A connection point between neurons, or between a neuron and an effector, is called a synapse. Here, electrical signals are converted into chemical signals and transmitted.
Basic Knowledge
Structure of a synapse: It consists of three parts: the presynaptic membrane(stores neurotransmitters in vesicles), the synaptic cleft(approximately 20〜50 nm), and the postsynaptic membrane(has receptor proteins).
Sequence of synaptic transmission:
① An action potential reaches the presynaptic membrane. ② Ca²⁺ flows in through the presynaptic membrane, and synaptic vesicles release neurotransmitters into the cleft by exocytosis. ③ Neurotransmitters bind to receptors on the postsynaptic membrane. ④ Ion channels in the postsynaptic membrane open, causing an excitatory(EPSP) or inhibitory(IPSP) change in membrane potential. ⑤ Neurotransmitters are removed by enzymatic breakdown and reuptake.
Main neurotransmitters: Acetylcholine(motor nerves・parasympathetic nerves)・noradrenaline(sympathetic nerve endings)・dopamine(reward system, deficiency→Parkinson's disease)・serotonin(mood・sleep)・glutamate(the main excitatory neurotransmitter)・GABA(the main inhibitory neurotransmitter).
Explain, from the viewpoint of structure, why synapses can transmit signals in only one direction.
Answer: Synaptic vesicles are present only on the presynaptic-membrane side, while receptors for neurotransmitters are present only on the postsynaptic-membrane side. Therefore, neurotransmitters can function only in the direction from the presynaptic membrane to the postsynaptic membrane, so synaptic transmission is always one-way.
- Synapse=three-part structure: presynaptic membrane・cleft(20〜50 nm)・postsynaptic membrane
- Ca²⁺ influx→synaptic-vesicle exocytosis→neurotransmitter release
- Synaptic transmission is one-way(vesicles are present only on the presynaptic side)
- Acetylcholine=motor nerves・parasympathetic nerves
- Noradrenaline=sympathetic nerve endings
- Dopamine deficiency=Parkinson's disease
- GABA=the main inhibitory neurotransmitter
Points to Note
① Transmission at a synapse is a chemical process. Conduction within a neuron is electrical. ② Distinguish between excitatory synapses(EPSP) and inhibitory synapses(IPSP). ③ Organophosphate pesticides inhibit cholinesterase, making it impossible to break down acetylcholine.
Practice
- What ion triggers the release of neurotransmitters at the presynaptic membrane?
- State the name of the main inhibitory neurotransmitter(abbreviation accepted).
- Which neurotransmitter is associated with Parkinson's disease?