6-2-8-synaptic-transmission

Transmission across a cholinergic synapse

• Synapses that use the neurotransmitter acetylcholine (ACh) are known as cholinergic synapses

• Nerve signals are transmitted between cells at cholinergic synapses as follows:

  1. an action potential arrives at the presynaptic cell and causes depolarisation of the membrane

  2. voltage-gated calcium ion channels open

  3. calcium ions diffuse into the presynaptic neurone

  4. vesicles containing ACh move towards and fuse with the presynaptic membrane, releasing ACh into the synaptic cleft

  5. ACh molecules diffuse across the synaptic cleft and bind to receptor proteins in the postsynaptic membrane

  6. sodium ion channels associated with the receptor proteins open, allowing sodium ions to diffuse into the postsynaptic neurone

  7. the postsynaptic membrane is depolarised, and if a threshold is reached then a new action potential is generated in the postsynaptic neurone

  8. the enzyme acetylcholinesterase catalyses the hydrolysis of ACh in the synaptic cleft

  9. the products of ACh breakdown are absorbed by the presynaptic cell, which uses them to produce more ACh

Synapses and unidirectionality

• Synapses ensure the one-way transmission of impulses

• Impulses can only pass in one direction at synapses because, e.g.:

  • calcium ion channels are only present in the membrane of the presynaptic cell

  • vesicles containing neurotransmitter molecules are only present in the presynaptic cell

  • receptors for the neurotransmitter are only present in the membrane of the postsynaptic cell

Summation at synapses

• In order for a new action potential to be generated in a postsynaptic cell, the depolarisation of the membrane must reach threshold potential; this will only happen if:

  • enough acetylcholine is released into the synaptic cleft

  • enough sodium ion channels are opened in the postsynaptic membrane

  • enough sodium ions enter the postsynaptic cell

• A single impulse arriving at a presynaptic cell may not release enough neurotransmitter to generate an action potential in the postsynaptic neurone

  • This allows the nervous system to filter out low level stimuli

• When multiple impulses arrive at a synapse together, this is more likely to initiate an action potential; this is known as summation, and it:

  • allows signals from different parts of the nervous system to be combined

  • enables the nervous system to detect signals that might otherwise be too weak

• There are two types of summation:

  • Temporal summation

  • Spatial summation

Temporal summation

• In temporal summation there is rapid, repeated release of neurotransmitters from one neurone

  • Temporal = time

• The first few impulses may not result in the release of enough neurotransmitter, but if stimulation continues then the volume of neurotransmitter in the cleft will build up until threshold is reached

Spatial summation

• In spatial summation multiple impulses arrive at the same time from several presynaptic cells

  • Spatial = in space

• The neurotransmitter molecules from several different presynaptic cells is enough to reach threshold potential

Inhibitory synapses

• Synapses can either be:

  • excitatory: they result in the initiation of a new action potential in the postsynaptic cell by causing an influx of positive ions

  • inhibitory: they prevent a new action potential in the postsynaptic cell by causing hyperpolarisation

• Inhibitory synapses function by lowering membrane potential, e.g. by causing an:

  • outflow of positive ions

    • Opening potassium ion (K⁺) channels in the membrane allows potassium ions to diffuse out of the cell

  • inflow of negative ions

    • Opening chloride ion (Cl^–) channels allows an influx of chloride ions

• A neurone may have input from both excitatory and inhibitory synapses, allowing complex information processing