Muscle Physiology & Major Skeletal Muscles

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SKELETAL MUSCLE PHYSIOLOGY (RECAP)
• Excitation at neuromuscular junction:
– \text{Ca^{2+}} influx → ACh release → motor end-plate depolarization → T-tubules.
– Sarcoplasmic reticulum releases \text{Ca^{2+}} → binds troponin → tropomyosin shifts → cross-bridge cycling.
• ATP: detaches myosin, re-cocks head.
• Relaxation: ACh breakdown, \text{Ca^{2+}} pumped back; rigor mortis = no ATP.
• Fiber types: slow-oxidative, fast-oxidative, fast-glycolytic.
• Motor unit recruitment → graded force.

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SMOOTH VS SKELETAL MUSCLE
Structure
• No myofibrils, striations, or T-tubules.
• Dense bodies link thin filaments; thick filaments interdigitate → cell shortens/twists.
Function
• Very slow contraction, highly fatigue-resistant.
Excitation
• Varicosities form diffuse junctions; transmitter may be ACh or others.
• Also activated by stretch, hormones (e.g., oxytocin), gap junction spread.

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MUSCLE SYSTEM & NAMING BASICS
• ~600 skeletal muscles, usually bilateral pairs.
Roles
• Agonist (prime mover) – main force.
• Antagonist – opposes.
• Synergist – assists/stabilizes.
Naming clues
1 Action (flexor, extensor)
2 Location (tibialis, orbicularis oris)
3 Attachments (sternocleidomastoid)
4 Fiber orientation (rectus = straight)
5 Shape (deltoid = triangle)
6 Size/length (maximus, brevis, longus).

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HEAD & FACE MUSCLES – KEY ACTIONS
• Epicranius (frontal + occipital bellies) – raises eyebrows.
• Temporalis – elevates mandible.
• Masseter – prime mandible elevator (chewing).
• Orbicularis oculi – forceful eye closure/wink.
• Corrugator supercilii – wrinkles brow (frown/confusion).
• Zygomaticus major/minor – elevates mouth corners (smile).
• Orbicularis oris – purses lips (kiss).
• Buccinator – compresses cheek (chewing, blowing).

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NECK & TRUNK MUSCLES
Neck
• Sternocleidomastoid – cervical flexion, lateral rotation ("no" muscle).
Shoulder/Chest
• Deltoid – prime shoulder abduction (+ ant part flexes, post extends).
• Pectoralis major – horizontal/adduction of humerus.
• Pectoralis minor – scapula protraction or rib elevation (forced inhale).
Breathing
• Diaphragm – contracts ↓ (inspiration); relaxes ↑ (expiration).
• External intercostals – elevate ribs (inspire).
• Internal intercostals – depress ribs (expire).
Abdomen
• External oblique – rotation, trunk flex assist.
• Internal oblique – as above, opposite fiber angle.
• Rectus abdominis – spine flexion (sit-ups).
• Transversus abdominis – compresses abdomen ("suck in").

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BACK & SHOULDER (POSTERIOR)
• Trapezius – scapula retraction; upper fibers elevate, lower depress.
• Levator scapulae – elevates scapula.
• Rhomboid major/minor – scapula retraction, slight elevation.
Rotator-cuff
• Supraspinatus – abduction start.
• Infraspinatus & Teres minor – lateral rotation.
• Subscapularis (deep, not pictured) – medial rotation.
• Teres major – medial rotation, adduction (not rotator-cuff).
• Latissimus dorsi – shoulder extension & posterior adduction (lat pull-down).

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ARM & FOREARM
Arm
• Biceps brachii – elbow flexion, forearm supination.
• Brachialis – prime elbow flexor.
• Triceps brachii – elbow extension (antagonist).
• Brachioradialis – synergist elbow flexion; landmark for forearm.
Anterior forearm (flexors)
• Pronator teres – forearm pronation.
• Flexor carpi radialis – wrist flexion, abduction.
• Palmaris longus – weak wrist flexor (absent ~15 %).
• Flexor carpi ulnaris – wrist flexion, adduction.
• Flexor digitorum superficialis – finger flexion (middle phalanges).
• Flexor digitorum profundus – finger flexion (distal phalanges).
• Flexor pollicis longus – thumb flexion.
Posterior forearm (extensors)
• Extensor carpi radialis longus/brevis – wrist extension, abduction.
• Extensor digitorum – finger extension.
• Extensor carpi ulnaris – wrist extension, adduction.
• Supinator – forearm supination.
• Abductor pollicis longus – thumb abduction.
• Extensor pollicis longus – thumb extension.

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LOWER LIMB (INTRO)
Gluteal group
• Gluteus maximus – hip extension; inserts into iliotibial (IT) tract.
• Gluteus medius & minimus – hip abduction, medial stability.
Hamstrings (posterior thigh)
• Biceps femoris (lateral).
• Semitendinosus (superficial medial).
• Semimembranosus (deep medial).
Actions: hip extension & knee flexion (posterior moves).
Anterior hip flexor
• Iliopsoas = iliacus + psoas major; prime hip flexion.

(End of covered content – lecture stopped at iliopsoas.)