Forelimb Nerves Research Articles

Patterns of afferent input to the lateral reticular nucleus of the cat.

1. 182 cells in the lateral reticular nucleus, LRN, identified by antidromic stimulation, were recorded extracellularly and tested for responses to stimulation of peripheral nerves and in some cases the contralateral sensorimotor cortex. 2. Although a majority of cells responded to stimulation of all 4 limbs, ipsilateral nerves tended to be more effective than the respective contralateral nerves, and forelimb nerves more effective than hind limb nerves. 3. Cells responding to nerve stimulation were found in the magnocellular as well as in the parvocellular part of LRN without any systematic localization of those cells which responded predominantly to forelimb or hind limb stimulation, respectively. 4. The latencies of both excitatory and inhibitory responses started at 4–6 ms for forelimb and 6–8 ms for hind limb stimulation. The patterns of latency distribution did not reveal any significant differences between ipsilateral and contralateral sides. 5. Most cells tested responded to stimulation of the deep radial nerve as well as the superficial radial nerve. The afferents responsible for the effects from the deep radial nerve had their thresholds in the range of group II afferents. In the few cases with effects at group I strength the responses had a longer latency. 6. The pattern of convergence from stimulation of the sensorimotor cortex and peripheral nerves, studied in 25 cells, failed to demonstrate a clear correlation of inputs from forelimb afferents and forelimb sensorimotor cortex or from hind limb afferents and hind limb sensorimotor cortex.

On the pharmacology of ascending, descending and recurrent postsynatic inhibition of the cuneo‐thalamic relay cells in the cat

1. In cats decerebrated or anaesthetized with pentobarbitone, cells of the middle third of the cuneate nucleus that were excited by tactile stimulation of the ipsilateral forelimb (responding to displacement of hairs, skin or joints) and inhibited by electrical stimulation of the contralateral pyramid, were invariably inhibited by electrical stimulation of the ipsilateral forepaw and the contralateral forelimb nerves.2. In 50% of the cats, the cells were more fully identified by placing electrodes stereotaxically in the contralateral medial lemniscus. Recurrent inhibition was always a concomitant of the antidromic action potential.3. The intensity and the duration of inhibition evoked by all of these pathways was totally resistant to iontophoretic and intravenous strychnine in doses at least 5 times that required to block completely the response of the same cells to iontophoretic glycine and was extremely sensitive to either iontophoretic bicuculline or picrotoxin.4. Although the inhibition was invariably sensitive to intravenous picrotoxin, no significant change occurred in the duration or intensity of the inhibition when bicuculline was administered intravenously (5 or 6 times) as repeated doses of 0.2 mg/kg.5. Postsynaptic inhibition in the cuneate may be mediated by gamma-aminobutyric acid released from the nerve terminals of a common pool of interneurones shared by ascending, descending and recurrent pathways. Since the receptors involved in this pathway are resistant to intravenous bicuculline, they may well be distinct from those responsible for changes in the primary afferent terminal excitability, usually believed to be associated with presynaptic inhibition.

Synaptic actions of peripheral nerve impulses upon Deiters neurones via the mossy fibre afferents.

1. The cerebellar integration of sensory inputs to Deiters neurones was investigated in decerebrate cats. In some preparations decerebration was combined with transection of the olivocerebellar fibres.2. In the latter preparations peripheral nerve impulses generally produced a response consisting of a sequence of the following post-synaptic potentials: (i) an initial e.p.s.p. (d(1)), (ii) early i.p.s.p. (h(1)), (iii) later i.p.s.p. (h(2)).3. The mean latencies of d(1), h(1) and h(2) were 5.7, 7.3 and 9.8 msec from the forelimb nerves, and 7.5, 9.0 and 13.4 msec from the hind limb nerves, respectively.4. The stimulus intensity-response relation indicates that the Group I muscle afferents as well as the low threshold cutaneous afferents contribute to the response.5. In the preparations with the intact inferior olive there were additional components of the post-synaptic potentials: a later e.p.s.p. (d(2)) and another later i.p.s.p. (h(3)), their mean latencies being 15.3 and 19.7 msec from the forelimb nerves, and 18.0 and 21.3 msec from the hind limb nerves, respectively.6. The d(1) and h(2) components were attributed to the mossy fibre afferents and d(2) and h(3) to the climbing fibres; d(1) and d(2) were due to excitation through the collaterals of the mossy and climbing fibres, and h(2) and h(3) to inhibition from Purkyne cells activated by the mossy and climbing fibres, respectively. h(1) was too early to be produced through the cerebellum, and was probably mediated by inhibitory neurones in the reticular formation.

