Human Physiology - William F. Ganong 2002

Functions of the Nervous System
Hearing and Balance
Vestibular Function

Response to Angular Acceleration

Angular acceleration in the plane of a given semicircular canal stimulates its crest. The inertia of the endoplasmatic fluid causes it to shift in the direction opposite to the rotation. The movement of the fluid deforms the cupula, which causes the bending of the Hair Cell processes (see Fig. 9-6). Once a constant rotational velocity is reached, the endolymph moves at the same speed as the entire body, and the cupula returns to its initial position. With a decrease in angular velocity, the cupula deforms in the direction opposite to the acceleration deformation. After 25–30 s, it returns to the middle position. The Displacement of the cupula in one direction usually causes an increase in impulse traffic in the nerve fibers extending from the corresponding crest, whereas displacement in the opposite direction inhibits neural activity (Fig. 9-17). Rotation causes maximum stimulation of the semicircular canal closest to the plane of rotation. Since the canals of one half of the HEAD are a mirror image of the canals of the other half, the endolymph moves toward the ampulla in one ear and away from the ampulla in the opposite ear. Therefore, the combination of impulses reaching the Brain differs and depends on both the direction and the plane of rotation. Linear acceleration presumably does not cause cupular displacements and does not stimulate the crests. It is known that the destruction of a certain part of the labyrinth leads to the compensation of the lost part's Functions by the preserved parts, which makes experimental study of labyrinth functions extremely difficult.

Class="center">Table 9-1. Most Common Tuning Fork Tests for Differentiating Neural and Conductive Hearing Loss

Fig. 9-17. Ampullary response to rotation. The timing of impulse generation by the ampullary crests of two semicircular canals in response to angular acceleration, uniform rotation, and angular deceleration (reproduced with permission from Adrian ED: Discharges from vestibular receptors in the cat. J Physiol [Lond] 1943;101:389).

The function of the vestibular nuclei is primarily associated with maintaining head posture. Descending Pathways from these nuclei provide correction of the head position relative to the neck and other body parts (see Chapter 12). Ascending connections with cranial nerve nuclei are mainly related to Eye Movements.

Nystagmus

Characteristic jerky eye movements at the beginning and end of rotation are called nystagmus. In fact, nystagmus is a reflexive response to ensure gaze fixation on stationary objects during body rotation, although it is not caused by visual impulses and has been observed in blind individuals. At the onset of rotation, the eyes begin to move slowly in the direction opposite to the rotation, ensuring visual fixation (vestibulo-ocular reflex). When a certain limit of rotation is reached, the eyeballs quickly jump to a new fixation point and resume movement in the direction opposite to the body's rotation. The slow component of nystagmus is caused by impulses from the labyrinth; the fast component is triggered by a specialized center in the Brainstem. Nystagmus is most commonly horizontal (i.e., the eyes move in a horizontal plane), but it can also be vertical, when the head is tilted from side to side during rotation, and rotational, when the head is tilted forward. The direction of eye movement in nystagmus is conventionally considered to be the direction of the fast component. As a rule, the direction of the fast component coincides with the direction of rotation; however, in the case of post-rotational nystagmus, which is observed due to cupular displacement upon cessation of rotation, it has the opposite direction. Clinical nystagmus can be observed in patients with brainstem lesions.

Response to Linear Acceleration

In mammals, the maculae of the utricle and saccule respond to linear acceleration. In general, the utricle detects horizontal acceleration, and the saccule detects vertical acceleration. Compared to the endolymph, otoliths have a higher density, so acceleration in any direction displaces them in the opposite direction, leading to the bending of hair cell processes and the activation of nerve fibers. Without head displacement, the maculae generate tonic discharges due to the EFFECT OF GRAVITY on the otoliths. These impulses participate in ensuring the correct posture of the head and body, as discussed in Chapter 12.

Although most responses to macular stimulation are reflexive, vestibular impulses reach the Cerebral Cortex. They likely provide conscious perception of movement and supply some of the information needed for spatial orientation of the body. Vertigo, caused by a sensation of rotation when none is actually occurring, is a major symptom indicating inflammation of a single labyrinth.

Caloric Stimulation

Semicircular canals can be stimulated by introducing Water into the external auditory meatus at a Temperature higher or lower than body temperature. The temperature difference induces convection currents in the endolymph, resulting in cupular displacement. The method of caloric (thermal) stimulation, sometimes used for diagnostic purposes, can cause nystagmus, dizziness, and nausea. To avoid these symptoms during the Treatment of ear infections, medications should be at body temperature.

Spatial Orientation

Spatial orientation depends partly on impulses originating from vestibular receptors, although visual signals are also important. Relevant information is provided by impulses from proprioceptors in joint capsules, which signal the relative positions of various body parts, as well as impulses from cutaneous exteroceptors, especially Touch and pressure receptors. These four groups of impulses are utilized by the cerebral cortex to continuously synthesize a representation of the body's orientation in space.

Motion Sickness

Nausea, changes in Blood pressure, sweating, pallor, and vomiting—the main symptoms of motion sickness—are caused by excessive stimulation of the vestibular apparatus. They are likely triggered by Reflexes transmitted through vestibular connections in the brainstem and the vestibulocerebellar (flocculonodular) lobe of the Cerebellum (see Chapter 12).

Space motion sickness—nausea, vomiting, and dizziness—develops in astronauts experiencing weightlessness for the first time and typically disappears after a few days of spaceflight. Its symptoms may recur upon returning to Earth's gravity due to increased gravitational forces. The cause of this is a mismatch between neural impulses caused by changes in specific PARTS OF THE vestibular apparatus and other gravity sensors, without a corresponding change in Other types of impulses that characterize body orientation in space.