Synaptic Transmission (AQA GCSE Psychology)

The process of synaptic transmission

• The junction at which two neurons meet is known as a synapse

• A synapse consists of a:

  • presynaptic knob

  • synaptic cleft 

  • postsynaptic membrane

• A signal begins as an electrical impulse within the presynaptic neuron which changes to a chemical messenger went sent across a synapse to another (postsynaptic) neuron

• When an electrical impulse arrives at the end of the axon on the presynaptic neuron (also called the pre-synaptic knob), chemical messengers called neurotransmitters are released from structures called vesicles at the presynaptic membrane

A synapse between a pre-synaptic neuron a post-synaptic neuron

Release and reuptake of neurotransmitters

• Neurotransmitters are released as a chemical substance from the presynaptic neuron after the action potential (electrical impulse) has occurred

• The synapse is the site of transmission between the two neurons (presynaptic across to postsynaptic)

• Neurotransmitters diffuse across the synaptic cleft and temporarily bind with receptor sites on the postsynaptic membrane (a bit like a baseball glove catching a ball)

• This action stimulates the postsynaptic neuron to generate another electrical impulse that then travels down the axon of the postsynaptic neuron

• The neurotransmitter molecules are then destroyed by enzymes or recycled to prevent continued stimulation of the second neuron, which could cause repeated impulses to be sent

• The re-absorbed neurotransmitter may be used again by the presynaptic neuron

Excitation & inhibition

Excitatory neurons

• Excitatory neurotransmitters are those which stimulate an action potential (electrical impulse) in the postsynaptic neuron

• Excitatory neurotransmitters increase the neuron’s positive charge i.e. making it more likely to ‘fire’

• Adrenaline is an excitatory neurotransmitter

Inhibitory neurons

• Inhibitory neurotransmitters are those which inhibit an action potential (electrical impulse) in the postsynaptic neuron

• Inhibitory neurotransmitters increase the neuron’s negative charge i.e. making it less likely to ‘fire’

• Serotonin is an excitatory neurotransmitter

Summary:

1. An action potential (electrical impulse) starts the process of neurotransmission

2. This electrical impulse travels down the axon

3. The electrical impulse turns into a chemical messenger (the neurotransmitter)

4. The neurotransmitter is released from the presynaptic neuron across the synaptic cleft

5. Receptor sites on the postsynaptic neuron receive the neurotransmitter

6. Any remaining molecules of the neurotransmitter are taken back up into the presynaptic neuron (reuptake) or are broken down by enzymes