Synaptic actions of peripheral nerve impulses upon Deiters neurones via the climbing fibre afferents.

1. The cerebellar integration of sensory inputs to Deiters neurones was studied in cats under Nembutal anaesthesia.2. Stimulation of peripheral nerves produced in the Deiters neurones a sequence of an initial excitatory post-synaptic potential (e.p.s.p.) and a later inhibitory post-synaptic potential (i.p.s.p.), or a relatively small e.p.s.p.3. The Deiters neurones were classified as forelimb (FL)- or hind limb (HL)-type cells according to the location of the most effective peripheral nerve. In the FL cells stimulation of the forelimb nerves produced the e.p.s.p.-i.p.s.p. sequence (dominant response), while stimulation of the hind limb nerves was ineffective or produced the small e.p.s.p. (non-dominant response). In contrast, in the HL cells the non-dominant response was evoked from the forelimb nerves, and the dominant response from the hind limb nerves.4. The stimulus intensity-response relation indicates that Group I and II muscle afferents and low and high threshold cutaneous afferents contribute to the dominant and non-dominant responses.5. Antidromic identification of these Deiters neurones revealed that 90% of the HL cells and 85% of the FL cells project to the lumbo-sacral and cervico-thoracic segments of the spinal cord, respectively, while 10% of the HL cells and 15% of the FL cells innervate the cervico-thoracic and lumbo-sacral segments, respectively.6. The mean latency of the e.p.s.p. evoked from the forelimb nerves was 14 msec in the FL cells and 13 msec in the HL cells, and the latency of the e.p.s.p. evoked from the hind limb nerves was 17 msec in the FL cells and 18 msec in the HL cells. The later i.p.s.p. regularly followed the onset of the e.p.s.p. with a delay of 3-5 msec.7. The dominant and non-dominant responses in both types of cells exhibited the following three characteristic features: (i) a strong depression after conditioning stimulation of the inferior olive, (ii) an increase of the inferior olivary excitability during the responses, and (iii) a striking frequency depression with stimulation at relatively low frequency (5-10/sec).8. Consequently it was concluded that all of the responses were produced through the climbing fibres originating from the inferior olive, the i.p.s.p.s due to inhibition from Purkyne cells activated by the climbing fibres and the e.p.s.p.s due to excitation from the collaterals of the climbing fibres.

The action of carbon dioxide on synaptic transmission in the cuneate nucleus

1. The excitability of synaptic structures in the cuneate nucleus was studied in eighteen decerebrate, unanaesthetized cats during acute changes in inspired P(CO2).2. Micro-electrode stimulation in the caudal half of the cuneate nucleus evoked antidromic and orthodromic responses which were recorded simultaneously from a forelimb nerve and from the medial lemniscus at the transected mid-brain surface.3. Increases in the concentration of inspired CO(2) (2-20%) progressively decreased the direct excitability of both the afferent fibre terminals (reflected in the antidromic potentials) and the cuneate relay neurones (reflected in the alpha wave of the orthodromic lemniscal response). Synaptically mediated responses, recorded as the beta component of the lemniscal potentials, were also depressed.4. The relation between input and output at the cuneate was determined by plotting antidromic against trans-synaptic (beta lemniscal) responses for different intensities of stimulation. The mean slope for logarithmic values of control potential amplitudes was 1.17 (+/- S.E. 0.13). It therefore appears that the transfer function for the cuneate is linear over a wide range.5. In the majority of experiments the input-output relation was unchanged or increased by raising P(CO2). It was concluded that the efficiency of synaptic transmission and release of transmitter appeared to be well maintained or possibly increased at individual active synapses during hypercarbia.6. The depressant action of CO(2) on afferent transmission can therefore be attributed largely to a block of impulse conduction in the primary afferent fibres.

The action of carbon dioxide on afferent transmission in the dorsal column-lemniscal system.

