Topic: Nervous system, neuronal transmission
Question: Define synapse, and enumerate the steps of synaptic transmission. What is the significance of synaptic delay?
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A synapse is a junction between two neurons or between a neuron and an effector cell. Synapses allow neurons to communicate with one another through the transmission of signals via neurotransmitters.
The steps of synaptic transmission are:
1. An action potential reaches the presynaptic terminal of the axon. This causes voltage-gated calcium channels to open.
2. Calcium ions enter the presynaptic terminal. This causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane.
3. Neurotransmitters are released into the synaptic cleft between the presynaptic and postsynaptic cells.
4. The neurotransmitters bind to specific receptors on the postsynaptic membrane. This binding causes the opening or closing of ion channels in the postsynaptic cell.
5. Excitatory receptors, like those binding glutamate, will open sodium channels causing depolarization. Inhibitory receptors, like those binding GABA, will open chloride channels causing hyperpolarization.
6. The changes in ion concentrations either depolarize or hyperpolarize the postsynaptic cell. If threshold is reached, an action potential will fire in the postsynaptic cell.
7. The neurotransmitters unbind from the receptors and are metabolized or reabsorbed for recycling. This ends the synaptic transmission.
The synaptic delay refers to the time between the arrival of the action potential in the presynaptic terminal and the generation of a postsynaptic potential in the postsynaptic cell. It represents the time required for the several steps involved in synaptic transmission to occur.
The significance of the synaptic delay is that it limits the frequency at which neurons can communicate with each other. Each step in the process, from the opening of voltage-gated calcium channels to the binding of neurotransmitters to receptors, takes time. The total synaptic delay, which can last between 0.5 to 20 milliseconds, depends on the particular synaptic pathway. It places an upper limit on the frequency of action potentials in communicating neurons.
In summary, synapses are connections between neurons that allow them to communicate via chemical or electrical signals. Synaptic transmission involves the release of neurotransmitters, binding to receptors, and changes in postsynaptic ion concentrations. The synaptic delay refers to the time required for synaptic transmission and limits the frequency of signaling between connected neurons.
