113. Which one of the following is NOT applicable for a chemical synapse?
A. Usually synaptic cleft is approx. 20 – 40 nm.
B. May release excitatory or inhibitory neurotransmitter.
C. Cannot modulate signal intensity.
D. There is a delay of signal propagation.

Detailed Explanation:

Chemical synapses are specialized junctions that allow neurons to communicate with each other or with other cell types such as muscle cells. Let’s break down the key characteristics of chemical synapses:

1. Synaptic Cleft Size (A): The synaptic cleft, the gap between two neurons, is typically around 20-40 nm. This small space is essential for the transmission of neurotransmitters between the pre- and post-synaptic neurons.

2. Neurotransmitter Release (B): Chemical synapses can release various neurotransmitters, which can be excitatory (such as glutamate) or inhibitory (such as GABA). These neurotransmitters regulate the electrical signals in the receiving neuron.

3. Signal Modulation (C): A key feature of chemical synapses is their ability to modulate signal intensity. The amount and type of neurotransmitter released, along with the receptor type on the postsynaptic neuron, allow for fine-tuning of the signal. This modulation can adjust the strength of the signal transmitted.

4. Signal Propagation Delay (D): Chemical synapses introduce a delay in signal propagation due to the time it takes for the neurotransmitter to cross the synaptic cleft and bind to receptors. This delay is typically around 1-5 milliseconds, in contrast to the much faster electrical transmission through gap junctions in electrical synapses.

Answer:

The statement C. “Cannot modulate signal intensity” is NOT applicable to a chemical synapse, as chemical synapses can modulate signal intensity depending on the neurotransmitter release and receptor interaction.

Thus, C is the correct answer.