The inner ear contains specialized inertial receptors that activate upon movement. They include the 3 paired semi-
Electrical signals derived from the activation of these various receptors play an important role in the overall balance network which integrates diverse signals from the somatosensory receptors located in the bones, muscle and skin, the cerebellum, visual signals from the eyes and higher cortical signals traveling down from specific regions of the brain associated with balance.
In addition to providing sensory clues about the direction of gravity, the position of the body in relation to the environment, the relative positions of individual parts of the body (e.g. head, arms, legs) and information concerning self movement versus movement of the environment, the so called afferent limb of the balance network, the inner ear receptors trigger descending reflexes that cause rapid adjustments to the body in response to changes these afferent signals. The most well studied of these reflexes is the vestibular ocular reflex (VOR), the vestibulo-
The balance network as we have just defined it is a self regulating highly adaptable integrative system. This means that in many cases injury, illness or dysregulation of the network typically results in compensation and adaptation with return to near normal function. However, the system can undergo maladaptive compensation resulting in profound effects of postural balance and ambulation.
There are a growing number of diagnostic tests designed to evaluate the function of thee inner ear. A fairly recent test is the Vestibular Evoked Myogenic Potential test
or VEMP. The VEMP uses an auditory stimulus that is believed to activate primarily otolith receptors in the inner ear. Research suggests that lower frequency tones (below 1000 hz) can activate otolith organs in the ear and the resulting muscle contraction (the myogenic potential) can be recorded in the neck muscles, specifically the (SCM), for the vestibule-
The muscle action potential recorded from the cervical musculature (SCM) is known as a cervical VEMP or cVEMP. The muscle action potential recorded from the inferior oblique muscle is known as the ocular VEMP of oVEMP.
The cVEMP is believed to be generated by preferential activation of the saccule otolith organ and is recorded from the SCM muscle on the same side as the stimulation. The oVEMP is believed to be generated by preferential activation of the utricle otolith organ and is recorded on the face below the eye contralateral to the side of stimulation. That is to say that stimulating the right ear with a tone that selectively activates the right saccule will send a polysynaptic signal that can be recorded at the right SCM in the neck. Stimulating the same ear (right) with a tone which more effectively stimulates the utricle, will send a polysynaptic signal that can be recorded on the face interior to the left eye.
Character of tones and preferential activation of utricle versus saccule organs.
A number of different types of auditory stimuli have been used to evoke muscle action potentials through vestibular stimulation. Clicks and tone bursts of various frequency ranges using both air conducted and bone conducted sounds have been used to elicit recordable VEMPs. Air conducted tones must be loud and may not be tolerated well by the patient. There is even the suggestion that the loud air conducted tone bursts may cause transient (and clinically insignificant) hearing loss.
Acoustic stimuli used to elicit VEMP were found to have an adverse effect on the cochlear function. A clinically relevant hearing loss was not found in our study in healthy adults. Subjective auditory symptoms were reversible within 24 hours.
Effects of acoustic stimuli used for vestibular evoked myogenic potential studies
on the cochlear function. Krause E(1), Mayerhofer A, Gürkov R, Drexl M, Braun T
Otol Neurotol. 2013 Sep;34(7):1186-
Bone conducted sounds may evoke otolith derived myogenic potentials at substantially lower decibel levels making this form of stimulation the preferred method to activate otolith organs.
“Sound was characterized by a band-
A utricular origin of frequency tuning to low-
Todd NP, Rosengren SM, Colebatch JG.
Neurosci Lett. 2009 Feb 27;451(3):175-
“Measurements were made using two separate pathways arising from the vestibular apparatus: to the neck using vestibular
evoked myogenic potentials (VEMPs), and to the eyes using ocular vestibular evoked myogenic potentials (OVEMPs). For both sound and vibration the two response pathways produced different tuning to pulse trains. The vestibulo-
pathway was characterised by a band-
contribute to the low-
Todd NP et al.
Neurosci Lett. 2009 Jul 10;458(1):43-