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Integrative and Comparative Biology Aug 2018During rapid locomotion, the vestibular inner ear provides head-motion signals that stabilize posture, gaze, and heading. Afferent nerve fibers from central and... (Review)
Review
During rapid locomotion, the vestibular inner ear provides head-motion signals that stabilize posture, gaze, and heading. Afferent nerve fibers from central and peripheral zones of vestibular sensory epithelia use temporal and rate encoding, respectively, to emphasize different aspects of head motion: central afferents adapt faster to sustained head position and favor higher stimulus frequencies, reflecting specializations at each stage from motion of the accessory structure to spike propagation to the brain. One specialization in amniotes is an unusual nonquantal synaptic mechanism by which type I hair cells transmit to large calyceal terminals of afferent neurons. The reduced synaptic delay of this mechanism may have evolved to serve reliable and fast input to reflex pathways that ensure stable locomotion on land.
Topics: Animals; Ear, Inner; Hair Cells, Vestibular; Neurons, Afferent; Signal Transduction; Vertebrates
PubMed: 29920589
DOI: 10.1093/icb/icy069 -
Current Opinion in Neurobiology Dec 2018Proprioceptive sensory input and descending supraspinal projections are two major inputs that feed into and influence spinal circuitry and locomotor behaviors. Here we... (Review)
Review
Proprioceptive sensory input and descending supraspinal projections are two major inputs that feed into and influence spinal circuitry and locomotor behaviors. Here we review their influence on each other during development and after spinal cord injury. We highlight developmental mechanisms of circuit formation as they relate to the sensory-motor circuit and its reciprocal interactions with local spinal interneurons, as well as competitive interactions between proprioceptive and descending supraspinal inputs in the setting of spinal cord injury.
Topics: Animals; Humans; Interneurons; Locomotion; Nerve Net; Neurons, Afferent; Neurons, Efferent; Proprioception; Spinal Cord; Spinal Cord Injuries
PubMed: 30205323
DOI: 10.1016/j.conb.2018.08.008 -
International Journal of Molecular... Jul 2023Capsaicin-sensitive peptidergic sensory nerves mediate triple actions: besides transmitting sensory and pain signals to the central nervous system (afferent function),...
Capsaicin-sensitive peptidergic sensory nerves mediate triple actions: besides transmitting sensory and pain signals to the central nervous system (afferent function), they also have local and systemic efferent functions [...].
Topics: Humans; Neurons, Afferent; Afferent Pathways; Peripheral Nervous System; Capsaicin; Pain; Inflammation
PubMed: 37569621
DOI: 10.3390/ijms241512243 -
Journal of Neurophysiology Dec 2019Semicircular canal afferent neurons transmit information about head rotation to the brain. Mathematical models of how they do this have coevolved with concepts of how... (Review)
Review
Semicircular canal afferent neurons transmit information about head rotation to the brain. Mathematical models of how they do this have coevolved with concepts of how brains perceive the world. A 19th-century "camera" metaphor, in which sensory neurons project an image of the world captured by sense organs into the brain, gave way to a 20th-century view of sensory nerves as communication channels providing inputs to dynamical control systems. Now, in the 21st century, brains are being modeled as Bayesian observers who infer what is happening in the world given noisy, incomplete, and distorted sense data. The semicircular canals of the vestibular apparatus provide an experimentally accessible, low-dimensional system for developing and testing dynamical Bayesian generative models of sense data. In this review, we summarize advances in mathematical modeling of information transmission by semicircular canal afferent sensory neurons since the first such model was proposed nearly a century ago. Models of information transmission by vestibular afferent neurons may provide a foundation for developing realistic models of how brains perceive the world by inferring the causes of sense data.
Topics: Animals; Models, Biological; Neurons, Afferent; Semicircular Canals; Vestibule, Labyrinth
PubMed: 31693427
DOI: 10.1152/jn.00087.2019 -
Experimental Physiology Jan 2024Our objective was to evaluate an ex vivo muscle-nerve preparation used to study mechanosensory signalling by low threshold mechanosensory receptors (LTMRs).... (Review)
Review
Our objective was to evaluate an ex vivo muscle-nerve preparation used to study mechanosensory signalling by low threshold mechanosensory receptors (LTMRs). Specifically, we aimed to assess how well the ex vivo preparation represents in vivo firing behaviours of the three major LTMR subtypes of muscle primary sensory afferents, namely type Ia and II muscle spindle (MS) afferents and type Ib tendon organ afferents. Using published procedures for ex vivo study of LTMRs in mouse hindlimb muscles, we replicated earlier reports on afferent firing in response to conventional stretch paradigms applied to non-contracting, that is passive, muscle. Relative to in vivo studies, stretch-evoked firing for confirmed MS afferents in the ex vivo preparation was markedly reduced in firing rate and deficient in encoding dynamic features of muscle stretch. These deficiencies precluded conventional means of discriminating type Ia and II afferents. Muscle afferents, including confirmed Ib afferents were often indistinguishable based on their similar firing responses to the same physiologically relevant stretch paradigms. These observations raise uncertainty about conclusions drawn from earlier ex vivo studies that either attribute findings to specific afferent types or suggest an absence of treatment effects on dynamic firing. However, we found that replacing the recording solution with bicarbonate buffer resulted in afferent firing rates and profiles more like those seen in vivo. Improving representation of the distinctive sensory encoding properties in ex vivo muscle-nerve preparations will promote accuracy in assigning molecular markers and mechanisms to heterogeneous types of muscle mechanosensory neurons.
