Sensory receptors with unmyelinated (C) fibers innervating the skin of the rabbit's ear

1985 ◽  
Vol 54 (3) ◽  
pp. 491-501 ◽  
Author(s):  
V. K. Shea ◽  
E. R. Perl
Keyword(s):  
Mechanical Stimulation ◽  
Receptive Fields ◽  
High Threshold ◽  
Cutaneous Stimulation ◽  
Great Auricular Nerve ◽  
C Fibers ◽  
C Fiber ◽  
Back Ground ◽  
Low Threshold ◽  
Stimulation Of

The cutaneous receptive properties of unmyelinated (C) fibers of the rabbit's great auricular nerve were determined by single-unit recordings. The majority of C-fiber units could be excited by cutaneous stimulation, and such sensory units fell into three major categories on the basis of responses to mechanical and thermal stimulation of their cutaneous receptive fields: low-threshold mechanoreceptors, nociceptors, or specific thermoreceptors. The majority of afferent elements were nociceptive, and all nociceptors responded to strong mechanical stimulation. Three types of nociceptors could be distinguished by their responses to thermal stimuli. Polymodal nociceptors responded to heat with thresholds of 40-55 degrees C and typically displayed enhanced responses or sensitization after noxious heating of their receptive fields. High-threshold mechanoreceptors failed to respond promptly to heat before noxious cutaneous stimulation which, however, elicited subsequent back-ground activity or sensitivity to heat. A third type of nociceptor responded to cold but not to heat. Low-threshold mechanoreceptors were identified by their brisk responses to very gentle, slowly moving mechanical stimulation of their receptive fields, and were readily distinguished from any element classified as nociceptive by their lower mechanical thresholds. Rapid innocuous warming or cooling excited some of the low-threshold mechanoreceptors. Specific thermoreceptors, both warming and cooling types, were rare, insensitive to mechanical stimulation, and responded to very slight changes in temperature. In contrast to the sensitization to heat, which was characteristic of most nociceptors, specific warming receptors displayed depressed thermal responses after noxious heating of their receptive fields. These results provide further evidence of the similarity of C-fiber receptors innervating hairy skin of different species. Some differences from past reports and additional features are described.

Electrophysiological characteristics of primate spinothalamic neurons with renal and somatic inputs

1989 ◽  
Vol 61 (6) ◽  
pp. 1121-1130 ◽  
Author(s):  
W. S. Ammons
Keyword(s):  
Nerve Stimulation ◽  
Receptive Fields ◽  
High Threshold ◽  
Spinothalamic Tract ◽  
Renal Nerve ◽  
C Fibers ◽  
Afferent Fibers ◽  
Renal Nerve Stimulation ◽  
Somatic Receptive Fields ◽  
Stimulation Of

1. Spinothalamic tract (STT) neurons in the T10-L3 segments were studied for responses to renal and somatic stimuli. A total of 90 neurons was studied in 25 alpha-chloralose anesthetized monkeys (Macaca fascicularis). All neurons were antidromically activated from the ventral posterior lateral nucleus of the thalamus. 2. Sixty-two cells were excited by renal nerve stimulation and six inhibited. Probability of locating cells with renal input was greatest in T11-L1. Cells were located in laminae I and IV-VII; however, most were located in laminae V-VII. Antidromic latencies averaged 4.61 +/- 0.32 (SE) ms, whereas antidromic conduction velocities averaged 43.23 +/- 9.03 m/s. 3. Cells with excitatory renal input received A delta input only (36 cells) or A delta- and C-fiber inputs (26 cells). Stimulation of A delta renal afferent fibers evoked bursts of 1-10 spikes/stimulus [mean 3.6 +/- 0.9 spikes/stimulus] with onset latencies of 10.7 +/- 0.5 ms. Stimulation of C-fibers evoked 1.3 +/- 0.5 spikes/stimulus with onset latencies of 61.7 +/- 11.1 ms. Magnitude of responses to A delta-fiber stimulation was greatest in T12 and decreased both rostrally and caudally. Inhibitory responses to renal nerve stimulation required activation of renal C-fibers. 4. All cells that responded to stimulation of renal afferent fibers received convergent inputs from somatic structures. Forty-four cells were classified as wide dynamic range, 10 were high threshold, 12 were high-threshold cells with inhibitory input from hair, 2 were deep, and 2 were low threshold. Somatic receptive fields were large and located on the flank and abdomen and/or the upper hindlimb. Fourteen cells had inhibitory receptive fields located on the contralateral hindlimb or one of the forearms. 5. It is concluded that T11-L1 STT cells in the monkey respond reliably to renal nerve stimulation. Thoracolumbar STT cells may thus play a role in pain that results from renal disease. The locations of the somatic receptive fields of the cells suggest that they are responsible for the referral of renal pain to the flank and abdomen.

