Effects of monocular deprivation on excitatory and inhibitory pathways in cat striate cortex

Summary The effects of monocular deprivation from contour vision were investigated in the striate cortex of cats. In addition to the receptive field (RF) properties of single cells responses to electrical stimulation of the deprived and the experienced optic nerve were analyzed: Evoked potentials as well as intra- and extracellularly recorded single unit responses were evaluated. The main goals were: 1. to determine to what extent the responses to electrical stimulation reflected the shift in ocular dominance apparent from the RF analysis, 2. to determine the relative effects of deprivation on excitatory and inhibitory responses and 3. to locate the site of impaired transmission in the pathway from the deprived eye. The results show that the responses to electrical stimulation reflect precisely the shift in ocular dominance apparent from the RF analysis. The evoked potentials elicited from the deprived nerve further indicate that deprivation had also affected the afferent system at the LGN level or (and) at the terminal field of the thalamo-cortical fibers. In contrast to the reduction of short latency excitatory responses to stimulation of the deprived nerve, oligosynaptic inhibition with latencies of 4–6 msec was equally well elicited by stimulation of either eye. The same was true for delayed excitatory responses which frequently occur with latencies between 40 and 80 msec after nerve stimulation. It is concluded from these results 1. that transmission between thalamic afferents and inhibitory interneurones in the cortex is less affected by deprivation than transmission in those pathways which relay cortical excitation, 2. that there is another deprivation resistant indirect pathway from the retina to the visual cortex which is probably relayed through mesencephalic structures and 3. that deprivation effects are not confined to transmission failure at the thalamo-cortical synapses but include alterations already at the presynaptic level.     

Functional organization of the corticofugal system from visual cortex to lateral geniculate nucleus in the cat

1. In the cat visual cortex (VC), electrophoretic glutamate application at a depth corresponding to layer VI may have excitatory or inhibitory effects on relay cells of the lateral geniculate nucleus (LGN). Corticofugal excitation was seen, if the receptive field centers (RFCs) of the VC neurons recorded at the application site were within 2.3 degrees of the RFCs of the LGN neurons under test. Inhibitory effects were seen if the RFCs of both cells were further apart up to 3.1 degrees. Glutamate application at more superficial cortical sites had no effect on LGN-neuron activity. 2. Cross-correlation analysis between spontaneous activities of simultaneously recorded VC and LGN neurons revealed excitatory cortico-geniculate connections in 18 pairs with RFCs separated by less than 1.7 degrees. In 15 pairs the peak latency of the excitation was 2--5 msec (3.4 msec in the average), 3 pairs showed long cortico-geniculate latencies (13--18 msec). The existence of a fast and slow cortico-geniculate system is suggested. 3. Inhibitory cortico-geniculate interaction was demonstrated with cross-correlation analysis in 8 pairs of which 4 had RFCs separated by more than 1.7 degrees. The onset latency of the inhibition was 2--7 msec except for 2 pairs with about 20 msec latency. 4. Most of the LGN neurons which were affected by cortical glutamate application or which showed an excitatory or inhibitory connection with a VC neurons were sustained cells, while the majority of VC neurons which were recorded in the effective glutamate application sites or which showed a significant interaction with LGN neurons in the cross-correlogram were binocularly driven and complex, with mostly large RFCs (mean diameter 3.5 degrees). They responded briskly to moving small spots as well as to moving slits. 5. It is concluded that the corticofugal excitatory effect is transmitted through monosynaptic links from VC neurons located in layer VI (complex cell) to LGN relay neurons (mostly sustained-cell) and this system is organized in a precise topographical manner. 6. In an Appendix neuron pairs which showed a positive correlation in the geniculo-cortical direction were described. The findings may support the view that complex as well as simple cells are drive monosynaptically from geniculo-cortical afferents of the sustained or transient type.     

