Skeletal Anatomy, Bone Histology & Muscle Physiology – Comprehensive Study Notes

Bone acts as a primary calcium store; when plasma $\mathrm{Ca^{2+}}$ drops, the body mobilizes calcium from bone.
Knowledge of bone structure allows paleontologists to reconstruct posture & form of extinct animals (e.g.
dinosaurs) from skin impressions & skeletal remains.

Axial Skeleton (mid-line structures)
- Skull (cranium + facial bones)
- Vertebral column
- Sternum & associated costal cartilage
Appendicular Skeleton (limbs & girdles)
- Pectoral (shoulder) girdle: scapula, clavicle, coracoid (in birds/reptiles)
- Pelvic girdle: fused coxal bone (ilium, ischium, pubis) forming acetabulum
- Front limb series: humerus → radius & ulna → carpals → metacarpals → phalanges
- Hind limb series: femur → tibia & fibula → tarsals → metatarsals → phalanges

Cervical: $C1-C7$
- C1 = Atlas (supports skull);
- C2 = Axis (odontoid process permits head rotation)
Thoracic: $T1-T12$ (articulate with ribs)
Lumbar: $L1-L5$ (large bodies, bear weight)
Sacrum: fusion of $5$ embryonic vertebrae
Coccyx: variable number of vestigial caudal vertebrae

Mammals share an extremely conserved limb plan → once human bones are memorised, cat, dog, kangaroo, etc. can be identified by positional analogy.
Carnivorans (cat/dog) are digitigrade—walk on toes (phalanges); metacarpals/metatarsals elevated.
Birds:
- Forelimb transformed into wing; fusion/reduction distal to wrist.
- Sternum possesses a deep keel for pectoralis attachment (flight muscles; "breast meat").
- Clavicles fuse into the V-shaped furcula (wishbone).
- Coracoid persists as strong strut between shoulder joint & sternum.
- Tarsometatarsus = fusion of distal tarsals + metatarsals; birds are also digitigrade.

Two expanded ends = epiphyses (contain spongy bone)
Middle shaft = diaphysis (compact bone surrounds marrow cavity)
Medullary cavity contains yellow marrow (lipid-rich)
Outer surface cloaked by fibrous periosteum; marrow cavity lined by endosteum

Repeating cylindrical units = osteons (Haversian systems)
- Central Haversian canal (blood vessels + nerves)
- Concentric lamellae of mineralised matrix
- Osteocytes reside in lacunae
- Canaliculi: tiny radiating channels connecting osteocytes for nutrient/waste diffusion.
Identify in lab: resembles concentric tree rings; often confused with seminiferous tubules—avoid mis-ID.

When a slide obviously shows bone, do not label it merely “connective tissue” or “skeletal”; state compact bone.
Distinguish muscle types via orientation:
- Longitudinal skeletal muscle → visible striations; cross-section lacks stripes.
- Cardiac muscle has striations and intercalated discs.
- Smooth muscle shows spindle-shaped, non-striated cells.

Skeletal Muscle: long, multinucleate, striated; voluntary (somatic nervous system); individual cell often called a “muscle fibre.”
Cardiac Muscle: branching, striated, single nucleus; intercalated discs; involuntary (autonomic).
Smooth Muscle: spindle-shaped, single nucleus, non-striated; involuntary (autonomic); lines viscera & vessels.

Z-Disc: boundary; anchors thin filaments.
Thin Filament
- Mostly actin (two helical chains of G-actin)
- Regulatory proteins: tropomyosin (covers myosin-binding sites) & troponin (binds $\mathrm{Ca^{2+}}$ ).
Thick Filament
- Composed of myosin; heads possess actin-binding site & ATPase site.

Cross-bridge formation – Myosin head (in "cocked" position) binds exposed actin site.
Power stroke – Release of $P_i$ causes head to pivot; thin filament slides toward centre.
Cross-bridge detachment – New ATP binds myosin, reducing affinity → head releases actin.
Reactivation – ATP hydrolysed: $\mathrm{ATP \rightarrow ADP + P_i}$ ; energy resets head to cocked state.

Concurrent cycles across thousands of heads shorten every sarcomere, hence the entire muscle.

Motor neuron AP reaches axon terminal; $\text{ACh}$ released → binds nicotinic receptors on muscle end-plate.
Local depolarisation triggers muscle fibre action potential that invaginates along T-tubules.
Depolarisation alters voltage-sensitive proteins linked to sarcoplasmic reticulum (SR).
Voltage-gated $\mathrm{Ca^{2+}}$ channels in SR open → $\mathrm{Ca^{2+}}$ floods cytosol.
$\mathrm{Ca^{2+}}$ binds troponin → tropomyosin shifts → actin sites exposed → cross-bridge cycling begins.
Termination: cessation of neural firing; SR Ca^{2+} pumps (ATP-dependent) sequester calcium → troponin releases $\mathrm{Ca^{2+}}$ → tropomyosin recovers sites → relaxation.

Upper limb: Biceps brachii (flexor) vs. Triceps brachii (extensor)
Lower limb: Hamstring group (flex knee) vs. Quadriceps femoris (extend knee)
Concept: muscles generally act in oppositional pairs to enable bidirectional joint control.

Post-mortem ATP synthesis stops → no ATP to detach myosin heads from actin.
Cross-bridges become locked; muscles stiffen (rigor) until proteolytic decay breaks down the contractile proteins.

Bone slide: look for concentric osteons; locate Haversian canal, lacunae, canaliculi.
Skeletal muscle (longitudinal): parallel fibres, clear striations, peripheral nuclei.
Cardiac muscle: same striations + distinct intercalated discs at cell junctions.
Smooth muscle: bundled spindles, single central nuclei, no stripes.
Always correlate orientation: striations visible only in longitudinal view of striated muscle.

All mammals (even giraffes) possess $7$ cervical vertebrae.
Sacrum derives from fusion of $5$ embryonic vertebrae.
Standard human vertebral formula: $7\,{\rm C} +12\,{\rm T}+5\,{\rm L}+5\,{\rm S}+\mathrm{(3{-}5)\,Co}$ .
Digitigrade stance = walking on phalanges; plantigrade = entire foot on ground (e.g. humans, bears).