How does a message. travel across the gap. at synapse?
At the synapse, the gap between two neurons is called the synaptic cleft. Neurotransmitters are chemical messengers that transmit signals across this gap. Here's how a message travels across the synaptic cleft:
1. Action Potential Arrival: When an action potential reaches the presynaptic neuron (the neuron sending the signal), it depolarizes the neuron's membrane.
2. Calcium Ion Influx: This depolarization opens voltage-gated calcium channels on the presynaptic membrane, allowing calcium ions to flow into the neuron.
3. Neurotransmitter Release: The influx of calcium ions triggers the release of neurotransmitters from specialized structures called synaptic vesicles. These vesicles fuse with the presynaptic membrane and release their neurotransmitter contents into the synaptic cleft.
4. Neurotransmitter Binding: The released neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic neuron (the neuron receiving the signal).
5. Postsynaptic Response: The binding of neurotransmitters to receptors on the postsynaptic neuron can have different effects depending on the type of neurotransmitter and receptor involved. Typically, it results in either the generation of an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).
- Excitatory Postsynaptic Potential (EPSP): If the neurotransmitter binding leads to the opening of ion channels that allow positively charged ions (such as sodium) to enter the postsynaptic neuron, it results in an EPSP. This depolarizes the postsynaptic membrane, making it more likely to reach the threshold potential and generate an action potential.
- Inhibitory Postsynaptic Potential (IPSP): Alternatively, if the neurotransmitter binding leads to the opening of ion channels that allow negatively charged ions (such as chloride) to enter the postsynaptic neuron or the efflux of positively charged ions (such as potassium), it results in an IPSP. This hyperpolarizes the postsynaptic membrane, making it less likely to reach the threshold potential and generate an action potential.
6. Action Potential Generation (or Inhibition): The combined effect of EPSPs and IPSPs determines whether the postsynaptic neuron reaches the threshold potential and generates an action potential. If the cumulative EPSPs are stronger than the IPSPs, the neuron will depolarize and generate an action potential, propagating the signal to the next neuron. If the IPSPs are dominant, the neuron will remain below the threshold potential, preventing the generation of an action potential.
This process of neurotransmitter release, binding, and postsynaptic response allows signals to be transmitted across the synaptic cleft, enabling communication between neurons and the processing of information in the nervous system.