Patterns of responses of neurons in cuneate nucleus to controlled mechanical stimulation of cutaneous velocity receptors in the cat

1980 ◽  
Vol 43 (6) ◽  
pp. 1673-1699 ◽  
Author(s):  
V. Golovchinsky
Keyword(s):  
Receptive Field ◽  
Mechanical Stimulation ◽  
Receptive Fields ◽  
Neuronal Firing ◽  
High Threshold ◽  
Pacinian Corpuscles ◽  
Sharp Separation ◽  
First Order ◽  
Discharge Patterns ◽  
Stimulation Of

1. The responses of single cuneate neurons to controled mechanical stimulation of skin were recorded in cats lightly anesthetized with a nitrous oxide-halothane mixture. The discharge patterns and peripheral receptive-field characteristics were studied in neurons driven by sensitive cutaneous mechanoreceptors, including slowly adapting skin mechanoreceptors. Virtually all cuneate neurons display maximum discharge during the velocity component of displacement. 2. Among cuneate neurons encountered in this study, approximately 46% were driven by guard hair mechanoreceptors, 15% were driven by field receptors, and 13% were driven by slowly adapting skin receptors. Neurons responding to stimulation of deep tissues (including claws) were not studied with controlled mechanical stimulation and accounted for 19%. The rest of the neurons were driven by Pacinian corpuscles, received afferent inputs from several different first-order afferents, or were not definitely identified. There was no clear evidence of down hair or high-threshold mechanoreceptor representation. 3. The discharge pattern in response to a constant-velocity stimulus proved most valuable in describing submodality classes of neurons driven by hair and field receptors since sensitivity of these neurons to dynamic and to static phases of stimulation constitute respective continua and, thus, preclude sharp separation into distinct groups. 4. The majority of neurons displayed response properties and receptive fields similar to those of first-order afferents. A minority of cells had receptive fields that were larger than those of primary afferents, with nearly identical modality and velocity characteristics throughout the receptive field. 5. Approximately 2% of recorded neurons displayed convergent properties not encountered in first-order afferents, including neurons driven from receptors of different modalities or from discontinuous receptive fields. 6. Inhibition of neuronal firing generated from outside the receptive field was rarely seen, possibly due to anesthetic conditions. In a small number of neurons, irregularities in the discharge were observed that might indicate inhibitory influences originating from within the receptive field.

Nociceptive responses of neurons in medial thalamus and their relationship to spinothalamic pathways

1978 ◽  
Vol 41 (6) ◽  
pp. 1592-1613 ◽  
Author(s):  
W. K. Dong ◽  
H. Ryu ◽  
I. H. Wagman
Keyword(s):  
Mechanical Stimulation ◽  
The Body ◽  
Direct Stimulation ◽  
Spinothalamic Tract ◽  
Medial Thalamus ◽  
C Fibers ◽  
Nociceptive Responses ◽  
C Fiber ◽  
Noxious Mechanical Stimulation ◽  
Stimulation Of

1. An extracellular study of the cat medial thalamus has revealed four types of somatosensory neurons. These were located primarily in the n. parafascicularis, n. subparafascicularis, and n. centralis lateralis; none were found in the n. centrum medianum. There was no functional segregation of neurons within each nucleus or between nuclei. Each type of neuron had large and often bilateral receptive areas. No somatotopic organization of neurons was found within the medial thalamus. 2. Noxious (N) and noxious-tap (NT) neurons comprising 72% of the sample (78 of 109 total) were considered to be nociceptive. N cells responded exclusively to noxious mechanical stimulation of skin, muscle fascia, tendons, and joints, and to direct stimulation of A-delta- and C-fiber groups in cutaneous, articular, and muscle nerves. NT cells responded to noxious and tap stimulation in a differential manner and to stimulation of the entire spectrum of A- and C-fibers. N and NT cells accurately signaled the duration of noxious mechanical stimulation. Their nociceptive responses were also graded as a function of both noxious stimulus intensity and the number of activated A-delta- and C-fibers. Stimulation of A- and C-fibers evoked, respectively, an inital burst and a late burst of discharges. A brief period of inhibition intervened between the initial and late bursts of NT cells. Prolonged afterdischarge was often observed following noxious natural stimulation or stimulation of A-delta- and C-fibers. The phenomenon of discharge "windup" was observed during iterative stimulation of C-fibers. 3. Tap (T) neurons (10%) responded only to brisk but innocuous taps applied to skin or underlying tissue. These cells were driven only by activation of A-alpha- and A-beta-fibers. The response to such stimulation was seen as an initial burst of discharges followed by an inhibitory period. 4. Inhibited (I) neurons (18%) had resting discharges that were inhibited by noxious stimuli and stimulation of A-beta- and C-fiber groups. 5. The results obtained from monitoring the peripherally evoked responses of nociceptive N and NT neurons before and after selective lesions of the spinal cord strongly suggested that the spinothalamic tracts were the only spinal projections mediating A- and C-fiber input to these cells. Each spinothalamic tract apparently carried information originating from both sides of the body.

