Is the knee-jerk reflex a conditioned reflex?

The knee-jerk reflex is an instinctive reflex of the human body. As long as you do not suffer from neurological diseases, you will normally have the knee-jerk reflex. The knee jerk reflex refers to the subconscious bounce of the knee after hitting the knee hard. To complete the knee jerk reflex, the nerves in various parts of the body need to work together and be controlled by the brain. So, is the knee-jerk reflex a conditioned reflex?

Introduction

The knee-jerk reflex is the simplest type of reflex. The basic method of neural regulation is reflex. The entire nerve conduction pathway from receiving stimulation to generating a reaction is called a reflex arc, which includes receptors, afferent nerves, nerve centers, efferent nerves, and effectors.

The nerve center of the knee-jerk reflex is a low-level nerve center located in the gray matter of the spinal cord. However, when the knee jerk reflex is completed, the nerves in the spinal cord that lead to the brain will transmit this nerve impulse to the brain, making people feel that their knees are tapped. The knee jerk reflex is completed first, and then the tap on the knee is felt, but almost simultaneously.

Stimulation of receptors in the thigh muscles at the knee (mechanoreceptors within the quadriceps tendon) triggers action potentials in sensory neurons that travel up the spinal cord (gray matter), where sensory neurons make direct synaptic connections with motor neurons. If the signal is strong enough, it can trigger an action potential in the motor neuron. When this action potential is transmitted to the thigh muscle (quadriceps femoris), the quadriceps femoris contracts and the biceps femoris relaxes, which can cause the knee jerk reflex. However, most reflexes are much more complex than the knee-jerk reflex and involve one or more interneurons in the spinal cord that connect sensory neurons to motor neurons.

Knee jerk test

The knee jerk reflex refers to the reaction of lightly tapping the knee tendon (ligament below the knee) when the knee is half-flexed and the calf is hanging freely, causing the quadriceps muscle to contract and the calf to kick forward rapidly. This reflex is a tendon reflex. Its receptors are muscle spindles that can sense mechanical stretch stimulation. Muscle spindles are general muscle fibers arranged in parallel, shaped like spindles, with both ends attached to the tendons (or extrafusal muscle fibers) and a connective tissue capsule on the outside. The capsule contains 2 to 12 specialized muscle fibers, the middle of which is filled with cell nuclei, has no transverse stripes, and can sense tensile stimulation. There are horizontal lines at both ends, which provide contraction force. When the tendon below the knee joint is tapped, the muscle is pulled quickly and the intrafusal muscle fibers contract, which stimulates the sensory part of the muscle spindle and releases nerve impulses, which are transmitted to the spinal cord (lumbar 2-4 segments) by the afferent nerve fibers in the femoral nerve. Tendon reflexes are monosynaptic reflexes in which the afferent nerve fiber contacts the cell body of the efferent neuron directly. The impulse is transmitted by the efferent fibers located in the femoral nerve to the motor endplate of the effector quadriceps muscle, causing the stretched muscle to contract and extend the calf forward. This reflex is usually affected by the higher parts of the central nervous system. The strength and speed of its reaction can reflect the functional state of the central nervous system. It is used clinically to examine diseases of the central nervous system.

Experimental procedures

Divide the subjects into groups of two and have them sit on a chair with one leg naturally resting on the other leg. The experimenter will use a rubber hammer or the inner edge of the palm to quickly tap the ligament below the knee of the subject's upper leg. Pay attention to the reaction of your calves.