The Process of Synaptic Transmission (AQA A Level Psychology)

The process of synaptic transmission

• The junction where two neurons meet is known as a synapse

• A synapse consists of:

  • the presynaptic knob:

    • A swelling at the end of the presynaptic neuron; it contains vesicles which hold neurotransmitters

  • the synaptic cleft:

    • The space in between the axon of one neuron and the dendrites of another; where the electrical signal is turned into a chemical messenger that can be detected by the next neuron

  • the postsynaptic membrane

    • The membrane receives a signal (and binds the neurotransmitter) from the presynaptic neuron

What happens at a synapse?

• Synaptic transmission involves a series of events

  1. A signal begins as an electrical impulse (an action potential) within the presynaptic neuron

  2. When the electrical impulse arrives at the end of the axon on the pre-synaptic knob, chemical messengers called neurotransmitters are released from structures called vesicles at the presynaptic membrane

  3. Neurotransmitters are released into the synaptic cleft as a chemical substance from the presynaptic neuron after the action potential has occurred

  4. Neurotransmitters diffuse across the synaptic cleft and temporarily bind with receptor sites on the postsynaptic membrane

  5. This action stimulates the postsynaptic neuron to generate another electrical impulse (action potential) that then travels down the axon of the postsynaptic neuron

  6. 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

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

Neurotransmitters

• Neurotransmitters are many and varied, each having a specific function with accompanying receptor sites ready to 'catch it' e.g.

  • Serotonin receptor 4 (5-HT₄R) plays an important role in regulating mood, particularly depression and anxiety

  • Dopamine receptors play an essential role in movement and the experience and anticipation of reward

    • dopamine receptors are located in the hippocampal dentate gyrus

• Neurotransmitters are either excitatory or inhibitory

Excitatory neurotransmitters

• Excitatory neurotransmitters are those which stimulate an action potential in the postsynaptic neuron

• Excitatory neurotransmitters increase the neuron’s positive charge

  • Increasing the neuron's positive charge makes it more likely to ‘fire’

• Adrenaline is an excitatory neurotransmitter

Inhibitory neurotransmitters

• Inhibitory neurotransmitters are those which inhibit an action potential in the postsynaptic neuron

• Inhibitory neurotransmitters increase the neuron’s negative charge

  • Increasing the neuron's negative charge makes it less likely to ‘fire’

• Serotonin is an inhibitory neurotransmitter