Influences of cortico-tectal and intertectal connections on visual responses in the cat's superior colliculus

Summary Our experiments, utilizing electrical shocks applied to the lateral- or supra-sylvian gyrus of the cortex, demonstrate an initially excitatory (latency 2–10 msec) but predominantly inhibitory influence of cortico-tectal afferents on the discharge of tectal neurons. Primary or secondary inhibition in tectal cells after cortical stimulation suppressed spontaneous or visually driven activity and limited the frequency of stimulation which tectal neurons could follow.The main influence of the contralateral colliculus on visual responses of tectal cells is inhibitory but again some principally monosynaptic intertectal connections evoked initial excitation (latency 3–10 msec) after electrical stimulation of the contralateral optic tract.Removal of the visual areas 17, 18 and 19 did not cause a loss of movement- or direction-selectivity in neurons of the superior colliculus. Cooling of the occipital cortex, while recording from direction-selective tectal neurons did not alter their essential response characteristics. The response to cortical shocks disappeared in tectal neurons during cooling but could be restored by rewarming of the cortex.It could not be confirmed in our experiments that excitation and movement- or direction-selectivity of neurons in the superior colliculus depend on a specific input from areas 17, 18 and 19 of the cortex.     

Centrifugal and centripetal connections of the cat visual cortex

In experiments on curarized cats unit responses in the dorsal lateral geniculate body to stimulation of various zones in area 17 of the visual cortex were analyzed. Of all cells tested 69% were found to respond antidromically and 8% orthodromically; in 7.6% of cells IPSPs occurred either after an initial antidromic spike or without it. The velocities of conduction of excitation along the corticopetal fibers of the optic radiation varied from 28 to 4.3 m/sec, but the three commonest groups of fibers had conduction velocities of 28-19, 14-12, and 10-9.5 m/sec. A difference between latent periods of antidromic responses of the same neurons was found to stimulation of different zones of the visual cortex; this indicates that axons of geniculo-cortical fibers split into several branches which form contacts with several neurons in area 17 of the visual cortex. The degree and possible mechanisms of cortical influences on neurons of the lateral geniculate body are discussed.Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 243–249, May–June, 1976.     

Reticulospinal connections with limb and axial motoneurons

Summary Responses of motoneurons supplying muscles of the forelimbs, hindlimbs, back, and neck to stimulation of the medial pontomedullary reticular formation were studied with intracellular recording in cere-bellectomized cats under chloralose anesthesia.Stimulation of the midline or of a reticular region consisting of nucleus reticularis (n.r.) pontis caudalis and the dorsorostral part of n.r. gigantocellularis produced monosynaptic excitation of ipsilateral motoneurons supplying axial muscles and flexor and extensor muscles in both proximal and distal parts of the limbs. This widespread excitation appears to have been produced by rapidly conducting medial reticulospinal fibers.Stimulation of a second region consisting of n.r. ventralis and the ventrocaudal part of n. r. gigantocellularis produced monosynaptic excitation of ipsilateral neck and back motoneurons but only longer latency, apparently multisynaptic excitation of limb motoneurons. Collision tests indicated that this monosynaptic excitation did not involve fibers descending along the midline. It therefore appears to have been produced by lateral reticulospinal fibers.Reticular stimulation also produced short latency, monosynaptic inhibition of neck motoneurons, long latency, apparently polysynaptic inhibition of limb motoneurons and intermediate latency inhibition of back motoneurons. The latencies and properties of inhibitory responses of back motoneurons indicated that they were produced either disynaptically by fast fibers or monosynaptically by slower fibers.The data indicate that the medial pontomedullary reticular formation can be divided into a number of different zones each with a distinct pattern of connections with somatic motoneurons. These include the dorsorostrally located medial reticulospinal projection area, from which direct excitation of a wide variety of motoneurons can be evoked, the ventrocaudally located lateral reticulospinal projection area from which direct excitation of neck and back and direct inhibition of neck motoneurons can be evoked and the dorsal strip of n.r. gigantocellularis which has direct excitatory and inhibitory actions only on neck motoneurons.Supported in part by NSF Grant BMS 7500487 and N. I. H. Grants EY 02249, EY 00100 and NS 02619     

Neuropharmacological properties of electrophysiologically identified,visually responsive neurones of the posterior lateral suprasylvian area

