Slowly Adapting Research Articles

Tangential torque tunes touch

Tangential torque tunes touch

Site of stimulation effects on the prevalence of the tactile motion aftereffect

The motion aftereffect (MAE) refers to the apparent motion of a stationary stimulus following adaptation to a continuously moving stimulus. There is a growing consensus that the fast adapting (FA) rather than the slowly adapting (SA) afferent units mediate the tactile version of the MAE. The present study investigated which FA units underlie the tactile MAE by measuring its prevalence, duration, and vividness on different skin areas that vary in their composition of FA units. Specifically, the right cheek, volar surface of the forearm, and volar surface of the hand were adapted using a ridged cylindrical drum, which rotated at 60 rpm for 120 s. Although there was no difference in duration or vividness between the skin surfaces tested, the tactile MAE was reported twice as often on the hand compared to the cheek and forearm, which did not differ significantly from one another. This suggests that the FA I units in the glabrous skin and the hair follicle and/or the FA I and field units in the hairy skin contribute to the tactile MAE.

Molecular Identification of Rapidly Adapting Mechanoreceptors and Their Developmental Dependence on Ret Signaling

In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the "early Ret(+)" DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles, and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret(+) neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.

Functional properties of low-threshold mechanoreceptive afferents in the human labial mucosa

We used microneurography to investigate the functional properties of low-threshold mechanoreceptive afferents innervating the oral mucosa of the inside of the lower lip. Impulse responses were recorded from the inferior alveolar nerve of four human subjects. The threshold force and receptive field boundaries were identified for 19 single mechanoreceptive afferents using thin filaments (von Frey hairs) that applied known forces to the mucosa. Most of the receptive fields were located close to the corners of the mouth. Twelve of the afferents were slowly adapting (SA) and the remaining seven units were fast adapting (FA). Two types of slowly adapting responses were observed, SA I and SA II. Four of the six SA II units were spontaneously active. The geometric mean value of the receptive field sizes was 4.20 mm(2) for the SA I units, 5.65 mm(2) for the SA II units, and 5.60 mm(2) for the FA I units. None of the FA afferents showed response properties characteristic of Pacinian-corpuscle type afferents (FA II units). All afferents showed low force threshold between 0.06 and 1 mN. The properties of the mechanoreceptors supplying the human labial mucosa appear more similar to those of the vermilion and facial skin of the lower lip than those supplying the mucosa of the dorsal tongue.

Response properties of trigeminal ganglion mechanosensitive neurons innervating the temporomandibular joint of the rabbit

The primary mechanosensitive neurons innervating the temporomandibular joint (TMJ neurons) may play an important role in controlling mandibular movement and position. The purpose of the study was to investigate the neurophysiological properties of TMJ neurons during passive movement of the isolated condyle in 55 rabbits and the intact condyle in 29 rabbits. Discharges of TMJ neurons from the trigeminal ganglion were recorded with a microelectrode as the isolated condyle was moved manually and by a computer-regulated mechanostimulator and as the intact condyle was manually stimulated. A total of 237 TMJ neurons were recorded rostrocaudally from the mandibular nerve area lateral to the maxillary region in the dorsal half of the trigeminal ganglion. Of the recorded TMJ units, 97% were slowly adapting (SA) and 67% of the SA units had an accompanying ongoing discharge. The proportion of adaptation types and appearance of ongoing discharges for the isolated condyle did not differ significantly from those for the intact condyle. Most of the TMJ units (89%) responded multidirectionally to the rostral and ventral movements of the isolated condyle. The discharge frequencies of the TMJ units increased as the condylar displacement and velocity increased within a 5-mm anterior displacement of the isolated condyle. Displacement of the isolated condyle influenced the discharge frequency of the units to a greater extent than the velocity of the condyle movement. No responses of TMJ units were observed during the descending ramp. Based on these results, we conclude that sensory information is transmitted by TMJ neurons encoding joint position, displacement and velocity in a physiological range of mandibular displacement.

Slowly adapting mechanoreceptors in the borders of the human fingernail encode fingertip forces.

