Static labyrinth: anatomy and actions

The position and structure of the maculae are shown in Figure 19.3. The utricular macula is relatively horizontal, the saccular macula is relatively vertical. The cuboidal cells lining the membranous labyrinth become columnar supporting cells in the maculae. Among the supporting cells are so-called hair cells, to which vestibular nerve endings are applied. Some hair cells are almost completely enclosed by large nerve endings, whereas others (phylogenetically older) receive only small contacts. At the cell bases are ribbon synapses, the synaptic vesicles being lined up along synaptic bars. Projecting from the free surface of each hair cell are about 100 stereocilia and, close to the cell margin, a single, long kinocilium. The hair cells discharge continuously, the resting rate being about 100 Hz.

Figure 19.3 Static labyrinth.

The cilia are embedded in a gelatinous matrix containing protein-bound calcium carbonate crystals called otoconia (‘ear sand’). (The term ‘otoliths’, when used, refers to the larger, ‘ear stones’ of reptiles.) The otoconia exert gravitational drag on the hair cells. Whenever kinocilia are dragged away from stereocilia, depolarization is facilitated. The macula has a central groove (striola) and the hair cell orientations have a mirror arrangement in relation to the groove. Electrical activity of hair cells is facilitated on one side of the groove by a given gravitational vector, and disfacilitated on the other side.

The maculae also respond to linear acceleration of the head in the horizontal plane (e.g. during walking) or in the vertical (gravitational) plane. Also, when the tilted head is stationary in a flexed or extended position, the facilitated half of the utricular macula discharges intensely in both ears. The saccular ones are more responsive when the head is held to the side.

The primary function of the static labyrinth is to signal the position of the head relative to the trunk. In response to this signal, the vestibular nuclei initiate compensatory movements, with the effect of maintaining the center of gravity between the feet (in standing) or just in front of the feet (during locomotion), and of keeping the head horizontal. These effects are mediated by the vestibulospinal tracts.

The lateral vestibulospinal tract, seen earlier in sections of medulla oblongata in Chapter 17, arises from large neurons in the lateral vestibular nucleus (of Deiters). The fibers descend in the anterior funiculus on the same side of the spinal cord and synapse upon extensor (antigravity) motor neurons. Both α and γ motor neurons are excited, and a significant part of the increased muscle tone is exerted by way of the gamma loop (Ch. 16). During standing, the tract is tonically active on both sides of the spinal cord. During walking, activity is selective for the quadriceps motor neurons of the leading leg; this commences following heel strike and continues during the stance phase (when the other leg is off the ground). Deiters’ nucleus is somatotopically organized, and the functionally appropriate neurons are selected by the flocculonodular lobe of the cerebellum. The flocculonodular lobe (Ch. 25) has two-way connections with all four vestibular nuclei.

Antigravity action is triggered mainly from the horizontal macula of the utricle. The vertical macula of the saccule, on the other hand, is maximally activated by a free fall. The shearing effect on the macula produces a powerful extensor thrust in anticipation of a hard landing.

A small, medial vestibulospinal tract arises in the medial and inferior vestibular nuclei (Figure 17.6). It descends bilaterally in the medial longitudinal fasciculus and terminates upon excitatory and inhibitory internuncials in the cervical spinal cord. It operates head-righting reflexes, which serve to keep the head – and the gaze – horizontal when the body is craned forward or to one side. Good examples of head-righting reflexes are to be seen around pool tables and in bowling alleys. An added twist can be provided, if required, by torsion of the eyeballs (up to 10°) within the orbital sockets. This eye-righting reflex is mediated by axons ascending the medial longitudinal fasciculus from the lateral vestibular nucleus to reach nuclei controlling the extraocular muscles. Evidence derived from unilateral vestibular destruction (Clinical Panel 19.1) indicates that the horizontal position of the eyes in the upright head is the result of a canceling effect of bilateral tonic activity in these Deitero-ocular pathways.

The medial vestibulospinal tract is also activated by the kinetic labyrinth.

The static labyrinth contributes to the sense of position. The sense of position of the body in space is normally provided by three sensory systems: the visual system, the conscious proprioceptive system, and the vestibular system. Deprived of one of the three, the individual can stand and walk by using information provided by the other two. Following loss of vision, for example, the subject can get about, although the constraints imposed by blindness are known to all. Following loss of conscious proprioception instead, the subject uses vision as a substitute for proprioceptive sense, and is disabled by closure of the eyes (sensory ataxia, Ch. 15). If the static labyrinths alone are active, closure of the eyes may lead to a heavy fall.

Kinetic labyrinth: anatomy and action

Basic features of macular epithelium are repeated in the three cristae. Again there are supporting cells, and hair cells to which vestibular nerve endings are applied. The kinocilia of the hair cells are long, penetrating into a gelatinous projection called the cupula (Figure 19.4). The cupula is bonded to the opposite wall of the ampulla.