Extracellular recordings have been made from 118 electrophysiologically identified neurones lying in the posterior lateral suprasylvian area (PLLS and PLMS) of cats anaesthetized with Nembutal. Eighty-one cells were activated synaptically by the electrical stimulation of cortical and subcortical sites known to be the sources of monosynaptic projections to the lateral suprasylvian area; latencies to such activations have been measured. The locations and sizes of the receptive fields of 55 neurones were determined. The direction sensitivity and ocularity of these cells also were examined. The effects of various pharmacological agonists and antagonists have been observed on visual responsiveness and synaptic excitability. The excitatory effects of subcortical (dorsal lateral geniculate nucleus and pulvinar nuclear complex) electrical stimulation on the activity of suprasylvian neurones were reduced substantially by the iontophoretic administration of atropine. Antagonists of the receptors for the excitatory amino acids reduced the effectiveness, on the single cell evoked activity, of stimulation of the ipsilateral 17/18 border region and contralateral homotopic lateral suprasylvian area. Both classes of antagonist reduced the magnitude of neuronal responses to photic stimulation, and these response attenuations were additive when the antagonists were ejected concurrently. All of the pharmacological effects were reversible and reproducible. These data lend support to the proposition that acetylcholine and an excitatory amino acid are mediators of synaptic transmission of cortical visual processes in the lateral suprasylvian area.     

Quantitative studies of intracellular postsynaptic potentials in the lateral geniculate nucleus of the cat with respect to optic tract stimulus response latencies

Summary LGN cells were intracellularly recorded with glass micropipettes. Electrical stimuli of different amplitude and frequency were applied to the optic tract close to the optic chiasm. The cells were classified according to stimulus response latencies of action potentials as belonging to class I (1.0–1.6 msec) or class II (1.7–3.0 msec).Class I EPSPs had shorter latencies (1.0–1.5 msec), durations (4–12 msec), rise times to peak (0.5–1.4 msec), and decay times (3.0–8.5 msec); the synaptic transmission time was on the average 0.41 msec. Class II EPSPs (1.6–2.6 msec latency) had longer durations (10–30 msec), rise times (1.6–3.7 msec), and decay times (9.0–25 msec); the synaptic transmission time was on the average 0.67 msec.With repetitive stimulation the EPSPs of latency class I revealed almost no stimulus frequency dependence between 1 and 120 Hz, while class II EPSPs decreased in amplitude between 30 and 70% with increasing frequency. Comparable temporal summation of excitation occurred in cells of both latency classes. Negative serial correlation coefficients of first order were found for consecutive EPSP amplitudes of all cells recorded for sufficient periods of time.The IPSPs were subdivided into two groups according to their optic tract response latency. Group 1 IPSPs had shorter latencies (2.0–2.6 msec), durations (15–50 msec), and times from the onset to maximal hyperpolarization (2.4–4.2 msec) than group 2 IPSPs (3.0–4.8 msec latency, 40–100 msec duration, 2.7–7.5 msec time from onset to extremum).The group 2 IPSPs decreased in amplitude by about 90% when the stimulus frequency was increased from 1 to 50 Hz, while the group 1 IPSPs displayed a comparable decrease in the frequency range between 50 and 120 Hz. Effective temporal summation was found in group 2 IPSPs in the frequency range below 70 Hz, and in group 1 IPSPs at stimulus frequencies between 70 and 120 Hz.The EPSP peak latencies and the latencies to the minimum of IPSPs proved to be invariant with respect to PSP amplitude and stimulus frequency in individual cells. The latencies to the extrema of EPSPs and IPSPs as well as the amplitude values were symmetrically distributed.     

The mode of synaptic linkage in the cerebro-ponto-cerebellar pathway of the cat. II. Responses of single cells in the pontine nuclei

Extracellular and intracellular recordings were made from single cells in the pontine nuclei (PN) of the cat. PN cells were identified by antidromic invasion from the cerebellum by stimulating either the brachium pontis (BP) or the white matter near the cerebellar nuclei. The cerebrally-induced impulses excited PN cells postsynaptically with a monosynaptic latency. Both fast and slow conducting cortical fibres contributed to the corticopontine excitation, so that the latency varied over a wide range. Measurements of the latencies for antidromic and corticopontine excitation and of the distances between stimulated sites permitted the calcuation of conduction velocities of PN cell axons and of their cortical input fibres. PN cells with fast conducting axons received convergence from both fast and slow cortical fibres, whereas PN cells with slow axons were innervated only by slow cortical fibres. The majority of PN cells were also excited by stimulating the medullary pyramid through collaterals of the pyramidal tract. Evidence of abundant pyramidal collaterals was provided by the collision technique. The functional role of the PN is discussed in connection with the cerebro-cerebellar loop circuits.     

