BY: Syed Irshad Murtaza Trainee Technologist Neurophysiology DeptThe Aga Khan University Karachi Date: 22-08-2012
The H-reflex is the electrical equivalent of the monosynaptic stretch reflex and is normally obtained in only a few muscles. The H (Hoffmanns) reflex, named after German neurologist Johann Hoffmann who first evoked the response in 1918.
The H-reflex (or Hoffmanns reflex) is a reflectory reaction of muscles after electrical stimulation of type Ia sensory fibers (Primary Afferent Fibers which constantly monitor how fast a muscle stretch changes) in their innervating nerves. The H- wave, is the expression of a monosynaptic reflex, which runs in afferents from the muscle and back again through efferents of the same muscle. OR
It is elicited by selectively stimulating the sensory Ia fibres (Primary Afferent Fibers which constantly monitor how fast a muscle stretch changes) of the posterior tibial or median nerve.
The stimulus travels along the Ia fibers, through the dorsal root ganglion, and is transmitted across the central synapse to the anterior horn cell which fires it down along the alpha (large lower motor neurons of the brainstem and spinal cord) motor axon to the muscle.
The H-Reflex which is monosynaptic reflex consistently obtained in normal adults only, by stimulating the tibial nerve sub-maximally, generally in the popliteal fossa, while recording from the gastrocnemius-soleus muscle, similar to the clinically elicited Achilles reflex. The H.Reflex can also be recorded in median nerve recording from the FCR muscle and stimulating in elbow, and from femoral nerve recording from quadriceps muscles (VM, RF, VL, VI)
To record the H-Reflex, G1 (active recording electrode) is placed 2-3 fingerbreadths distal to the soleus over the two bellies of the gastrocnemius muscle and G2 (Reference Electrode) is placed over the Achilles tendon (usually 14cm distal to G1). Ground electrode is generally placed between G1 and Stimulating electrode (stimulator cathode)