (e.g., elbow joint).

Mechanical Advantage vs. Disadvantage

• Mechanical Advantage: Small force moves large load; fulcrum close to load.

• Mechanical Disadvantage: Greater force needed; fulcrum farther from load.

Muscle Strains

A muscle strain occurs when a muscle is overstretched or subjected to excessive contractile force, resulting in tears ranging from minor to complete rupture.

• Symptoms: Pain, limited movement, swelling, bruising.

• Treatment: PRICE method (Protect, Rest, Ice, Compression, Elevation), medications, physical therapy, or surgery.

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 learning.

Muscles of Facial Expression

Overview

Facial muscles often insert into skin or connective tissue, producing expressions by moving skin and facial structures.

• Epicranius: Frontalis and occipitalis connected by epicranial aponeurosis; elevates eyebrows.

• Orbicularis oculi: Closes eyelids.

• Levator palpebrae superioris: Opens eyelids.

• Zygomaticus major/minor: Smiling.

• Risorius: Smirk.

• Orbicularis oris: Controls lips.

• Depressor anguli oris, depressor labii inferioris, mentalis: Frowning, pouting.

• Buccinator: Sucking, whistling.

• Platysma: Jaw-dropping, tightens neck skin.

Muscle	Action
Epicranius	Elevates eyebrows
Orbicularis oculi	Closes eyelids
Zygomaticus major/minor	Smiling
Risorius	Smirk
Orbicularis oris	Controls lips
Buccinator	Sucking, whistling
Platysma	Tightens neck skin
Depressor anguli oris	Frowning

Extrinsic Eye Muscles

Overview

Six extrinsic 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 trochlea.

• Inferior oblique: Rotates eye superiorly and laterally.

Muscle	Origin/Insertion	Action
Medial rectus	Common tendinous ring	Moves eye medially
Lateral rectus	Common tendinous ring	Moves eye laterally
Superior rectus	Common tendinous ring	Moves eye superiorly/medially
Inferior rectus	Common tendinous ring	Moves eye inferiorly/medially
Superior oblique	Trochlea	Rotates eye inferiorly/laterally
Inferior oblique	Anterior maxilla	Rotates eye superiorly/laterally

Muscles of Mastication

Overview

Muscles responsible for chewing include:

• Masseter: Elevates mandible.

• Temporalis: Elevates mandible.

• Medial pterygoid: Elevates mandible, synergist.

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

Muscle	Origin/Insertion	Action
Masseter	Zygomatic arch/mandible	Elevates mandible
Temporalis	Temporal fossa/mandible	Elevates mandible
Medial pterygoid	Pterygoid plate/mandible	Elevates/protracts mandible
Lateral pterygoid	Pterygoid plate/mandible	Depresses/protracts mandible

Muscles of Swallowing

Pushing Food to Posterior Oral Cavity

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

• Genioglossus: Protrudes tongue.

• Hyoglossus: Depresses tongue.

• Styloglossus: Elevates/retracts tongue.

Muscle	Action
Genioglossus	Protrudes tongue
Hyoglossus	Depresses tongue
Styloglossus	Elevates/retracts tongue

Pushing Food into the Esophagus

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

• Pharyngeal constrictor: Sequential contraction moves food into esophagus.

Muscle	Action
Sternohyoid	Depresses hyoid bone
Omohyoid	Depresses hyoid bone
Sternothyroid	Depresses thyroid cartilage
Thyrohyoid	Depresses hyoid bone
Pharyngeal constrictor	Pushes food into esophagus

Muscles That Move the Head and Neck

Overview

• Sternocleidomastoid: Rotates head, flexes neck.

• Scalenes: Lateral flexion, elevate ribs in deep breathing.

• Trapezius: Superior part extends head.

• Splenius capitus/cervicis: Rotation and extension of head/neck.

Muscle	Action
Sternocleidomastoid	Rotates head, flexes neck
Scalenes	Lateral flexion, elevate ribs
Trapezius	Extends head
Splenius capitus/cervicis	Rotation/extension

Muscles of the Vertebral Column

Erector Spinae Muscle Group

Postural muscles and extensors of the vertebral column, divided into three columns:

• 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.

Muscle	Action
Iliocostalis	Extension/lateral flexion
Longissimus	Extension/rotation
Spinalis	Extension
Semispinalis	Extension/rotation
Multifidus	Extension/rotation
Rotatores	Extension/rotation
Quadratus lumborum	Extension/lateral flexion

Muscles of Ventilation

Overview

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

• External intercostals: Raise/spread ribs for inspiration.

• Internal intercostals: Depress ribs for forced expiration.

