Question

Discuss the events that occur at a synapse that lead to generating an action potential that is able to “jump” from one neuron to the next. Be sure to discuss the role of the action potential traveling down the axon, influx of Calcium ions, neurotransmitters, post synaptic ion channels, and reuptake.

Answer 1

Answer: When the neuron is at rest, the concentration of Na+ ions is ten times greater outside the cell than inside of the cell, the concentration of K+ ions is more inside the cell than outside. Due to this inside of the neuronal cell is more negative than the ECF (extracellular fluid). The charge is -70mV, which is the resting membrane potential.

On receiving a stimulus through the sensory receptor, sodium channels of the neuron open up, resulting in influx of sodium ions, the cell is said to be depolarized now, as more and more sodium ions come inside the cell, the membrane potential changes to +30mV. This depolarization of the cell spreads over and this action potential reaches to the axon terminal.

Synapse is the gap between the adjacent neurons, when the action potential reaches the axon terminal, it needs to be transmitted to the adjacent neuron through the synapse, and there are two ways in which the action potential jumps from one neuron to another i.e. electrical synapse and chemical synapse.

Electrical synapse: in this the action potential travels through the gap junctions which are narrow gaps between the neurons. The neurons are at a distance of 3.8nm from each other. While for chemical synapse the distance between the neurons is 20-40nm. The electrical synapse is present where the fast response is needed such as in defence reflexes.

Chemical synapse: Mostly neurons communicate through chemical synapse; this takes place with the help of certain chemicals known as neurotransmitters.

The neurotransmitters are chemicals which are synthesized and stored in the vesicles at the axonal knob till the action potential arrives. The neurotransmitters are stored in the vesicles near the pre synaptic membrane; these vesicles are bound to Calcium sensitive vesicle membrane proteins which are in turn bound to microtubules and other members of the cytoskeleton.

When the action potential reaches the axon terminal, the voltage dependent Calcium channels of the membrane open and there is influx of Calcium ions, these ions trigger the release of the neurotransmitters stored into the vesicles in the synaptic cleft (space within the synapse)

Once the neurotransmitters are released they bind to the receptors present on the post synaptic membrane of the adjacent neuron and open up the sodium channels (post synaptic ion channels) or close the sodium channel in the adjacent neuron depending whether the neurotransmitters are excitatory or inhibitory.

After the recognition of the neurotransmitter, it comes back to the synaptic cleft where some neurotransmitters are removed by special transporter proteins, which bring them back for reuptake by the presynaptic membrane where it is repackaged into the vesicle or are chemically inactivated. This is done to prevent constant stimulation and excessive firing of the action potential.

Example: Acetylcholine is a neurotransmitter, after it passes on the depolarisation signal to the adjacent neuron it is inactivated by acetylcholine esterase.