Three ascending spinal pathways in the dorsal part of the lateral funiculus

Current Principles of Motor Control, with Special Reference to Vertebrate Locomotion

The vertebrate control of locomotion involves all levels of the nervous system from cortex to the spinal cord. Here, we aim to cover all main aspects of this complex behavior, from the operation of the microcircuits in the spinal cord to the systems and behavioral levels and extend from mammalian locomotion to the basic undulatory movements of lamprey and fish. The cellular basis of propulsion represents the core of the control system, and it involves the spinal central pattern generator networks (CPGs) controlling the timing of different muscles, the sensory compensation for perturbations, and the brain stem command systems controlling the level of activity of the CPGs and the speed of locomotion. The forebrain and in particular the basal ganglia are involved in determining which motor programs should be recruited at a given point of time and can both initiate and stop locomotor activity. The propulsive control system needs to be integrated with the postural control system to maintain body orientation. Moreover, the locomotor movements need to be steered so that the subject approaches the goal of the locomotor episode, or avoids colliding with elements in the environment or simply escapes at high speed. These different aspects will all be covered in the review.

Dynamic synchronization of ongoing neuronal activity across spinal segments regulates sensory information flow

Previous studies on the correlation between spontaneous cord dorsum potentials recorded in the lumbar spinal segments of anaesthetized cats suggested the operation of a population of dorsal horn neurones that modulates, in a differential manner, transmission along pathways mediating Ib non-reciprocal postsynaptic inhibition and pathways mediating primary afferent depolarization and presynaptic inhibition. In order to gain further insight into the possible neuronal mechanisms that underlie this process, we have measured changes in the correlation between the spontaneous activity of individual dorsal horn neurones and the cord dorsum potentials associated with intermittent activation of these inhibitory pathways. We found that high levels of neuronal synchronization within the dorsal horn are associated with states of incremented activity along the pathways mediating presynaptic inhibition relative to pathways mediating Ib postsynaptic inhibition. It is suggested that ongoing changes in the patterns of functional connectivity within a distributed ensemble of dorsal horn neurones play a relevant role in the state-dependent modulation of impulse transmission along inhibitory pathways, among them those involved in the central control of sensory information. This feature would allow the same neuronal network to be involved in different functional tasks.

Rhythmic activity of feline dorsal and ventral spinocerebellar tract neurons during fictive motor actions

Neurons of the dorsal spinocerebellar tracts (DSCT) have been described to be rhythmically active during walking on a treadmill in decerebrate cats, but this activity ceased following deafferentation of the hindlimb. This observation supported the hypothesis that DSCT neurons primarily relay the activity of hindlimb afferents during locomotion, but lack input from the spinal central pattern generator. The ventral spinocerebellar tract (VSCT) neurons, on the other hand, were found to be active during actual locomotion (on a treadmill) even after deafferentation, as well as during fictive locomotion (without phasic afferent feedback). In this study, we compared the activity of DSCT and VSCT neurons during fictive rhythmic motor behaviors. We used decerebrate cat preparations in which fictive motor tasks can be evoked while the animal is paralyzed and there is no rhythmic sensory input from hindlimb nerves. Spinocerebellar tract cells with cell bodies located in the lumbar segments were identified by electrophysiological techniques and examined by extra- and intracellular microelectrode recordings. During fictive locomotion, 57/81 DSCT and 30/30 VSCT neurons showed phasic, cycle-related activity. During fictive scratch, 19/29 DSCT neurons showed activity related to the scratch cycle. We provide evidence for the first time that locomotor and scratch drive potentials are present not only in VSCT, but also in the majority of DSCT neurons. These results demonstrate that both spinocerebellar tracts receive input from the central pattern generator circuitry, often sufficient to elicit firing in the absence of sensory input.

The Camino intracranial pressure device in clinical practice. Assessment in a 1000 cases

BACKGROUND

Intracranial pressure (ICP) monitoring has become standard in the management of neurocritical patients. A variety of monitoring techniques and devices are available, each offering advantages and disadvantages. Analysis of large populations has never been performed.

PATIENTS AND METHODS

A prospective study was designed to evaluate the Camino fiberoptic intraparenchymal cerebral pressure monitor for complications and accuracy.

