Expression patterns of HNK-1 carbohydrate and serotonin in sea urchin, amphioxus, and lamprey, with reference to the possible evolutionary origin of the neural crest

We examined deuterostome invertebrates, the sea urchin and amphioxus, and an extant primitive vertebrate, the lamprey, for the presence of structures expressing the HNK-1 carbohydrate and serotonin. In sea urchin embryos and larvae, HNK-1 positive cells were localized in the ciliary bands and in their precursor ectoderm. Serotonergic cells were exclusively observed in the apical organs. In juvenile amphioxus, primary sensory neurons in the dorsal nerve cords were HNK-1 immunoreactive. The juvenile amphioxus nerve cords contained anti-serotonin immunoreactive nerve fibers that seem to be the Rohde axons extending from amphioxus interneurons, the Rohde cells. In lamprey embryos, migrating neural crest cells and primary sensory neurons, including Rohon-Beard cells, expressed the HNK-1 carbohydrate. Lamprey larvae (ammocoetes) contained cell aggregates expressing both the HNK-1 carbohydrate and serotonin in the pronephros and in the regions adjacent to the gut epithelium. Some of these cell aggregates were present in the anti-serotonin positive visceral motor nerve net. Motor neurons and Müller fibers were serotonergic in ammocoetes. Comparison of the expression patterns of HNK-1 carbohydrate among sea urchins, amphioxus and lampreys seem to suggest the possible evolutionary origin of the neural crest, that is, ciliary bands in dipleurula-type ancestors evolved into primary sensory neurons in chordate ancestors, as inferred from Garstang's auricularia hypothesis, and the neural crest originated from the primary sensory neurons.     

Sensory Innervation of the Raccoon Forepaw: 2. Response Properties and Classification of Slowly Adapting Fibers

Physiological recordings were made from 136 slowly adapting (SA) fibers in the median and ulnar nerves that innervate the glabrous skin of the raccoon. It was found that wetting the skin produced large increases in fiber responsiveness and decreases in threshold. Their responses decreased rapidly with slight displacements of the stimulus away from the center of the receptive field. Responses also decreased with increases in the diameter of the tip of the stimulus probe. The length of time that an SA fiber responded to a prolonged indentation was related to the magnitude of the indentation, and was greater after wetting of the skin. The absence of any clear and consistent grouping of fibers into moderately SA (MSA) and very SA (VSA) units argues against the existence of two types of SA receptors differing in this property. However, the distinction between SA I and SA II fibers that has been made in other species was confirmed in the raccoon.     

Control of respiratory motor pattern by sensory neurons in spinal cord of lamprey

Summary Extracellular stimulation over the dorsal funiculus in the spinal cord of lampreys was found to selectively activate prolonged episodes of fictive arousal respiration (Figs. 1, 3). The induced episodes showed comparable increases in cycle frequency and motoneuron burst duration to the spontaneous arousal pattern observed in isolated brain preparations (Fig. 2). Intracellular stimulation of primary sensory neurons with axons in the dorsal funiculus, called dorsal cells, also elicited the arousal pattern (Fig. 4). Mechanoreceptive dorsal cells respond to cutaneous stimulation. When mechanical stimuli were applied to the skin of intact lampreys (Fig. 6) or to lampreys with ipsilateral vagotomy, arousal respiration was induced (Figs. 7, 8). Bilateral, but not unilateral, trigeminal lesion blocked dorsal cell induction of the arousal response (Fig. 5). Spontaneous arousal respiration was recorded from intact, unrestrained lampreys (Fig. 9). These results suggest that fictive arousal respiration is the in vitro correlate of natural arousal respiration in lampreys, and that one mechanism leading to arousal respiration may be the activity of sensory dorsal cells. A model for respiratory motor pattern switching in lamprey is proposed. The model suggests that the normal and arousal patterns are produced by separately engaging rostral or caudal pattern generators in the medulla, rather than by modifying one pattern generator (Fig. 10).     

