Bipolar Neuron

They are bipolar neurons with a dendritic process ending in an apical swelling called an olfactory knob, which is exposed to the outside world, and an axon that projects through the cribriform plate into the olfactory bulb.

From: Encyclopedia of Gastroenterology , 2004

Cranial Nerves

Ann B. Butler , in Encyclopedia of the Human Brain, 2002

II.B.1.a Cochlear Division

The bipolar neurons of the cochlear division of VIII innervate the hair cells of the cochlear organ of Corti. The apical surfaces of the hair cells are studded with stereocilia and border the inner chamber of the cochlea, the scala media, which is filled with endolymph. A tectorial membrane overlies the stereocilia. An outer chamber, formed by the scala vestibuli and scala tympani, contains perilymph, and the scala tympani portion is separated from the organ of Corti by a basilar membrane. Sound waves cause vibration of the perilymph and resultant displacement of the basilar membrane, which in turn pushes the stereocilia against the tectorial membrane, bending them and opening ion channels. Influx of potassium from the endolymph through the opened channels causes depolarization of the receptor cell. The frequency of the auditory stimuli is tonotopically mapped along the length of the cochlear spiral, with best responses to high frequencies occurring at its base and those to lower frequencies at more apical locations. The bipolar neurons preserve the tonotopic map for relay to the cochlear nuclei and then throughout the ascending auditory pathway. They also encode intensity by their discharge rate.

Cell bodies of cochlear bipolar neurons lie within the spiral ganglion, named for the shape of the cochlea. Their central processes enter the lateral aspect of the brain stem at a caudal pontine level and terminate in the dorsal and ventral cochlear nuclei. The cochlear nuclei project to multiple sites, including the superior olivary nuclear complex in the pons and the inferior colliculus in the midbrain roof. The superior olivary complex (SO) receives bilateral input mainly from the so-called bushy cells of the ventral cochlear nucleus; SO neurons are coincidence detectors that utilize the time delay between the inputs from the two sides in order to compute the location in space of the sound source. The ascending auditory projections predominantly originate from pyramidal neurons within the dorsal cochlear nucleus and from the superior olivary complex and pass via the lateral lemniscus to the inferior colliculus, which in turn projects to the medial geniculate body of the dorsal thalamus. The latter projects to auditory cortex in the temporal lobe. Damage to the cochlear division of VIII results in dysfunction (such as experiencing a buzzing sound) and/or deafness.

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Special Senses—Vision and Hearing

Bruce M. Carlson MD, PhD , in The Human Body, 2019

Retinal Output

Both photoreceptor and bipolar neurons have very short axons and consequently don't require action potentials for transmission of impulses. Retinal ganglion cells, on the other hand, send long axons into the central nervous system (CNS) proper and thus operate more like typical neurons. The axons emanating from the retinal ganglion cells converge upon the optic disk (see Fig. 7.3), which represents the starting point of the optic nerve (cranial nerve II).

The optic nerve is organized more like a tract in the brain than a peripheral nerve. There is a reason for this. During embryonic development, most of the eye arises as an outpocketing of the diencephalon of the brain, and the optic nerve is the continuous connection between brain and eye. Each eye sends an optic nerve to a region at the base of the diencephalon (the optic chiasm), where they converge and then separate (Fig. 7.14A).

Figure 7.14. Visual fields and central pathways. (A) Primary visual pathways. (B) Central connections of visual fields and their functions.

Within the optic nerve are axons that go directly to a variety of locations within the brain. At the optic chiasm, axons carrying primary visual signals can take two pathways. Those that originate on the lateral sides of the retina leave the optic chiasm to enter the optic tract on the same side. In contrast, those axons that arise on the medial side of the retina cross over to the optic tract on the other side (see Fig. 7.14A). Retinal axons entering the optic tracts can lead to several functional destinations (Fig. 7.14B). A small number go to the hypothalamus, where they are involved in the regulation of light–dark rhythms. Another group feeds into the midbrain region called the pretectum. These axons are part of a circuit that connects with autonomic nerve fibers to control the pupillary light reflex. Yet another group synapses in the superior colliculus of the midbrain. These are involved in the coordination between head and eye movements and visual targets.

