Marco Schieppati

Interhemispheric transfer of voluntary motor commands in man

The lack of callosal fibres between homotopic areas of the hand in the sensorimotor cortex in man and its possible functional correlates were investigated in normal adult subjects by comparing simple reaction times (RTs) of voluntary movements triggered by a somaesthetic stimulus to the same or opposite side of the body. An air jet was delivered to the skin of distal (index finger) or proximal (shoulder) ipsi- or contralateral zones, and triggered voluntary extension of the index finger or flexion of the forearm. The RT was measured from the arrival of the stimulus to the skin to the onset of the surface EMG of the muscle extensor indicis proprius or biceps brachii. The RTs of the contralateral finger movements triggered by either proximal or distal skin stimuli were significantly longer than the RTs of the corresponding ipsilateral movements (mean difference 11.72 and 15.10 msec respectively). When the task was flexion of the forearm, the differences in RTs between contra- and ipsilateral movements were instead compatible with a transcallosal transfer (mean difference about 2 msec in both cases). It is concluded that transcallosal connections between hand sensorimotor areas are conceivably absent also in man. Furthermore, the delay in contralateral distal performance appears to be due to a lack of transfer of the command through the motor areas, rather than to a lack of transfer of the triggering cutaneous afferent information to the performing hemisphere.

Effects of stimulus intensity, cervical cord tractotomies and cerebellectomy on somatosensory evoked potentials from skin and muscle afferents of cat hind limb

The somatosensory evoked potentials (SEPs) recorded from the sensory cortex were investigated by using graded stimulation of skin and muscle nerves from contralateral hind limb in the cat. Sections were made of the middle cervical cord to assess the pathways involved in mediating SEPs evoked by large and small diameter fibers. Dorsal column (DC) section caused a decrease of SEPs from skin group I afferents, and a small increase in those from group I muscle afferents. A subsequent section of dorso-lateral fasciculus (DLF) further decreased SEPs from skin and eliminated SEPs from muscle, evoked at low stimulus intensity. When the stimulus recruited group III fibres, SEPs were still present after DC and DLF section, both from skin and muscle nerves. Section of ALT in addition to DC confirmed a major role played by DLF (mainly spino-cervical tract of Morin) in transmitting impulses from muscle afferents; the role of DLF in mediating potentials evoked from skin is less remarkable than that of DC. Cerebellectomy did not change any SEP, however evoked. Previous results in the literature are discussed, taking into account the methodologies employed by various authors, and the possible interactions among pathways mediating SEPs.

Basal forebrain and hypothalamic influences upon brain stem neurons

Intracellular recordings were performed from bulbar and caudopontine (BcP), rostropontine (rP) and mesencephalic (Mes) neurons in acute encéphale isolé preparations during low- and high-frequency basal forebrain (BF) and hypothalamic (Hyp) stimulation. Low-frequency BF stimulation induced long-latency (7-8 msec) EPSPs on 31% of BcP neurons. High-frequency stimulation of the same regions induced excitation in 58% of them. No inhibition was observed. The descending influences on BcP neurons disappeared after bilateral acute sensorimotor decortication. Low- and high-frequency BF stimulation influenced a very low percentage of rP and Mes neurons. The only effect was a long-latency slow rising EPSP with or without spikes. No inhibition was found in these neurons. Low- and high-frequency Hyp stimulation produced short-latency EPSPs with or without spikes on a large percentage of BcP, rP and Mes neurons. The effect was more constant on rostral (rP) than on caudal (BcP) neurons and more evident when high-frequency stimulation was applied. At high frequency the activation of brain stem neurons paralleled an EEG desynchronization. IPSPs were seen in a low percentage of Mes and rP neurons. Mes neurons could be also activated antidromically. The results suggest that BF regions do not antagonize directly the mesencephalic activating system and may induce electrocortical synchronization through the activation of caudal brain stem neurons. This circuit involves sensorimotor cortical neurons. The facilitatory effect of Hyp stimulation on rostral brain stem neurons indicates that the EEG desynchronization which follows high-frequency Hyp activation may be mediated by the ascending reticular system. Other mechanisms mainly at the diencephalo-telecephalic level may however be involved in this effect.

