Which of the following is not a part of knee jerk reflex ?

Options

A. Afferent pathway
B. Muscle spindle
C. Efferent pathway
D. Interneuron

Answer

**Step-by-Step Solution:**

1. **Understanding the Knee Jerk Reflex**: The knee jerk reflex, also known as the patellar reflex, is a simple reflex action that occurs when the patellar tendon is tapped, causing the knee to jerk forward. This reflex is a protective mechanism that helps maintain posture and balance.

2. **Identifying the Components of the Reflex Arc**: The knee jerk reflex involves several components:
- **Afferent Pathway**: This is the sensory pathway that carries the sensory information from the knee to the spinal cord. It is made up of sensory neurons that detect the stretch in the muscle.
- **Muscle Spindle**: This is a sensory receptor located within the muscle that detects changes in muscle length (stretch) and the rate of that change. It plays a crucial role in the knee jerk reflex.
- **Efferent Pathway**: This is the motor pathway that carries the response signal from the spinal cord back to the muscle, causing it to contract.
- **Interneuron**: This is a neuron that acts as a connector between the sensory and motor neurons. However, in the knee jerk reflex, there is typically no interneuron involved, as it is a monosynaptic reflex.

3. **Analyzing the Options**: The options given in the question are:
- Afferent pathway
- Muscle spindle
- Efferent pathway
- Interneuron

From the analysis:
- The **afferent pathway** is involved in carrying the sensory information to the spinal cord.
- The **muscle spindle** is a key component that detects the stretch.
- The **efferent pathway** sends the response signal back to the muscle.
- The **interneuron** is not involved in the knee jerk reflex as it is a monosynaptic reflex.

4. **Conclusion**: The component that is **not a part of the knee jerk reflex** is the **interneuron**.

**Final Answer**: The correct option is **Interneuron** (Option D).

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