scholarly journals Optimal delineation of single C-tactile and C-nociceptive afferents in humans by latency slowing

2017 ◽  
Vol 117 (4) ◽  
pp. 1608-1614 ◽  
Author(s):  
Roger H. Watkins ◽  
Johan Wessberg ◽  
Helena Backlund Wasling ◽  
James P. Dunham ◽  
Håkan Olausson ◽  
...  
Keyword(s):  
Electrical Stimulation ◽  
Receptive Field ◽  
Mechanical Stimulation ◽  
Receptive Fields ◽  
High Threshold ◽  
Field Stimulation ◽  
Affective Touch ◽  
Wide Range ◽  
Low Threshold ◽  
Repetitive Electrical Stimulation

C-mechanoreceptors in humans comprise a population of unmyelinated afferents exhibiting a wide range of mechanical sensitivities. C-mechanoreceptors are putatively divided into those signaling gentle touch (C-tactile afferents, CTs) and nociception (C-mechanosensitive nociceptors, CMs), giving rise to positive and negative affect, respectively. We sought to distinguish, compare, and contrast the properties of a population of human C-mechanoreceptors to see how fundamental the divisions between these putative subpopulations are. We used microneurography to record from individual afferents in humans and applied electrical and mechanical stimulation to their receptive fields. We show that C-mechanoreceptors can be distinguished unequivocally into two putative populations, comprising CTs and CMs, by electrically evoked spike latency changes (slowing). After both natural mechanical stimulation and repetitive electrical stimulation there was markedly less latency slowing in CTs compared with CMs. Electrical receptive field stimulation, which bypasses the receptor end organ, was most effective in classifying C-mechanoreceptors, as responses to mechanical receptive field stimulation overlapped somewhat, which may lead to misclassification. Furthermore, we report a subclass of low-threshold CM responding to gentle mechanical stimulation and a potential subclass of CT afferent displaying burst firing. We show that substantial differences exist in the mechanisms governing axonal conduction between CTs and CMs. We provide clear electrophysiological “signatures” (extent of latency slowing) that can be used in unequivocally identifying populations of C-mechanoreceptors in single-unit and multiunit microneurography studies and in translational animal research into affective touch. Additionally, these differential mechanisms may be pharmacologically targetable for separate modulation of positive and negative affective touch information. NEW & NOTEWORTHY Human skin encodes a plethora of touch interactions, and affective tactile information is primarily signaled by slowly conducting C-mechanoreceptive afferents. We show that electrical stimulation of low-threshold C-tactile afferents produces markedly different patterns of activity compared with high-threshold C-mechanoreceptive nociceptors, although the populations overlap in their responses to mechanical stimulation. This fundamental distinction demonstrates a divergence in affective touch signaling from the first stage of sensory processing, having implications for the processing of interpersonal touch.

scholarly journals Selective inhibition of meningeal nociceptors by botulinum neurotoxin type A: Therapeutic implications for migraine and other pains

Cephalalgia ◽  
2014 ◽  
Vol 34 (11) ◽  
pp. 853-869 ◽  
Author(s):  
Rami Burstein ◽  
XiChun Zhang ◽  
Dan Levy ◽  
K Roger Aoki ◽  
Mitchell F Brin
Keyword(s):  
Mechanical Stimulation ◽  
Botulinum Neurotoxin ◽  
Receptive Fields ◽  
Type A ◽  
Selective Inhibition ◽  
High Threshold ◽  
Mechanical Hypersensitivity ◽  
Therapeutic Implications ◽  
Botulinum Neurotoxin Type A ◽  
Stimulation Of

