Interactions of nigrostriate synaptic transmission, iontophoretic O-methylated phenethylamines, dopamine, apomorphine and acetylcholine.

Recordings were made from, and drugs applied to, neurons in the caudate nucleus of unanesthetized cats, using multibarrel micropipette electrodes. The substantia nigra was stimulated by sterotactically placed electrodes. Three O-methylated derivatives of dopamine, meta-methoxyphenethylamine (m-MPEA), para-methosy-phenethylamine (p-MPEA) and 3,4-demethoxyphenethylamine (DIMPEA), inhibited most, excited a few, and had no detectable effect on a substantial number of the cells upon which they were tested. A statistically significant correlation was found between the effects of dopamine (DA) and the three O-methylated derivatives on the same populations of cells. Iontophoretic release of the O-methylated derivatives could not prevent the actions of DA, nor could it block synaptically mediated effects of the nigrostriate pathway. It is concluded that the three O-methylated products are partial agonists of DA. The findings are difficult to reconcile with the suggestion that the experimental parkinsonian-like symptoms caused by O-methylated phenethylamines are the consequence of blockade of dopaminergic synapses. No correlation, negative or positive, was found between the effects of DA and of acetylcholine (ACh). The findings do not support the theory that balanced sets of antagonistic synapses, one dopaminergic, the other cholinervic, operate upon individual neurons in the caudate nucleus. Apomorphine and dopamine were shown to have similar effects on a substantial number of neurons, even though the onset and offset of the effect of apomorphine were slower than those of DA. This observation agrees with the suggestion that some of the central effects of apomorphine are due to an action at dopaminoceptive receptor sites.

Interactions among convergent inputs to somatosensory cortex neurons.

Postsynaptic potentials (psps) produced by electrical stimulation of 4 forelimb nerves were recorded intracellularly from neurons in the primary somatosensory cortex of sodium pentobarbital anesthetized cats. Convergent inputs were found from nerves subserving different modalities and different regions of the forelimb. Psps from separate afferent sources usually did not sum linearly but rather interacted with one another. These interactions could have occurred at the cortical level or earlier in the ascending pathways and are interpreted with regards to the control of somatosensory responsiveness by multiple converging inputs.

Peripheral input pathways projecting to the motor cortex in the cat.

The possibility that the motor cortex receives peripheral input directly from the thalamus was examined using the evoked potential method and the following results were obtained. Potentials in the motor cortex evoked by stimulation of superficial radial (SR) or group II deep radial (DR) nerve were neither abolished nor delayed by ablation of the sensory cortex. Potentials in the motor cortex evoked by stimulation of group II DR nerve were most severely reduced by interruption of the spinocervcial tract. Potentials evoked by stimulation of SR nerve were more severely reduced in the sensory cortex than in the motor cortex by section of the dorsal funiculus or cooling of the cuneate nucleus. The size of evoked potentials in the motor cortex increased rapidly when stimulus intensity to DR nerve exceeded the threshold to group II fibers. The results suggest that some inputs from the SR and group II DR nerves reach the motor cortex without a relay through the sensory cortex.

Intracortical mechanisms for the recruitment of motor cortex neurons.

Neurons project out of motor cortex to the spinal cord and to other targets. Not all projection neurons recruit in the same way during behavior, but instead recruitment patterns depend on the projection target of the neurons. The problem is to understand how neurons projecting to different targets are recruited selectively. We have investigated possible mechanisms for the recruitment of motor cortex neurons with electrophysiological approaches in anesthetized cats. To determine if neurons projecting out of motor cortex to different targets have selective input connectivity from extrinsic sources we electrically stimulated corticocortical, callosal and thalamocortical pathways. Subthreshold effects of input pathways were detected by monitoring latency variations of antidromic responses. Intracortical connections to identified output neurons were evaluated by cross-correlation and a new variation of the antidromic latency method. Output neurons in different layers along single electrode tracks usually had different inputs from extrinsic sources. Neurons in close proximity were most likely to share the same inputs, especially when they projected axons to the same target. These results support the conclusion that combinations of inputs from extrinsic sources could selectively recruit efferent neurons from separate cortical layers or from within groups of nearby neurons, according to the target of their axonal projections. In contrast, the data on intracortical connectivity suggest that common drive causes a more synchronous activation of nearby cortical neurons. Combining the conclusions on effects of inputs from extrinsic and intracortical sources leads to the speculation that motor cortex neurons that might at one time be recruited selectively by action of extrinsic afferent pathways to cortex could at another time be bound into synchrony by a common drive shared with their neighbours.

