The response of some sympathetic neurones to volleys in various afferent nerves

1. Responses of sympathetic neurones to various afferent inputs have been analysed both in anaesthetized cats with intact neuraxis and in spinal cats.2. In anaesthetized cats electrical stimulation of low threshold group III afferent fibres in skin and muscle nerves inhibited sympathetic neurones and gave depressor responses. The silent period following a sympathetic reflex discharge is most likely due to inhibition brought on by a particular subgroup within this afferent group.3. High threshold group III and group IV afferent fibres excited sympathetic neurones and elicited pressor responses.4. Sympathetic reflex arcs could be temporally facilitated during the somatic afferent induced inhibition, by group IV, and to a lesser extent by group III volleys.5. Section of the spinal cord shortened the time course and lessened the degree of group III inhibition suggesting that suprasegmental pathways are involved in the long-lasting depression following a reflex.6. Baroreceptor afferent stimulation inhibited group III- and group IV- evoked reflexes even with some temporal facilitation in the pathway.7. The special properties of the sympathetic reflex arcs and the relation of the results to other work on B.P. reflexes are discussed. It is suggested that the group III and IV muscle afferents have a chemoreceptor function and are responsible for mediation of the pressor reflex during muscle exercise.     

Reflex responses evoked in the adrenal sympathetic nerve to electrical stimulation of somatic afferent nerves in the rat

The present study was initiated to determine the role of somatic A (myelinated) and C (unmyelinated) afferent fibers in both responses of increases and decreases in adrenal sympathetic nerve activities during repetitive mechanical pinching and brushing stimulations of the skin in anesthetized rats with central nervous system (CNS) intact. Accordingly, changes in adrenal sympathetic nerve activity resulting from repetitive and single shock electrical stimulation of various spinal afferent nerves, especially the 13th thoracic (Th13) spinal nerve and the sural nerve, were examined in urethane/chloralose-anesthetized rats. Repetitive electrical stimulation of A afferent fibers in Th13 spinal or sural nerve decreased the adrenal nerve activity similarly as brushing stimulation of skin of the lower chest or hindlimb did, while repetitive stimulation of A plus C afferent fibers of those nerves increased the adrenal nerve activity as pinching stimulation of those skins did. Single shock stimulation of spinal afferent nerves evoked various reflex components in the adrenal nerve: an initial depression of spontaneous activity (the early depression); the following reflex discharge due to activation of A afferent fibers (the A-reflex); a subsequent reflex discharge due to activation of C afferent fibers (the C-reflex); and following post-excitatory depressions. These reflexes seem to be mediated mainly via supraspinal pathways since they were abolished by spinal transection at the C1-2 level. Although the supraspinal A- and C-reflexes could be elicited from stimulation of a wide variety of spinal segmental afferent levels, the early depression was more prominent when afferents at spinal segments closer to the level of adrenal nerve outflow were excited. It is suggested that the decreased responses of the adrenal nerve during repetitive electrical stimulation of A afferent nerve fibers are attributable to summation of both the early depression and post-excitatory depression evoked by single shock stimulation, while the increased responses during repetitive stimulation of A plus C afferent fibers are attributable to summation of the C-reflex after single shock stimulation. In spinalized rats, repetitive stimulation of Th13 always increased the adrenal nerve activities regardless of whether A fibers alone or A plus C fibers were stimulated, just as brushing and pinching of the lower chest skin always increased them. The increased responses in spinal animals seem to be related to the fact that single electrical stimuli of Th13 produced A- and C-reflexes of spinal origin without clear depressions.     

Effects of sympathetic stimulation on C-fibre responses in rabbit

Unmyelinated C-fibre responses to electrical stimulation were recorded in common peroneal, sural and tibial nerves of rabbits. Three distinct C elevations, here called C1, C2 and C3, were recorded. C2 is probably of somatic origin because it was depressed due to collision by peripheral stimulation of cutaneous receptors. The conduction velocity of C3 corresponded to that of sympathetic post-ganglionic fibres. During sympathetic trunk stimulation the A-fibre responses were not significantly changed while C responses, especially C2, were reduced in amplitude and slightly delayed. The C-fibre responses were also influenced by intra-arterial infusion of noradrenaline. In most cases, the latency of the response was increased. The effect of sympathetic stimulation was completely blocked by hexamethonium, and partly blocked by phentolamine, an adrenergic alpha-receptor blocking agent which also blocked the effect of noradrenaline. The findings suggest that there are adrenergic receptors distributed along unmyelinated somatic afferent fibres. Sympathetic activity may release noradrenaline in the peripheral nerve, resulting in changed conductive properties in unmyelinated fibres transmitting sensory information.     