Background Meningeal and other trigeminal nociceptors are thought to play important roles in the initiation of migraine headache. Currently, the only approved peripherally administered chronic migraine prophylactic drug is onabotulinumtoxinA. The purpose of this study was to determine how botulinum neurotoxin type A (BoNT-A) affects naïve and sensitized meningeal nociceptors. Material and methods Using electrophysiological techniques, we identified 43 C- and 36 Aδ-meningeal nociceptors, and measured their spontaneous and evoked firing before and after BoNT-A administration to intracranial dura and extracranial suture-receptive fields. Results As a rule, BoNT-A inhibited C- but not Aδ-meningeal nociceptors. When applied to nonsensitized C-units, BoNT-A inhibited responses to mechanical stimulation of the dura with suprathreshold forces. When applied to sensitized units, BoNT-A reversed mechanical hypersensitivity. When applied before sensitization, BoNT-A prevented development of mechanical hypersensitivity. When applied extracranially to suture branches of intracranial meningeal nociceptors, BoNT-A inhibited the mechanical responsiveness of the suture branch but not dural axon. In contrast, BoNT-A did not inhibit C-unit responses to mechanical stimulation of the dura with threshold forces, or their spontaneous activity. Discussion The study provides evidence for the ability of BoNT-A to inhibit mechanical nociception in peripheral trigeminovascular neurons. These findings suggest that BoNT-A interferes with neuronal surface expression of high-threshold mechanosensitive ion channels linked preferentially to mechanical pain by preventing their fusion into the nerve terminal membrane.

Excitation of primate spinothalamic neurons by cutaneous C-fiber volleys

1979 ◽  
Vol 42 (5) ◽  
pp. 1354-1369 ◽  
Author(s):  
J. M. Chung ◽  
D. R. Kenshalo ◽  
K. D. Gerhart ◽  
W. D. Willis
Keyword(s):  
Dynamic Range ◽  
High Threshold ◽  
Mechanical Stimuli ◽  
Spinothalamic Tract ◽  
Noxious Heat ◽  
C Fibers ◽  
C Fiber ◽  
Excitatory Action ◽  
Low Threshold ◽  
Lateral Nucleus

1. The responses of spinothalamic tract cells in the lumbosacral spinal cords of anesthetized monkeys were examined following electrical stimulation of the sural nerve or the application of noxious thermal and mechanical stimuli to the skin on the lateral aspect of the foot. 2. The spinothalamic tract neurons were classified as wide dynamic range (WDR), high-threshold (HT), or low-threshold (LT) cells on the basis of their responses to mechanical stimuli. 3. All of the WDR and HT spinothalamic tract cells tested responded to volleys in A- and C-fibers. However, strong C-fiber responses were more common in HT than in WDR cells. 4. The responses atributed to C-fibers were graded with the size of the C-fiber volley. The latencies of the responses attributed to C-fibers indicated that the fastest afferents involved had a mean conduction velocity of 0.9 m/s. The responses remained after anodal blockade of conduction in A-fibers. 5. Temporal summation of the responses of spinothalamic tract cells was demonstrated both to brief trains of stimuli at 33 Hz and to single stimuli repeated at 1- to 2-s intervals. The latter phenomenon is often called "windup." 6. The responses of several spinothalamic tract cells to noxious heat pulses could still be elicited during anodal blockade of conduction in A-fibers. Similarly, it was possible to demonstrate an excitatory action of noxious mechanical stimuli despite interference with conduction in A-fibers by anodal current. 7. The cells investigated were located either in the marginal zone or in the layers of the dorsal horn equivalent to Rexed's laminae IV-VI in the cat. The cells were generally activated antidromically from the caudal part of the ventral posterior lateral nucleus of the thalamus.

Neurons in ventrobasal region of cat thalamus selectively responsive to noxious mechanical stimulation

1983 ◽  
Vol 49 (3) ◽  
pp. 662-673 ◽  
Author(s):  
C. N. Honda ◽  
S. Mense ◽  
E. R. Perl
Keyword(s):  
Sciatic Nerve ◽  
Mechanical Stimulation ◽  
Receptive Fields ◽  
High Threshold ◽  
Noxious Stimulation ◽  
Nociceptive Neurons ◽  
Subcutaneous Tissues ◽  
Alpha Beta ◽  
Lateral Nucleus ◽  
Stimulation Of

