Patellar Reflex, also called knee-jerk, is a stretch reflex associated with quadriceps femoris muscle stretching.
how it works _
The stretch is created by a blow upon the patellar tendon (positioned just below the Patella). This blow, usually performed with a specific tendon hammer, activates the muscle spindle in the quadriceps femoris muscle.
Muscle spindle is a particular type of sensory receptor, normally embedded in muscle fibers, able to detect changes in the length of the muscle itself. Once “activated” by the external blow, this receptor sends a signal to the spinal cord. Instead of involving higher nervous centres (it would take too long), at the level of the spinal cord an alpha-motor neuron is immediately activated.
The alpha-motor neuron conducts an efferent impulse directly back to the quadriceps femoris muscle, leading to its contraction. At the same time, an inhibitory interneuron relaxes the hamstring muscle, which is the quadricep’s antagonistic muscle.
The result of such coordinated contraction-relaxation, causes the “kick movement” of the leg. In normal health conditions, the leg extends once and then comes back to rest. It only takes about 50 milliseconds between the tap and the start of the leg kick.
why it is useful _
Patellar Reflex is a proprioceptive reflex which helps keeping posture and balance. The fact that everything “happens” at the level of the spinal cord, without involving higher nervous centres, allows for instance to keep balance without effort (actually, one does not have to focus on keeping an upright position). Energies are saved for more complex activities. Moreover, Patellar Reflex helps avoiding strong muscle contractions which could tear the tendon.
clinical interest _
As said, there is no interneuron in the pathway leading to contraction of the quadriceps muscle. Patellar Reflex can be used, for example, to check the conditions of the connections between the spinal cord and the muscles.
The absence or decrease of the Patellar Reflex is known as Westphal’s sign. On the other hand, multiple oscillation of the leg following the blow may be a symptom of cerebellar diseases.
source: contents taken from Wikipedia’s page