RESULTS

Between 1992-2004 one thousand consecutive patients had a fiberoptic ICP monitor placed. The most frequent indication for monitoring was severe head injury (697 cases). The average duration of ICP monitoring was 184.6 +/- 94.3 hours; the range was 16-581 hours. Zero drift (range, -17 to 21 mm Hg; mean 7.3 +/- 5.1) was recorded after the devices were removed from 624 patients. Mechanical complications such as: breakage of the optical fiber (n = 17); dislocations of the fixation screw (n = 15) or the probe (n = 13); and failure of ICP recording for unknown reasons (n = 4) were found in 49 Camino devices.

CONCLUSIONS

The Camino ICP sensor remains one of the most popular ICP monitoring devices for use in critical neurosurgical patients. The system offers reliable ICP measurements in an acceptable percentage of device complications and the advantage of in vivo recalibration. The incidence of technical complications was low and similar to others devices.

Proton sensitivity Ca2+ permeability and molecular basis of acid-sensing ion channels expressed in glabrous and hairy skin afferents

We contrasted the physiology and peripheral targets of subclassified nociceptive and nonnociceptive afferents that express acid-sensing ion channel (ASIC)-like currents. The threshold for current activation was similar in eight distinct cell subclasses regardless of functional modality (pH 6.8). When potency was determined from concentration-response curves, nonnociceptors exhibited currents with significantly greater potency than that of all but one class of nociceptors (pH50 = 6.54 and 6.75 vs. 6.20-6.34). In nonnociceptive cells, acid transduction was also confined to a very narrow range (0.1-0.3 vs. 0.8-1.4 pH units for nociceptors). Simultaneous whole cell recording and ratiometric imaging of three peptidergic nociceptive classes were consistent with the expression of Ca2+ -permeable ASICs. Sensitivity to psalmotoxin and flurbiprofen indicated the presence of Ca2+ -permeable ASIC1a. Immunocytochemistry on these subclassified populations revealed a differential distribution of five ASIC proteins consistent with Ca2+ permeability and differential kinetics of proton-gated currents (type 5: ASIC1a, 1b, 2a, 2b, 3; type 8a: ASIC1a, 1b, 3; type 8b: ASIC1a, 1b, 2a, 2b, 3). Using DiI tracing, we found that nociceptive classes had discrete peripheral targets. ASIC-expressing types 8a and 9 projected to hairy skin, but only types 8a and 13 projected to glabrous skin. Non-ASIC-expressing types 2 and 4 were present only in hairy skin. We conclude that ASIC-expressing nociceptors differ from ASIC-expressing nonnociceptors mainly by range of proton reactivity. ASIC- as well as non-ASIC-expressing nociceptors have highly distinct cutaneous targets, and only one class was consistent with the existence of a generic C polymodal nociceptor (type 8a).

Interlimb reflex activity after spinal cord injury in man: strengthening response patterns are consistent with ongoing synaptic plasticity

OBJECTIVE

Previous reports from our laboratory have described short-latency contractions in muscles of the distal upper limb following stimulation of lower limb nerves or skin in persons with injury to the cervical spinal cord. It takes 6 or more months for interlimb reflexes (ILR) to appear following acute spinal cord injury (SCI), suggesting they might be due to new synaptic interconnections between lower limb sensory afferents and motoneurons in the cervical enlargement. In this study, we asked if once formed, the strength of these synaptic connections increased over time, a finding that would be consistent with the above hypothesis.

METHODS

We studied persons with sub-acute and/or chronic cervical SCI. ILR were elicited by brief trains of electrical pulses applied to the skin overlying the tibial nerve at the back of the knee. Responses were quantified based on their presence or absence in different upper limb muscles. We also generated peri-stimulus time histograms for single motor unit response latency, probability, and peak duration. Comparisons of these parameters were made in subjects at sub-acute versus chronic stages post-injury.

RESULTS

In persons with sub-acute SCI, the probability of seeing ILR in a given muscle of the forearm or hand was low at first, but increased substantially over the next 1-2 years. Motor unit responses at this sub-acute stage had a prolonged and variable latency, with a lower absolute response probability, compared to findings from subjects with chronic (i.e. stable) SCI.

CONCLUSIONS

Our findings demonstrate that interlimb reflex activity, once established after SCI, shows signs of strengthening synaptic contacts between afferent and efferent components, consistent with ongoing synaptic plasticity.

SIGNIFICANCE

Neurons within the adult human spinal cord caudal to a lesion site are not static, but appear to be capable of developing novel-yet highly efficacious-synaptic contacts following trauma-induced partial denervation. In this case, such contacts between ascending afferents and cervical motoneurons do not appear to provide any functional benefit to the subject. In fact their presence may limit the regenerative effort of supraspinal pathways which originally innervated these motoneurons, should effort in animal models to promote regeneration across the lesion epicenter be successfully translated to humans with chronic SCI.