Topics: Mice; Animals; Muscle Spindles; Tendons; Signal Transduction; Neurons; Neurons, Afferent
PubMed: 37119460
DOI: 10.1113/EP090763 -
Current Opinion in Pharmacology Dec 2016Vagal afferent neurons (VANs) play an important role in the control of food intake by signaling nutrient type and quantity to the brain. Recent findings are broadening... (Review)
Review
Vagal afferent neurons (VANs) play an important role in the control of food intake by signaling nutrient type and quantity to the brain. Recent findings are broadening our view of how VANs impact not only food intake but also energy homeostasis. This review focuses exclusively on studies of the vagus nerve from the past 2 years that highlight major new advancements in the field. We firstly discuss evidence that VANs can directly sense nutrients, and we consider new insights into mechanisms affecting sensing of gastric distension and signaling by gastrointestinal hormones ghrelin and GLP1. We discuss evidence that disrupting vagal afferent signaling increases long-term control of food intake and body weight management, and the importance of this gut-brain pathway in mediating beneficial effects of bariatric surgery. We conclude by highlighting novel roles for vagal afferent neurons in circadian rhythm, thermogenesis, and reward that may provide insight into mechanisms by which VAN nutrient sensing controls long-term control of energy homeostasis.
Topics: Animals; Body Weight; Circadian Rhythm; Eating; Energy Metabolism; Ghrelin; Glucagon-Like Peptide 1; Homeostasis; Humans; Neurons, Afferent; Reward; Signal Transduction; Vagus Nerve
PubMed: 27591963
DOI: 10.1016/j.coph.2016.08.007 -
Anatomical Record (Hoboken, N.J. : 2007) Mar 2019The VIII nerve is formed by sensory neurons that innervate the inner ear, i.e., the vestibular and the auditory receptors. Neurons of the auditory portion, the cochlear... (Review)
Review
The VIII nerve is formed by sensory neurons that innervate the inner ear, i.e., the vestibular and the auditory receptors. Neurons of the auditory portion, the cochlear afferent fibers that innervate the sensory hair cells of the organ of Corti, have their somas in the cochlear spiral ganglion where two types of neurons can be distinguished. Afferent Type-I neurons are the 95% of the total population. Bipolar and myelinated fibers, each one innervates only one cochlear inner hair cell (IHC). In contrast, afferent Type-II neurons are only the 5% of the spiral ganglion population. They are pseudounipolar and unmyelinated fibers and innervate the cochlear outer hair cells (OHC) so that one afferent Type-II fiber contacts with multiple OHCs, but each OHC only receives one contact from one Type-II neuron. Both types of VIII nerve fibers are glutamatergic, but these asymmetric innervations of the cochlear sensory cells could suggest that the IHC codifies the truly auditory message but the OHC only informs about mechanical aspects of the state of the organ of Corti. In fact, the central nervous system (CNS) has control over the information transmitted by the Type-I neuron by means of axons from the superior olivary complex that innervate them to modulate, filter and/or inhibit the entry of auditory message to CNS. The aim of this paper is to review the current knowledge about the anatomy and physiology of the auditory portion of the VIII nerve. Anat Rec, 302:463-471, 2019. © 2018 Wiley Periodicals, Inc.
Topics: Afferent Pathways; Animals; Auditory Pathways; Axons; Cochlea; Cochlear Nerve; Hair Cells, Auditory; Mice; Mice, Inbred C57BL; Nerve Fibers; Neurons; Spiral Ganglion
PubMed: 29659185
DOI: 10.1002/ar.23815 -
American Journal of Physiology.... Sep 2019The distal colon is innervated by the splanchnic and pelvic nerves, which relay into the thoracolumbar and lumbosacral spinal cord, respectively. Although the peripheral...
