Thecalorictestismoreusefulbecauseitcandistinguishbetweenmalfunctionofthelabyrinthsonthetwosides.Theneckisbentbackwardabout60degreessothatthetwohorizontalcanalsareessentiallyvertical.Ifwarmwaterisintroducedintotheleftear,theendolymphintheouterportionoftheloopoftheleftsemicircularcanaltendstoriseasthespecificgravityoftheendolymphdecreasesbecauseofheating.Thissetsupaconvectionflowofendolymph,andasaresult,thekinociliaoftheleftampullarycresthaircellsaredeflectedtowardtheutricle,asiftheheadhadrotatedtotheleft;thedischargeoftheafferentfibersthatsupplythiscanalincreases;andnystagmusoccurswiththefastphasetowardtheleft.Thenystagmusproducesasensationthattheenvironmentisspinningtotheright,andthepersontendstofalltotheright.Theoppositeeffectsareproducedifcoldwaterisplacedintheear.AmnemonicexpressionthatcanhelpinrememberingthedirectionofthenystagmusinthecalorictestisCOWS (“coldopposite,warmsame”).Inotherwords,coldwaterinoneearresultsinafastphaseofnystagmustowardtheoppositeside,andwarmwatercausesafastphasetowardthesameside.
posterior parietal areas). Activity in the superior colliculus is related to computation of the direction and amplitude of the saccade. Indeed, the deep layers of the superior colliculus contain a topographic motor map of saccade locations. From the superior colliculus, information is forwarded to distinct sites for control of horizontal and vertical saccades, referred to as the horizontal and vertical gaze centers, respectively. The horizontal gaze center consists of neurons in the paramedian pontine reticular formation, in the vicinity of the abducens nucleus (
Fig. 9.28A ). The vertical gaze center is located in the reticular formation of the midbrain: specifically, the rostral interstitial nucleus of the medial longitudinal fasciculus and the interstitial nucleus of Cajal. Because the circuitry and operation of the horizontal gaze center are better understood than those of the vertical gaze center, it is discussed here in detail. However, cells showing analogous activity patterns have been described in the vertical gaze center.
Fig. 9.28A is an overview of the neural circuitry by which saccades are generated, and
Fig. 9.28B shows the activity of certain types of neurons found in the gaze center that are responsible for horizontal saccades. Each horizontal gaze center has excitatory burst neurons that project to motor neurons in the ipsilateral abducens nucleus and to the internuclear neurons (which excites medial rectus motor
CHAPTER 9 Organization of Motor Function neurons in the contralateral oculomotor nucleus). It also has inhibitory burst neurons that inhibit the contralateral abducens. These burst neurons are capable of extremely high bursts of spikes (up to 1000 Hz). Moreover, the gaze center has neurons showing tonic activity and burst-tonic activity.
Normally, both inhibitory and excitatory burst neurons are inhibited by omnipause neurons located in the nucleus of the dorsal raphe. When a saccade is to be made, activity from the frontal eye fields or the superior colliculus, or both, leads to inhibition of the omnipause cells and excitation of the burst cells on the contralateral side. The resulting high-frequency bursts in the excitatory burst neurons provide a powerful drive to motor neurons of the ipsilateral lateral rectus and contralateral medial rectus muscles (see
Fig. 9.28A ); at the same time, inhibitory burst neurons enable relaxation of the antagonists. The initial bursts of these neurons allow strong contraction of the appropriate extraocular muscles, which overcomes the viscosity of the extraocular muscle and enables rapid movement to occur.