Characterization of cutaneous primary afferent fibers excited by acetic acid in a model of nociception in frogs

Acetic acid applied to the hind limb of a frog evokes nocifensive behaviors, including a vigorous wiping of the exposed skin, referred to as the wiping response. The aim of this study was to examine the responses of cutaneous primary afferent fibers in frogs to acetic acid (pH 2.84-1.42) applied topically to the skin. Conventional electrophysiological methods were used to record neuronal activity from single identified primary afferent fibers with cutaneous receptive fields on the hind limb. Fibers were classified according to their conduction velocities and responses evoked by mechanical and thermal (heat and cold) stimuli. One hundred and twenty-two mechanosensitive afferent fibers were studied (44 Abeta, 60 Adelta, and 18 C fibers). Thirty-nine percent of all fibers were excited by acetic acid, but a greater percentage of Adelta (52%) and C fibers (44%) were excited than Abeta fibers (20%). Evoked responses of fibers increased with increasingly more acidic pH until the greatest responses were evoked by acetic acid at pH 2.59-2.41. Application of acetic acid at pHs <2.41 evoked less excitation, suggesting that fibers became desensitized. Similar percentages of nociceptors and low-threshold mechanoreceptors were excited by acetic acid. Thus primary afferent fibers were excited by acetic acid at pHs that have been shown to evoke the wiping response in our previous study. The results of the present study suggest that the model of acetic acid-induced nociception in frogs may be useful for studying the mechanisms by which tissue acidosis produces pain.

Subcutaneous versus intracutaneous injections of sterile water for labour analgesia: a comparison of perceived pain during administration

OBJECTIVE

To investigate whether, during injections of sterile water, there is any difference in perceived pain between intracutaneous and subcutaneous injections.

DESIGN

Blind controlled trial with cross-over design.

SETTING

Göteborg and Skövde, Sweden.

PARTICIPANTS

One hundred healthy female volunteers.

METHODS

The women were randomised into two groups and subjected to two trials, within one week of each other. During the first trial one group (n = 50) received the intracutaneous injection first, followed by the subcutaneous injection. The second group (n = 50) was given the subcutaneous injection first, followed by intracutaneous injection. In both groups all the injections were given in reverse order during the second trial.

MAIN OUTCOME MEASURES

Experienced pain during the administration of sterile water injections, measured by visual analogue scale.

RESULTS

The analysis showed intracutaneous injections to be significantly more painful than subcutaneous injections, even after adjusting for injection day and for left/right site of injection (mean 60.8 vs 41.3, P < 0.001).

CONCLUSIONS

The findings suggest that the less painful subcutaneous injection technique should be used.

Pruritus: more scratch than itch

Itch is one of the major symptoms of skin disease although it remains poorly studied. Little is known about its mediators or the neurological processes involved in either the detection of an itch stimulus or the induction of the main response to itch, scratch. This lack of knowledge may be due to the subjective nature of the sensation itself and the related difficulties in quantifying it, and is compounded by the absence of a convincing animal model. Defining itch as that sensation which provokes the desire to scratch provides two approaches to measurement, that of itch itself, and the behavioural response, scratch. The measurement of itch itself traditionally involves the use of questionnaires or visual analogue scale, both of which rely on the dubious assumption that the subject is able to relate their experiences accurately. By contrast experimental induction of itch and measurement of areas of allokinesis around application sites may provide a more reliable and repeatable method of itch quantification. Recent advances in two areas that may prove relevant are discussed: new technological improvements in movement meters and compatible software; and some recently described animal models.

The venous footpump: influence on tissue perfusion and prevention of venous thrombosis

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Local anaesthesia of the skin

Local anaesthesia of intact skin is of growing importance given the increased use of minor surgical procedures on an out-patient basis. Specific pain receptors (nociceptors) are responsible for sensing cutaneous pain. Mechanisms of pain perception and the role of local anaesthetics in reversibly blocking nociception are considered. Effective routes of administration are dermal infiltration or, more recently, anaesthesia by topical application of specifically formulated preparations which promote percutaneous absorption of the drug (percutaneous local anaesthesia). Although many other topical anaesthetic products are also available these do not permit the anaesthetic to reach the nociceptors underlying the stratum corneum and are therefore only suitable for mucosal anaesthesia or for application to damaged skin. The chemical characteristics of local anaesthesia drugs are identified. Those agents suitable for cutaneous anaesthesia are reviewed with respect to potency, onset and duration of anaesthesia and possible systemic or local adverse reactions.