Influence of somatosensory cortex on different classes of cat motor cortex output neuron.

1. Multiple output pathways originate from motor cortex. In this study on cats, six classes of corticofugal neurons were identified by antidromic activation. Corticocallosal neurons of layer III were activated antidromically by stimulation of contralateral motor cortex. Layer V neurons were identified by antidromic activation from cerebral peduncle, red nucleus, lateral reticular nucleus of medulla, or spinal cord. Corticothalamic neurons were identified in layer VI. All the identified neurons were tested for input from primary somatosensory cortex. 2. Neurons of all corticofugal groups received excitatory inputs from primary somatosensory cortex. The shortest latency corticocortical effects of 1.2-2.5 ms were found for corticocallosal neurons of layer III, and for layer V neurons which projected axons through the cerebral peduncle, to red nucleus, and to spinal cord. 3. Nearby neurons, projecting to the same of different targets, were affected nonuniformly by corticocortical inputs. This finding supports the conclusion that specificity of afferent connections within cerebral cortex is not determined by anatomic segregation of cell bodies nor by projection target of efferent neurons. 4. These selectively distributed input connectivities suggest that even a small region of motor cortex could send different signals to its diverse targets.

The distribution of corticocortical, thalamocortical, and callosal inputs on identified motor cortex output neurons: mechanisms for their selective recruitment.

Motor cortex neurons were identified antidromically in anesthetized cats by their axonal projections to one of six targets: (1) somatosensory cortex, (2) opposite motor cortex, (3) red nucleus, (4) lateral reticular nucleus, (5) spinal cord, and (6) ventrolateral thalamus. Three inputs to motor cortex were tested for their influences on the identified cortical efferent neurons. The tested inputs originated from ipsilateral somatosensory cortex, opposite motor cortex, and ventral thalamus. Subthreshold effects of input pathways were detected by monitoring latency variations of antidromic responses. The three afferent sources, when activated by electrical stimulation, were not equally effective on motor cortex neurons. Ipsilateral corticocortical and thalamocortical excitation were found for the majority of neurons; the influenced proportions ranged from 55% to 100%, according to the target of the output neurons. Effects from the opposite hemisphere were found for only 5% to 30% of the neurons in the same projection classes. Many neurons (36 of 81, or 44%) were excited from more than one source, but few (5 of 37, or 14%) were influenced by all three possible sources of input, even in small regions of cortex innervated by all three of the inputs. Among 19 electrode tracks where all three inputs were present, there were only 2 tracks where all the neurons shared the same combination of inputs. Even for neurons in closest anatomical proximity ("clusters"), it was unusual (only 7 of 25 clusters) for all the neurons to have the same input pattern. Among the seven clusters where all the neurons shared the same input pattern, five of the clusters projected to the same target. These variable combinations of inputs to motor cortex neurons support the conclusion that efferent neurons could be recruited selectively from separate cortical layers or from within clusters of nearby neurons, according to the target of their axonal projection.

Segmental and supraspinal input to cells of origin of non-primary fibres in the feline dorsal columns.

1. The synaptic input to ascending tract cells with axons in the dorsal columns was investigated using intracellular recording. 2. E.p.s.p.s evoked by stimulation of the lateral funiculus were analysed to test for the possibility of collateral connexions between spino-cervical tract cells and dorsal column cells. Three groups of fibres were found to contribute to such e.p.s.p.s: fibres which terminated or originated between spinal segments C3-4 and C1, or Th9 and C3-4 and cortico-spinal tract fibres. The latencies and thresholds of e.p.s.p.s evoked by stimulation of the first group of fibres were compatible with their origin via axon collaterals of spino-cervical tract cells. The occurrence of these e.p.s.p.s in dorsal column cells which were disynaptically excited from cutaneous afferents further corroborated this possibility. 3. E.P.S.P.S of specifically cervical origin were also found in some other neurones in the dorsal horn, probably segmental interneurones, but were absent in spinocervical tract cells. 4. Convergence of group I muscle afferents (possibly both group Ia and group Ib) and cutaneous afferents was found in about 50% of the dorsal column cells. The shortest latency e.p.s.p.s from cutaneous and group I afferents were evoked with segmental delays indicating monosynaptic and disynaptic coupling. 5. I.p.s.p.s were evoked from cutaneous and group I muscle afferents in either the same or different nerves as those from which the e.p.s.p.s were elicited. Excitatory potentials were, however, dominating.

Intrinsic discharge patterns and somatosensory inputs for neurons in raccoon primary somatosensory cortex.