There are clusters of slowly adapting (SA) mechanoreceptors in the skin folds bordering the nail. These "SA-IInail" afferents, which constitute nearly one fifth of the tactile afferents innervating the fingertip, possess the general discharge characteristics of slowly adapting type II (SA-II) tactile afferents located elsewhere in the glabrous skin of the human hand. Little is known about the signals in the SA-IInail afferents when the fingertips interact with objects. Here we show that SA-IInail afferents reliably respond to fingertip forces comparable to those arising in everyday manipulations. Using a flat stimulus surface, we applied forces to the finger pad while recording impulse activity in 17 SA-IInail afferents. Ramp-and-hold forces (amplitude 4 N, rate 10 N/s) were applied normal to the skin, and at 10, 20, or 30 degrees from the normal in eight radial directions with reference to the primary site of contact (25 force directions in total). All afferents responded to the force stimuli, and the responsiveness of all but one afferents was broadly tuned to a preferred direction of force. The preferred directions among afferents were distributed all around the angular space, suggesting that the population of SA-IInail afferents could encode force direction. We conclude that signals in the population of SA-IInail afferents terminating in the nail walls contain vectorial information about fingertip forces. The particular tactile features of contacted surfaces would less influence force-related signals in SA-IInail afferents than force-related signals present in afferents terminating in the volar skin areas that directly contact objects.

Hyper-excitability in low threshold mechanical A fibers is potentially involved in scorpion BmK sting pain

In the present study, using the single fiber recording technique, we found that BmK I, the main toxic component in scorpion Buthus martensi Karsch (BmK) venom, induced dramatic increase in excitability of rapidly adapting (RA) and type I slowly adapting (SAI) low threshold mechanical A fibers of rat. Five micrograms BmK I (691 nmol, in 10 μl saline) administrated to the receptive fields induced spontaneous activity in 80% of RA and SAI fibers, increased the response to 10 g–10 s stimulation at about 20 times and altered the firing pattern to burst mode with maximal NS (number of spikes in burst) averaging from all fibers studied as many as 59. The increase in the excitability of RA and SAI fibers did not recover completely in 2 h. Our finding suggests that the gigantic abnormal activity in low threshold mechanical A fibers is involved in BmK scorpion sting pain, and the experimental model of BmK scorpion sting pain can be used to study A-fiber related central pathway which is important for relief of refractory neuropathic pain likewise.

The Representation of Stimulus Orientation in the Early Stages of Somatosensory Processing

At an early stage of processing, a stimulus is represented as a set of contours. In the representation of form, a critical feature of these local contours is their orientation. In the present study, we investigate the representation of orientation at the somatosensory periphery and in primary somatosensory cortex. We record the responses of mechanoreceptive afferents and of neurons in areas 3b and 1 to oriented bars and edges using a variety of stimulus conditions. We find that orientation is not explicitly represented in the responses of single afferents, but a large proportion of orientation detectors (approximately 50%) can be found in areas 3b and 1. Many neurons in both areas exhibit orientation tuning that is preserved across modes of stimulus presentation (scanned vs indented) and is relatively insensitive to other stimulus parameters, such as amplitude and speed, and to the nature of the stimulus, bar or edge. Orientation-selective neurons tend to be more SA (slowly adapting)-like than RA (rapidly adapting)-like, and the strength of the orientation signal is strongest during the sustained portion of the response to a statically indented bar. The most orientation-selective neurons in SI are comparable in sensitivity with that measured in humans. Finally, responses of SI neurons to bars and edges can be modeled with a high degree of accuracy using Gaussian or Gabor filters. The similarity in the representations of orientation in the visual and somatosensory systems suggests that analogous neural mechanisms mediate early visual and tactile form processing.

Two psychophysical channels of whisker deflection in rats align with two neuronal classes of primary afferents.