The distal colon is innervated by the splanchnic and pelvic nerves, which relay into the thoracolumbar and lumbosacral spinal cord, respectively. Although the peripheral properties of the colonic afferent nerves within these pathways are well studied, their input into the spinal cord remain ill defined. The use of dual retrograde tracing from the colon wall and lumen, in conjunction with in vivo colorectal distension and spinal neuronal activation labeling with phosphorylated MAPK ERK 1/2 (pERK), allowed us to identify thoracolumbar and lumbosacral spinal cord circuits processing colonic afferent input. In the thoracolumbar dorsal horn, central projections of colonic afferents were primarily labeled from the wall of the colon and localized in laminae I and V. In contrast, lumbosacral projections were identified from both lumen and wall tracing, present within various dorsal horn laminae, collateral tracts, and the dorsal gray commissure. Nonnoxious in vivo colorectal distension evoked significant neuronal activation (pERK-immunoreactivity) within the lumbosacral dorsal horn but not in thoracolumbar regions. However, noxious in vivo colorectal distension evoked significant neuronal activation in both the thoracolumbar and lumbosacral dorsal horn, with the distribution of activated neurons correlating to the pattern of traced projections. Dorsal horn neurons activated by colorectal distension were identified as possible populations of projection neurons or excitatory and inhibitory interneurons based on their neurochemistry. Our findings demonstrate how colonic afferents in splanchnic and pelvic pathways differentially relay mechanosensory information into the spinal cord and contribute to the recruitment of spinal cord pathways processing non-noxious and noxious stimuli. In mice, retrograde tracing from the colon wall and lumen was used to identify unique populations of afferent neurons and central projections within the spinal cord dorsal horn. We show that there are pronounced differences between the spinal cord regions in the distribution pattern of colonic afferent central projections and the pattern of dorsal horn neuron activation evoked by colorectal distension. These findings demonstrate how colonic afferent input influences spinal processing of colonic mechanosensation.
Topics: Afferent Pathways; Animals; Colon; Male; Mice, Inbred C57BL; Neurons, Afferent; Posterior Horn Cells; Spinal Cord
PubMed: 31188624
DOI: 10.1152/ajpgi.00013.2019 -
Hearing Research Jun 2016The spiral ganglion neurons (SGNs) are the first action potential generating neurons in the auditory pathway. The type I SGNs contact the sensory inner hair cells via... (Review)
Review
The spiral ganglion neurons (SGNs) are the first action potential generating neurons in the auditory pathway. The type I SGNs contact the sensory inner hair cells via their peripheral dendrites and relay auditory information to the brainstem via their central axon fibers. Individual afferent fibers show differences in response properties that are essential for normal hearing. The mechanisms that give rise to the heterogeneity of afferent responses are very poorly understood but are likely already in place at the peripheral dendrites where synapses are formed and action potentials are generated. To identify these molecular mechanisms, this review synthesizes a variety of literature and comprehensively outlines the cellular and molecular components positioned to regulate SGN afferent dendrite excitability, especially following glutamate release. These components include 1) proteins of the SGN postsynapses and neighboring supporting cells that together shape glutamatergic signaling, 2) the ion channels and transporters that determine the intrinsic excitability of the SGN afferent dendrites, and 3) the neurotransmitter receptors that extrinsically modify this excitability via synaptic input from the lateral olivocochlear efferents. This cellular and molecular machinery, together with presynaptic specializations of the inner hair cells, can be collectively referred to as the type I afferent signaling complex. As this review underscores, interactions of this signaling complex determine excitability of the SGN afferent dendrites and the afferent fiber responses. Moreover, this complex establishes the environmental milieu critical for the development and maintenance of the SGN afferent dendrites and synapses. Motivated by these important functions, this review also indicates areas of future research to elucidate the contributions of the afferent signaling complex to both normal hearing and also hearing loss.
Topics: Acetylcholine; Action Potentials; Animals; Animals, Newborn; Cochlea; Dendrites; Dopamine; Ganglia, Spinal; Gerbillinae; Guinea Pigs; Hair Cells, Auditory, Inner; Hearing Loss; Humans; Mice; Neurons, Afferent; Rats; Signal Transduction; Spiral Ganglion; Synapses; Synaptic Potentials; gamma-Aminobutyric Acid
PubMed: 27018296
DOI: 10.1016/j.heares.2016.03.011 -
Journal of Neurophysiology Aug 2018Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are... (Review)
Review
Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via γ-motoneurons (fusimotor neurons), that control their sensitivity to stretch. Much of what we know about human muscles spindles comes from studying the behavior of their afferents via intraneural microelectrodes (microneurography) inserted into accessible peripheral nerves. We review the functional properties of human muscle spindles, comparing and contrasting with what we know about the functions of muscle spindles studied in experimental animals. As in the cat, many human muscle spindles possess a background discharge that is related to the degree of muscle stretch, but mean firing rates are much lower (~10 Hz). They can faithfully encode changes in muscle fascicle length in passive conditions, but higher level extraction of information is required by the central nervous system to measure changes in muscle length during muscle contraction. Moreover, although there is some evidence supporting independent control of human muscle spindles via fusimotor neurons, any effects are modest compared with the clearly independent control of fusimotor neurons observed in the cat.
Topics: Action Potentials; Animals; Humans; Motor Neurons, Gamma; Muscle Contraction; Muscle Spindles; Neurons, Afferent; Proprioception
PubMed: 29668385
DOI: 10.1152/jn.00071.2018