1. Discharge patterns of neurons are regulated by synaptic inputs and by intrinsic membrane properties such as their complement of ionic conductances. Discharge patterns evoked by synaptic inputs are often used to identify the source and modality of sensory input. However, the interpretation of these discharge patterns may be complicated if different neurons respond to the same synaptic input with a variety of discharge patterns due to differences in intrinsic membrane properties. The purposes of this study were 1) to investigate intrinsic discharge patterns of neurons in primary somatosensory cortex of raccoon in vivo and 2) to use somatosensory postsynaptic potentials evoked by stimulation of forepaw digits to determine thalamocortical connectivity for the same neurons. 2. Conventional intracellular recordings with sharp electrodes were made from 121 neurons in the cortical representation of glabrous skin of digit four (d4). Intracellular injection of identical current pulses (100-120 ms in duration) elicited various patterns of discharge in different neurons. Neurons were classified on the basis of these intrinsic patterns of discharge, rates of spike adaptation, and characteristics of spike waveforms. Three main groups were identified: regular spiking (RS) neurons, intrinsic bursting (IB) neurons, and fast spiking (FS) neurons. Subclasses were identified for the RS and IB groups. 3. Neurons were tested for somatosensory inputs by stimulating electrically d3, d4, and d5. Excitatory postsynaptic potentials (EPSPs) were elicited in 100% of the neurons by electrical stimulation of d4, the "on-focus" digit. EPSPs were usually followed by inhibitory postsynaptic potentials (IPSPs). Many neurons (41%) responded with EPSP-IPSP sequences after stimulation of d3 or d5, the "off-focus" digits. 4. Latencies of somatosensory EPSPs and IPSPs were used to determine the synaptic order in the cortical circuitry of RS, IB, and FS neurons. EPSPs with monosynaptic thalamocortical latencies were recorded in RS, IB, and FS neurons. 5. We conclude that precise patterns of neural discharge in primary somatosensory cortex cannot be reliable estimates of sensory inputs reaching these neurons because patterns of discharge are so strongly influenced by intrinsic membrane properties. Ionic conductances governing patterns of neuronal discharge seem almost identical in intact cortex of raccoon, rat, and cat, and in slices of rodent cortex, because similar patterns of discharge are found. The consistency of patterns of discharge across species and types of preparation suggests that these intrinsic membrane properties are a general property of cerebral cortical neurons and should be considered when evaluation sensory coding by these neurons.

Further study on the excitation of pyramidal tract cells by intracortical microstimulation.

The effective spread of stimulating current for pyramidal tract (PT) cells and fibers was studied using a method of cancelling the shock artifacts and the following results were obtained: 1. The excitability of PT axon collaterals was as high as that of PT cells. 2. These axon collaterals extended as far as 1.0 mm horizontally from the PT cells. 3. The low threshold area for activation of a given PT cell was as wide as 3--4 mm2 on the surface of the cortex. 4. Intracortical microstimulation (ICMS) delivered to the PT cell layer produced direct (D) and indirect (I) descending volleys in the pyramidal tract, but ICMS to the superficial layer (III) produced only I-waves. 5. These I-waves grew significantly larger after 15--20 msec from the start of the train of stimuli. 6. It is concluded that either surface stimulation, or short train of ICMS is inadequate for delineating fine localization of motor function within the cortex. Longer train (30--40 msec) with high frequency pulses (300--400 cy/sec) can produce muscle contraction with much smaller currents, increasing the accuracy of measuring the localization of motor function.

Convergence of sensory inputs upon projection neurons of somatosensory cortex.

Cortico-cortical neurons and pyramidal tract neurons of the cat were tested for convergent inputs from forelimb afferents. Neurons were recorded in cortical areas 1, 2, and 3a. Consideration was given to both suprathreshold and subthreshold inputs evoked by electrical stimulation of forelimb nerves. Individual cortico-cortical neurons and also pyramidal tract neurons were characterized by convergence of multiple somatosensory inputs from different regions of skin, from several muscle groups, and between group I deep afferents and low threshold cutaneous afferents. Certain patterns of afferent input varied with cytoarchitectonic area. There was, however, no difference between area 3a and areas 1-2 in the incidence of cross-modality convergence in the form of input from cutaneous and also deep nerves. Many of the inputs were subthreshold. Arguments are presented that these inputs, though subthreshold, must be considered for a role in cortical information processing. The convergent nature of the sensory inputs is discussed in relation to the proposed specificities of cortical columns. The patterns of afferent inputs reaching cortico-cortical neurons seem to be appropriate for them to have a role in the formation of sensory fields of motor cortex neurons. PT neurons of somatosensory cortex have possible roles as modifiers of ascending sensory systems, however, the convergent input which these PT neurons receive argues against a simple relationship between the modality of peripheral stimuli influencing them and the modality of the ascending tract neurons under their descending control.