The rat whisker system has evolved into in an excellent model system for sensory processing from the periphery to cortical stages. However, to elucidate how sensory processing finally relates to percepts, methods to assess psychophysical performance pertaining to precise stimulus kinematics are needed. Here, we present a head-fixed, behaving rat preparation that allowed us to measure detectability of a single whisker deflection as a function of amplitude and peak velocity. We found that velocity thresholds for detection of small-amplitude stimuli (<3 degrees) were considerably higher than for detection of large-amplitude stimuli (>3 degrees). This finding suggests the existence of two psychophysical channels mediating detection of whisker deflection: one channel exhibiting high amplitude and low velocity thresholds (W1), and the other channel exhibiting high velocity and low amplitude thresholds (W2). The correspondence of W1 to slowly adapting (SA) and W2 to rapidly adapting (RA) neuronal classes in the trigeminal ganglion was revealed in acute neurophysiological experiments. Neurometric plots of SA and RA cells were closely aligned to psychophysical performance in the corresponding W1 and W2 parameter ranges. Interestingly, neurometric data of SA cells fit the behavior best if it was based on a short time window integrating action potentials during the initial phasic response, in contrast to integrating across the tonic portion of the response. This suggests that detection performance in both channels is based on the assessment of very few spikes in their corresponding groups of primary afferents.

Mapping the Gates. Focus on “Relationship Between Physiological Response Type (RA and SA) and Vibrissal Receptive Field of Neurons Within the Rat Trigeminal Ganglion”

Four decades ago, Hubel and Wiesel demonstrated that cortical neurons show a response specificity that does not exist in the thalamus or retina and triggered a universal pursuit after cortical mechanisms of perception. Under this pursuit hid an implicit assumption that whatever is needed to be known

GABA and glycine in synaptic microcircuits associated with physiologically characterized primary afferents of cat trigeminal principal nucleus

Previous studies suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by GABA-mediated presynaptic mechanisms in the trigeminal principal sensory nucleus (Vp) than other sensory nuclei. However, it is unknown if such a mechanism is common to functionally different classes of primary afferent in the same nucleus or across the nuclei. To address these issues, the present study focused on synaptic microcircuits associated with slowly adapting (SA) mechanosensory afferents innervating the periodontal ligaments in the cat Vp and attempted to examine GABA, glycine, and glutamate immunoreactivity in axon terminals involved in the circuits. Afferents were physiologically characterized before injection of horseradish peroxidase (HRP) and preparation for electron microscopy. HRP-labeled afferent boutons were serially sectioned and immunostained with antibodies against GABA, glycine, and glutamate using a postembedding immunogold method. All the afferent boutons examined contacted non-primary dendrites and they were frequently postsynaptic to unlabeled axons (p-endings). Axodendritic and axoaxonic contacts per afferent bouton were 1.3 (46/35) and 2.0 (70/35), respectively. Most p-endings were immunoreactive for GABA (63/70) and also glycine was co-stained in the majority of the p-endings (49/63). Thirty percent of p-endings with the colocalization of GABA and glycine participated in synaptic triads where a p-ending formed a synapse with the same dendrite as the afferent bouton. None of the p-endings was immunoreactive for glutamate. Most afferent boutons were enriched with glutamate but were immunonegative for GABA and glycine. This study provides evidence suggesting that transmission from SA afferents is strongly controlled presynaptically by GABAergic interneurons with colocalized glycine, and that a proportion of these interneurons, involved in synaptic triads, may also have postsynaptic inhibitory actions on target neurons of the SA afferents.

Quantitative Analyses of Dynamic Strain Sensitivity in Human Skin Mechanoreceptors

Microneurographical recordings from 24 slowly adapting (SA) and 16 fast adapting (FA) cutaneous mechanoreceptor afferents were obtained in the human radial nerve. Most of the afferents innervated the hairy skin on the back of the hand. The afferents' receptive fields were subjected to controlled strains in a ramp-and-hold fashion with strain velocities from 1 to 64%.s(-1), i.e., strain velocities within most of the physiological range. For all unit types, the mean variation in response onset approached 1 ms for strain velocities >8%.s(-1). Except at the highest strain velocities, the first spike in a typical SAIII unit was evoked at strains <0.5% and a typical SAII unit began to discharge at <1% skin strain. Skin strain velocity had a profound effect on the discharge rates of all classes of afferents. The "typical" peak discharge rate at the highest strain velocity studied was 50-95 imp/s(-1) depending on unit type. Excellent fits were obtained for both SA and FA units when their responses to ramp stretches were modeled by simple power functions (r2 > 0.9 for 95% of the units). SAIII units grouped with SAII with respect to onset latency and onset variation but with SAI units with respect to dynamic strain sensitivity. Because both SA and FA skin afferents respond strongly, quickly, and accurately to skin strain changes, they all seem to be able to provide useful information about movement-related skin strain changes and therefore contribute to proprioception and kinesthesia.