Neural crest origin of mammalian Merkel cells

Here, we provide evidence for the neural crest origin of mammalian Merkel cells. Together with nerve terminals, Merkel cells form slowly adapting cutaneous mechanoreceptors that transduce steady indentation in hairy and glabrous skin. We have determined the ontogenetic origin of Merkel cells in Wnt1-cre/R26R compound transgenic mice, in which neural crest cells are marked indelibly. Merkel cells in whiskers and interfollicular locations express the transgene, beta-galactosidase, identifying them as neural crest descendants. We thus conclude that murine Merkel cells originate from the neural crest.     

Morphological and electrophysiological examination of olfactory sensory neurons during the early developmental prolarval stage of the sea lamprey Petromyzon marinus L

This study examined olfactory sensory neuron morphology and physiological responsiveness in newly hatched sea lamprey, Petromyzon marinus L. These prolarvae hatch shortly after neural tube formation, and stay within nests for approximately 18 days, before moving downstream to silty areas where they burrow, feed and pass to the larval stage. To explore the possibility that the olfactory system is functioning during this prolarval stage, morphological and physiological development of olfactory sensory neurons was examined. The nasal cavity contained an olfactory epithelium with ciliated olfactory sensory neurons. Axons formed aggregates in the basal portion of the olfactory epithelium and spanned the narrow distance between the olfactory epithelium and the brain. The presence of asymmetric synapses with agranular vesicles within fibers in the brain, adjacent to the olfactory epithelium suggests that there was synaptic connectivity between olfactory sensory axons and the brain. Neural recordings from the surface of the olfactory epithelium showed responses following the application of L-arginine, taurocholic acid, petromyzonol sulfate (a lamprey migratory pheromone), and water conditioned by conspecifics. These results suggest that lampreys may respond to olfactory sensory input during the prolarval stage.     

Correlation of receptive field position of mechanosensory neurons and the strength of their connections to AP neurons in the CNS of the leech (Whitmania pigra)

An identified neuron of unknown function in the CNS of the leech, the anterior pagoda (AP) cell, receives multiple synaptic inputs from mechanosensory neurons that innervate the skin. Impulses in touch (T), pressure (P) and nociceptive (N) sensory cells on both sides of the ganglion produced electrical coupling potentials on both AP cells. Sensory cells with receptive fields contralateral to the cell body of the AP neuron always gave rise to larger synaptic potentials. In addition sensory cells supplying dorsal skin gave rise to larger synaptic potentials than those with lateral or ventral fields. It is suggested that integration by the AP cell can provide information about the position of mechanical stimuli impinging on the body wall of the animal.     

Interaction among different sensory units within a single fungiform papilla in the frog tongue

The possible interaction among different sensory units in the frog tongue was studied using several single papillae dually innervated by the medial and lateral branches of the glossopharyngeal (IXth) nerve. The afferent activity in one branch exposed to NaCl stimulation of the papilla revealed marked inhibition after antidromic electrical stimulation (100 Hz, 30 s, and 3 V) of the other branch. The degree of inhibition depended on the number of sensory responses observed in the electrically stimulated branch as well as the nature of the stimulated sensory units. Statistical analysis suggested that antidromic activation of gustatory units conducting the responses to NaCl and quinine and slowly adapting mechanosensitive units produced a large antidromic inhibition amounting to 19-25%, but that of gustatory units conducting the responses to acetic acid and rapidly adapting mechanosensitive units gave rise to only a slight inhibition. To examine the differential effects of these sensory units in antidromic inhibition, antidromic impulses were evoked by chemical stimulation of the adjacent papilla neuronally connected with the dually innervated papilla under study. Antidromic volleys of impulses elicited by NaCl or quinine stimulation produced a large inhibition of the afferent activity in the other branch, as induced by NaCl stimulation of the dually innervated papilla. Plausible mechanisms of synaptic interaction in peripheral gustatory systems are considered.     

