Bror Alstermark

Genetic dissection of the circuit for hand dexterity in primates

It is generally accepted that the direct connection from the motor cortex to spinal motor neurons is responsible for dexterous hand movements in primates. However, the role of the ‘phylogenetically older’ indirect pathways from the motor cortex to motor neurons, mediated by spinal interneurons, remains elusive. Here we used a novel double-infection technique to interrupt the transmission through the propriospinal neurons (PNs), which act as a relay of the indirect pathway in macaque monkeys (Macaca fuscata and Macaca mulatta). The PNs were double infected by injection of a highly efficient retrograde gene-transfer vector into their target area and subsequent injection of adeno-associated viral vector at the location of cell somata. This method enabled reversible expression of green fluorescent protein (GFP)-tagged tetanus neurotoxin, thereby permitting the selective and temporal blockade of the motor cortex–PN–motor neuron pathway. This treatment impaired reach and grasp movements, revealing a critical role for the PN-mediated pathway in the control of hand dexterity. Anti-GFP immunohistochemistry visualized the cell bodies and axonal trajectories of the blocked PNs, which confirmed their anatomical connection to motor neurons. This pathway-selective and reversible technique for blocking neural transmission does not depend on cell-specific promoters or transgenic techniques, and is a new and powerful tool for functional dissection in system-level neuroscience studies.

Skilled reaching relies on a V2a propriospinal internal copy circuit

The precision of skilled forelimb movement has long been presumed to rely on rapid feedback corrections triggered by internally directed copies of outgoing motor commands, but the functional relevance of inferred internal copy circuits has remained unclear. One class of spinal interneurons implicated in the control of mammalian forelimb movement, cervical propriospinal neurons (PNs), has the potential to convey an internal copy of premotor signals through dual innervation of forelimb-innervating motor neurons and precerebellar neurons of the lateral reticular nucleus. Here we examine whether the PN internal copy pathway functions in the control of goal-directed reaching. In mice, PNs include a genetically accessible subpopulation of cervical V2a interneurons, and their targeted ablation perturbs reaching while leaving intact other elements of forelimb movement. Moreover, optogenetic activation of the PN internal copy branch recruits a rapid cerebellar feedback loop that modulates forelimb motor neuron activity and severely disrupts reaching kinematics. Our findings implicate V2a PNs as the focus of an internal copy pathway assigned to the rapid updating of motor output during reaching behaviour.

Circuits for Skilled Reaching and Grasping

From an evolutionary perspective, it is clear that basic motor functions such as locomotion and posture are largely controlled by neural circuitries residing in the spinal cord and brain-stem. The control of voluntary movements such as skillful reaching and grasping is generally considered to be governed by neural circuitries in the motor cortex that connect directly to motoneurons via the corticomotoneuronal (CM) pathway. The CM pathway may act together with several brain-stem systems that also act directly with motoneurons. This simple view was challenged by work in the cat, which lacks the direct CM system, showing that the motor commands for reaching and grasping could be mediated via spinal interneurons with input from the motor-cortex and brain-stem systems. It was further demonstrated that the spinal interneurons mediating the descending commands for reaching and grasping constitute separate and distinct populations from those involved in locomotion and posture. The aim of this review is to describe populations of spinal interneurons that are involved in the control of skilled reaching and grasping in the cat, monkey, and human.

Integration in descending motor pathways controlling the forelimb in the cat. 17. Axonal projection and termination of C3-C4 propriospinal neurones in the C6-Th1 segments.

SummaryCollateralization and termination of single C3-C4 propriospinal neurones (PNs) have been studied in the C6-Th1 segments of the cat using two methods: threshold mapping for antidromic activation of C3-C4 PNs and intra-axonal injection of horseradish peroxidase. Low threshold points for antidromic activation of C3-C4 PNs were found in the region of different motor nuclei in lamina IX both at one level and at different segmental levels, in all parts of lamina VII, in the lateral part of lamina VI and in the dorsal and ventral parts of lamina VIII. Collaterals were found from C6 to Th1. A marked decrease of conduction velocity of the stem axon occurred in the caudal region of termination, while it was almost constant in the rostral region of termination. HRP was injected iontophoretically in C6-Th1 into stem axons of neurones, which were activated antidromically from the ventral part of the lateral funiculus in C5/C6, from the lateral reticular nucleus (LRN) and monosynaptically from the corticospinal fibres (stimulated in the contralateral pyramid) which were transected in C5/C6. Reconstruction of successfully stained stem axons, revealed collaterals with terminals on presumed motoneurones in different parts of lamina IX and on interneurones in laminae IV–VIII. These findings confirm previous results which showed monosynaptic projections from C3-C4 PNs to forelimb motoneurones and Ia inhibitory interneurones. With respect to termination in the region of the motoneurones in lamina IX and in the region of Ia inhibitory interneurones in lamina VII, three patterns were found: 1) termination mainly in lamina IX (n=1) 2) termination in laminae IX and VII (n=15) and 3) termination mainly in lamina VII (n=2). However, in some cases the same stem axon gave off collaterals which terminated either on motoneurones in lamina IX or on presumed Ia inhibitory interneurones in lamina VII. Furthermore, when the stem axons had collaterals which terminated in different motor nuclei only some of these collaterals had additional terminations on presumed Ia inhibitory interneurones. This result suggest that C3-C4 PNs do not follow a strict Ia pattern of reciprocal innervation. It is tentatively proposed that the difference of innervation may be related to the type of multi-joint movement, such as target-reaching with the forelimb, which has been shown to be controlled by the C3-C4 PNs. Termination in laminae VI, VIII and different parts of lamina VII indicates that C3-C4 PNs also project to other types of neurones than motoneurones and Ia inhibitory interneurones. Injection of wheat germ agglutinated horseradish peroxidase (WGA-HRP) laterally in laminae VI-VII in C3 and C4 caused anterograde labelling of axonal bundles from neurones in these segments. Labelled axons were found mainly in the lateral funiculus with the highest density in the ventral part. These axons could be traced throughout the forelimb segments and also to the LRN.

