Intracellular current administration evokes rapid, graded, and bidirectional mechanical responses of isolated outer hair cells from the mammalian inner ear. The cells become shorter in response to depolarizing and longer in response to hyperpolarizing currents in the synaptic end of the cell. The cells respond with either an increase or decrease in length to transcellular alternating current stimulation. The direction of the movement with transcellular stimuli appears to be frequency dependent. Iontophoretic application of acetylcholine to the synaptic end of the cell decreases its length. The microarchitecture of the organ of Corti permits length changes of outer hair cells in a manner that could significantly influence the mechanics of the cochlear partition and thereby contribute to the exquisite sensitivity of mammalian hearing.

1. Single-unit responses in the dorsal cochlear nucleus of unanesthetized, decerebrate cats have been divided into two categoreis. These have been differentiated on the basis of responses to best-frequency tones. Type IV units responded to best-frequency tones with excitation from threshold to about 20 or 30 dB above threshold; at higher levels, their response was inhibitory. In a few cases, the excitatory area near threshold was not seen and in a few others, the response became excitatory again at high levels. Type IV units could be divided into two groups based on the length of time that inhibition was maintained in response to long tones. Type IV units are not seen in anesthetized cats. 2. Type II/III units responded to best-frequency tones of all levels with excitation. Nonmonotonic rate versus level functions were seen in type II/III units, but they were of much less drastic character; the discharge rate of nonmonotonic type II/III units was still well above spontaneous rate for tones 50 dB above threshold. Type II/III units defined in this way were found to have, on the average, lower rates of spontaneous activity and higher thresholds than type IV units. 3. Type II/III units responded weakly to broad-band noise in comparison to auditory nerve fibers and many of them did not respond at all to noise. Type IV units, with best frequencies above 0.9 kHz, gave excitatory responses to noise. 4. The inhibitory response areas of type IV units could be divided into two areas: a central inhibitory area in the vicinity of best frequency where on- and off-discharges and afterdischarges were seen; and inhibitory side bands at higher and lower frequencies where simple inhibitory responses were seen. In four units, it was possible to show that the central inhibitory area was converted to an excitatory area after administration of an anesthetic dose of pentobarbital. 5. Most type II/III and type IV units could be excited or inhibited by stimuli in the contralateral ear. Broad-band noise was a more effective contralateral stimulus than tones at the ipsilateral best frequency. 6. On the basis of the properties of type II/III and type IV cells, it is suggested that type II/III responses are recorded from interneurons which provide a large share of the inhibitory imput to type IV cells.

1. Recordings were made from single nerve fibers in barbiturate-anesthetized cats in the midline trapezoid body, a location that permits selective sampling of efferent cells of the ventral cochlear nucleus. Single units were localized to either the dorsal or ventral components of the trapezoid body. The fibers were physiologically classified on the basis of their peristimulus time histograms (PSTH) and receptive-field properties. In addition, low characteristic frequency (CF) units were probed for rapid rate and phase shifts with increases in intensity. The projection patterns of some fibers were traced by iontophoresing horseradish peroxidase (HRP) into their axons. 2. HRP-labeled fibers most likely originated from globular bushy cells of the ventral cochlear nucleus in that they sent a large branch into the contralateral medial nucleus of the trapezoid body which terminated in a calyceal ending and an ipsilateral branch into the lateral nucleus of the trapezoid body. A thin branch, usually starting from the large branch, wound its way through the medial nucleus of the trapezoid body to its termination in the ventral nucleus of the trapezoid body. Additional branches from the parent axon could pass through medial periolivary groups throughout the rostrocaudal extent of the superior olivary complex. The parent fiber was traced as far as the ventral lateral lemniscus where it faded before reaching its termination. 3. The majority of units were recorded in the ventral component of the trapezoid body. Although the ventral component is comprised of both large and small diameter fibers, our sample was biased to the larger diameter fibers representing the activity of axons originating from globular bushy cells in the ventral cochlear nucleus. Ventral component units were not tonotopically arrayed and had CFs that spanned the audible range for cats. HRP labeling of ventral component axons revealed that the section of the axon traveling through the midline shifted its dorsal-ventral location. This pattern was compatible with the lack of tonotopy found in the ventral component. Recordings were also made from the dorsal component of the trapezoid body, which contained medium diameter axons. These axons originated from spherical bushy cells in the ventral cochlear nucleus. Dorsal component units were tonotopically arrayed and had CFs less than 7 kHz. 4. Cells were characterized by their PSTH at CF. Primary-like and phase-locked units constituted most of the dorsal component units.(ABSTRACT TRUNCATED AT 400 WORDS)

1. Voltage-dependent properties of isolated guinea pig outer hair cells (OHCs) were investigated using whole-cell recording. An inward current was detected in approximately 10% of the cells. This inward current was identified as belonging to the voltage-activated sodium current family on the basis of its high sensitivity to tetrodotoxin and the effect of substitution of impermeant ions. Although this is the first report of a sodium current in the mammalian cochlea, it differs from the classical neuronal sodium current by having a variable magnitude from cell to cell and an inactivation that is shifted to hyperpolarized potentials. The sensory processing role of hair cells in general and outer hair cells in particular could be disrupted by the presence of a regenerative voltage-dependent current. The functional role of the OHC sodium channels is puzzling, particularly as they may be silent in vivo.