Reflexes

Biological Foundation

Reflexes occur via a reflex arc that is specific to each individual reflex. This arc is the complete neural circuit from stimulus to response for that particular reflex. The arc includes the receptor, the point on or in the body where the stimulus is received, followed by the afferent pathway, the incoming neural path the impulse takes to the spinal column. From there, the impulse is processed and sent back to the response site via the efferent pathway, the outgoing neural path, to the effector site where the response occurs.

Reflexes can be monosynaptic or polysynaptic. Monosynaptic reflexes involve only one neuronal synapse, the gap, or space, between neurons across which an electrical impulse passes with the help of neurotransmitters. Common stretch reflexes, like the patellar tendon reflex, are monosynaptic. This well-known knee jerk reflex is a common part of physical exams as the doctor tests the health of the spinal column or the central nervous system in general. In healthy individuals, a tap on the patellar tendon results in the lower leg jerking forward abruptly. A delayed or minimal response to this stimulus would indicate a need for more in-depth testing and could be indicative of a pathological condition. Examples of similar stretch reflexes can be found at the ankle (e.g., tap the Achilles tendon for a plantar flexion of the foot), the elbow (tap the triceps tendon with the lower arm hanging loosely for a contraction of the triceps muscles), or the biceps (tap the biceps for a contraction of the biceps muscles). These stretch reflexes are also examples of proprioceptive, deep, or somatic reflexes where the stimulus leads to a response via the skeletal muscles of the body. A systematic examination of these and other reflexes can be a useful medical tool as initial measures of the health of various aspects of the central nervous system. Most reflexes, however, are polysynaptic.

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