1. A survey was made of neurons located in the ventral posterior lateral nucleus of the cat thalamus and its immediate vicinity for elements with specifically nociceptive properties. 2. Pipette microelectrodes filled with a dye solution were used to obtain extracellular recordings of unitary activity in 34 animals anesthetized with chloralose. 3. The great majority of the over 1,000 different single units responding to sciatic nerve stimulation noted in this series of experiments could also be excited by innocuous mechanical stimulation of skin or subcutaneous tissues. An infrequent but consistently noted group of units excited by A-alpha beta delta sciatic nerve volleys did not respond to innocuous mechanical manipulation or A-alpha beta sciatic nerve volleys; they were excited only by either noxious levels of mechanical stimulation or when volleys included the activity of more slowly conducting myelinated fibers. The latencies of such "high-threshold" units to sciatic volleys were longer than those of the other units. 4. Histologically identified recording sites marked by dye were recovered for 17 high-threshold units. Twelve of the 17 could be excited by noxious manipulations of restricted parts of the contralateral hindlimb. Nine of the 12 had cutaneous receptive fields, whereas 3 responded only to stimulation of subcutaneous tissues. None of the 17 high-threshold units evidenced additional discharges that could be correlated with the C-fiber component of sciatic nerve volleys. 5. The high-threshold units typically exhibited a low level of irregular background activity, which increased on repeated noxious stimulation of the peripheral receptive fields. Tactile units of the same or adjacent penetrations usually had a much greater degree of ongoing activity, often marked by bursts at a relatively high frequency. 6. The recording sites for the 17 high-threshold neurons were located dorsal and ventrolateral to the core of the ventrobasal nuclei and were not found in the midst of the low-threshold, cutaneous, mechanoreceptive population. During vertical stereotaxic penetrations, high-threshold units were noted dorsal or ventral to the location of ventrobasal tactile units in a pattern consistent with the core's somatotopic arrangement. 7. These results support the concept that the cat ventrolateral thalamus receives a small but distinct selectively nociceptive projection. The nociceptive neurons appear to be located in a shell that surrounds the main tactile projection to the ventral posterior lateral nucleus and that retains at least part of the topographic arrangement characteristic of the tactile core. Presumably, this projection is part of an organization identifying and localizing noxious stimulation.

Response characteristics of neurons in the ventral posterior lateral nucleus of the monkey thalamus

1986 ◽  
Vol 56 (2) ◽  
pp. 370-390 ◽  
Author(s):  
J. M. Chung ◽  
K. H. Lee ◽  
D. J. Surmeier ◽  
L. S. Sorkin ◽  
J. Kim ◽  
...  
Keyword(s):  
Dynamic Range ◽  
Large Population ◽  
Receptive Fields ◽  
Reticular Nucleus ◽  
High Threshold ◽  
Thalamic Neurons ◽  
Response Characteristics ◽  
C Fibers ◽  
C Fiber ◽  
Lateral Nucleus

The activity of 132 neurons in the caudal part of the ventral posterior lateral nucleus (VPLc) of the thalamus was recorded from 23 anesthetized monkeys. All single thalamic units that could be excited by electrical search stimuli applied to the contralateral sciatic nerve were investigated. Responses of these cells to mechanical, thermal, and electrical stimuli applied in the periphery indicated that at least half of the sampled cells were nociceptive. Based on responses to graded mechanical stimuli applied to the periphery, 110 of the sampled cells that received a predominant input from cutaneous receptive fields were classified. There were 56 low-threshold, 39 wide dynamic range, and 15 high-threshold cells. The same neurons were also classified into five mechanical types based on a cluster analysis: types 1-5 contained 25, 34, 17, 10, and 24 cells, respectively. The fact that about half the population of cells belonged to either the wide dynamic or the high threshold group (or mechanical types 3-5) suggested that a large population of VPLc neurons respond to mechanical nociceptive stimuli either exclusively or preferentially. Responses of 63 thalamic neurons were tested to noxious heat pulses applied to their cutaneous receptive fields with a contact thermostimulator. Of these, 47 cells were excited, whereas only 16 cells did not respond. The peripheral nerve that innervated the receptive field of each of 82 thalamic neurons was stimulated with graded strengths to activate A fibers only or both A and C fibers. All tested cells responded to peripheral A fiber volleys. In addition, 42 of these cells responded to peripheral C fiber volleys. The C fiber responses could be either short lasting (a few hundreds of milliseconds) or long lasting (up to several seconds). The recording sites of 80 cells were reconstructed. Of these, 78 were in the VPLc nucleus and the remaining two were in the reticular nucleus of the thalamus. No obvious relationship between the response characteristics and the locations of the cells within the VPLc nucleus was found. Sampled thalamic units had a variety of sources of input from the periphery, including both cutaneous and/or deep tissue receptive fields. The majority of the cells, however, had exclusively cutaneous receptive fields. The sizes of the cutaneous receptive fields were often very small, so that nearly half (41%) of the receptive fields of cells sampled occupied an area of skin smaller than half the foot.(ABSTRACT TRUNCATED AT 400 WORDS)