1. Transmission in the lemniscal afferent pathway was studied in thirteen decerebrate, unanaesthetized cats while changing the concentration of inspired P(CO2).2. 2-20% CO(2), when inhaled for </= 10 min, raised the mean tissue P(CO2), recorded from the surface of the medulla or cerebellum, from 22 torr to between 26 and 93 torr.3. Medial lemniscal potentials were evoked by stimulation of a forelimb nerve and recorded from the transected surface of the contralateral mid-brain. The transmission of supramaximal responses was progressively and reversibly depressed as tissue levels of P(CO2) were raised and lowered. The time course of the changes in transmission corresponded closely to changes in the tissue P(CO2).4. The amplitude of the main early lemniscal peak was decreased to 80% by the inhalation of 20% CO(2). A late component of the lemniscal response, presumably due to repetitive firing and conduction in smaller fibres and polysynaptic pathways, was more affected than the early main response.5. The failure of transmission was unrelated to threshold changes in the peripheral nerve, since potentials recorded close to the site of stimulation showed no changes.6. Increases in the transmission time of the main lemniscal potentials were uniform and < 10% during the administration of CO(2) and did not appear to contribute to amplitude changes.7. The inhibition of afferent transmission from one nerve by a preceding conditioning volley in a second nerve was not altered by hypercarbia.8. It is concluded that CO(2) has a blocking action on afferent transmission in the pre-thalamic lemniscal system. The site of this block may be at the synapses and/or other regions of low safety factor in the afferent fibres.

Cerebellar Purkinje cell responses to afferent inputs. II. Mossy fiber activation

(1) The mossy fiber pathway to cerebellar Purkinje cells was activated in decerebrate and lightly thiopental anesthetized cats by electrical stimulation of the cerebellar cortex surface, the lateral reticular nucleus (LRN), and various hind- and forelimb nerves. (2) On-beam activation of parallel fibers could be accomplished by any of the 3 types of stimulations used. Such on-beam activation rarely evoked more than a single spike in Purkinje cells, and nerver more than three, in either preparation. The average number of spikes evoked per stimulus graded in a delicate manner when the strength of the stimulus at any of the 3 sites was increased. (3) Similarly, inhibition of Purkinje cells firing could be produced by either on-or off-beam activation of parallel fiber by any of the 3 stimulations. This inhibition was of a highly variable duration, at times greater than 200 msec, and also graded with stimulus strength. (4) Surface stimulation could be arranged at times to produce sudden, ungraded inhibition, presumably by direct excitation of Golgi cells. Direct excitation of mossy fibers with surface stimulation was also at times observed. (5) The latency of the LRN evoked response in Purkinje cells was of the order of 3.0–6.0 msec. A slow negative field potential was sometimes observed following the N 2 potential. (6) A hypothesis is put forward that the mossy fiber pathway carries rapidly changing proprioceptive information from peripheral receptors which enables Purkinje cells to compute appropriate motor responses.

Cerebellar Purkinje cell responses to afferent inputs. I. Climbing fiber activation

(1) The climbing fiber pathway to the cerebellum was activated in decerebrate and lightly thiopental anesthetized cats by electrical stimulation of the inferior olive and various hind- and forelimb nerves. (2) Climbing fiber activation is found to produce both excitatory and inhibitory effects upon Purkinje cell firing. Salient features of each are described. (3) The excitatory effect represented by the climbing fiber response is separable from the pause in firing which may follow it. Either can occur without the other and the duration of the pause grades with the strength of a stimulus applied to the inferior olive. The duration of the pause which may follow spontaneous climbing fiber responses varies from next to nothing up to about 800 msec. (4) Both extracellular tests and intracellular recordings indicate that the larger pauses are not accompanied by membrane depolarization. (5) It is concluded that a significant role in the generation of these pauses in firing is played by inhibitory pathways in the cerebellar cortex which are activated by climbing fibers. These pathways comprise inhibitory interneurons and Purkinje cell axon collaterals. (6) A possible functional role for climbing fibers is discussed.