Termination patterns of identified group II and III afferent fibres from deep tissues in the spinal cord of the cat

In chloralose-anaesthetized cats, the impulse activity of single afferent fibres supplying receptors in the deep tissues of the hindlimb (fasciae, muscles, ligaments, joint capsules) was recorded using micropipettes filled with a solution of horseradish peroxidase. Only myelinated fibres with conduction velocities up to 40 m/s (Group III and Group II units) were studied, i.e. fast conducting afferent fibres from muscle spindles and tendon organs were excluded. The fibres were functionally characterized with the use of mechanical stimuli such as local pressure and joint movements. The results show that a relationship exists between the functional properties of a given afferent unit and the location of its terminals in the spinal cord. Since the conduction velocity and hence the diameter of the fibres was similar in all the units studied, these factors appear not to be of importance for determining the pattern of spinal termination. Out of 84 units, 42 were classified as high-threshold mechanosensitive, 26 as low-threshold mechanosensitive, and 16 as secondary endings from muscle spindles. Following physiological identification the fibres were ionophoretically injected with horseradish peroxidase and their trajectory in the white and gray matter of the spinal cord visualized histologically with diaminobenzidine. High-threshold mechanosensitive units took a lateral course in the posterior funiculus and usually did not bifurcate. They exhibited two different patterns of spinal termination, one being characterized by terminal arborizations in both lamina I and deeper laminae (mostly IV/V), the other one by an exclusive projection to lamina I. Low-threshold mechanosensitive units often showed a bifurcation in the posterior funiculus and did not have a uniform termination pattern. The main areas of termination were lamina II and laminae IV-VI. The slowly conducting secondary endings from muscle spindles projected mainly to laminae VI and VII with additional collaterals entering the ventral horn. They thus had a termination pattern similar to that reported for fast conducting afferent fibres (above 50 m/s) from muscle spindle secondary endings. With the exception of one high-threshold mechanosensitive unit none of the stained fibres possessed terminal arborization and boutons in lamina III. It is concluded that different types of Group II and III primary afferent fibres from deep tissues exhibit different patterns of spinal termination.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcitonin gene-related peptide immunostained axons provide evidence for fine primary afferent fibers in the dorsal and dorsolateral funiculi of the rat spinal cord

The hypothesis being tested in the present paper is that there are large numbers of fine primary afferent axons in the dorsal and dorsolateral funiculi of the lumbar spinal cord of the rat. The data show numerous calcitonin gene-related peptide labeled fine myelinated and unmyelinated axons in these funiculi. Approximately 95% of the labeled axons disappear after dorsal rhizotomy. Accordingly, the hypothesis is confirmed. Thus it is becoming apparent that fine primary afferent fibers are more widely distributed in spinal white matter than had been previously recognized. Implications are that it is not possible to find areas in the spinal white matter that contain only large myelinated sensory axons and that significant numbers of fine primary afferent fibers will be lost even if lesions are restricted to the dorsal funiculus. The sizable population of fine myelinated primary afferent axons in the dorsal funiculus is emphasized. An obvious question, suggested by significant differences in average diameters of the axons in the different pathways, is whether there are differences in the types of information carried by the fine afferent fibers in their different locations in the white matter of the lumbar cord.

Supraspinal nocifensive responses of cats: spinal cord pathways, monoamines, and modulation