Failure of sympathetic stimulation to affect responsiveness of rabbit polymodal nociceptors

1985 ◽  
Vol 54 (3) ◽  
pp. 513-519 ◽  
Author(s):  
V. K. Shea ◽  
E. R. Perl
Keyword(s):  
Impulse Activity ◽  
Receptive Fields ◽  
Testing Temperature ◽  
Maximum Temperature ◽  
Sympathetic Stimulation ◽  
Great Auricular Nerve ◽  
C Fibers ◽  
Cervical Sympathetic ◽  
Polymodal Nociceptor ◽  
Stimulation Of

The responses of polymodal nociceptors with unmyelinated (C) fibers of the rabbit's great auricular nerve were examined with and without intermittent stimulation of the ipsilateral cervical sympathetic trunk. The receptive field of each polymodal nociceptor was heated twice in a stepwise manner from 30 to 50 or 55 degrees C in 5 degree C steps. For each unit, one heating trial was a control trial and the other was accompanied by sympathetic stimulation. The order of the control and sympathetic stimulation trials and the maximum testing temperature were varied systematically among the units examined. The initial responses of polymodal nociceptors in the first heating trial in the presence of sympathetic stimulation were similar to the responses of units whose first heating was a control trial. Units whose receptive fields were tested to a maximum temperature of 50 degrees C in the first trial displayed enhanced responses to heat in their second trial (sensitization), while units tested initially to 55 degrees C responded less briskly during their second heating trial (depression). However, the occurrence of sympathetic stimulation in the second heating trials had no apparent effect on the responses to heat of sensitized or depressed elements. Alterations in the numbers of impulses, instantaneous frequency, or pattern of impulse activity of individual units could not be attributed to sympathetic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of transmission in rostral trigeminal sensory nuclei during chewing

1986 ◽  
Vol 55 (1) ◽  
pp. 56-75 ◽  
Author(s):  
K. A. Olsson ◽  
K. Sasamoto ◽  
J. P. Lund
Keyword(s):  
Receptive Field ◽  
Peripheral Nerve ◽  
Sensorimotor Cortex ◽  
Receptive Fields ◽  
Border Zone ◽  
High Threshold ◽  
Motor Nucleus ◽  
Lateral Border ◽  
Low Threshold ◽  
Stimulation Of

Eighty-one sensory neurons in the rostral trigeminal sensory nuclei (main sensory nucleus, nucleus oralis, and the lateral border zone of the motor nucleus) were recorded in urethan-anesthetized rabbits before and during mastication. Receptive-field characteristics were described, and responses evoked by electrical stimulation of the inferior alveolar and infraorbital nerves, sensorimotor cortex, and thalamus were recorded. Forty-four percent of neurons were stimulated by the movements of mastication; nevertheless, evidence is presented that the excitability of the 49 neurons that receive low-threshold mechanoreceptor inputs is depressed during mastication for the following reasons: The spontaneous activity of seven cells was inhibited during movement. The probability of firing in response to stimulation of the peripheral nerve on sensorimotor cortex was decreased during mastication. There was usually a corresponding increase in the latency of the action potentials. Injections of local anesthetic (prilocaine hydrochloride, 4%) into the receptive field of the neuron did not prevent the decrease in excitability during mastication. Fourteen neurons that received inputs from periodontal pressoreceptors were recorded medial to most of the low-threshold group. The excitability of six of these was reduced during jaw closure and during the occlusal phase of movement, that is, within the period in which they would be activated by pressure on the teeth. The rest were tonically suppressed. Eighteen neurons recorded in the lateral border zone of the motor nucleus had receptive fields that were of high threshold or were undefined. They responded to stimulation of the peripheral nerve at high threshold. The excitability of most of these neurons was strongly phase modulated during mastication. They were most excitable during jaw closure or during the occlusal phase of movement and inexcitable during opening. The excitability of the others was tonically depressed. In most cases, the changes in excitability described did not seem to be due to the patterns of activity of the neurons that were generated by the movements. We conclude that the pattern elaborated by the central pattern generator includes selective modifications of sensory transmission. One reason for this is to suppress reflex responses to low-threshold inputs while maintaining the protective response to tissue damage.

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