Unidirectional transmission of nerve impulse through nerve fibre is due to the fact that

Options

A. Nerve fibre is insulated by a medullary sheath
B. Sodium pump start operating only at the cyton and then continues into the nerve fibre
C. Neurotransmitters are released by dendrites and not by axon endings
D. Neurotransmitters are released by the axon endings and not by dendrites

Answer

### Step-by-Step Solution

1. **Understanding the Structure of a Neuron**:
- A neuron consists of several parts: the cell body (soma), dendrites, axon, and axon terminals. The axon is often covered by a myelin sheath, which is segmented by nodes of Ranvier.

2. **Role of the Myelin Sheath**:
- The myelin sheath acts as an insulating layer around the axon, which helps in speeding up the transmission of nerve impulses. It prevents the loss of electrical signals and allows for faster communication between neurons.

3. **Transmission of Nerve Impulses**:
- Nerve impulses are transmitted from one neuron to another through synapses. The axon terminal of one neuron releases neurotransmitters into the synaptic cleft, which then bind to receptors on the dendrites of the next neuron.

4. **Direction of Impulse Transmission**:
- The transmission of nerve impulses is unidirectional, meaning it travels in one direction—from the axon terminal of the presynaptic neuron to the dendrites of the postsynaptic neuron. This is because neurotransmitters are released from the axon terminals and not from the dendrites.

5. **Evaluating the Options**:
- Option 1: "Nerve fiber is insulated by myelin sheath" - While true, it does not explain unidirectional transmission.
- Option 2: "Sodium pump starts operating only at the cytosol" - This is incorrect regarding the context of impulse transmission.
- Option 3: "Neurotransmitters are released by dendrites" - This is incorrect; neurotransmitters are released by axon terminals.
- Option 4: "Neurotransmitters are released by axon endings and not by dendrites" - This is correct and explains the unidirectional nature of impulse transmission.

6. **Conclusion**:
- The correct answer is that unidirectional transmission of nerve impulse through a nerve fiber is due to the fact that neurotransmitters are released by axon endings and not by dendrites.