Physiological properties and patterns of projection in the cortico-cortical connections from the second somatosensory cortex to the motor cortex, area 4 gamma, in the cat

Modulation of the ∽20-Hz motor-cortex rhythm to passive movement and tactile stimulation

BACKGROUND

Integration of afferent somatosensory input with motor-cortex output is essential for accurate movements. Prior studies have shown that tactile input modulates motor-cortex excitability, which is reflected in the reactivity of the ∽ 20-Hz motor-cortex rhythm. ∽ 20-Hz rebound is connected to inhibition or deactivation of motor cortex whereas suppression has been associated with increased motor cortex activity. Although tactile sense carries important information for controlling voluntary actions, proprioception likely provides the most essential feedback for motor control.

METHODS

To clarify how passive movement modulates motor-cortex excitability, we studied with magnetoencephalography (MEG) the amplitudes and peak latencies of suppression and rebound of the ∽ 20-Hz rhythm elicited by tactile stimulation and passive movement of right and left index fingers in 22 healthy volunteers.

RESULTS

Passive movement elicited a stronger and more robust ∽ 20-Hz rebound than tactile stimulation. In contrast, the suppression amplitudes did not differ between the two stimulus types.

CONCLUSION

Our findings suggest that suppression and rebound represent activity of two functionally distinct neuronal populations. The ∽ 20-Hz rebound to passive movement could be a suitable tool to study the functional state of the motor cortex both in healthy subjects and in patients with motor disorders.

Effect of afferent input on motor cortex excitability during stroke recovery

OBJECTIVE

Afferent input is proposed to mediate its effect on motor functions by modulating the excitability of the motor cortex. We aimed to clarify - in a longitudinal study - how afferent input affects motor cortex excitability after stroke and how it is associated with recovery of hand function.

METHODS

The motor cortex excitability was studied by measuring the reactivity of the motor cortex beta rhythm to somatosensory stimulation. We recorded the amplitude of the suppression and subsequent rebound of the beta oscillations during tactile finger stimulation with MEG in 23 first-ever stroke patients within one week and at 1 and 3 months after stroke, with concomitant evaluation of hand function.

RESULTS

The strength of the beta rhythm rebound, suggested to reflect decreased motor cortex excitability, was weak in the affected hemisphere after stroke and it was subsequently increased during recovery. The rebound strength correlated with hand function tests in all recordings.

CONCLUSION

Motor cortex excitability is modulated by afferent input after stroke. The motor cortex excitability is increased in the AH acutely after stroke and decreases in parallel with recovery of hand function.

SIGNIFICANCE

The results implicate the importance of parallel recovery of both sensory and motor systems in functional recovery after stroke.

Signals from the ventrolateral thalamus to the motor cortex during locomotion

The activity of the motor cortex during locomotion is profoundly modulated in the rhythm of strides. The source of modulation is not known. In this study we examined the activity of one of the major sources of afferent input to the motor cortex, the ventrolateral thalamus (VL). Experiments were conducted in chronically implanted cats with an extracellular single-neuron recording technique. VL neurons projecting to the motor cortex were identified by antidromic responses. During locomotion, the activity of 92% of neurons was modulated in the rhythm of strides; 67% of cells discharged one activity burst per stride, a pattern typical for the motor cortex. The characteristics of these discharges in most VL neurons appeared to be well suited to contribute to the locomotion-related activity of the motor cortex. In addition to simple locomotion, we examined VL activity during walking on a horizontal ladder, a task that requires vision for correct foot placement. Upon transition from simple to ladder locomotion, the activity of most VL neurons exhibited the same changes that have been reported for the motor cortex, i.e., an increase in the strength of stride-related modulation and shortening of the discharge duration. Five modes of integration of simple and ladder locomotion-related information were recognized in the VL. We suggest that, in addition to contributing to the locomotion-related activity in the motor cortex during simple locomotion, the VL integrates and transmits signals needed for correct foot placement on a complex terrain to the motor cortex.