Transformation of somatic afferent volleys across the prethalamic and thalamic components of the lemniscal system during the rapid eye movements of sleep

1. 1. The orthodromic lemniscal response recorded from the contralateral medial lemniscus to single shock stimulation of the forelimb nerves is phasically depressed during the bursts of rapid eye movement (REM) typical of desynchronized sleep. On the contrary, the post-synaptic potential evoked from the nucleus ventralis postero-lateralis on single shcok stimulation of the medial lemniscus is greatly facilitated during REM. 2. 2. While the depression of prethalamic lemniscal responses occurs only during large bursts of REM, facilitation of the thalamic responses elicited by lemniscal stimulation appears not only during large but also during small bursts of REM. 3. 3. Comparison of the lemniscal and thalamic responses induced by cutaneous afferent volleys during the same periods of desynchronized sleep indicated that facilitation of thalamic responses generally occurred during small bursts of REM, which were usually not associated with changes in amplitude of the lemniscal responses. 4. 4. On the contrary, during large bursts of REM the post-synaptic facilitation of thalamic responses was unable to compensate for the reduced transmission of the somatic afferent volley through the dorsal column nuclei and the final effect was a reduction of the orthodromic thalamic volley. 5. 5. Previous observations indicate that muscular contractions occur during bursts of REM. It is postulated that some part of the efferent activity giving rise to contractions of the limb musculature during REM is fed into the somatic sensory system, particularly the nucleus ventralis postero-lateralis, where it interacts with the incoming somatic information filtered at dorsal column level.

Afferent connexions to group I activated cells in the main cuneate nucleus of the cat.

1. Extracellular recordings were made from a total of 240 group I activated cells in the main cuneate nucleus. Cuneothalamic relay neurones (128) were identified by antidromic stimulation of the medial lemniscus in the ventrobasal thalamic complex.2. A majority of the relay neurones were activated by afferents in only one of six dissected forelimb nerves innervating muscle groups at various joints. Even among afferents from adjacent synergistic muscles, convergence to individual neurones was infrequent.3. Some of the relay neurones received excitation from group II muscle afferents in the same nerve that provided group I excitation. Excitation from group II muscle afferents in other nerves was uncommon. Some neurones were weakly excited by cutaneous volleys.4. Inhibition of group I relay cells was produced from cutaneous afferents and group II muscle afferents. Weak inhibition was sometimes observed from group I afferents. The relay cells were also inhibited by stimulation of the cerebral cortex with a focus around the lateral end of the cruciate sulcus. A good correspondence was found between the inhibition and the depolarization of group I afferent terminals in the cuneate nucleus.5. A majority of the group I activated cells not antidromically activated from the ventrobasal complex (;non-relay cells') were excited by cortical stimulation. Excitation from cutaneous afferents and group II muscle afferents was frequently found among these cells.6. The group I activated cells were found almost exclusively in the ventral part of the nucleus.7. The pattern of convergence found in eleven group I activated cells in the dorsal horn of the spinal cord from C 2 to C 4 is described.

Excitation of group I activated thalamocortical relay neurones in the cat.

1. Extracellular recordings have been made from 134 group I activated neurones in the ventrobasal thalamic complex. One hundred of the neurones were identified as thalamocortical relay neurones by antidromic activation from the projection areas for group I afferents.2. The discharge evoked by group I volleys showed characteristic fluctuations in latency and number of spikes. The mechanisms underlying these phenomena are discussed.3. Half of the relay neurones were activated from afferents in only one of six dissected forelimb nerves innervating muscle groups at the various forelimb joints. Two-thirds of the relay neurones were activated from at least two adjacent synergistic muscles.4. The pattern of group I convergence in the thalamic neurones is compared with that at the cuneate and cortical levels of the group I pathway to the cerebral cortex. It is suggested that integration of information from synergistic muscles occurs at the thalamic level and that integration of information from muscle groups of more unrelated function occurs at the cortical level.5. A few of the thalamocortical cells (15%) were activated by group II muscle afferents, often in the same nerve as that which provided group I excitation. Weakly linked excitation from cutaneous afferents was observed in 39% of the neurones.6. Stimulation of the group I projection area in the first sensorimotor cortex at moderately high stimulus frequencies produced a trans-synaptic excitation in most of the group I activated cells.

