The vestibular system is the apparatus present in the bony labyrinth of the inner ear. This system contributes to a sense of balance and spatial orientation and it is involved in coordinating movement with balance. The vestibular system is made up of semicircular canals and the otoliths which send information to the brain. The brain uses this information and enables the person to understand the position and acceleration of the body. The activation of the vestibular system leads to feeling dizzy after someone spins around like a top. Riding on a fast roller coaster or having a bumpy plane ride may stimulate the vestibular system which then leads to motion sickness.
Structure of Vestibular system
The vestibular apparatus is present in the petrous part of the temporal bone. It consists of a system of membranous tubes and chambers. It has two parts: the semicircular canals, which are involved in dynamic equilibrium; and the otoliths, which are involved in static equilibrium.
- Semicircular canals
The vestibular system has three semicircular canals that are present at the right angle to each other. Every semicircular canal has a dilated sac called the osseous ampulla which is present at the end of the canal. The sac is more than twice the diameter of the canal. Each sac contains an ampulla crest which consists of a gelatinous cap and hair cells.
The position and orientation of these canals are of utmost importance because they represent all three planes in space. A different canal is stimulated by the movement of the head in a different plane. When the head is rotated around a vertical axis, the horizontal semicircular canal responds to it by the movement of the fluid present in the canal. The movement of fluid in the anterior and posterior semicircular canals corresponds to the movement of the head in the sagittal and the frontal plane.
- Otolithic organs
The vestibular system has two otolithic organs: the utricle and the saccule. Both the utricle and the saccule contain some hair cells and some supporting cells called macula. The macula has many stereocilia and a true cilium called a kinocilium. The macula of the utricle lies on its inferior surface in a horizontal plane and determines the orientation of the head when the head is upright. On the other hand, the macula of the saccule lies in a vertical plane and it detects the orientation of the head when the person is lying down. The macula has many small calcium carbonate crystals called otoconia which enhance the sense of gravity and motion.
Maintenance of Dynamic equilibrium
The vestibular system detects the angular acceleration and maintains the dynamic equilibrium through three semicircular canals. When the head is rotated in any direction, the endolymph stays stationary because of its inertia while the semicircular ducts move. In this way, the relative motion of the fluid is in the opposite direction of the head movement. The fluid deflects the jellylike structure cupula in the opposite direction which bends and stimulates the hair cells by bending their cilia.
When the cupula is in its resting position, the hair cell emits 100 impulses per second. When the rotation begins and the hair cell bends to one side, the rate of discharge increases greatly. The discharge gradually subsides with the continuation of rotation and comes back to the resting level. This reduction in discharge happens because the cupula returns to its normal position in 5 to 20 seconds because of its own elastic recoil. The semicircular canals transmit signals of opposite polarities during the beginning and stopping of rotation.
The semicircular canals only detect the beginning and stopping of the rotation of a person’s head in different directions. The function of the semicircular canal is not to maintain the equilibrium. Yet, loss of the semicircular canal function is associated with poor equilibrium while performing rapid body movements. The semicircular have a predictive function which means that they predict that disequilibrium is going to occur. The equilibrium centers utilize this information and make appropriate preventive adjustments. In this way, the semicircular canals help a person to maintain balance.
This usually happens when a person is running forward and suddenly begins to turn on one side, the semicircular canals cause the nervous system to make appropriate adjustments to maintain the balance. On the other hand, the maculae of the utricle and saccule cannot detect the loss of balance until after it has occurred.
Maintenance of Static equilibrium
The vestibular system detects the linear acceleration and maintains the static equilibrium through otolithic organs: the utricle and the saccule. The hair cells present in the maculae of the utricle and the saccule are all oriented in different directions. This causes stimulation of different hair cells with the different positions of the head. The stimulation of the hair cells causes the brain to understand the position of the head with respect to the pull of gravity. This leads to the excitation of the vestibular, cerebellar, and reticular motor nerve systems of the brain which then maintain proper equilibrium.
The maculae of the utricle are in an approximately horizontal plane. Because of this position, they are more useful in providing information about the side-to-side tilt of the head and the position of the head in an upright position. Conversely, the saccular maculae are in parallel vertical planes and provide information about head orientation during forward and backward tilting of the head and also when the person is lying down.
When the body of a person accelerates, the small calcium carbonate crystals called otoconia fall backward on the cilia of hair cells. This happens because otoconia have greater mass inertia than the surrounding fluid. This information is sent to the central nervous system and the person feels as though he or she were falling backward. The central nervous system causes the body to lean forward until the anterior shift of otoconia exactly equals the tendency to fall backward. That is how the central nervous system senses the state of a proper equilibrium and leans the body no further.
The maculae detect the linear acceleration, but they do not detect linear velocity. When the runners begin to run, they lean forward to maintain their balance. but when they achieve a certain running speed, there is no need to lean forward. Then the runners lean forward not because of the maculae but because of the air resistance against their bodies.