Contribution of different limb controllers to modulation of motor cortex neurons during locomotion

During locomotion, neurons in motor cortex exhibit profound step-related frequency modulation. The source of this modulation is unclear. The aim of this study was to reveal the contribution of different limb controllers (locomotor mechanisms of individual limbs) to the periodic modulation of motor cortex neurons during locomotion. Experiments were conducted in chronically instrumented cats. The activity of single neurons was recorded during regular quadrupedal locomotion (control), as well as when only one pair of limbs (fore, hind, right, or left) was walking while another pair was standing. Comparison of the modulation patterns in these neurons (their discharge profile with respect to the step cycle) during control and different bipedal locomotor tasks revealed several groups of neurons that receive distinct combinations of inputs from different limb controllers. In the majority (73%) of neurons from the forelimb area of motor cortex, modulation during control was determined exclusively by forelimb controllers (right, left, or both), while in the minority (27%), hindlimb controllers also contributed. By contrast, only in 30% of neurons from the hindlimb area was modulation determined exclusively by hindlimb controllers (right or both), while in 70% of them, the controllers of forelimbs also contributed. We suggest that such organization of inputs allows the motor cortex to contribute to the right-left limbs' coordination within each of the girdles during locomotion, and that it also allows hindlimb neurons to participate in coordination of the movements of the hindlimbs with those of the forelimbs.

Activation in parietal operculum parallels motor recovery in stroke

Motor recovery after stroke requires continuous interaction of motor and somatosensory systems. Integration of somatosensory feedback with motor programs is needed for the automatic adjustment of the speed, range, and strength of the movement. We recorded somatosensory evoked fields (SEFs) to tactile finger stimulation with whole-scalp magnetoencephalography in 23 acute stroke patients at 1 week, 1 month, and 3 months after stroke to investigate how deficits in the somatosensory cortical network affect motor recovery. SEFs were generated in the contralateral primary somatosensory cortex (SI) and in the bilateral parietal opercula (PO) in controls and patients. In the patients, SI amplitude or latency did not correlate with any of the functional outcome measures used. In contrast, the contralateral PO (cPO) amplitude to the affected hand stimuli correlated significantly with hand function in the acute phase and during recovery; the weaker the PO activation, the clumsier the hand was. At 1 and 3 months, enhancement of the cPO activation paralleled the improvement of the hand function. Whole-scalp magnetoencephalography measurements revealed that dysfunction of somatosensory cortical areas distant from the ischemic lesion may affect the motor recovery. Activation strength of the PO paralleled motor recovery after stroke, suggesting that the PO area is an important hub in mediating modulatory afferent input to motor cortex.

Quantitative characteristics of the associative projections of field 4y to subfields of the sensorimotor and parietal cortex of the cat

The Nauta-Gygax method was used to study the ipsilateral associative connections of motor cortex field 4y after local electrolytic lesioning of this zone. The relative quantitative distribution of associative fibers running from field 4y to somatosensory areas I and II, the motor cortex, and the parietal cortex was determined. The greatest projections of field 4y were found to be directed to field 2pri (the secondary somatosensory zone) and field 5ab. Occasional degenerative fibers passed to fields 1, 2, 3a, and 3b of the primary somatosensory zone of the cortex. Efferent fibers from field 4y were not directed to fields 4fu, 4sfu, 6aa, 6ab, or 6ifu. It is suggested that the morphological basis of motor reactions mediated by field 4y is not provided by the fundal (4fu, 4sfu, 6ifu) or premotor (6aa, 6ab) fields but by field 2pri and 5ab, with which it has more extensive connections.

Long-term physical exercise and somatosensory event-related potentials

We have compared the occurrence patterns of somatosensory event-related potentials (ERPs) in athletes (soccer players) and non-athletes. ERPs were elicited by two oddball tasks following separate somatosensory stimulation at the median nerve (upper-limb task) and at the tibial nerve (lower-limb task). In the athlete group the N140 amplitudes were larger during upper- and lower-limb tasks and the P300 amplitude and latency were larger and shorter, respectively, during the lower-limb task compared with non-athletes. On the other hand, no significant differences in the P300 amplitude and latency during the upper-limb task were observed between the athlete and non-athlete groups. These results indicate that plastic changes in somatosensory processing might be induced by performing physical exercises that require attention and skilled movements.

Inhibition of the human primary motor area by painful heat stimulation of the skin

OBJECTIVE

To prove whether painful cutaneous stimuli can affect specifically the motor cortex excitability.