Effect of nociceptive stimulation of signal transmission from low-threshold cutaneous afferents in the somatosensory system

In cats anesthetized with chloralose nociceptive heating of the skin of the foot to 44–60°C led to a two- to fourfold increase in amplitude of primary cortical responses to direct stimulation of neurons of the spinocervical tract receiving information from the heated area of skin, but did not affect primary responses evoked by stimulation of axons of these neurons in the dorsolateral funiculus, and actually inhibited the response to stimulation of the nerve innervating the heated area of skin. Inhibition was accompanied by depolarization of central terminal of low-threshold fibers of this nerve: During heating the amplitude of the antidromic discharges evoked in the nerve by stimulation of its presynaptic endings in the spinal cord was increased two- to threefold. After abolition of presynaptic depolarization with picrotoxin (0.2–0.7 mg/kg, intravenously) or as a result of asphyxia, nociceptive heating acquired the ability to facilitate primary responses arising as a result of stimulation of the nerve also. The amplitude of the responses was increased under these circumstances by 3–20 times. It is concluded that acute nociceptive stimulation causes such powerful presynaptic inhibition of impulse transmission from low-threshold fibers of the cutaneous nerve that it virtually abolishes the facilitating effect of nociceptive impulses on sensory neurons of the spinal cord. It is suggested that it is this inhibitory mechanism which prevents the development of hyperalgesia during acute nociceptive stimulation.Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 621–627, November–December, 1981.     

Functional Properties of Slowly Adapting Mechanoreceptors in Cat Footpad Skin

The functional properties of slowly adapting (SA) afferent fibers innervating cat footpad skin were examined. Measurements were taken of receptive field area; spontaneous activity (< 1 impulse/sec); the slope of the stimulus-response curve for steady indentations up to 2 mm in amplitude; variability of the interimpulse intervals, as measured by the coefficient of variation of time interval histograms; decay of the response to steady indentation; and sensitivity to sinusoidal vibration (most sensitive at 5-10 Hz). Where comparable tests were performed on glabrous and hairy skin SA fibers, the functional properties of those in glabrous skin more closely resembled SAI fibers than SAII fibers. Additional results from glabrous skin SA fibers suggest that it is distortion of the nerve endings rather than steady indentation or compression that leads to a brisk response. On the measures described above, there appeared to be only one functional class of SA fiber innervating the cat footpad skin.     

Effects of stimulus duration on neuronal response properties in the somatosensory cortex of the star-nosed mole

Star-nosed moles have a series of mechanosensory appendages surrounding each nostril. Each appendage is covered with sensory organs (Eimer's organs) containing both rapidly adapting and slowly adapting mechanoreceptors and each appendage is represented in primary somatosensory cortex (S1) by a single cortical module. When the skin surface of an appendage is depressed, neurons in the corresponding module in S1 respond in either a transient or sustained fashion. The aim of this study was to characterize and compare the responses of these two classes of neurons to both short (5 or 20 ms) and long (500 ms) mechanosensory stimulation. Activity from neurons in the representation of appendage 11, the somatosensory fovea, was recorded while delivering mechanosensory stimuli to the corresponding skin surface. Transient and sustained neurons had different levels of spontaneous activity and different responses to both short and long mechanosensory stimulation. Neurons with sustained responses had a significantly higher spontaneous firing rate than neurons with transient responses. Transient neurons responded to a 5 ms stimulus with excitation followed by suppression of discharge whereas sustained neurons did not exhibit post-excitatory suppression. Rather, responses of sustained neurons to 5 ms stimuli lasted several hundred milliseconds. Consequently sustained responses contained significantly more spikes than transient responses. These experiments suggest contact to the appendages causes two distinct firing patterns in cortex regardless of the duration of the stimulus. The sustained and transient responses could reflect either the activity of fundamentally different classes of neurons or activity in distinct subcortical and cortical networks.     

