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Sensory neuron
These carry messages from the peripheral nervous system to the CNS. They have long dendrites and short axons
Relay neuron
These connect the sensory neurons to the motor or other neurons of these. They have short dendrites and short axons
Motor neuron
These connect the central nervous system to effectors such as muscles and glands. They have short dendrites and long axons
The structure of a neuron- Cell body
Neurons vary in size from less than a millimetre to up to a metre long, but all share the same basic structure.
Includes a nucleus, which contains the genetic material of the cell
The structure of a neuron- Dendrites
These are branch-like structures that protrude from the cell body. These carry nerve impulses from neighbouring neurons towards the cell body
The structure of a neuron- Axon
These carry the impulses away from a cell body down the length of the neuron. It is covered in a fatty layer of myelin sheath that protects it and speeds up electrical transmission. If the myelin sheath was continuous this would have the reverse effect and slow down the electrical impulse. Thus, the myelin sheath is segmented by gaps called nodes of Ranvier. These speed up the transmission of the impulse by forcing to 'jump' across the gaps along this.
The structure of a neuron- Terminal buttons
These are at the end of the axons. They communicate with the next neuron in the chain across a gap known as the synapse.
Location of neurons
The cell bodies of motor neurons may be in the central nervous system but they have long axons which form part of the peripheral nervous system.
Sensory neurons are located outside of the CNS, in the PNS in clusters as ganglia.
Relay neurons make up 97% of all neurons and most are found within the brain and the visual system.
Electrical transmission- the firing of a neuron
When a neuron is in a resting state the inside of the cell is negatively charged compared to the outside. When a neuron is activated by a stimulus, the inside of the cell becomes positively charged for a split second causing an action potential to occur. This creates an electrical impulse that travels down the axon towards the end of the neuron.
Synaptic transmission
- The process by which neighbouring neurons communicate with each other by sending chemical message across the gap (the synapse) that separates them.
Chemical transmission
Neurons communicate with each other within groups known as neural networks. Each neuron is separated from the next as tiny gaps called the synapse.
Signals within neurons are transmitted electrically. However, signals between neurons are transmitted chemically across the synapse.
When the electrical impulse reaches the end of the neuron (the presynaptic terminal) it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles.
Neurotransmitters
- Brain chemicals released from synaptic vesicles that relay signals across the synapse from one neuron to another. They can be broadly divided into those that perform an excitatory function and those that perform an inhibitory function.
Excitation and inhibition
Neurotransmitters have either these effects on the neighbouring neuron.
For instance, the neurotransmitter serotonin causes inhibition in the receiving neuron, resulting in the neuron becoming more negatively charged and less likely to fire.
In contrast, adrenaline (an element of the stress response which is both a hormone and a neurotransmitter) causes excitation of the postsynaptic neuron by increasing its positive charge and making it more likely to fire.
Summation
Whether a postsynaptic neuronal fires is decided by the process of summation.
The excitatory and inhibitory influences are summed: if the net effect is inhibitory then the postsynaptic neuron is less likely to fire. If the net effect is excitatory it is more likely to fire.
The action potential of the postsynaptic neuron is only triggered if the sum of the excitatory and inhibitory signals at any one time reaches the threshold.
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