Efferent Zones Research Articles

The morphology of gills of Haliotis tuberculata (Linnaeus, 1758)

AbstractManganaro, M., Laurà, R., Guerrera, M.C., Lanteri, G., Zaccone, D. and Marino, F. 2011. The morphology of gills of Haliotis tuberculata (Linnaeus, 1758). —Acta Zoologica (Stockholm) 93: 436–443.Although the morphology of abalone gills has been studied by some authors, up to date no data are available about the gills of Haliotis tuberculata. This study was carried out, by light and electron microscopy, on 10 wild adult H. tuberculata. Gills lamellae produce an undulated surface increasing the area in contact with water. At the level of skeletal rods, we observed a joint‐like structure that allows a checked movement. The left ctenidium is always decidedly larger than the right, probably because of the enormous size of the shell muscle. The cilia permit oxygenated water that leaves the afferent border and is thrust away at the tips of the lamellae by the extremely long cilia. Ciliary movement may take part in sweeping mucous secretions to capture extraneous particles and remove them from the gills. Three types of mucous cells are distributed along the epithelium of the afferent and efferent zones of the gill filament. They seem to play a role in the cleansing of gills in coordination with the muscle contraction and ciliary movement. The presence of microvilli on particular cells reflects their role associated with the absorption of substances from the environment. A haemolymphatic vessel is located in the central zone of the gill filament. The backbone of the haemolymphatic vessel is a chitino‐like structure, which gives support to the gills.

Patterns of spatio-temporal correlations in the neural activity of the cat motor cortex during trained forelimb movements

In order to study how neurons in the primary motor cortex (MI) are dynamically linked together during skilled movement, we recorded simultaneously from many cortical neurons in cats trained to perform a reaching and retrieval task using their forelimbs. Analysis of task-related spike activity in the MI of the hemisphere contralateral to the reaching forelimb (in identified forelimb or hindlimb representations) recorded through chronically implanted microwires, was followed by pairwise evaluation of temporally correlated activity in these neurons during task performance using shuffle corrected cross-correlograms. Over many months of recording, a variety of task-related modulations of neural activities were observed in individual efferent zones. Positively correlated activity (mainly narrow peaks at zero or short latencies) was seen during task performance frequently between neurons recorded within the forelimb representation of MI, rarely within the hindlimb area of MI, and never between forelimb and hindlimb areas. Correlated activity was frequently observed between neurons with different patterns of task-related activity or preferential activity during different task elements (reaching, feeding, etc.), and located in efferent zones with dissimilar representation as defined by intracortical microstimulation. The observed synchronization of action potentials among selected but functionally varied groups of MI neurons possibly reflects dynamic recruitment of network connections between efferent zones during skilled movement.

Quantitative analyses of neurons projecting to primary motor cortex zones controlling limb movements in the rat

The objective was to determine if projections of single neurons to primary motor cortex preferentially terminate in several efferent zones that could form synergies for the execution of limb movements. Intracortical microstimulation was used to identify zones evoking hip flexion (HF), elbow flexion (EF), and both plantarflexion (PF) and dorsiflexion (DF) about the ankle. Histological examination showed that the zones from which some movements were evoked extended beyond the agranular cortex into granular cortex. Fluorogold, Fast blue, and propridium iodide or rhodamine-labeled dextran were injected into three of these four efferent zones in each rat. There was a virtual absence of multiple-labeled cells despite having an intermingling of different-colored cells of which 15% in frontal cortex were less than 1.2 mm away from a neighboring neuron that projected to a different efferent zone. This suggests that single neurons projecting to the motor cortex do not hard-wire specific synergies but rather project to single efferent zones in order to offer the greatest degree of freedom for the generation of movements. The distribution of ventral posterolateral and ventrolateral thalamic nucleus labeling depended on whether the injections were in granular or agranular cortex. Conversely, frontal cortex projections to motor efferent zones were made irrespective of their location in either granular or agranular cortex and thereby supporting their presumed role in the control of movements. Hindlimb motor cortex injections yielded retrograde labeling that extended into the more localised distribution of frontal cortex neurons retrogradely labeled from forelimb injections. This may allow hindlimb movements to be synchronized by forelimb movements during walking on challenging terrain.

