Christoph Fromm

Pyramidal tract neurons in somatosensory cortex: central and peripheral inputs during voluntary movement

Recordings with pyramidal tract neurons (PTNs) in the primary somatosensory cortex of the monkey show that these neurons have 3 properties in common with PTNs of primary motor cortex: (1) they exhibit discharge prior to the onset of voluntary movement, (2) their discharge frequency varies as a function of strength of muscular contraction, and (3) they show reflex responses to afferent stimuli that occur during movement. These findings support the view that in addition to its widely recognized role in somesthetic perception, somatosensory cortex has a direct role in the control of movement.

Corticostriatal cells in comparison with pyramidal tract neurons: contrasting properties in the behaving monkey

Antidromically identified neurons projecting to the putamen (CPNs) and pyramidal tract neurons (PTNs) were recorded from motor and premotor cortex of a monkey which performed a load-bearing task with the wrist. CPNs appeared as a uniform population with very slowly conducting axons and low spontaneous activity. In contrast to PTNs, they exhibited weak, mostly insignificant correlation with graded steady-state forces, responded to torque perturbations with remarkably long latency, and seemed to discharge much later with active movement. Collateral branching of PTNs to the putamen was found to be infrequent (1%). We suggest that the putamen receives a cortical message that is strikingly different from that sent down the pyramidal tract.

Depression of the recurrent inhibition of extensor motoneurons by the action of group II afferents

The influence of varying the muscular afferent fiber input on both the normal firing rate (Fn) and the amount of recurrent inhibition (Fn-Fi) induced by a constant ventral root stimulation was investigated on tonic extensor motoneurons recorded from ventral root filaments in decerebrate cats. The afferent input was varied by graded electrical stimulation of the gastrocnemius nerves and by vibrating the triceps surae muscle (100 mum amplitude). When the input consisted solely of impulses in Ia afferents, as was the case during vibration, the mean recurrent inhibition Fn-Fi was 2.3 times greater than during nerve tetanization at 1.8 times threshold of group I (TI). This strength generally excited all group I and some low-threshold group II afferents. Between 1.8 TI and 8 TI, Fn-Fi decreased by some 50%. The average Fn increased slightly and motoneurons with a phasic discharge pattern were recruited when the stimulus strength was raised so as to excite group II afferents; these cells were never recruited during vibration and nerve tetanization at 1.8 TI. The results indicate the possibility of a disinhibitory action of secondary muscle spindle afferents on extensor motoneurons by reducing the recurrent inhibition.

Changes of steady state activity in motor cortex consistent with the length-tension relation of muscle

Steady state activity of motor cortex (MI) neurons and muscles was examined in relation to joint position. Two monkeys performed either isometric or load-bearing isotonic contractions, at different joint positions and during variation of steady torque. In either condition, MI steady state firing rates were found to be related to the amount of muscular excitation necessary to adjust muscle tension to length at any given position and load. The results obtained from 526 neurons (including pyramidal tract neurons) demonstrate for 206 neurons a correlate of the length-tension relation of muscle in the motor cortex.

Length-dependent autogenetic inhibition of extensor ?-motoneurones in the decerebrate cat

SummaryThe effects of stretching the triceps surae muscle on the resting discharge of functionally isolated and identified γ-efferents in the medial gastrocnemius nerve were studied in intercollicular decerebrate cats. Extension of the muscle decreases the activity of 9 out of 25 γ-motoneurones. This inhibition becomes apparent at about 8 mm muscle length and increases with increasing extension. Only those γ-efferents exhibiting this length-dependent inhibition are found to be inhibited by repetitive antidromic stimulation of the ventral root or the homonymous muscle nerve. We suggest that inhibition of fusimotor neurones by muscle stretch is at least partly mediated by the recurrent collaterals of the α-motoneurones activated by the stretch. 11 γ-motoneurones were predominantly excited by muscle stretch applied to the detached Achilles tendon as well as by squeezing the tendon. Nociceptors within the tendon or surrounding fascia are probably responsible.

Activity in the Precentral Motor Areas After Presentation of Targets for Delayed Reaching Movements Varies with the Initial Arm Position

We studied single unit activity in the primary motor and premotor cortex during preparation for unrestrained arm movements that the monkey made to a right or left visual target. ‘Set‐related’ activity occurring during an instructed delay period could not only vary with the target location, but also with the initial arm posture which was changed via a motor‐driven forearm support between two positions. Modulation with arm displacement of some cells was contingent upon prior instruction, i.e. it occurred only if a target for the upcoming movement was presented, and for the majority of neurons it was different from the effects caused by the same passive arm shift when being applied before the visual instruction cue. Furthermore, responses to presentation of the same target were found to depend on the starting position of the hand. These findings suggest that both kinaesthetic and visual information interact in motor cortical cells in an appropriate manner for elaborating the movement trajectory.

