neuroanatomy

Vestibular Reflexesand Concussions
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Vestibular Reflexes and Concussions
Lauren Bachand
Professors Becker and Liau
April 30, 2014

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Introduction
In this paper I will be researching the vestibular system and the role it plays in
maintaining balance and postural alignment. I will first exam the vestibular system as a whole
and the major structures that make up the vestibular system. Next I will examine how these
structures and the vestibular system works and functions under normal conditions. I will then
address the two primary reflexes involved in balance, the vestibulo-ocular reflex (VOR) and the
vestibular spinal reflex (VSR). Finally, I will explore how these reflexes and their structures are
affected during and post-concussions. Specifically I will investigate the vestibular and
equilibrium complications that arise after a concussion, most commonly referred to as Post-
concussion Syndrome.
The Vestibular System
The vestibular system is defined as, “a proprioceptive, somato-sensory system which
mediates the special functions of posture maintenance, muscle tone, equilibrium, and the
coordination of head and eye movements” (Patestas &Gartner, 2006, p.318). It is responsible for
detecting angular acceleration (the rotation of the head), spatial orientation (the posture and
location of the head in relationship to the body and gravitational force), and interpreting sensory
input from the vestibular, visual and proprioceptive systems via the nervous system and
cerebellum. This sensory input then generates muscle control and motor responses to maintain
equilibrium. All of this is done by unconsciously taking sensory information from the: eyes,
inner ears, and vestibular spinal tract; then sending the information through the nervous system,

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which as a result generates motor responses to initiate posture, muscle tone, muscle control, and
vestibular reflexes for balance maintenance.
There are multiple structures that are involved in the vestibular system which contribute
to equilibrium maintenance; however the primary structures that I am going to focus on are the
vestibular apparatus and its components, and the vestibular nerve. Patestas and Gartner (2006)
refer and define the vestibular apparatus as a structure that “resides in the inner ear and is
composed of bony labyrinths and membranous labyrinths” (p. 319). As explained by Patestas
and Gartner (2006) the vestibular system’s primary function is detecting head motions and
orientation. Receptors in the vestibular system are responsible for picking up on sensory
information within the inner ear and relaying it to the vestibular nuclei and vestibular nerve.
The Bony Labyrinth of the vestibular apparatus is made up of three major components:
the cochlea, semicircular canals, and the vestibule. For the purpose of this paper I am going to
omit the cochlea since it plays no major or primary role in equilibrium maintenance and focus
only on the two balance-related components, the semicircular canals and the vestibule.
The semicircular canals are actually comprised of three distinct canals in the inner ear.
These include the anterior, posterior, and lateral canals. Each individual canal is positioned a
specific way and plays an independent role when it comes to interpreting head movements and
orientations. Each type of canal is part of a set, one residing in each ear, and works
simultaneously with the contralateral partner. For example, when the head rotates, receptors in
the semicircular canals in both ears respond to the stimuli. The receptors on one side are then
activated while the contralateral receptors are inhibited.

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When it comes to determining head movement and orientation, the semicircular canals
respond to three planes of rotational movement: pitch, yaw, and roll. Pitch refers to forward and
backwards tilting of the head and involves the anterior and posterior canals. Parestas and Gartner
(2006) describe this rotation as moving the head as if saying “yes”. Yaw refers to rotating the
head side to side and involves the lateral canals. This move is described by Parestas and Gartner
(2006) as moving the head as if saying “no”. Roll refers to tilting the head from one shoulder to
the other, and also involves the posterior and anterior canals.
The second structure, the vestibule is located at the end of the semicircular canals and
contains the utricle and saccule membrane sacs. These membrane sacs play a contributing role in
balance and make up the membranous labyrinth. The other structures that reside in the
membranous labyrinth are the semicircular ducts. All of the named structures of the membranous
labyrinth contain endolymph.
Inside the vestibular apparatus are vestibular receptors: hair cells and otoliths. They are
found in five different ampullares: three in the crista ampullaris that reside at the end of the
semicircular canals, and one in each of the utricles and saccules. The hair cells are made up of
numerous motile microvilli and one immotile kinocilium and are found immersed in endolymph.
As explained by Parestas and Gartner (2006) whenever the head moves the endolymph (a
gelatinous fluid) that surrounds the hair cells responds and shifts. This shift in the endolymph
mechanically moves the microvilli and kinocilium which is translated into electrical signals that
are then sent to the brainstem and cerebellum via the cranial vestibular nerve.
Otoliths are also receptors that are responsible for receiving sensory stimuli and relaying
sensory information to the cerebellum and nervous system. The otoliths, however, sit on the

