From a surface level, reflexes do not appear very complicated. Most people have been to a doctor’s office and had their knee hit with a rubber tool in hopes that the leg will slightly kick. This action is called a reflex. Yet how does that slight force applied to the knee cause a kick without a conscious command from the brain? The topic of reflexes becomes even more complicated when examining cranial nerve, human infant, and post—spinal cord injury reflexes.
Simply defined, a reflex movement is an involuntary, rapid response to a given stimulus. The purpose of these quick, automatic movements is to avoid pain or injury. In the example of the knee-jerk reflex, a sensory nerve quickly transmits information about the force on the patellar ligament to the spinal cord. Nerves within the spinal cord relay a “contract now” motor signal to the quadriceps. The resulting “kick” is a way to relieve tension on the patellar tendon and prevent injury of the connected muscles or knee structure.
Anatomy and Physiology
It is useful to separate reflexes into monosynaptic and polysynaptic categories. The monosynaptic category is most commonly discussed through the lens of the knee-jerk reflex. Others in this category include the biceps, triceps, brachioradialis, and Achilles reflexes. To recap, a stretch sensation is detected by a sensory neuron in the muscle body. This signal is relayed to the spinal cord and a single synapse exists between the sensory fibers and the motor unit neurons. From this point, a signal to contract is relayed to a muscle in the hopes of reducing the stretch.
Several important concepts exist in this monosynaptic model. First, how does the sensory neuron “detect” stretch? The answer can be found by examining the neuronal plasma membrane. Once there is an actual physical stretch, special chemical channels are opened in the neuronal cell membrane and the action potential signal of “stretch” is relayed to the spinal cord by traveling the length of the neuron’s axon. Second, how does this reflex occur so quickly? This is because sensory and motor neurons are not all created equally. The fastest conducting neurons (Ia sensory and α-motor neurons) are utilized in these circuits in order to reduce the chance of injury from excessive muscle stretch. Lastly, how does the reflex reduce the muscle tone of opposing muscle groups, like the hamstrings in the leg? Each monosynaptic sensory signal not only activates specific motor neurons, but also simultaneously antagonizes/relaxes other motor neurons. This allows for a specific action based on a single sensory input.
The neurological examination is a systematic method used by health care providers and particularly by neurologists to look for abnormalities or lesions in the nervous system. The neurological examination contains several broad rubrics: (1) the mental status examination, (2) the cranial nerve examination, (3) the reflex and motor examination, (4) the coordination and gait examination, and (5) the sensory examination. This examination provides a robust method to test for nervous system function. This sidebar focuses on why a health care provider tests a patient’s reflexes.
Simply defined, a reflex movement is an involuntary, rapid response to a given stimulus. The purpose of these quick, automatic movements is to avoid pain or injury. In the example of the knee-jerk reflex, a sensory nerve quickly transmits information about the force on the patellar ligament (a ligament attached to the kneecap) to the spinal cord. Nerves within the spinal cord relay a “contract now” motor signal to the quadriceps. The resulting “kick” is a way to relieve tension on the patellar tendon and prevent injury to the connected muscles or knee structure.
Reflex hammer (can be purchased at Amazon.com) to test all four extremities
Something to sit on that will allow the person’s legs to dangle
Patellar reflex (knee-jerk reflex): Have the person sit on the edge of a table or desk so that their legs are dangling. Using the rubber end of the reflex hammer, gently tap the right quadriceps tendon that is located just below the right kneecap. The person’s right leg should automatically jerk and then swing back and forth one or two times. If there is no reaction, tap just to the left or right of the first tap. If no reflex response is observed, the person might be focused on the knee so that no reflex takes place. Ask the person to interlock their hands and focus on pulling them apart while the rubber hammer lightly hits the knee. Repeat this test with the left leg and compare the responses.
Babinski reflex (plantar reflex): Have the person remove their shoes and socks, and then sit on the edge of a table or desk so that their legs are dangling. Using the rubber end of the reflex hammer, drag the hammer from the person’s heel to the toes. The normal response is for the toes to move down in contraction. The great toe moving up is an abnormal response called the Babinski sign.
Biceps-jerk reflex: Ask the person to relax their arm and hold their elbow at a 90-degree angle. Gently tap the biceps tendon (front of the upper arm). Repeat this test with the other arm and compare the responses.
Nicholas Breitnauer and Audrey S. Yee
With an understanding of the monosynaptic reflex concept, polysynaptic reflexes accomplish an end result, such as movement, just with more intermediate neuronal synapses. The central pattern generator (CPG) is one example. A conscious decision is made to walk, at which point the CPG is initiated. Parts of the complex reflex exist in the various areas of the brain that are involved in motion as well as in the spinal cord. All work in symphony in order to coordinate the various movements involved in walking so a person can resume thinking of other matters.
Another example of a polysynaptic reflex is called the “flexor withdrawal.” Imagine touching a hot stove or stepping on a sharp tack. Almost without thinking, the hand is quickly withdrawn and the foot is lifted up. Pain fibers (Ic) sense these types of noxious stimuli—though not as fast as stretch receptors—and synapse with interneurons in the spinal cord. These neurons amplify or mute pain before they synapse onto a motor fiber. Interestingly, the story gets a little more complicated than simply flexing to withdraw a hand or foot. The other limb simultaneously receives a signal from the contralateral spinal cord neurons to extend certain muscle groups. This is important, for example, to maintain balance if one foot is lifted up into the air.