• Sternocleidomastoid, scalenes: Assist in forced inspiration.

Muscle	Action
Diaphragm	Inspiration
External intercostals	Elevate ribs
Internal intercostals	Depress ribs

Abdominal Muscles

Overview

• Rectus abdominis: Flexes trunk, enclosed by rectus sheath, forms "six-pack".

• External/internal obliques: Rotate/flex trunk laterally.

• Transversus abdominis: Compresses abdominal cavity, increases intra-abdominal pressure.

Muscle	Action
Rectus abdominis	Flexes trunk
External oblique	Rotates/flexes trunk
Internal oblique	Rotates/flexes trunk
Transversus abdominis	Compresses abdominal cavity

Muscles of the Pelvic Diaphragm, Urogenital Diaphragm, and Perineum

Overview

• Pelvic diaphragm: Levator ani (pubococcygeus, iliococcygeus, ischiococcygeus), supports pelvic organs.

• Perineum: Contains external genitalia, anus.

• Urogenital diaphragm: External urethral sphincter, deep transverse perineal muscle.

• External anal sphincter: Voluntary control of defecation.

• Bulbospongiosus, ischiocavernosus: Aid erection, expel semen.

Urinary Incontinence and Kegel Exercises

Overview

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

• Kegel exercises: Strengthen levator ani, prevent pelvic organ prolapse, recommended for both men and women.

Muscles That Move the Scapula at the Pectoral Girdle

Overview

• Serratus anterior: Protracts, rotates scapula.

• Pectoralis minor: Protracts, depresses scapula.

• Trapezius: Elevates, retracts, depresses scapula.

• Levator scapulae: Elevates scapula.

• Rhomboid major/minor: Retract scapula.

Muscle	Action
Serratus anterior	Protracts scapula
Pectoralis minor	Protracts/depresses scapula
Trapezius	Elevates/retracts/depresses scapula
Levator scapulae	Elevates scapula
Rhomboid major/minor	Retract scapula

Muscles That Move the Arm at the Shoulder Joint

Overview

• Pectoralis major: Flexes, adducts, internally rotates arm.

• Coracobrachialis: Assists flexion/adduction.

• Deltoid: Flexion, abduction, extension.

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

• Teres major: Assists latissimus dorsi.

• Rotator cuff: Stabilizes shoulder (teres minor, supraspinatus, infraspinatus, subscapularis).

Muscle	Action
Pectoralis major	Flexes/adducts/rotates arm
Deltoid	Abducts/flexes/extends arm
Latissimus dorsi	Extends/adducts/rotates arm
Teres major	Extends/adducts/rotates arm
Rotator cuff	Stabilizes shoulder

Muscles That Move the Forearm and Hand

Muscles That Move the Forearm at the Elbow Joint

• Biceps brachii: Flexes, supinates forearm.

• Brachialis: Agonist of elbow flexion.

• Brachioradialis: Assists flexion.

• Triceps brachii: Extends forearm.

• Anconeus: Assists extension.

Muscles That Move the Hand and Fingers

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

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

• Intrinsic hand muscles: Abductor pollicis brevis, flexor pollicis brevis, lumbricals, interossei.

Muscles of the Hip, Thigh, Knee, and Leg

Anterior and Medial Muscles

• Iliopsoas: Thigh flexion.

• Pectineus: Assists flexion/adduction.

• Adductor group: Adducts thigh.

• Sartorius: Flexion, abduction, lateral rotation.

• Quadriceps femoris: Extension at knee, thigh flexion.

Muscle	Action
Iliopsoas	Flexes thigh
Pectineus	Flexes/adducts thigh
Adductor group	Adducts thigh
Sartorius	Flexes/abducts/laterally rotates thigh
Quadriceps femoris	Extends leg at knee

Posterior Muscles

• Gluteal group: Extends, abducts, rotates thigh.

• Hamstrings: Extend thigh/leg.

Muscles of the Ankle, Foot, and Toes

Muscles That Move the Foot

• Tibialis anterior, extensor digitorum longus: Dorsiflexion.

• Extensor hallucis longus: Assists dorsiflexion.

• Fibularis longus/brevis: Eversion.

• Triceps surae (gastrocnemius, soleus): Plantarflexion.

Muscles That Move the Toes

• Flexor hallucis longus: Flexes hallux.

• Flexor digitorum longus: Flexes digits 2–5.

• Intrinsic foot muscles: Support arches, skilled movements.

The Big Picture of Muscle Movement

Muscle movement involves coordinated actions of multiple muscle groups, lever systems, and connective tissues to produce efficient and controlled motion throughout the body.