The morphology of spinocervical tract neurones revealed by intracellular injection of horseradish peroxidase (cat)

1. The morphology of physiologically identified spinocervical tract neurones was studied using the intracellular injection of horseradish peroxidase in anaesthetized cats.

2. Thirty-six spinocervical tract neurones were reconstructed from serial sections of the lumbosacral spinal cord, cut in either the transverse or longitudinal planes.

3. Horseradish peroxidase provided a more complete picture of the dendrites of spinocervical tract neurones than earlier experiments using Procion Yellow injection (Brown, House, Rose & Snow, 1976a). The longitudinal (rostro-caudal) spread of dendrites from an individual cell was much greater in the present material; neurones in the medial parts of the dorsal horn had dendrites extending for about 500 μm from the soma (1 mm total spread) and neurones in the lateral horn had dendrites extending for about 1 mm from the soma (2 mm total spread). However, the conclusions of the earlier work, on the medio-lateral and dorso-ventral extents of dendritic trees, together with the shapes of dendritic trees viewed as reconstructions in the transverse plane, have been confirmed. Dendrites of spinocervical tract cells barely entered lamina II of Rexed: they often ran in the longitudinal direction along the border between laminae II and III for several hundred μm. Dendritic spines were observed on many spinocervical tract neurones.

4. Horseradish peroxidase reaction product stained up to 2·5 cm of the axon of spinocervical tract neurones. Axons usually pursued a tortuous path through the grey matter close to the cell body, giving off up to six collaterals before entering the ipsilateral dorsolateral funiculus. In the funiculus, further collaterals often arose at distances of up to 5·5 mm from the soma; these collaterals ran back into the dorsal horn. Collaterals could be traced sometimes to presumed terminal boutons. The majority of collateral terminal arborizations was between the level of the cell body and 500 μm ventral to it (in laminae IV and V). They were, however, in the same medio-lateral and rostro-caudal region as the dendritic tree of the parent cell.

5. It is concluded that the spinocervical tract must now be considered as having a segmental function, in addition to its function of forwarding information towards the cerebral cortex.

     

Responses of spinocervical tract neurones to noxious stimulation of the skin.

1. Activity of single spinocervical tract neurones has been recorded in the lumbar spinal cord of chloralose anaesthetized or decerebrated cats. Reversible spinalization was produced by cold block at L3. Sensitivity of these neurones to noxious stimulation was studied by heating their cutaneous receptive fields above 40-45 degrees C. 2. Most of the units were located in lamina IV of the dorsal horn and had their receptive fields in the ipsilateral foot. All but one of the studied neurones were excited by moving hairs or by gentle mechanical stimulation of the skin. 3. Eighty-four % of the units were affected by noxious stimuli and three kinds of response were obtained: (i) 61% were excited (E-cells) by noxious heat; (ii) 19% were inhibited (I-cells); and (iii) 19% gave a mixed response reversing from excitatory to inhibitory (EI-cells). 4. E-cells had axons with a wider range of conduction velocities than the rest and also received the strongest descending inhibition from supraspinal structures. 5. The recording sites of EI-cells were located in the medial third of the dorsal horn whereas E- and I-cells were distributed over the full width of the dorsal horn. 6. The possible role of the spinocervical tract in nociception is discussed.     

Vibrotactile coding capacities of spinocervical tract neurons in the cat

The response characteristics and tactile coding capacities of individual dorsal horn neurons, in particular, those of the spinocervical tract (SCT), have been examined in the anesthetized cat. Twenty one of 38 neurons studied were confirmed SCT neurons based on antidromic activation procedures. All had tactile receptive fields on the hairy skin of the hindlimb. Most (29/38) could also be activated transynaptically by electrical stimulation of the cervical dorsal columns, suggesting that a common set of tactile primary afferent fibers may provide the input for both the dorsal column-lemniscal pathway and for parallel ascending pathways, such as the SCT. All but 3 of the 38 neurons studied displayed a pure dynamic sensitivity to controlled tactile stimuli but were unable to sustain their responsiveness throughout 1s trains of vibration at vibration frequencies exceeding 5-10 Hz. Stimulus-response relations revealed a very limited capacity of individual SCT neurons to signal, in a graded way, the intensity parameter of the vibrotactile stimulus. Furthermore, because of their inability to respond on a cycle-by-cycle pattern at vibration frequencies >5-10 Hz, these neurons were unable to provide any useful signal of vibration frequency beyond the very narrow bandwidth of approximately 5-10 Hz. Similar limitations were observed in the responsiveness of these neurons to repetitive forms of antidromic and transynaptic inputs generated by electrical stimulation of the spinal cord. In summary, the observed limitations on the vibrotactile bandwidth of SCT neurons and on the precision and fidelity of their temporal signaling, suggest that SCT neurons could serve as little more than coarse event detectors in tactile sensibility, in contrast to DCN neurons the bandwidth of vibrotactile responsiveness of which may extend beyond 400 Hz and is therefore broader by approximately 40-50 times than that of SCT neurons.     

