1. The neuron and the divisions of the nervous system
Neuron (nerve cell): the structural and functional unit of the nervous system.
- Dendrites: short branches that receive impulses and carry them towards the cell body.
- Cell body (cyton): contains the nucleus and granules; controls the cell.
- Axon: a single long fibre carrying impulses away from the cell body; covered by a fatty myelin sheath (insulates, speeds conduction) broken at nodes of Ranvier; ends in branched axon endings.
- Synapse: the junction between the axon ending of one neuron and the dendrite of the next; a chemical released by the axon ending carries the impulse across, so impulses pass in one direction only.
Kinds of neuron: sensory (receptor → CNS), motor (CNS → effector), relay (within the CNS).
| Division | Parts | Role |
|---|---|---|
| Central (CNS) | Brain and spinal cord | Receives, interprets, decides, commands |
| Peripheral (PNS) | 12 pairs of cranial nerves, 31 pairs of spinal nerves | Connects the CNS with receptors and effectors |
| Autonomic (ANS) | Nerves to internal organs | Controls involuntary activities: heartbeat, gut, glands, pupil |
Key terms
- Neuron
Nerve cell; unit of the nervous system.
- Myelin sheath
Fatty covering of an axon that insulates it and speeds the impulse.
- Synapse
Junction between two neurons where a chemical carries the impulse across.
- Autonomic nervous system
Part controlling involuntary organs.
2. The brain
The brain lies in the skull (cranium). Its main parts:
| Part | Position | Functions | Effect of damage |
|---|---|---|---|
| Cerebrum | Largest; two folded hemispheres joined by the corpus callosum | Thinking, memory, intelligence, speech, voluntary actions, interpreting sensations | Loss of memory, speech or voluntary movement |
| Cerebellum | Back, below the cerebrum | Balance, posture, coordination of muscle movements | Staggering, clumsy movements |
| Thalamus | Deep in the centre | Relays sensory impulses (except smell) to the cerebrum | Disturbed sensations |
| Hypothalamus | Below the thalamus | Body temperature, hunger, thirst, sleep; controls the pituitary | Temperature or appetite out of control |
| Pons | Bulge on the brain stem | Connects parts of the brain; helps regulate breathing | Disturbed breathing |
| Medulla oblongata | Lowest part, continues as the spinal cord | Heartbeat, breathing, blood pressure, swallowing, coughing, sneezing, vomiting | Often fatal |
Key terms
- Cerebrum
Largest part of the brain; thinking and voluntary actions.
- Cerebellum
Part of the brain that maintains balance.
- Hypothalamus
Part regulating temperature, hunger and thirst.
- Medulla oblongata
Lowest part of the brain; controls heartbeat and breathing.
3. The spinal cord; grey and white matter
The spinal cord runs from the medulla oblongata down through the vertebral column, which protects it. It has a central canal.
- Grey matter = cell bodies of neurons; white matter = nerve fibres with white myelin.
- In the cerebrum: grey matter outside (cortex), white matter inside.
- In the spinal cord: white matter outside, grey matter inside (butterfly-shaped).
Functions of the spinal cord: the white matter conducts impulses to and from the brain; the grey matter is the centre for spinal reflexes. Spinal nerves leave it through a dorsal (sensory) root with a ganglion and a ventral (motor) root.
Key terms
- Grey matter
Nervous tissue made of cell bodies.
- White matter
Nervous tissue made of myelinated fibres.
4. Voluntary, involuntary and reflex actions; the reflex arc
| Action | Features | Examples |
|---|---|---|
| Voluntary | At will, after thinking; controlled by the cerebrum; slower | Writing, walking, speaking |
| Involuntary | Not under the will; controlled by the medulla and autonomic system | Heartbeat, breathing, peristalsis |
| Reflex | Sudden, automatic response to a stimulus, without thinking | Knee-jerk, sneezing, coughing, blinking |
Reflex arc — the pathway of a reflex: receptor → sensory neuron → relay neuron (grey matter of the spinal cord) → motor neuron → effector (muscle or gland).
- Spinal reflexes (knee-jerk, withdrawal from a hot object) pass through the spinal cord; reflexes of the brain (blinking, narrowing of the pupil) through the brain.
- Pain is felt after the withdrawal, because the impulse takes longer to reach the brain.
- Reflexes protect the body by their speed and leave the brain free for other work.
Key terms
- Reflex action
Quick, automatic response to a stimulus without thinking.
- Reflex arc
Pathway of impulses in a reflex action.
- Effector
Muscle or gland that responds.
5. Natural and acquired reflexes
| Natural (unconditioned) | Acquired (conditioned) |
|---|---|
| Inborn, inherited | Learnt after birth |
| No practice needed | Developed by repeated experience |
| Same in all members of a species | Differ from person to person; may be lost |
| Sneezing, blinking, coughing, a baby sucking, salivation when food is in the mouth | Braking at a red light, typing, cycling, salivation at the sight or smell of a favourite food |
Pavlov's experiment: a bell was rung each time a dog was fed. After many repeats the bell alone made the dog salivate — a conditioned reflex. Acquired reflexes need the cerebrum (learning and memory).
| Reflex | Natural or acquired |
|---|---|
| Blinking | Natural |
| Typing without looking | Acquired |
Key terms
- Natural reflex
An inborn reflex needing no learning.
- Acquired reflex
A reflex learnt by repeated experience.
Exam tips
Common mistakes
- Reversing the direction of the impulse: dendrites carry it in, the axon carries it out.
- Writing that the cerebrum has grey matter inside: it is the spinal cord that has grey matter inside.
- Calling the knee-jerk a voluntary action, or a learnt skill a natural reflex.
- Leaving out the relay neuron from a reflex arc.
How marks are usually awarded
- 1 mark: a term (synapse, reflex arc, cyton), a brain part for a function, an odd one out with its category.
- 2 marks: two differences, an effect of damage with its reason.
- 3 marks: a labelled neuron or reflex arc; functions of three brain parts.