Spinal branching of pyramidal tract neurons in the monkey.

The branching pattern of individual pyramidal tract (PT) neurons of the monkey motor cortex was studied by activating these neurons antidromically from within the cervical motor nuclei and also from other regions of the spinal cord. 1. Fifty-four neurons were activated from motor nuclei in the cervical cord. Twenty-eight of these were activated from one segment and six (11%) were activated from motor nuclei of different segments. The remaining 20 neurons were activated from motor nuclei and also from unspecified region(s) of the gray matter. 2. Another 156 neurons were activated from unspecified regions(s) of cervical gray matter which could have been motor nuclei or outside the nuclei, and 64 of these were activated from more than one segment. 3. The branching patterns of PT neurons sending axons directly to motor nuclei innervating distal forelimb muscles suggested that they branch less than the rest of PT neurons.

Projection from area 3a to the motor cortex by neurons activated from group I muscle afferents.

Two receiving areas in the pericruciate cortex are known for inputs from group I muscle afferents of forelimb nerves. One focus is near the postcruciate dimple of area 3a, and the other in the lateral sigmoid gyrus of the motor cortex (area 4gamma). The cortico-cortical projection of area 3a to 4gamma, and the relay by this projection of group I muscle afferent input to the motor cortex were investigated in cats. The following results were obtained. 1. Seventy-four neurons within area 3a were antidromically activated by intracortical microstimulation of the motor cortex. 2. Although excitation evoked by stimulation of group I muscle afferents could be demonstrated for only a few (8 of 48) cortico-cortical neurons in extracellular recordings, due to the methodological limitations discussed, this input evoked EPSPs in 8 of 9 cortico-cortical neurons recorded intracellularly. Therefore, it is likely that the majority of neurons projecting from area 3a to the motor cortex have an excitatory synaptic input from group I afferents. 3. Neurons projecting from area 3a to the motor cortex were most commonly found in cortical layer III, although some were found in layer V. 4. Five of nine pyramidal tract neurons of area 3a had a strong excitatory synaptic input from group I muscle afferents. 5. A new type of pyramidal tract neuron was found which has cortico-cortical axon collaterals connecting the two cytoarchitectonic regions. These various neurons may be part of a feedback system from muscle afferents to the motor cortex.

Crossed disynaptic inhibition of sacral motoneurones.

1. Intracellular recording was made from motoneurones in lower sacral (S2 and S3) segments of the spinal cord in cats, to analyse the neuronal organization of the inhibition evoked in these motoneurones from contralateral afferents. 2. It was confirmed that stimulation of the lowest threshold afferents of contralateral dorsal roots evokes i.p.s.p.s with latencies similar to those of disynaptic i.p.s.p.s. evoked from group Ia muscle spindle afferents in limb motoneurones. 3. The crossed disynaptic i.p.s.p.s in sacral motoneurones were found to be mediated by interneurones which are themselves inhibited by Renshaw cells, these interneurones and Renshaw cells being activated from the dorsal and ventral roots respectively, on the side of the body opposite to the location of the inhibited motoneurones. 4. In unanaesthetized decerebrate preparations crossed recurrent facilitation of sacral motoneurones was evoked with a time course similar to that of recurrent facilitation of lumbar motoneurones. It was taken to indicate a tonic inhibition of sacral motoneurones by interneurones responsible for their crossed disynaptic inhibition, and a disinhibition following stimulation of contralateral ventral roots. 5. In anaesthetized preparations crossed recurrent inhibition appeared, instead of the recurrent facilitation, in more than one half of the tested motoneurones. 6. A comparison of the input from ipsilateral and contralateral afferents to identified motoneurones of tail muscles with the input to pudendal motoneurones led to the conclusion that crossed disynaptic inhibition is evoked specifically in tail motoneurones. 7. Intracellular staining of sacral motoneurones with horseradish peroxidase revealed that the tail motoneurones and others with crossed disynaptic inhibition differ from the pudendal motoneurones in their location and in a number of morphological features; tail motoneurones are larger, they have differently directed dendrites and they show more extensively branched initial axon collaterals which appeared to ramify only within the ventral and lateral parts of the ipsilateral ventral horn. 8. One Renshaw cell which was stained with horseradish peroxidase was found to project contralaterally, after giving a number of axon collaterals ipsilaterally.