post-synaptic excitation and inhibition from primary afferents in neurones of the spinocervical tract

1. Intra- and extracellular recordings were made from cells of the spinocervical tract in the lumbosacral spinal cord. A convergence of monosynaptic excitatory post-synaptic potentials (EPSPs) and disynaptic inhibitory post-synaptic potentials (IPSPs) was a general pattern of effects from the low threshold cutaneous fibres. Unitary IPSPs, probably mediated via the same disynaptic path, were evoked by light touch of hairs, which was also the adequate stimulus for exciting the cells. The receptive field for unitary IPSPs was closely related to the excitatory receptive field but was eccentric, not of a surround type.

2. EPSPs, IPSPs, or both, were evoked from the flexor reflex afferents in the great majority of neurones. Disynaptic IPSPs may be evoked from the interosseous nerve. No effects were produced by volleys in group I muscle afferents.

3. It is suggested, on the basis of the spatial organization of the excitatory and inhibitory receptive skin fields, that the spinocervical tract may give information regarding the direction of tactile stimuli.

     

Indications for coupling between feline spinocervical tract neurones and midlumbar interneurones

The possibility of collateral segmental actions of spinocervical tract (SCT) neurones upon interneurones with input from cutaneous and group II muscle afferents was investigated in deeply anaesthetized cats. To this end, intracellular and/or extracellular recordings were made from 35 dorsal horn and 15 intermediate zone interneurones in midlumbar segments of the spinal cord and effects of stimulation of the ipsilateral dorso-lateral funiculus (DLF) at C3 and C1 levels, i.e. below and above the lateral cervical nucleus where axons of SCT cells terminate, were compared. The stimuli applied at the C3 segment were within the range of stimuli (50–100 μA) required for antidromic activation of SCT neurones in the same experiment. Those applied at the C1 segment (200–500 μA) were at least 3 times stronger than C3 stimuli. Under the same experimental conditions, long ascending and descending tract neurones (dorsal spino-cerebellar and rubro-spinal tract neurones) with axons in the DLF were activated at similar thresholds from the C1 and C3 segments. Intracellular recordings were made from 29 interneurnoes of which 19 (65%) were dorsal horn and 10 (35%) were intermediate zone interneurones. Excitatory postsynaptic potentials (EPSPs) evoked by single stimuli applied at the C3 segment, but not the C1 segment, were found in 14 (48%) of those interneurones; their latencies (3.0–5.7 ms) and frequency following with only minimal temporal facilitation were as required for potentials being evoked monosynaptically by the fastest conducting SCT neurones. Extracellular recordings were made from 30 interneurones (24 dorsal horn and 6 intermediate zone interneurones), and in these neurones spike potentials induced from the C3, but not from the C1 segment, were evoked only by short trains of stimuli. However, their latencies from the first effective stimulus (4.3–5.4 ms) were compatible with mono- or oligosynaptically mediated collateral actions of SCT neurones. They were found in 10 (33%) of the 30 investigated interneurones. Similar effects of C3 stimuli were found in similar proportions of dorsal horn interneurones and intermediate zone interneurones. Indications were also found for synaptic actions evoked by C3 stimuli that could not be attributed to direct collateral actions of SCT neurones. In some intracellularly recorded dorsal horn interneurones, short-latency EPSPs were evoked from the C3 segment by the 2nd or 3rd stimulus in the train, but not by single stimuli. In other dorsal horn and intermediate zone interneurones, inhibitory postsynaptic potentials (IPSPs) were evoked from the C3 segment at minimal latencies (2.7–3.2 ms), which might be too short to allow their mediation via SCT neurones. We conclude that SCT neurones might be used to forward information from muscle group II and cutaneous afferents not only to neurones in the lateral cervical nucleus and via them to thalamus and cerebral cortex but also to interneurones in spinal reflex pathways. Thereby reflex actions evoked from group II and cutaneous afferents might be co-ordinated with responses mediated by supraspinal neurones. We conclude also that dorsal horn and intermediate zone mid-lumbar interneurones might contribute to the previously reported di-and poly-synaptic excitation or inhibition of postsynaptic dorsal column (PSDC), spinothalamic tract (STT) and spinomesencephalic tract (SMT) neurones by collateral actions of SCT cells. Thereby these interneurones might contribute to the co-ordination of responses mediated by various populations of supraspinal neurones. Received: 18 November 1996 / Accepted: 1 September 1997     

