Muscle Anatomy & Physiology – Sarcomere to Whole-Muscle Lecture

Sarcomere Model & Micro-Anatomy

• Model shown is a single myofibril stripped of its sarcoplasmic reticulum (SR) and T-tubules so the contractile units (sarcomeres) are fully exposed.
• One sarcomere is defined as the segment between two adjacent Z-lines / Z-discs (zig-zag plates anchoring thin filaments).

Bands, Zones & Lines (relate to light-microscope striations)

• A-Band – any region that contains thick filaments (myosin); always the same length; appears dark.
• I-Band – regions containing only thin filaments (actin) on either side of a Z-line; light band; each sarcomere contributes two half I-bands.
• H-Zone – central subsection of the A-band that lacks thin-filament overlap; looks like a stacked “H” in some micrographs.
• M-Line – protein mesh in the exact centre; anchors thick filaments.
• Zone of Overlap – where thin (red on model) and thick (blue) filaments interdigitate; its width increases during contraction and decreases on relaxation.

Contraction vs. Relaxation (Demonstrated by Two Positions of the Model)

• Contracted position – thin filaments pulled farther into A-band; Z-lines closer together; wider zone of overlap; sarcomere visibly shortened.
• Relaxed position – thin filaments further apart; minimal overlap; sarcomere lengthened.

Filament Composition

• Thick filaments – myosin molecules; each has outward-projecting myosin heads (cross-bridges).
• Thin filaments – two helical chains of actin sub-units (individual red “beads”).

Regulatory Proteins (Colour-coded on close-up)

• Tropomyosin (yellow strand) – lies in actin’s groove, blocking myosin-binding sites in a resting fibre.
• Troponin (green 3-part complexes) – Ca²⁺-sensitive; shifts tropomyosin away when Ca²⁺ binds, permitting cross-bridge formation.

Connective-Tissue Wrappings of Skeletal Muscle (external to fibre)

• Endomysium – delicate areolar C.T. surrounding a single muscle fibre (visible on earlier whole-fibre model).
• Perimysium (misspoken in video as “paramycium”) – fibrous sheath encasing a fascicle (bundle of fibres).
• Epimysium – dense irregular C.T. that envelopes the entire named muscle; continuous with tendons & aponeuroses.

Neuromuscular Junction (NMJ) & Motor Neuron Anatomy

Gross View

• Whole model depicts a single motor neuron terminating on a skeletal muscle fibre → this interface is the neuromuscular junction.

Axon & Insulation

• Axon – long, thin conducting process; only a sliver visible within the model.
• Myelin Sheath – multilayer lipid insulation surrounding the axon.
– Formed by successive wrappings of individual Schwann cells (PNS glia).
– Schwann Cell Nucleus – flattened oval lying just under the sheath surface.

Terminal Region

• Axon Terminal / Synaptic Terminal – distal unmyelinated segment that contacts the muscle.
• Terminal Branches – numerous small twig-like endings that increase contact area.
• Synaptic End Bulb (Synaptic Bulb) – enlarged knob at each terminal branch containing transmitter vesicles.
• Synapse (Synaptic Cleft) – microscopic gap between synaptic bulb membrane and folded sarcolemma.
• Synaptic Vesicles – membrane spheres filled with neurotransmitter (acetylcholine, ACh, in skeletal muscle).
• Motor End Plate – highly folded sarcolemma directly across the cleft; houses ACh receptors; also called folded sarcolemma.
• Entire structural triad (bulb + cleft + end plate) = Neuromuscular Junction.

Fundamental Joint / Whole-Body Movements (Review List)

(Familiar from bone-unit lab; identical terminology used for muscle unit)
• Flexion – decrease joint angle. • Extension – increase joint angle. • Hyperextension – beyond anatomical position.
• Abduction vs. Adduction – movement away from vs. toward midline (or long axis of limb/digit).
• Circumduction – conical motion at ball-and-socket joints (shoulder, hip).
• Rotation – bone spins around its own longitudinal axis (e.g., atlas–axis, humerus, femur).
• Special Movements –
– Elevation / Depression (scapula, mandible)
– Protraction / Retraction (jaw, scapula)
– Inversion / Eversion (foot)
– Dorsiflexion / Plantar-flexion (ankle)
– Supination / Pronation (forearm)
– Opposition (thumb to fingers)

How Muscles Are Named – Guiding Logic (pre-reading reminder)

• Location (e.g., temporalis covers temporal bone).
• Shape (deltoid = triangular; trapezius = trapezoid).
• Size (gluteus maximus vs. minimus; adductor longus vs. brevis).
• Direction of fibers (rectus = straight; oblique = angled).
• Number of heads (biceps = 2, triceps = 3).
• Attachments – origins & insertions explicitly cited in some names (sternocleidomastoid = sternum + clavicle → mastoid).
• Action (flexor, extensor, adductor, pronator, etc.).

Learning Tip stated in video: while memorising a muscle, visualise its attachments and then imagine what shortens → that gesture is the muscle’s action.

