Electrical thresholds of unimpaled corticospinal cells in the cat

The Cortical Motor System in the Domestic Pig: Origin and Termination of the Corticospinal Tract and Cortico-Brainstem Projections

The anatomy of the cortical motor system and its relationship to motor repertoire in artiodactyls is for the most part unknown. We studied the origin and termination of the corticospinal tract (CST) and cortico-brainstem projections in domestic pigs. Pyramidal neurons were retrogradely labeled by injecting aminostilbamidine in the spinal segment C1. After identifying the dual origin of the porcine CST in the primary motor cortex (M1) and premotor cortex (PM), the axons descending from those regions to the spinal cord and brainstem were anterogradely labeled by unilateral injections of dextran alexa-594 in M1 and dextran alexa-488 in PM. Numerous corticospinal projections from M1 and PM were detected up to T6 spinal segment and showed a similar pattern of decussation and distribution in the white matter funiculi and the gray matter laminae. They terminated mostly on dendrites of the lateral intermediate laminae and the internal basilar nucleus, and some innervated the ventromedial laminae, but were essentially absent in lateral laminae IX. Corticofugal axons terminated predominantly ipsilaterally in the midbrain and bilaterally in the medulla oblongata. Most corticorubral projections arose from M1, whereas the mesencephalic reticular formation, superior colliculus, lateral reticular nucleus, gigantocellular reticular nucleus, and raphe received abundant axonal contacts from both M1 and PM. Our data suggest that the porcine cortical motor system has some common features with that of primates and humans and may control posture and movement through parallel motor descending pathways. However, less cortical regions project to the spinal cord in pigs, and the CST neither seems to reach the lumbar enlargement nor to have a significant direct innervation of cervical, foreleg motoneurons.

The distribution of monosynaptic excitation from the pyramidal tract and from primary spindle afferents to motoneurones of the baboon's hand and forearm

1. Intracellular records were obtained from motoneurones innervating muscles of the baboon's forearm and hand. Monosynaptic excitatory postsynaptic potentials (EPSPs) were elicited by stimulation of motor cortex (CM EPSPs) and peripheral nerves (Ia EPSPs).2. CM EPSPs were larger on average in motoneurones innervating intrinsic hand muscles and extensor digitorum communis (EDC) than in neurones of other forearm muscles.3. Among motoneurones of the median nerve, the CM EPSP tended to be larger for cells with more rapidly conducting axons than for those with more slowly conducting axons. Among motoneurones of EDC the opposite tendency was found.4. The afferent fibres responsible for the Ia EPSP nearly always had a lower stimulus threshold than that of motor axons in the same nerve. Some observations were made concerning the distribution of heteronymous Ia EPSPs.5. Among motoneurones of a given nerve, those with large Ia EPSPs tended to receive larger CM EPSPs than did cells in which the Ia EPSP was small.6. The results are discussed in relation to problems concerning the pyramidal control of hand and finger movement.

Epicortical electrical mapping of motor areas in primates

The neocortical motor areas of monkeys were discovered by epicortical electrical stimulation. Ferrier in 1875 drew non-overlapping circles for face, limbs and tail. His 'centres' were postcentral as well as precentral. In the 1880s Horsley added an arm and face area on the medical surface (where Penfield's supplementary motor area is now ensconced). At the turn of the century the newborn science of cortical architectonics discovered striking differences between precentral and postcentral areas. Sherrington got no responses from the postcentral gyrus of apes. At mid-century, however, Woolsey reinstated the postcentral motor map (his Sm 1). The discrepancies between these classical maps could probably be explained by sensitivity to levels of anaesthesia and to arbitrarily chosen configurations of stimuli whose intracortical actions remained obscure. Some of the modes of action of epicortical stimulation have since been worked out. These are relevant to the results of electrical and magnetic stimulation of the human brain through scalp and skull. Today's research is differentiating the functions of the precentral and postcentral areas (4, 6PM, 6SMA, 3a, 3b, 1): at the microscopic level, by studies of sampled neurons whose discharges can be related to specific components of learnt motor performances, and whose connectivities can be traced by electroanatomical and microscopical labelling of their perikarya, axons and synapses; at the macroscopic level, by studies of Bereitschaftspotential and regional cerebral metabolism in motor performances in intact humans.

