[A] : In mammals, ends of presynaptic fibres are enlarged to form synaptic Knobs which are separated by a synaptic cleft from postsynaptic neurons. Each synaptic knob contains many mitochondria and small gran- ule-Iike synaptic vesicles filled with minute packets of the chemical that transmits im- pulses to postsynaptic neurons.
[R]: The chemical transmitters are called neurohumors which proportionate to `K^+` in- flux.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To analyze the assertion and reason statements provided in the question, we can break down the concepts step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that in mammals, the ends of presynaptic fibers are enlarged to form synaptic knobs.
- These synaptic knobs are separated from postsynaptic neurons by a space known as the synaptic cleft.
- Each synaptic knob contains many mitochondria and synaptic vesicles filled with neurotransmitters, which are chemicals that transmit impulses to the postsynaptic neurons.

2. **Analyzing the Structure**:
- The presynaptic neuron ends in a structure called the axon terminal, which is swollen to form the synaptic knob.
- Within the synaptic knob, synaptic vesicles store neurotransmitters (e.g., acetylcholine).
- The synaptic cleft is the gap between the presynaptic and postsynaptic membranes.

3. **Understanding the Role of Neurotransmitters**:
- When an action potential reaches the synaptic knob, neurotransmitters are released into the synaptic cleft.
- These neurotransmitters bind to receptors on the postsynaptic membrane, leading to the generation of a new action potential in the postsynaptic neuron.

4. **Understanding the Reason (R)**:
- The reason states that the chemical transmitters are called neurohumors, which are proportional to K+ influx.
- This statement is misleading because the primary neurotransmitter action involves sodium (Na+) influx rather than potassium (K+) influx.

5. **Conclusion**:
- The assertion (A) is true as it accurately describes the structure and function of synaptic knobs and neurotransmitter release.
- The reason (R) is false because it incorrectly associates neurotransmitter action with K+ influx instead of Na+ influx.
- Therefore, the correct answer is that A is true and R is false.

### Final Answer:
- Assertion (A) is true, and Reason (R) is false.

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