These experiments were conducted to determine (1) whether dorsal and ventral ascending spinal pathways can each mediate unlearned supraspinal nocifensive responses of cats to noxious thermal stimuli and (2) whether interrupting the spinal projection of supraspinal monoaminergic neurons alters the excitability and natural modulation of these responses. In partially restrained cats, thermal pulses (greater than or equal to 47 degrees C) delivered to the hindlimbs of intact cats or rostral to lesions of the thoracic spinal cord elicited abrupt body movements and interruption of eating (or of exploring for) liquified food. These electronically monitored responses automatically terminated the stimulus. Natural modulation of responsiveness was produced by delivering food and thermal stimuli simultaneously; this reduced response probability by an average of 41%. Complete transection of the thoracic spinal cord eliminated both thermally elicited responses and orienting responses to noxious and tactile mechanical stimulation of the hindlimbs. Ventral bilateral thoracic spinal cord lesions that spared only the dorsal funiculus and portions of the dorsolateral funiculus (three cats) significantly reduced orienting responses to all mechanical hindlimb stimuli and reduced, but did not eliminate, movement and interrupt responses to noxious thermal hindlimb stimuli. Response latency was unaffected. Food-induced response suppression persisted although lumbar spinal cord concentrations of serotonin (5HT) and norepinephrine (NE) were markedly reduced. A bilateral lesion of the dorsal funiculi and dorsal portions of the dorsolateral funiculi (one cat) also reduced nocifensive responsiveness, but only the NE concentration in lumbar spinal cord was reduced significantly relative to a matched cervical sample. In contrast, deep bilateral lesions of the dorsolateral funiculi (two cats) produced an increase in the probability of movement and interrupt responses without affecting either response latency or food-induced response suppression. Lumbar spinal cord concentrations of NE and, in one cat, 5HT were reduced. We conclude that (1) the dorsal and ventral spinal funiculi are each sufficient to initiate and necessary to maintain normal supraspinally organized nocifensive behavior in the cat; (2) descending monoaminergic pathways are not necessary for the phasic modulation of these responses; and (3) the tonic excitability, but not the phasic modulation, of these responses is determined in part by fibers in the dorsolateral funiculus.

Sensory input to cells of origin of uncrossed spinocerebellar tract located below Clarke's column in the cat

1. Sensory inputs to and locations of uncrossed spinocerebellar tract neurones in the lower lumbar cord were studied in chloralose-anaesthetized cats. 2. Neurones with axons ascending in the ipsilateral thoracic funiculi and projecting to the cerebellum were found mainly dorsal to the central canal (laminae V and VI) in the L5-L6 segments, i.e. at levels caudal to Clarke's column. Axons considered to originate from these cells were located in the dorsal half of the lateral funiculus at the level of L2, intermingled with axons of the dorsal spinocerebellar tract originating at the levels of Clarke's column. 3. Synaptic actions of primary afferents on neurones with antidromic invasion following stimuli applied to ipsilateral thoracic funiculi or to the cerebellum were investigated using intracellular or extracellular recording in the caudal lumbar segments. 4. Monosynaptic excitatory effects were evoked by electrical stimulation of group I muscle afferents of the hindlimb ipsilateral to the cell body. The majority of neurones received monosynaptic excitation from two or more muscles, predominantly extensors. They were frequently co-excited by group Ia muscle spindle and group Ib tendon organ afferents. 5. Volleys in cutaneous afferents produced excitation with short central latencies. In addition to the monosynaptic and disynaptic excitation from low-threshold cutaneous afferents, there were indications of monosynaptic effects from slightly slower conducting fibres. The majority of these neurones also received monosynaptic excitation from group I muscle afferents. Neurones with cutaneous input tended to be located more dorsally compared with those responding only to muscle afferents. 6. Volleys in joint afferents produced monosynaptic excitatory postsynaptic potentials (EPSPs) in the neurones with EPSPs from group I or group I and cutaneous afferents. 7. Some neurones were disynaptically inhibited from group I muscle afferents. Convergence of monosynaptic group I excitation and disynaptic group I inhibition occurred in varieties of patterns. 8. Polysynaptic excitation, inhibition or mixed effects of both were evoked from ipsilateral cutaneous afferents and high-threshold muscle and joint afferents, whereas effects from the controlateral afferents were feeble.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary afferent and propriospinal fibers in the rat dorsal and dorsolateral funiculi

The purpose of this study is to determine the numbers of primary afferent and propriospinal fibers in the dorsal and dorsolateral funiculi of the rat. The reason for concentrating on these areas is that they contain large numbers of unmyelinated axons. Our data are axonal numbers from the S2 segment of spinal cord in animals that had unilateral dorsal rhizotomies or spinal cord isolations. The major conclusions are 1) that 23% of the primary afferent fibers in the dorsal funiculus are unmyelinated; 2) that there are approximately 12,500 unmyelinated primary afferent fibers in the dorsolateral funiculus, which is more than the number of primary afferent fibers in the dorsal funiculus and tract of Lissauer combined, and 3) that approximately 25% of the axons in the dorsal funiculus and 44% of the axons in the dorsolateral funiculus are propriospinal. These data modify and extend previous ideas of the organization of spinal white matter.

