Which of the following is not involved in kneejerk reflex ?

Options

A. Muscle spindle
B. Motor neuron
C. Brain
D. Inter neurons

Answer

**Step-by-Step Solution:**

1. **Understanding the Knee-Jerk Reflex:**
- The knee-jerk reflex, also known as the patellar reflex, is a type of reflex action that occurs when the patellar tendon is tapped just below the kneecap. This action causes the quadriceps muscle to contract, resulting in the leg kicking forward.

2. **Identifying the Components Involved:**
- The knee-jerk reflex involves several components:
- **Muscle Spindle:** This is a stretch receptor located in the muscle that detects the stretch when the tendon is tapped.
- **Motor Neuron:** This neuron carries the impulse from the spinal cord to the quadriceps muscle, causing it to contract.
- **Interneurons:** These neurons can be involved in reflex pathways, but in the case of the knee-jerk reflex, they are not necessary as it is a monosynaptic reflex (involving only one synapse between the sensory neuron and the motor neuron).

3. **Evaluating the Options:**
- **Option 1: Muscle Spindle** - Involved in detecting the stretch; therefore, it is part of the reflex.
- **Option 2: Motor Neuron** - Responsible for the contraction of the muscle; hence, it is also involved.
- **Option 3: Brain** - The knee-jerk reflex operates at the spinal cord level and does not require input from the brain, making this option a candidate for being not involved.
- **Option 4: Interneurons** - While not essential for the knee-jerk reflex, they can be involved in other reflexes but are not necessary for this specific reflex.

4. **Conclusion:**
- The component that is not involved in the knee-jerk reflex is the **Brain** (Option 3). The reflex action is mediated by the spinal cord without any input from the brain.

**Final Answer:**
The correct answer is **Option 3: Brain**.

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