Tactile sensory function in the forearm of the monotreme Tachyglossus aculeatus.

Peripheral tactile neural mechanisms in the forepaw of the echidna (Tachyglossus aculeatus, from the order Monotremata) were investigated to establish the extent of correspondence or divergence that has emerged over the widely different evolutionary paths taken by monotreme and placental mammals. Electrophysiological recordings were made in anesthetized echidnas from 29 single tactile sensory nerve fibers isolated in fine strands of the median or ulnar nerves of the forearm. Controlled tactile stimuli were applied to the forepaw glabrous skin to classify fibers, initially, into two broad divisions, according to their responses to static skin displacement. One displayed slowly adapting (SA) response properties, and the other showed a selective sensitivity to the dynamic components of the skin displacement. The SA class was made up of low-threshold SA fibers and other less sensitive SA fibers, and the purely dynamically sensitive tactile fibers could be subdivided according to vibrotactile sensitivity and receptive field characteristics into a rapidly adapting (RA) class, sensitive to low-frequency (< or =50-Hz) vibration, that resembled a corresponding RA class in placental species, and another class, sensitive to a broader range of vibrotactile frequencies (approximately 50-300 Hz), that may represent a monotreme equivalent of the Pacinian corpuscle (PC)-related fiber class of placental mammals. The differential tactile sensitivity of the three principal fiber classes and their individual coding characteristics, determined by quantitative stimulus-response analysis, indicate, first, that this triad of fiber classes can subserve high-acuity tactile signalling from the echidna footpad and, second, that peripheral tactile sensory mechanisms are highly conserved across evolutionarily divergent mammalian orders.

Orofacial mechanoreceptors in humans: encoding characteristics and responses during natural orofacial behaviors

We used microneurography to characterize stimulus-encoding properties of low-threshold mechanoreceptive afferents in human orofacial tissues. Signals were recorded from single afferents in the infraorbital, lingual and inferior alveolar nerves while localized, controlled, mechanical stimuli were delivered to the facial skin, lips, oral mucosa and teeth. We likewise analyzed activity in these afferents during orofacial behaviors such as speech, chewing and biting. The afferents in the soft tissues functionally resemble four types described in the human hand: hair follicle afferents, slowly adapting (SA) type I and type II afferents and fast adapting (FA) type I afferents. Afferents in the facial skin, lips and buccal mucosa respond not only to contact with environmental objects, but also to contact between the lips, changes in air pressure generated for speech sounds, and to facial skin and mucosa deformations that accompany lip and jaw movements associated with chewing and swallowing. Hence, in addition to exteroceptive information, these afferents provide proprioceptive information. In contrast, afferents terminating superficially in the tongue do not signal proprioceptive information about tongue movements in this manner. They only respond when the receptive field is brought into contact with other intraoral structures or objects, e.g. the teeth or food. All human periodontal afferents adapt slowly to maintained tooth loads. Populations of periodontal afferents encode information about both which teeth are loaded and the direction of forces applied to individual teeth. Most afferents exhibit a markedly curved relationship between discharge rate and force amplitude, featuring the highest sensitivity to changes in tooth load at low forces (below 1 N). Accordingly, periodontal afferents efficiently encode tooth load when subjects first contact, hold, and gently manipulate food by the teeth. In contrast, only a minority of the afferents encodes the rapid and strong force increase generated when biting through food. We conclude, that humans use periodontal afferent signals to control jaw actions associated with intraoral manipulation of food rather than exertion of jaw power actions.

ATP suppresses activity in slowly adapting but not rapidly adapting mechanoreceptors in toad skin.

