Contents
1 · Nervous System Overview
A network of specialised cells (neurones) that rapidly coordinates responses to stimuli by transmitting electrical impulses. In humans it consists of the central nervous system (CNS: brain and spinal cord) and the peripheral nervous system (PNS: sensory and motor neurones connecting the CNS to the rest of the body).
The pathway for a voluntary response: Stimulus → Receptor → Sensory neurone → CNS (brain) → Motor neurone → Effector → Response
The nervous system allows rapid, short-lived, and highly localised responses — in contrast to the hormonal system, which is slower but longer-lasting.
2 · Neurone Structure
| Type | Function | Key features |
|---|---|---|
| Sensory neurone | Carries impulses from receptor to CNS | Long dendron, short axon, cell body in middle |
| Motor neurone | Carries impulses from CNS to effector | Short dendrites, long axon, cell body at one end |
| Relay (connector) neurone | Connects sensory and motor neurones in CNS | Many short processes; found only in brain and spinal cord |
Parts of a Neurone
- Cell body: Contains nucleus and cytoplasm; metabolic centre.
- Dendrites/dendron: Short extensions that receive impulses and carry them toward the cell body.
- Axon: Long extension that carries impulses away from the cell body.
- Myelin sheath: Fatty insulating layer formed by Schwann cells; speeds up impulse transmission and prevents impulses from "leaking" to adjacent neurones.
- Nodes of Ranvier: Gaps in the myelin sheath where the impulse "jumps", further increasing conduction speed.
Drawing questions on neurones are common. Always label: cell body, nucleus, dendrites, axon, myelin sheath. For a motor neurone, show the cell body at one end with the axon extending away from it.
3 · Synapses
A junction between two neurones (or between a neurone and an effector). There is a tiny gap called the synaptic cleft. Impulses cross the synapse chemically, not electrically.
How a Synapse Works
- An electrical impulse arrives at the pre-synaptic membrane.
- Synaptic vesicles fuse with the pre-synaptic membrane and release neurotransmitters (e.g. acetylcholine) into the synaptic cleft.
- Neurotransmitters diffuse across the cleft and bind to receptors on the post-synaptic membrane.
- This generates a new electrical impulse in the post-synaptic neurone.
- Neurotransmitters are broken down by enzymes (e.g. acetylcholinesterase) or reabsorbed to prevent continuous stimulation.
Synapses ensure impulses travel in ONE direction only — because neurotransmitter vesicles are only on the pre-synaptic side, and receptors only on the post-synaptic side. This is a very common "explain why" exam question.
4 · The Reflex Arc
A rapid, involuntary (automatic) response to a stimulus that does not involve conscious thought. It is coordinated by the spinal cord (not the brain), allowing a faster response.
The reflex arc pathway: Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector
The brain is informed of the reflex via sensory neurones going up the spinal cord, but this happens after the response has already occurred — which is why you pull your hand away from a hot object before you consciously feel pain.
1. Pain receptors in the skin (fingertip) detect the stimulus (heat/pain) and generate an impulse.
2. The impulse is carried along the sensory neurone to the spinal cord.
3. In the spinal cord, the relay neurone connects the sensory neurone to the motor neurone.
4. The impulse is carried along the motor neurone to the effector (muscle in the arm), which contracts, pulling the hand away.
Many students say the reflex arc goes through the brain. It goes through the spinal cord. The brain is only notified — it does not direct the response.
5 · The Brain
| Region | Functions |
|---|---|
| Cerebrum | Conscious thought, memory, language, voluntary movement, sensory perception, personality |
| Cerebellum | Balance and coordination of movement; fine motor control |
| Medulla oblongata | Controls autonomic (involuntary) functions: breathing rate, heart rate, peristalsis |
| Hypothalamus | Thermoregulation, water balance (osmoregulation), hunger, sleep, controls the pituitary gland |
| Pituitary gland | "Master gland" — releases hormones that control other endocrine glands |
6 · The Eye
Key Structures and Functions
| Structure | Function |
|---|---|
| Cornea | Transparent layer; refracts (bends) most of the incoming light |
| Iris | Controls the size of the pupil; regulates the amount of light entering the eye |
| Pupil | The opening in the iris; dilates in dim light, constricts in bright light |
| Lens | Fine-tunes focusing by changing shape (accommodation) |
| Ciliary muscles | Contract/relax to change lens shape for focusing |
| Suspensory ligaments | Attach lens to ciliary body; taut when ciliary muscles relax (viewing distant object) |
| Retina | Contains photoreceptors (rods and cones); converts light into electrical impulses |
| Fovea (yellow spot) | Highest concentration of cones; point of sharpest vision |
| Blind spot | Where the optic nerve exits; no photoreceptors, so no image formed here |
| Optic nerve | Carries electrical impulses from retina to the brain |
Accommodation (Focusing)
| Near object | Distant object | |
|---|---|---|
| Ciliary muscles | Contract | Relax |
| Suspensory ligaments | Slack/loose | Taut/tense |
| Lens shape | Fat (more curved) | Thin (less curved) |
| Refraction | More (shorter focal length) | Less (longer focal length) |
Students say "the lens gets bigger/smaller". The lens changes shape (fatter or thinner), not size. Also, for a near object the ciliary muscles contract — which is the opposite of what many students expect.
7 · The Ear
The ear has two roles: hearing and balance.
Hearing Pathway
Sound waves → ear canal → tympanic membrane (eardrum) vibrates → three ossicles (malleus, incus, stapes) amplify and transmit vibrations → oval window → fluid in the cochlea vibrates → hair cells in cochlea convert vibrations to electrical impulses → auditory nerve → brain.
Balance
The semicircular canals (three canals at right angles to each other) detect rotational movement. The utricle and saccule detect linear acceleration and gravity (static balance).
8 · Nervous vs Hormonal Coordination
| Feature | Nervous system | Hormonal system |
|---|---|---|
| Speed | Very fast (milliseconds) | Slower (seconds to minutes) |
| Signal type | Electrical impulse | Chemical (hormone in blood) |
| Duration | Short-lived | Long-lasting |
| Specificity | Specific (target organ via nerve) | Widespread (all cells with receptors) |
| Transmission | Along neurones | Via bloodstream |
🧠 Quick Practice Quiz
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