Action potentials in parasympathetic and sympathetic efferent fibres to the trachea and lungs of dogs and cats

1. Action potentials were recorded from seventy-four single and twenty-nine small multifibre nerve strands efferent to the trachea and lungs of cats and dogs. From the pathway (vagal or sympathetic), spontaneous activity, conduction velocity and responses to various interventions the efferent fibres were classified in the following way.2. Group I, vagal. These had a mean conduction velocity of 9.7 m/sec, and had a respiratory but seldom a cardiac rhythm. Their discharge was inhibited during hypertension caused by injections of adrenaline and during inflation of the lungs, but was increased during tracheal occlusion, stimulation of peripheral chemoreceptors and irritation of the larynx. The fibres are thought to be constrictor to the airways.3. Group II, sympathetic. These had a mean conduction velocity of 0.85 m/sec and usually had inspiratory and cardiac rhythms. Their discharge usually responded qualitatively as that of group I fibres to the various interventions, but with clear quantitative differences. They are divided into three subgroups on the basis of their responses to injections of adrenaline and to asphyxial stimuli.4. Group III, vagal and sympathetic. These had a mean conduction velocity of 9.0 m/sec, very slow discharge rates and often an expiratory and cardiac modulation. They were activated during hypertension due to adrenaline and often by tracheal occlusion, chemoreceptor stimulation, laryngeal irritation and lung inflation. Their motor action is unknown.5. Group IV, vagal and sympathetic. These had conspicuous cardiac rhythms resembling those of vascular baroreceptors, but their discharge could not be correlated with arterial blood pressure. Their mean conduction velocity was 6.6 m/sec. Some were active after combined vagotomy and sympathectomy. Together with some unclassified fibres, those of group IV are thought to be aberrant afferent nerves or collateral afferent branches, and possibly to subserve local reflexes.6. The results are discussed in relation to nervous control of effector tissues in the airways and autonomic nervous control generally.     

Intrafascicular recordings of afferent multi-unit activity from the human supraorbital nerve

Intrafascicular recordings of afferent multi-unit activity were obtained from the supraorbital nerve in 40 healthy human volunteers, using tungsten micro-electrodes inserted percutaneously at the eyebrow. Seventy-seven fascicular receptive fields were mapped; their area ranged from 2 to 76 cm², with a median of 19 cm². The smallest fields were found in the eyebrow region, and the largest on the scalp. In response to non-painful electrical intradermal stimulation, the conduction velocity of the fastest nerve fibres was calculated to be 40 ± 2 m s^-1 (mean ± SEM), and the later part of the afferent volley corresponded to a velocity of 20 ± 1 m s^-1. The responses to skin indentation indicated that the density of mechanoreceptive innervation was higher in the lower part of the forehead than in the upper/posterior part of the innervation territory of the nerve. A rapid mechanical tap on the forehead and scalp evoked two major afferent volleys corresponding to the on- and off-phase of the stimulus. Manipulation of hairs resulted not only in dynamic responses to hair movement, but also in a static discharge during sustained hair displacement. When a fascicular field included the eyebrow region, skin stretching during blinking movements evoked distinct afferent activity. Following mechanical and electrical stimuli there were no signs of ‘trigeminal antidromic potentials’ of the type described in the cat and monkey.     

Encoding of visceral noxious stimuli in the discharge patterns of visceral afferent fibres from the colon