Reflex pathways from group II muscle afferents. 3. Secondary spindle afferents and the FRA: a new hypothesis

A hypothesis is forwarded regarding the role of secondary spindle afferents and the FRA (flexor reflex afferents) in motor control. The hypothesis is based on evidence (cf. Lundberg et al. 1987a, b) summarized in 9 introductory paragraphs. Group II excitation. It is postulated that subsets of excitatory group II interneurones (transmitting disynaptic group II excitation to motoneurones) may be used by the brain to mediate motor commands. It is assumed that the brain selects subsets of interneurones with convergence of secondary afferents from muscles whose activity is required for the movement. During movements depending on coactivation of static gamma-motoneurones impulses in secondary afferents may servo-control transmission to alpha-motoneurones at an interneuronal level. The large group II unitary EPSPs in interneurones are taken to indicate that, given an adequate interneuronal excitability, impulses in single secondary afferents may fire the interneurone and produce EPSPs in motoneurones; interneuronal transmission would then be equivalent to that in a monosynaptic pathway but with impulses from different muscles combining into one line. It is postulated that impulses in the FRA are evoked by the active movements and that the role of the multisensory convergence from the FRA onto the group II interneurones is to provide the high background excitability which allows the secondary spindle afferents to operate as outlined above. The working hypothesis is put forward that a movement governed by the excitatory group II interneurones is initiated by descending activation of these interneurones, but is maintained in a later phase by the combined effect of FRA activity evoked by the movement and by spindle secondaries activated by descending activation of static gamma-motoneurones. As in the original "follow up length servo" hypothesis (Rossi 1927; Merton 1953), we assume that a movement at least in a certain phase can be governed from the brain solely or mainly via static gamma-motoneurones. However, our hypothesis implies that the excitatory group II reflex connexions have a strength which does not allow transmission to motoneurones at rest and that the increase in the gain of transmission during an active movement is supplied by the movement itself. Group II inhibition. It is suggested that the inhibitory reflex pathways like the excitatory ones have subsets of interneurones with limited group II convergence. When higher centres utilize a subset of excitatory group II interneurones to evoke a given movement, there may mobilize inhibitory subsets to inhibit muscles not required in the movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptive field profiles and integrative properties of spinocervical tract cells in the cat

The receptive fields of sixteen spinocervical tract (s.c.t.) cells whose responses were recorded extracellularly were mapped using discrete and uniform jets of air given at equally spaced locations on the clipped fur of cats anaesthetized with chloralose. All the cells whose receptive fields were on the thigh or upper hind limb showed approximately unimodal gradients of sensitivity to stimulation within their excitatory receptive fields. The response magnitudes declined steadily as the stimuli were moved sequentially from the centres to the peripheries of the fields and abrupt edges were not found. Spatial summation from within the excitatory receptive field was studied in twelve s.c.t. cells. These cells showed a poor ability to summate the responses to two spatially separated air jets when these stimuli were applied simultaneously within their receptive fields. No significant summation was found in twenty-five out of thirty-one trials and in six of these trials (four cells) the responses were significantly reduced. Summation was found in six trials (four cells). Lack of summation or response reduction was more prevalent when the individual response levels were low (less than impulses stimulus-1). These results are discussed in relation to similar findings for cells of somatosensory relay nuclei and cortex.

Numbers of myelinated and unmyelinated axons in the dorsal, lateral, and ventral funiculi of the white matter of the S2 segment of cat spinal cord

The present work determines the numbers of myelinated and unmyelinated axons in the dorsal, lateral, and ventral funiculi of the S2 segment of the cat spinal cord. The major finding is that unmyelinated axons are almost as numerous as myelinated axons in these pathways. The myelinated axons tend to be distributed uniformly, although there is a slight concentration of these fibers in the dorsal part of the lateral funiculus. By contrast, the unmyelinated fibers, although found in significant numbers in all parts of these funiculi, concentrate in the dorsal part of the lateral funiculus and in the dorsal funiculus. Of particular note are the unmyelinated fibers in the dorsal funiculus, because it is highly likely that some of these are sensory. The findings in this study will serve as a basis for experimental studies to determine the numbers, locations, and types of unmyelinated fibers in the white matter of the mammalian cord.