Inhibition of spinocervical tract discharges from localized areas of the sensorimotor cortex in the cat.

1. Intracortical microstimualtion (ICMS) was applied within the sensorimotor cortex of cats anaesthetized with chloralose. 2. The effects of the ICMS were examined on the number of impulses in spinocervical tract (SCT) cells (recorded extracellularly in the contralateral lumbosacral spinal cord) evoked by peripheral stimulation. 3. Inhibition of SCT discharges was produced by ICMS in two distinct regions of the sensorimotor cortex. 4. One inhibitory regions was in part of cytoarchitectonic area 4 gamma in the upper bank of the cruciate sulcus. It sometimes extended caudally into area 4 delta, medially into area 3 alpha and/or rostrally into the part of area 4 gamma on the caudal lip of the cruciate sulcus. 5. The other inhibitory region was in the medial part of the posterior sigmoid gyrus and included parts of areas 3 alpha, 3 beta, 1, 5 alpha and 5 beta. 6. Most inhibitory sites were in cortical layers III, V and VI. 7. No regions were found in which ICMS consistently caused facilitation of SCT discharges.     

Responses of spinocervical tract neurones to natural stimulation of identified cutaneous receptors

Summary Microelectrode recordings were made from ascending fibres of the spinocervical tract in spinal, decerebrate and anaesthetized cats. Three types of unit were recognised in spinal cats on the basis of their response to mechanical stimulation; units excited by 1. hair movement; 2. hair movement and skin pressure; 3. pressure and pinch of the skin. Five types were recognised in decerebrate and anaesthetized cats; units excited by 1. movement of guard hairs and skin pressure; 2. movement of tylotrich hairs; 3. movement of all the hairs and skin pressure; 4. pressure and pinch of the skin; 5. units which could not be influenced from the periphery. The presence or absence of inhibitory fields and the mean rate of spontaneous discharges depended on the type of preparation and the type of unit. The differences between spinal and decerebrate or anaesthetized cats suggest that a descending neuronal system, intact in the decerebrate and anaesthetized animals, operates to control the input to the spinocervical tract.The mean frequency of response of units sensitive to hair movement was related to the velocity of hair movement by a power function. All units responded with an increased frequency of discharge to heating the skin to high temperatures, the degree of response depending on the preparation and type of unit. Some units responded with an increased discharge to low skin temperatures.Part of the apparatus used was supplied by a Royal Society Grant-in-aid of Scientific Investigations to A.G.B.Supported by a U.S.P.H.S. Postdoctoral Fellowship (F2 NB 34, 495) from the National Institute of Neurological Diseases and Blindness. Present address: Department of Pharmacology, College of Medicine, University of Utah, Salt Lake City, Utah, USA.     

Descending influences on the responses of spinocervical tract neurones to chemical stimulation of fine muscle afferents

