18.1 The Neuron — Structural Unit
The neuron is the structural & functional unit of the nervous system, specialised to receive, conduct and transmit impulses.
- Cell body (cyton) — contains the nucleus, cytoplasm and granular Nissl bodies.
- Dendrites — short branched processes that bring impulses toward the cell body.
- Axon — single long process carrying impulses away; ends in synaptic knobs with neurotransmitter vesicles.
- Myelin sheath (made by Schwann cells) insulates the axon; gaps are the nodes of Ranvier.
Types of neurons (by structure)
| Type | Processes | Example |
|---|
| Multipolar | One axon, many dendrites | Cerebral cortex |
| Bipolar | One axon, one dendrite | Retina of eye |
| Unipolar | Cell body with one axon | Embryonic stage |
🧠 Memory Hook
"Dendrites Deliver in, Axons Away"
Dendrites carry impulses toward the cyton; the axon carries them away.
18.2 Resting Potential
A resting (non-conducting) neuron membrane is polarised at about −70 mV (inside negative relative to outside).
- The Na⁺/K⁺ pump pushes 3 Na⁺ out and 2 K⁺ in, using ATP.
- The resting membrane is more permeable to K⁺ than Na⁺; the axoplasm has high K⁺ and low Na⁺, the outside the reverse.
Resting potential ≈ −70 mV (polarised)
Pump: 3 Na⁺ OUT ↔ 2 K⁺ IN (active, ATP-driven)
18.3 Action Potential & Impulse Conduction
A stimulus above threshold triggers an action potential — a transient reversal of membrane polarity.
Resting −70 mV→Stimulus: Na⁺ channels open, Na⁺ rushes IN→Depolarisation (+30 mV)→K⁺ moves OUT — repolarisation→Na⁺/K⁺ pump restores resting state
Conduction speed
- Myelinated fibres: the impulse jumps node to node — saltatory conduction (much faster).
- Non-myelinated fibres: the impulse moves as a continuous wave (slower).
🧠 Memory Hook
"Na⁺ IN to fire, K⁺ OUT to reset"
Sodium influx = depolarisation (firing); potassium efflux = repolarisation (reset).
18.4 The Synapse
A synapse is the junction between two neurons. Two types exist.
| Electrical synapse | Chemical synapse |
|---|
| Membranes very close; current flows directly | Separated by a synaptic cleft |
| Transmission very fast | Uses a neurotransmitter (slower) |
Chemical transmission
Impulse reaches axon terminal→Ca²⁺ enters; vesicles release acetylcholine→Neurotransmitter crosses the cleft→Binds receptors on post-synaptic membrane→New impulse generated
Acetylcholine is a common neurotransmitter.
18.5 Organisation of the Nervous System
The human neural system has a central and a peripheral division.
| Division | Includes |
|---|
| Central NS (CNS) | Brain + spinal cord — integration |
| Peripheral NS (PNS) | Nerves linking CNS to body |
- PNS fibres: afferent (sensory) carry impulses to the CNS; efferent (motor) carry commands away.
- PNS is functionally split into the somatic (voluntary, skeletal muscle) and autonomic systems.
- The autonomic system has antagonistic sympathetic ("fight/flight") and parasympathetic ("rest/digest") parts.
Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.
18.6 Protection of the Brain — Meninges & CSF
The brain sits in the cranium and is wrapped in three meninges.
Dura mater (tough, outer)→Arachnoid (middle, web-like)→Pia mater (delicate, inner)
- Cerebrospinal fluid (CSF) fills the ventricles and the space around the brain, cushioning it and supplying nutrients.
🧠 Memory Hook
"D-A-P from outside in"
Meninges order outermost → innermost: Dura → Arachnoid → Pia.
18.7 The Brain — Forebrain
The brain has three regions: forebrain, midbrain and hindbrain.
Cerebrum
- Largest part; two hemispheres joined by the corpus callosum.
- Outer cerebral cortex is grey matter (cell bodies); folds (gyri) and grooves (sulci) increase its surface area.
- Four lobes — frontal (thinking, voluntary movement), parietal (touch), temporal (hearing), occipital (vision).
Thalamus & Hypothalamus
- Thalamus — major relay/coordination centre for sensory and motor signals.
- Hypothalamus — controls body temperature, hunger, thirst, sleep, emotions and the pituitary gland; secretes neurohormones.
- The limbic system (amygdala, hippocampus) governs emotions, motivation and memory.
18.8 The Brain — Midbrain & Hindbrain
The midbrain lies between the thalamus and pons; its corpora quadrigemina control visual and auditory reflexes; the cerebral aqueduct passes through it.
Hindbrain
| Part | Function |
|---|
| Cerebellum | Coordination, balance, posture and precision of movement |
| Pons | Relay between brain regions; helps regulate respiration |
| Medulla oblongata | Controls respiration, heartbeat, blood pressure and gastric secretion |
The medulla, pons and midbrain together form the brain stem, connecting the brain to the spinal cord.
18.9 Reflex Action & Reflex Arc
A reflex action is a rapid, automatic, involuntary response to a stimulus, mediated by the spinal cord.
Receptor→Sensory (afferent) neuron→Spinal cord / interneuron→Motor (efferent) neuron→Effector (muscle/gland)
- The pathway is the reflex arc; the knee-jerk is a simple example.
- It protects the body by acting faster than conscious thought.
🧠 Memory Hook
"Some Cars Move (R→S→C→M→E)"
Receptor → Sensory neuron → CNS → Motor neuron → Effector.
18.10 The Eye — Photoreception
The eye has three coats and converts light into nerve impulses.
| Layer | Features |
|---|
| Sclera (outer) | Tough white coat; transparent cornea in front |
| Choroid (middle) | Pigmented, vascular; forms ciliary body & iris; the pupil is its aperture |
| Retina (inner) | Contains photoreceptors — rods & cones |
- The lens (held by the ciliary body) focuses light; aqueous humor lies in front and vitreous humor behind it.
Rods vs Cones
| Rods | Cones |
|---|
| Light | Dim light (night) | Bright light |
| Colour | No | Yes (3 types) |
| Pigment | Rhodopsin (visual purple) | Photopsin/iodopsin |
- Fovea (yellow spot) — packed with cones, point of sharpest vision.
- Blind spot — where the optic nerve leaves; no photoreceptors.
🧠 Memory Hook
"Cones = Colour · Rods = Rhodopsin at night"
Cones see colour in bright light; rods (rhodopsin) work in dim light.
18.11 The Ear — Hearing & Balance
The ear serves both hearing and equilibrium and has three regions.
| Region | Parts |
|---|
| Outer ear | Pinna, external auditory canal, tympanic membrane (eardrum) |
| Middle ear | Ear ossicles — malleus, incus, stapes; Eustachian tube |
| Inner ear | Cochlea (organ of Corti — hearing) and vestibular apparatus (balance) |
- The organ of Corti on the basilar membrane holds hair cells that convert sound into impulses.
- The vestibular apparatus = three semicircular canals + utricle + saccule, sensing position and movement.
- The Eustachian tube equalises air pressure across the eardrum.
🧠 Memory Hook
"MIS — Malleus, Incus, Stapes"
The three ear ossicles in order (hammer, anvil, stirrup).