VESTIBULOPATHY – NEUROLOGICAL IMPLICATIONS IN BALANCE DISORDERS
The brain is receptor driven via afferent projections to cerebellar and thalamic nuclei. Simply stated, the cerebellum is the neurological interface between incoming and outgoing messages; i.e., body – brain / brain – body. Peripheral receptors such as the retina of the eyes, the cochlear receptors for sound, the pheromonal receptors for the sense of smell, all contribute to the central integrative state of the brain and all “touch base” in the thalamus. From the thalamus the messages are sent to whatever lobe of the brain that is specific for that particular sensation. The greatest input into the brain and nervous system is from environmental stressors that are constant; i.e., those related to the earth’s gravitational pull on muscle spindle cells, Golgi tendon organs, the gamma motor apparatus, and joint mechanoreceptors. An additional population of sensory receptors is found in the VESTIBULAR APPARATUS in the inner ear to provide CONSTANT appraisal of our orientation in the earth’s gravitational field. Since the only constant source of activation is gravity, plasticity (learned behavior) in this labyrinthine mechanism provides the foundation for BALANCE, reflex midline shunt (core) stability; conjugate movement of the eyes and vertebral motion segments, and the establishment of cervical and lumbar curves. It is the absence of activation of GRAVITATIONAL receptors that renders astronauts ataxic after a couple of months in outer space, where there is no gravity.
Activation of gravitational receptors (afferentation) initiates the central transmission through primary afferent nerves to synapse on the second order projections to summate in the cerebellum and contralateral thalamus (everything, except the sense of smell, crosses over). The thalamus is the “terminal” of all sensory input to the brain, without which adequate human perception cannot exist. Integrity of thalamic neurons is vital to our well-being and can only be guaranteed through constant activation.
The function of all neurons is to promote survivability through the receipt and transmission of information as a consequence of their frequency of firing.
Fortunately, we only have two sides from which receptors transmit information from one side of the nervous system to the other side of the brain; i.e. into the ipsilateral cerebellum and contralateral thalamus. The brain is capable of modulating its own input by regulating the sensitivity of sensory systems, largely through the alpha/gamma mechanism of the musculoskeletal system. A mismatch in input to the brain, will lead to a mismatch in function between the two hemispheres, and can lead to altered cognition and perception of one’s world. It is a processing imbalance. If the two systems can’t get “online” simultaneously, the imbalance, not the malfunction (diagnosis), is the problem.
The vestibular systems consists of six semicircular canals in the inner ear, which dictate, more than anything else, our sense of balance. There are three canals on each side. These canals, via the cerebellum, are connected to the muscles that move our eyes during non-volitional movement, as in tracking, smooth pursuit, etc. Any head movement to either side automatically generates eye movement to the opposite side via semicircular canal activation. If one moves the head to the right when looking at an object, the eyes will move to the left; i.e. the right semicircular canal will inhibit, or shut off, the left semicircular canal. If you move your head down and to the left, your eyes will turn up and to the right when things are working as they should. If there is aberrant input from one side, the vestibular system will interpret that as movement and the eyes will comply with the perception, a conditon known as Nystagmus and consequent vertigo.