Responses in the inferior olive to stimulation of the cerebellar and cerebral cortices in the cat

1. Extracellular field potentials and single unit responses have been recorded from the inferior olive of the cat following stimulation of the surface of the contralateral paramedian lobule of the cerebellum, and of the ipsilateral cerebral cortex. 2. Cerebellar stimulation results in antidromic invasion of inferior olivary neurones via the climbing fibres. These responses are followed by synaptic discharges which may be generated through climbing fibre recurrent collaterals. 3. Precise histological controls have shown that these responses to stimulation of the paramedian lobule are located in the ventral lamella of the principal olive. 4. Unifocal stimulation of the sensori-motor cortex with surface-anodal pulses evokes synaptically generated discharges of neurones in the central lamella, with a latency of 8-9 msec. The area of cortex yielding responses has been mapped at chosen stimulus intensities and the limitations of the maps have been discussed. 5. It has been shown that the initial excitatory responses obtained from either cortex are followed by an inhibition which lasts about 100 msec, and gives way to a period of recovery or facilitation. This, in turn, is succeeded by a further period of inhibition. Possible neural substrates for these changes have been discussed.

Motor outflow to cervical motoneurons from raccoon motorsensory cortex

Motor outflow from forelimb motorsensory cortex (MsI) to forelimb muscle motoneurons in raccoon has been investigated using three approaches: 1) determination of latencies for cortically evoked efferent discharge in forelimb nerves; 2) determination of latencies for cortical facilitation of forelimb monosynaptic reflexes; and 3) intracellular recording of cortically evoked synaptic potentials. All three approaches indicated a major polysynaptic pathway (minimally disynaptic) for corticofugal facilitation or inhibition of cervical motoneurons. Suggestive evidence for a monosynaptic connection between forelimb MsI and cervical motoneurons was found for only one motoneurons. Nevertheless, the motor pathway between MsI and cervical motoneurons was shown to be more efficacious (defined on the basis of central delays) than in the cat under similar experimental conditions. The results are discussed in terms of organization changes in forelimb MsI which appear to be related to the extent to which certain mammals use their forelimbs for manipulating and exploring objects.

Reevaluation of motor cortex and of sensorimotor overlap in cerebral cortex of albino rats

The organization of motor cortex and the sensorimotor overlap zone was examined by in-depth electrical stimulation using micromapping procedures in rats. The cutaneous somatic sensory, as well as the efferent motor projections to the hindlimb and forelimb sensorimotor overlap zone were studied in the same animals. Low-threshold movements were elicited from portions of 3 architectonic areas: the lateral agranular, dysgranular and granular areas. Cutaneous light touch projections occur only within the granular area. Cutaneous projections to, and motor projections from individual punctures in the granular overlap zone did not always involve homologous body parts. The total motor cortex exhibits a general musculotopic pattern of organization.

Excitation and inhibition of marginal layer and interstitial interneurons in cat nucleus caudalis by mechanical stimuli

In cats anesthetized with urethane, the caudal medulla was stabilized in preparation for intracellular recording from interstitial neurons in the descending tract of the trigeminal nerve and from neurons in lamina I of nucleus caudalis. Glass micropipets (10-50 M ohms) were advanced from the surface to a maximum depth of 350 micrometer. When DC potential shifts occurred, it was found that mechanical stimuli to the face generated bursts of positive-going spikes, followed in some cases by inhibitory postsynaptic potentials (IPSPs). Subdermal electrical stimulation of the face in each receptive field almost always enabled the same neuron to be driven electrically. Recordings were classified as from primary afferent fibers or from interneurons. Primary fibers had a purely positive spike, with a latency varying by no more than 0.05 msec, and could follow stimulation at 500 Hz. The mean latency for the fibers was 1.87 +/- 0.06 msec (n=75), and their absolute refractory period was 0.42 +/- 0.02 msec (n=36). Recordings were classed as from interneurons if there was an IPSP or the latency was at least 4 msec, with a variation of latency of at lest 0.5 msec. Responses thought to be monosynaptically driven had a mean latency of 2.09 +/0 0.07 msec (n=32) and could follow pairs of stimuli at a mean minimum interval of 0.70 +/- 0.06 msec (n=20). Responses thought to be polysynaptically driven had a mean latency of 7.9 +/- 1.08 msec (n=49) and a mean interstimulus interval of 2.96 +/- 0.84 (n=20). Most responses were generated by brushing the face (n=87), some by pressure on the face (n=25), and a few by pinching the skin of the face (n=6). Interneuron responses were most commonly recorded in the first 200 micrometer of the descending tract, and this position was confirmed by the injection of pontamine sky blue and the examination of frozen sections. The recordings were thought to be from dendrites of marginal and interstitial cells or the somas of interstitial cells. The IPSPs which followed spike potentials could only follow stimuli at 10 Hz or less. The failure appeared to be at a primary afferent synapse upon an inhibitory interneuron. Collision tests between mechanically evoked and electrically evoked responses showed long-lasting inhibition of the response to electrical stimulation after collision. Presynaptic inhibition exerted on the primary afferent excitation was suggested as the explanation.

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.