1. In cats, extracellular micro-electrode recordings were made from axons of the spinocervical tract (s.c.t.) in both the decerebrate state and during cold block of the spinal cord (reversible spinal state) to examine the effects of intra-arterial injection of algesic agents (bradykinin, potassium, 5-hydroxytryptamine) into the gastrocnemius-soleus (g.s.) muscle on the discharge behaviour of s.c.t. neurones.2. In the decerebrate state without cooling the spinal cord 13% of the cells (eleven out of eighty-three) responded to intra-arterial injection of bradykinin, 33% (twenty-two out of sixty-nine) to 5-hydroxytryptamine, and 38% (thirty-five out of ninety-one) to potassium injection.3. The general time course and the latency of the responses of s.c.t. cells induced by injection of pain-producing substances into the g.s. muscle reflect in many respects the activations of g.s. group III and group IV primary afferent units studied previously.4. For twenty-seven s.c.t. neurones the period of recording was long enough to record the responses of the same cell to injections of algesic agents in both the decerebrate and the reversible spinal state. In the reversible spinal state 83% (nineteen out of twenty-three) of the s.c.t. neurones tested with all the three substances responded to at least one of the algesic agents. In the decerebrate state the percentage was lower (39%).5. Reversible spinalization led not only to a significant increase in the number of s.c.t. neurones responding to the algesic agents used but also to an increase in the magnitude of the chemically induced responses.6. The mean latency of the responses of neurones that were activated in both preparations were shorter in the reversible spinal state than in the decerebrate state.7. Control experiments showed that the responses to bradykinin and potassium were entirely due to the nervous outflow from the g.s. muscle. In contrast, intra-arterially applied 5-hydroxytryptamine influenced the s.c.t. cells via unknown additional sites of action.8. The results indicate that muscular group III and/or group IV units excitable by algesic substances do project on to neurones of the spinocervical tract. Furthermore it is concluded that the responses of s.c.t. neurones to activation of fine muscle afferents by algesic agents are subject to a descending control similar to the well known descending modulation of their responsiveness to cutaneous input. Therefore, in addition to serving as a cutaneous pathway the spinocervical tract may take part in muscular nociception.     

Electrophysiological evidence that spinomesencephalic neurons in the cat may be excited via spinocervical tract collaterals

 Extracellular microelectrode recordings were made from spinomesencephalic tract (SMT) neurons in the lumbosacral spinal cord of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The SMT cells were antidromically fired from the posterolateral parts of the superior colliculus and the intercollicular region, were located in laminae IV to VIII, and had response properties and axonal conduction velocities similar to those described previously. The effects of stimulating the dorsolateral funiculus of the cervical cord at C3 and rostral C1, below and above the termination of spinocervical tract (SCT) axons in the lateral cervical nucleus, were examined on 33 SMT cells. The strength of stimulation was adjusted so that at C3 it was above threshold for antidromic activation of SCT cells and at C1 was below threshold for activation of the same cells. Seven (21%) SMT neurons were excited from C3 but not from C1. The remaining 26 (79%) were excited from both C3 and rostral C1 and 23 (70% of these) were excited significantly more from C3. That is, 91% of the total sample were either excited only from C3 or more strongly from C3 than from rostral C1. We discuss the possible neuronal systems involved and conclude that the greater excitatory effects from C3 are most likely due to antidromic activation of the SCT. The shortest latency effects from C3 indicate a monosynaptic linkage between SCT cells with the fastest axons and the SMT. The longer latency actions may be due to monosynaptic connexions from SCT cells with slower conducting axons, to di- or polysynaptic actions from SCT cells with fast axons, or a combination of both. SMT cells are another population of spinal neurons, in addition to postsynaptic dorsal column, spinothalamic and dorsal horn spinocerebellar neurons, which receive excitation via SCT collaterals. Received: 21 May 1996 / Accepted: 21 March 1997     

Effect of neonatal denervation of hindlimb digits on the somatotopic organization of lumbar spinocervical tract neurons in the cat.