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surface of the endolymph instead of residing inside the gelatinous matter. Like the hair cells,
they are pulled by gravity by the acceleration of the head. When they are pulled they stimulate
the hair cells.
The macula utriculia and macula sacculi are receptors whose primary functions are
described by Parestas and Gartner (2006) as determining “spatial orientation of the head in space
relative to gravity and linear acceleration or deceleration” (p. 323). The macula utriculia is
stimulated by linear acceleration and deceleration on the horizontal plane and the macula sacculi
is stimulated by linear acceleration and deceleration on the vertical plane.
The final structure of the vestibular system I am going to analyze is the vestibular never
with is portrayed by Parestas and Gartner (2006) as a group of bipolar neurons that branch out
from the cristate ampulluris, macula utriculia and macula sacculi. It is a division of the
vestibulocochlear cranial nerve and synapses from the hair cells to the brainstem, where the
vestibular nerve fibers enter the vestibular nuclei. The vestibular nuclei are then responsible for:
receiving sensory input from the semicircular canals and send information to the Medial
Longitudinal Fasciculus (MLF) and medial vestibulospinal tract, which are responslible for the
vestibular ocular reflex (VOR); receiving sensory input from the utricle and saccule and relaying
the input to motorneurons in order to make postural adjustments (the vestibulospinal reflex); and
receiving sensory input from the semicircular canals and projecting the information to the
reticular formation and cerebellum.
Normative Functioning

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Now I am going to exam how these structures function normally. I have already briefly
addressed how these structures function normally, but I will now address how they all work
collaboratively to maintain balance.
As I stated before, when the head is tilted, the gravitational forced cause the membranes
to slant and as a result, deflect hair cells. These hair cells are stimulated in one ear and inhibited
in the other side. The stimulated hair cells activate the vestibular nerve dendrites. The vestibular
nerve then releases sensory information to the MLF and the vestibulospinal tracts. This results in
reflex activity. These reflexes allow for balance and postural compensations by adjusting visual
focus and stimulating anti-gravitational extensor and flexor muscles in the limbs, neck, and torso.
The Vestibular Ocular Reflex and Vestibulospinal Reflex
This leads me to the next focus of my paper: the vestibular ocular reflex and the
vestibulospinal reflex. Both reflexes play a crucial role when it comes to perceiving and
maintaining balance. As explained in Denham’s lecture (2014), both reflexes are autonomic and
therefore disruption can lead to dizziness, instability, and imbalance. For the purpose of the
purpose of this paper I will examine the role and basic normative functions of these reflexes so
that they can be applied to the evaluation of balance problems that arise after a concussion (mild
traumatic brain injury).
Parestas and Gartner (2006) explain the function of the vestibular ocular reflex as, “when
the head is turned information is transferred from the canal- specific vestibular system via the
reflex circuit connections to the abducens and ocularmotor nuclei, which innervate extraocular
muscles controlling horizontal eye movement” (p.329). As stated previously, when the head
turns to the right endolymph flows in the semicircular canals in both ears. Because of inerta the

