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Angel Ariel Caputi
American gymnotiformes and African mormyriformes have evolved an active sensory system using a self-generated electric field as a carrier of signals. Objects polarized by the discharge of a specialized electric organ project their images on the skin where electroreceptors tuned to the time course of the self-generated field transduce local signals carrying information about impedance, shape, size, and location of objects, as well as electrocommunication messages, and encode them as primary afferents trains of spikes. This system is articulated with other cutaneous systems (passive electroreception and mechanoception) as well as proprioception informing the shape of the fish’s body. Primary afferents project on the electrosensory lobe where electrosensory signals are compared with expectation signals resulting from the integration of recent past electrosensory, other sensory, and, in the case of mormyriformes, electro- and skeleton-motor corollary discharges. This ensemble of signals converges on the apical dendrites of the principal cells where a working memory of the recent past, and therefore predictable input, is continuously built up and updated as a pattern of synaptic weights. The efferent neurons of the electrosensory lobe also project to the torus and indirectly to other brainstem nuclei that implement automatic electro- and skeleton-motor behaviors. Finally, the torus projects via the preglomerular nucleus to the telencephalon where cognitive functions, including “electroperception” of shape-, size- and impedance-related features of objects, recognition of conspecifics, perception based decisions, learning, and abstraction, are organized.
Jeffrey R. Holt and Gwenaëlle S.G. Géléoc
The organs of the vertebrate inner ear respond to a variety of mechanical stimuli: semicircular canals are sensitive to angular velocity, the saccule and utricle respond to linear acceleration (including gravity), and the cochlea is sensitive to airborne vibration, or sound. The ontogenically related lateral line organs, spaced along the sides of aquatic vertebrates, sense water movement. All these organs have a common receptor cell type, which is called the hair cell, for the bundle of enlarged microvilli protruding from its apical surface. In different organs, specialized accessory structures serve to collect, filter, and then deliver these physical stimuli to the hair bundles. The proximal stimulus for all hair cells is deflection of the mechanosensitive hair bundle. Hair cells convert mechanical information contained within the temporal pattern of hair bundle deflections into electrical signals, which they transmit to the brain for interpretation.
Age-related hearing loss affects over half of the elderly population, yet it remains poorly understood. Natural aging can cause the input to the brain from the cochlea to be progressively compromised in most individuals, but in many cases the cochlea has relatively normal sensitivity and yet people have an increasingly difficult time processing complex auditory stimuli. The two main deficits are in sound localization and temporal processing, which lead to poor speech perception. Animal models have shown that there are multiple changes in the brainstem, midbrain, and thalamic auditory areas as a function of age, giving rise to an alteration in the excitatory/inhibitory balance of these neurons. This alteration is manifest in the cerebral cortex as higher spontaneous and driven firing rates, as well as broader spatial and temporal tuning. These alterations in cortical responses could underlie the hearing and speech processing deficits that are common in the aged population.
Natalia Duque-Wilckens and Brian C. Trainor
Aggressive behavior plays an essential role in survival and reproduction across animal species—it has been observed in insects, fish, reptiles, and mammals including humans. Even though specific aggressive behaviors are quite heterogeneous across species, many of the underlying mechanisms modulating aggression are highly conserved. For example, in a variety of species arginine vasopressin (AVP) and its homologue vasotocin in the hypothalamus, play an important role in regulating aggressive behaviorssuch as territorial and inter male aggression. Similarly in the medial amygdala, activation of a subpopulation of GABAergic neurons promotes aggression, while the prefrontal cortex exerts inhibitory control over aggressive behaviors. An important caveat in the aggression literature is that it is focused primarily on males, probably because in most species males are more aggressive than females. However, female aggression is also highly prevalent in many contexts, as it can affect access to resources such as mates, food, and offspring survival. Although it is likely that many underlying mechanisms are shared between sexes, there is sex specific variation in aggression, type, magnitude, and contexts, which suggests that there are important sex differences in how aggression is regulated. For example, while AVP acts to modulate aggression in both male and female hamsters, it increases male aggression but decreases female aggression. These differences can occur at the extent of neurotransmitter or hormones release, sensitivity (i.e., receptor expression), and/or molecular responses.