Corticofugal influences on transmission to the dorsal spinocerebellar tract from hindlimb primary afferents

Summary Effects from the cerebral cortex on neurones of the dorsal spinocerebellar tract (DSCT) were examined: I. In group I units (units receiving monosynaptic excitation from group I fibres) repetitive stimulation of the contralateral sensorimotor cortex usually inhibited impulse transmission from the primary afferents. The inhibition had a latency of 10–20 msec and lasted for 82-100 msec or more. Discharges induced by muscle stretch were also inhibited by the cortical stimulation. DSCT units belonging to extensors and flexors were both inhibited from the cortex. In a small percentage of group I units the inhibition was preceded by a shorter-lasting excitation. 2. FRA units (units receiving excitation from cutaneous and/or high threshold muscle afferents) were typically excited by the cortical stimulation. The excitation was often followed by a period of depression of transmission from the periphery. 3. It is suggested from the effective cortical area and experiments with lesions in the medullary pyramid and in the spinal cord that the inhibition in group I units and the excitation of FRA units are both mediated by the corticospinal tract.Experiments were also made to determine the level where the cell body of a given DSCT unit is located, and the results from 56 units are presented.     

Synaptic actions on mitral and tufted cells elicited by olfactory nerve volleys in the rabbit.

1. A unitary study has been carried out of mitral and tufted cell responses to olfactory nerve volleys in the olfactory bulb of rabbits lightly anaesthetized with urethane-chloralose. 2. With volleys of different strengths, some mitral cells responded with a spike whose latency decreased considerably as the strength increased (elastic response); other cells responded at an invariant latency (inelastic response). The former may reflect diffuse olfactory nerve inputs to the dendritic tufts in the olfactory glomeruli, while tha latter may reflect input from discrete bundles of fibres. 3. The shortest spike latencies are consistent with monosynaptic excitation by the olfactory nerves; longer latencies may be due to longer pathways through the nerves, or polysynaptic pathways within the glomerular layer. 4. Facilitation, in terms of lower threshold and shorter spike latency, was found when testing with paired volleys of weak intensity at relatively short intervals (less than 40 msec). Suppression, in terms of raised threshold, longer latency and briefer repetitive discharges, was found at intervals up to several hundred msec. The facilitation and suppression are consistent with the hypothesis of synaptic excitation and inhibition, respectively, mediated through interneurones in the olfactory bulb. 5. Presumed tufted cells were similar in response properties to identified mitral cells. 6. Intracellular recordings revealed long-lasting hyperpolarization and in some cases, an initial depolarization leading to spike initiation, in response to an olfactory nerve volley.     

Synaptic linkage in the vestibulo-ocular and cerebello-vestibular pathways to the VIth nucleus in the rabbit

Summary Intra- and extra-cellular responses were recorded with glass microelectrodes from motoneurons in the VIth cranial nuclei of anesthesized rabbits. VIth nucleus motoneurons were identified by their antidromic activation from the VIth nerve. In these motoneurons stimulation of the ipsilateral VIIIth nerve produced IPSPs with disynaptic latencies (mean and S.D., 1.08 ± 0.1 msec) while stimulation of the contralateral VIIIth nerve produced EPSPs with disynaptic latencies (mean and S.D., 1.20 ± 0.18 msec). Correspondingly, direct stimulation of the ipsilateral medial vestibular nucleus (MV), produced IPSPs with monosynaptic latencies (mean and S.D., 0.61±0.15 msec) while direct stimulation of the contralateral MV produced EPSPs with monosynaptic latencies (mean and S.D., 0.61±0.09 msec). Further, with the recording electrode placed within the VIth nucleus to observe the extracellular potentials corresponding to the intracellularly recorded IPSPs and EPSPs, the medulla was systematically tracked with a monopolar stimulating electrode. It was demonstrated that the inhibitory relay cells could be effectively stimulated in the rostral half of the ipsilateral MV and the excitatory relay cells in the rostral half of the contralateral MV.Pharmacological investigation suggested that the inhibitory transmitter involved in the vestibular inhibition is gamma amino-butyric acid or a related substance.Electric stimulation of the flocculus produced a prominant depression in the inhibitory vestibulo-ocular reflex pathway to the VIth nucleus, while the excitatory pathway was free of any similar flocculus inhibition.     

