Ch 18 covers the nervous system in detail — neuron structure, generation and transmission of nerve impulses, synaptic function, brain structure, reflex arcs, and the sensory organs (eye and ear).
Neuron: dendrites → cell body → axon → synaptic terminals. Resting potential: −70 mV (Na⁺/K⁺ pump). Action potential: stimulus → Na⁺ channels open → depolarisation (+30 mV) → K⁺ channels open → repolarisation → refractory period. Propagation along axon (saltatory in myelinated). Synapse: axon terminal releases neurotransmitter (acetylcholine) → diffuses across synaptic cleft → binds receptors on postsynaptic membrane → generates new impulse.
Brain: cerebrum (cognition, motor, sensory), cerebellum (balance, coordination), diencephalon (thalamus, hypothalamus), brainstem (medulla — vital functions). Spinal cord: reflexes, relay. Eye: cornea → lens → retina (rods for dim light, cones for colour — RGB). Accommodation: ciliary muscles adjust lens. Ear: external (pinna) → middle (ossicles) → inner (cochlea — hearing, semicircular canals — balance).
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When an action potential reaches the synaptic knob, voltage-gated Ca²⁺ channels open. Ca²⁺ enters and triggers vesicles containing neurotransmitter (e.g., acetylcholine) to fuse with the membrane and release their contents into the synaptic cleft. The neurotransmitter diffuses across the cleft, binds to receptors on the post-synaptic membrane, opening ion channels and generating a new action potential.
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