What Is the Knee Jerk Reflex and How Does the Reflex Arc Work
Reflex actions are important neurological responses that cause you to react without thinking. The signal doesn't reach your brain but your spinal cord receives the signal promptly to initiate an action. However, sometimes the action may be disrupted. For diagnosing this problem, a common reflex test, a pendular knee jerk reflex test (a common knee jerk test) is conducted.
The knee jerk reflex, also known as the pendular knee reflex, is an action of kicking in response to a sharp pat on the patellar tendon. The patellar tendon lies just below the knee cap. The kicking movement is initiated by the stretch receptor which influences your quadriceps to stretch. This stretch receptor lies above the knee cap area.
The pendular knee jerk reflex test is done when you would sit keeping the knees bent. It is suggested to sit in a crossed-leg way so that the foot is suspended. This helps to stimulate the knee reflex to kick the leg straight away without any interruption.
Knee Jerk Reflex Explained
The mystery is how does the tendon jerk reflex occur? Why does the knee muscle stretch after the tap of the tendon? This mechanism involves a series of signals and neurons and muscle fibers that are responsible for the reflex action.
Let Us Learn the Steps By Which, Gradually, the Tendon Jerk Reflex is Initiated.
1. First Step
As soon as the tapping on the tendon occurs, the stretch receptor located in the upper portion of the leg recognizes and receives the signal of the tapping. This is the primary step of the jerk reflex.
2. Second Step
This signal received by the receptor is transmitted to the sensory neuron. Via the sensory neuron cell body, the signal passes and reaches the spinal cord.
3. Third Step
Now, here the signal is passed on to the motor neuron cell body via an interneuron that connects the sensory and motor neurons. This is the time to transform the signal into a jerk reflex action.
4. Fourth Step
In this step, the motor neuron passes the signal from the spinal cord directly to the muscles of action. These muscles are located in the upper leg portion. These two muscles are the biceps femoris muscle and quadriceps femoris muscles.
5. Fifth Step
Here comes the final step of the reflex action. The signal makes the biceps femoris muscle relax while the quadriceps femoris muscle stretches. This reaction causes the kicking off the leg completing the process of knee reflex action.
These are the main steps that make you kick your leg when the doctor taps the tendons below the knee cap with a hammer. Thus the complete procedure of knee jerk reflex is explained.
Two Types of Reflex Actions
We can divide the reflex actions depending on the number of neurons involved in the procedure. They are-
Monosynaptic reflex action
Polysynaptic reflex action
A monosynaptic neuron uses one synapsis to transmit the signal. It involves a sensory neuron, a motor neuron, and one interneuron to transform the signal into an action. Also, only one action is initiated by the monosynaptic reflex action.
On the other hand, polysynaptic reflex includes two or more interneurons connecting one or more sensory and motor neurons. Here, the human being can simultaneously show more than one action to react to an event.
Now, can you tell which reflex is expressed in a patellar knee jerk reflex action? Yes, it is a monosynaptic reflex as one interneuron is involved in causing a single action.
Knee Jerk Analysis
Why is a knee-jerk test done? This is an excellent and easy way for diagnosing some problems related to the knee jerk nerve in humans, especially, kids. When people fail to react promptly to an event, this shows a lack of reflex stimulation.
To study this problem, the doctors hit on the tendon below the knee cap with a tiny hammer. If the knee-jerk nerves are working, the person would immediately kick off his leg. If he fails to react promptly and his legs don't stretch, this is detected as a difficulty in the spinal cord nervous system. Also, it shows the sign of neuromuscular disorders.
This reflex action test is also conducted in other parts of the body like the ankle, biceps, triceps, abdomen, and forearm. These are places that are supposed to stretch as a consequence of the hit of a hammer if the neurons, muscles, and spinal cord systems are functioning properly.
Did You Know?
A simple knee-jerk test can tell a lot about your body functions. It is not only used to see if there are any problems in the sensors, neurons, muscles, or PNS but can tell about diseases. Reduced and slow action during the test is a sign of Westphal's disease.
On the other hand, if the leg oscillates as a reaction, it shows a trace of cerebral diseases. Exaggerated reflex is a mark of hyperthyroidism, nervousness, and anxiety issues.
FAQs on Knee Jerk Reflex and Its Mechanism
1. What is the knee jerk reflex?
The knee jerk reflex is a rapid, involuntary extension of the lower leg in response to tapping the patellar tendon. It is a type of stretch reflex that helps maintain posture and muscle tone. When the tendon below the kneecap is tapped, it stretches the quadriceps muscle, triggering an automatic reflex action through the spinal cord without conscious control.
2. How does the knee jerk reflex work step by step?
The knee jerk reflex works through a simple spinal reflex arc that produces an immediate muscle contraction.
- Tap on the patellar tendon stretches the quadriceps muscle.
- Muscle spindles detect the stretch and generate nerve impulses.
- Sensory neurons carry impulses to the spinal cord.
- The sensory neuron directly synapses with a motor neuron (monosynaptic reflex).
- The motor neuron stimulates the quadriceps to contract, extending the lower leg.
3. Why is the knee jerk reflex called a monosynaptic reflex?
The knee jerk reflex is called a monosynaptic reflex because it involves only one synapse between a sensory neuron and a motor neuron. In the spinal cord, the sensory neuron directly connects to the motor neuron without an interneuron. This single synapse makes the reflex extremely fast compared to polysynaptic reflexes.
4. What is the function of the knee jerk reflex?
The primary function of the knee jerk reflex is to maintain posture and muscle tone by preventing overstretching of the quadriceps muscle.
- Helps stabilize the body while standing.
- Maintains balance during walking.
- Protects muscles from excessive stretch.
5. Which nerve is involved in the knee jerk reflex?
The femoral nerve is the main nerve involved in the knee jerk reflex. It carries sensory signals from the quadriceps muscle to the spinal cord and motor signals back to the muscle. The reflex is primarily integrated at the L2–L4 spinal cord segments.
6. What happens if the knee jerk reflex is absent?
An absent knee jerk reflex usually indicates damage to the sensory nerve, motor nerve, or spinal cord segments (L2–L4). Possible causes include:
- Peripheral neuropathy
- Spinal cord injury
- Lower motor neuron lesions
7. Why do doctors test the knee jerk reflex?
Doctors test the knee jerk reflex to evaluate the integrity of the spinal cord and peripheral nervous system. A reflex hammer tap checks:
- Proper sensory and motor nerve function
- Spinal cord segment health (L2–L4)
- Signs of upper or lower motor neuron disorders
8. What receptors are involved in the knee jerk reflex?
The receptors involved in the knee jerk reflex are muscle spindles, which detect muscle stretch. These specialized sensory receptors are located within the quadriceps muscle. When the muscle is stretched, muscle spindles generate nerve impulses that initiate the reflex arc.
9. What is the difference between the knee jerk reflex and a withdrawal reflex?
The knee jerk reflex is a monosynaptic stretch reflex, while the withdrawal reflex is a polysynaptic reflex triggered by pain.
- Knee jerk reflex: Involves one synapse and maintains posture.
- Withdrawal reflex: Involves interneurons and removes a body part from a painful stimulus.
10. Is the knee jerk reflex controlled by the brain?
The knee jerk reflex is primarily controlled by the spinal cord and does not require direct brain involvement for the response. The reflex action occurs at the spinal level for speed. However, the brain can modulate or influence the strength of the reflex through descending neural pathways.
