Anatomy and Physiology / Unit 6: The Nervous System and Senses
6.3 Synapses and Neurotransmitters
Notes
A synapse is the junction between a neuron and another cell. Most are chemical.
- An action potential reaches the axon terminal, opening voltage-gated calcium channels. Calcium causes vesicles to release neurotransmitter by exocytosis into the synaptic cleft. The neurotransmitter binds receptors on the postsynaptic cell, opening ion channels.
- Excitatory transmitters depolarize the postsynaptic cell; inhibitory ones hyperpolarize it. Signals are combined (summation).
- The transmitter is then removed by breakdown (acetylcholinesterase breaks ACh), reuptake or diffusion.
- Key transmitters: acetylcholine (muscle contraction, memory), dopamine (movement and reward; low in Parkinson's disease), serotonin (mood), GABA (main inhibitory transmitter), glutamate (main excitatory).
- Example: drugs that block acetylcholinesterase leave ACh active longer.
Practice
Which ion entering the axon terminal triggers the release of neurotransmitter?
Calcium entry causes vesicles to fuse with the membrane and release transmitter.
A patient has tremors and slow movement due to loss of dopamine-producing neurons. This disease is most likely:
Parkinson's disease involves degeneration of dopamine neurons in the basal nuclei.
Why is it important that enzymes or reuptake remove neurotransmitter from the cleft?
Without removal the postsynaptic cell would stay stimulated.
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