Which of the following is not involved in knee-jerk reflex ?

Options

A. (a) Muscle spindle
B. (b) Motor neuron
C. (c) Brain
D. (d) Inter neurons

Answer

To determine which of the following options is not involved in the knee-jerk reflex, we need to understand the components of the reflex arc associated with this reflex.

### Step-by-Step Solution:

1. **Understanding the Knee-Jerk Reflex**:
- The knee-jerk reflex, also known as the patellar reflex, is an involuntary response that occurs when the patellar tendon is tapped. This action stretches the tendon and the associated muscle, leading to a reflexive contraction of the quadriceps muscle.

2. **Identifying Components of the Reflex Arc**:
- The reflex arc consists of several key components:
- **Receptors**: Muscle spindles in the quadriceps muscle detect the stretch.
- **Sensory Neurons**: These neurons carry the sensory information from the muscle spindles to the spinal cord.
- **Interneurons**: Located in the spinal cord, these neurons facilitate communication between sensory and motor neurons.
- **Motor Neurons**: These neurons send signals from the spinal cord to the quadriceps muscle, causing it to contract.
- **Effectors**: The muscle that responds to the motor neuron signal (in this case, the quadriceps).

3. **Analyzing the Options**:
- **Option A: Muscle Spindles** - Involved in detecting stretch; therefore, they are part of the reflex.
- **Option B: Motor Neurons** - These are essential for transmitting signals to the muscle; hence, they are involved.
- **Option C: Brain** - The knee-jerk reflex operates at the spinal cord level and does not require brain involvement for the reflex action.
- **Option D: Interneurons** - These are crucial for communication within the spinal cord and are involved in the reflex.

4. **Conclusion**:
- Based on the analysis, the only component listed that is not involved in the knee-jerk reflex is the **brain**. Therefore, the correct answer is **Option C: Brain**.

### Final Answer:
**C: Brain**

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