H reflex

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The H-reflex is an electroneurographic method of neurophysiology . It is named after the physiologist Paul Hoffmann (1884–1962) from Freiburg im Breisgau . The H-reflex is triggered by an electrical stimulus from a peripheral mixed nerve and the response is measured as a muscle action potential .

The method is used for diagnostics in neurology in order to prove the structural integrity of the reflex arc. In humans, only a few nerves have the prerequisites for derivation. A classic is the triggering of an H-reflex through electrical stimulation of the tibial nerve in the hollow of the knee and measurement of the triggered muscle action potential of the soleus muscle of the calf.

Muscle reflexes can be triggered by stretching a muscle. Here are muscle spindles energized, the electrical discharges afferent fibers to the somata of motoneurons send in the spinal cord that supply the stretched muscle. The motor neurons are excited and in turn send discharges via their efferent axons back to the muscle, which then contracts. This closes the reflex arc.

Since a standardized stretching of a muscle is difficult to achieve in routine diagnostics, instead of muscle stretching, the electrical stimulation of the afferent fibers to the motor neuron is used for the H-reflex. However, since motor and sensory fibers in a peripheral nerve are always mixed with one another, selective excitation of the afferents does not initially seem possible. The electrical stimulus triggers excitation in both the afferent and the efferent axons.

Irrespective of the function of the respective axon, this excitation spreads in both the antidromic and the orthodromic direction. This means that in a motor axon the excitation wave runs on the one hand in the "normal" direction ( orthodromic ) to the muscle and triggers a first contraction there; on the other hand, the excitation runs antidromic (backwards) to the soma of the motor neuron in the spinal cord and triggers a discharge of the soma membrane out. In the sensitive nerve fibers, antidromic excitation will run back to the muscle spindles, but will not trigger any relevant effect there.

The orthodromic excitation runs to the spinal cord and is switched to the motor neuron via a direct synapse ( monosynaptic ), which then sends a forwarded action potential to the muscle via the efferent axon, which can be measured as a change in tension in the muscle. However, since the motor neuron or axon has already been excited at the same time by the efferent, antidromic excitation, the afferent excitation hits a refractory membrane and cannot be transmitted further. A reflex cannot then be measured. This means that a reflex response can only be triggered if it is possible to stimulate the afferent fibers at least partially selectively. A special feature of some nerves can now be used precisely for this purpose. Afferent fibers are thicker than the motor axons in some nerves. Since, due to the physical properties ( nerve conduction velocity ), the thick axons can be excited with lower stimuli in the case of electrical stimulation, an area can be reached in which predominantly afferent fibers are stimulated by slowly increasing the stimulus intensity. The H-reflex can be derived from this area.

The measurement of the H-reflex is indicated especially for the diagnosis of the proximal parts of the peripheral nerves. The H-reflex especially of the tibial nerve is examined for cone- cauda equina syndrome , root compression syndromes , damage to the leg nerve plexus , sciatic nerve lesions or Guillain-Barré polyradiculitis .

literature

  • P. Hoffmann: Contributions to the knowledge of human reflexes with special consideration of electrical phenomena. In: Arch Anat Physiol. 1, 1910, pp. 223-246.
  • P. Hoffmann: About the relationship between tendon reflexes and voluntary movement to tone. In: Z Biol. 68, 1918, pp. 351-370.
  • Paul Hoffmann: Investigations into the self-reflexes (tendon reflexes) of human muscles. Julius Springer, Berlin 1922.
  • Katja Müller: Investigation of the excitability of spinal motor neurons during propofol mononesthesia: H-reflex and H-reflex recovery (interstimulus interval-dependent recovery) . Dissertation. Humboldt-Univ., Berlin 2006, DNB 978794605 .