Question

Describe the steps in a chemical synapse from the arrival of an action potential at the pre-synaptic membrane to the formation of a graded potential on the post-synaptic membrane. Include as much biological detail as you can.

Answer 1

​​​​​​Answer:

Chemical synapse is the biological junction between two neurons which provides a space to tranfer signal from one neuron to another and to non-neuronal cells like cells in muscles, glands etc.

In chemical synapse, information is passed from a presynaptic nerve cell to a postsynaptic nerve cell. The terminal of pre-synaptic cell is a specialised area that contains neurotransmitters (small membrane-bound sperical vesicles called synaptic vesicles filled with chemical). Synaptic vesicles are present at the plasma membrane of pre-synaptic cells , these regions are called active zones.

There is postsynaptic cell immediately after the pre synaptic cell membrane and it contains neurotransmitter receptors ar regions called postsynaptic region present on the dendrites of post-synaptic cell. There is an elaborate complex of interlinked proteins called the postsynaptic density (PSD) behind the postsynaptic membrane. These Proteins are involved in monitoring the movement of neurotransmitter receptors and activity of neurotransmitter receptors. The following events occur in chemical synapse region between two neurons.

1. The process begins when a wave of electrochemical excitation reaches the synapse region presynaptic cell. This wave is generated at dendrite of this cell and called action potential.
2. Electrical depolarization of the membrane occurs at the synapse which open the channels for transport of calcium ions through the presynaptic membrane which are present at the exterior of membrane.
3. Due to this, calcium ions rapidly move into the presynaptic cell and concentration of calcium ions increases in the interior of the cell.
4. Due to high concentration of calcium ions, calcium-sensitive proteins attached to synaptic vesicles are activated.
5. Due to this, shape of these proteins changes and cause the synaptic vesicles to fuse with the presynaptic membrane and vesicles gets open and release their neurotransmitter content into the synaptic region between the two neurons.
6. The neurotransmitters reach to post synaptic membrane and there they bind to chemical receptor molecules present on the post-synaptic membrane.
7. Due to this binding, receptor molecules get activated.
8. These receptors open the ion channels in the membrane of post synaptic cell and cause the ions to enter or exit the post-synaptic cell which changes transmembrane potential and results in change in potential. this newly fromed potential is called graded potential.
9. Due to thermal vibration, the neurotransmitter molecules eventually detach from the receptor molecules.
10. These neurotransmitter are then either reabsorbed by the presynaptic cell, and repackaged for future release or are boken down due to metabolic activities.