Functional properties of neurons in the primate tongue primary motor cortex during swallowing.

Recent studies conducted in our laboratory have suggested that the tongue primary motor cortex (i.e., tongue-MI) plays a critical role in the control of voluntary tongue movements in the primate. However, the possible involvement of tongue-MI in semiautomatic tongue movements, such as those in swallowing, remains unknown. Therefore the present study was undertaken in attempts to address whether tongue-MI plays a role in the semiautomatic tongue movements produced during swallowing. Extracellular single neuron recordings were obtained from tongue-MI, defined by intracortical microstimulation (ICMS), in two awake monkeys as they performed three types of swallowing (swallowing of a juice reward after successful tongue task performance, nontask-related swallowing of a liquid bolus, and nontask-related swallowing of a solid bolus) as well as a trained tongue-protrusion task. Electromyographic activity was recorded simultaneously from various orofacial and laryngeal muscles. In addition, the afferent input to each tongue-MI neuron and ICMS-evoked motor output characteristics at each neuronal recording site were determined. Neurons were considered to show swallow and/or tongue-protrusion task-related activity if a statistically significant difference in firing rate was seen in association with these behaviors compared with that observed during a control pretrial period. Of a total of 80 neurons recorded along 40 microelectrode penetrations in the ICMS-defined tongue-MI, 69% showed significant alterations of activity in relation to the swallowing of a juice reward, whereas 66% exhibited significant modulations of firing in association with performance of the trained tongue-protrusion task. Moreover, 48% showed significant alterations of firing in relation to both swallowing and the tongue-protrusion task. These findings suggest that the region of cortex involved in swallowing includes MI and that tongue-MI may play a role in the regulation of semiautomatic tongue movement, in addition to trained motor behavior. Swallow-related tongue-MI neurons exhibited a variety of swallow-related activity patterns and were distributed throughout the ICMS-defined tongue-MI at sites where ICMS evoked a variety of types of tongue movements. These findings are consistent with the view that multiple efferent zones for the production of tongue movements are activated in swallowing. Many swallow-related tongue-MI neurons had an orofacial mechanoreceptive field, particularly on the tongue dorsum, supporting the view that afferent inputs may be involved in the regulation of the swallowing synergy.

Microstimulation of primate motor thalamus: somatotopic organization and differential distribution of evoked motor responses among subnuclei.

1. The functional organization of motor responses to microstimulation throughout the primate "motor" thalamus including nucleus ventralis lateralis, pars oralis (VLo); nucleus ventralis posterior lateralis, pars oralis (VPLo); nucleus ventralis lateralis, pars caudalis (VLc); and portions of ventralis anterior (VA) and area X, was systematically studied in awake monkeys. A total of 2,021 sites were examined for their response to microstimulation. Of these, 1,123 were histologically verified as to their location within the motor thalamus. At or near each site, isolated neurons were examined for their responses to somatosensory examination and active movement (n = 1,272). This study was carried out as part of a larger study examining the responses of neurons in the motor thalamus to somatosensory examination, torque-induced limb perturbations, and active movement in a visuomotor step-tracking task. 2. Microstimulation at < or = 40 microA evoked movements in the contralateral limbs, trunk, or face. Evoked movements of the limb were generally maximal about a single joint. 3. There was a differential response to microstimulation between subnuclei of the motor thalamus. In order of decreasing frequency, the percentages of sites within each subnucleus from which movements were evoked were as follows: VPLo, 93% (449 of 483); VLo, 21% (57 of 272); VLc, 11% (15 of 140); VA, 1% (1 of 85); and reticular nucleus, 0% (0 of 65). In VPLc, 44% (34 of 78) of sites examined were microexcitable. However, these were almost all within 500 microns of the border of VPLo, suggesting they may have occurred as a result of current spread to adjacent VPLo. Although area X was not sampled in its entirety, it did not appear to be microexcitable. 4. Microexcitable responses had a somatotopic organization, similar to that for neuronal responses to sensorimotor examination, with leg responses found most laterally and arm and face responses found progressively more medially. 5. Zones in VPLo generally ranging from 500 to 1,500 microns were found in which microstimulation resulted in the same motor response. These microexcitable zones resemble those described for the striatum and were termed thalamic microexcitable zones (TMZ). TMZs also resemble cortical efferent zones in that both are somatotopically organized, may affect a single muscle or group of muscles, have low thresholds for microstimulation with sharp boundaries that lie adjacent to other microexcitable zones with the opposite effects, and are of approximately the same dimension. 6. This study suggest that a fundamental unit of motor organization, i.e., single muscle or joint, is preserved at the thalamic level in the form of TMZs, and that these fundamental units of organization may contribute to the modular organization of the cortex.