1. The effect of transection and ligation of the digital nerves of either one (toe 3) or two (toe 3 and toe 4) hindpaw digits, in the first postnatal week, on the tactile receptive fields (RFs) of spinocervical tract (SCT) neurons was studied in adult, alpha-chloralose-anesthetized cats. Immediately before recording, the digital nerves of the corresponding digit(s) of the opposite, intact hindpaw were transected, and the neonatally lesioned digital nerves were recut proximal to the transection neuroma. 2. In the medial part of the dorsal horn at the L6-L7 level, the digits of the hindlimb are represented in the RFs of SCT cells in a precise axial sequence from the most medial digit (toe 2) rostrally to the most lateral digit (toe 5) caudally. Acute denervation of one or two digits in the adult produced an area in the ipsilateral dorsal horn in which SCT cells lacked any RFs. When acute denervation was restricted to a single digit, the unresponsive region of dorsal horn was approximately 3 mm in length, and when two digits were denervated the unresponsive zone was approximately 6 mm long. Because the representation of the toes of the left hindpaw is a mirror image of that of the right, the rostrocaudal extent and position of the region of unresponsive SCT cells was used to assess the location of the borders of the chronically deprived region on the opposite side of the cord. 3. In all cats examined after neonatal denervation of toe 3, most (89%) of the SCT cells sampled within the chronically deprived toe 3 representation had RFs. These RFs were either on toe 2 (44%) or toe 4 (18%), and a large proportion of cells (38%) had multiple RFs with components on both toe 2 and toe 4. In most cases the cells fired briskly to displacement of hairs or light touch of the skin within these RFs. SCT cells with a RF on toe 2 and/or toe 4 were found throughout the whole 3-mm length of the chronically deprived toe 3 region, but cells with a RF on toe 2 were more commonly found than cells with a RF on toe 4 at axial distances greater than or equal to 1.5 mm from the boundary of the normal representations of the respective digit. 4. After chronic, neonatal denervation of both toe 3 and toe 4, 59% of SCT cells sampled overall had RFs, but there was a large degree of interanimal variation in the proportion of unresponsive neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Fine structure of normal and degenerating primary afferent boutons associated with characterized spinocervical tract neurons in the cat

Spinocervical tract neurons in the dorsal horn of the cat spinal cord were intracellularly stained with horseradish peroxidase. The neurons came from one intact animal and from animals with dorsal rhizotomies (L3-S2) 3, 5, 10, 28 and 42 days previously. The morphology of terminals associated with spinocervical tract neurons was examined in a combined light and electron microscopical study. Some terminals containing agranular, circular vesicles degenerated as a result of deafferentation; these are therefore the terminals forming monosynaptic inputs to the neurons from primary afferent fibres. Other terminals containing agranular circular vesicles and terminals containing ovoid agranular vesicles survived deafferentation; these boutons therefore do not originate from primary afferent fibres.     

Pyramidal tract of the cat: axon size and morphology

Summary The purpose of this work was to determine the number and morphology of pyramidal tract (PT) axons in the cat, using electron microscopy, modern methods of fixation, and computer-assisted morphometric analysis. Sections taken at the level of the medullary pyramids in three animals were fixed and magnified up to 10,000 x to produce photomicrographs. Morphological data were entered into computer files for analysis by tracing axon perimeters on micrographs mounted on a digitizer tablet. The number of axons per PT averaged 415,000, of which 88% were myelinated and 12% were unmyelinated. 90% of the myelinated axons fell in the diameter range 0.5–4.5 m. Axons larger than 9 m diameter accounted for 1% of the total; the largest were 20–23 m. Myelinated axon mean diameter was 1.98 m; because of the skewed distribution, with many small axons and a few very large axons, median diameter was 1.60 m. Size distribution was relatively uniform throughout the PT cross section, with all sizes represented in all regions. However, the more medial regions had a higher proportion of small fibers than the more lateral regions: mean medial diameter was 1.85 m while mean lateral diameter was 2.09 m. Myelin sheath thickness averaged 7.9% of fiber diameter for axons up to 11 m, but was constant at 0.9 m for larger fibers. Myelinated fibers were distorted from the circular shape in cross section, with a mean circularity index (or form factor) of 0.85, which implies that the fibers could swell about 15% without rupture of the cell membrane. Unmyelinated fibers averaged 0.18 m diameter (range 0.05–0.6 m); the largest unmyelinated axons were larger than the smallest myelinated axons. It is concluded that previous work greatly underestimated the number of axons in the cat pyramidal tract.     

Descending and segmental inhibition of transmission through the spinocervical tract

1. Micro-electrode recordings were made from axons of the spinocervical tract in unanaesthetized decerebrate-spinal cats.2. The effects of stimulation of (1) descending systems at the level of the upper cervical spinal cord and (2) hind limb cutaneous nerves, on discharges of spinocervical tract neurones were examined.3. Effects were obtained from bilateral spinal cord regions in the dorsolateral funiculi and the most medial and ventral parts of the ventral funiculi and also from the dorsal columns in the upper cervical region even though the columns had been transected at low thoracic and upper lumbar levels.4. Stimulation of either descending or segmental systems inhibited spontaneous and evoked responses. Facilitation was not seen. The inhibition had a time course of up to 250 msec, with maximal action at 20-40 msec and was greatest for polysynaptic responses or those evoked from the smaller myelinated cutaneous axons.5. It is suggested that the descending and segmental systems converge on to common inhibitory interneurones.     

