The reflex examination is important for several reasons. Reflex changes may be the earliest and most subtle indication of a disturbance in neurologic function. The testing of reflexes is the most objective part of the neurologic examination. Reflexes are under voluntary control to a lesser extent than most other parts of the neurologic examination, and reflex abnormalities are difficult to simulate. They are not as dependent on the attention, cooperation, or intelligence of the patient and can be evaluated in patients who cannot or will not cooperate with other parts of the examination. In such circumstances, the integrity of the motor and sensory systems can sometimes be appraised more adequately by the reflex examination than by other means. Although the reflex examination is an essential component, it is only one part of the neurologic examination and must be evaluated in the context of the other findings.
A reflex is an involuntary response to a sensory stimulus. Afferent impulses arising in a sensory organ produce a response in an effector organ. There are segmental and suprasegmental components. The segmental component is a local reflex center in the spinal cord or brainstem and its afferent and efferent connections. The suprasegmental component is made up of the descending central pathways that control, modulate, and regulate the segmental activity. Disease of the suprasegmental pathways may increase the activity of some reflexes, decrease the activity of others, and cause reflexes to appear that are not normally seen. A reflex response may be motor, sensory, or autonomic.
The stimulus is received by the receptor, which may be a sensory ending in the skin, mucous membranes, muscle, tendon, or periosteum, or, in special types of reflexes, in the retina, cochlea, vestibular apparatus, olfactory mucosa, gustatory bulbs, or viscera. Receptor stimulation initiates an impulse that travels along the afferent pathway to the central nervous system (CNS), where there is a synapse in a reflex center that activates the cell body of the efferent neuron. The efferent neuron transmits the impulse to the effector: the cell, muscle, gland, or blood vessel that then responds. A disturbance in function of part of the reflex arc—the receptor, afferent limb, reflex center, efferent limb, or effector apparatus—will disrupt the reflex arc, causing a decrease or loss of the reflex.
Most reflexes investigated clinically are more complex than the primitive reflex response just described. All parts of the nervous system are intimately connected; it is rare for one part to react without affecting or being affected by other parts. Almost immediately on entering the CNS, the afferent fiber sends collaterals to cells at higher and lower levels on the same and opposite side. Activation of an agonist muscle group is accompanied by inhibition of the antagonist muscle group (Sherrington’s law of reciprocal innervation); when the extensors of a limb are contracted, the flexors are relaxed. Association pathways may carry the impulse to the cerebral cortex for either reflex or voluntary modification of the response. Complex reflexes involve connections between various segments on the same and opposite sides of the spinal cord, brainstem, and brain. The more complex the reflex, the greater the number of associated neurons and mechanisms involved. Stronger stimuli cause the excitation of a greater number of neurons: the phenomenon of irradiation.
Reflex activity is essential to normal functioning. Nociceptive reflexes help avoid injurious stimuli. Reflex activity is important in maintaining the body in its daily environment, in sustaining an upright position, in standing and walking, and in moving the extremities. It is an integral part of the response to visual, gustatory, auditory, and vestibular stimulation, and it is important in visceral functions.
Reflexes have been named in various ways: according to the site of elicitation, the body part stimulated, the muscles involved, the part of the body that responds, the ensuing movements, the joint acted on, or the nerve involved. Many carry the names of one or more individuals who are said to have first described them. Hundreds of reflexes have been identified. Because many are not clinically important and it is impractical to test all the reflexes routinely, only those more important for clinical diagnosis will be described. The majority of these are muscle responses. Reflex abnormalities due to disease involving the descending motor pathways are often clinically referred to as upper motor neuron, corticospinal, or pyramidal signs, but the abnormalities likely result from dysfunction of related motor pathways rather than the corticospinal tract proper (see Chapter 25).