Afferent fibres, in the inferior splanchnic nerves and lumbar white rami, which supply the colon and its mesentery in the cat, were investigated for their responses to distension and contraction of the colon and to local pressure applied to colon and its mesentery. 1) 63% (177 out of 287) of the axons had resting activity (median 0.3 imp/s). These axons were either unmyelinated (conduction velocity below 2 m/s) or thin myelinated (conduction velocity below 18 m/s). Most axons without resting activity (N=95 out of 106 axons) conducted at less than 1.4 m/s, and most were probably sympathetic efferents. 2) 76 out of 80 afferent units with resting activity (95%) and 8 out of 27 units without (30%) were excited by distension of the colon. The thresholds were largely at intraluminal pressures of around 25 mm Hg or higher. 3) Most afferent units (87%) responded with an increased steady state discharge throughout the distension with or without initial dynamic response. The rest of the afferent units responded only with a transient discharged to distension. 4) Most afferent units reacted in a graded manner to variable intraluminal pressures. 5) In only 43% of the distension-sensitive afferent units could mechanoreceptive sites be located on the wall of the colon or in the mesentery. The majority of the afferent units had one mechanoreceptive site only, some had two. 6) Afferent units reacting to colon distension were also excited by contraction of the colon. 7) The excitability spectrum of visceral afferent fibres in the inferior splanchnic nerves, which are activated by colon distension, suggests that these units are involved in visceral nociception from the colon.     

Sympathetic and noradrenaline effects on C-fibre transmission: single-unit analysis

Single afferent unmyelinated fibres were dissected from the otherwise intact sural nerve in anesthetized rabbits. The sympathetic trunk could be stimulated via electrodes implanted through the abdomen. The response in single C fibres was elicited by electrical stimulation in the cutaneous innervation area of the fibre. Sympathetic stimulation (8 Hz, 1 ms pulses, 5 mA for 60 s) increased the latency in all tested C fibres (2.0% +/- 0.8%, mean +/- SD, n = 17). In 48% of the units the amplitude of the action potential decreased (26.4% +/- 12.3%) during sympathetic stimulation. Infusion of noradrenaline (5 micrograms min-1) increased (7.7% +/- 4.1%) the latency in all units and increased (36.9% +/- 29.8%) the amplitude of 25% of the units. The effects of sympathetic stimulation and noradrenaline infusion were blocked by pre-treatment with phentolamine (3 mg kg-1 i.v.). The results suggest that catecholamines change the membrane properties of unmyelinated fibres.     

Afferent innervation of the guinea-pig's ureter

The electrical activity of 41 mechanosensitive afferent units was recored using anin vitro preparation of the guinea-pig's ureter. The conduction velocities of these fibres were found to be in the C-fibre range. Only 4 of them responded to contractions of the ureter. The activation threshold of the units to intraluminal (i.l.) pressure varied between 3 and 50 mm Hg. It is concluded that some of these afferent fibres might be involved in the signalling of nociceptive events.     

Characteristics of spinal neurones responding to cutaneous myelinated and unmyelinated fibres

1. Spike discharges were recorded from neurones in the lumbar spinal cord in cats anaesthetized by barbiturate.2. The neurones were examined systematically for various physiological parameters and for their location. Especially the neurones situated in the dorsal horn were classified for the following parameters: mono- or polysynaptic linkage to myelinated afferents; type of natural stimuli which excited the neurones; depth from the cord surface; number of impulses discharged upon a cutaneous A fibre stimulus; steady-state discharge in the absence of intentional stimulation.3. All neurones were also tested as to whether or not they responded to volleys in cutaneous C fibres. Of 111 units which were activated by the A fibres in nerves from the hairy skin, 57 (= 51%) responded to C volleys in those nerves too.4. By blocking conduction in the A fibres using polarizing currents it was shown that the responses to C fibre volleys were partially or totally suppressed by a preceding discharge of the neurone in response to an A volley. Using search stimuli which were suprathreshold for C fibres one cell out of 36 could be found which responded only to afferent volleys in C fibres.5. About half of all neurones were shown to be connected monosynaptically to cutaneous A fibres, as was judged from the synaptic delay. The other half were polysynaptically linked to the A fibres. Both mono- and polysynaptic neurones were found in all layers of the dorsal horn. About 15% of the cells had additional input from muscle Group II and/or III fibres via polysynaptic pathways.6. Subdividing the A and A+C responsive neurones according to their mono- (M) or polysynaptic (P) connexions yielded the following sub-samples: MC, 39%; PC, 15%; MA, 13%; PA, 33%. Most MC neurones had, and most PA units had not, a spontaneous discharge. About half of the PA cells could not be driven by natural skin stimulation. The majority of MC units responded specifically to movement of hairs.7. A model was proposed hypothesizing two pathways in the dorsal horn, one showing convergence of A and C fibres and the other not. Some relations concerning other observations on C fibre effects were discussed.     