The largest group of visual axons synapses in the lateral geniculate nucleus of the thalamus, where they connect with nerve fibers that pass through white matter of the cerebral cortex as the optic radiations, (see Fig. 7.14A), which terminate in the visual cortex of the occipital lobe of the cerebrum. From the retina to the visual cortex, the axons maintain a strict topographical order, so that in the visual cortex retinal images are accurately represented. Much processing occurs within the visual cortex, and association projections from there go to many other regions of the cerebral cortex, where further processing and integration into conscious actions take place.

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Development of the Auditory Organ (Johnston's Organ) in Drosophila

Andrew P. Jarman , in Development of Auditory and Vestibular Systems, 2014

8 Structure of JO Neurons

Chordotonal neurons are bipolar neurons with an unbranched dendrite that has at its tip a cilium specialized as a mechanosensory organelle. The latter is the key feature that distinguishes these neurons for their particular function. The cilium (also known as the dendrite outer segment) extends from a pair of basal bodies at the tip of the dendrite inner segment ( Fig. 2.7). In most chordotonal neurons it has a regular axoneme structure of 9+0 microtubule doublets extending from the distal basal body. The tip of the cilium ends within the extracellular cap and is probably connected to it via the zona pellucida (ZP) domain protein, NompA (Chung et al., 2001) (Fig. 2.7). At a point along the cilium is the ciliary dilation, which is seen by transmission election microscopy (TEM) to contain a paracrystalline core (Todi et al., 2004). RempA and DCX-EMAP proteins accumulate at the ciliary dilation (Bechstedt et al., 2010; Lee et al., 2008). RempA is required for formation of the dilation (see below), while DCX-EMAP is proposed to be part of the mechanotransduction apparatus. Disruption of the dilation in rempA mutants causes mechanosensory malfunction (Lee et al., 2008). The dilation may therefore play a direct role in mechanotransduction. In addition, the ciliary dilation marks the boundary between two structurally and functionally distinct proximal and distal zones. These zones are perturbed in rempA mutants. Extending from the basal bodies into the cell soma is a prominent ciliary rootlet, most likely formed by Rootletin protein, the product of the CG6129 gene (Laurençon et al., 2007). The rootlet is thought to be involved in maintaining neuronal integrity in the face of mechanical stress (K. Styczynska-Soczka and Jarman, unpublished data).

FIGURE 2.7. The chordotonal mechanosensory cilium.

(A) Schematic of the tip of the chordotonal neuron dendrite showing the sensory cilium (taken from Figure 2.2). Some of the proteins localized to each part are shown in colored text. Sas-4 (spindle assembly abnormal) and PLP (pericentrin-like protein) are centrosomal/basal body proteins. Eys (eyes shut) is a secreted protein that forms a prominent luminal band surrounding the cilium. The other proteins are mentioned in the text. (B) Immunofluorescence of pupal JO chordotonal neurons expressing a GFP-tagged axonemal dynein light intermediate chain (DNALI1) marking the proximal (motile) cilia zone (green) and with NompC marking the distal (sensory) zone (magenta).

 Image courtesy of D. J. Moore.

The mechanosensory mechanism is only partially known. The primary mechanosensory molecule is thought to be the TRPN channel encoded by nompC, which localizes to the membrane of the distal ciliary zone (Fig. 2.7) (Cheng et al., 2010; Effertz et al., 2011). The evidence suggests that it is expressed in and required for the subset of JO neurons that responds specifically to sound (Effertz et al., 2011; Kamikouchi et al., 2009; Sun et al., 2009). The protein has ankyrin repeats that have been proposed to form a coiled spring. The membrane of the proximal ciliary zone contains TRPV channels formed by proteins encoded by inactive (iav) and nanchung (nan) (Gong et al., 2004; Kim et al., 2003). These genes are essential for chordotonal neuron function and appear to be involved in membrane potential propagation downstream of the initial mechanotransduction event and also in gain control of the nonlinear neuronal response to sound (Göpfert et al., 2006). Unlike nompC, the TRPV genes are expressed in all JO neurons (Kamikouchi et al., 2009; Sun et al., 2009).