Cutaneous overexpression of NT-3 increases sensory and sympathetic neuron number and enhances touch dome and hair follicle innervation

Target-derived influences of nerve growth factor on neuronal survival and differentiation are well documented, though effects of other neurotrophins are less clear. To examine the influence of NT-3 neurotrophin overexpression in a target tissue of sensory and sympathetic neurons, transgenic mice were isolated that overexpress NT- 3 in the epidermis. Overexpression of NT-3 led to a 42% increase in the number of dorsal root ganglia sensory neurons, a 70% increase in the number of trigeminal sensory neurons, and a 32% increase in sympathetic neurons. Elevated NT-3 also caused enlargement of touch dome mechanoreceptor units, sensory end organs innervated by slowly adapting type 1 (SA1) neurons. The enlarged touch dome units of the transgenics had an increased number of associated Merkel cells, cells at which SA1s terminate. An additional alteration of skin innervation in NT-3 transgenics was an increased density of myelinated circular endings associated with the piloneural complex. The enhancement of innervation to the skin was accompanied by a doubling in the number of sensory neurons expressing trkC. In addition, measures of nerve fibers in cross- sectional profiles of cutaneous saphenous nerves of transgenics showed a 60% increase in myelinated fibers. These results indicate that in vivo overexpression of NT-3 by the epidermis enhances the number of sensory and sympathetic neurons and the development of selected sensory endings of the skin.     

Cutaneous sensory afferents recorded from the nervus intramandibularis ofGallus gallus vardomesticus

Summary The responses of single sensory afferent nerve fibres were recorded from small nerve bundles of the intramandibular nerve of the chicken following thermal and mechanical stimulation of the beak. Thermoreceptors, nociceptors and mechanoreceptors were identified and their responses characterized.Of the thermoreceptors identified 11 units were classified as cold receptors, which responded to cooling the receptive field by increasing the discharge rate and had conduction velocities in the range 0.83 to 4.4 m/s. Only one warm unit was identified.Two classes of nociceptors were identified: mechano-thermal (polymodal) nociceptors and high threshold mechanical nociceptors. The discharge characteristics and stimulus-response curves of both types were described. While the mechanothermal nociceptors were exclusively C-fibres (c.v. 0.4 to 1.86 m/s), the high threshold mechanoreceptors contained both C and A delta fibres (c.v. 1 to 5.5 m/s). Thermal response thresholds for the mechano-thermal units ranged from 41 to 50 °C with mechanical thresholds of 2 to over 50 g. Mechanical thresholds for the high threshold units ranged from 5 to over 50 g.The mechanoreceptors were either slowly or rapidly adapting. The pattern of response together with stimulus-response curves were presented for the slowly adapting units. Conduction velocities of the slowly adapting units varied from 0.7 to 20 m/s and mechanical threshold from 0.1 to 2 g. On the basis of their response to a vibrating, and a ramp-and-hold mechanical stimulus, the rapidly adapting units were divided into Herbst and Grandry units with only the Herbst units responding accurately to the vibrating stimulus. Both units had fibres conducting in the 50 m/s range with thresholds in the 0.1 to 10 g range.The results are discussed in relation to the receptors found in other avian species and mammalian peripheral sensory afferents.Abbreviations c.v. conduction velocity - RA rapidly adapting (receptors) - SA slowly adapting (receptors)     

Sensory responses of dorsal cells in the lamprey brain

Summary Sensory responses were evoked in 17/33 dorsal cells in the brain of the lampreyIchthyomyzon unicuspis by mechanical stimulation of the dorsolateral skin of the sucker and head. Fifteen of the responsive cells tested were classified as pressuresensitive and two as nociceptive. Thus, some of the dorsal cells which are situated in the medulla of the lamprey and which project into the branches of the trigeminal nerves have sensory functions similar to those of dorsal cells in the spinal cord.     

