Ruili Xie, Tessa-Jonne F. Ropp, Michael R. Kasten, and Paul B. Manis
Hearing loss generally occurs in the auditory periphery but leads to changes in the central auditory system. Noise-induced hearing loss (NIHL) and age-related hearing loss (ARHL) affect neurons in the ventral cochlear nucleus (VCN) at both the cellular and systems levels. In response to a decrease in auditory nerve activity associated with hearing loss, the large synaptic endings of the auditory nerve, the endbulbs of Held, undergo simplification of their structure and the volume of the postsynaptic bushy neurons decreases. A major functional change shared by NIHL and ARHL is the development of asynchronous transmitter release at endbulb synapses during periods of high afferent firing. Compensatory adjustements in transmitter release, including changes in release probability and quantal content, have also been reported. The excitability of the bushy cells undergoes subtle changes in the long-term, although short-term, reversible changes in excitability may also occur. These changes are not consistently observed across all models of hearing loss, suggesting that the time course of hearing loss, and potential developmental effects, may influence endbulb transmission in multiple ways. NIHL can alter the representation of the loudness of tonal stimuli by VCN neurons and is accompanied by changes in spontaneous activity in VCN neurons. However, little is known about the representation of more complex stimuli. The relationship between mechanistic changes in VCN neurons with noise-induced or age-related hearing loss, the accompanying change in sensory coding, and the reversibility of changes with the reintroduction of auditory nerve activity are areas that deserve further thoughtful exploration.
Donald M. Caspary and Daniel A. Llano
As arguably the third most common malady of industrialized populations, age-related hearing loss is associated with social isolation and depression in a subset of the population that will approach 25% by 2050. Development of behavioral or pharmacotherapeutic approaches to prevent or delay the onset of age-related hearing loss and mitigate the impact of hearing loss of speech understanding requires a better understanding of age-related changes that occur in the central auditory processor. This chapter critically reviews and discusses changes that occur in the auditory brainstem and thalamus with increased age. It briefly discusses age-related cellular changes that occur de novo within the central auditory system versus deafferentation plasticity and animal models of aging. Subsections discuss the cochlear nucleus, superior olivary complex, inferior colliculus, and the medial geniculate body with an emphasis on age-related changes in neurotransmission and how these changes could underpin the observed loss of precise temporal processing with increased age.
Romuald Nargeot and Alexis Bédécarrats
Behaviors of invertebrates can be modified by associative learning in a similar manner to those of vertebrates. Two simple forms of associative learning, Pavlovian and operant conditioning, allow animals to establish a predictive relationship between two events. Here we summarize five decades of studies of behavioral, cellular, and subcellular changes that are induced by these two learning paradigms in different invertebrate animal models. A comparative description of circuitry, neuronal elements, and properties that contribute to these conditioning procedures will be drawn to decipher common and distinguishing features of the learning processes. We will illustrate that similar circuits, synaptic and neuronal membrane plasticity, and similar molecular sites of detection of association are implicated in both forms of conditioning. However, evidence will also suggest that passively responding and endogenous dynamic properties of central networks and/or their constituent neurons might differentially contribute to Pavlovian and operant learning.
Yun Doo Chung and Jeongmi Lee
Hearing in invertebrates has evolved independently as an adaptation to avoid predators or to mediate intraspecific communication. Although many invertebrate groups are able to respond to sound stimuli, insects are the only group in which hearing is widely used. Therefore, we will focus here on the auditory systems of some well-known insect models. Appearance of the ability to perceive sound in insects is presumably a quite recent event in evolution. As a result of independent evolution, diverse types of hearing organs are evolved in insects. Here we will introduce basic features of insect ears and the mechanisms through which sound stimuli are converted into neuronal electric signals. We will also summarize our current understanding of neural processing of auditory information, including tonotopy, sound localization, and pattern recognition.
Nina Kraus and Trent Nicol
The encoding of speech and music in the auditory brainstem is available at the human scalp via the auditory-evoked frequency following response. The FFR, primarily reflecting activity in the inferior colliculus, may be evoked by speech or music stimulation and represents the combined activity of sensorimotor, cognitive, and reward centers in the brain. Its response properties, like the inferior colliculus itself, are influenced by long-term experience with sound. The transparency, individual-level reliability, and ability to gauge neural plasticity provide the researcher and clinician a powerful probe of auditory processing in the human brainstem. With it, we have learned a great deal about how mechanisms of decline, deprivation, and enrichment affect the processing of complex signals such as music and speech in the human brainstem.
