B1.2.1 The Nervous System

Humans react to their surroundings using their nervous system.

1. Stimulus - change in surroundings ->

2. Receptors- cells that detect stimuli

2. Nervous system - detects stimuli and coordinates response ->

3. Response - change in organism's behaviour as a result of stimulus

Eye - light
Ear - change in position (balance), sound
Nose -  chemicals in air (smell)
Tongue - chemicals in solids and liquids (taste)
Skin (different cells for each stimulus) - touch, pressure, pain, temperature

Light receptor cells have a cytoplasm, cell membrane, and nucleus.

The human CNS/Central Nervous System consists of the spinal cord and brain.

When a receptor is stimulated, it sends a signal along the neurones (nerve cells) to the brain, which then coordinates a response.

Neurones - nerve cells that carry information as tiny electrical signals. There are three different types:
1. Sensory - signals from receptors to the CNS
2. Relay - messages from one part of the CNS to another
3. Motor - signals from the CNS to effectors

Effectors - Muscles that respond by contracting
               - Glands that respond by secreting chemicals e.g. hormones

Synapse - TIny gap where two neurones meet. Signals cross synapses using chemicals

1. A neurone releases the chemical into the gap
2. The chemical diffuses across the gap
3. Stimulates the next neurone to transmit an electrical signal

Reflex action - involuntary (automatic) action with survival value. Bypasses the brain and goes through the CNS.

1. Receptor cell responds to a stimulus 2. Receptor along a sensory neurone -> CNS 3. (At a synapse) sensory neurone -> relay neurone
4. Relay neurone -> motor neurone
5. Motor neurone -> effector