Paul E. Nachtigall
Toothed whales and dolphins, odontocete cetaceans, produce very loud biosonar sounds in order to navigate and to locate and catch their prey of fish and squid. Underwater biosonar was not discovered until after 1950, but the initial experiments demonstrated a unique sensory modality that could find small targets far away and distinguish between objects buried in mud that differed only by the metal from which they were made. Dolphins determine the distance to their prey by evaluating very small time differences between the outgoing signal and the echo return. The type of outgoing signal varies greatly from low frequency, explosively loud sperm whale clicks, to frequency modulated mid-frequency beaked whale sounds, to very high frequency (over 100 kHz) harbor porpoise signals. All appear to be made by specialized pneumatic phonic lips closely connected to sound projecting fatty melons that focus sound before sending out narrow echolocation sound beams. The frequency of most hearing is matched to echolocation, with the areas of best hearing of the animals being the areas of principal outgoing signal frequency. The sensation levels of hearing are under the animal’s control with “automatic gain control” operating to assure the best hearing of the echo returns. Angular localization of the bottlenose dolphins, for discriminating the minimum audible angles of clicks, is less than one degree in both the horizontal and vertical directions. This remarkable localization performance has yet to be fully explained, but new hypotheses of gular pathways, shaded receiver models, and internal pinnae may provide some explanations as a theory of auditory localization in the odontocetes develops.
Nicolas Dallière, Lindy Holden-Dye, James Dillon, Vincent O'Connor, and Robert J. Walker
The microscopic free-living nematode worm Caenorhabditis elegans was the first metazoan to have its genome sequenced and for many decades has served as a genetically tractable model for the investigation of neural mechanisms of behavioral plasticity. Many of its behaviors involve the detection of its food, bacteria, which are ingested and transported to the intestine by a muscular pharynx. The structure of the pharynx and the circuitry of the pharyngeal nervous system that regulates pharyngeal activity have been described in some detail. This has provided a platform for understanding how this simple organism finely tunes its feeding behavior in response to the changing availability and quality of its food, and in the context of its own nutritional status. This resonates with fundamental principles of energy homeostasis that occur throughout the animal kingdom.
Douglas K. Reilly and Jagan Srinivasan
To survive, animals must properly sense their surrounding environment. The types of sensation that allow for detecting these changes can be categorized as tactile, thermal, aural, or olfactory. Olfaction is one of the most primitive senses, involving the detection of environmental chemical cues. Organisms must sense and discriminate between abiotic and biogenic cues, necessitating a system that can react and respond to changes quickly. The nematode, Caenorhabditis elegans, offers a unique set of tools for studying the biology of olfactory sensation.
The olfactory system in C. elegans is comprised of 14 pairs of amphid neurons in the head and two pairs of phasmid neurons in the tail. The male nervous system contains an additional 89 neurons, many of which are exposed to the environment and contribute to olfaction. The cues sensed by these olfactory neurons initiate a multitude of responses, ranging from developmental changes to behavioral responses. Environmental cues might initiate entry into or exit from a long-lived alternative larval developmental stage (dauer), or pheromonal stimuli may attract sexually mature mates, or repel conspecifics in crowded environments. C. elegans are also capable of sensing abiotic stimuli, exhibiting attraction and repulsion to diverse classes of chemicals. Unlike canonical mammalian olfactory neurons, C. elegans chemosensory neurons express more than one receptor per cell. This enables detection of hundreds of chemical structures and concentrations by a chemosensory nervous system with few cells. However, each neuron detects certain classes of olfactory cues, and, combined with their synaptic pathways, elicit similar responses (i.e., aversive behaviors). The functional architecture of this chemosensory system is capable of supporting the development and behavior of nematodes in a manner efficient enough to allow for the genus to have a cosmopolitan distribution.
In response to changes in metabolic demand, the cardiovascular and respiratory systems are regulated in a highly coordinated fashion, such that both ventilation and cardiac output increase in a parallel fashion, thus maintaining a relatively constant level of arterial blood PO2, PCO2, and pH. In addition, external alerting stimuli that trigger defensive or orienting behavioral responses also trigger coordinated cardiorespiratory changes that are appropriate for the particular behavior. Furthermore, environmental challenges such as hypoxia or submersion evoke complex cardiovascular and respiratory response that have the effect of increasing oxygen uptake and/or conserving the available oxygen.
