If the ligament directly below the kneecap is struck lightly by the edge of the hand or by a doctor's rubber hammer, knee jerk reflex occurs. This stretch reflex is

Options

A. Polysnaptic
B. Withdrawl
C. Monosynaptic
D. Bisynaptic

Answer

**Step-by-Step Solution:**

1. **Understanding the Reflex Action**: The knee jerk reflex occurs when the ligament below the kneecap is struck. This action triggers a reflex response in the body.

2. **Types of Reflexes**: Reflex actions can be categorized into two main types based on the number of synapses involved: monosynaptic and polysynaptic reflexes.
- **Monosynaptic Reflex**: Involves only one synapse between the sensory neuron and the motor neuron. There are no interneurons involved.
- **Polysynaptic Reflex**: Involves one or more interneurons between the sensory neuron and the motor neuron, resulting in multiple synapses.

3. **Identifying the Knee Jerk Reflex**: The knee jerk reflex is a classic example of a monosynaptic reflex. When the patellar ligament is tapped, the sensory neurons send a signal directly to the spinal cord, where it synapses with the motor neuron that causes the quadriceps muscle to contract, resulting in the leg kicking out.

4. **Eliminating Incorrect Options**:
- **Polysynaptic**: Incorrect because the knee jerk reflex does not involve interneurons.
- **Withdrawal Reflex**: This is an example of a polysynaptic reflex, as it involves multiple synapses and interneurons.
- **Bisynaptic Reflex**: This term is not commonly used in the classification of reflexes, making it an incorrect option.

5. **Conclusion**: Based on the above analysis, the knee jerk reflex is classified as a monosynaptic reflex.

**Final Answer**: The stretch reflex is **monosynaptic**.

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