Neuroendocrine and behavioral responses to weak electric fields in adult sea lampreys (Petromyzon marinus)

We characterized the behavioral and neuroendocrine responses of adult sea lampreys (Petromyzon marinus) to weak electric fields. Adult sea lampreys, captured during upstream spawning migration, exhibited limited active behaviors during exposure to weak electric fields and spent the most time attached to the wall of the testing arena near the cathode (−). For adult male sea lampreys, exposure to weak electric fields resulted in increased lamprey (l) GnRH-I mRNA expression but decreased lGnRH-I immunoreactivities in the forebrain, and decreased Jun (a neuronal activation marker) mRNA levels in the brain stem. Similar effects were not observed in the brains of female sea lampreys after weak electric field stimulation. The influence of electroreception on forebrain lGnRH suggests that electroreception may modulate the reproductive systems in adult male sea lampreys. The changes in Jun expression may be associated with swimming inhibition during weak electric field stimulation. The results for adult sea lampreys are the opposite of those obtained using parasitic-stage sea lampreys, which displayed increased activity during and after cathodal stimulation. Our results demonstrate that adult sea lampreys are sensitive to weak electric fields, which may play a role in reproduction. They also suggest that electrical stimuli mediate different behaviors in feeding-stage and spawning-stage sea lampreys.     

Peripheral synapses in Limulus chemoreceptors

1. Contact chemoreceptors on spines of Limulus prosomal appendages responded to stimulation by amino acid solutions with slowly adapting activity. 2. Mixtures of amino acids elicited stronger responses than equal concentrations of single amino acids. 3. Single unit responses consisted of irregularly timed, intermittent, bursts of action potentials recorded from the axons of the primary chemoreceptor neurons. 4. These axons send collateral branches to neural plexuses located at points peripheral to the recording sites. 5. Putative chemical synapses among the collaterals are seen and suggested as the site for implementing the second order nature of the responses.     

Spinal dorsal column afferent fiber composition in the pigeon: an electrophysiological investigation

Summary The spinal dorsal column of homing pigeons (Colomba livia) was investigated electrophysiologically by recording responses from individual afferent fibers at a high cervical level (segments C4-C5) to mechanical stimulation of wing skin and deep tissue. Of 157 afferent fibers 134 were cutaneous afferents. The remainder were afferents of deep receptors.Thirty of the cutaneous afferents were slowly adapting and 87 rapidly adapting (17 not identified). Rapidly adapting afferents were studied with regard to Pacinianlike characteristics (Herbst corpuscles in birds; vibration sensitive receptors). Of 43 rapidly adapting afferents 38 were classified as afferents of vibration sensitive Herbst corpuscles and 5 as non vibration sensitive rapidly adapting afferents; 44 afferents could not be studied sufficiently with regard to vibrational stimuli. The vibration sensitive Herbst corpuscle afferents had U-shaped vibrational tuning curves and responded best to vibration frequencies of 300 to 400 Hz. The 11 threshold for 300 Hz vibration ranged from 2 to 36 um. Herbst corpuscle afferents always showed strong phase coupling to the stimulus cycle.Afferents of deep receptors showed slowly adapting responses to firm pressure or movements of limbs and were classified as joint receptors. No muscle spindle afferents were encountered.Primary afferent fibers were identified in 89 cases (80 cutaneous and 9 deep), postsynaptic elements in 15 cases (11 cutaneous, 4 deep). Only slowly adapting responses were found in postsynaptic fibers.Abbreviations CV coefficient of variation - EI entrainment index - INTH interval histogram - PSTH peristimulus time histogram - RA rapidly adapting - SA slowly adapting     

Central mechanisms of vascular headaches

The intracranial blood vessels supplying the dura and brain are innervated by sensory afferents from the trigeminal nerve. These fibers are believed to be responsible for conveying the pain associated with vascular head pain such as migraines. This paper reviews recently published data describing the existence of neurons within the cat trigeminal nucleus and thalamus that respond to electrical stimulation of the middle meningeal artery and superior sagittal sinus. Almost all of these neurons receive convergent input from the facial skin and most of the receptive fields include the periorbital region. On the basis of their cutaneous inputs, most of the neurons are classified as nociceptive. The characteristics of these cerebrovascular-activated neurons are consistent with their role in mediating vascular head pains and with the typical referral of such pains in man to the orbital region. This paper also presents preliminary results of recordings from rat trigeminal ganglion neurons activated by electrical stimulation of the middle meningeal artery and sagittal sinus. The latencies of activation of these neurons are indicative of conduction in slowly conducting myelinated axons and in unmyelinated axons. Some of the neurons could also be activated by mechanical stimuli applied to the vessels.     