Long-latency, nonreciprocal reflex responses of antagonistic hind limb muscles after cutaneous nerve stimulation in the cat

Abstract The time course of changes in monosynaptic reflex amplitude, after conditioning from both ipsi- and contralateral sural nerves at different stimulus strengths, was studied on two antagonistic motoneuronal pools acting on ankle muscles in spinal cats. Attention was focused on late effects, namely those appearing after a dely of more than 30 ms from the cutaneous stimulus. With low-threshold afferent activation, at conditioning-test intervals to 30 ms, the ipsilateral extensor monosynaptic reflex, recorded from the proximal stump of L7 to S1 ventral roots, showed marked inhibition; at longer intervals, a late facilitation period (LFP) lasting to 100 ms was observed. Increasing stimulus strength did not modify the time course of reflex excitability, but might enhance the amount of the facilitatory effecct. On the flexor monosynaptic reflex, sural conditioning induced, after the expected early facilitation, a second facilitatory period, starting at about 30 ms and recovering at about 130 ms. The excitability of antagonistic contralateral motoneuronal pools was also influenced, showing again a LFP with the same time course. The LFP was present after stimulation of the sural and saphenous nerves and was absent after stimulation of a muscle nerve. These late, long-lasting and nonreciprocal facilitatory effects on flexor and extensor ipsilateral motoneurons were quite distinct from the early reciprocal responses, and were evoked by large cutaneous fibers. An interpretation is put forward in light of the primary afferent hyperpolarization of Ia afferent terminals. A correlation is tentatively proposed with the mechanism subserving the stumbling corrective reaction.

Influences of locus ceruleus, raphe dorsalis, and periaqueductal gray matter on somatosensory-recipient thalamic nuclei

Spontaneous and evoked discharge of neurons in the nucleus ventralis posterolateralis (VPL) and spontaneous discharge of neurons in the posterior group and nucleus lateralis posterior (LP) were conditioned by brief trains of stimuli to the locus ceruleus (LC), raphe dorsalis (RD), and periaqueductal gray matter (PAG) in cats anesthetized with pentobarbital or ketamine. Stimulation of LC and RD was without effect on VPL neurons, but induced a long-latency, long-lasting inhibition of LP neurons. Stimulation of the PAG induced marked inhibition of the firing of neurons in all three thalamic nuclei. No differences were found between cats anesthetized with ketamine or pentobarbital.

Hypothalamic and amygdaloid influences upon sensorimotor cortical neurons

Basal forebrain (BF) stimulation induces in intact brain EEG synchronization 2°, behavioural inhibition and sleep 21, while posterior hypothalamus (HYP) yields EEG desynchronization and arousal 1,11,17,18. Also, stimulation of the amygdaloid nuclei elicits in unanaesthetized animals complex behavioural reactions 7, accompanied by EEG synchronization and desynchronization4, 5. Several considerations10,15 (see also refs. 2, 13 and 16) indicate that these subcortical effects on the EEG are mediated by the brain stem reticular system. However, tonic EEG desynchronization is still elicitable by anterior and posterior hypothalamic stimulation after complete acute and chronic transection of the brain stem 1, and stimulation of dorsal amygdala is able to induce, in the same preparation, desynchronization of ipsilateral hemisphere 1. The aim of this investigation has been to study any postsynaptic changes in sensorimotor cortical neurons during hypothalamic and amygdaloid stimulation. The HRP technique has in fact revealed the existence of direct connections ascending from various hypothalamic regions3, s and amygdaloid nuclei lz to sensorimotor cortical neurons in cats. The experiments were carried out in 34 acute enc6phale isol6 cats curarized, artificially ventilated and with bilateral pneumothorax. The operation was carried out under ether anaesthesia. The surgical and pressure points were then locally anaesthetized with Novocaine. Stimulation was performed with bipolar electrodes stereotaxically oriented and implanted in basal forebrain regions (A 13.5-14; L 2.5; H --3, --4), posterior hypothalamus (A 8.5; L 2-3; H--3) , basal (A 13; L 9; H-7) and dorsal (A 12; L 8; H --3) amygdala. Low (7-10/sec) and high (70-200/sec) frequency stimulation was used (0.1-0.5 msec duration, 80-120 #A intensity). Identification of PT and non-PT neurons was made through antidromic activation by stimulation of bulbar pyramids 19.