Many of these genes were identified in genetic screens for abnormal touch responses (Kernan et al., 1994) or deafness (Eberl et al., 2000). More recently, a transcriptome analysis has uncovered a large number of new genes that are potentially involved in mechanosensation in JO neurons (Senthilan et al., 2012). This entailed a transcriptomic comparison of second antennal segments from wild type adults with those from atonal mutant flies, in which JO is not formed. Some 274 JO-expressed genes were identified, although the proportion of these expressed in the chordotonal neurons as opposed to the support cells is unknown. When a sample of 42 new genes was tested genetically, about half were found to be required for hearing. Of particular interest is the unexpected finding that JO expresses significant numbers of genes previously associated with visual responses (including rhodopsins) as well as gustatory ionotropic receptors.

JO is not simply a passive auditory sensor. It generates low amplitude oscillations at its optimum receptive frequency (200 Hz), which enhance sensitivity to low amplitude sounds of that frequency (i.e., the response to sound of increasing amplitude is nonlinear) (Göpfert et al., 2005; Göpfert and Robert, 2003). This positive feedback is reminiscent of active amplification in the vertebrate cochlea. In both vertebrates and insects, the mechanism that promotes this amplification resides in the motility of the mechanosensory cells themselves (Robles and Ruggero, 2001).

Although the mechanism involved is not definitively known, there is much evidence to suggest that motion generation occurs within the sensory cilia themselves. The proximal ciliary zone has a key characteristic of motile cilia: the presence of axonemal dynein motors on the microtubules of the axoneme. Electron microscopy shows that the proximal ciliary zone is decorated with both inner and outer dynein arms (Newton et al., 2012) (Fig. 2.7). Indeed Drosophila orthologues of inner and outer arm axonemal dynein components are exclusively expressed in chordotonal neurons (Newton et al., 2012; Senthilan et al., 2012), and the proteins localize to the proximal zone (Fig. 2.7B) along with the TRPV channels, which are thought to provide gain control for the active amplification (Göpfert et al., 2006). Thus the mechanism is a remarkable parallel to amplification in vertebrate cochlear hair cells (the so-called cochlear amplifier (Robles and Ruggero, 2001)). Although the latter is due to the action of myosin motors, it seems that many aspects of active amplification may be evolutionarily conserved (Robert and Göpfert, 2002). The insect paradigm may provide a more accessible model for physiological analysis of this process.

The ciliary motility apparatus may not only be linked to motion generation. It could also be involved in biophysical properties associated with mechanotransduction such as stiffness and adaptation. Such roles may explain why ciliary motility is required not only for hearing but also for the proprioceptive functions of chordotonal organs (both in JO and elsewhere). Indeed, axonemal dynein mutants have proprioceptive defects including loss of negative gravitactic behavior (Moore et al., 2013).

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Clinical Neuroanatomy

Brian D. Loftus MD , ... Igor M. Cherches MD , in Neurology Secrets (Fifth Edition), 2010

OLFACTION

179 What are the olfactory receptor cells?

The receptor cells are bipolar neurons that pass from the olfactory mucosa through the cribriform plate to the olfactory bulb. Collectively, the central processes of the olfactory receptor cells constitute cranial nerve I.

180 What is the anatomy of the olfactory pathway?

1.

In the olfactory bulb, the axons of receptor cells synapse on dendrites of mitral and tufted cells (forming a glomerulus).

2.

The axons of mitral and tufted cells compose the olfactory tract, which soon divides into medial and lateral stria. Medial stria fibers cross to the contralateral side via the anterior commissure, while the lateral stria fibers terminate in the anterior perforated substance, amygdaloid complex, and lateral olfactory gyrus (which is the primary olfactory cortex).

3.

From the lateral olfactory gyrus (prepiriform area), fibers project to the entorhinal cortex, the medial dorsal nucleus of the thalamus, and the hypothalamus.

181 What is unique about the projection of olfactory information to the cerebral cortex?

Unlike other sensory modalities, olfaction reaches the cortex without relay through the thalamus.

182 What are the most common causes of anosmia?

1.