Receptive field organization and response properties of spinal neurones with axons ascending the dorsal columns in the cat

Micro-electrode recordings were made from single post-synaptic axons in the dorsal columns of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The recordings were made from the L5 segment and the axons were shown to project to the upper cervical level. Forty-eight units were recorded and the axons had conduction velocities of 22-61 ms-1, averaging 38.3 ms-1. Excitatory receptive fields were complex in many units, being made up of clearly defined, separate, low and high threshold areas. The receptive fields were often discontinuous. Only a few units behaved as if they received excitatory input from a single class of mechanoreceptors. A minority (13%) of units had labile, excitatory receptive fields that expanded in size during the recording period. About 40% of the units had inhibitory receptive fields. These were of two main types: either small and within or adjacent to the excitatory field, or large and separated from or adjacent to the excitatory field. The great majority of units had resting discharges upon isolation and these consisted of single impulses or bursts of impulses at short intervals separated by longer, irregular periods. The time course of inhibition produced by electrical stimulation of cutaneous nerves suggested presynaptic inhibitory components to the inhibition. Some inhibitory curves were very prolonged with maxima at about 100 ms and total durations of up to 400 ms. The complexity of the receptive field organization in these dorsal horn neurones is discussed, as is their possible significance as input neurones to the dorsal column nuclei.

Numbers of axons in lateral and ventral funiculi of rat sacral spinal cord

The present study determines the numbers of myelinated and unmyelinated axons in the ventral and lateral funiculi of rat sacral spinal cord. On average, there are 55,000 myelinated and 110,000 unmyelinated axons in the lateral funiculus and 26,000 myelinated and 9,000 unmyelinated axons in the ventral funiculus at these levels. These figures combined with data from earlier studies of the posterior funiculus and the tract of Lissauer give approximate figures of 88,500 myelinated and 131,500 unmyelinated axons for the entire white matter of one side of the rat sacral spinal cord. Thus unmyelinated axons predominate in the white matter of the rat sacral spinal cord. The majority of axons, particularly the unmyelinated axons, are located in the lateral funiculus. The axons are concentrated in the dorsolateral part of the lateral funiculus, and so the dorsal part of the lateral funiculus, often referred to as the dorsolateral funiculus, contains more than half the fibers in the white matter of the spinal cord. A small nick in the dorsal and lateral part of the lateral funiculus, which is often done for various experimental reasons, could thus remove 40% of the axons in the white matter of rat sacral spinal cord. The data reported in the present paper will serve as a basis for future studies on the white matter of the isolated spinal cord.

Convergence of muscle spindle afferents on single neurons of the cat dorsal spino-cerebellar tract and their synaptic efficacy

By means of tungsten microelectrodes, action potentials from axons within the dorsal spino-cerebellar tract (DSCT) and from muscle spindle afferents were recorded. A quantitative study was performed in monomuscular DSCT neurons which were excited predominantly by Ia fibers originating in the gastrocnemius muscles. In some experiments single Ia fibers were stimulated electrically while the impulse sequence of a DSCT neuron postsynaptic to the respective afferent fiber was recorded. The gastrocnemius DSCT neurons receive excitatory inputs from 10-18 Ia muscle spindle afferents. The efficacy of each of these inputs is very similar. Thus the neuronal activation decreased approximately linearly with the number of the excitatory afferents cut. Cross-correlograms between the impulse sequence of a Ia gastrocnemius muscle spindle afferent and a DSCT neuron postsynaptic to it exhibited an increased discharge probability of the DSCT neuron from 3-4 ms to 10 ms after the Ia action potential. With increasing impulse rates of the Ia afferent fibers, the excitatory efficacy of the single action potential decreased, but the overall excitation increased with the presynaptic discharge frequency, according to a hyperbolic function. This effect was tested by electrical stimulation of a single Ia axon exciting the DSCT neuron recorded. Interval histograms computed from DSCT neuron impulse trains at steady stretch conditions were predominantly monomodal. They can be well approximated by a Gaussian distribution. The coefficient of variation was independent of the mean activity. At impulse rates above 25 imp X s-1 a negative correlation between successive intervals was observed in first order joint interval diagrams. With an increasing mean discharge rate this correlation (expressed as the serial linear correlation coefficient of the first order r1,2) became stronger up to--0.62 at 90 imp X s-1. Only in a few neurons did the higher order linear correlation coefficients deviate significantly from zero. In 15% of the observed histograms double discharging (mean interval 3-5 ms) produced bimodal distributions. Under steady-state conditions the response of Ia-activated DSCT cells are linearly related to muscle stretch within a middle range of extensions. The differences between Ia impulse pattern and DSCT neuron impulse pattern at steady stretch are discussed. The number of large dendrites of the principal cells in the nucleus dorsalis (Clarke's column) corresponds to the number of excitatory afferent muscle fibers. It is assumed that each excitatory Ia axon sends one axon collateral to the DSCT neuron, forming a climbing type terminal mainly on one of the large dendrites of a DSCT cell.

