Study Guide: The Muscular System (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: Fascicles run parallel to the muscle's long axis (e.g., Sartorius).

• Convergent: Broad origin, fascicles converge to a single tendon (e.g., Pectoralis major).

• Circular: Fascicles form rings, act as sphincters (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: Multiple tendons, fascicles join together (e.g., Deltoid).

Naming Muscles

Muscles are named based on size, location, shape, appearance, position, function, or structural features.

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

• Location: Directional terms (e.g., Superior, Inferior, Medial, Lateral), regional anatomy (e.g., Pectoralis minor)

• Attachment: Named for structures attached (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, allow for control.

• Synergists: Assist agonists, guide movement.

• Fixators: Stabilize bones, enhance efficiency.

Muscle Origin and Insertion

Muscle attachments are classified as origin (fixed point) and insertion (moveable point). 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 and Knots

Muscle Strain occurs when a muscle is overstretched or overloaded, resulting in tears. Symptoms include pain, swelling, and limited movement. Treatment follows the PRICE method: Protect, Rest, Ice, Compression, Elevation.

Myofascial Trigger Point (Muscle Knot): Localized area of irritability/inflammation in fascia, causing sustained contraction and pain. Treatment includes massage, anti-inflammatory medications, muscle relaxants, and stretching.

Studying Muscles: Regional Overview

Muscles are categorized by region:

• Head, neck, and vertebral column

• Trunk and pelvic floor

• Pectoral girdle and upper limb

• Hip and lower limb

Muscles of Facial Expression

Overview

Facial muscles often insert into skin or connective tissue, allowing for expressive movements.

• Epicranius: Frontalis and occipitalis, elevates eyebrows/forehead.

• Orbicularis oculi: Closes eyelids.

• Levator palpebrae superioris: Opens eyelids.

• Corrugator supercilii: Squinting.

• Levator labii superioris: Elevates upper lip.

• Zygomaticus major/minor: Smiling.

• Risorius: Lateral movement of mouth.

• Orbicularis oris: Controls lips.

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

• Buccinator: Cheek inward (sucking, chewing).

• Platysma: Jaw-dropping, tightens neck skin.

Table: Muscles of Facial Expression

Muscle	Action
Epicranius	Elevates eyebrows/forehead
Orbicularis oculi	Closes eyelids
Zygomaticus major/minor	Smiling
Buccinator	Cheek inward
Platysma	Jaw-dropping
Additional info: See image_15 for full table and directional arrows.

Extrinsic Eye Muscles

Overview

Six extrinsic muscles control eye movement, originating from the common tendinous ring.

• Medial rectus: Moves eye medially

• Lateral rectus: Moves eye laterally

• Superior rectus: Moves eye superiorly/medially

• Inferior rectus: Moves eye inferiorly/medially

• Superior oblique: Rotates eye inferiorly/laterally

• Inferior oblique: Rotates eye superiorly/laterally

Table: Extrinsic Eye and Orbit Muscles

Muscle	Action
Medial rectus	Moves eye medially
Lateral rectus	Moves eye laterally
Superior rectus	Moves eye superiorly/medially
Inferior rectus	Moves eye inferiorly/medially
Superior oblique	Rotates eye inferiorly/laterally
Inferior oblique	Rotates eye superiorly/laterally
Additional info: See image_17 for full table and directional arrows.

Muscles of Mastication

Overview

Muscles responsible for chewing and moving the jaw.

• Masseter: Elevates mandible

• Temporalis: Elevates mandible

• Medial pterygoid: Elevates mandible, synergist

• Lateral pterygoid: Depresses/protracts mandible, side-to-side movement

Table: Muscles of Mastication

Muscle	Action
Masseter	Elevates mandible
Temporalis	Elevates mandible
Medial pterygoid	Elevates/protracts mandible
Lateral pterygoid	Depresses/protracts mandible
Additional info: See image_19 for full table and directional arrows.

Muscles of Swallowing

Overview

Muscles elevate the hyoid bone, tongue, and floor of the mouth to facilitate swallowing.

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

• Genioglossus: Protrudes tongue

• Hyoglossus: Depresses tongue

• Styloglossus: Elevates/retracts tongue

Table: Muscles of the Tongue

Muscle	Action
Genioglossus	Protrudes tongue
Hyoglossus	Depresses tongue
Styloglossus	Elevates/retracts tongue
Additional info: See image_22 for full table and directional arrows.

Pushing Food into the Esophagus

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

• Pharyngeal constrictor: Sequential contraction moves food

Table: Muscles of Swallowing

Muscle	Action
Digastric	Elevates hyoid, depresses mandible
Mylohyoid	Elevates hyoid/floor of mouth
Pharyngeal constrictor	Pushes food into esophagus
Additional info: See image_24 for full table and directional arrows.

Muscles That Move the Head and Neck

Overview

• Sternocleidomastoid: Rotates head, flexes neck

• Scalenes: Lateral flexion, elevate ribs in deep breathing

• Trapezius: Extends head

• Splenius capitus/cervicis: Rotate/extend head and neck

Table: Muscles That Move the Head and Neck

Muscle	Action
Sternocleidomastoid	Rotates/flexes head
Scalenes	Lateral flexion, rib elevation
Splenius capitus/cervicis	Rotate/extend head/neck
Additional info: See image_27 for full table and directional arrows.