Rhinitis/nasal congestion

2.

Smoking

3.

Head injury

4.

Craniotomy

5.

Subarachnoid hemorrhage

6.

Meningiomas of the olfactory groove

7.

Zinc and vitamin A deficiency

8.

Hypothyroidism

9.

Congenital disorders (Kallmann's syndrome)

10.

Dementing diseases (Alzheimer's, Parkinson's)

11.

Multiple sclerosis

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Autonomic-Hypothalamic-Limbic Systems

David L. Felten MD, PhD , ... Mary Summo Maida PhD , in Netter's Atlas of Neuroscience (Third Edition), 2016

16.38 Olfactory Pathways

Primary sensory axons from bipolar neurons pass through the cribriform plate and synapse in the olfactory glomeruli in the glomerular layer of the olfactory bulb. The glomeruli are the functional units for processing specific odor information. The olfactory nerve fibers synapse on the dendrites of the tufted and mitral cells, the secondary sensory neurons that give rise to the olfactory tract projections. Periglomerular cells are interneurons that interconnect the glomeruli. Granule cells modulate the excitability of tufted and mitral cells. Centrifugal connections (from serotonergic raphe nuclei and the noradrenergic locus coeruleus) modulate activity in the glomeruli and periglomerular cells. The olfactory tract bypasses the thalamus and projects to the anterior olfactory nucleus, the nucleus accumbens, the primary olfactory cortex (in the uncus), the amygdala, the periamygdaloid cortex, and the lateral entorhinal cortex. The olfactory cortex has interconnections with the orbitofrontal cortex, the insular cortex, the hippocampus, and the lateral hypothalamus.

Clinical Point

The olfactory bulb and tract can be damaged by meningiomas of the olfactory groove or, less commonly, of the sphenoid ridge. These tumors produce Foster-Kennedy syndrome, which consists of ipsilateral anosmia, ipsilateral optic atrophy resulting from direct pressure, and papilledema caused by increased intracranial pressure. If the ipsilateral optic nerve is completely atrophic, papilledema will not be observed on that side. The olfactory bulb and tract also can be damaged by tumors of the frontal bone, pituitary tumors with frontal extension, frontal tumors such as gliomas that act as mass lesions, aneurysms at the circle of Willis, and meningitis. These conditions are distinguished from the olfactory groove meningiomas by the additional symptoms they cause.

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Auditory System, Peripheral

Eric Javel , in Encyclopedia of the Neurological Sciences, 2003

Spiral ganglion and auditory nerve

Hair cells are connected to bipolar neurons of the spiral ganglion. The axons or central processes of the spiral ganglion collect in the modiolus or central core of the cochlea and form the cochlear branch of the statoacoustic or eighth cranial nerve. These terminate in the cochlear nucleus, a brainstem center. The cochlear nerve in humans is composed of the axons of ∼30,000 spiral ganglion cells. These axons, often called auditory nerve fibers (ANFs), transmit to the brain all information that exists about sound. ANFs exhibit all-or-none responses or action potentials and possess homogeneous physiological response characteristics. ANFs exhibit spontaneous activity (action potentials or "spikes" that occur in the absence of a stimulus) that forms two or perhaps three groups. ANFs with high spontaneous discharge rates (20–150  spikes/sec) typically exhibit the lowest thresholds to sound and comprise ∼60% of the ANF population. ANFs with low and medium spontaneous rates (0–20   spikes/sec) normally have higher thresholds and comprise the remainder of the population. ANFs are tonic responders that exhibit a moderate amount of adaptation. That is, discharges occur throughout the duration of an input signal, but discharge rates tend to be greatest at response onset, usually settling to a lower, steady-state rate within 100   msec. Consistent with the fact that the basilar membrane performs mechanical frequency filtering on sound, each ANF exhibits a tuning curve (threshold plotted as a function of frequency). The frequency to which an ANF is most sensitive, the characteristic frequency (CF), reflects the cochlear location at which it connects to an IHC. Thus, ANFs emanating from the cochlear base are tuned to high frequencies, and ANFs emanating from the cochlear apex are tuned to low frequencies. Tuning curves (Fig. 5) have two portions—(i) a sensitive, sharply tuned "tip" that contains the CF and is determined by the active process and (ii) a high-threshold, broadly tuned "tail" that reflects passive basilar membrane tuning. Deficits involving OHCs reduce sensitivity in tuning curve tips but have little effect on tail sensitivity. In the extreme case of complete loss of OHC function, ANF tuning curves exhibit no tip and possess only a high-threshold, broadly tuned tail.