The brain mechanisms that are responsible for generating coordinated cardiorespiratory responses can be divided into reflex mechanisms and feedforward (central command) mechanisms. Reflexes that regulate cardiorespiratory function arise from a wide variety of internal receptors, and include those that signal changes in blood pressure, the level of blood oxygenation, respiratory activity, and metabolic activity. In most cases more than one reflex is activated, so that the ultimate cardiorespiratory response depends upon the interaction between different reflexes. The essential central pathways that subserve these reflexes are largely located within the brainstem and spinal cord, although they can be powerfully modulated by descending inputs arising from higher levels of the brain. The brain defense mechanisms that regulate the cardiorespiratory responses to external threatening stimuli (e.g., the sight, sound, or odor of a predator) are highly complex, and include both subcortical and cortical systems. The subcortical system, which includes the basal ganglia and midbrain colliculi as essential components, is phylogenetically ancient and generates immediate coordinated cardiorespiratory and motor responses to external stimuli. In contrast, the defense system that includes the cortex, hypothalamus, and limbic system evolved at a later time, and is better adapted to generating coordinated responses to external stimuli that involve cognitive appraisal.
Z. Yan Wang and Clifton W. Ragsdale
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Neuroscience. Please check back later for the full article.
Cephalopod molluscs have the largest invertebrate nervous systems and demonstrate a commensurate repertoire of behaviors, including a sophisticated and extremely rapid adaptive coloration system, extraordinary facility in problem solving, and dramatic visually guided predation capabilities. These complex behaviors have long fascinated humans, and have spurred over a century of research on cephalopod nervous systems.
Extant cephalopods are morphologically diverse and can be divided into the nautiloids, which retain an external shell; and the coleoids, which include octopus, squid, and cuttlefish. Despite their divergence ~400 mya, nautiloids and coleoids share some underlying organizational principles and anatomical features in their nervous systems, including central brain masses situated around the esophagus flanked by a pair of optic lobes. The soft-bodied coleoid cephalopods have, however, developed many striking innovations, such as chemosensory suckers that line the arms, dozens of sharply distinguished lobes within the central brain, and a pair of neuroendocrine organs called the optic gland. Recent developments in cephalopod biology, including the availability of the octopus genome, have led to new insights into cephalopod nervous system organization, including the striking finding that the functional convergence of the cephalopod nervous system with that of vertebrates is paralleled by convergence at the molecular level.
Anna Di Cosmo and Gianluca Polese
Within the Phylum Mollusca, cephalopods encompass a small and complex group of exclusively marine animals that live in all the oceans of the world with the exception of the Black and Caspian seas. They are distributed from shallow waters down into the deep sea, occupying a wide range of ecological niches. They are dominant predators and themselves prey with high visual capability and well-developed vestibular, auditory, and tactile systems. Nevertheless, their perceptions are chemically facilitated, so that water-soluble and volatile odorants are the key mediators of many physiological and behavioral events.
For cephalopods as well as the other aquatic animals, chemical cues convey a remarkable amount of information critical to social interaction, habitat selection, defense, prey localization, courtship and mating, affecting not only individual behavior and population-level processes, but also community organization and ecosystem function. Cephalopods possess chemosensory systems that have anatomical similarities to the olfactory systems of land-based animals, but the molecules perceived from distance are different because their water solubility is of importance. Many insoluble molecules that are detected from distance on land must, in an aquatic system, be perceived by direct contact with the odour source. Most of the studies regarding olfaction in cephalopods have been performed considering only waterborne molecules detected by the “olfactory organs.” However cephalopods are also equipped with “gustatory systems” consisting of receptors distributed on the arm suckers in octopods, buccal lips in decapods, and tentacles in nautiluses.
To date, what is known about the olfactory organ in cephalopods comes from studies on nautiloids and coleoids (decapods and octopods). In the nautiloid’s olfactory system, there is a pair of rhinophores located below each eye and open to the environment with a tiny pore, whereas in coleoids a small pit of ciliated cells is present on either side of the head below the eyes close to the mantle edge.