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flow moves to the left, causing the hair cells in the right horizontal canal ampulla to be
stimulated, creating neural activity in the right crista ampulluris. At the same time, the hair cells
in the corresponding left canal depolarize and inhibit the neural activity on the left side. When
the hair cells are activated, the vestibular nerve is stimulated and impulses are sent to the
vestibular nucleus. From there sensory information from the right side is sent to the contralateral
abducence nucleus. The information is then sent along to the left lateral rectus and MLF, which
synapse to with the ocularmotor neurons. The right medial rectus is activated and as a result the
eyes reflex with simultaneous horizontal movements in order to maintain focus of and image on
the retina. This is known as a nystagmus which is when the eyes reflexively try to maintain
visual fixations through visual compensation.
The vestibulospinal reflex (VSR), as defined by Herdman (2007), is “responsible for
generating compensatory body movements in order to maintain head and posture stability,
thereby preventing falls” (p.2). Similar to the VOR, the VSR works by stimulating muscle
contraction and relaxation in order to maintain balance and posture alignment. The VSR relies on
sensory information from the otoliths and also involves the: semicircular canals, vestibular nerve,
lateral and medial vestibulospinal tracts, reticulospinal tract, and anti-gravity muscles. As
addressed by Herdman (2007), the reticulospinal tract is assumed to be involved in maintaining
blanace by stimulating reflexes that allow for postural adjustments as a result of visual, auditory
or tactile stimuli (p.11).
The vestibulospinal reflex occurs, like the VOR, when the head is tilted and endolymphic
fluid deflects otoliths and hair cells. This information is sent through the vestibular nerve to the
vestibular nucleus where the sensory information is projected to the spinal cord via the medial
and lateral vestibulospinal tracts. These tracts then branch off and synapse with motorneurons

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which in response activate extensor muscle reflexes to the ipsalateral side, and flexor muscle
reflexes to the contralateral side. As a result of these contractions and relaxations in the muscles,
the individual is able to make the correct postural adjustments in order to maintain balance.
In addition to their functions, the anatomies of these reflexes are important to keep in
mind. The cerebellum and brainstem are both part of the hindbrain. The cerebellum is posterior
to the brainstem and is inferior to the occipital lobe. The brainstem (made up of the pons and
medulla) is anterior to the cerebellum and the spinal cord originates from the base of the
medulla. The MLF involved in the VOR is located near the midline of the brainstem. The medial
and lateral vestibulospinal tracts, involved in the VSR, originate from the upper medulla and
continue down the spinal cord where they terminate and synapse onto interneurons.
Defining Concussions
Now that I have examined the normative functions of the vestibular system and its
reflexes, I am going to examine and define concussions. I will then connect the concussions back
to the vestibular system by addressing the effect concussions have on the vestibular reflexes and
how complications can arise post-concussion, causing prolonged vestibular symptoms.
A concussion is defined as “minor traumatic brain injury caused by violent jarring or
shaking of the brain, which results in the disturbance of brain function” (A.D.A.M Medical
Encyclopedia, 2013). The cause of a concussion can include, but is not limited to, direct or
indirect collision, and whiplash.
The main aspect of a concussion I am going to focus on is the coup-countercoup. This is
when the head or body experiences a direct or indirect collision, causing the brain to crash into
the interior walls of the skull. As a result, contusions (bleeding and swelling of the brain tissue)

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may occur, most commonly on the frontal and occipital lobes of the brain- as addressed by
Schreck and McCaffrey (2008). In some instances as addressed in the documentary, Head
Games (2012), there may even be direct damage to the cerebellum.
In addition to contusions, another serious event occurs in the brain when there is a
concussion. When the brain moves and collides with the interior walls of the cranium, various
areas of the cerebral cortex shift and move at different velocities. This causes the axons in the
brain to stretch and, if the force is strong or consistent enough, break from their cell body. This is
referred to as axonal shearing, and can lead to communication and sensory transmission
problems between neurons. This is double-fold since when the axons break and degenerate they
release toxic levels of neurotransmitters, causing the surrounding neurons to become affected
and leads to further neuron destruction. Along with the cerebral cortex shifting, the brainstem
also experiences movement and can cause stress and damage to the brainstem.
Finally, it is important to remember that the neck and spine are affected during a
concussion. When the head rotated forwards and then backwards in a whiplash motion, torque
and strain is applied to the neck, affecting the C1 and C2 vertebrae. The force of the rotation can
cause problems such as subluxation of the C1 and C2 vertebrae, ligament damage, or nerve
pinching.
Vestibular Reflexes and Concussions
Now that I have examined both the vestibular reflexes and concussions, I will begin to
connect the two together and describe the role of the vestibular system when it comes to
concussions, and how damage to the structure of the vestibular system can lead to prolonged
symptoms.

