# GCSE Science/The Nervous System
## The nervous system

The nervous system consists of the brain, spinal cord and relay neurones (Central nervous system) and peripheral nervous system (sensory neurones and motor neurones). The nervous system lets the organism react to the environment and surroundings and coordinate their behaviour.

## The senses

Sense organs contain receptors that are sensitive to stimuli. Typical stimuli are due to changes in chemical composition, mechanical effects, heat, sound and light. Stimuli are the changes that are detected. Receptors detect the change.
There are five sense organs that are studied at this stage, the nose, tongue, eyes, ears and skin.
The nose has smell receptors that are sensitive to a wide range of chemical stimuli. Much of what is called 'taste' is actually smell; the range of flavours in food is very limited if the nose is blocked.
The tongue has taste receptors that are sensitive to chemical stimuli. There are four basic tastes: bitter, salt, sweet and sour.
The eyes have receptors that are sensitive to light, the ears have receptors that are sensitive to sound and the skin has receptors that are sensitive to temperature, touch, pressure and stretch.

### The eye

Light from the surroundings enters the eye through the pupil. It is focussed by the cornea and lens so that it forms an image on the retina. The cornea performs crude focussing and the lens performs active, fine focussing. The eye is filled with transparent liquid (the humours).
The iris controls the size of the pupil and so controls how much light gets into the eye. In bright light the circular muscles in the iris contract, this makes the pupil smaller and allows less light into the eye. In dim light the radial muscles contract, this makes the pupil larger and allows more light into the eye.
The lens changes shape to allow the eye to focus on things at various distances. The shape of the lens is changed by the action of the ciliary muscles. The ciliary muscles form a circle that is attached to the lens by the suspensory ligaments. To focus on distant objects the ciliary muscles relax which pulls the suspensory ligaments tight and makes the lens thinner. To focus on nearby objects the ciliary muscles contract which allows the suspensory ligaments to relax so that the lens forms a more spherical shape.
The retina contains cells that are sensitive to light. These cells are called rods and cones.
The rods are more sensitive than the cones but do not provide any colour information. In dim light our view of the world is largely provided by rods and appears as a black and white image.
There are three types of cone and these are each sensitive to different colours of light. The cones are especially densely packed in the part of the retina called the fovea; this dense packing means that the fovea is most sensitive to fine detail in the image.
The part of the eye where the optic nerve enters is called the blind spot. The blind spot does not contain any light receptors (it has no rods or cones).

### Hearing

Sound is nothing more than vibrations in the air around us. The ears are the sensory organs that can distinguish both the pitch and loudness of those vibrations. Vibrations enter the ear through the auditory canal. The vibrations cause the tympanum, or eardrum, to vibrate. These vibrations are picked up by three tiny bones, commonly called the hammer, anvil, and stirrup. The last of these bones, the stirrup, transmits the vibrations to the oval window. Vibrations of the oval window create pressure waves in the fluid filled cochlea of the inner ear. The cochlea is lined with tiny hair cells that are pushed back and forth by these pressure waves. In response to these movements, the hair cells produce nerve impulses that are sent to the brain through the cochlear nerve.

### Balance

Your ears contain structures that help your central nervous system maintain your balance. Withn the inner ear just above the cochlea are three tiny canals at right angles to each other. They are called semicircular canals because each forms a half circle. The semicircular canals and the two tiny sacs located behind them monitor the position of your body. The semicircular canals and the sacs are filled with fluid and lined with hair cells. At the head changes position, the fluid in the canals also changes position. This causes the hair on the hair cells to bend. This action, in turn, sends impulses to the brain that enable it to determine body motion and position.

## Neurones

Neurones are cells that are specialised to transmit electrical impulses around the body. They consist of three principal components: the dendrites, the cell body and the axon. Each neurone has its own nucleus. (The US spelling of neurone is "neuron").
There are three types of neurone: sensory neurones, relay neurones and motor neurones.
Neurones connect to other neurones by means of synapses. Nerve impulses travel down the axon to one side of the synapse where the electrical signal causes chemicals to be released. These chemicals diffuse across the gap and generate an electrical impulse in the neurone on the other side of the synapse.

## The Central Nervous System

The central nervous system is composed of the brain and spinal cord. It contains huge numbers of neurones that process data and provide output signals to effectors in the body.
The effectors are muscles and glands that respond to signals from the central nervous system. Muscles respond with movements and glands respond by secreting substances such as fluids and chemicals. Effectors produce a response to change.

### Reflex arcs

The nervous system has many reflexes. These are automatic responses to stimuli. Very quick responses are managed by Reflex arcs. A reflex arc consists of the following components:
* Stimulus
* Receptor
* Sensory neurone
* Relay neurone
* Motor neurone
* Effector
* Response
A typical reflex arc called the "pain reflex" is illustrated.
In the pain reflex the stimulus is mechanical damage, the receptor is a "pain receptor", the pain receptor generates an electrical impulse in a sensory neurone which creates an electrical impulse in a relay neurone, which in turn creates an electrical impulse in a motor neurone. The impulse in the motor neurone stimulates a muscle, which is an effector, and this creates movement away from the painful stimulus as a response.
Another reflex is the "patellar" or "knee jerk reflex" where a blow beneath the knee-cap makes the leg straighten. The contraction of the pupil in response to bright light is yet another reflex.