To test the possible role of ATP in transducing or modulating touch sensation, an isolated skin-nerve preparation from the toad, Bufo marinus, perfused on the inner side, was used to examine the effects of ATP on slowly adapting (SA) and rapidly adapting (RA) mechanoreceptors, identified by ramp and hold indentation of the skin. ATP (1-50 mM) did not generate impulse activity in either SA or RA mechanoreceptors nor increase their responses to mechanical stimulation. Instead, ATP (> or =5 mM) reversibly and selectively suppressed impulse discharge from SA mechanoreceptors. Our results indicate that while ATP may modulate the responses of specific touch receptors, it is not involved in transducing touch into nerve impulses.

Response properties of periodontal mechanoreceptors in rats, in vitro

Single unit activities of the inferior alveolar nerve evoked by calibrated von Frey stimuli (1.1, 2.9, 7.8, 11.8, and 17.2 mN) on the periodontal ligaments of the mandibular molars or incisors were recorded in an in vitro jaw-nerve preparation of Wistar albino rats. The data of 55 (lower incisor) and 100 (lower molars) units were collected in the present study. Both rapidly (RA) and slowly adapting (SA) type units were found in the incisors, and most of these units were innervated by A β fibers. While all the units of the molars were of RA types, the innervated fibers of two-thirds (67/100) of the units have been identified as Aδ fibers. The response patterns of the RA type were subdivided into three types (ON, OFF or ON–OFF type) both in the incisors and the molars. While von Frey thresholds of all incisor units were 11.8 mN except one unit that was 7.8 mN , those of the molars varied from 2.9 to 11.8 mN. In the molars, a majority of afferents innervated the periodontal ligaments of more than one tooth. This study suggests that response properties of periodontal mechanoreceptors are different between the incisors and the molars in rats, suggesting that these receptors have different functions in the regulation of mastication.

Encoding of Direction of Fingertip Forces by Human Tactile Afferents

In most manipulations, we use our fingertips to apply time-varying forces to the target object in controlled directions. Here we used microneurography to assess how single tactile afferents encode the direction of fingertip forces at magnitudes, rates, and directions comparable to those arising in everyday manipulations. Using a flat stimulus surface, we applied forces to a standard site on the fingertip while recording impulse activity in 196 tactile afferents with receptive fields distributed over the entire terminal phalanx. Forces were applied in one of five directions: normal force and forces at a 20 degrees angle from the normal in the radial, distal, ulnar, or proximal directions. Nearly all afferents responded, and the responses in most slowly adapting (SA)-I, SA-II, and fast adapting (FA)-I afferents were broadly tuned to a preferred direction of force. Among afferents of each type, the preferred directions were distributed in all angular directions with reference to the stimulation site, but not uniformly. The SA-I population was biased for tangential force components in the distal direction, the SA-II population was biased in the proximal direction, and the FA-I population was biased in the proximal and radial directions. Anisotropic mechanical properties of the fingertip and the spatial relationship between the receptive field center of the afferent and the stimulus site appeared to influence the preferred direction in a manner dependent on afferent type. We conclude that tactile afferents from the whole terminal phalanx potentially contribute to the encoding of direction of fingertip forces similar to those that occur when subjects manipulate objects under natural conditions.

Functional characteristics of the parallel SI- and SII-projecting neurons of the thalamic ventral posterior nucleus in the marmoset.