Any discussion of reflexes would be incomplete without mentioning those unique to human infants. Babies need certain “preprogrammed” responses to ensure survival early in life. Examples include those useful in eating such as the “suck” and “rooting” reflexes: an object placed in a baby’s mouth immediately initiates sucking, an object lightly touching an infant’s cheek causes the baby to turn toward that side. Interestingly, these reflexes and many more begin to fade before the baby’s first birthday. An important explanation arises from the changes that occur in the central nervous system during this time period. A baby continues to myelinate neurons—speeding conduction—and making more mature neuronal connections as learning takes place. These two factors lead to a slow dampening and eventual cessation of these reflexes.
Ultimately, the example of human infant reflexes drives home a final point: the conscious human brain is able to regulate reflexes. The antagonizing neurochemical from the cortex can, for example, minimize or eliminate a knee-jerk reflex. Occasionally, patients will be so focused on the knee that no reflex takes place. They might then be asked to interlock their hands and focus on pulling them apart while the rubber hammer lightly hits their knee. The shift in focus from the knee to the hands will allow the spinal cord reflex to occur in the absence of the cortex’s inhibition.
A change in the corneal blink reflex with prolonged contact usage is another example of the cortex’s effect on reflexes. The corneal reflex can be experienced when a hand or object comes close to or actually touches the eye. Almost without control, both eyelids blink forcefully and rapidly (approximately within 10 milliseconds). This reflex involves the sensory trigeminal nerve with a quick relay to both facial nerve nuclei in the brainstem. Interestingly, the reflex slowly diminishes with the constant touching of an eye during contact usage. A person will eventually be able to place and remove contacts without a blink. It is thought the conscious attention on this process reduces the reflex.
Reflexes in Disease and Injury
Knowledge of normal reflex patterns helps enlighten understanding of reflex patterns in disease/injury states. Spinal cord injuries are extraordinarily unfortunate, but teach a great deal about reflex concepts. Once a spinal cord is injured, any function governed below that site is affected. The vertebrae overlying the region divide the spinal cord’s anatomy into four general segments: cervical, thoracic, lumbar, and sacral. Imagine a spinal cord injury around the 12th thoracic to the first lumbar section or T12—L1. At this level, the area around the waist and below would be impaired from both a sensation and motor standpoint.
Spinal shock—a term used to describe loss of motor function and sensation with eventual return of reflexes—begins instantly. The first 24 hours of injury would leave the lower limbs hyporeflexive and hypotonic. Reflexes such as the knee-jerk would return during days one to three, but then become hyperreflexive over the next few weeks. Finally, the muscles would become tighter during the weeks to years after the injury leading to a “hypertonic” state. This hypertonicity can be explained by unregulated neuron regeneration at and below the injury.
The return and progressive increase of the reflexes below the spinal cord injury reinforce concepts from normal reflexes. Polysynaptic reflexes return first due to the simple reason that more neurons contribute to these complex actions. The hyperreflexive state exists in the absence of central nervous system regulation in a manner analogous to human infant reflexes. Additionally, the attempt of spinal cord neurons to reestablish a connection leads to more connections below the injury, leading to a stronger, more powerful circuit for the mono- and polysynaptic reflexes.
Injuries within the central nervous system (CNS) are identified and localized with an understanding of cranial nerve reflexes. Cranial nerve reflexes fall into the polysynaptic category due to interneurons that take sensory signals and transmit motor signals to bilateral sides. For instance, a bright penlight in one eye causes both pupils to constrict. If the light causes only the eye with the light to constrict, there is a problem with the motor neuron on the contralateral side. If there is no pupilary constriction when a penlight is shone, the problem likely exists with that ipsilateral optic nerve. This understanding can help identify problems with the nerves or with the brain itself.
In addition to injuries to the CNS, back injuries can be localized with an understanding of reflexes. Consider a person with significant lower back pain after lifting a heavy object. Is this a medical emergency? Upon close examination, it appears there is no Achilles reflex on the right side compared to the left. This subtle finding, taken with other evidence, might compel the doctor to order a magnetic resonance imaging (MRI) of the patient’s lower back. It would be feared that the heavy lifting caused an intervertebral disk to bulge outward into the spinal canal and push against the right-sided S1 nerve root. Surgery to repair the herniated disk is often warranted in these situations.
Reflexes and Prosthetics
Limb prosthetics is a field currently encountering rapid growth and improvement. The demand continues to grow for more responsive, advanced devices. Integrating reflex response into these devices is currently in the early stages of position adjustment. For example, consider picking up a can of soup. If the person’s grip is not strong enough, the can will begin to slip. A typical response would be to tighten the grip or potentially rapidly lower the arm in hopes of not letting the soup can completely slip away. Current prosthetic devices are designed to give rapid muscular feedback in order to replicate the subconscious reaction to maintain the grip.
Future applications of reflex understanding to artificial devices are myriad. Could a device eventually be able to sense temperature and lead to a “flexor-extensor” reflex? Or might that same device be able to respond to signals arising from the contralateral side, leading to a completion of the polysynaptic reflex?
See also: Blink Reflex; Neurological Examination
Alberstone, C. D., E. C. Benzel, I. M. Najm, & M. P. Steinmetz. (2009). Anatomic diagnosis of neurologic diagnosis. New York, NY: Thieme.
Costanzo, Linda S. (2011). Board review series: Physiology (5th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.