Convergence of sensory inputs in somatosensory cortex: interactions from separate afferent sources.

Intracellular recording techniques were used to test for cross-modality and topographic convergence among inputs to area 3a of cerebral cortex. Recordings were made within the projection area of group I afferent fibers of the deep radial nerve in barbiturate-anesthetized cats. Epsps were evoked in 90% of neurons (81/90) by electrical stimulation of more than one nerve of the contralateral forelimb. The deep radial nerve evoked the shortest latency epsps within this region of cortex and the only ones likely to be mediated by a monosynaptic thalamocortical pathway. However, the epsps evoked from other forelimb nerves (of deep or cutaneous origin) had mean latencies only a few milliseconds (1.3-3.0 ms) longer. Furthermore, there were a variety of interactions among inputs from separate afferent sources. The observed interactions included spatial facilitation, occlusion and afferent inhibition. The consequence of these interactions was that neuronal responses were shaped by combinations of effects from different topographic regions of the forelimb or of different modalities. The findings are interpreted as indicating a sharing of neurons among pathways to cortical neurons from separate afferent sources. Interactions between ascending pathways by way of such shared neurons may contribute to the modulation or plasticity of somatosensory responsiveness during behavior or after deafferentation.

Multiple inputs to a population of thalamocortical neurons projecting to cat somatosensory cortex.

Ninety thalamocortical (TC) neurons were recorded extracellularly in ventrobasal thalamus of halothane-anesthetized cats. Projections of all of these neurons to specific subdivisions of somatosensory cortex were identified by their antidromic invasion following intracortical microstimulation restricted to these subdivisions. Collision-extinction tests were used to document excitatory inputs to TC neurons from afferent fibers of forelimb nerves stimulated electrically. Thirty-nine TC neurons (43% of sample) were excited from at least one forelimb nerve. Fifteen TC neurons were activated from two or more forelimb nerves. Combinations of effective nerves included ones innervating topographically different regions of forelimb. Neurons projecting to area 1-2 were least likely to be activated from more than one nerve. Seven TC neurons activated by electrical stimulation of nerve trunks were tested also with two distinct forms of mechanical somatic stimuli, i.e., hair bending and vibration, and with intradermal electrical stimuli. These tests revealed convergent inputs from hairy and glabrous skin. We conclude that there is a population of neurons, located in ventrobasal thalamus, which is capable of conveying multiple inputs to each of the subdivisions of primary somatosensory cortex. These neurons could be involved in forming properties of feature-extracting neurons of somatosensory cortex.

Electrophysiological study of corticocaudate projections in cats.

The potential waves evoked in the caudate nucleus (CN) of cats by stimulation of the cerebral neocortex were sterotactically recorded. The head and the body of the caudate nucleus were systematically explored. Stimulation of the ipsilateral sigmoid gyrus and the orbitofrontal cortex evoked waves with the largest amplitude in the CN. Smaller potentials were evoked from the ipsilateral ectosylvian and suprasylvian gyri and from the sigmoid gyrus on the contralateral side. Antidromic conduction from the caudate nucleus to the cortex demonstrated the directness of the corticocaudate pathway. By stimulating the white matter and by amking lesions, the corticocaudate pathway was shown to pass, in part, through the subcallosal fasciculus and, in part, through the internal capsule. Corticocaudate connections were shown to be separate from the fibers of the corticospinal tract. A staggered and extensively overlapping topographic progression of the corticocaudate projections was demonstrable along the antero-posterior axis, but was less evident in the medio-lateral direction. It was concluded that the intranuclear distribution of functional synaptic connections must be more profusely branched than was suspected from anatomical data.

Responses of cat motor cortex neurons to cortico-cortical and somatosensory inputs.

Intracellular techniques were used to investigate a cortico-cortical path from sensory cortex to motor cortex of cats. Cortico-cortical epsps were evoked in motor cortex neurons by microstimulation of area 3a. Epsps with latencies between 1.2 and 2.4 ms were identified as monosynaptic. These short latency cortico-cortical effects were recorded in layers II through VI of the motor cortex. Neurons with monosynaptic cortico-cortical epsps also received excitatory inputs from forelimb nerves, usually from both muscle and cutaneous afferent fibers. The epsps evoked from forelimb nerves in motor cortex neurons were preceded by neural activity in somatosensory cortex. Time delays between arrival of inputs in sensory cortex and in motor cortex were compared to the latencies of cortico-cortical epsps in the same motor cortex neurons. It was apparent that the timing was appropriate for the identified cortico-cortical path to have relayed some sensory inputs to motor cortex.