Responses of sacral visceral afferents from the lower urinary tract,colon and anus to mechanical stimulation

The discharge characteristics of sacral visceral afferents supplying the urinary bladder, urethra, colon and anus to mechanical stimuli were analyzed in the anaesthetized cat. The stimuli used were passive distension (urinary bladder, colon), isovolumetric contraction (urinary bladder), movements of the urethral catheter and mechanical shearing stimuli (mucosal skin of the anal canal). (1) In total 245 afferent units which projected in the pelvic nerve were isolated from the sacral dorsal roots. From one of the following organs, urinary bladder, colon, urethra and anus 117 afferent units were activated. By these stimuli from the bladder, urethra and anus 122 afferent units could not be activated, and as far as tested also not from the colon; in 6 afferent units the classification was unclear. (2) Afferent units from the urinary bladder and the colon responded consistently to passive distension of the respective organ. The units from the urinary bladder showed graded responses at intraluminal pressures of about 10–70 mm Hg and responded also to isovolumetric contractions of the organ. The thresholds of the units from the bladder to passive distension and contraction varied from about 5 to 20 mm Hg intravesical pressure. (3) The afferent units from the urethra and the anus did not react or showed some weak phasic and irregular responses to distension and contraction applied to the urinary bladder or to distension of the colon. They were consistently excited by low threshold mechanical stimulation of the urethra and anus, respectively. (4) The axons from the bladder, urethra and anus were presumably myelinated (conduction velocity above 2 m/s) and conducted at 10.3±6.1 m/s (n=34, mean±SD), 26.3±9.3 m/s (n=13) and 9.5±5.1 m/s (n=37), respectively. The axons from the colon conducted at about 0.5 to 16 m/s (n=20), 13 of them conducting at less than 2 m/s. About 75% of the axons which could not be activated by mechanical stimulation of the visceral organs were presumably unmyelinated (conduction velocity below 2 m/s). (5) Some ongoing activity was found in 9 out of 26 afferent units from the anus but, with one exception, the afferent units from the bladder, urethra and colon were silent. (6) It is concluded that the pelvic afferent units from the urinary bladder, urethra, colon and anus consist of distinct populations with characteristic response patterns. There is no indication from this investigation that the urinary bladder is supplied by sacral afferents which are only recruited at high intravesical pressures during passive distension and isovolumetric contractions and which are possibly associated with pain.Supported by the Deutsche Forschungsgemeinschaft     

Functional evidence for the differential control of superficial and deep blood vessels by sympathetic vasoconstrictor and primary afferent vasodilator fibres in rat hairless skin

We quantitatively investigated sympathetic vasoconstriction and antidromic vasodilation mediated by small-diameter primary afferents on the plantar hairless skin of the hindpaws in Wistar rats using laser Doppler (LD) flowmetry and an infrared thermometer. Sympathetic vasoconstriction was elicited by electrical stimulation of the centrally cut ipsilateral lumbar sympathetic trunk (LST) with 50-s trains at 0.1–20 Hz. Antidromic vasodilation was evoked by electrical stimulation of the dorsal root (DR) L5 with 20-s or 50-s trains at 1–4 Hz. Cutting the LST resulted in increases in skin temperature (SKT) by 6.1±1.0° C (mean±SEM) and in LD flow by 128±20%. Stimulation of the LST resulted in a graded decrease in LD flow and SKT that was most pronounced between 0 and 0.1 Hz. However, DR stimulation evoked a large increase in LD flow but only little change in SKT in rats with sectioned LST. When the DR was stimulated either in animals with intact LST or during continuous stimulation of vasoconstrictor fibres in the sectioned LST, i.e. while baseline temperature was relatively low (26.3±1.1° C), DR stimulation still resulted in large increases in LD flow, but only minor changes in SKT. These results suggest that blood flow through both deep and superficial layers of rat hairless skin is regulated by activity in sympathetic postganglionic vasoconstrictor fibres, whereas small-diameter primary afferent fibres appear to influence predominantly the blood flow through superficial layers of rat plantar skin. Received: 22 April 1997/Accepted: 11 July 1997     

Single unit responses and the total afferent outflow from the cat's foot pad upon mechanical stimulation