Input-output organization in the hand area of the human motor cortex

The primate motor cortex consists of efferent zones which receive sensory information from a portion of limb in close anatomical relation to the muscle to which they project. To investigate a similar input-output relation in humans, we studied the effect of tactile stimuli on the size of motor evoked potentials (MEPs) elicited by transcranial magnetic and electrical stimulation. For tactile stimuli we applied air to the skin. The sizes of MEP of 3 finger muscles (flexor pollicis brevis, first dorsal interosseous, and abductor digiti minimi) with and without air stimuli were compared. Air stimuli applied to the tip of one finger facilitated mainly the magnetically evoked MEP of a muscle attached to that finger. A less obvious facilitatory effect was observed when giving stimuli on the dorsal aspect of the hand. Air stimuli had no facilitatory effect on electrically evoked MEPs. In one subject, there was no facilitatory effect even to magnetically evoked MEPs. In this subject, D-waves had the lowest threshold for magnetic and electrical stimulation. These results suggested that the effect was produced at the cortical level. This effect may correspond to the input-output relation found in the primate motor cortex.

Functional properties of single neurons in the face primary motor cortex of the primate. I. Input and output features of tongue motor cortex.

1. We have recently demonstrated that reversible, cooling-induced inactivation of the face motor cortex results in a severe impairment in the ability of monkeys (Macaca fascicularis) to perform a tongue-protrusion task but produces only relatively minor effects on the performance of a biting task by the same monkeys. To establish a neuronal correlate for these different behavioral relations, the present study has detailed the afferent input and intracortical microstimulation (ICMS)-defined output features of a population of face motor cortical neurons, and in a subsequent study we have documented the activities of the same population of neurons during the performance of the tongue-protrusion and biting tasks. 2. Of the 231 single neurons recorded within the face motor cortex, 163 were located at sites from which ICMS (less than or equal to 20 microA) could evoke tongue movements (i.e., "tongue-MI" sites) at the lowest threshold for eliciting orofacial movements. The remainder were located at sites from which ICMS evoked jaw movements ("jaw-MI" sites), face movements ("face-MI" sites), or at a few sites, tongue movements and, at the same threshold intensity, either a jaw movement or a facial movement. 3. We confirmed the general organizational features of the face motor cortex that have been defined in previous studies, but we documented in detail the organizational features for tongue-MI. Thus we found that tongue movements were well represented, whereas jaw-closing movements were poorly represented; the representations for face, jaw, and tongue movements were overlapped; the same ICMS-evoked tongue movement could be multiply represented within tongue-MI; tongue-MI was characterized by a prominent input from superficial mechanosensory afferents, whereas there was little evidence for deep input; a close spatial match was found between ICMS-defined motor output and somatosensory afferent input for tongue-MI. 4. A variety of tongue movements could be evoked by ICMS at tongue-MI sites and were categorized into protrusion, retrusion, laterally directed, and other types of tongue movement. Low-threshold (i.e., less than or equal to 5 microA) ICMS-defined tongue-MI sites, which were considered to represent "efferent zones" projecting relatively directly to motoneurons, were reconstructed three dimensionally to provide insights into the spatial organization of tongue-MI. Examples of each of the four low-threshold efferent-zone categories were usually found throughout the ICMS-defined tongue-MI without any apparent preferential distribution. Furthermore, different low-threshold efferent-zone categories had close spatial relationships to each other in cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional properties of single neurons in the face primary motor cortex of the primate. III. Relations with different directions of trained tongue protrusion.