Lack of involvement of fusimotor activation in movements of the foot produced by electrical stimulation of monkey cerebral cortex

1. Contractions of the small muscles of the foot producing flexion and adduction of the hallux were elicited by brief trains of electrical stimulation of a motor point on the precentral gyrus of anaesthetized monkeys and these contractions were recorded myographically.2. The cortical stimulus intensities necessary to produce minimal muscle contractions were measured for different frequencies of stimulation at the cortical point, and the latency of the minimal muscle contraction was measured in each case.3. Section of all the relevant lumbar and sacral dorsal roots had no effect on the threshold stimulus currents necessary to produce minimal contractions or on the latencies of these responses. Hence, in the anaesthetized monkey, the power of the cortico-fusimotor activity stirred up by electrical stimulation of the cortex is inadequate to influence significantly the motor responses of the most accessible muscles.

Early facilitation at corticomotoneuronal synapses

1. Corticomotoneuronal EPSPs have been generated in lumbar motoneurones of the monkey by single and paired corticospinal volleys. The facilitation of the second of a pair of EPSPs with respect to the size of the first has been measured.2. The relationship between the degree of facilitation of the second response and the interval between the two volleys has been studied. Average facilitation of minimal EPSPs was found to be maximal about 2 msec after the arrival of the corticospinal volley and to decay roughly exponentially thereafter with a time constant of about 10 msec.3. The degree of facilitation varied from one minimal corticomotoneuronal EPSP to another but this facilitation was not statistically correlated with the time course of the individual EPSPs.4. Significant facilitation (0.4) was still present 10 msec after a corticospinal volley so that this phenomenon could play a part in the initiation of motoneuronal discharge by corticospinal activity at natural frequencies of the order of 100 impulses/sec.

Projection to cerebral cortex of group I muscle afferents from the cat's hind limb

1. Muscle afferent projections from the contralateral hind limb to the postsigmoid gyrus of the cerebral cortex were investigated in cats anaesthetized with chloralose. The evoked potentials were recorded from the cortical surface or from deeper layers by penetrating micro-electrodes. Graded electrical stimulation of the nerves was used.2. Group Ia as well as Ib muscle afferents from the contralateral quadriceps, posterior biceps-semitendinosus, gastrocnemius-soleus and deep peroneal muscles projected to two different loci in the postsigmoid gyrus. One of these was located on the dorsal surface of the hemisphere 4-5 mm lateral to the mid line and 1-3 mm posterior to the cruciate sulcus, thus rostro-medial to the postcruciate dimple. The other was located on the medial surface of the hemisphere adjacent to the cruciate sulcus. There was no overlap between the two loci.3. There was no significant difference in thresholds or latencies of the Group I responses in the two loci. The latency was short and similar to that of the potential evoked by the cutaneous afferents in the somatosensory primary projection areas.4. The Group Ib path was largely independent of the Ia path, because a maximal Group I volley evoked a response, when the Ia path was made refractory by simultaneous stimulation with a maximal Ia volley at 20 per second.5. The cortical potentials evoked by the Group I muscle afferents from the contralateral hind limb did not change after transection of the dorsal columns at C1-C3 levels but disappeared after a superficial section in the dorsolateral fascicle at C1 level. The responses were not affected by cerebellectomy. It was concluded that the path travelled with the dorsal spinocerebellar tract or utilized brain stem collaterals of this tract.6. Group II muscle afferents evoked a response near the border of the Group I loci, but not in the positions where the Group I responses were maximal in amplitude.7. The receptor origin of the stimulated Group I afferents, the location of the medullary relay in the Group I path and the destination of the efferent outflow from the Group I loci were discussed.

Thresholds of cortical activation of muslce spindles and alpha motoneurones of the baboon's hand