Muscles of the Vertebral Column

Overview

• Erector spinae: Extension, lateral flexion of vertebral column

• Iliocostalis: Lateral column

• Longissimus: Middle column

• Spinalis: Medial column

• Transversospinal group: Extension, rotation

• Quadratus lumborum: Extension, lateral flexion

Table: Muscles of the Vertebral Column

Muscle	Action
Erector spinae	Extension/lateral flexion
Transversospinal group	Extension/rotation
Quadratus lumborum	Extension/lateral flexion
Additional info: See image_29 for full table and directional arrows.

Muscles of Ventilation

Overview

• Diaphragm: Contracts/flattens for inspiration, relaxes for expiration

• External intercostals: Elevate/spread ribs (inspiration)

• Internal intercostals: Depress ribs (forced expiration)

• Sternocleidomastoid, scalenes: Assist forced inspiration

Table: Muscles of Ventilation

Muscle	Action
Diaphragm	Inspiration
External intercostals	Elevate ribs
Internal intercostals	Depress ribs
Additional info: See image_31 for full table and directional arrows.

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, raises intra-abdominal pressure

Table: Abdominal Muscles

Muscle	Action
Rectus abdominis	Flexes trunk
External oblique	Rotates/flexes trunk
Transversus abdominis	Compresses abdominal cavity
Additional info: See image_33 for full table and directional arrows.

Muscles of the Pelvic Diaphragm, Urogenital Diaphragm, and Perineum

Overview

• Pelvic floor (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: 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

Muscles That Move the Arm at the Shoulder Joint

Overview

• Pectoralis major: Flexes, adducts, internally rotates arm

• Coracobrachialis: Assists flexion/adduction

• Deltoid: Abduction, flexion, extension

• Latissimus dorsi: Extends, adducts, internally rotates arm

• Teres major: Assists latissimus dorsi

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

Muscles That Move the Forearm and Hand

Overview

• Biceps brachii: Flexes/supinates forearm

• Brachialis: Agonist of elbow flexion

• Brachioradialis: Assists flexion

• Triceps brachii: Extends forearm

• Anconeus: Assists extension

Flexors and Extensors of Hand and Fingers

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

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

Intrinsic Hand Muscles

• Abductor pollicis brevis: Abducts thumb

• Flexor pollicis brevis: Flexes thumb

• Lumbricals, dorsal/palmar interossei: Move fingers

Muscles of the Hip, Thigh, Knee, and Leg

Overview

• Iliopsoas: Thigh flexion

• Pectineus: Assists flexion/adduction

• Adductor group: Adduct thigh

• Gracilis: Adducts thigh

• Sartorius: Flexion, abduction, lateral rotation

• Quadriceps femoris: Extension at knee

Gluteal and Hamstring Groups

• Gluteus maximus: Extends, abducts, laterally rotates thigh

• Gluteus medius/minimus: Abduct/medially rotate thigh

• Piriformis, obturator internus/externus, gemelli, quadratus femoris: Lateral rotation

• Hamstrings: Extend thigh/leg (semitendinosus, semimembranosus, biceps femoris)

Muscles of the Ankle, Foot, and Toes

Overview

• Tibialis anterior, extensor digitorum longus: Dorsiflexion

• Extensor hallucis longus: Assists dorsiflexion

• Fibularis longus/brevis: Eversion

• Gastrocnemius, soleus: Plantarflexion, form calcaneal tendon

• Flexor hallucis longus, flexor digitorum longus: Flex toes

• Intrinsic foot muscles: Support arches, skilled movements

Calcaneal Tendon Injuries

Overview

• Gastrocnemius and soleus form a second-class lever, generating high force.

• Calcaneal tendon injuries are common in sports; symptoms include pain, inflammation, and loss of plantarflexion.

• Simmonds Test: Squeeze gastrocnemius; if tendon is intact, foot plantarflexes.

• Treatment: Rest, physical therapy, surgical repair if needed.

The Big Picture of Muscle Movement

Muscle movement involves coordinated actions of multiple muscle groups, lever systems, and nervous system control to produce efficient and purposeful motion.

Summary Table: Muscle Groups and Functions

Region	Main Muscles	Function
Head/Neck	Sternocleidomastoid, scalenes, trapezius	Head/neck movement
Trunk	Rectus abdominis, obliques, diaphragm	Flexion, rotation, breathing
Pectoral girdle	Serratus anterior, trapezius, rhomboids	Scapular movement
Upper limb	Biceps brachii, triceps brachii, forearm flexors/extensors	Arm/hand movement
Hip/lower limb	Iliopsoas, quadriceps, hamstrings, gluteals	Thigh/leg movement
Foot/toes	Tibialis anterior, gastrocnemius, intrinsic foot muscles	Foot/toe movement

Additional info: Tables and images referenced above provide detailed muscle names, origins, insertions, and actions for exam preparation.