| Sounds in real-world situations seldom occur in isolation. The temporal features of biological sounds just like mating calls of frogs, ultrasonic calls of echolocating bats, vocal communication of human being, are all complicated. In all vertebrates, their lower brainstem contains a number of parallel auditory pathways that provide excitatory or inhibitory outputs differing in their temporal discharge patterns and latencies. These pathways converge in the auditory midbrain, where many neurons are tuned to biologically important parameters of sound. The inferior colliculus is a nexus in the mammalian auditory system where the ascending fibers from 10 or more of the lower auditory nuclei make an obligatory synaptic connection. The inferior colliculus is the major source of auditory inputs to the thalamocortical pathway. In addition, the inferior colliculus of echolocating bats is a major source of inputs to areas involved in motor coordination, include the superior colliculus, an area responsible for orienting movements, and the pontine gray, a source of inputs to the cerebellum.Echolocating bats rely on their sense of hearing for obtaining information about their external world. In keeping with their reliance on hearing, their auditory systems are not only well developed, but are also proportionately much larger than are the auditory systems of other mammals. Bat is an important species of animals in auditory studies. Plenty of research works have been done in bats, such as ethology, neurophysiology and neuroanatomy. The vocalizations of bats have specific temporal features that are potential sources of information for echolocation. These features include the duration of the pulses, the rate of frequency change within a pulse, and the interval between pulses. The time difference between pulse and echo is a potential cue for determining the distance of a reflective object.In the first part experiment of our present studies, we investigated the effects of GABA-mediated inhibition on intensity and frequency sensitivity of the inferior colliculus of the big brown bat, with the string of the different repetition rate of soundpulse trains to imitate the repetition rate of echoes which the echolocating bats receive from the objects in the state of nature. In the second part experiment of our studies, we investigated the characteristics of responses to two tones in different intensity and interval. It is important to understand the mechanisms of auditory system how to process and integrate the temporal information of different temporally patterned sounds. 1. Effects of GABAergic inhibition on the responses of inferior collicular neuronsto temporally patterned pulse trainsExperimental subjects comprised 13 big brown bats (Eptesucus fuscus). Electrophysiological technique and iontophoresis of drug were used in the following investigations. The repetitive rates of acoustic signal trains used in the experiment were 10 pps (pulse per second), 30 pps, 90 pps. Tone pulses of 4 ms duration (including a rise-fall time of 0.5 ms) were presented to isolate the single 1C neuron and recode its best frequency (BF), minimum threshold (MT), and latency and receding depth. Each procedure had its own post-stimulus time (PST) histograms. Then the rate-intensity functions (RIF) of units were constructed by changing the intensity from the MT to the highest loud level of the loudspeaker. The isointensity frequency tuning curves (IFTC) were constructed by changing the frequency at 20 dB above the MT.I. Effects of GABA on Temporal Discharge Patterns of 1C Neurons to Acoustic Pulse TrainWe studied the responses of 53 units. The responses of 45(78%) 1C neurons tested were changed during the application of bicuculline, a competitive antagonist for y-aminobutyric acid-A (GABAA) receptors. These neurons responded to every pulse of 10 pps and 30 pps sound train before and during bicuculline application. But when the pulse repetition rate of sound train was 90 pps, the responses of the neurons changed a lot. During bicuculline ap...
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