When a person moves his head in different directions or changes his body movements rapidly, the vestibulo-ocular reflexes (VOR) helps the person to stabilize the direction of their eyes. It produces reflex eye movements that are equal and opposite in direction to the movement of the head. VOR is very fast and accurate and it allows people to stabilize their gaze while doing physical activities like running, walking, playing, and driving. The vestibular system controls all six pairs of eye muscles through specific connections to oculomotor nuclei. Studies have shown that the saccule along with anterior and posterior canals control eye movements in a vertical plane. The horizontal canals and utricle play a role in controlling the eye movements in a horizontal plane.
For example, while reading a book, when the head is turned leftward, the horizontal semicircular canals send signals to vestibular nuclei. This vestibular nucleus excites the contralateral abducens nucleus which innervates the right lateral rectus muscle through the sixth cranial nerve. The contraction of the right lateral muscle occurs. It also sends a signal to the ipsilateral oculomotor nucleus, which innervates the medial rectus muscle through the third cranial nerve. The oculomotor nucleus excites the left medial rectus. In this way, the resulting eye movement in the opposite direction of the head movement stabilizes the gaze on the book.
The vestibulospinal tract is a descending neural tract in the CNS which relays the information from vestibular nuclei to motor neurons. It plays an important role in maintaining upright posture and balance, stabilizing the gaze, and realization of orientation and motion. The tract consists of two sub pathways.
- Medial vestibulospinal tract
The medial vestibulospinal tract originates in the Schwalbe’s nucleus which is also called the medial vestibular nucleus. These fibers join with the ipsilateral and contralateral medial longitudinal fasciculus. These fibers run down in the anterior funiculus to cervical spinal cord segments. The medial vestibulospinal tract innervates the supporting muscles of the head and neck. It helps in the stabilization of head position and plays an important role in coordinating head movements and eye movements.
- Lateral vestibulospinal tract
The lateral vestibulospinal tract originates in the Deiters’ nucleus or the lateral vestibular nucleus in the pons. This tract is found in the lateral funiculus of the spinal cord. The fibers of the lateral vestibulospinal tract descend uncrossed in the anterior part of the lateral funiculus and terminate at the interneurons of laminae VII and VIII. It relays the signals to the motor neurons in antigravity muscles in the legs that help in maintaining upright posture and balance.
The vestibulospinal tract plays an important part in maintaining homeostasis through some important reflexes. The examples are:
- Vestibulospinal reflex
- Tonic labyrinthine reflex
- Righting reflex
- Labyrinthitis and Vestibular Neuritis
Vestibular neuritis and labyrinthitis are disorders resulting from a bacterial or viral infection. The infection inflames the inner ear and the labyrinth, or the nerves present between the ear and the brain. The result is disruption of the transmission of sensory information from the ear to the brain. Common symptoms of this disorder may include vertigo, difficulties with balance or hearing, and dizziness.
- Benign Paroxysmal Positional Vertigo (BPPV)
Benign paroxysmal positional vertigo (BPPV) causes vertigo, dizziness, loss of balance, and other symptoms. This happens due to debris made up of small crystals of calcium carbonate that has accumulated within a part of the inner ear. When the crystals are displaced, they shift with the movement of the head and send false signals to the brain.
- Age-related dizziness and imbalance
Dizziness and failure to maintain balance in the elderly are usually the results of problems with the vestibular, central, and vision systems. Neuropathy, psychological causes, and unknown (idiopathic) causes may also result in these symptoms. The most common cause of dizziness in older people is vestibular disorder.
The vestibular system is present in the inner ear and consists of three semicircular canals and the otolithic organs. It plays an important role in maintaining static and dynamic equilibrium and contributes to a sense of orientation. The semicircular canals are present at a right angle to each other and detect motion of the head in all three planes. The horizontal canal responds to movement in a vertical direction and the anterior and posterior canals respond to the movement in a sagittal and frontal plane. The two otolithic organs determine the orientation of the head when the head is upright or when the person is lying down.
The semicircular canals play a predictive role in maintaining dynamic equilibrium. They detect the head movement because of the flow of the endolymph in opposite direction to the movement which causes a discharge. It predicts that the disequilibrium is going to occur and upon receiving the information, the central nervous system makes preventive changes. The otolithic organs play a role in maintaining static equilibrium. The stimulation of hair cells present in the maculae of the utricle and the saccule provides information to the brain about the position of the head.
There are two types of vestibulospinal tracts that are present in the central nervous system. The medial vestibulospinal tract innervates the muscles of the neck and plays a role in stabilizing the gaze. The lateral vestibulospinal tract innervates the antigravity muscles in the legs and helps in maintaining posture and balance. The disorders of the vestibular system are caused by viral and bacterial infections, tumors, accumulation of debris in the inner ear, and age-related changes. The common symptoms of a vestibular disorder include dizziness, vertigo, and failure to maintain posture and balance.
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