Motor recovery after serial spinal cord lesions of defined descending pathways in cats

The food-taking movement by which a cat uses its forepaw to take a piece of food and bring it to its mouth normally depends on the cortico- (CS) and rubrospinal (RS) tracts and disappears when they are transected in C5; a slow reappearance over months is due to bulbospinal (BS) take-over. After complete CS transection but minimal RS transection, food-taking remains. If, one month later, the RS tract is completely transected, food-taking is not abolished as it is when transection is made in one session. It is permanently abolished after a third transection of the ventral quadrant in C2. It is suggested that the food-taking remaining after the first lesion is due to combined RS and BS activity and that the RS tract induces the BS neurones to contribute to the extent that they can take over when the RS tract is completely transected.

The C3–C4 propriospinal system in the cat and monkey: a spinal pre-motoneuronal centre for voluntary motor control

This review deals with a spinal interneuronal system, denoted the C3–C4 propriospinal system, which is unique in the sense that it so far represents the only spinal interneuronal system for which it has been possible to demonstrate a command mediating role for voluntary movements. The C3–C4 propriospinal neurones govern target reaching and can update the descending cortical command when a fast correction is required of the movement trajectory and also integrate signals generated from the forelimb to control deceleration and termination of reaching.

Motor command for precision grip in the macaque monkey can be mediated by spinal interneurons

In motor control, the general view is still that spinal interneurons mainly contribute to reflexes and automatic movements. The question raised here is whether spinal interneurons can mediate the cortical command for independent finger movements, like a precision grip between the thumb and index finger in the macaque monkey, or if this function depends exclusively on a direct corticomotoneuronal pathway. This study is a followup of a previous report (Sasaki et al. J Neurophysiol 92: 3142-3147, 2004) in which we trained macaque monkeys to pick a small piece of sweet potato from a cylinder by a precision grip between the index finger and thumb. We have now isolated one spinal interneuronal system, the C3-C4 propriospinal interneurons with projection to hand and arm motoneurons. In the previous study, the lateral corticospinal tract (CST) was interrupted in C4/C5 (input intact to the C3-C4 propriospinal interneurons), and in this study, the CST was interrupted in C2 (input abolished). The precision grip could be performed within the first 15 days after a CST lesion in C4/C5 but not in C2. We conclude that C3-C4 propriospinal interneurons also can carry the command for precision grip.

Skilled digit movements in feline and primate--recovery after selective spinal cord lesions.

Recovery of voluntary movements after partial spinal cord injury depends, in part, on a take‐over of function via unlesioned pathways. Using precise forelimb movements in the cat as model, spinal pathways contributing to motor restitution have been investigated in more detail. The food‐taking movement by which the cat graSPS a morsel of food with the digits and brings it to the mouth is governed by interneurones in the forelimb segments (C6‐Th1) and is normally controlled via the cortico‐ and rubrospinal tracts. Food‐taking disappears after transection of these pathways in the dorsal part of the lateral funiculus (DLF) in C5/C6, but then recovers during a period of 2–3 weeks. Experiments with double lesions showed that the recovery depends on a take‐over via ipsilateral ventral systems; a ventrally descending pathway, most probably cortico‐reticulospinal, and a pathway via propriospinal neurones in the C3–C4 segments. It is postulated that the recovery involves a plastic reorganization of these systems. Dexterous finger movements in the macaque monkey are generally considered to depend on the monosynaptic cortico‐motoneuronal (CM) connexion, which is lacking in the cat. Such movements are abolished after pyramidotomy at the level of the trapezoid body. However, experiments with transection of the corticospinal tract in the DLF and partly ventral part of the lateral funiculus in C5, showed a fast (1–28 days) recovery of precision grip and, to some extent, independent finger movements. Deficits in preshaping during the final approach to the morsel as well as lack of force were observed. A C5 DLF lesion spares corticofugal pathways to the brainstem and upper cervical segments. It is suggested that indirect corticomotoneuronal pathways may provide for recovery of dexterous finger movements and that the role of CM pathways for such movements should be broadened to include not only the monosynaptic connexion.