1. In previous papers we presented evidence pointing to an important role for face motor cortex in the control of tongue movements. Intracortical microstimulation (ICMS) at many sites within face motor cortex evoked different types of tongue movement, and many neurons at these "tongue-MI" sites received intraoral mechanosensitive afferent input, and their activity was related to a tongue-protrusion task performed by a monkey. In view of the synergistic action of the various tongue muscles during tongue movement, we hypothesized that these different tongue-MI sites are recruited to effect the appropriate change in tongue shape and position during a tongue-protrusion movement. A prediction from this hypothesis is that variations in the direction of a tongue-protrusion movement should be associated with variations in the activities within the different tongue-MI efferent zones. Differences in efferent-zone activity should be reflected in differences in the firing rates of neurons that are located at these tongue-MI sites. 2. We trained two monkeys to perform a tongue-protrusion task at each of three directions. The tongue-protrusion task transducer was positioned at 0 degrees (Ts), 30 degrees to the left (Tlt), or 30 degrees to the right (Trt) from the midsagittal plane; the latter two positions were termed asymmetric tongue-protrusion task positions. Single neurons were recorded from tongue-MI during trials of tongue-protrusion task at each of two or three of the above positions. Some of the neurons were also studied during a biting task. In addition, neurons were tested for possible mechanosensitive afferent input. 3. Of the 66 neurons studied, 31 (45%) exhibited directional relations; that is, the change in firing rate between the pretrial period (PTP) and the task period for the tongue-protrusion task was significantly different for each neuron depending on the direction in which the activity of the neuron was studied. 4. The "directional" neurons exhibited a single preferred direction of firing in that the mean firing rate during one direction of tongue-protrusion task was significantly greater than for any other direction. Of the 20 neurons studied at all three directions of tongue-protrusion task, the mean firing rate of each of 18 was highest at one of the asymmetric positions, and 12 of these 18 neurons exhibited a monotonic decrease in absolute firing frequency from one asymmetric task direction to the other. 5. Thirteen of the neurons were also studied while the monkey performed the biting task. Most tongue-MI directional neurons were not related to the biting task.(ABSTRACT TRUNCATED AT 400 WORDS)

Low threshold unilateral and bilateral facial movements evoked by motor cortex stimulation in cats

Organizational features of the ipsilateral representation in the cat face motor cortex were investigated by using the technique of intracortical microstimulation (< 30 μA). In 4 intact animals, 61 efferent zones controlling facial muscles were identified. They were devoted to contralateral muscles (contralateral efferent zones; n = 35), ipsilateral muscles (ipsilateral efferent zones; n = 8) or symmetrical muscles of both sides (bilateral efferent zones; n = 18). Contralateral efferent zones were found within both the rostral part of the coronal gyrus and the lateral bank of the presylvian sulcus, whereas ipsilateral and bilateral efferent zones were exclusively localized to the rostromedical region of the face motor cortex. Movement thresholds proved to be lowest at the contralateral efferent zones and highest at the ipsilateral efferent zones, with intermediate values at the bilateral efferent zones. Latencies of suprathreshold EMG responses evoked from contralateral and ipsilateral efferent zones were the shortest and the longest, respectively; intermediate values were found upon stimulation of sites from bilateral efferent zones. The efferent zones identified in 3 lesioned animals (two cats with contralateral motor cortex ablation and one cat with transection of the rostral two thirds of the corpus callosum) were contralateral ( n = 14), bilateral ( n = 13), and ipsilateral ( n = 9). Mean thresholds of effective sites from bilateral and ipsilateral efferent zones in lesioned preparations were not significantly higher than those in intact animals. This would thus suggest that extracallosal pathways may account for ipsilateral responses.