Responses of cat dorsal spino-cerebellar tract neurons to sinusoidal stretching of the gastrocnemius muscle

A defined class of cells within the nucleus dorsalis (Clarke's column) receives excitatory input from Ia afferents of mainly one muscle. Action potentials were recorded from axons of these cells (DSCT neurons) which are excited by Ia afferents of the gastrocnemius muscles. We investigated the response to sinusoidal muscle stretch over a wide range of amplitudes (10 microns-4 mm) and frequencies (0.1-130 Hz) in the deefferented preparation. The dynamic stretch was superimposed on a moderate static muscle stretch to ensure that the muscle was not slack during the phase of release. The response up to 10 Hz was displayed as PST histograms (cycle histograms) and a sinewave of stretch frequency was fitted to the PST histograms to define amplitude and phase of a response sinewave. At a constant frequency of about 3 Hz, the relation between stretch amplitude and response amplitude could well be described by decelerating intensity functions: the hyperbolic or tanh log function and a modified power function (exponent 0.48 +/- 0.12). The phase lead of the response sinewave increased with increasing stretch amplitudes of up to 0.5 mm and then decreased. At constant stretch amplitudes of 0.5-2.0 mm the frequency response was investigated. In relation to stretch frequencies between 0.1 and 1 Hz an increase in the response amplitude of 4.4 dB was observed and an increase for 13 dB/decade between 3 and 10 Hz. At 0.1 Hz the phase of the response sinewave was 48 degrees in advance and increased to a maximum lead of 89 degrees at 6-8 Hz. Above 10 Hz the positions of the responding action potentials with respect to the stretch cycle were used to define a phase, which was in advance up to 60 Hz but decreased and changed to a phase lag at higher frequencies. If in PST histograms no periods of silence occurred during the phase of stretch release, the mean discharge rate was found to be independent of the sinusoidal stretching. If the pauses were present the mean rate increased with increasing stretch frequencies or amplitudes.

Dendritic trees and cutaneous receptive fields of adjacent spinocervical tract neurones in the cat

1. The relationship between the dendritic trees and the receptive fields of adjacent spinocervical tract neurones were studied using the intracellular injection of horseradish peroxidase in chloralose-anaesthetized, paralysed, cats. 2. Fourteen pairs of neurones were successfully stained and the receptive fields of twelve pairs were also defined. Five of the pairs were 'rostrocaudal pairs', i.e. in line within +/- 50 micrometer of each other in the rostrocaudal axis of the cord. Nine of the pairs were 'mediolateral pairs', i.e. in line within +/- 50 micrometer of each other in the mediolateral axis of the cord. 3. For twelve pairs of stained neurones, for which the receptive fields were described, a correspondence was found between the organization of their dendritic trees and their receptive fields. 4. All five rostrocaudal pairs of neurones had interdigitating dendritic trees together with overlapping receptive fields. 5. Of the seven mediolateral pairs, four had receptive fields which overlapped and interdigitating dendritic trees. The remaining three had separate receptive fields and non-interdigitating dendritic trees. 6. The results are discussed in relation to previous work on the columnar organization of the receptive fields of s.c.t. neurones and the columnar organization of hair follicle afferent fibre terminals in the spinal cord. The hypothesis is advanced that the receptive fields of most s.c.t. neurones are defined by their connexions with hair follicle afferent fibres such that along the mediolateral axis spatial separation of the dendritic trees of s.c.t. neurones is crucial for receptive field separation.

Projections of primary afferent fibres from muscle to neurones of the spinocervical tract of the cat

1. Micro-electrode recordings were made from axons in the spinocervical tract of decerebrate-spinal cats. 2. The effect of electrical stimulation of afferent fibres in muscle nerves was examined. 3. Stimulation of group III fibres in muscle nerves excited hair only and hair and pressure units in the spinocervical tract. Hair and pressure units also responded to electrical stimulation of group IV fibres. 4. Units of the spinocervical tract excited by stimulation of muscle nerves had receptive fields in the skin overlying the relevant muscles. 5. Stimulation of group III fibres in muscle nerves produced long lasting inhibition of spinocervical tract units. In two such units the inhibitory action of afferent myelinated fibres was enhanced by increasing the stimulus strength so that group IV fibres were recruited.