Andrew J. Todd and Fan Wang
Nociceptive primary afferents detect stimuli that are normally perceived as painful, and these afferents form synapses in the dorsal horn of the spinal cord and the spinal trigeminal nucleus. Here they are involved in highly complex neuronal circuits involving projection neurons belonging to the anterolateral tract (ALT) and interneurons, which modulate the incoming sensory information. The ALT neurons convey somatosensory information to a variety of brain regions that are involved in the various aspects of the pain experience. A spinothalamic-cortical pathway provides input to several regions of the cerebral cortex, including the first and second somatosensory areas (S1, S2), the insula and the cingluate cortex. These regions are thought be responsible for the sensory-discriminative aspects of pain (S1), pain-related learning (S2), the autonomic and motivational responses (insula), and the negative affect (cingulate). Another ascending system, The spinoparabrachial-limbic pathway targets a variety of brain regions, including the amygdala, and is likely involved in the affective component of pain. A descending system that includes the limbic system, the periaqueductal gray matter of the midbrain, the locus coeruleus, and the rostral ventral medulla, can suppress pain, and this operates partly through the monoamine transmitters noradrenaline and serotonin which are released in the spinal and trigeminal dorsal horn.
Changes in the Inferior Colliculus Associated with Hearing Loss: Noise-Induced Hearing Loss, Age-Related Hearing Loss, Tinnitus and Hyperacusis
Alan R. Palmer and Joel I. Berger
The inferior colliculus is an important auditory relay center that undergoes fundamental changes following hearing loss, whether noise induced (NIHL) or age related (ARHL). These changes may contribute to the induction or maintenance of phenomena such as tinnitus (phantom auditory sensations) and hyperacusis (increased sensitivity to sound). Here, we outline changes that can occur in the inferior colliculus following damage to the periphery and/or as a result of the ageing process, both immediate and long-term, and attempt to disentangle which changes relate to either tinnitus or hyperacusis, as opposed to solely hearing loss. Understanding these changes is ultimately important to reversing the underlying pathology and treating these conditions.
Reception of chemicals via olfaction and gustation are prerequisites to find, distinguish, and recognize food and mates and to avoid dangers. Several receptor gene superfamilies are employed in arthropod chemosensation: inverse 7-transmembrane (7-TM) gustatory and olfactory receptors (GRs, ORs), 3-TM ionotropic glutamate-related receptors (IRs), receptor-guanylyl cyclases, transient receptor potential ion channels, and epithelial sodium channels. Some of these receptor gene families have ancient origins and expanded in several taxa, producing very large, variant gene families adapted to the respectively relevant odor ligands in species-specific environments. Biochemical and electrophysiological studies in situ as well as molecular genetics found evidence for G-protein-dependent signal transduction cascades for ORs, GRs, and IRs, suggesting that signal amplification is paramount for chemical senses. In contrast, heterologous expression studies argued for primarily ionotropic transduction as a prerequisite to interstimulus intervals in the range of microseconds.
Important cnidarian contributions to our understanding of nervous system evolution may be found in the arrangement of conducting systems and their interactions. We see multiple, diffuse systems that interact to produce specific behaviors, the compression of conducting systems into compact directional or bidirectional conduction systems, and accumulation of multiple compressed conducting systems into integrating structures like nerve rings. We even see ganglion-like rhopalia that contain bilateral and directional conducting pathways. We now know that this compression and specificity of connections is controlled by conserved sets of genetic commands similar to those found in bilateral animals, and likely in common ancestors. This gradation in centralization is only limited in a directed pathway by the unique radial symmetry of cnidarians. Based on the compression of cnidarian conducting systems into integrating centers (nerve rings and rhopalia), the primary hurdle to cephalization is body symmetry. Medusoid cnidarians possess multiple “brains” connected by conducting systems that, by necessity, are nonpolarized.
Donata Oertel, Xiao-Jie Cao, and Alberto Recio-Spinoso
Plasticity in neuronal circuits is essential for optimizing connections as animals develop and for adapting to injuries and aging, but it can also distort the processing, as well as compromise the conveyance of ongoing sensory information. This chapter summarizes evidence from electrophysiological studies in slices and in vivo that shows how remarkably robust signaling is in principal cells of the ventral cochlear nucleus. Even in the face of short-term plasticity, these neurons signal rapidly and with temporal precision. They can relay ongoing acoustic information from the cochlea to the brain largely independently of sounds to which they were exposed previously.