Effects on the ventral spinocerebellar tract neurones from deiters' nucleus and the medial longitudinal fascicle in the cat.

Effects from the vestibulospinal tract (VST) and from fibres descending in the medial longitudinal fascicle (MLF) on the cells of origin of the ventral spinocerebellar tract (VSCT) have been studied with intracellular recording. Out of 110 VSCT neurones, the VST evoked monosynaptic EPSPs in 27, di- or polysynaptic EPSPs in 56 and disynaptic IPSPs in 26. In 93 tested VSCT cells, MLF stimulation evoked monosynaptic EPSPs in 26, monosynaptic IPSPs in 2, di- or polysynaptic EPSPs in 25 and disynaptic IPSPs in 21. Convergence of monosynaptic EPSPs from VST and MLF was found in a small proportion of cells whereas the two descending pathways evoked reciprocal effects in another small group of neurones. Convergence of monosynaptic EPSPs from VST or MLF and from group I afferents was also modest. In 9 VSCT neurones there was convergence of monosynaptic excitation and disynaptic inhibition from the vestibulospinal tract and the same pattern from MLF was recorded in 9 neurones. The results are discussed in view of the hypothesis that VSCT neurones carry information on the interneuronal ttransmission in the spinal cord.     

Effects of descending impulses on transmission through the spinocervical tract

1. Micro-electrode recordings were made from ascending axons of the spinocervical tract in unanaesthetized decerebrate cats before, during and after reversible cold block of impulse conduction in the spinal cord rostral to the recording site.2. Most units (forty-one of forty-four) fell into one of four categories as defined by their evoked responses to mechanical stimulation of identified cutaneous receptors. These categories were; Type I excited by movement of tylotrichs (hairs) in the decerebrate preparation but by movement of all types of hairs after block of descending impulses; Type II excited by movement of guard hairs and usually weakly by pressure in the decerebrate state but by movement of all types of hairs and by pressure in the spinal state; Type III excited by movement of all types of hairs and often by pressure in the decerebrate animal but by movement of all types of hairs and always by pressure in the spinal animal; Type IV weakly excited by heavy pressure or with no receptive field in the decerebrate state but excited by pressure in the spinal state.3. The descending influences depressed the spontaneous activity and the evoked responses to harmful stimuli.4. The descending influences depressed inhibitory inputs from segmental levels.5. The functional significance of the descending control is discussed.     

Post-tetanic hyperpolarization produced by an electrogenic pump in dorsal spinocerebellar tract neurones of the cat

1. Hyperpolarization following single and repetitive excitation of dorsal spinocerebellar tract (DSCT) neurones of the cat was studied by intracellular recording.2. Hyperpolarization following an antidromic action potential consisted of an initial, brief phase (undershoot) and a late, prolonged phase. The latter hyperpolarization was independent of the membrane potential, whereas the former was reversed in polarity by hyperpolarizing pulses applied across the DSCT cell membrane.3. DSCT neurones showed a prolonged hyperpolarization after a train of antidromic action potentials. The amplitude and duration of the hyperpolarization were dependent on the number and the frequency of action potentials. A similar hyperpolarization was observed following a train of impulses evoked by depolarizing pulses applied through the intracellular electrode.4. There was no detectable conductance change during the post-tetanic hyperpolarization. The latter showed no reversal potential when the membrane potential was altered.5. The half-decay time of the post-tetanic hyperpolarization was lengthened when the cord temperature was lowered. The temperature coefficient (Q(10)) was 2.4 within the range of 31-40 degrees C.6. The amplitude of the undershoot following each action potential was assumed to provide a criterion for the accumulation of the extracellular K(+). Alterations in the amplitude of undershoots during repetitive excitation suggested that the duration of post-tetanic hyperpolarization depends on the accumulation of extracellular K(+) as well as of intracellular Na(+) associated with a train of impulses.7. It is suggested that post-tetanic hyperpolarization is produced by an electrogenic sodium pump. A possible significance of such a hyperpolarization in impulse coding is discussed.