Neurones
- A neurones has a long fibre known as an axon
- The axon is insulated by a fatty sheath with small, uninsulated sections along its length (called nodes of Ranvier)
- The sheath is made of myelin, a substance that is made by specialised cells known as Schwann cells
- Myelin is made when Schwann cells wrap themselves around the axon along its length
- This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next
- This means that less time is wasted transferring the impulse from one cell to another
- Their cell bodies contain many extensions called dendrites
- This means they can connect to many other neurones and receive impulses from them, forming a network for easy communication
A Myelinated Neurone Diagram
An example of a neurone
- There are three main types of neurone: sensory, relay and motor
- Sensory neurones carry impulses from receptors to the CNS (brain or spinal cord)
- Intermediate (aka relay) neurones are found entirely within the CNS and connect sensory and motor neurones
- Motor neurones carry impulses from the CNS to effectors (muscles or glands)
The Three Types of Neurone Diagram
The three types of neurone – the red line shows the direction of impulses. Note that axons always carry nerve impulses away from the cell body
- Each type of neurone has a slightly different structure
- Motor neurones have:
- A large cell body at one end, that lies within the spinal cord or brain
- A nucleus that is always in its cell body
- Many highly-branched dendrites extend from the cell body, providing a large surface area for the axon terminals of other neurones
- Sensory neurones have the same basic structure as motor neurones, but have:
- A cell body that branches off in the middle of the cell - it may be near the source of stimuli or in a swelling of a spinal nerve known as a ganglion
Reflex arc
- Sensory neurones, intermediate (relay) neurones and motor neurones work together to bring about a response to a stimulus
- A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain
- As it does not involve the brain, a reflex response is quicker than any other type of nervous response
- Examples of simple reflex actions that are coordinated by these pathways are:
- Removing the hand rapidly from a sharp or hot object
- Blinking
- Focusing the eye on an object
- Controlling how much light enters the eye
The Reflex Action Diagram
How sensory neurones, intermediate (relay) neurones and motor neurones work together to carry out a reflex action
In the example above:
- A pin (the stimulus) is detected by a pain receptor in the skin
- The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
- Electrical impulses are passed on to an intermediate neurone in the spinal cord
- The intermediate neurone connects to the motor neurone and passes the impulses on
- The motor neurone carries the impulses to the muscle in the leg (the effector)
- The impulses cause the muscle (the quadriceps in this case) to contract and pull the leg up and away from the sharp object (the response)
The Pathway of a Reflex Arc Diagram
The pathway of a reflex arc
Examiner Tip
You may be asked to identify the different types of neurone in a diagram. It can be helpful to memorise the key differences between them – such as the location and size of the cell body.