Afferent nerve groups and sympathetic reflex pathways

Summary In chloralose-anesthetized cats, two reflex sympathetic discharges were recorded from lumbar white rami after single shock stimulation of peripheral afferent nerves: an early discharge associated with a spinal reflex pathway, and a late discharge associated with a supraspinal pathway. The following relationship between afferent fibers and reflex pathways were noted: (1) Excitation of only Group I hindlimb muscle fibers did not evoke reflex sympathetic discharges. (2) Excitation of hindlimb Group II plus fibers, or of Group Aα or Aβ hind- or forelimb cutaneous, or visceral fibers evoked only the supraspinal reflex. Beyond its threshold the late response increased with stimulus strength. (3) Stimulus strengths greater than threshold for Group III hindlimb muscle, or Group Aδ hind- or forelimb cutaneous, or visceral fibers evoked the spinal sympathetic reflex. The discharge increased with stimulus strength. (4) Supraspinal sympathetic reflexes were present in all fore- and hindlimb studies. Spinal reflexes were consistently found in hindlimb nerve studies but in only one out of four forelimb nerve studies. (5) Stimuli strong enough to excite C fibers often caused increases in the late reflex sympathetic discharge. Such increases might be due to a C-fiber activated spinal pathway reflex. (6) Strychnine or tetanic stimulation of the peripheral nerves led to increased amplitudes of both early and late sympathetic responses to succeeding single shocks. (7) Stimulation of Aα or Aβ cutaneous afferent evoked an early sympathetic reflex in chronic spinal cats. Excitation of smaller fibers increased the amplitude of this reflex but did not evoke a late reflex.

Control by brain stem reticular formation of sensory transmission in burdach nucleus analysis of single units

In cats anesthetized with Nembutal spontaneous and evoked activity of single units in the cuneate nucleus was extracellularly recorded and analyzed. CTR cells were activated by stimulation of contralateral medial lemniscus and ipsilateral forelimb nerves ( RS, U, M). They might also be excited throughout bulbopontine reticular formation. The stimulation of bulbopontine reticular structures could reduce or abolish the activity evoked in CTR neurons by stimulation of peripheral nerve whereas it never modified the activity antidromically evoked in CTR neurons by lemniscal stimulation. The interneurons were activated by stimuli to the contralateral sensorimotor cortex, reticular structures and cutaneous nerves ( RS, U, M). Most of them showed a convergence of different impulses on a single cell. From these results it can be deduced that the pontobulbar reticular formation has a presynaptic inhibitory effect on afferent transmission to cuneate neurons, presumably by interneurons located in the nucleus itself.

Functional organization of long, second-order afferents in the dorsal funiculus.

Second-order neurones activated from forelimb nerves and ascending in the deep part of the dorsal funiculus from the eighth to the first cervical segentm were investigated in the cat by axonal recording. These neurones form a previously unknown tract denoted the ascending dorsal funiculus tract, ADFT. The ADFT neurones receive excitation from cutaneous afferents as well as high threshold afferents in deep nerves. The excitation is supplied by afferents from hair, touch, and possibly also pain receptors in the skin, and from receptors in deep connective tissue. There is no excitation from muscle spindles or from specific joint receptors. The cutaneous receptive fields are small and lack inhibitory surround.

Analysis of electrical potentials evoked in the cerebellar anterior lobe by stimulation of hindlimb and forelimb nerves.

Responses were evoked in the anterior lobe of the cerebellum by volleys in group I and II fibers of forelimb and hindlimb nerves — cutaneous, muscular, joint and fascial. These responses have been observed along microelectrode tracks that traverse the whole depth of the anterior lobe. These tracks have been identified in histological sections, and the recording sites along these tracks have been determined. It has been shown that there are many distinguishing features for the responses produced by the two types of afferent input to the cerebellum: climbing fibers and mossy fibers. The depth profiles are of particular importance in the differentiation of the CF and MF responses, and they correspond to those already determined for the exposed surface areas of the cerebellar cortex. As would be expected from the distribution of synapses by the CF fibers to the Purkinje cell dendrites, there is a maximum extracellular negativity deep in the molecular layer with sources superficial and deep thereto. In contrast, the mossy fiber input produces a powerful synaptic excitation in the granular layer, which is recorded there as a negative wave (N2). The mossy fiber input by sequential relay also produces a negative wave (N3) in the molecular layer. This wave is distinguished from the CF-evoked negative wave because it is not reversed in the fissura and the adjacent superficial molecular layer. An important distinguishing feature of the MF- and CF-evoked responses is that the latencies of the former are shorter by 6–12 msec for forelimb nerves and by 9–15 msec for hindlimb nerves. It is thus possible to measure the sizes of the MF and CF responses in the same traces. Another distinguishing feature is the failure of the CF responses with stimulus frequencies of 5–15/sec, whereas the MF-evoked potentials are well maintained above 15/sec. Also CF-evoked responses show much more size and latency variance than the MF-evoked responses, and often the facilitation of two or three volleys is required in order to evoke a stable CF response. By utilizing these various tests it is always possible to distinguish between the CF- and the MF-evoked responses recorded along the microelectrode tracks in the anterior lobe.