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As mentioned previously, there are a few things that occur during a concussion that can
lead to balance and postural complications after a concussion. The first is localized damage to
the brain tissue. The second is the shifting of the cerebral cortex and axonal shearing. The third is
strain and damage to the cerebellum and brainstem. And finally, there is the violent torque that
occurs in the neck during a concussion.
When the brain experiences contusions, there is swelling and bleeding in the brain tissue
which can lead to localized damage. The swelling and bleeding (hematoma) can lead to problems
of their own, causing dangerous intracranial pressure which can lead to compressing brain tissue
and neurons. In addition to the intracranial pressure, directional blows and swelling to the frontal
and occipital lobes, which effect sensory and visual processing, can cause neuron damage. In
addition to the occipital and frontal lobes, the cerebellum may also receive localized damage
which will cause interference in both VOR and VSR sensory signals (Miremami, J., Talauliker,
P., Harrison, J., & Lifshitz, J. 2014).
Next the shifting of the cerebral cortex during coup-countrecoup, can lead to axonal
shearing. This axonal shearing result in the loss of neurons, causing delayed of decreased
communication of sensory information. Sensory impulses along the vestibulospinal tracts and
MLF can become affected causing delays in the VOR and VSR ((Miremami, J., Talauliker, P.,
Harrison, J., & Lifshitz, J. 2014).
The shifting of the brain inside the cranium can also lead to damage of the brainstem,
particularly to the nerves and tracts of ascending and descending pathways. Serious damage to
the area can lead to paralysis and become fatal.

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Finally, when an individual experiences an impact, causing a concussion, there is torque
in the neck, affecting the C1 and C2 vertebrae of the spine. Strain to these areas can lead to nerve
damage. C1 and C2 are located just under the skull and protect the base of the brainstem and
spinal cord. As previously mentioned, the VSR sensory tracts run through this area of the spine
and brainstem. Damage, subluxation, or swelling to this area can lead to delays or decreased
transmission of sensory impulses down the descending pathways, as addressed by (Herdman
&Shumway-Cook, 2007). This can result in reflex complications.
Both the vestibulospinal tract and MLF reside in the medulla and the vestibulospinal
tracts continue down to the spinal cord. It could be concluded that if there were to be any damage
to the C1 and C2 due to a concussion, these neural pathways could be impacted leading to
sensory and reflex delays. Since the VOR is responsible for visual focus and eye tracking, which
prevents “dizziness”, it can be concluded that damage to the sensory tract may cause vestibular
symptoms and post-concussion complications. In addition to delayed responses, “fast, intense
rotation can overwhelm the receptors, sending a flood of signals to the brain, with consequent
dizziness and or nausea” (Patestas& Gartner, 2006, p.325). This inconsistency in sensory
information can lead to common symptoms such as vertigo, and nausea causing the individual to
feel instable and perceive himself to be spinning. As a result the individual will have a difficult
time with maintaining balance. In addition to the VOR complications, and VSR damage may
cause postural problems. Since the VSR is responsible for contracting and relaxing extensor and
flexor muscles in order to maintain postural alignment, damage could result in sensory and
synaptic delays causing extensor rigidity or inability to compensate for weight shifts in the body.
Conclusion

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Based upon this research one can conclude that the vestibular system and its reflexes play
a major and contributing role in the maintenance of equilibrium. It can also be concluded that
when there is a deficiency in the reflexes as a result of a concussion, there will be numerous
vestibular symptoms and complications experienced by the post-concussive patient.
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https://www.youtube.com/watch?v=SiPRa3r-Big
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Injury Success Books.
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vestibular disorders. Neuroclinical North America. 1990; 2(8): 441-457.
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brain-injured pateints. Journal of head trauma rehabilitation, 4(5), 51-62.
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