Effects of early monocular deprivation on response properties and afferents of nucleus of the optic tract in the ferret

Summary Effects of early monocular deprivation on visual response properties of neurons in the nucleus of the optic tract (NOT) were studied in six adult ferrets. Retinal input to NOT was investigated by orthodromic electrical stimulation of optic chiasm and optic nerves. Electrical stimulation of the ipsilateral primary visual cortex was applied to reveal the presence of a cortical pathway to NOT. All 75 neurons studied in the NOT displayed the typical strongly direction-specific response to horizontal stimulus motion; they were activated by ipsiversively directed motion (i.e. motion towards the recorded hemisphere) similar to NOT-cells in animals with normal visual experience. When tested binocularly most of the NOT-cells preferred velocities of 10 or 20 deg/s, revealing no significant difference from animals reared with normal binocular experience. The most pronounced effect of monocular deprivation was observed on ocular dominance: In the hemisphere contralateral to the non-deprived eye, NOT-cells were almost exclusively driven through the contralateral eye. In the hemisphere contralateral to the deprived eye, three of the six animals studied showed a marked dominance of the ipsilateral, non-deprived eye. In the other three animals, most neurons were binocularly activated, but over all they were significantly more strongly activated by the ipsilateral eye than found in normal animals. In four animals, dependence of ocular dominance on stimulus velocity was tested in the NOT contralateral to the deprived eye. In one of them, neurons were almost exclusively driven by the ipsilateral, non-deprived eye, irrespective of stimulus velocity. In the other animals, ocular dominance shifted from contralateral to ipsilateral with increasing stimulus velocities. Electrical stimulation of the optic chiasm revealed a mean latency of 5.53 ± 0.48 ms. In both hemispheres, NOT-units could only be activated by stimulation of the contralateral optic nerve. Thus, no significant difference in the retinofugal conduction velocities from the deprived and the normal nerve could be detected. Of 52 cells studied, 28 (= 54%) could be activated by stimulation of primary visual cortex, mean latency being 3.9± 1.7 ms. No significant difference in the percentage of cortically excitable cells between the two hemispheres as well as compared to normal animals was found (contralateral to the deprived eye: 67%, contralateral to the non-deprived eye: 53%). Therefore, cortical projections to NOT seem not to be affected by monocular deprivation. The effects of monocular deprivation in the ferret NOT, especially on ocular dominance and cortical input, are compared to the results previously described for the cat.     

Behavior related to trained increase in visual cortex excitability

Operant conditioning techniques were utilized to train cats to increase the amplitude of the short latency (5 msec) cortical response evoked by electrical stimulation of the optic radiation fibers. The cats learned to produce larger evoked potentials, and two distinctly different behaviors were associated with such increase. The first was a general relaxation of skeletal activity. The second was a brisk movement of the head, associated with a transient increase in excitability, lasting several hundred msec, with the maximum effect occurring about 250 msec after a warning stimulus.     

Cells of origin of the occipito-pontine projection in the cat: Functional properties and intracortical location

Summary Extracellular records were taken from the areas 17, 18 and 19 of anaesthetized and immobilized cats. Of the 350 cells recorded in the cortical layers IV–VI, 22 responded with an antidromic action potential to electrical stimulation of the basal pontine region. Antidromic latencies ranged between 1.5–8.8 msec (mean and SD: 3.98±1.83 msec). The recording sites of these cells were in the cortical layer V. In the areas 17 and 18 these cells were detected only in peripheral parts of the visual field representation.Some of these cells were tested with visual stimulation. They had complex receptive fields of large sizes, they received input from either eye, and their orientation selectivity was relatively low. The sample of cells had neither a preference for a certain orientation nor for any particular degree of directional specifity nor for a certain range of movement velocity.Dr. Donate-Oliver was supported by the exchange program between the Spanish national research council and the Max-Planck-Society, FRG.     

