The human nervous systemOxford AQA IGCSE Biology: Revision notes
Section 1
How is the nervous system organised?
The nervous system is divided into two main parts:
| Component | Structure | Function |
|---|---|---|
| Central Nervous System (CNS) | Brain and spinal cord | Processes information and coordinates responses |
| Peripheral Nervous System (PNS) | Sensory and motor nerves | Connects CNS to the rest of the body |
The brain is the control centre and processes sensory information. The spinal cord acts as a highway for nerve impulses between the brain and the rest of the body. Peripheral nerves branch out from the CNS and carry signals to and from organs, muscles, and receptors.
The brain contains four key regions:
- Cerebral cortex: controls voluntary actions, memory, language, and consciousness
- Cerebellum: coordinates muscle movement and maintains balance and posture
- Medulla: controls involuntary (automatic) functions like heart rate, breathing, and blood pressure
- Hypothalamus: maintains homeostasis by regulating body temperature, hunger, and hormone release
Examiners expect you to know the specific functions of each brain region. When asked about the medulla, always mention automatic functions; for the cerebellum, always link to coordination and balance.
Section 2
What are neurones and how do they transmit signals?
Neurones are specialised nerve cells that transmit electrical impulses. There are three types:
| Type | Structure | Function |
|---|---|---|
| Sensory neurone | Long dendrite, short axon | Carries impulses from receptors to CNS |
| Relay neurone | Found in CNS | Connects sensory to motor neurones; processes information |
| Motor neurone | Short dendrite, long axon | Carries impulses from CNS to effectors (muscles/glands) |
All neurones have a cell body containing the nucleus, and an axon that transmits impulses. Impulses travel as electrical signals along the axon.
At a synapse, the gap between two neurones, transmission becomes chemical:
- The impulse arrives at the synaptic knob (axon terminal)
- Neurotransmitter molecules are released into the synaptic cleft
- Neurotransmitters bind to receptors on the next neurone's membrane
- This triggers a new electrical impulse in the next neurone
- Neurotransmitters are then broken down or reabsorbed
This chemical transmission ensures impulses can only travel in one direction (unidirectional transmission).
Think of a synapse like a post office: the sending neurone is the sender, neurotransmitters are the letters, the synaptic cleft is the gap they cross, and the receiving neurone is the recipient.
Students often confuse the direction of transmission—sensory neurones always carry impulses TO the CNS (from receptors), while motor neurones always carry impulses FROM the CNS (to effectors).
Section 3
What is a reflex arc and why is it important?
A reflex arc is the pathway taken by a nerve impulse during a reflex action. It bypasses the brain, allowing a rapid automatic response to a stimulus.
The reflex arc pathway:
- Stimulus is detected by a receptor (e.g. sensory cell in skin)
- Sensory neurone carries impulse to spinal cord
- Relay neurone in spinal cord processes the signal
- Motor neurone carries impulse from spinal cord to effector
- Effector (muscle or gland) produces a response (e.g. muscle contraction)
Importance of reflex actions:
- Speed: Responses occur without waiting for brain processing; critical in emergencies (e.g. withdrawing from hot objects)
- Survival: Automatic responses protect the body from danger
- Efficiency: Frees the brain to focus on other tasks while essential responses happen automatically
- Involuntary: Occur without conscious control or decision-making
Common examples include the withdrawal reflex (hand away from flame) and the pupil reflex (iris adjusting to light).
Examiners mark heavily on the correct sequence of the reflex arc. Always remember: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. Write the pathway in this order in your exam.
Hand touches hot plate: heat is the stimulus, touch receptors in skin detect it, sensory neurone sends impulse to spinal cord, relay neurone processes it instantly, motor neurone activates arm muscles, hand withdraws (response)—all in milliseconds, before the brain even becomes aware.
Section 4
How does the eye work as a sense organ?
The eye is a sense organ that converts light into electrical impulses interpreted as vision.