Mossy and climbing fibre organization on the anterior lobe of the cerebellum activated by forelimb and hindlimb areas of the sensorimotor cortex.

Stimulation of forelimb and hindlimb areas of the sensorimotor cortex in the cat evokes in the lobus anterior of the cerebellum an early response at latency of 3–3.5 msec due to the mossy fibre input (MF) and a later response at latency of 13–16 msec due to the climbing fibre (CF) input. In the pars intermedia these two types of responses are organized in a somatotopic manner: the hindlimb area projects in lobuli HIV and HIII whereas the forelimb area projects to lobulus HV. In the vermis a somatotopic arrangement is less clear. Both forelimb and hindlimb areas of the sensorimotor cortex project to lobuli III, IV and V: on a maintained somatotopy in a caudo-rostral direction there is a tendency for the hindlimb area of the sensorimotor cortex to be well represented in a longitudinal strip close to the paravermal sulcus. This arrangement in the vermis is evident for the CF pathways, but more difficult to demonstrate for the MF pathways. The forelimb area of the sensorimotor cortex projects to those areas of the lobus anterior impinged upon by the forelimb nerves through both the MF and CF systems and the same holds true for the hindlimb area and the hindlimb nerves.

Termination and functional organization of the ventral spino-olivocerebellar path.

1. The spino-olivocerebellar path ascending through the ventral funiculus (VF-SOCP) was investigated in decerebrate cats with the cord transected in the third cervical segment except for the left ventral funiculus. The climbing fibre responses evoked in Purkinje cells were studied by recording from single cells and by recording the mass activity at the cerebellar surface or in the molecular layer.2. In the spinal cord the forelimb component of the VF-SOCP occupies a medial part and the hind limb component a lateral part of the ventral funiculus.3. The main projection of the limb nerves through the VF-SOCP is to the lateral two thirds of the vermis of the anterior lobe. The projection area consists of three sagittally arranged bands: a lateral band receiving olivary axons activated exclusively from the ipsilateral hind limb, an intermediate band receiving olivary axons activated bilaterally from the hind limbs, and a medial band receiving olivary axons activated bilaterally from the forelimbs. Some olivary neurones are activated from all four limbs.4. The latency of the climbing fibre responses was about 22 msec on stimulation of ipsilateral hind limb nerves and about 20 msec on stimulation of ipsilateral forelimb nerves. The responses evoked from contralateral nerves had a latency which was about 3 msec longer than the latency of the corresponding ipsilateral responses.5. The olivary neurones were usually activated from all tested muscle and skin nerves in the limb(s) constituting the receptive field. Cutaneous afferents and groups II and III muscle afferents were responsible for the excitation elicited by single shock stimulation of the nerves. Brief repetitive stimulation revealed additional activation from mainly Ib, but also Ia, afferents in ipsilateral hind limb nerves.6. Natural stimulation of receptors evoked responses in about half of the olivary neurones tested. The responses were elicited by strong pressure against deep structures. Inhibitory effects were seldom observed.

Antagonism of presynaptic inhibition in the cuneate nucleus by picrotoxin.

