Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item Sculpting Sounds: INTRINSIC PHYSIOLOGY AND INHIBITORY ANATOMY OF THE AVIAN AUDITORY BRAINSTEM(2024) Baldassano, James; MacLeod, Katrina; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Soundwaves are rapidly modulated, multi-dimensional stimuli. The cochlea decomposes these signals into frequency and intensity information which is conveyed via the auditory nerve into the brain. How does the brain manage to extract these multidimensional signals from auditory nerve activity? How does it sculpt this input so that both the microsecond precision of “where?” and the spectrotemporal modulations of “what?” are encoded with high fidelity? Birds are powerful models for studying early auditory processing because they interact with sounds similarly to mammals but have a simpler neuronal architecture. We describe the intrinsic physiology and anatomy and auditory brainstem neurons involved in spectrotemporal processing. In birds, the auditory nerve synapses onto two anatomically distinct cochlear nuclei, cochlear nucleus magnocellularis (NM) which encodes frequency/timing information, and the more heterogeneous cochlear nucleus angularis (NA) which encodes intensity information. NA has been shown to encode the acoustic envelope, likely through a subset of neurons that respond preferentially to modulations in their inputs via an adaptive spike threshold. We first examined the intrinsic basis of this adaptive threshold and found that a dendrotoxin-sensitive low threshold potassium conductance is responsible for it. In addition to the intrinsic properties of neurons, inhibition sculpts a number of auditory processes. The majority of inhibition in the avian auditory brainstem originates in the superior olivary nucleus (SON), which has multiple response types & projects either to multiple lower order ipsilateral nuclei, including NA & NM, or to the contralateral SON. Retrograde labeling experiments have demonstrated that these projections originate from distinct populations of SON neurons, however it is not clear if there is a relationship between response types and postsynaptic target. We used in vitro electrophysiology and neuronal reconstruction to establish a relationship between response types and targets. While the function of inhibition is well documented in timing circuits, its role in intensity processing is less clear. We used dynamic clamp to model inhibitory conductances while recording from NA neurons in vitro to determine how inhibition impacts the range of inputs that a NA neuron can encode before its firing rate saturates.Item INHIBITION IN THE CRAYFISH LATERAL GIANT CIRCUIT(2020) Winter, Lucy Soda Venuti; Herberholz, Jens; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Inhibition is critical for the proper functioning of neural circuits. Crayfish present a unique opportunity for the study of inhibition. Crustaceans have been used extensively as model organisms, and many important neuroscientific phenomena were originally described in crayfish or other crustaceans. Their escape responses, mediated by giant fibers, have received particular attention. The lateral giant system has been mapped out in great detail, and every synapse between the receptors that stimulate it to the muscles it recruits is known. Quite surprisingly, despite this extensive knowledge of the excitatory portions of the circuit, its inhibitors are still poorly understood. The lateral giant interneuron is a particularly good target in which to study inhibition, as it receives three unique types of inhibition. Its firing causes a rapid autoinhibition of the neuron, its general excitability is modulated by tonic inhibition, and its selectivity to sudden, phasic stimuli is partially mediated by sensory-evoked inhibition. While the existence and very basic characterization of these forms of inhibition have been described, the mechanisms and details of all three remain elusive. Here I present data which aids substantially in our understanding of these inhibitory systems. I show that the lateral giant’s autoinhibition is mediated by both GABA and glutamate, and that the axon of lateral giant neuron responds to these inhibitory neurotransmitters. I also pharmacologically characterize the inhibitory inputs evoked by its sensory afferents, and show that the neuron is sensitive to THIP, a compound which is selective for receptor subtypes that mediate tonic inhibition. In addition, I utilize alcohol exposure to uncover these mechanisms, allowing it to be used to interpret the recently discovered social modulation of alcohol’s effect that is seen in crayfish, and aiding in our understanding of alcohol’s interplay with cellular inhibition.