Revision - GCSE biology - the nervous system and the eye

The nervous system

The nervous system is made up of 5 different sense organs

  • The eyes
  • The ears
  • The nose
  • The tongue
  • The skin

Each of these organs has a stimulus which they are sensitive to

Organ Receptor Stimuli
Eyes Light receptors (Rods and cones) Light and colour
Ears Sound and balance receptors Sound and balance
Nose Taste and smell receptors Chemical stimuli
Skin Touch, pressure and heat receptors Pressure and heat
Tongue Taste receptors Bitter, salt, sweet and sour (chemical stimuli)

The entire CNS is covered by some sort of bone (the brain by the cranium and the spinal chord by the spine)


Turn energy (e.g. light energy) into impulse.

The central nervous system (CNS) is made up of the spinal chord and the brain. In a reflex action sensory neurones carry information from a receptor to the CNS, where it is transferred over the synapse from the end of the synaptic knob (the end of the sensory neurone) to the dendrites of the relay neurone. The relay neurone then carries the impulse along and transfers it from its synaptic knobs to the dendrites of the motor neurone, where it travels to the muscles.

The dendrites here are attached to receptors   The dendrites here are attached to muscle (i.e. and effector)


In all of these the axon is the nerve fibre which transmit the impulse. The myelin sheath acts as an insulator and stops the impulse breaking out.


The synapse

The synapse is a gap where the electrical signal is transformed into chemical signals which diffuse over the gap. Synapses connect two neurones together. Once the chemical has been transferred across the gap the chemicals set off a new impulse along the second neurone. The synapses lie in the grey matter of the spinal chord, and are used in reflex actions. These chemical messages are slowed down by some drugs, and sped up by other (stimulants speed it up, while depressants such as alcohol slow it down)


The reflex arc

This allows very quick responses because reflexes are automatic, and are used to protect the body from injury.

Remember that a stimulus causes a receptor to send an impulse, and an impulse causes effectors to give a response.

Within our body there are voluntary and autonomic nervous systems.

The eye

  • The pupil lets light through into the retina. The back of the eye is filled with the vitreous humour. This keeps the eyes shape. The retina is covered in rods and cones – rods detect light while cones detect colour (cones are less effective in dim light, while rods are good in dim light).
  • The fovea or yellow spot is a concentration of rods and cones and is where the light ideally focuses.
  • The choroid absorbs the light and stops it being reflected back, and also has a copious blood supply which provides the retina with oxygen and food.
  • Rods contain a substances called visual purple which is broken down by tiny amounts of light. It then reforms while the information is sent to the brain. This is why you can see dimly lit objects out of the corner of your eye, you are using the edge were there are more rods. Seeing in the dark is done using visual purple, and this is why after coming out of bright light into a dark room you can’t see much, because the visual purple needs a couple of minutes to reform after being completely broken down by the light.
  • In bright light the circular muscles contract to close the iris up and make the pupil smaller, letting less light into the eye. (NB the radial muscle relaxes also)
  • In dim light the radial muscles contract pulling the iris open so the pupils get bigger and more light gets in. (radial = dim). (NB the circular muscle relaxes also).
  • To focus on distant objects the ciliary muscle relaxes which means the suspensory ligaments pull tight making the lens thin. This means it refracts less light so less light is bent.
  • To focus on near objects the ciliary muscles contract, which means the suspensory ligaments are slackened, which means the lens becomes fat meaning it refracts more light. This movement of the eye muscles is called accommodation.
  • A short sighted person needs concave spectacles because they cannot focus on things far away, so by wearing a concave lens the light is refracted slightly out before being refracted back in to the correct place. A long sighted person is the opposite, they can only see things far away, but not close, so they wear convex glasses. The condition astigmatism is when the cornea and/or lens are un-evenly surfaced, so the light reflects into one place and then another
  • The tear gland gives out a fluid which acts like a windscreen cleaning fluid. It contains the enzyme lysozyme which kills germs. The eyelids and eyelashes stop sweat and dust running into the eye

Also See

Here are the other comprehensive GCSE Biology notes by Prometheus: