Musculoskeletal System — Muscle Contraction

Types of muscle:

Skeletal — voluntary, striated, multinucleated

Cardiac — involuntary, striated, intercalated discs, autorhythmic

Smooth — involuntary, non-striated, found in organs

Sliding filament theory:

Myosin (thick filament) walks along actin (thin filament)

Sarcomere shortens → muscle contracts

A band (myosin) stays same length; I band and H zone shorten

Excitation-contraction coupling:

Motor neuron releases acetylcholine → depolarization → action potential travels along T-tubules → Ca²⁺ released from sarcoplasmic reticulum → Ca²⁺ binds troponin → tropomyosin moves off actin → myosin binds actin → power stroke

ATP role in contraction:

ATP needed to detach myosin from actin (NOT to contract)

Rigor mortis: no ATP after death → myosin locked to actin

Bone types: Long, short, flat, irregular, sesamoid. Compact (cortical) vs. cancellous (spongy) bone. Osteoblasts build; osteoclasts resorb bone.

Reference:

Wikipedia: Sliding Filament Theory

https://en.wikipedia.org/wiki/Sliding_filament_theory