Muscular System: Muscle Types and Organization

Muscle Types

Feature	Skeletal	Cardiac	Smooth
Location	Attached to skeleton	Heart wall	Hollow organs, vessels
Cell shape	Long, cylindrical	Branched, cylindrical	Spindle-shaped
Striations	Yes	Yes	No
Nuclei	Multiple, peripheral	Single, central	Single, central
Control	Voluntary	Involuntary	Involuntary
Gap junctions	No	Yes (intercalated discs)	Yes
Pacemaker activity	No	Yes (SA node)	Yes (some organs)
Regeneration	Limited	None	Good

Skeletal Muscle Structure

Organization Levels

Level	Description
Whole muscle	Epimysium (outer connective tissue sheath)
Fascicle	Perimysium (surrounds bundles of fibers)
Muscle fiber (cell)	Endomysium (surrounds individual fiber)
Myofibril	Contractile organelle (1-2 microns diameter)
Sarcomere	Basic contractile unit (between Z-discs)

Sarcomere Structure

Zone	Contents
A band	Thick filaments (myosin) + overlapping thin filaments
I band	Thin filaments only (actin)
H zone	Thick filaments only (center of A band)
M line	Central attachment for thick filaments
Z disc	Boundary between sarcomeres (actin attachment)

Muscle Fiber Types

Type	Name	Color	Metabolism	Fatigue	Use
I	Slow oxidative	Red	Aerobic	Resistant	Posture, endurance
IIa	Fast oxidative-glycolytic	Red/pink	Mixed	Moderate	Walking, cycling
IIx (IIb)	Fast glycolytic	White	Anaerobic	Easily	Sprinting, power

Muscle Architecture

Type	Description	Example
Fusiform	Parallel fibers, tapered ends	Biceps brachii
Pennate (unipennate)	Fibers at angle to tendon	Extensor digitorum longus
Pennate (bipennate)	Fibers on both sides of tendon	Rectus femoris
Pennate (multipennate)	Multiple tendon intersections	Deltoid
Circular	Concentric rings	Orbicularis oris
Convergent	Broad origin, narrow insertion	Pectoralis major

Major Skeletal Muscles

Head and Neck

Muscle	Action	Innervation
Masseter	Jaw closure (chewing)	CN V3
Temporalis	Jaw closure	CN V3
Sternocleidomastoid	Head rotation, flexion	CN XI
Trapezius (upper)	Shoulder elevation	CN XI, C3-C4

Back

Muscle	Action	Innervation
Trapezius	Scapular retraction, elevation	CN XI, C3-C4
Latissimus dorsi	Shoulder extension, adduction, internal rotation	Thoracodorsal (C6-C8)
Erector spinae	Spinal extension	Dorsal rami
Rhomboids	Scapular retraction	Dorsal scapular (C5)

Chest

Muscle	Action	Innervation
Pectoralis major	Shoulder adduction, internal rotation, flexion	Medial/lateral pectoral
Pectoralis minor	Scapular protraction, depression	Medial pectoral
Serratus anterior	Scapular protraction (boxer muscle)	Long thoracic (C5-C7)

Shoulder

Muscle	Action	Innervation
Deltoid	Shoulder abduction (all fibers)	Axillary (C5-C6)
Supraspinatus	Shoulder abduction (initiation)	Suprascapular
Infraspinatus	External rotation	Suprascapular
Teres minor	External rotation	Axillary
Subscapularis	Internal rotation	Upper/lower subscapular

Arm

Muscle	Action	Innervation
Biceps brachii	Elbow flexion, forearm supination	Musculocutaneous (C5-C6)
Brachialis	Elbow flexion (primary)	Musculocutaneous, radial
Triceps brachii	Elbow extension	Radial (C6-C8)
Brachioradialis	Elbow flexion	Radial

Forearm

Compartment	Muscles	Action	Nerve
Anterior (flexor)	FCU, FCR, FDS, FDP, PL, PT	Wrist/finger flexion	Median (most), ulnar (FCU)
Posterior (extensor)	ECRL, ECRB, ECU, EDC, EDM, EIP, APL, EPB, EPL	Wrist/finger extension	Radial (deep branch)