A comparison of the responses of frog skin receptors to mechanical and electrical stimulation

1. The latencies of spike responses evoked alternatively by brief mechanical (M) and electrical (E) pulses applied to single mechanoreceptive terminals in frog skin were compared on the same receptor.2. Latency was found to be a maximum at threshold and to decrease with increased stimulus strength for both modes of excitation, but at all strengths M latency exceeded E latency. Mean maximum and minimum values for M latency were 4.8 and 2.85 msec; for E latency the maximum was 2.8 and minimum 2.3 msec.3. At high frequency and strength of E stimulation there was an abrupt and marked shortening of latency to a fixed minimum value which ranged from 0.5 to 1.2 msec (mean 0.8). This was taken to be the response of the parent myelinated axon excited directly. The gap (1.5 msec) between the minimum value for the receptor response (2.3 msec) and the axonal response (0.8 msec) was taken to represent conduction time in the terminal branches of the sensory axon.4. The response latency for excitation of the sensory terminal was also dependent on the duration of the stimulus pulse, but whereas the latency range for the M stimulus could be greatly extended that for the E stimulus was only slightly affected by increase in pulse duration.5. The responses evoked by direct currents were complex, and consisted of an early brief discharge at the start of a cathodal current followed after a delay of 5-30 sec by a prolonged multi-fibre discharge which out-lasted the stimulus. It is proposed that the sensory terminal is rapidly accommodating to current flow and that the delayed discharge is due to release of chemical material.6. It is suggested that delay in mechanical excitation may be due to non-rigid coupling of the receptor terminal to the skin tissues.     

Responses of cortical neurones to stimulation of the visual afferent radiations

Summary Extracellular spikes of visual cortical neurones in unanesthetized cats were recorded. The latency after electric stimuli of the optic radiation to the onset of firing of cells with concentric fields, simple receptive fields and complex receptive fields was measured.Simple receptive field neurones had response latencies averaging 0.93 msec longer than neurones with concentric fields. The majority of the latter represented responses of geniculate axones terminating in the visual cortex and a few were action potentials of first-order cortical cells. Cells with complex receptive fields had longer latencies than simple field neurones and often showed prolonged cessation of firing following electrical stimulation.Many neurones showed a period of firing arrest after radiation shock. The duration and variability of this period also varied according to receptive field type.The data were consistent with the hypothesis that neurones with simple receptive fields receive the initial contact of incoming geniculocalcarine afferent fibres. Complex receptive field neurones appear to derive their input from other cortical cells rather than from the direct geniculocortical connections.A preliminary report of this research was presented to the German Physiological Society and an abstract published in Pflügers Arch. ges. Physiol. 294, 56 (1967).     

The short-latency projection from the baboon's motor cortex to fusimotor neurones of the forearm and hand

1. The corticospinal connexions that are responsible for the firing of fusimotor impulses at short latency in response to brief, high-frequency stimulation of the baboon's motor cortex have been investigated by micro-electrode recording from antidromically identified fusimotor neurones of the forearm and hand.2. Of nineteen fusimotor neurones investigated by intracellular recording, six showed EPSPs at monosynaptic latency.3. In extracellular records, the latency of firing of an early fusimotor impulse was always too long to be explained by monosynaptic excitation by the corticospinal D volley, but could be explained by monosynaptic excitation from an I volley.4. Four of the nineteen intracellularly recorded fusimotor neurones showed short-latency (?disynaptic) IPSPs in response to brief high frequency cortical bursts.     

Comparison of human motor cortical projections to abdominal muscles and intrinsic muscles of the hand