Electromechanical model for object roughness perception during finger sliding

Touch allows us to gather abundant information in the world around us. However, how sensory cells embedded in the fingers convey texture information into their firing patterns is still poorly understood. Here, we develop an electromechanical model for roughness perception by incorporating main ingredients such as voltage-gated ion channels, active ion pumps, mechanosensitive channels, and cell deformation. The model reveals that sensory cells can convey texture wavelengths into the period of their firing patterns as the finger slides across object surfaces, but they can only convey a limited range of texture wavelengths. We also show that an increase in sliding speed broadens the decoding wavelength range at the cost of reduction of lower perception limits. Thus, a smaller sliding speed and a bigger contact force may be needed to successfully discern a smooth surface, consistent with previous psychophysical observations. Moreover, we show that cells with slowly adapting mechanosensitive channels can still fire action potentials under static loadings, indicating that slowly adapting mechanosensitive channels may contribute to the perception of coarse textures under static touch. Our work thus provides a new theoretical framework to study roughness perception and may have important implications for the design of electronic skin, artificial touch, and haptic interfaces.     

Neural pathways and innervation of cnidocytes in tentacles of sea anemones

Our previously published studies are here reviewed detailing neuro-cnidocyte synapses, demonstrating putative neurotransmitter substances, and identifying complex neural pathways in sea anemones. Synapses were traced to their contacts on nematocytes and spirocytes by transmission electron microscopy of serial thin sections of tentacles. In five animals, cells containing microbasic p-mastigophores had synapses with clear vesicles, whereas cells containing basitrichous isorhizas had synapses with dense-cored vesicles, providing preliminary evidence for a selectivity of neurotransmitter types for different nematocysts. Either clear or dense-cored synaptic vesicles were also present at neuro-spirocyte contacts. Antho-RFamide immunoreactivity occurred in some anthozoan synaptic vesicles and immunogold labeling of serotonin was found at a neuro-spirocyte synapse. Neural pathways included direct innervation of spirocytes by sensory cells, sequential neuro-neuro-spirocyte and neuro-neuro-nematocyte synapses and reciprocal synapses involving axons of both sensory cells and ganglion cells. Such synaptic patterns resemble neuro-effector pathways found in higher animals and lay to rest the independent effector hypothesis for cnidocyte discharge in tentacles of sea anemones.     

Studies of Mechanoreceptors in Skin of the Snout of the Echidna Tachyglossus aculeatus

The echidna Tachyglossus aculeatus, together with the platypus, belongs to the monotremes, a group of mammals with a number of reptilian characteristics. A structure unique to the skin of monotremes is the push rod—a compacted column of epidermal cells that is 20 μm wide and 100 μm long with its tip at the skin surface, and that is able to move relatively independently of adjacent tissue. At the base of each push rod is a cluster of encapsulated nerve endings. Push rods are common in skin of the snout and have been postulated to have a mechanosensory function. Experiments were carried out on four anesthetized echidnas with the aim of determining the function of push rods. Recordings made from the infraorbital nerve, which supplies the skin of the upper jaw, yielded responses from a total of 46 afferents. Two were electroreceptors; the others were mechanoreceptors. Within the group of mechanoreceptors with rapidly adapting responses, three responded to high-frequency vibration and resembled pacinian corpuscles. There were 26 slowly adapting (SA) mechanoreceptors, which, based on the regularity of their discharge, could be divided into two groups: SA I or Merkel type, and SA II or Ruffini type. SA I receptors had very discrete receptive fields with diameters of 100 μm. The receptive fields of two SA I receptors were marked, and after histological processing, one was seen to lie near two push rods. It is concluded that mechanoreceptor responses in the echidna's snout skin resemble those in other mammals in many aspects. We could not unequivocally associate responses to mechanical stimulation with the push rods.