Positionsempfindlichkeit und fusimotorische Aktivierung prätibialer Muskelspindelendigungen vor und nach Deafferentierung

SummaryIn decerebrate cats the position sensitivity of pretibial muscle spindle endings was obtained by plotting impulse frequency against extension. Fusimotor activation, reflexly induced mainly by ipsilateral muscle nerve stimulation, does not change the slope of the approximately linear frequency-extension relation, but shifts the whole curve bodily upwards. This holds true only if neither the dorsal roots nor the ventral roots are cut. However after unilateral deafferentation fusimotor activation, now induced by contralateral muscle nerve stimulation, increases the slope constants of the endings. We assume that extension applied to the muscle not isolated from the spinal cord leads to inhibition of the fusimotoneurones with the outcome that the slope of the frequency-extension relation is kept constant even during fusimotor activation.ZusammenfassungBei dezerebierten Katzen wurde die Positionsempfindlichkeit prätibialer Muskelspindelendigungen durch Auftragung der Impulsfrequenz gegen die Muskelextension bestimmt. Fusimotorische Aktivierung — reflektorisch meistens durch ipsilaterale Muskelnervenreizung ausgelöst — verändert die Anstiegssteilheit der ungefähr linearen Frequenz-Dehnungsbeziehung nicht, sondern verschiebt die Kurve im ganzen nach oben. Dies gilt nur, wenn weder die Dorsalnoch die Ventralwurzeln durchschnitten sind. Nach einseitiger Deafferentierung indes nimmt die Steilheitskonstante der Endigungen bei fusimotorischer Aktivierung — jetzt durch contralaterale Muskelnervenreizung ausgelöst — zu. Wir nehmen an, daß die Extension eines Muskels, der nicht vom Rückenmark isoliert ist, die Fusimotoneurone hemmt, so daß die Anstiegssteilheit selbst bei fusimotorischer Aktivierung konstant bleibt.

Central Olfactory Connections in the Microsmatic Marmoset Monkey (Callithrix jacchus)

The mammalian primary olfactory system consists of a set of different telencephalic structures, including paleo-, archi-, periarchi- and mesocortical components. We present the first characterisation of the normal and connectional anatomy of the primary olfactory cortex of the common marmoset, a microsmatic simian species increasingly used in primate research. The centrifugal and centripetal bulbar projections were determined by injections of the anterograde and retrograde tracer wheat germ agglutinin-conjugated horseradish peroxidase and fluorescent dyes into the ipsilateral main olfactory bulb. The efferent projections of the marmoset bulb are organised entirely ipsilaterally and are established via a rudimentary medial olfactory tract and the dominant lateral olfactory tract. Target areas are the anterior olfactory nucleus, the entire prepiriform cortex, ventral tenia tecta, periamygdaloid cortex and the rostral part of the entorhinal cortex. The bulbar axons predominantly terminate in the outer part of layer I. The anterior olfactory nucleus receives a weak additional input within layer II and III, which is not found in macrosmatic rodents. Further anterograde labelling was found in the endopiriform nucleus deep under the prepiriform cortex and within an anterolateral strip of the olfactory tubercle. However, control injections into the olfactory tubercle suggest that the marmoset olfactory tubercle receives a bisynaptic olfactory input only. Retrograde labelling after bulb injections revealed that, except for the olfactory tubercle, all primary olfactory cortices contributed to an ipsilateral bulbopetal feedback projection. Like in rodents, the only bulbopetal projection organised bilaterally in the marmoset is maintained by the anterior olfactory nucleus. With few exceptions, the projections of the marmoset olfactory brain are organised similarly to that of the macaque monkey or those of macrosmatic species.

Inhibition of extensorγ motoneurons by antagonistic primary and secondary spindle afferents

SummaryAbout 2/3 of the γ efferents isolated from the medial gastrocnemius nerve were inhibited by longitudinal high-frequency vibration applied to the tendons of the non-contracting pretibial flexors (decerebrate cats). The inhibition appeared at 15–25 μm amplitude of vibration and increased up to a maximum at nearly 100 μm. Increasing the frequency of vibration from 100 to 300 Hz increased the inhibition. The reflex effects elicited by muscle vibration corresponded well in incidence and magnitude with those evoked by tetanization of the deep peroneal nerve at group I stimulus strength. The reflex disappeared when the nerve supply of the vibrated muscles was cut. The sensitivity of some pretibial proprioceptors to vibration was also tested. It is concluded that primary spindle endings of the pretibial flexors inhibit the extensorγ motoneurons. Some findings hint at a spinal pathway involving I a inhibitory interneurons.In addition, an inhibitory action of pretibial group II afferents, probably secondary spindle endings, on extensor γ efferents was demonstrated.The described fusimotor inhibition by antagonistic muscle spindle afferents is a further example of α-γ-linkage.