Overactive
Facial Nerves
Contribute to Tinnitus Intensity
by Barry Keate
A
new study from the University of Michigan, Kresge
Hearing Research Institute, has found when the cochlea
is damaged by loud noise or other causes, sensory
nerves in the face and neck become overactive and
contribute to the noise we know as tinnitus.
The direct implication of this research is that present-day
therapies, such as acupuncture, when applied to the
nerves in the face and neck, should have a beneficial
effect on tinnitus loudness.
Susan Shore, PhD and her team at the Kresge Hearing
Institute conducted the study on guinea pigs. It was
published in the European Journal of Neuroscience.
They conducted the research on 18 healthy female guinea
pigs. Six were control animals and 12 were hearing
impaired caused by exposure to loud broadband noise.
The researchers studied responses in the dorsal cochlear
nucleus (DCN). The DCN is a neuron lobe that is the
first relay station in the auditory system for signals
from the cochlea on their way to the auditory cortex.
It also contains multisensory neurons that receive
signals from the trigeminal nerve system that contains
sensory information from the face and neck. This sensory
information includes touch, vibration, skin temperature
and pain.
The dorsal cochlear nucleus is bi-modal in this sense:
it integrates nerve signals from the auditory nerve
as well as from the sensory nerves in the face and
neck and sends them to the brain.
In the study, Shore and her team measured the pattern
of activity of neurons in the brains of normal and
deafened guinea pigs. They used a 16-electrode array
to measure signals from the trigeminal (sensory) nerve
and multisensory neurons in the dorsal cochlear nucleus.
When they compared results in the two groups, they
found clear differences in trigeminal nerve activity.
They knew from earlier research that some neurons
in the cochlear nucleus became hyperactive after hearing
damage, and this hyperactivity has been linked to
tinnitus in animals.This study showed that only those
neurons that received sensory input from the trigeminal
nerve become hyperactive.
“In
this study, we showed that when there is hearing loss,
other parts of the brain that normally convey signals
to the cochlear nucleus have an enhanced effect”
says Shore. “When you take one source of excitation
away, another source comes in to make up for it.”
The resulting nerve firings in the cochlear nucleus
create the din of tinnitus.
This is analogous to those who have lost their vision
and auditory signals become more enhanced resulting
in acute hearing. Here the auditory component is damaged
and sensory information is enhanced resulting in hyperactivity
and increased tinnitus.
Acupuncture,
the ancient Chinese system of balancing the life forces
in the body, may be very helpful for calming the neuronal
overload in the dorsal cochlear nucleus.
 Many
individuals with temporo-mandibular joint dysfunction
(TMJ), a condition that causes frequent pain in the
jaw, experience tinnitus. Shore’s research could
lead to a better understanding of this link. In people
with TMJ, the sensory system is disrupted and inflamed.
Shore says that it’s possible that in this situation,
as in hearing loss, sensory neurons stir excessive
neuronal activity in the cochlear nucleus.
Author’s note:
It is my understanding that the condition of TMJ causes
stress in the ligament that connects the temporo-mandibular
joint with the inner ear. Treatment for this condition
currently emphasizes relaxation techniques designed
to reduce the inflammation, exactly what Dr. Shore
is describing. You can read a full discussion of TMJ
in our tinnitus library.
References:
-
SE
Shore, S Koehler, M Oldakowski, LF Hughes, S Syed.
Dorsal cochlear nucleus responses to somatosensory
stimulation are enhanced after noise-induced hearing
loss. European Journal of Neuroscience, Vol. 27,
pp. 155-168, 2008.
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