Summary The properties of mechanosensitive units with large myelinated afferents were determined in the hairless skin of the central pad of the cat's hind foot, and the total afferent outflow from this region after short skin indentations and during constant force stimuli was measured in the plantar nerves.Basically three types of mechanosensitive units with afferent conduction velocities above 40 m/s were found: (a) receptors with the properties of Pacinian corpuscles (PC-receptors); (b) receptors which showed burst discharges for up to 500 ms after the onset of a constant force stimulus (RA-receptors); and (c) receptors which discharged throughout a constant force stimulus (SA-receptors).The afferent conduction velocities of these units were in the same range as those of receptors from the surrounding hairy skin. A considerable proportion of receptors from both skin areas had no collaterals in the dorsal columns.The afferent outflow after short skin indentations of up to 5 displacement consisted of impulses from PC-receptors only. Stimuli of 20 recruited between 50 and 100 afferent units of which less than 10% were other than PC-units. During constant force stimuli the afferent outflow came from SA-receptors only. Ten seconds after stimulus onset a 500 g stimulus evoked an afferent discharge of about 1000 imp/s and a 1000 g stimulus of about 1700 imp/s. At all times a power function of the form F=K · (S–S₀)ⁿ related the afferent discharge F to the stimulus intensity S. The exponents were around n=0.5 and tended to increase in the course of the stimulus.This work was supported by the Deutsche Forschungsgemeinschaft.     

Activity of aortic chemoreceptors during electrical stimulation of the stellate ganglion in the cat

1. In anaesthetized cats, action potentials from aortic chemoreceptors were recorded during electrical stimulation of preganglionic sympathetic fibres to the decentralized right stellate ganglion. The rate of discharge in afferents in the ipsilateral but not in the contralateral aortic nerve increased when stimulus frequency was 4/sec or higher.

2. The post-ganglionic fibres in the stimulated pathway originate in the right stellate ganglion. They leave the ganglion in the caudal limb of the ansa subclavia, and the results suggest that the ipsilateral aortic nerve and its branches distribute sympathetic fibres as well as afferents to aortic bodies.

3. In contrast, the rate of discharge of chemoreceptor fibres in the contralateral aortic nerve fell as blood pressure increased during sympathetic stimulation. Sympathetic pathways to the aortic bodies can maintain or increase chemoreceptor discharge during hypertension elicited by sympathetic activation.

     

Sympathetic baroreflex control of vascular resistance in comfortably warm man. Analyses of neurogenic constrictor responses in the resting forearm and in its separate skeletal muscle and skin tissue compartments

Resting forearm vascular resistance changes elicited in male volunteers by graded reflex sympathetic activation evoked by graded lower body negative pressure (LBNP) were studied at room temperatures of 24–25 and 2C–21 7deg;C. The latter condition caused strong suppression of skin flow and permitted preferential analysis of muscle responses and, by comparison with responses at 24–25 7deg;C, secondary estimation of circulatory reactions in the skin. Short-lasting LBNP-bouts (1.5 min) allowed analyses of reflex vascular reactions to high and barely tolerated LBNP (85 mmHg) and thereby to high levels of circulatory stress and sympathetic nerve discharge, yet without risks for the subjects under study. Both muscle and skin reacted intensely and in a graded manner to graded sympathetic activation with very pronounced resistance change (74–77% flow decline; 350–400% resistance rise above control level) at high LBNP. Therefore, the sympathetic vasomotor fibres can exert a very potent control of vascular resistance both in skeletal muscle and in skin under thermoneutral conditions, and both tissues apparently can serve as major targets for powerful sympathetic homeostatic baroreflexes. Evidence indicated that this control is exerted from both low-pressure cardiopulmonary and high-pressure arterial baroreceptor areas. These conclusions deviate from previous literature, in which baroreflex sympathetic vasoconstriction in the human limb has been proposed to be more or less selectively mediated from cardiopulmonary receptors and, further, muscle to respond fully already at mild circulatory stress without further constriction if the stimulus is increased.     

