Molly Webster

Spiral ganglion neuron loss following organ of corti loss: A quantitative study

The packing density of spiral ganglion neurons was measured in 2.5- and 13-15-month-old guinea pigs, in guinea pigs at various times after drug-deafening or acoustic trauma, and in Waltzing guinea pigs of various ages. Analysis of variance and Duncans new multiple range tests were used to determine significant differences between treatment/survival groups. Spiral ganglion neurons in young and old normal ears did not have significantly different packing densities. Drug-deaf guinea pigs showed a significant loss of neurons by 2 weeks following treatment, a further significant loss by 2 months, and a marginally significant loss between 4 and 8 months. The neuronal population was then stable through 15 months, at about 13% of normal. Acoustic trauma ears showed the first significant loss isn the lower second turn at 1 month. Long-term (12-14 months post-exposure) trauma ears were highly variable. Waltzers lost about 50% of their normal neuronal population between 4 and 8 months; they showed an unexpected greater-than-normal density at 2 months, possible explanations of which are discussed. Thus, loss of the organ of Corti from various causes results in a slow but progressive loss of spiral ganglion neurons, the time course of which varies with the type of cochlear insult.

Effects of neonatal conductive hearing loss on brain stem auditory nuclei.

Both postnatal auditory deprivation and experimentally produced conductive hearing losses in mice result in incomplete maturation of most brain stem auditory neurons. The affected groups are: octopus cell, globular cell, small spherical cell, and large spherical cell groups in ventral cochlear nuclei; and the lateral superior olive and medial nucleus of the trapezoid body of the superior olivary complex. When 45 days of auditory deprivation are followed by 45 days of normal acoustic stimulation, there is incomplete maturation of neurons in: multipolar cell, globular cell, small spherical cell, and large spherical cell groups in ventral cochlear nuclei; lateral superior olive and medial nucleus of trapezoid body in superior olivary complex; and central nucleus of inferior colliculus. A critical period exists when adequate sound stimulation is needed for full development of these neurons.

Adaptive Value of Hearing and Vision in Kangaroo Rat Predator Avoidance

The predatory strikes of a sidewinder rattlesnake (Crotalus cerastes) can be avoided by kangaroo rats (Dipodomys merriami) in the normal condition and also after either of two surgical manipulations: (1) removal of the eyes, or (2) reduction of middle ear volume. When both these operations are performed, however, the kangaroo rat is struck by the rattlesnake with relative ease.Reduction of middle ear volume alone in a natural population of kangaroo rats resulted in a high loss of animals with reduced middle ear volume; most of them disappeared from the population during the dark phase of the moon. It is concluded that both vision and audition play significant adaptive roles in predator avoidance.

Kangaroo Rat Auditory Thresholds before and after Middle Ear Reduction

Four kangaroo rats were conditioned by shock avoidance and their auditory thresholds determined from 125 through 8,000 Hz. The middle ears were reduced and thresholds re-determined. Normal mean thresh

Cochlear Nerve Projections following Organ of Corti Destruction

Experimental organ of Corti destruction results in (1) secondary loss of all type I spiral ganglion neurons, (2) development of type III spiral ganglion neurons, (3) degeneration of most cochlear nerve myelinated fibers, and (4) terminal degeneration in the ventral and dorsal cochlear nuclei. The first signs of degenerative changes occur by eight days after organ of Corti destruction and degeneration debris remains until 28 weeks after destruction.

Multipolar residual spiral ganglion neurons

Guinea pigs drug deafened with a combination of kanamycin (400 mg/kg S.C.) followed 2 h later with ethacrynic acid (40 mg/kg I.V.) have a stable number of spiral ganglion neurons by 8 months post‐treatment which is 13% of the normal spiral ganglion population [M. Webster and D. B. Webster, Brain Res. 212, 17–30 (1981)]. Three μm serial sections of the spiral ganglion of such drug deafened guinea pigs sacrificed either 9 months or 16 months following treatment were cut. Each section was photographed and the profile of each neuron was traced onto acrylic sheets. Stacking the acrylic sheets allowed a three‐dimensional visualization of each neuron. In the 9‐month post‐treatment animals, the spiral ganglion neuronal types were: 6% unipolar, 53% bipolar, 38% multipolar, and 3% questionable. In the 16‐month post‐treatment animals, the spiral ganglion neuronal types were: 3% unipolar, 17% bipolar, 78% multipolar, and 2% questionable. These data suggest that following long term organ of Corti loss, the residual spiral ganglion neurons sprout new processes. [Work supported by NIH grants NS‐12510 and NS‐11647.]

Brainstem auditory nuclei after sound deprivation

Neuron size and neuron packing were measured in three groups of 45‐d CBA/J mice. A deprived group was used with a “devocalized” mother in a sound chamber with ambient noise below their auditory thresholds. An operated group had their external auditory meati and tympanic membranes removed at three days of age. A normal group served as controls. Neurons of the globular cell area of the cochlear nuclei and medial nuclei of the trapezoid body were significantly smaller (p<0.01) in the deprived and operated groups than in the control group. No neuron size differences were found in the other hindbrain and midbrain auditory cell groups. No differences were found in cell packing except in the central region of the dorsal cochlear nucleus where there were fewer neurons per unit area in deprived and operated mice than in normal mice (p<0.02). The data suggest that congenital conductive hearing losses may adversely affect development of parts of the central auditory system.