Hand Intrinsics

Muscle Group	Action	Nerve
Thenar (APB, FPB, OPB, AP)	Thumb opposition, abduction, flexion	Median (recurrent branch)
Hypothenar (ADM, FDM, ODM)	Little finger movement	Ulnar
Lumbricals	MCP flexion, IP extension	Median (1-2), Ulnar (3-4)
Interossei (palmar 3, dorsal 4)	Finger adduction (palmar), abduction (dorsal)	Ulnar

Abdomen

Muscle	Action	Innervation
Rectus abdominis	Trunk flexion	T6-T12 intercostal
External oblique	Trunk rotation (contralateral), flexion	T6-T12 intercostal
Internal oblique	Trunk rotation (ipsilateral), flexion	T6-T12 intercostal
Transversus abdominis	Compresses abdomen (deepest)	T6-T12 intercostal

Pelvic Floor

Muscle	Function	Innervation
Levator ani	Pelvic floor support	S2-S4 (pudendal)
Pubococcygeus	(Part of levator ani)	S2-S4
Iliococcygeus	(Part of levator ani)	S2-S4

Hip and Thigh

Compartment	Muscles	Action	Nerve
Anterior (flexors)	Iliopsoas, sartorius, rectus femoris	Hip flexion	Femoral (L2-L4)
Posterior (extensors)	Gluteus maximus, hamstrings (semitendinosus, semimembranosus, biceps femoris)	Hip extension	Inferior gluteal (L5-S2), sciatic
Medial (adductors)	Adductor longus/brevis/magnus, gracilis, pectineus	Hip adduction	Obturator (L2-L4)
Lateral (abductors)	Gluteus medius, gluteus minimus, TFL	Hip abduction	Superior gluteal (L4-S1)

Leg

Compartment	Muscles	Action	Nerve
Anterior	Tibialis anterior, EHL, EDL, peroneus tertius	Dorsiflexion, toe extension	Deep peroneal (L4-S1)
Lateral	Peroneus longus, peroneus brevis	Eversion	Superficial peroneal (L5-S1)
Posterior (superficial)	Gastrocnemius, soleus, plantaris	Plantarflexion	Tibial (S1-S2)
Posterior (deep)	Tibialis posterior, FDL, FHL	Plantarflexion, toe flexion	Tibial (L5-S2)

Foot Intrinsics

Muscle Layer	Muscles	Action
First	Abductor hallucis, FDB, abductor digiti minimi	Toe abduction, flexion
Second	Quadratus plantae, lumbricals	Toe flexion
Third	FHB, adductor hallucis, FDM	Great toe flexion/adduction, little toe flexion
Fourth	Interossei (plantar 3, dorsal 4)	Toe adduction/abduction

Neuromuscular Junction

Component	Description
Motor neuron	Alpha motor neuron (anterior horn)
Axon terminal	Branches to multiple muscle fibers (motor unit)
Synaptic cleft	~50 nm gap between nerve and muscle
Motor end plate	Specialized muscle membrane with ACh receptors
Acetylcholine (ACh)	Neurotransmitter (released from vesicles)
Nicotinic receptor	Ligand-gated Na+/K+ channel on muscle

Motor unit: One alpha motor neuron + all the muscle fibers it innervates

Fiber Type	Fibers per Motor Unit
Type I (slow)	10-100 (fine control)
Type II (fast)	300-2000 (power)
Extraocular	3-5 (very fine control)

Cardiac Muscle

• Striated, involuntary
• Branched cells connected by intercalated discs
• Intercalated discs: Gap junctions (electrical coupling) + desmosomes (mechanical adhesion)
• Spontaneous depolarization (autorhythmicity)
• Long refractory period (avoids tetanus)

Smooth Muscle

• No striations, involuntary
• Two types: Multi-unit (iris, airways) and single-unit (GI tract, uterus)
• Single-unit: Gap junctions, pacemaker activity
• Slow, sustained contractions
• Can maintain tension with minimal ATP (latch state)