Figure 5. ANF tuning curves for fibers tuned to 2000   Hz (left) and 15,000   Hz (right). The y axis is threshold in decibels sound pressure level.

Input–output functions of ANFs (Fig. 6) are described by four parameters: spontaneous rate, threshold, maximum or saturation discharge rate, and dynamic range. Thresholds at CF for ANFs with high spontaneous rates generally follow the behavioral sensitivity curve, and thresholds at CF for ANFs with low spontaneous rates progressively increase by 10–40   dB as the spontaneous rate decreases to zero. Maximum discharge rates for ANFs range from 100 to 400   spikes/sec and, like threshold, correlate with spontaneous rate. That is, the higher the spontaneous rate, the higher the maximum discharge rate tends to be. Dynamic range refers to the intensity range over which an ANF's response increases from the resting rate to the maximum rate. Dynamic ranges for most ANFs are typically 20–30   dB, but some low-spontaneous-rate ANFs can have dynamic ranges up to 70   dB.

Figure 6. Input–output functions of ANFs with different spontaneous rates.

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Reproduction and Development

V. Hartenstein , in Comprehensive Molecular Insect Science, 2005

1.11.1.4 Type II Multidendritic Sensilla

Besides the sensilla that are innervated by bipolar neurons, the insect sensory system comprises another set of multidendritic subepidermal neurons which act as receptors for pressure and stretch ( Finlayson, 1976; Frye, 2001). A subset of these sensory neurons is accompanied by specialized ligament cells by which they stretch out alongside muscles; examples are the wing hinge stretch receptors and pleural stretch receptors. Most multidendritic neurons are naked, extending complex dendritic arbors at the basal surface of the epidermis (Figure 5a). These neurons, which have been identified in soft-bodied insects (i.e., mostly larval stages of holometabolans) are called dendritic arbor (da) neurons. In Manduca and Drosophila, four classes (α, β, γ, and δ) with different dendritic complexity, axonal termination, and physiological properties were defined (Grueber et al., 2001, 2002; Figure 5b). Members of one class completely tile the epidermis. Thus, dendritic branches of a given da neuron form a dense mesh in a given domain of the epidermis. Another da neuron of the same class innervates a domain that is contiguous, yet nonoverlapping with the first one. In this manner, the entire epidermis is completely covered with the innervation domains of the da neurons of a given class.

Figure 5. Multidendritic (Type II) neurons. (a) Dye filling of subepidermal multidendritic neuron (β class) in Manduca. (b) Line drawings of multidendritic neurons (ddaB, ddaC, ddaD, IdaC; For nomenclature, see Figure 7a) that represent the four morphologically defined classes (Grueber et al., 2001). Neurons of class α have a small dendritic arbor with a few branches. Class β neurons are small, but possess highly branched dendrites. Class γ is large and possesses low branch density; δ is large and has high branch density. Scale bar = 100 μm. ((a, b): Reproduced from Grueber, W.B., Graubard, K., Truman, J.W., 2001. Tiling of the body wall by multidendritic sensory neurons in Manduca Sexta. J. Comp. Neurol. 440, 271–283.)

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Smell and Taste

S.S. Schiffman , in Encyclopedia of Gerontology (Second Edition), 2007

Smell

Olfactory receptor neurons (ORNs) are bipolar neurons that are activated when airborne molecules in inspired air bind to olfactory receptors (ORs) expressed on their cilia. The ORs belong to a G-protein-coupled receptor superfamily. The ORNs are located high within the nasal vault in the olfactory epithelium. The olfactory epithelium is situated on the undersurface of the cribriform plate of the ethmoid bone, on the medial surface of the superior and middle turbinates, and on the upper nasal septum. The turbinates are scrolled spongy bones in the nasal passages that create airflow patterns that allow volatile compounds inhaled through the nares to reach the olfactory epithelium. ORs in the olfactory epithelium can also be activated through 'retronasal olfaction' when molecules from the oral cavity (e.g., from food) pass up through the nasopharynx into the nose.

