Performance of the avian inner ear

The cerebellum in maintenance of a motor skill: a hierarchy of brain and spinal cord plasticity underlies H-reflex conditioning

Operant conditioning of the H-reflex, the electrical analog of the spinal stretch reflex, is a simple model of skill acquisition and involves plasticity in the spinal cord. Previous work showed that the cerebellum is essential for down-conditioning the H-reflex. This study asks whether the cerebellum is also essential for maintaining down-conditioning. After rats decreased the soleus H-reflex over 50 d in response to the down-conditioning protocol, the cerebellar output nuclei dentate and interpositus (DIN) were ablated, and down-conditioning continued for 50-100 more days. In naive (i.e., unconditioned) rats, DIN ablation itself has no significant long-term effect on H-reflex size. During down-conditioning prior to DIN ablation, eight Sprague-Dawley rats decreased the H-reflex to 57% (+/-4 SEM) of control. It rose after ablation, stabilizing within 2 d at about 75% and remaining there until approximately 40 d after ablation. It then rose to approximately 130%, where it remained through the end of study 100 d after ablation. Thus, DIN ablation in down-conditioned rats caused an immediate increase and a delayed increase in the H-reflex. The final result was an H-reflex significantly larger than that prior to down-conditioning. Combined with previous work, these remarkable results suggest that the spinal cord plasticity directly responsible for down-conditioning, which survives only 5-10 d on its own, is maintained by supraspinal plasticity that survives approximately 40 d after loss of cerebellar output. Thus, H-reflex conditioning seems to depend on a hierarchy of brain and spinal cord plasticity to which the cerebellum makes an essential contribution.

Ablation of cerebellar nuclei prevents H-reflex down-conditioning in rats

While studies of cerebellar involvement in learning and memory have described plasticity within the cerebellum, its role in acquisition of plasticity elsewhere in the CNS is largely unexplored. This study set out to determine whether the cerebellum is needed for acquisition of the spinal cord plasticity that underlies operantly conditioned decrease in the H-reflex, the electrical analog of the spinal stretch reflex. Rats in which the cerebellar output nuclei dentate and interpositus (DIN) had been ablated were exposed for 50 d to the H-reflex down-conditioning protocol. DIN ablation, which in itself had no significant long-term effect on H-reflex size, entirely prevented acquisition of a smaller H-reflex. Since previous studies show that corticospinal tract (CST) transection also prevents down-conditioning while transection of the rubrospinal tract and other major descending tracts does not, this result implies that DIN output that affects cortex is essential for generation of the CST activity that induces the spinal cord plasticity, which is, in turn, directly responsible for the smaller H-reflex. The result extends the role of the cerebellum in learning and memory to include participation in induction of plasticity elsewhere in the CNS, specifically in the spinal cord. The cerebellum might simply support processes in sensorimotor cortex or elsewhere that change the spinal cord, or the cerebellum itself might undergo plasticity similar to that occurring with vestibulo-ocular reflex (VOR) or eyeblink conditioning.

Effects of salicylates and aminoglycosides on spontaneous otoacoustic emissions in the Tokay gecko

The high sensitivity and sharp frequency discrimination of hearing depend on mechanical amplification in the cochlea. To explore the basis of this active process, we examined the pharmacological sensitivity of spontaneous otoacoustic emissions (SOAEs) in a lizard, the Tokay gecko. In a quiet environment, each ear produced a complex but stable pattern of emissions. These SOAEs were reversibly modulated by drugs that affect mammalian otoacoustic emissions, the salicylates and the aminoglycoside antibiotics. The effect of a single i.p. injection of sodium salicylate depended on the initial power of the emissions: ears with strong control SOAEs displayed suppression at all frequencies, whereas those with weak control emissions showed enhancement. Repeated oral administration of acetylsalicylic acid reduced all emissions. Single i.p. doses of gentamicin or kanamycin suppressed SOAEs below 2.6 kHz, while modulating those above 2.6 kHz in either of two ways. For ears whose emission power at 2.6-5.2 kHz encompassed more than half of the total, individual emissions displayed facilitation as great as 35-fold. For the remaining ears, emissions dropped to as little as one-sixth of their initial values. The similarity of the responses of reptilian and mammalian cochleas to pharmacological intervention provides further evidence for a common mechanism of cochlear amplification.

Spontaneous otoacoustic emissions in the barn owl

Spontaneous otoacoustic emissions (SOAE) were studied in a bird, the barn owl. They were found in 79% of the ears investigated, and each emitting ear generated on average 1.9 emissions. Their peak sound-pressure levels lay between -5.8 and 10.3 dB, and their centre frequencies between 2.3 and 10.5 kHz. The SOAE originated primarily in the upper quarter of the animal's hearing range, and derived from a specialized area previously described as being within an auditory fovea. Indeed, 93% of the emissions had centre frequencies above 7.5 kHz. The median of the frequency distances between neighbouring SOAE was 406 Hz (0.058 oct). The 3 dB bandwidth of the emissions depended on their amplitude above the noise: for SOAE whose level exceeded 10 dB above the noise floor, the 3 dB bandwidths ranged between 4.5 and 11.4 Hz. SOAE frequencies were temperature sensitive. Raising the temperature shifted the emissions to higher frequencies, and vice versa (the frequency shifted on average 0.039 oct/degrees C). External tones could suppress the level of SOAE, an effect that was highly tuned. For SOAE with frequencies between 2.5 and 10.5 kHz, the Q(10dB) values of 2 dB iso-suppression tuning curves (STC) varied from 1.07 to 10.40. The best thresholds of 2 dB STC were generally below 15 dB SPL.