Figure 19.4 (A) A rightward head turn activates nerve endings in the right lateral semicircular canal, resulting in contraction of the left lateral and right medial rectus muscles. (B) The nerve endings in the cupula are excited by passive displacement of the cupula toward the ampulla. Impulse traffic increases along parent bipolar neurons whose central fibers excite the medial and superior vestibular nuclei.

The cristae are sensitive to angular acceleration of the labyrinths. Angular acceleration occurs during rotary ‘yes’ and ‘no’ movements of the head. The endolymph tends to lag behind because of its inertia, and the cupula balloons like a sail when thrust against it. The disposition of the kinocilia is uniform across each crista, and is such that the lateral ampullary crista is facilitated by cupular displacement toward the utricle; the superior and posterior cristae are facilitated by cupular displacement away from the utricle. In practical terms, the right lateral ampulla is activated by turning the head to the right; both superior ampullae are activated by flexion of the head; and both posterior ampullae by extension of the head.

Afferents from the cristae terminate in the medial and superior vestibular nuclei. As with the macular afferents, there are two-way connections with the flocculonodular lobe of the cerebellum.

The function of the kinetic labyrinth is to provide information for compensatory movements of the eyes in response to movement of the head. Vestibulo-ocular reflexes operate to maintain the gaze on a selected target. A simple example is our ability to gaze at the period (full stop) at the end of a sentence, while moving the head about. The two eyes move conjugately, i.e. in parallel.

The horizontal vestibulo-ocular reflex response to a rightward turn of the head is depicted in Figures 19.4 and 19.5, and described in their captions. Appropriate point-to-point connections also exist between the vestibular nuclei and gaze centers in the midbrain for similar reflexes in the vertical plane.

Figure 19.5 Under cerebellar guidance, the right medial vestibular nucleus responds to a rightward head turn by sending impulses to the contralateral paramedian pontine reticular formation (PPRF, Figure 17.15). The PPRF selects abducens motor neurons supplying the left lateral rectus, and sends internuclear fibers up the right medial longitudinal fasciculus to the right oculomotor nucleus, where they seek out motor neurons serving the right medial rectus.

(Not shown is the superior vestibular nucleus, which sends ipsilateral fibers having the function of inhibiting motor neurons to the two antagonist recti.)

In order to control the vestibulo-ocular reflexes, the cerebellum is informed about the initial position of the head in relation to the trunk. This information is provided by a great wealth of muscle spindles in the deep muscles surrounding the cervical vertebral column. The spindle afferents enter the rostral spinocerebellar tract and relay in the accessory cuneate nucleus on each side (Figure 17.12).

Nystagmus

A horizontal vestibulo-ocular reflex can be elicited artificially by warming or cooling the endolymph in the semicircular canals. In routine tests of vestibular function, advantage is taken of the proximity of the lateral semicircular canal to the middle ear. The canal is angled at 30° to the horizontal plane. Tilting the head back by 60° brings the canal into the vertical plane, with the ampulla uppermost. In the warm caloric test, water at 44°C is then instilled into the ear. The air in the middle ear is heated, and heat transfer to the lateral canal produces convection currents within the endolymph. Whether through displacement of the cupula or by some other mechanism, the crista of the warmer lateral ampulla becomes more active than its opposite number. The result is a slow drift of the eyes away from the stimulated side. It is as if the head had been turned to the side being tested. The drift is followed by a recovery phase in which the eyes snap back to the resting position. These slow and fast phases are repeated several times per second. This is vestibular nystagmus. The direction of the nystagmus is named in accordance with the fast phase because of the obvious ‘beat’. A warm caloric test applied to the right ear should produce a right-beating nystagmus (‘nystagmus to the right’).

Subjectively, nystagmus is accompanied by vertigo – a sense of rotation of self in relation to the external world, or vice versa.

Unilateral and bilateral vestibular syndromes are considered in Clinical Panel 19.1. A vascular syndrome involving the vestibular system in the medulla oblongata is described in Clinical Panel 19.2.

Figure CP 19.2.1 Lateral medullary infarct (shaded).

Vestibulocortical connections

Second-order sensory neurons project from the vestibular nuclei mainly to the ipsilateral thalamus. The fibers relay via the ventral posterior nucleus to the parieto-insular vestibular cortex (PIVC) and to the adjacent region of the superior temporal gyrus, as shown in Figure 19.6. However, the PIVC cannot be named as a primary sensory area because it is in fact multisensory, receiving visual and tactile inputs as well as vestibular. (By analogy, PET studies of tactile sensation show activity throughout most of the parietal lobe, but we know from other sources that the postcentral gyrus is the primary area, being the take-off point for analysis in the posterior parietal cortex.)

Figure 19.6 Parieto-insular vestibular cortex (blue). I–III, short insular gyri; IV, V, long insular gyri (IV and V are commonly fused as a single, Y-shaped long insular gyrus).

The relationship of PIVC activity to hemisphere dominance is discussed in Chapter 32.