Summary Percutaneous electrical stimulation of the motor cortex was used to activate rapidly conducting corticofugal pathways to human abdominal muscles. Following cortical stimulation the response latencies for the abdominal muscles were similar to those for limb muscles which are a similar distance from the motor cortex. Cortically evoked responses recorded from the abdominal muscles had the same latency and similar amplitude during several voluntary tasks including expiration, expulsive manoeuvres and trunk flexion. Responses could also be evoked when the chemical drive to breathe was increased by rebreathing. In addition, the properties of the cortical projection to muscles of the abdominal wall were directly compared with those of the projection to the intrinsic muscles of the hand. The latencies of responses in abdominal muscles and intrinsic muscles of the hand were measured during static contractions over a range of strengths in the same subjects (0–100% maximal voluntary contraction, MVC). For both muscle groups, cortically evoked muscle responses of minimal latency occurred when background contractions reached 10–20% MVC with responses of maximal amplitude at 60% MVC. The variability in latency of fifty consecutive responses were similar for the two muscle groups. Furthermore, post-stimulus time histograms for 4 rectus abdominis motoneurones revealed a brief initial excitatory peak of 1.15ms duration (range 0.96–1.34ms) following cortical stimulation. The characteristics of this peak are the same as reported for motoneurones of intrinsic hand muscles. These findings demonstrate a powerful rapidly conducting pathway from the motor cortex to the human abdominal muscles. This pathway has many of the same properties as the monosynaptic corticospinal projection to the distal muscles of the upper limb.     

Mediation of late excitation from human hand muscles via parallel group II spinal and group I transcortical pathways

This study addresses the question of the origin of the long-latency responses evoked in flexors in the forearm by afferents from human hand muscles. The effects of electrical stimuli to the ulnar nerve at wrist level were assessed in healthy subjects using post-stimulus time histograms for flexor digitorum superficialis and flexor carpi radialis (FCR) single motor units (eight subjects) and the modulation of the ongoing rectified FCR EMG (19 subjects). Ulnar stimulation evoked four successive peaks of heteronymous excitation that were not produced by purely cutaneous stimuli: a monosynaptic Ia excitation, a second group I excitation attributable to a propriospinally mediated effect, and two late peaks. The first long-latency excitation occurred 8–13 ms after monosynaptic latency and had a high-threshold (1.2–1.5 × motor threshold). When the conditioning stimulation was applied at a more distal site and when the ulnar nerve was cooled, the latency of this late excitation increased more than the latency of monosynaptic Ia excitation. This late response was not evoked in the contralateral FCR of one patient with bilateral corticospinal projections to FCR motoneurones. Finally, oral tizanidine suppressed the long-latency high-threshold excitation but not the early low-threshold group I responses. These results suggest that the late high-threshold response is mediated through a spinal pathway fed by muscle spindle group II afferents. The second long-latency excitation, less frequently observed (but probably underestimated), occurred 16–18 ms after monosynaptic latency, had a low threshold indicating a group I effect, and was not suppressed by tizanidine. It is suggested that this latest excitation involves a transcortical pathway.     

The paramedian reticular nucleus: a site of inhibitory interaction between projections from fastigial nucleus and carotid sinus nerve acting on blood pressure

1. The interaction between the pressor response to electrical stimulation of the fastigial nucleus (FN), the fastigial pressor response (FPR), and the depressor response to electrical stimulation of the carotid sinus nerve (CSN) was examined in paralysed anaesthetized cats.2. Blood pressure responses evoked by electrical stimulation of the FN and the CSN were mutually inhibitory and summed algebraically.3. The FPR was augmented after denervation of buffer nerves. Lesions of the FN did not alter the depressor response to stimulation of the CSN.4. Bilateral electrolytic lesions of the paramedian reticular nucleus abolished both the FPR and the CSN depressor response without altering base line pressure.5. With micro-electrode recording neurones were discovered within the paramedian reticular nucleus which responded to electrical stimulation of the FN or the CSN. These neurones were polysynaptically excited by stimulation of either the FN or the CSN but rarely from both, and could be further subdivided into cells responding with either a single spike or a burst discharge.6. The interaction between the FN and the CSN projections to the paramedian reticular nucleus was examined by conditioning-test studies. Eleven per cent of FN- and CSN-units were inhibited by conditioning stimulation of the heteronymous input. The interaction was exclusively inhibitory and observed only in units with latencies > 4 msec and having burst responses. The latency for inhibition was > 20 msec, peaked around 100 msec and lasted up to 300 msec.7. We conclude that the FRP is buffered by baroreceptors and that there is a mutually inhibitory interaction between projections from the FN and the CSN acting on sympathetic vasomotor neurones. The paramedian reticular nucleus appears to be an important site for the interaction.8. The findings support the view that interneurones mediating pressor and depressor responses are intermixed within the medial reticular formation of the medulla.