Question

Explain transmission of a nerve signal across a synapse. (page \(281)\)

Short answer

Nerve signal transmission across a synapse involves the following steps: an action potential travels along the presynaptic neuron and triggers the opening of voltage-gated calcium channels at the synaptic terminal. Calcium ions enter, causing vesicles to release neurotransmitters into the synaptic cleft. Neurotransmitters bind to specific receptors on the postsynaptic neuron, opening ion channels and generating an excitatory or inhibitory postsynaptic potential. If the threshold value is reached, an action potential is triggered in the postsynaptic neuron. Finally, neurotransmitters are removed from the synaptic cleft to terminate the signal transmission.

Step-by-step solution

Introduction to Nerve Signal Transmission

Nerve signal transmission is the process through which electrical signals are transmitted between nerve cells to facilitate communication within the nervous system. This involves a specialized junction called a synapse, which allows a signal to cross from one nerve cell to another.

Propagation of an Action Potential

When a nerve signal is initiated, an electrical charge called an action potential travels along the neuron's axon. The action potential is a result of depolarization and repolarization of the neuron's membrane due to the movement of ions (mainly sodium and potassium) through specialized protein channels.

Arrival at the Synaptic Terminal

Once the action potential reaches the synaptic terminal (axon terminal) of the presynaptic neuron, it triggers the opening of voltage-gated calcium channels. This allows calcium ions to enter the neuron.

Release of Neurotransmitters

The influx of calcium ions inside the synaptic terminal causes vesicles containing neurotransmitters (chemical messengers that transmit signals) to fuse with the plasma membrane. This results in the release of neurotransmitters from the presynaptic neuron into the synaptic cleft, a tiny gap between the two neurons.

Binding to Receptors

Neurotransmitters released into the synaptic cleft diffuse across the gap and bind to specific receptors on the postsynaptic neuron's dendrites. This binding generates a response in the postsynaptic neuron, either leading to excitation (depolarization) or inhibition (hyperpolarization) depending on the type of receptor and neurotransmitter involved.

Generation of a Postsynaptic Potential

Once neurotransmitters bind to their receptors, this opens ion channels on the postsynaptic neuron's membrane. The flow of ions generates either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP), depending on whether the neuron is depolarized or hyperpolarized.

Integration and Propagation of the New Signal

If the postsynaptic neuron receives enough EPSPs to overcome the inhibitory effects of IPSPs, it will reach the threshold value that triggers an action potential in this neuron. This new action potential will then be propagated along the postsynaptic neuron, effectively allowing the nerve signal to cross the synapse and continue transmitting the signal to the next neuron or target cell.

Termination of the Signal at the Synapse

To terminate the signal transmission, the neurotransmitter molecules are removed from the synaptic cleft. This can be achieved through reuptake by the presynaptic neuron, degradation by enzymes, or diffusion away from the synapse. This process ensures that the signal does not continue to stimulate the postsynaptic neuron and helps maintain the sensitivity of synaptic connections. By understanding these steps, students will have a clear and comprehensive overview of how a nerve signal is transmitted across a synapse in the nervous system.