Key structures and their functions:
| Structure | Function |
|---|---|
| Cornea | Clear outer layer that refracts (bends) light rays |
| Iris | Coloured muscular diaphragm that controls pupil size to regulate light entry |
| Lens | Refracts light further and focuses it onto the retina; changes shape for accommodation |
| Retina | Light-sensitive tissue at the back of the eye; contains rod and cone cells |
| Rod cells | Photoreceptors sensitive to low light; detect black and white only |
| Cone cells | Photoreceptors sensitive to bright light; detect colour |
| Optic nerve | Carries visual impulses from the retina to the brain |
| Ciliary muscles | Contract or relax to change lens shape during accommodation |
| Suspensory ligaments | Attach lens to ciliary muscles; control lens tension |
Accommodation is the eye's ability to focus on objects at different distances:
- Viewing near objects: Ciliary muscles contract, suspensory ligaments relax, lens becomes fatter, increased refraction focuses the image on the retina
- Viewing distant objects: Ciliary muscles relax, suspensory ligaments tighten, lens becomes thinner, decreased refraction focuses the image on the retina
When explaining accommodation, examiners expect you to name both the ciliary muscles AND suspensory ligaments, and describe the thickness change of the lens. Saying 'the lens changes shape' alone is insufficient.
The iris and pupil work like a camera aperture: in bright light, the iris contracts to make the pupil smaller (reducing light entry); in dim light, the iris relaxes to make the pupil larger (allowing more light in).
Section 5
What are long-sightedness and short-sightedness?
Defects of vision occur when light is not focused precisely on the retina.
| Condition | Cause | Effect | Correction |
|---|---|---|---|
| Short-sightedness (myopia) | Lens too curved OR eyeball too long | Cannot focus on distant objects clearly; nearby objects appear clear | Concave (minus) lens spreads rays outward before entering the eye, moving focal point back onto the retina |
| Long-sightedness (hyperopia) | Lens too flat OR eyeball too short | Cannot focus on nearby objects clearly; distant objects appear clear | Convex (plus) lens converges rays inward before entering the eye, moving focal point forward onto the retina |
How corrective lenses work:
- Concave lenses (used for short-sightedness) are thinner in the centre and thicker at the edges; they diverge (spread out) light rays, effectively moving the image back onto the retina
- Convex lenses (used for long-sightedness) are thicker in the centre and thinner at the edges; they converge (bring together) light rays, effectively moving the image forward onto the retina
Both types of lens ensure light focuses exactly on the retina, allowing a clear image to be formed and transmitted to the brain via the optic nerve.
Students often reverse which lens corrects which condition. Remember: short-sightedness (myopia) uses a concave (minus/diverging) lens; long-sightedness (hyperopia) uses a convex (plus/converging) lens.
A short-sighted person cannot read a distant blackboard but can read a book held close. Light from the board focuses in front of their retina. A concave lens spreads these rays outward, moving the focal point back onto the retina, making the board legible.
Must Know
- The nervous system has two parts: the CNS (brain and spinal cord) and the PNS (peripheral nerves); the brain contains the cerebral cortex (voluntary control), cerebellum (coordination), medulla (automatic functions), and hypothalamus (homeostasis)
- Three types of neurone transmit impulses: sensory (receptors to CNS), relay (in CNS), and motor (CNS to effectors); impulses cross synapses via neurotransmitters released from the synaptic knob into the synaptic cleft
- Reflex arc pathway: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response; reflex actions are fast, automatic, and bypass the brain for survival
- The eye focuses light using the cornea and lens onto the retina, which contains rod cells (low light, black/white) and cone cells (bright light, colour); the optic nerve transmits the image to the brain
- Accommodation occurs when ciliary muscles contract (viewing near objects, lens thickens) or relax (viewing distant objects, lens thins), controlled by suspensory ligaments
- Short-sightedness (light focuses in front of retina) is corrected with a concave lens; long-sightedness (light focuses behind retina) is corrected with a convex lens
That's the notes covered.
Carry on to the next subtopic.