Named Skeletal Muscles Mentioned (Master List)

Scalp & Face

• Frontalis – forehead (frontal belly of occipitofrontalis)
• Occipitalis – over occipital bone (occipital belly)
• Epicranial Aponeurosis / Gallia Aponeurotica – broad tendon linking the two bellies
• Temporalis – fan-shaped over temporal bone
• Orbicularis Oculi – concentric around eyes; closes eyelids
• Nasalis – compresses bridge/widens nostrils
• Orbicularis Oris – puckers lips
• Masseter – major jaw closer; cheek
• Depressor Anguli Oris – pulls mouth corners inferiorly
• Platysma – superficial neck “sheet”; tenses skin, depresses jaw

Neck & Anterior Thorax

• Sternocleidomastoid – sternum + clavicle → mastoid; neck flexion/rotation
• Scalenes – deep lateral neck flexors (ant., middle, post.)
• Omohyoid – scapula (“omo”) → hyoid
• Sternohyoid – sternum → hyoid
• Pectoralis Major – clavicle/sternum/ribs → humerus; arm flexion/medial rotation
• Serratus Anterior – ribs 1-8 → medial scapula; protracts scapula
• Deltoid – clavicle/acromion/spine → humerus; arm abduction

Upper Limb – Anterior Arm & Forearm

• Biceps Brachii – scapula → radial tuberosity; forearm supination/flexion
• Brachialis – humerus → ulna; primary forearm flexor
• Pronator Teres – pronates forearm
• Brachioradialis – humerus → distal radius; synergist flexor
• Flexor Carpi Radialis – wrist flexor, radial deviation
• Flexor Digitorum Superficialis – flexes fingers 2-5
• Flexor Carpi Ulnaris – wrist flexor, ulnar deviation

Upper Limb – Posterior Arm & Forearm

• Triceps Brachii (long, lateral, medial heads) – forearm extensor
• Anconeus – assists elbow extension
• Extensor Carpi Radialis Brevis – wrist extender, radial side, short
• Extensor Digitorum – extends fingers
• Extensor Carpi Ulnaris – wrist extender, ulnar side
• Flexor Carpi Ulnaris – visible from posterior as it wraps
• Abductor Pollicis Longus – abducts thumb
• Extensor Pollicis Brevis – extends thumb

Trunk & Abdominal Wall

• Trapezius – large kite; elevates, retracts scapula
• Latissimus Dorsi – broad lower back; arm extension/adduction
• External Obliques – superficial abdominal wall; trunk rotation
• Rectus Abdominis – “six-pack”; trunk flexion

Pelvis & Gluteal Region

• Gluteus Maximus – primary hip extensor
• Gluteus Medius – abducts/medially rotates thigh (deep/superior to maximus)
• Tensor Fasciae Latae – anterior iliac crest → iliotibial tract; steadies thigh

Hip Flexors & Medial Thigh

• Iliopsoas (Psoas Major + Iliacus) – chief hip flexor
• Pectineus – adducts/flexes thigh
• Adductor Longus – thigh adductor
• (Brief mention) Adductor Brevis – short adductor, barely visible
• Gracilis – long medial strap; adduction, knee flexion
• Sartorius – longest muscle, crosses hip & knee (“tailor’s position”)

Anterior Thigh (Quadriceps Group)

• Rectus Femoris – crosses hip & knee; powerful extender
• Vastus Lateralis – lateral quad
• Vastus Medialis – medial quad
• Patella – sesamoid embedded in quadriceps tendon; continues inferiorly as patellar ligament

Posterior Thigh (Hamstrings)

• Biceps Femoris – lateral; long & short heads
• Semitendinosus – superficial medial hamstring
• Semimembranosus – deep to semitendinosus

Leg – Anterior, Lateral, Posterior Compartments

• Tibialis Anterior – dorsiflexes/inverts foot
• Fibularis (Peroneus) Longus – plantar-flexes/everts foot
• Gastrocnemius – calf; plantar flexion, knee flexion
• Soleus – deep to gastrocnemius; postural plantar flexor
• Plantaris – small muscle with long tendon between gastroc & soleus
• Flexor Digitorum Longus – deep posterior; flexes toes 2-5 (model note: not visible externally)
• Calcaneal (Achilles) Tendon – fusion of gastrocnemius, soleus, plantaris tendons → calcaneus

Practical / Exam Connections & Tips

• Any protein/molecule (Actin, Myosin, Tropomyosin, Troponin) can be tested in both histology & physiology components (Chapter 10 content).
• Connective tissue layers are best asked via histological or diagrammatic images because physical models for epi- & perimysium are lacking.
• Origins & Insertions will be specifically drilled in a future video; innervations reserved for nervous-system unit.
• When studying large charts, correlate each muscle’s name with: (1) location, (2) attachment(s), (3) action, then test yourself by visualising the joint motion and palpating on your own body.

Real-World / Clinical Relevance

• Understanding zones of overlap & H-zone shrinkage underpins the sliding-filament mechanism and clinical phenomena like rigor mortis.
• Damage to NMJ (e.g., myasthenia gravis) targets the motor end plate ACh receptors discussed here.
• Connective-tissue sheaths transmit force to tendons; pathology such as compartment syndrome involves these fascial planes.
• Structural muscle knowledge aids injection site selection (e.g., gluteus medius for IM shots).