METHODS

The electromyographic (EMG) responses, recorded from the first dorsal interosseous muscle after either transcranial magnetic or electric anodal stimulation of the primary motor (MI) cortex, was conditioned by both painful and non-painful CO2 laser stimuli delivered on the hand skin.

RESULTS

Painful CO2 laser stimuli reduced the amplitude of the EMG responses evoked by the transcranial magnetic stimulation of both the contralateral and ipsilateral MI areas. This inhibitory effect followed the arrival of the nociceptive inputs to cerebral cortex. Instead, the EMG response amplitude was not significantly modified either when it was evoked by the motor cortex anodal stimulation or when non-painful CO2 laser pulses were used as conditioning stimuli.

CONCLUSIONS

Since the magnetic stimulation leads to transynaptic activation of pyramidal neurons, while the anodal stimulation activates directly cortico-spinal axons, the differential effect of the noxious stimuli on the EMG responses evoked by the two motor cortex stimulation techniques suggests that the observed inhibitory effect has a cortical origin. The bilateral cortical representation of pain explains why the painful CO2 laser stimuli showed a conditioning effect on MI area of both hemispheres. Non-painful CO2 laser pulses did not produce any effect, thus suggesting that the reduction of the MI excitability was specifically due to the activation of nociceptive afferents.

Cortical activation during oesophageal stimulation: a neuromagnetic study

We investigated the neuromagnetic responses to mechanical stimulation of the oesophagus. In six healthy right-handed volunteers (mean age 31.6 years) the proximal and distal oesophagus were stimulated by electronically controlled pump-inflation of a silicone balloon once every 4.5-5.5 sec (dwell time 145 msec). The balloon volume was adjusted to induce different sensation levels (i) just above threshold of perception, (ii) strong sensation and (iii) painful sensation. Evoked magnetic brain responses were recorded time-locked to stimulus onset with a Neuromag-122TM whole-head neuromagnetometer and modelled as equivalent current diploe (ECD) sources. ECDs were superimposed on individual magnetic resonance imaging (MRI) scans. Magnetic brain responses following distal oesophageal stimulation were adequately explained by a time-varying 2-4 dipole model with unilateral or bilateral sources in second somatosensory cortex and later sources in the frontal cortex. With increasing stimulus intensities, latencies of the sources decreased and amplitudes increased. Proximal oesophageal stimulation led to activation of source areas spatially similar to those of distal oesophageal stimulation but with shorter response latencies. Both painful and nonpainful mechanical stimulation of the oesophagus activate the second somatosensory cortex (SII). Evidence for topographic organization of oesophageal afferents in SII is poor.

Different cortical organization of visceral and somatic sensation in humans

Sensory stimuli from the visceral domain exhibit perceptual characteristics different from stimuli applied to the body surface. Compared with somatosensation there is not much known about the cortical projection and functional organization of visceral sensation in humans. In this study, we determined the cortical areas activated by non-painful electrical stimulation of visceral afferents in the distal oesophagus, and somatosensory afferents in the median nerve and the lip in seven healthy volunteers using whole-head magnetoencephalography. Stimulation of somatosensory afferents elicited short-latency responses (approximately 20-60 ms) in the primary somatosensory cortex (SI) contralateral (median nerve) or bilateral (lip) to the stimulated side, and long-latency responses (approximately 60-160 ms) bilaterally in the second somatosensory cortex (SII). In contrast, stimulation of visceral oesophageal afferents did not evoke discernible responses in SI but well reproducible bilateral SII responses (approximately 70-190 ms) in close vicinity to long-latency SII responses following median nerve and lip stimuli. Psychophysically, temporal discrimination of successive stimuli became worse with increasing stimulus repetition rates (0.25 Hz, 0.5 Hz, 1 Hz, 2 Hz) only for visceral oesophageal, but not for somatosensory median nerve stimuli. Correspondingly, amplitudes of the first cortical response to oesophageal stimulation emerging in the SII cortex declined with increasing stimulus repetition rates whereas the earliest cortical response elicited by median nerve stimuli (20 ms SI response) remained unaffected by the stimulus frequency. Our results indicate that visceral afferents from the oesophagus primarily project to the SII cortex and, unlike somatosensory afferents, lack a significant SI representation. We propose that this cortical projection pattern forms the neurophysiological basis of the low temporal and spatial resolution of conscious visceral sensation.