Trigeminal excitation of dorsal neck motoneurones in the cat

SummaryExcitation of dorsal neck motoneurones evoked by electrical stimulation of primary trigeminal afferents in the Gasserian ganglion has been investigated with intracellular recording from α-motoneurones in the cat. Single stimulation in the Gasserian ganglion ipsi-and contralateral to the recording side evoked excitatory postsynaptic potentials (EPSPs) in motoneurones innervating the lateral head flexor muscle splenius (SPL) and the head elevator muscles biventer cervicis and complexus (BCC). The gasserian EPSPs were composed of early and late components which gave the EPSPs a hump-like shape. A short train of stimuli, consisting of two to three volleys, evoked temporal facilitation of both the early and late EPSP components. The latencies of the gasserian EPSPs ranged from 1.6 to 3.6 ms in SPL motoneurones and from 1.6 to 5.8 ms among BCC motoneurones. A rather similar latency distribution between 1.6 and 2.4 ms was found for ipsi- and contralateral EPSPs in SPL and BCC motoneurones, which is compatible with a minimal disynaptic linkage between primary trigeminal afferents and neck motoneurones. Systematic transections of the ipsi- and contralateral trigeminal tracts were performed in the brain stem between 3 and 12 mm rostral to the level of obex. The results demonstrate that both the ipsi- and contralateral disynaptic and late gasserian EPSPs can be mediated via trigeminospinal neurones which take their origin in the nucleus trigeminalis spinalis oralis. Transection of the midline showed that the contralateral trigeminospinal neurones cross in the brain stem. Systematic tracking in and around the ipsilateral trigeminal nuclei demonstrated that the axons of ipsilateral trigeminospinal neurones descend just medial to and/or in the medial part of the nucleus. Spinal cord lesions revealed a location of the axons of the ipsilateral trigeminospinal neurones in the lateral and ventral funiculi. Interaction between the ipsi- and contralateral gasserian EPSPs showed complete summation of the disynaptic EPSP component, while the late components were occluded by about 45%. These results show that the disynaptic EPSPs are mediated by separate trigeminospinal neurones from the ipsi- and contralateral side, while about half of the late EPSPs are mediated by common neurones which receive strong bilateral excitation from commissural neurones in the trigeminal nuclei. Spatial facilitation was found in the late gasserian EPSP but not in the disynaptic gasserian EPSP by conditioning stimulation of cortico- and tectofugal fibres. Disynaptic pyramidal and tectal EPSPs, which are mediated by reticulospinal neurones, were facilitated by a single stimulation in the gasserian ganglion at an optimal interval of 2 ms. It is suggested that primary trigeminal afferents can excite the reticulospinal neurones via a disynaptic trigeminoreticular pathway.

Pyramidal effects in dorsal neck motoneurones of the cat.

The effects of contralateral pyramidal stimulation have been investigated with intracellular recording from cat alpha‐motoneurones that innervate the dorsal neck musculature. A short train of stimuli evoked three types of synaptic effects: predominant excitation or inhibition and mixed effects characterized chiefly by early excitation followed by inhibition. Latency measurements indicated a minimal disynaptic linkage for excitation and for inhibition. Splenius motoneurones received primarily excitation whereas biventer cervicis‐complexus motoneurones received a more varied input characterized by mixed effects or inhibition. Following transection of the pyramid just rostral to the decussation (lower pyramidal lesion) pyramidal stimulation above the lesion still produced disynaptic excitation and longer latency (possibly trisynaptic) inhibition. Pyramidal stimulation just caudal to this transection evoked inhibition with a minimal disynaptic latency, as well as longer latency excitation. The incidence of longer latency excitation was found to be reduced in cats with corticospinal tract transections at the level of the second cervical spinal segment. No post‐synaptic potentials were evoked by pyramidal stimulation rostral to a pyramidal transection at the level of the trapezoid body. It is suggested that disynaptic excitation evoked by pyramidal stimulation above the lower pyramidal lesion is mediated by medullary reticulospinal neurones possessing monosynaptic excitatory connexions with neck motoneurones. Longer latency excitation appears to be mediated by neurones that receive corticospinal tract input and are located in the spinal segments containing the neck motoneurones. Disynaptic inhibition is mediated by neurones likely to be situated between the second cervical spinal segment and the level of the lower pyramidal lesion. The results also suggest that the first neurone in the chain mediating longer latency inhibition is located in the brain stem. The differences in pyramidal synaptic input between splenius and biventer cervicis‐complexus motoneurones are considered in relation to the roles these muscles may serve in head position control.