H-Reflex

The H- reflex is an electro euro graphical method of neurophysiology. It is named after the physiologist Paul Hoffmann (1884-1962) in Freiburg im Breisgau. The H- reflex is triggered by an electrical stimulation of a peripheral mixed nerves and the response as muscle action potential measured.

The method is used for diagnosis in neurology in order to demonstrate the structural integrity of the proprioceptive reflex arc. In humans, the conditions for the derivation are given only a few nerves. Classic is the triggering of an H- reflex by electrical stimulation of the tibial nerve in the popliteal fossa and measurement of the triggered muscle action potential of the soleus muscle of the calf.

Tendon reflexes can be triggered by straining a muscle. Moreover, muscle spindles are excited that send electrical discharges via afferent fibers to the somata of motor neurons in the spinal cord that supply the stretched muscle. The motor neurons are excited and in turn send efferent discharges over their axons to the muscle back, the contracted thereupon. Thus, the reflex arc is closed.

Since a standardized stretching of a muscle is difficult to achieve in routine diagnostics, using the H - reflex instead of muscle stretching the electrical stimulation of the afferent fibers to the motor neuron. But as always proceed in a mixed peripheral nerves with each motor and sensory fibers, a selective excitation of the afferents initially seems impossible. The electrical stimulus triggers both the afferent and efferent axons from the also a thrill. This excitation propagates independently of the function of each axon from both antidromic and orthodromic in direction. That is, in a motor axon, the excitation wave one hand runs in "normal" direction ( " orthodromically " ) to the muscle where it triggers a first contraction, on the other hand runs the excitation " antidromic " (backward) to the soma of the motor neuron in the spinal cord where it triggers a discharge from the Somamembran. In the sensory nerve fibers is run according antidromic excitation back to the muscle spindles, but cause no relevant effect there. However, the orthodromic excitation passes to the spinal cord and is switched via a direct synapse ( " monosynaptic " ) on the motor neuron, which then a continuous -led action potential via the efferent axon sends to the muscle, which can be measured as muscle contraction. However, since the motor neuron or axon has already been energized simultaneously by the efferent, antidromic excitation, meets the afferent excitation on a refractory membrane and can not be propagated. A reflex is then not measurable. That is, only if it is possible at least in part to selectively excite the afferent fibers, triggering a reflex response is possible. A special feature of some nerves can now be exploited to do just that. Afferent fibers are thicker than the motor axons in some nerves. Since the thick axons with lower currents can be excited in an electrical irritation due to the physical properties ( nerve ), an area can be achieved by slowly increasing the stimulus intensity, are excited in the predominantly afferent fibers. In this area can be derived of the H- reflex.

The measurement of the H- reflex is particularly indicated for the diagnosis of proximal portions of the peripheral nerves. The H- reflex of the tibial nerve is specifically studied in cone - cauda syndrome, nerve root compression syndromes, damage to the leg nerves braid Ischiadikusläsionen or the polyradiculitis Guillain-Barré.