Which of the following statements regarding impulse transmission is incorrect ?
I . Chemicals called neurotransmitters are involved in impulse transmission across the synapses.
II. The membranes of the pre - synaptic and post - synaptic neuron are separated by an empty space called the synaptic cleft.
III. The released neurotransmitters bind to the specific receptors present on the pre - synaptic membrane.
IV. The new potential which develops is always excitatory.

Options

A. II and III
B. I, II and IV
C. I and III
D. II,III and IV

Answer

To determine which statement regarding impulse transmission is incorrect, let's analyze each statement step by step:

1. **Statement I**: "Chemicals called neurotransmitters are involved in impulse transmission across the synapses."
- This statement is **correct**. Neurotransmitters are indeed the chemicals that facilitate the transmission of nerve impulses across the synaptic gap between neurons.

2. **Statement II**: "The membranes of the pre-synaptic and post-synaptic neuron are separated by an empty space called the synaptic cleft."
- This statement is **correct**. The synaptic cleft is the small gap between the pre-synaptic neuron (the neuron sending the signal) and the post-synaptic neuron (the neuron receiving the signal).

3. **Statement III**: "The released neurotransmitters bind to the specific receptors present on the pre-synaptic membrane."
- This statement is **incorrect**. The neurotransmitters actually bind to specific receptors on the **post-synaptic membrane**, not the pre-synaptic membrane.

4. **Statement IV**: "The new potential which develops is always excitatory."
- This statement is **incorrect**. The potential that develops can be either excitatory or inhibitory, depending on the type of neurotransmitter released and the receptors activated.

Now, summarizing the analysis:
- Statement I is correct.
- Statement II is correct.
- Statement III is incorrect.
- Statement IV is incorrect.

Thus, the incorrect statements are III and IV.

### Conclusion:
The answer to the question is that Statements III and IV are incorrect regarding impulse transmission.

### Final Answer:
The incorrect statements regarding impulse transmission are III and IV.