Cortico-cortical connections from somatosensory areas to the motor area of the cortex following peripheral nerve lesion in the cat

Cortico-cortical connections of the forelimb digital area of the motor cortex (MCx) following peripheral nerve lesion were examined using horseradish peroxidase (HRP) in adult cats. A small amount of HRP injection was made into the digital area of the MCx in the control animals with intact nerves. HRP labelled cells were found in the first somatosensory area (SI; areas 1-2, 3a and 3b), secondary somatosensory area (SII) and area 5 of the ipsilateral cortex. In addition, a small number of HRP labelled cells were found in the contralateral cortex located in area 4 (MCx) and area 3a of SI. In the experimental animals, peripheral nerves were cut and after 2 or 3 months of survival, HRP was injected to the corresponding area of the MCx. The HRP labelled cells were found in SI (areas 2, 3a, 3b), SII, SIII, SIV and SV and in areas Id and 5 of the ipsilateral cortex. Furthermore, HRP-labelled cells were found in area 4 of the MCx, in SI (3a), SII, SIV and Id on the contralateral side of the cortex. HRP-labelled cells were located in layers II, III and V. Most of these cells were found in layer III.

Long-lasting potentiation in the secondary somatosensory cortex affects motor control: assessment by H-reflex

We investigated descending projections from the secondary somatosensory cortex to the feline spinal cord and the effects of long-lasting potentiation in secondary somatosensory cortex on the activities of motoneurons of the cat. Electrophysiological examinations revealed that the low-intensity subthreshold secondary somatosensory cortex stimulation could change the H-Reflex induced by radial nerve stimulation. The H-wave amplitudes, recorded in wrist flexor muscles, were enhanced when the intervals from secondary somatosensory cortex to radial nerve stimuli were altered from 0 to 30 ms (initial excitation, 146 +/- 11% (mean +/- S.E.M.) of the control value). In contrast, the H-waves were suppressed with intervals longer than 30 ms (80 +/- 3%). The descending pathways from secondary somatosensory cortex to the spinal cord were assessed using an immunohistochemical technique. c-Fos and Zif268 proteins, induced by stimulation of the hand-represented secondary somatosensory cortex areas, could thus express in activated cervical neurons. The density of labeled cells was significantly higher in the seventh and eighth cervical segments than in other levels. The great majority of positive cells were distributed in the lateral part of the contralateral ventral horn and their somas ranged from 10 to 50 microns in size. Finally, we examined the effects of long-lasting potentiation, induced by high-frequency stimulation of the ventral posterolateral thalamic nucleus, on the activities of spinal motoneurons. Long-lasting potentiation altered the previously observed effects of secondary somatosensory cortex stimulation on the H-wave amplitude. The secondary somatosensory cortex-conditioned initial excitation of the H-reflex was enhanced (from 139 to 175%, P < 0.05), while late suppression was completely blocked (from 74 to 112%, P < 0.01). In conclusion, the descending pathways from secondary somatosensory cortex to the spinal cord modulated the H-reflex, and long-lasting potentiation in secondary somatosensory cortex affected this modulation. We have previously reported that corticocortical inputs from primary to secondary somatosensory cortex is required for induction of long-lasting potentiation in secondary somatosensory cortex. Taken together, the present study suggests that cortical plasticity in secondary somatosensory cortex amplifies somatic inputs from primary somatosensory cortex as a means of adaptive motor control by the sensory system.

Low-threshold motor effects produced by stimulation of the facial area of the fifth somatosensory cortex in the cat

The motor effective sites of the fifth somatosensory cortex (SV) in the cat were mapped in detail by using unit recording and intracortical microstimulation (ICMS) techniques. The motor effective sites for facial muscle contraction were identified using stimulus currents of less than 30 microA. Of the 257 effective sites detected, 49% were activated by stimulus currents of less than 20 microA and of these, 51% responded to stimulus currents of less than 10 microA. ICMS within the facial area of the SV neuron produced contralateral eye-blinking, the lowest threshold current for which was 2 microA and ICMS within the SV neurons produced whisker movements, the minimum threshold current for which was 4 microA. Furthermore, stimulation of some SV neurons at a threshold current as low as 4 microA produced whisker movements and some responded to both visual and cutaneous stimuli. Ablation of areas 6a beta, 3a, SII, SIII and the motor cortex did not eliminate or reduce the low-threshold responses elicited by this weak stimulation of the SV. These motor effective areas receive input from the contralateral cutaneous of the surrounding muscle motor effective region. Our results suggest that the described effect is independent of motor effective areas.