The axons of the bipolar ORNs pass through small foramina (natural openings) in the cribriform plate joined together in fascicles (bundles), where they synapse in intricate neural masses called glomeruli in the olfactory bulb. Each ORN axon innervates only a single glomerulus. There is considerable convergence at the level of the olfactory bulb, with millions of ORNs converging on far fewer glomeruli. During the aging process, the glomeruli atrophy as fibers degenerate and disappear such that the olfactory bulb takes on a moth-eaten appearance. ORNs are vulnerable to trauma (e.g., blows to head, domestic falls, automobile accidents, and assaults) due to shearing of the axons by the ethmoid bone.

Axons from two principal cell types (mitral and tufted cells) emerge from the olfactory bulb to form the lateral olfactory tract, which subsequently projects to the anterior olfactory nucleus, the olfactory tubercle, the prepyriform cortex, and the amygdala, which are known collectively as the olfactory cortex. Many of these structures constitute the so-called 'limbic system' of the brain, which also processes emotions and memories. The neuroanatomical overlap between neurons that mediate olfaction and emotions provides an anatomical basis for the capacity of odors to produce hedonic responses. Olfactory information is ultimately transmitted to the hypothalamus, an area of the brain that is intricately involved in eating and nutrition.

The direct accessibility of ORNs to airborne agents makes them vulnerable to toxins and infectious agents that occur in breathed air. This vulnerability to the damage from the external environments is probably why ORNs, unlike most other neurons, have the ability to regenerate from a precursor population. ORNs turn over every 30 days on average; they are replaced from a stem cell population of basal cells.

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Cranial Nerve VIII

Timothy C. Hain , in Textbook of Clinical Neurology (Third Edition), 2007

Vestibular Ganglion and Nerve

Vestibular nerve fibers are the afferent projections from the bipolar neurons of Scarpa's ganglion. The vestibular nerve transmits afferent signals from the labyrinths through the internal auditory canal (IAC). In addition to the vestibular nerve, the IAC also contains the cochlear nerve (hearing), the facial nerve, the nervus intermedius (a branch of the facial nerve), and the labyrinthine artery. The IAC travels through the petrous portion of the temporal bone to open into the posterior fossa at the level of the pons. The vestibular nerve enters the brain stem at the pontomedullary junction and contains two divisions, the superior and inferior vestibular nerves. The superior vestibular nerve innervates the utricle, as well as the superior and lateral canals. The inferior vestibular nerve innervates the posterior canal and the saccule. Because a common disorder of the vestibular nerve, vestibular neuritis, often spares the inferior division, clinical situations can arise in which there is a partial injury to the vestibular nerve. 4

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The Human Hypothalamus: Neuropsychiatric Disorders

Maria Graciela Cersosimo , ... Gabriela B. Raina , in Handbook of Clinical Neurology, 2021

Olfactory epithelium

Olfactory receptor cells located in the nasal cavity are first-order bipolar neurons composed of a cell body, a peripheral, and a central process ( Hinds et al., 1984; Doty, 2009, 2012; Patel and Pinto, 2014). The peripheral process or dendrite extends to the epithelial surface ending in numerous receptor cilia in contact with the nasal mucus (Hinds et al., 1984; Doty, 2012). Unmyelinated axons of bipolar cells gather in bundles making up the olfactory nerve fascicles which project through the ethmoid bone toward the olfactory bulb (Hinds et al., 1984; Doty, 2009, 2012; Patel and Pinto, 2014). The olfactory epithelium (OE) has basal cells that are stem cells capable of regenerating different cell types that compose the OE when become damaged (Doty, 2009, 2012). In addition, there are other elements such as sustentacular and microvillar cells and Bowman's glands (Hinds et al., 1984; Doty, 2009, 2012; Patel and Pinto, 2014).

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