Otoacoustic emissions, hair cells, and myosin motors

The stereovillar bundles of hair cells show active movements that may be generated by the putative myosin-actin interactions underlying hair-cell adaptation. Such movement is a possible candidate for the generation of spontaneous otoacoustic emissions (SOAE) in the ear canal of nonmammals. In the basilar papilla of certain lizard families, most hair cells are not coupled by a tectorial membrane, making it easier to assign the energy in emission peaks to defined groups of hair cells. We have studied 62 SOAE in the Bahamian Anole Anolis sagrei, which has about 140 hair cells with "free-standing" bundles in the high-frequency area of its papilla. Individual SOAE peaks were traced to between 3 and 38 hair cells, and the mean power output per hair cell was calculated to be 141 aW. The number of bundle myosins putatively involved in the generation of each SOAE was estimated and the force generated by each myosin at 1 kHz calculated to be approximately 0.1 pN. The data support the idea that hair cells generate emissions and suggest that myosin produces sufficient power to be the emission motor.

A functional map of the pigeon basilar papilla: correlation of the properties of single auditory nerve fibres and their peripheral origin

The purpose of the investigation was to correlate the functional properties of primary auditory fibres with the location of appertaining receptor cells in the avian basilar papilla. The functional properties of 425 single afferent fibres from the auditory nerve of adult pigeons were measured. The peripheral innervation site of 39 fibres was identified by intracellular labelling and correlated with the fibre's functional properties. Mean spontaneous firing rate (SR, 0.1-250/s) was distributed monomodally (mean: 91 +/- 47/s) but not normally. Characteristic frequencies (CFs) were in the range of 0.02-4 kHz. SR, threshold at CF (4-76 dB SPL) and sharpness of tuning (Q10 dB, 0.1-8.8) varied systematically with CF. For a given CF there was a strong correlation of threshold and Q10 dB and of threshold and SR. Labelled fibres innervated different hair cell types over 93% of the length and 97% of the width of the basilar papilla. The majority of fibres innervated hair cells located between 30 and 70% distance from the apex and 0 and 30% distance from the neural edge of the papilla. CFs are mapped tonotopically from high at the base to low at the apex of the papilla, with a mean mapping constant of 0.63 +/- 0.05 mm/octave (in vivo). The highest CF at the base extrapolates to 5.98 +/- 1.17 kHz. The lowest CF mapped at the apex is 0.021 kHz. From the data, together with data from mechanical measurements (Gummer et al., 1987), a frequency-place function of the pigeon papilla was calculated. Transverse gradients of threshold at CF and of Q10 dB were observed across the width of the papilla. Thresholds were lowest and sharpness of tuning was highest above the neural limbus at a distance of 23% from the neural edge of the papilla. Hair cells in this sensitive strip are the tallest and narrowest ones across the width of the papilla. They are packed most densely and receive the largest number of afferent fibres. Fibres innervating (mostly short) hair cells on the free basilar membrane were spontaneously active and responsive to sound. Their Q10 dB was less than average but their sensitivity and SR were comparable to the mean population values. It is concluded that functional properties change gradually not only along the length but also across the width of the pigeon basilar papilla. The results support the idea that sharp frequency tuning of avian primary auditory fibres involves tuning mechanisms supplementary to the tuning of the free part of the basilar membrane.

Filtering of distortion-product otoacoustic emissions in the inner ear of birds and lizards

When the output magnitude of more than one order of distortion-product otoacoustic emission (DPOAE) is measured, they reach their maximum at the same DPOAE frequency. This fact led several authors to the assumption that, subsequent to their generation in the cochlea, the DPOAE are band-pass filtered. It was suggested that the tectorial membrane is the structure responsible for this filtering. In this report, we show that the same kind of "DPOAE tuning' is shown by animals which have hearing organs with tectorial structures of very different morphology, or even with no tectorial membrane at all. We therefore conclude that it is unlikely that the filter is the tectorial membrane.

Stiffness changes in chick hair bundles following in vitro overstimulation

1. The in vitro effect of intense stimulation on the micromechanical stiffness of hair cell sensory hair bundles was studied at three locations on the chick basilar papilla. Threshold levels of hair bundle motion, produced by a water jet stimulus, were examined before and after exposure to a 300 Hz water jet stimulus set at 25 dB above the pre-exposure threshold level. 2. Threshold levels of motion were systematically examined in 8 unexposed control cells. The level of water jet stimulus needed to achieve the detection threshold of motion remained constant in these cells when periodically tested over a 36.5-min interval. 3. Post-exposure changes in the motion detection threshold of hair bundles were examined in 82 hair bundles, and a number of effects were identified: 2.4% of the hair bundles showed no threshold changes; 31.7% of the hair bundles had threshold shifts which indicated an increase in stiffness; 18.3% exhibited a threshold shift that indicated a decrease in hair bundle stiffness, but with no recovery; and 47.6% had thresholds that indicated a decrease in hair bundle stiffness with recovery to pre-exposure levels within 16-18 min. 4. The results suggest that chick hair bundles exhibit complex and varied responses to overstimulation which are very different from that seen in the mammal.