Raphe magnus inhibition of feline T1-T4 spinoreticular tract cell responses to visceral and somatic inputs

Background activity of spinoreticular tract neurons in the T1-T4 segments was on average inhibited 80% by electrical stimulation of nucleus raphe magnus. Nucleus raphe magnus stimulation inhibited responses of spinoreticular tract neurons to somatic input produced by touching the skin and hair (innocuous stimulus) or pinching the skin and muscle (noxious stimulus). Inhibition of responses to noxious and innocuous somatic inputs was not significantly different. Inhibition produced during nucleus raphe magnus stimulation was less effective when the activity of spinoreticular tract cells increased. This relationship was consistent for both background activity and responses to somatic noxious or innocuous input. Nucleus raphe magnus stimulation inhibited responses of spinoreticular tract neurons to visceral input produced by electrical stimulation of cardiopulmonary sympathetic afferent fibers. Responses to C-fiber sympathetic afferent fibers were more effectively inhibited than were responses to A-delta sympathetic afferent fibers. In conclusion, stimulation of the nucleus raphe magnus inhibits T1-T4 spinoreticular tract neuronal responses to visceral and somatic inputs. Since spinoreticular neurons project to the medullary reticular formation, activation of the nucleus raphe magnus could modulate affective-motivational behavior and cardiovascular adjustments that often occur during angina pectoris.     

Characteristics, specificity, and efferent control of frog cutaneous cold receptors

1. Thermal stimulation of frog skin produces a discharge in afferents in the dorsocutaneous nerve. The characteristics of this response have been examined with regard to static and dynamic sensitivity to thermal stimuli and to mechanical sensitivity. Frog cutaneous receptors respond only to cooling, with no response to warming through the same thermal range.2. The static temperature at which these receptors are maximally active is about 24 degrees C for Rana pipiens and about 27 degrees C for R. catesbiana.3. The dynamic sensitivity of frog cutaneous receptors is linearly related to both stimulus slope and magnitude. Maximum dynamic sensitivity was between -90 and -120 impulses/ degrees C.sec.4. Antidromic occlusion experiments demonstrate relative insensitivity of these receptors to tonic mechanical stimulation. At high stimulus intensities, however, larger fibres are recruited into the response; this recruitment of action potentials of larger amplitude is a linear function of both stimulus slope and magnitude.5. Spike heights are linearly related to conduction velocities in the dorsocutaneous nerve; tonic mechanoreceptors have a mean spike height of 28.4+/-0.6 muV and conduction velocities about 6-8 m/sec, whereas these temperature sensitive receptors have spike heights 15.8+/-0.4 muV and conduction velocities about 3-4 m/sec.6. Maximum dynamic sensitivity skin is increased following stimulation of the first or second sympathetic ganglion. This increase is both marked and progressive, reaching a maximal enhancement of about 150-160% control at a stimulus rate of 5 stimuli/train, each train delivered once every 5 sec.7. Static sensitivity of the cold receptors is also increased following sympathetic stimulation. This increased sensitivity is shown by both increased discharge rate within the same thermal range and by decreased temperature of maximum static sensitivity.8. Sympathetic modulation of dynamic thermal sensitivity is mimicked by epinephrine and norepinephrine in doses of 10(-6)-10(-7) g/ml. Ephedrine, another adrenergic agonist, also mimics the enhancement of cold receptors by sympathetic stimulation.9. Larger fibres are recruited to account for the increased sensitivity of thermoreceptors following sympathetic stimulation and epinephrine application.10. Propranolol and phentolamine both block the enhancement of the response by sympathetic stimulation, but propranolol blocks the response of the receptor to thermal stimulation as well. Reserpine pre-treatment blocks the effect of sympathetic stimulation on the cold response.     

Synaptic responses to mechanical stimulation in calyceal and bouton type vestibular afferents studied in an isolated preparation of semicircular canal ampullae of chicken

