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

Options

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

Explanation

**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 leg to kick out involuntarily.

2. **Identifying Components of the Reflex Arc**: The reflex arc involved in the knee-jerk reflex includes several components:
- **Muscle Spindle**: These are sensory receptors located in the muscles that detect stretch.
- **Motor Neuron**: This neuron carries the signal from the spinal cord to the muscle, causing it to contract.
- **Interneuron**: These neurons connect sensory and motor neurons within the spinal cord, although in the knee-jerk reflex, they are not essential.
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