Effects of deep barbiturate coma on acute spinal cord injury in the cat

The effects of barbiturate administration on experimental balloon-induced spinal cord injury were tested in cats. Somatosensory evoked potentials from sciatic nerve stimulation were obtained before trauma and every 60 minutes after it up to the sixth hour, when the animals were killed. Eight cats received no barbiturate treatment. On histologic examination the traumatic lesion was found to be extensive (mean, 72.8% of total cross section of the cord area), sparing dorsal columns only in six cats. Somatosensory evoked potentials were absent in two cats and profoundly modified (that is, the late waves were absent) in six cats at the sixth hour. Eight cats were given a continuous infusion for 1 hour of intravenous thiopental sodium (total dose, 65-90 mg/kg) starting 30 minutes after trauma. In these eight cats, the extent of the traumatic lesion was significantly reduced (8.8% of the cord area). Among them, three animals presented with unaltered somatosensory evoked potentials (that is, with the presence of both primary components and late waves) at the sixth hour. It was concluded that thiopental sodium improves the response of the spinal cord to trauma, both at an anatomic and at a functional level.

Effects of thoracic dorsal rhizotomy or vagotomy on inspiratory muscle activity at various levels of chemical drive.

The relationship between relative peak activity (moving average EMG) of the diaphragm (Adi) and of the cranial (2nd and 3rd) external intercostal or parasternal muscles (Aic) was assessed during rebreathing in animals before and after bilateral thoracic (T1-T4) dorsal rhizotomy (TDR) and/or bilateral vagotomy (VGT). The relationship had the form Aic=a Adib under all conditions. In intact rabbits and cats mean values for b were 1.48 and 1.79, respectively, a being unity by definition. Neither TDR nor VGT changed b; a decreased to about 0.15 with TDR and halved with VGT only if performed before TDR. Selective reflex facilitation of inspiratory intercostals with occlusions at FRC was observed after VGT and was abolished by TDR. Neither VGT nor TDR affected Adi time course. Hence: (1) central command to alpha-motoneurones of the major inspiratory muscles differs; (2) proprioceptive feedback markedly increases external intercostal activity, apparently by multiplying Aic due to central command to alpha-motoneurones by a factor independent of chemical drive; (3) vagally mediated augmentation of Aic depends entirely on intact proprioceptive feedback. The possible role of fusimotor drive is discussed.

Mesencephalic and bulbar reticular formation influences on somatosensory transmission through the thalamus

The influence of the brain stem reticular formation (RF) on transfer of somatosensory information has been studied in intact cats and in cats with a chronic hemisection of the brain stem at the pretrigeminal level. An air-jet applied to the hairy skin receptive field evoked the discharge of thalamocortical relay cells in nucleus ventralis postero-lateralis, extracellularly recorded. Conditioning stimuli were brief trains of electrical pulses to mesencephalic (MRF) and bulbar (BRF) reticular formation. In intact cats both MRF and BRF induced in a small percentage of cells slight facilitation or inhibition of evoked discharge. In pretrigeminal cats MRF stimulation increased the probability of discharge in response to peripheral stimuli, whereas BRF stimulation induced a striking decrease in evoked firing in a great percentage of neurones. It is suggested that RF activation can decrease the incoming peripheral volley by means of its caudalmost part, while it is able to enhance thalamic response by way of the more rostral structures.

Changes in the pause in muscle spindle discharge during a sequence of twitches.

Abstract A study was made of the change in duration of the pauses in discharge from spindle afferents occurring during a series of isometric twitches of the cats triceps surae muscle. These were induced by stimulation of the muscle nerve at frequencies of 0.1 to 1.0 Hz. At frequencies of 0.2 Hz and higher, the duration of the pause exhibited by group II and tonic and phasic group Ia afferents decreased progressively until stabilizing within 5 to 15 s. The time course of this change was largely independent of the frequency of stimulation, but the eventual duration in the steady state was strongly dependent on the frequency, being shorter at higher rates. At a given frequency, the pause was shorter at longer muscle lengths. The decrease in pause duration was not accompanied by a parallel change in duration or amplitude of the muscle tension responses, indicating that it arose from some alteration intrinsic to the spindle organ. We suggest that the progressive change in sensory discharge could serve the initial reinforcement of repetitive movements.