Physiology — Neuromuscular & Sensory — USMLE Step 1 Notes
Free, high-yield revision notes for USMLE Step 1. Read here, or drill the same material as questions and flashcards in the app.
These are high-yield revision notes written for first-order recall. A revision aid, not editorially reviewed and not clinical advice — verify against a primary source before relying on anything clinically.
Membrane & action potential
- The resting membrane potential (~-70 mV) is set by K+ permeability and maintained by the Na+/K+ ATPase (3 Na out, 2 K in).
- Action potential: threshold → voltage-gated Na+ influx (depolarisation) → Na+ channel inactivation and K+ efflux (repolarisation) → brief hyperpolarisation. It is all-or-none.
- Myelin speeds conduction by saltatory conduction between nodes of Ranvier.
Synaptic & neuromuscular transmission
- An action potential opens presynaptic voltage-gated calcium channels → neurotransmitter release → postsynaptic receptor binding.
- At the neuromuscular junction, acetylcholine acts on nicotinic receptors; acetylcholinesterase terminates the signal.
Skeletal muscle contraction
- Excitation-contraction coupling: AP down T-tubules → Ca2+ released from sarcoplasmic reticulum → Ca2+ binds troponin C → tropomyosin moves → actin-myosin cross-bridges cycle (needs ATP).
- Sliding filament theory: filaments slide, the sarcomere shortens; the A band stays constant while the I band and H zone narrow.
Muscle types compared
| Trigger for Ca2+ rise | Notes | |
|---|---|---|
| Skeletal | SR (voluntary, T-tubule coupled) | Troponin; no gap junctions |
| Cardiac | SR + calcium-induced calcium release | Troponin; gap junctions, automaticity |
| Smooth | Extracellular + SR → calmodulin | Myosin light-chain kinase; slow, sustained |
Reflexes & proprioception
- Muscle spindle senses stretch (stretch reflex, e.g. knee jerk); the Golgi tendon organ senses tension and inhibits contraction to protect the muscle.
- A reflex arc = receptor → afferent → (interneuron) → efferent → effector.
Special senses
- Vision: phototransduction in rods/cones — light bleaches rhodopsin, closing Na+ channels and hyperpolarising the cell (rods = dim light, cones = colour/acuity).
- Hearing: the cochlea is tonotopic — the base detects high frequencies, the apex low; hair-cell stereocilia transduce sound.
- Chemoreception: taste and smell use receptor cells; olfaction projects to cortex without a thalamic relay.
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Synaptic detail & muscle energetics
- EPSPs and IPSPs summate spatially and temporally to threshold; myelination and axon diameter set conduction velocity.
- Muscle fibre types: slow-oxidative (type I, endurance, red, mitochondria-rich) vs fast-glycolytic (type II, power, white).
- Energy: immediate (ATP/creatine phosphate), anaerobic glycolysis (lactate), then oxidative phosphorylation with sustained activity.
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