The functional organization of the primate somatosensory system at thalamocortical levels has been a matter of controversy, in particular, over the extent to which the primary and secondary somatosensory cortical areas, SI and SII, are organized in parallel or serial neural networks for the processing of tactile information. This issue was investigated for the marmoset monkey by recording from 55 single tactile-sensitive neurons in the lateral division of the ventral posterior nucleus of the thalamus (VPL) with a projection to either SI or SII, identified with the use of the antidromic collision technique. Neurons activated from the hand and distal forearm were classified according to their peripheral source of input and characterized in terms of their functional capacities to determine whether the direct thalamic input can account for tactile processing in both SI and SII. Both the SI- and SII-projecting samples contained a slowly adapting (SA) class of neurons, sensitive to static skin displacement, and purely dynamically sensitive tactile neurons that could be subdivided into two classes. One was most sensitive to high-frequency (> or =100 Hz) cutaneous vibration whose input appeared to be derived from Pacinian sources, while the other was sensitive to lower frequency vibration (< or =100 Hz) or trains of rectangular mechanical pulse stimuli, that appeared to receive its input from rapidly adapting (RA) afferent fibers presumed to be associated with intradermal tactile receptors. There appeared to be no systematic differences in functional capacities between SI- and SII-projecting neurons of each of these three classes, based on receptive field characteristics, on the form of stimulus-response relations, and on measures derived from these relations. These measures included threshold and responsiveness values, bandwidths of vibrational sensitivity, and the capacity for responding to cutaneous vibrotactile stimuli with phase-locked, temporally patterned impulse activity. The analysis indicates that low-threshold, high-acuity tactile information is conveyed directly to both SI and SII from overlapping regions within the thalamic VP nucleus. This direct confirmation of a parallel functional projection to both SI and SII in the marmoset is consistent with our separate studies at the cortical level that demonstrate first, that tactile responsiveness in SII largely survives the SI inactivation and second, that SI responsiveness is largely independent of SII. It therefore reinforces the evidence that SI and SII occupy a hierarchically equivalent network for tactile processing.

Characterization of tactile afferent fibers in the hand of the marmoset monkey.

The marmoset monkey, Callithrix jacchus, has increasingly been the subject of experiments for the analysis of somatosensory system function in simian primates. However, as response properties of the mechanoreceptive afferent fibers supplying the skin have not been characterized for this primate, the present study was undertaken to classify fibers innervating the glabrous skin of the marmoset hand and determine whether they resembled those described for other mammalian species, including cat, macaque monkey, and human subjects. Forty-seven tactile afferent fibers with receptive fields (RFs) on the glabrous skin of the hand were isolated in fine median and ulnar nerve strands. Controlled tactile stimuli, including static indentation and skin vibration, were used to classify fibers. Twenty-six (55%) responded to static indentation in a sustained manner and were designated slowly adapting (SA) fibers, while 21 (45%) were selectively sensitive to the dynamic components of the stimulus. The SA fibers had well-defined boundaries to their RFs, lacked spontaneous activity in most cases (23/26 fibers), had an irregular pattern of discharge to static skin indentation, and displayed graded response levels as a function of indentation amplitude, attributes that were consistent with the properties of slowly adapting type I (SAI) fibers described in other species. The dynamically sensitive afferent fibers could be subdivided into two distinct functional classes, based on their responses to vibrotactile stimulation. The majority (15/21) responded best to lower frequency vibration (~10-50 Hz) and had small RFs, whereas the second class responded preferentially to higher frequency vibration (50-700 Hz) with maximal sensitivity at ~200-300 Hz. These two classes resembled, respectively, the rapidly adapting (RA) and Pacinian corpuscle-related (PC) fiber classes found in other species, and like them, responded to vibration with tightly phase-locked patterns of response over a wide range of frequencies. The results demonstrate that the functional classes of tactile afferent fibers that supply the glabrous skin in the marmoset monkey appear to correspond with those described previously for the cat and macaque monkey, and are similar to those supplying the human hand and fingers, although the SA fibers in the human hand appear to fall into two classes, the SAI and SAII fibers. With the increasing use of the marmoset monkey as a primate model for somatosensory system studies, these data now allow tactile neurons identified at central locations, such as the cerebral cortex and thalamus, to be classified in relation to inputs from the peripheral classes identified in the present study.

Different spatial distributions of sodium channels in the slowly and rapidly adapting stretch receptor neuron of the crayfish

Inward Na + currents were studied, using a two-microelectrode intracellular voltage-clamp technique, in the slowly adapting (SA) and rapidly adapting (RA) stretch receptor neurons of the crayfish after the axons were cut at different distances from the soma. In the SA neuron, inward Na + currents were recorded in the soma even when the axon was cut as close as 100 μm from the center of the soma, indicating the presence of Na + channels in these parts. Also, two populations of Na + channels seem to exist in the SA neuron. In the RA neuron, only minute Na + currents were observed if the axon was shorter than 250 μm. The results strongly indicate that the voltage-gated Na + channels in the SA and RA neurons have different distributions and that the difference in the spatial distribution of Na + channel types may be important for the difference in firing properties in the two types of neurons.