Summary Relationships between the response patterns of semicircular canal afferents to mechanical stimulation and the morphologies of their peripheral endings were investigated in an isolated preparation of the anterior semicircular canal ampulla of chicken, using a combination of electrical recording with intracellular injections of Lucifer Yellow CH. The hair bundle mechanical stimulus was applied in a diffuse manner by a glass rod vibrating in the nearby bathing medium. Two types of spike discharge patterns and postsynaptic potentials were recorded. One type was found exclusively in the bouton type afferent and demonstrated a phasic increase of firing frequency and transient depolarizing postsynaptic potentials at the beginning of mechanical stimulation. These synaptic potentials were also observed spontaneously and their amplitudes were increased by membrane hyperpolarization. The other type was found exclusively in afferents with calyceal endings and showed a tonic increase of spiking frequency and depolarizing DC postsynaptic potentials with superimposing AC responses at the frequency of the mechanical stimulation. Amplitudes of postsynaptic potentials were increased by hyperpolarization. Hair cells generated depolarizing DC transduction potentials superimposed with AC potentials at frequency of the mechanical stimulation. The spontaneous spike discharging patterns of afferent nerve fibres were classified either as a regular type (CV < 0.10) or as an irregular type (CV > 0.25) on the basis of coefficient of variation (CV) of interspike intervals. The spontaneous firing rate of regular units was higher than that of irregular units. Several membrane characteristics are different between these two types of afferent fibers; irregular units had short membrane time constants and fast spikes associated with clear spike-afterhyperpolarization. These features fit with the fact that irregular units tend to have phasic responses to mechanical stimulation while regular units typically have tonic responses. Irregular units had bouton endings with an average axonal diameter thicker than the regular units which had calix endings.     

Functional characteristics of mechanoreceptors in sinus hair follicles of the cat

1. The discharge of impulses in afferent fibres dissected from the infraorbital and ulnar nerves of anaesthetized cats was recorded during controlled movements of the maxillary and carpal sinus hairs.2. Four main types of afferent units were identified. Two had slowly adapting responses characteristic of the epidermal type I, and dermal type II mechanoreceptors of the hairy skin. Two rapidly adapting responses to movement of the sinus hairs were found, one with a high velocity threshold and another with a low velocity threshold.3. The slowly adapting units showed a power relationship between the degree of displacement of the hair and the mean interspike interval of the response. Slowly adapting units also exhibited a power relationship between the velocity of displacement of a hair and the mean interspike interval of the response.4. The conduction velocities of all types of afferent units were measured and fell in the range of the Aalpha, fast myelinated fibres.5. Movements of the carpal sinus hairs yielded both types of slowly adapting response recorded in fibres of the ulnar nerve directly innervating the carpal sinus hair follicles, and rapidly adapting responses from Pacinian corpuscles, found in close association with, but external to, these follicles.6. On the basis of the findings in this study and the results of anatomical investigations of the receptor structures in the sinus hair follicle a correlation between the distinguishable afferent responses and the morphologically identifiable nerve endings has been proposed.     

The effect of electrical stimulation of the corticospinal tract on motor units of the human biceps brachii

In healthy human subjects, descending motor pathways including the corticospinal tract were stimulated electrically at the level of the cervicomedullary junction to determine the effects on the discharge of motoneurones innervating the biceps brachii. Post-stimulus time histograms (PSTHs) were constructed for 15 single motor units following electrical stimulation of the corticospinal tract and for 11 units following electrical stimulation of large diameter afferents at the brachial plexus. Responses were assessed during weak voluntary contraction. Both types of stimulation produced a single peak at short latency in the PSTH (mean 8.5 and 8.7 ms, respectively) and of short duration (< 1.4 ms). In separate studies, we compared the latency of the responses to electrical stimulation of the corticospinal tract in the relaxed muscle with that in the contracting muscle. The latency was the same in the two conditions when the intensity of the stimulation was adjusted so that responses of the same size could be compared. Estimates of the descending conduction velocity and measurements of presumed peripheral conduction time suggest that there is less than 0.5 ms for spinal events (including synaptic delays). We propose that in response to electrical stimulation of the descending tract fibres, biceps motoneurones receive a large excitatory input with minimal dispersion and it presumably contains a dominant monosynaptic component.     

Quantitative relationship between afferent activity and efferent impulses in viscero-cardiac reflexes

Summary Viscerocardiac reflexes were elicited in sympathectomized frogs and efferent electrical activity in the cardiac vagal fibres was recorded. The amplitude and the duration of the efferent discharges was found to vary according to the number of simultaneously excited afferent fibres. Relatively weak afferent activity was accompanied by a brief discharge of low amplitude. On the other hand, increase of afferent activity led to a greater amplitude and to a longer duration of the efferent discharge in the vagal cardiac fibres. These changes in electrical activity were associated with reflex changes (inhibition or stimulation) of cardiac function. We conclude that inhibition or stimulation of cardiac function is determined by the number of stimultaneously excited efferent vagal fibres.(Presented by Active Member AMN SSSR A. V. Lebedinskii) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 58, No. 7, pp. 12–15, July, 1964