Synapses
Synaptic Cleft: Intercellular space between the axon terminal and the dendrite. Approximately 20-30nm across.
Presynaptic Neurone: Neurone along which the action potential has travelled.
Postsynaptic Neurone: Neurone which receives the impulse.
Synaptic Knob: Swollen end of the axon, containing many mitochondria, and smooth endoplasmic reticulum to synthesise neurotransmitters.
Synaptic Vesicles: Vesicles containing neurotransmitters, which fuse to the membrane releasing neurotransmitters into the cleft.
Neurotransmitter Receptors: Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane.
Types of Neurotransmitter
Presynaptic Neurone: Neurone along which the action potential has travelled.
Postsynaptic Neurone: Neurone which receives the impulse.
Synaptic Knob: Swollen end of the axon, containing many mitochondria, and smooth endoplasmic reticulum to synthesise neurotransmitters.
Synaptic Vesicles: Vesicles containing neurotransmitters, which fuse to the membrane releasing neurotransmitters into the cleft.
Neurotransmitter Receptors: Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane.
Types of Neurotransmitter
- Excitatory neurotransmitters cause the postsynaptic neurone to depolarise, triggering an action potential. An example of an excitatory neurotransmitter is acetylcholine.
- Inhibitory neurotransmitters result in hyperpolarization of the postsynaptic neurone, meaning an action potential is unable to be triggered. An example of an inhibitory neurotransmitter is GABA.
Cholinergic Synapses
- Use acetylcholine neurotransmitter.
- Used at relay-effector junctions in the CNS, causing muscles to contract.
- Some of the most common synapses in the human body.
- Ensure impulses are unidirectional, so cannot be accidentally reversed.
- Allow one neurone to transmit an impulse to multiple other neurones at the synapse, creating multiple simultaneous responses.
- Multiple neurones can feed into one synapse causing multiple stimuli to elicit one unified response.
- Each stimulation causes the same amount of neurotransmitter to be released into the synapse.
- In some synapses, a single impulse is not enough to trigger a response, so multiple impulses are required to breach the threshold before the impulse is passed on.
- This is summation.
- This is summation.
- Spatial Summation: multiple presynaptic neurones connect to a single postsynaptic neurone, and when both presynaptic neurones release neurotransmitters the threshold is breached an impulse is passed on.
- Temporal Summation: A single presynaptic neurone releases lots of neurotransmitters into the synapse as a result of multiple stimulations over a short period of time, building up concentration until threshold is breached.
Effects of Drugs
- Many recreational and medical drugs work by inhibiting or stimulating synapses.
- Drugs which stimulate the nervous system create enhanced response
- They can mimic the shape of the neurotransmitter, triggering action potentials.
- They can stimulate the release of more neurotransmitters.
- They can inhibit enzymes which break down neurotransmitters causing prolonged stimulation.
- Drugs which inhibit the nervous systems result in a reduced response.
- They can block receptors to stop any neurotransmitters triggering action potential.
- They can bind to receptors and change their shape.