1. Much current thinking about voluntary movement assumes that the segmental gamma loops can function as a servomechanism operated by the brain. However, the alpha motoneurones of the baboon's hand receive a powerful monosynaptic (CM) projection from the precentral gyrus. If servo-driving from the same cortical area is to be possible, it must project independently to the fusimotor neurones and have sufficient power to increase the afferent signalling from the muscle spindles. The cortical thresholds for contraction of m. extensor digitorum communis and for acceleration of the discharges of its muscle spindles have therefore been compared.2. Significant results in this context require that the spindles studied be coupled in parallel with the responding extrafusal muscle fibres. Many spindles were not unloaded by the submaximal contractions evoked by cortical stimulation, although all so tested were unloaded by maximal motor nerve twitches. Reasons are given for thinking that such apparent lack of parallel coupling is an artifact of complex intramuscular anatomy and limitation of shortening by ;isometric' myography.3. A brief burst of corticospinal volleys at 500/sec, which is specially effective in exciting alpha motoneurones over the CM projection, failed to excite spindle afferents at or below the threshold for a cortical ;twitch'.4. In a few epileptiform discharges, bursts of spindle acceleration occurred independently of the clonic contractions. A relatively direct and independent cortico-fusimotor (CF) projection may therefore exist.5. Prolonged near-threshold stimulation at 50-100/sec, which allows time for temporal summation in the less direct projections (e.g. cortico-interneuronal, cortico-rubro-spinal) and does not cause frequency-potentiation at CM synapses, gives abundant evidence of independent alpha and fusimotor projections, whose actions hardly outlast the stimulation period.6. Although independent CF projections would permit servo-driving in natural movements of the hand (given adequate loop gain), there has been no evidence of servo-driving by cortical stimulation or in the spontaneous contractions of light anaesthesia.7. Independent projections would provide for controlled alphagamma co-excitation in the servo-governing of natural movements (Matthews, 1964).8. Evidence is reviewed that the CM projection itself may be part of an important control loop for voluntary movement in primates. A corollary would be a diminished importance of CF projections for segmental loops and an increased importance for maintaining the spindle input to cortical loops.

Types of unitary response and correlation with the field potential profile during activation of the avian optic tectum

1. Unitary responses were recorded in the lateral tectum of the pigeon, with electrolyte-filled micropipettes after electrical stimulation of the optic nerve-head.2. Optic nerve fibre spikes could be recognized by their conformation, fixed latency, brief recovery times, and location in the superficial tectum. Their action potentials were either triphasic with a prominent second phase, or monophasic positive.3. The optic nerve consists of small myelinated fibres conducting at 5.3-8.0 m/sec. These axons probably have diameters in the order of 1.6-2.2 mu.4. The fibre spikes were localized to the N-zone and R-zone. None was recorded deeper. Most of the fibre spikes preceded the tectal N-wave.5. One hundred and fifty-six post-synaptically fired cells were recorded. These had a diphasic positive-negative conformation, and were fired at variable latency.6. One hundred and forty of these cells fired a single spike to each stimulus to the optic nerve-head. Even the most stably fired cells could be proved to be trans-synaptically activated by the evidence of non-collision.7. Sixteen of the 156 cells fired repetitively to single stimuli to the optic nerve-head.8. Evidence could be obtained that afferent inhibition operates upon tectal cells.9. Cells in the N-zone were fired earliest in the 3 msec interval, corresponding to the rising phase of the tectal N-wave. By comparison, cells in the P-zone were not fired in the 3 msec interval, and the proportion fired in the 4 msec interval was reduced. Cell firing in the P-zone must be produced by tectal interneurones.10. Cells were present in the N-zone with recovery times below 5 msec. No cells in the P-zone had recovery times below 5 msec.11. A clear correlation could be made between the distribution of fibre and cell spikes, and the field potential profile. A correlation could also be made between the timing and recovery time of cells in the N-zone and P-zone. The unitary records show that the tectum is activated radially by the retinotectal pathway.

Cortical actions on hypoglossal motoneurones in cats: a proposed role for a common internuncial cell

1. Intracellular records were made of the synaptic potentials produced in hypoglossal motoneurones when electrical stimuli were applied to the cat's motor cortex.2. The depolarizing synaptic responses reached their maxima in steps similar to those produced in the same motoneurones by suprathreshold stimulation of the lingual nerve.3. Interneurones in or near the spinal trigeminal nucleus were caused to discharge by cortical shocks adequate to produce synaptic potentials in hypoglossal motoneurones. The latencies and patterns of repetitive responses in these interneurones were consistent with their possible role as internuncial cells in the motor pathway from cerebral cortex to hypoglossal motoneurones.4. Individual interneurones in or near the spinal trigeminal nucleus could be caused to discharge both by stimulation of the lingual nerve and by stimulation of the motor cortex. This convergence suggests a common function for the internuncial cell in reflexly and cortically induced excitation of hypoglossal motoneurones and would allow the internuncial cell to be a site for integration of spatially separated excitatory influences.

Responses of the pyramidal tract to stimulation of the baboon's motor cortex

1. The arm area of the baboon's precentral motor cortex was stimulated by brief surface-anodal pulses, and the discharge of the corticospinal tract (the ;pyramidal tract waves') was recorded by an electrode resting on the dorsolateral surface of the cervical spinal cord.2. Some properties of the pyramidal tract waves were described, and they were also studied in relation to the firing of single cortico spinal fibres.3. The results led to the conclusion that the later pyramidal tract waves (the ;I waves') were almost exclusively due to a semi-synchronous repetitive discharge of the same fast cortico spinal fibres as those responsible for the initial wave (the ;D wave').4. Some problems concerning the origin and significance of the I waves were discussed.