AN afferent volley in a cutaneous forelimb nerve produces on the surface of the cuneate nucleus a characteristic electrical response. This consists of a brief initial spike potential indicating the arrival of the primary afferent volley, followed by a negative wave (N wave) lasting about 4–5 msec and a prolonged positivity (P wave) reaching a maximum in 20–30 msec and lasting for about 200 msec1. Similarly, N and P waves result from electrical stimulation of the contralateral sensorimotor cortex2. The N wave produced by the afferent stimulus is thought to be caused by synaptically induced depolarization of the cuneate cells by the ascending volley in the dorsal column3. Systematic investigation of the P waves produced by cutaneous volleys and by cortical stimulation led Andersen et al.3 to suggest that both are caused by prolonged depolarization of cuneate tract fibres close to their synaptic terminals in the cuneate nucleus. This suggestion was further supported by excitability testing of these presynaptic fibres at their terminals and along their length, and by intracellular recording from these fibres4. With the technique of extracellular single cell recording, Jabbur and Towe5–8 had shown earlier that the sensorimotor cortex (primarily the contralateral) produced excitatory and inhibitory influences on cuneate and gracile neurones. With the additional technique of antidromic activation of dorsal column neurones after medial lemniscal stimulation9,10, two types of neurones have been identified. These are direct projection neurones, or dorsal column-thalamic relay neurones, which are depressed by antecedent descending volleys from the sensorimotor cortex, and interneurones, through which the cortex exerts presynaptic (and sometimes post-synaptic11) inhibition on the afferent inflow passing through the dorsal column nuclei. There is histological evidence for the presence of these corticofugal projections12.

Effects of chloralose anesthesia on spinal reflexes.

1. Experiments were carried out on 25 adult cats under spinal transection at the C1 level in order to extend observations on the chloralose jerky muscular contraction and to analyze the effects of chloralose on spinal reflexes.2. Chloralose (25-35mg/kg) was administered intravenously to spinal cats. These cats did not exhibit generalized jerky muscular reactions, even when a large dosage of 100mg/kg was administered.Blood pressure and heart rate were not changed by chloralose anesthesia at a dosage level lower than 45mg/kg. The following results were observed: 1) Slight augmentation of the monosynaptic reflex response, with a reduction in amplitude of polysynaptic reflex responses. 2) Reduction of the inhibitory influence of the sural nerve on the extensor monosynaptic reflex response. 3) Reduction of the Renshaw cell recurrent inhibition to the motoneurons. 4) Reduction of potential from the cord dorsum (N2 and P), dorsal root, and dorsal root reflex responses. 5) Reduction of augmentation of antidromic potentials of the primary afferent fibers following sural nerves timulation. 6) Reduction of the interlimb reflex response and augmentation of the local lumbosacral monosynaptic reflex response to the forelimb nerve stimulation, without alteration of the initial diminution.3. The observations suggest that the chloralose has a depressing action on the spinal polysynaptic reflex process and/or interneurons. Neural mechanisms of the chloralose jerky muscular contraction could not be due to mechanisms in the spinal cord.

Activity of neurons of somatosensory cortex during local strychnine poisoning

The types of re sponse of neurons in the somatosensory cortex to peripheral stimulation and the character of their reaction to direct electrical stimulation has already been examined (2, 3). A common component of all the individual responses of these neurons to these forms of stimulation was a temporary quiescence, probably due to inhibition. Since observations (4, 8, 9) have shown that strychnine abolishes the phenomena of inhibition, it was interesting to study the changes in the character of the responses to peripheral stimulation during local strychnine poisoning. It might also be expected that the application of strychnine, by abolishing inhibitory influences, would in turn facilitate excitation of the neurons of the surface layers of the cortex. The activity evoked in this manner would readily be distinguished by its selectivity and its greater potency from the activity evoked by direct electrical stimulation of the cortex. It was assumed that by performing experiments along these lines a more meaningful analysis could be made of interneuronal relationships in the primary somatosensory cortex. METHOD Cats were anesthetized with Nembutal (50 mg/kg, intraperitoneall y). Preparatory operations included tracheotemy, the application of stimulating electrodes to forelimb nerves, drainage of the 4th ventricle, and exposure of the sigmoid gyri of both hemispheres. Neurons in circumscribed areas of the posterior sigmoid gyri corresponding to the site of primary projection of forelimb nerves were investigated. Strychnine poisoning was produced by application of filter paper soaked in a 1% solution of strychnine nitrate to the area of the cortex to be investigated. The potentials of the neurons were detected by glass microelectrodes filled with 3 M KC1 with a resistance of 5-15 M~. The surface corticogram was recorded with a silver ball electrode. Potentials from the microelectrode were passed through a cathode follower (while those from the macroelectrode were passed directly)to a UBP 1-02 amplifier and, with a "Meopta" tape winder, were photographed from the screen of a D-581 twin-beam oscilloscope. The grid current of the cathode follower was less than 10-11 A. The nerves were stimulated with 0.5-msec rectangular pulses, applied through radio-frequency output from EST-7 generators.