Question

In a synapse, chemical signal is transmitted from
A) Dendritic end of one neuron to axonal end of another neuron
B) Axon to cell body of the same neuron
C) Cell body to axonal end of the same neuron
D) Axonal end of one neuron to dendritic end of another neuron

Answer 1

Impulse transmission in a neuron is unidirectional. Each part of the neuronal cell is designed to make sure the message goes in the forward direction and across the chemical synapse between two nerve cells.

Complete Answer:
- When the wave of depolarisation hits the end of the axon of a neuron, this causes synaptic vesicles to fuse with the end of the axon membrane, or the presynaptic membrane. Fusing of synaptic vesicles releases neurotransmitters into the synaptic cleft. These then attach to receptors on the dendritic ends of another neuron, the postsynaptic membrane. Attaching of receptors opens up various channels in the postsynaptic membrane, initiating the wave of depolarisation in the second neuron.
- Axons do not turn back and release neurotransmitters to their own cell bodies. The cell bodies themselves do not have receptors for neurotransmitters, nor are they capable of releasing neurotransmitters. They do transmit the signal down the membrane through the progressive depolarisation-repolarisation process.
- Dendritic ends are numerous, and pick up signals from other neurons or sensory organs, send them towards the cell body, and this in turn continues down the axonal membrane. The axon, on the other hand, directs the signal towards a particular neuron or group of neurons.

Hence the correct answer is option A.

Note: There are some instances where an axon may synapse with the soma or cell body of a postsynaptic neuron, or even the axon of a postsynaptic neuron. These are called axosomatic and axoaxonic synapses respectively.