Area 3a in the cat. II. Projections to the motor cortex and their relations to other corticocortical connections

It is well known that area 3a in the cat may monosynaptically influence the activity of neurons in the motor cortex. Much less information is available, however, on the anatomy of these connections. By using single or combined injections of different retrograde axonal tracers, we investigated the topography (horizontal and laminar) of area 3a neurons projecting to the motor cortex, and the anatomical relationships between these neurons and those projecting to other areas (2, 5, and SII) which, in turn, project to the motor cortex. Area 3a projects to all parts of area 4 gamma, to area 4 delta, and to the agranular area 6 in the lateral bank of the presylvian sulcus (area 6 alpha gamma), but not to other parts of areas 4 and 6. This projection exhibits a loose topographic organization along the mediolateral dimension of area 3a, and, in many cases, arises predominantly from the rostral half of this area. Although single small injections in the motor cortex produced two or more separate patches of retrograde labeling in 3a, after simultaneous injections of fluorochromes in two separate loci there often appeared in area 3a overlapping populations of neurons which were labeled retrogradely by each of the dyes, but with very few double-labeled neurons. In horseradish peroxidase (HRP) cases, 72% of area 3a neurons projecting to area 4 gamma were distributed in supragranular layers (mainly layer III), although the proportion of labeling in infragranular layers was larger when using fluorescent dyes. Double-labeled cells predominated in infragranular layers. These results have a bearing upon the functional roles that have been attributed to area 3a, as a cortical locus involved in muscle sensation, and a cortical relay to the motor cortex of rapid feedback information from muscle activity during movement.

Fifth somatosensory cortex (SV) representation of the whole body surface in the medial bank of the anterior suprasylvian sulcus of the cat

The physiological properties of neurons in the medial bank of the anterior suprasylvian sulcus (ASSS-m) of the cat's cortex were studied using unit recording techniques. Receptive fields (RFs) on the face are represented in the most rostral aspects of the ASSS-m. Of these neurons, 84% responded to light touch of the skin on the contralateral region of the face and 12% responded to mechanical stimulation of facial hair. In addition, 4% of the neurons responded to light touch to the skin or mechanical stimulation of the hair on the contralateral face and also to visual stimuli. The RFs of neurons responsive to the hindlimb and tail are located in the most caudal aspects of the ASSS-m. 22% of these neurons responded to the light touch to the skin and 78% responded to movement of hair. The RFs of neurons responsive to the trunk area in the ASSS-m are located between the facial and hindlimb regions. Of these neurons, 2% responded to light touch of the skin and 98% responded to movements of hair. Some neurons which responded to stimulation of hair or skin on the trunk included forelimb and/or hindlimb areas. In addition, some neurons had RFs on both sides of the trunk including the shoulder area. These regions were in area 5a. Various features of representation in ASSS-m distinguish this region from other somatosensory areas. We designate the ASSS-m as the fifth somatosensory cortex (SV).

Cortical potentials associated with voluntary biting movement in humans

We compared the distribution of Bereitschaftspotentials (BPs) on both sides of the scalp preceding jaw biting movements in order to identify the relationship between the cortical regions and the activation of the masseter muscle in 10 healthy subjects. The BPs were recorded from the midline-central, central and temporal areas of the scalp according to the international 10-20 system, preceding self-paced biting on one side. The cortical negative potentials began 1.0 approximately 1.5 s before the EMG onset of the masseter muscle. All of these negative potentials could be considered to be BPs, and the additional negative slope component (NS) occurred 70 approximately 80 ms before the EMG onset of the masseter muscle. The BPs were detected from all the recorded regions of the scalp, while the NS was observed only from the bilateral temporal area. The amplitudes of BPs and NSs were largest in the temporal areas (T3 and T4) that were ipsilateral to the biting. The rates of occurrence of NS at T3 and T4 ipsilateral temporal areas were 80% and 60%, respectively. These results suggest that unilateral biting movements may be controlled mainly from the ipsilateral hemisphere.