Normal balance comes from appropriate brainstem and cerebellar integration of three sensory systems: vestibular, visual, and proprioceptive (Table 21.2). Incorrect sensory signals or inappropriate integration of the sensory signals gives rise to dizziness and vertigo.

The vestibular system comprises end organs adjacent to each cochlea (three semicircular canals (SCC) , utricle, and saccule), vestibular nerves, and vestibular nuclei located in the dorsal medulla at the floor of the fourth ventricle and midline cerebellum (Fig. 21.1). The vestibular system divides into two major components. First, the vestibulo-spinal system alters body position in response to changes in gravity. Changes in gravity location are detected by the bending of hair cells in the macula of the utricle and saccule when there is movement of otoconia (tiny calcium carbonate crystals imbedded in a gelatinous matrix). Impulses sent via the vestibular nerve to vestibular nuclei are processed and then transmitted to anterior horn cells of antigravity muscles to maintain stable body posture. Changes in posture usually occur without an individual’s awareness.

Second, the vestibulo-ocular system maintains steady eye position in space during head movement. Angular acceleration is detected by one or more pairs of the semicircular canals that are located at right angles to each other. Head rotation bends SCC hair cells in the endolymph sending a change in the baseline frequency of nerve signals to brainstem vestibular nuclei. The signals are integrated resulting in appropriate signals transmitted via CN 3, 4, and 6 to move the eyes equally in the opposite direction of head rotation. Thus, during head movement, the world does not appear to move. Individuals with vestibulo-ocular reflex (VOR) dysfunction complain of vertigo when they move their head.

The visual system locates the horizon and detects head movement from the horizon. It also sends feedback (retinal slip) information to the vestibular nuclei regarding the integrity of vestibulo-ocular reflex. The visual system is comprised of eyes, optic nerves, lateral geniculate nuclei, optic radiations, visual cortexes, and pathways from the lateral geniculate bodies and occipital cortex to vestibular nuclei. The visual system seldom causes primary dizziness . However, it is the major compensating system when other sensory systems are impaired. As such, patients commonly have good balance during the day but feel off balance, dizzy, and fall at night when they have diminished vision.

The proprioceptive system delivers knowledge on the foot position detecting and compensating for leg and foot movement (sway). Joint position sensors located in the feet transmit changes in the foot position via small myelinated peripheral nerves to the spinal cord. Information then rises to the vestibular nuclei via the posterior columns. Nerve impulses sent from joint position sensors in the feet are important to maintain balance while standing and walking. Dysfunction of this system does not lead to vertigo but to a feeling of dizziness and being off balance (dysequilibrium) when standing and walking that improves with lying or sitting.

In summary, vestibular nuclei integrate signals from vestibular, visual, and proprioceptive systems to trigger appropriate changes in posture to maintain balance and to alter the eye position in order to keep the world steady during head movement. Paired vestibular nuclei in dorsal lateral medulla, flocculus, and nodulus cerebellar lobes receive and integrate afferent sensory signals. Efferent signals travel via the medial and lateral vestibulo-spinal tracts to anterior horn neurons of antigravity muscles and to the sensorimotor cortex for conscious knowledge of the body position and balance.