Late Segmental Responses
Late segmental potentials are electrodiagnostically-elicited responses in muscle after stimulation of motor nerves. They have been termed “late-potentials” because they take substantially longer to appear than the direct responses to stimulation of motor nerves. There are two distinct types of late responses, the H-reflex and the F-response.
The first type of “late response” is called the H-reflex, in honor of Johann Hoffmann, who first described this response in 1918. It is basically the electrophysiologic equivalent of the muscle stretch reflex. The H-reflex is most commonly tested by electrical stimulation of the tibial nerve, with recordings from the soleus muscle.
Electrical stimulation depolarizes the largest, most heavily myelinated nerve fibers at a lower stimulus intensity than is required to activate other smaller nerve fibers. Since the largest nerve fibers in a peripheral nerve are those arising from muscle stretch receptors, there should be a stimulus intensity that activates muscle stretch afferent nerve fibers without directly activating many motor nerve axons. When muscle stretch sensory fibers are stimulated, a monosynaptic reflex contraction will be elicited in the muscle. Theoretically, this reflex can be elicited from virtually any muscle but is very difficult to elicit. However, in clinical practice, only the triceps surae muscle produces H-reflexes that are reliable and clinically useful. This test evaluates the integrity of the reflex arc from the tibial nerve through the spinal cord and back to the triceps surae. Damage to any portion of the reflex arc, including the sciatic nerve or the S1 sensory or motor nerve root, can result in loss or slowing of the reflex response. Additionally, the amplitude of response (expressed as a ratio of the reflex response to the maximum direct motor response – the so-called H/Mmax) is also reliable enough to be of diagnostic value. Since the H-reflex is mediated primarily over the S1 nerve root (just like the ankle jerk reflex), it is a sensitive test for S1 radiculopathy. However, once the reflex arc has been damaged, it often does not return to normal. H-reflex is precisely quantified (both in terms of latency and amplitude) and, therefore, may be a more useful index to follow with time or treatment of sciatic nerve and S1 nerve root disfunctions.
The second type of late potential is the F-response. While the CMAP usually appears within several milliseconds (depending on how close the stimulus point is to the muscle), depending on the stimulus site, another response can be normally recorded in the muscle approximately 25-55 milliseconds later. Since this response was first recorded in foot muscles, it came to be known as the F-response. Over time it was found that this late response was not a reflex in the usual definition. The electrical impulse is trasmitted proximally along the motor axon from the site of initiation of the action potential. When this antidromic depolarization reaches the motor neurons in the spinal cord, a percentage of these motor neurons are activated a second time. This results in an electrical signal being conducted in the orthodromic direction from the spinal cord to the muscles innervated by the nerve. This second, later activation produces a small muscle contraction, the F-response. Because the number of motor neurons that are re-activated is somewhat unpredictable, the amplitude of this signal is variable and, therefore, amplitude measurements are usually not used. However, delay in the F-response indicates some slowing of conduction of the motor axon. Since the F-response traverses more proximal portions of the motor axons, it may be useful in the investigation of proximal nerve pathology such as root pathology seen in radiculopathy, AIDP or CIDP. The F–response is also very helpful in the confirmation of demyelinative peripheral neuropathies.
The blink reflex (BR) is a generalised phenomenon in mammals. Its teleological protective eye function is perhaps the reason why the BR can be provoked by a multitude of stimuli. The electrically elicited BR is an exteroceptive-nociceptive reflex recorded on the orbicularis oculi muscle and formed by three components: the two principal ones, R1 and R2, of well-known characteristics, and a third, R3, of increasing interest. The trigeminal afferent limb reaches the facial efferent one by means of a long and quite complex central pathway located at the brainstem bulbopontine level. The suprasegmental influences upon the reflex, coming mainly from the cerebral cortex and basal ganglia, as well as the impairment caused by their damage, are very important. For this reason BR study can provide crucial information about the afferent and efferent pathways and are excellent physiological tools for assessment of cranial nerve nuclei and the functional integrity of suprasegmental structures. Even the techniques of habituation and suppression-recovery of the BR are useful to measure its excitability and serve in practice for the pathophysiological study of numerous diseases. Several factors can influence the reflex, such as sleep. The strategic position of the neural structures of the BR, in an area involved in the gating of the various sensory-motor systems and the relative ease to its evaluation with common methodology used in clinical neurophysiology, makes the BR an essential tool for the diagnosis and pathophysiological insight into an important number of human neurological disorders.
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