How Does the Knee Jerk Reflex Help Assess Nervous System Health?
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: Definition, Steps & Role in Biology
1. What is the knee-jerk reflex and why is it important?
The knee-jerk reflex, also known as the patellar reflex, is an involuntary and rapid kicking motion of the lower leg in response to a sharp tap on the patellar tendon, located just below the kneecap. It is a classic example of a stretch reflex. Its primary importance is in maintaining proper posture and balance by preventing the overstretching of the quadriceps muscle, which helps stabilise the knee joint during movement.
2. What is the complete pathway of the knee-jerk reflex arc?
The neural pathway of the knee-jerk reflex arc is a rapid, five-step process:
- Stimulus: A reflex hammer taps the patellar tendon, stretching the quadriceps muscle.
- Receptor: Muscle spindles within the quadriceps detect this stretch.
- Afferent Pathway: A sensory neuron sends an electrical impulse from the muscle spindle to the spinal cord.
- Integration Center: In the spinal cord, the sensory neuron directly synapses with a motor neuron.
- Efferent Pathway: The motor neuron transmits a nerve impulse back to the quadriceps muscle.
- Response: The quadriceps muscle contracts, causing the lower leg to kick forward.
3. Does the brain have a role in the knee-jerk reflex?
No, the brain is not directly involved in executing the knee-jerk reflex. It is a spinal reflex, meaning the neural circuit is contained entirely within the spinal cord. This allows for an extremely fast reaction time as the signal does not need to travel to the brain and back. However, the brain can exert a conscious influence, such as voluntarily suppressing or exaggerating the reflex.
4. Why is the knee-jerk reflex considered a monosynaptic reflex?
The knee-jerk reflex is classified as monosynaptic because its reflex arc involves only a single synapse. The sensory neuron that detects the muscle stretch forms a direct connection with the motor neuron that triggers the muscle contraction. There are no interneurons, or intermediary nerve cells, in this specific pathway, making it one of the simplest and fastest reflexes in the human body.
5. How does the monosynaptic knee-jerk reflex differ from a polysynaptic reflex?
The key difference lies in the number of synapses in the reflex arc.
- Monosynaptic Reflex (Knee-Jerk): Involves a single synapse directly between a sensory and a motor neuron. The response is a simple contraction of one muscle.
- Polysynaptic Reflex (Withdrawal Reflex): Involves one or more interneurons that connect the sensory and motor neurons. This allows for more complex responses, such as contracting one set of muscles (e.g., flexors) while simultaneously relaxing the opposing set (e.g., extensors).
6. Which specific nerve and spinal cord segments are evaluated by the knee-jerk test?
The knee-jerk reflex test is used to assess the integrity of the femoral nerve and the corresponding spinal cord segments at the L2, L3, and L4 levels. An abnormal (exaggerated, diminished, or absent) response can indicate a potential neurological issue or lesion within this specific part of the nervous system.
7. What is the clinical significance of an exaggerated or absent knee-jerk reflex?
The response of the knee-jerk reflex provides valuable clinical information:
- An exaggerated reflex (hyperreflexia) often suggests a lesion in the upper motor neurons (in the brain or spinal cord), which normally exert an inhibitory effect on reflexes.
- An absent or diminished reflex (hyporeflexia) can indicate damage to the lower motor neurons, such as the femoral nerve, the nerve roots (L2-L4), or the muscle itself.
8. Beyond a simple kick, what is the functional purpose of the knee-jerk reflex in daily life?
The primary purpose of the knee-jerk reflex is to automatically maintain posture and balance without conscious thought. For example, if you stumble or your knee buckles slightly while walking, the quadriceps muscle stretches. This reflex instantly triggers it to contract, straightening the knee and preventing a fall. It continuously makes minor adjustments to muscle tone to keep you upright and stable.
