Hnéviðbragðið

Hnéviðbragðið, commonly known as the knee-jerk reflex, is a basic physiological response that occurs when the patellar tendon is tapped. This stimulus causes a rapid and involuntary extension of the leg at the knee. It is a monosynaptic reflex, meaning it involves only one synapse between the sensory neuron and the motor neuron.

When the patellar tendon is struck with a reflex hammer, it stretches the quadriceps femoris muscle. Sensory receptors within this muscle, called muscle spindles, detect the stretch. These receptors send a signal along a sensory neuron to the spinal cord. In the spinal cord, the sensory neuron directly synapses with a motor neuron. The motor neuron then sends a signal back to the quadriceps femoris muscle, causing it to contract and extend the leg.

The knee-jerk reflex is a vital diagnostic tool for neurologists. An exaggerated or absent reflex can indicate problems with the nervous system, such as nerve damage or spinal cord injury. The reflex is a classic example of a stretch reflex and demonstrates the basic principles of reflex arcs in the nervous system. It is involuntary, meaning it cannot be consciously controlled or suppressed. This rapid response is crucial for maintaining posture and balance, preventing falls when the body is unexpectedly perturbed.