Fishes have an impressive complement of hydrodynamic and acoustic sensors, commonly referred to as the lateral-line and inner-ear sense organs The basic receptor elements are the hair cells, which detect the minute displacements imparted to their apical ciliary bundles (Fig 41a) The directional sensitivity of the individual receptor cells is indicated by the asymmetric position of the single kinocilium relative to the several rows of stereocilia Morphologically, the hair cells of the various sensory clusters are strikingly uniform Their diversity in function is determined mainly by the peripheral structures coupling the ciliary bundles to the physical world that the animals inhabit

Hydrodynamic and Acoustic Field Detection

During natural movement on the earth’s surface, we are seldom aware of vestibular sensations, and when we do become aware of them, they usually signify an unnatural stimulus or an abnormality of vestibular function. With specially contrived conditions of observation, relationships between acceleratory stimuli and vestibular sensations and perceptions can be described quantitatively and qualitatively. These relationships constitute the primary subject matter of this chapter, but to appreciate this material in relation to daily experiences, we must first consider why vestibular sensations do not typically achieve conscious awareness during natural voluntary head and body movement.

Psychophysics of Vestibular Sensation

The sections in this article are:


1
Peripheral Receptor Mechanisms
1.1
Morphology of Vestibular Apparatus

1.2
Mechanisms of Sensory Transduction

1.3
Resting Discharge

1.4
Fiber Caliber and Afferent Response

1.5
Physiology of Semicircular Canals

1.6
Physiology of Otolith Organs




2
Centrifugal Systems
2.1
Efferent System

2.2
Sympathetic Innervation

2.3
Receptor-Receptor Neurons




3
Vestibular Nuclei
3.1
Functional Organization

3.2
Physiology of Canal-Related Secondary Neurons

3.3
Physiology of Otolith-Related Secondary Neurons

3.4
Sensitivity to Nonlabyrinthine Inputs

3.5
Vestibuloocular Reflexes

3.6
Canal-Related Reflexes

3.7
Otolith-Related Reflexes

3.8
Barbecue Nystagmus

3.9
Vestibulospinal Relations

3.10
Lateral Vestibulospinal Tract

3.11
Medial Vestibulospinal Tract

3.12
Functional Considerations

3.13
Vestibulocerebellar Relations

3.14
Inputs to Vestibulocerebellum

3.15
Cerebellar Influences on Vestibular Pathways




4
Vestibulocortical Projections
4.1
Cortical Representation

4.2
Thalamic Representation and Brain Stem Pathways




5
Plasticity in Vestibular Pathways
5.1
Compensation from Labyrinthectomy

5.2
Habituation

5.3
Adaptive Control of Vestibuloocular Reflex

The Vestibular System

This chapter examines the vestibular organs from a systems point of view in an attempt to describe their primary functions as well as the integrated operations of their essential elements. The examination is guided in large part by the analogy which is shown to exist between the vestibular system and the human-contrived inertial guidance system currently utilized in aerospace and submarine navigation. The operation of both systems can be shown to be based upon the same laws of physics, to embody analogous elements, to perform similar operations, and to provide analogous information.

A Systems Concept of the Vestibular Organs

It has long been known that, in contrast to the range of variation of body dimensions, the semicircular canals of different animals are approximately similar in size. In the present study, measurements of the internal radius ( r ) and radius of curvature ( R ) of the endolymphatic canal were made in 87 species, comprising 46 mammals, 17 birds, 17 fishes and 7 reptiles, using specimens, and photographic records of specimens, already available. Using the principles of dynamical similarity, theoretical arguments are adduced which predict that if Steinhausen’s (1933) interpretation of semicircular canal function is correct there should be very slow increases, systematic but discreet, of r and R with body mass ( m ). The results of the measurements largely confirmed the theoretical prediction, the actual relations obtained for all species together being: log 10 100 r 2 = 0·0945 (±0·0549) log 10 m +0·2519 and log 10 100 R = 0·0761 (±0·0402) log 10 m + 2·3797 (where r and R are measured in mm and m in kg), similar relations being obtained for each class examined. It is shown that the observed changes in r and R can to a large extent account respectively for the changes in canal sensitivity and the time constant of cupular return which the theory suggests should be called for by changes in animal shape and body mass.

O. Lowenstein

Papers

The mechanical analysis of the responses from the end‐organs of the horizontal semicircular canal in the isolated elasmobranch labyrinth