Patellar Reflex Physiology

Lab Report
3 pages (750 words)


The aim of this experiment was to familiarize students with the techniques of writing a lab report and to understand how the patellar reflex helps in preventing the stretching of quadriceps. Stretch reflexes are defined as special muscle reflexes that prevent the muscles from increasing in length, which may cause damage to the muscles fibers. Stretch reflex occurs when the muscle spindle receptor stretches followed by a series of events. In the process, the motor neuron is activated followed by the contraction of stretched muscles, as well as the supporting muscles, which is caused by efferent impulses of alpha motor neurons. Finally, inhibition of the antagonist muscles occurs, which is caused by the synapse of afferent fibers with the interneurons (green) (Surhone, Tennoe and Henssonow, 2010).

In stretch reflex, the muscles are attached to the tendons that hold them to the bone. Between the tendon and the muscle, there is a lower motor neuron and a muscle spindle that is highly sensitive to stretch. The lower motor neurons cause contraction of the muscle, which releases the upper motor neurons, as well as other parts of the CNS in order to perform other significant functions. According to Meriab (2003), stretch reflexes involve a motor activity and a single synapse on the same side of the body, which means that they are both monosynaptic, as well as ipsilateral. Example of a stretch reflex is the patellar reflex, also known as the Knee-Jerk reflex, which is a spinal reflex i.e. the neural circuit goes to the spinal cord and not the brain. Patellar reflex helps to prevent quadriceps from stretching (Surhone, Tennoe and Henssonow, 2010).

Conduction velocity refers to a test done to find out how fast electrical signals can travel through a nerve in the body. It is done by placing surface electrodes over the nerves, on the skin, at different locations, where each patch produces a mild electrical impulse that stimulates the nerve. The results of the electrical activities of the nerve are recorded, and the distance between the electrodes are used to establish the speed of the nerve signals (Jensen, 2003). This test is done so as to detect the damage or destruction of the nerve, which may also be used to determine diseases of the muscle or nerve such as myopathy and myasthenia gravis among others. Therefore, patellar reflex is performed in order detect any damage in the nerve; for instance, slow patellar reflex response is a sign of defect in nerve conduction (Meriab, 2003).


For the procedures used in this experiment, refer to Lab 11, activity 3, in the Anatomy and physiology Lab Manual.


Meriab (2003) argues that, Jendrassik maneuver heightens the patellar reflex by countering the normal decreasing inhibitory brainstem inputs to reflex arc interneurons. However, from the graph above, Jendrassik maneuver led to a decrease in patellar reflex, which is a sign of defect in nerve conduction. According to Jensen (2003), magnitude of the patellar reflex is higher; when the contractile tone in the extensors muscle is high. This is due to the presence of certain conditions like mental excitement that increases the tone of the muscles, which raise the extent of the patellar reflex. An increase in mental excitement leads to an increase in the velocity of the patellar reflex as shown in the graph above. Consequently, a decrease in mental excitement such as being in a restful state or sleep leads to a decrease in the patellar reflex. Moreover, studies have shown that fatigue leads to a decrease in the patellar reflex.

In summary, patellar reflex (Knee-Jerk reflex) is a spinal reflex in which the neural circuit goes to the spinal cord and not the brain. It helps to prevent quadriceps femoral nerves from stretching; hence, its biological purpose is to keep people from hurting themselves. Conduction velocity is done to detect any damage or destruction of the nerve, which may be used to determine diseases of the muscle or nerve. Therefore, the experiment helped students understand how patellar reflex helps in preventing the stretching of quadriceps.


Jensen, M. (2003). GC1135 Human Anatomy & Physiology. Human Reflex lab.  Retrieved April           24, 2012, from

Meriab, E. N. (2003). Human Anatomy and physiology, 6th Edition. San Francisco:          Benjamin/cummings Publishing Company, Inc.

Surhone, L., Tennoe, M. and Henssonow, S. (2010). Patellar Reflex. Saarbrücken: VDM Verlag            Dr. Mueller AG & Company Kg.


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