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Unit 3 • Homeostasis, Neurons, Muscles

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Set point logic: think thermostat; negative feedback restores, positive feedback accelerates to completion.
AP directionality: refractory period keeps signal forward; saltatory conduction speeds via myelin.
Neurotransmitter clearance: reuptake, degradation, diffusion—common reasoning step in MCQ.
Muscle fatigue links: ATP availability, Ca²⁺ handling, pH changes; connect to sliding filament checkpoints.

Homeostasis Vocab

Homeostasis: maintain internal balance; sensors + integrator + effector.

Negative feedback: reverses change (insulin/glucagon, thermoregulation).

Positive feedback: amplifies until endpoint (oxytocin, clotting).

Feedback inhibition: pathway end product inhibits earlier enzyme.

Action Potential Sequence

1

Rest: leak K⁺ + Na⁺/K⁺ pump keep -70 mV.

2

Threshold reached: ligand-gated or graded potentials depolarize membrane.

3

Voltage-gated Na⁺ channels open → rapid depolarization.

4

Na⁺ channels inactivate; voltage-gated K⁺ channels open → repolarization.

5

K⁺ efflux overshoots (hyperpolarization); pump + leak reset resting potential.

Myelin → saltatory conduction at nodes of Ranvier. Absolute refractory = no new AP; relative needs stronger depolarization.

Muscle & ACh

Sliding filament: Ca²⁺ binds troponin → tropomyosin shifts → myosin binds actin → power stroke. ATP binds to release; ATP hydrolysis cocks myosin.

ACh at neuromuscular junction opens ligand-gated Na⁺ channels → muscle AP travels T-tubules → Ca²⁺ release from SR.

Fatigue links: ATP depletion, lactic acid (pH), inorganic phosphate buildup interfering with cross-bridge cycling.

Quorum Sensing

Autoinducers accumulate with density; threshold triggers community genes (biofilm, virulence, luminescence). Both intraspecies and interspecies signals exist.