Section 1: The Neuron — Structure and Classification
The neuron is the fundamental unit of the nervous system. Its cell body contains the nucleus and Nissl granules (rough ER for protein synthesis). Dendrites receive signals; the single axon transmits them away from the soma. The axon hillock (junction of soma and axon) is the trigger zone for action potential generation. Neurons are myelinated (in PNS: Schwann cells; in CNS: oligodendrocytes) with gaps called nodes of Ranvier. Functionally, neurons are sensory/afferent (receptor → CNS), motor/efferent (CNS → effector), or interneurons (within CNS). Structurally: unipolar, bipolar (special senses), or multipolar (most CNS neurons).
Section 2: The Nerve Impulse — Electrochemical Basis
Resting potential: -70 mV (inside negative), maintained by the Na+/K+ pump (3 Na+ out, 2 K+ in) and K+ leak channels. Action potential: threshold stimulus → voltage-gated Na+ channels open → Na+ influx → depolarization to +30 mV → Na+ channels inactivate, K+ channels open → K+ efflux → repolarization → brief hyperpolarization → resting state restored. Obeys the all-or-none principle. In myelinated neurons, saltatory conduction (node to node jumping) achieves 15-120 m/s vs 0.5-2 m/s in unmyelinated fibres.
Section 3: The Synapse — Chemical Transmission
Action potential arrives at axon terminal → voltage-gated Ca2+ channels open → Ca2+ influx → exocytosis of neurotransmitters from synaptic vesicles into the 20 nm synaptic cleft → NT diffuses and binds postsynaptic receptors → new impulse generated. Signal terminated by acetylcholinesterase (for ACh) or reuptake. Key NTs: ACh (NMJ, all ANS preganglionic, parasympathetic postganglionic), norepinephrine (sympathetic postganglionic), dopamine (substantia nigra → basal ganglia; deficit = Parkinson's), serotonin (raphe nuclei; deficit = depression), GABA (inhibitory), glutamate (excitatory).
Section 4: The Brain — Structure and Functions
Forebrain: cerebrum (four lobes; initiates voluntary movement, higher cognition), thalamus (sensory relay, except smell), hypothalamus (homeostasis, pituitary control). Midbrain: visual and auditory reflex relay (superior and inferior colliculi). Hindbrain: cerebellum (coordinates movement, balance, posture — does NOT initiate), pons (relay, pneumotaxic centre), medulla oblongata (vital autonomic centres: cardiovascular, respiratory, vomiting, swallowing). Brain protected by cranium and three meninges (dura, arachnoid, pia); subarachnoid space contains CSF.
Section 5: The Spinal Cord and Reflex Arc
The spinal cord (31 spinal nerve pairs) serves as a two-way conduit between brain and body, and as a reflex centre. Reflex arc: receptor → sensory neuron → interneuron (spinal cord integration) → motor neuron → effector. The knee-jerk reflex is monosynaptic (no interneuron). Dorsal horn = sensory processing; ventral horn = motor neuron cell bodies; dorsal root ganglion = sensory neuron cell bodies.
Section 6: Autonomic Nervous System
Somatic NS: single motor neuron → skeletal muscle (voluntary). ANS: two-neuron chain (preganglionic + postganglionic) → visceral effectors (involuntary). Sympathetic: close ganglia, norepinephrine (postganglionic), fight/flight effects. Parasympathetic: distal ganglia, ACh (postganglionic), rest/digest effects.
Section 7: The Eye and Vision
Light path: cornea → pupil → lens (accommodation via ciliary muscles) → retina. Rods: 120M, rhodopsin, dim light, peripheral. Cones: 6-7M, iodopsin, colour, fovea. Fovea = sharpest vision. Optic disc = blind spot (no photoreceptors, optic nerve exit). Eye defects: myopia (concave lens), hypermetropia (convex), presbyopia (bifocal), astigmatism (cylindrical), cataract (surgery), glaucoma .
Section 8: The Ear — Hearing and Balance
Hearing: pinna → external canal → tympanic membrane → malleus → incus → stapes (smallest bone) → oval window → cochlear fluid → basilar membrane → hair cells of organ of Corti → auditory nerve (CN VIII) → brain. Ossicles amplify ~20x. Balance: vestibular apparatus — 3 semicircular canals (detect rotation) + utricle and saccule (detect linear acceleration and head tilt).