Anatomy · Short notes

Musculoskeletal System: Short Notes & Cheat Sheet

Bones and their types, bone cells and ossification, joints, the vertebral column, muscle types and muscle contraction.

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Short notes

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Types of synovial joint

Joint shape decides the range of movement: ball-and-socket joints move in every direction, plane joints only glide.

Bones

The adult skeleton has 206 bones: 80 axial (skull, vertebral column, ribs, sternum) and 126 appendicular (limbs and girdles). Babies have more, which later fuse.
Bone types: long (femur — the longest and strongest), short (carpals), flat (skull, sternum), irregular (vertebrae) and sesamoid (patella — the largest sesamoid). The stapes is the smallest bone.
Osteoblasts build bone, osteoclasts (from monocytes) resorb it, and osteocytes maintain it.
Compact bone is organised into osteons (Haversian systems); spongy bone contains red marrow.
Intramembranous ossification forms flat skull bones and the clavicle (the first bone to ossify). Endochondral ossification forms long bones, which grow at the epiphyseal plate.

Joints and the spine

Fibrous joints (skull sutures) barely move. Cartilaginous joints (pubic symphysis, intervertebral discs) move slightly. Synovial joints move freely.
Synovial joint types: ball-and-socket (hip, shoulder), hinge (elbow, knee), pivot (atlanto-axial, proximal radioulnar), condyloid (wrist), saddle (thumb carpometacarpal) and plane (intercarpal).
The vertebral column has 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral and 4 fused coccygeal.
Tendons join muscle to bone; ligaments join bone to bone.

Muscle

Skeletal muscle is striated and voluntary with many nuclei per cell. Cardiac muscle is striated and involuntary with intercalated discs. Smooth muscle is non-striated and involuntary.
Largest muscle: gluteus maximus. Longest: sartorius. Smallest: stapedius.
The rotator cuff (SITS) is supraspinatus, infraspinatus, teres minor and subscapularis.
Contraction follows the sliding filament model: calcium binds troponin C, exposing actin so myosin heads can bind. ATP is needed for myosin to detach, so without ATP muscles stay rigid (rigor mortis).
The sarcomere runs from one Z line to the next and is the unit of contraction.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Record holders

Record	Structure
Longest, strongest bone	Femur
Smallest bone	Stapes
Largest sesamoid bone	Patella
First bone to ossify	Clavicle
Largest muscle	Gluteus maximus
Longest muscle	Sartorius
Smallest muscle	Stapedius

Vertebrae

Region	Number
Cervical	7
Thoracic	12
Lumbar	5
Sacral (fused)	5
Coccygeal (fused)	4

Memory aids

Breakfast 7, lunch 12, dinner 5: 7 cervical, 12 thoracic and 5 lumbar vertebrae.
SITS: Rotator cuff: Supraspinatus, Infraspinatus, Teres minor, Subscapularis.

Exam traps

The adult has 206 bones; a newborn has more, which later fuse.
The knee is a modified hinge joint, not a ball-and-socket joint.
Osteoclasts break bone down; osteoblasts build it (B for build).
The clavicle is the first bone to start ossifying, by the intramembranous route.

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