Further investigations of the efferent linkage of the supplementary motor area (SMA) with the spinal cord in the monkey.

Intracortical microstimulation of the supplementary motor area (SMA) was studied in awake monkeys. With short trains of micropulses, contralateral muscle twitches mainly in shoulder and proximal arm muscles were elicited. There was an indication of a rostro-caudal representation of distal to proximal forelimb, trunk, and proximal to distal hindlimb muscles. However, an intermingling of efferent zones was much more prominent as compared to the precentral motor cortex (MI). All efferent zones to the spinal cord were clustered in the caudal half of the SMA, and we failed to detect face and ocular movements (except at one stimulation site) when microstimulating the rostral portions of the SMA. Single micropulses were also injected in efferent zones of the microexcitable cortex in order to investigate post-pulse facilitation of sustained EMG activity. For motor cortex (MI) stimulation, post-pulse facilitation was prominent and observed in 14 of 17 tested stimulation sites. The incidence of facilitation in comparable muscles obtained with SMA stimulation in comparable muscles obtained with SMA stimulation was only 20 out of 54 tests. The onset latencies of EMG modulation obtained from the two areas were in the same range but the amount of modulation in the SMA was less conspicuous than in MI. These results indicate that the SMA has oligo- or possibly even monosynaptic connections with motoneurones, but that these connections are less dense than those from MI.

Is the hindlimb representation of the rat's cortex a ‘sensorimotor amalgam’?

In the rat, the hindlimb representation of the sensorimotor cortex is characterized by the presence of large pyramids in the fifth layer and a dense granular layer (‘sensorimotor amalgam’). The objective was to investigate in the rat, whether or not the efferent zones to the gastrocnemius muscle and the proprioceptive feedback projection from that muscle to the cortex are co-extensive. To this end, the proprioceptive zone was mapped by means of field potentials and single unit discharges evoked by controlled longitudinal displacements of the gastrocnemius tendon. The efferent zones to the gastrocnemius muscle were mapped by means of intracortical microstimulation (ICMS; < 30μA). The proprioceptive zone occupied a territory extending from 1.0 to 2.5 mm caudal to the bregma and from 2.0 to 3.0 mm lateral from the midline. The response properties were similar to those observed previously in area 3a of monkeys. For sinusoidal displacements threshold amplitude decreased with increasing strecth frequency. The modal value of response latency was 7 ms, the shortest latency 4 ms. The ICMS zone lay 0.5 to 1.5 mm caudal to bregma having an overlap of 0.5 mm with the proprioceptive region. The proprioceptive as well as the motor areas lay within the granular cortex, but overlapped only to a small extent.

The callosal system of the superior parietal lobule in the monkey.

The callosal connections of the superior parietal lobule, area 5 of Brodmann, were studied in macaque monkeys (M. nemestrina and M. fascicularis) using anatomical techniques based on both anterograde and retrograde axoplasmic transport of wheat-germ-agglutinin-conjugated horseradish peroxidase. From sagittal sections, two-dimensional flattened computer reconstructions of the volumes of cortical tissue containing callosal-projecting neurons (callosal efferent zone) and/or callosal terminal axons (callosal terminal territory) were obtained. Callosal zones were found in area 5, including the supplementary sensory area, in a limited part of area 6, i.e., in the supplementary motor area, in area 7b, in the cortex of the dorsal bank of the sylvian fissure, and in a limited part of area 7a, in the cortex of the upper third of the rostral bank of the superior temporal sulcus. Callosal neurons in all cortical areas studied, though with regional variations, predominated in layer IIIb, but were also very numerous in layers VI and V. They were rare in other cortical laminae. In the cortical regions projecting heterotopically to area 5, the tangential distribution of callosal neurons was discontinuous because of the presence of large acallosal regions. These were not observed in area 5, although here the distribution of callosal neurons waxed and waned in the tangential cortical plane. Callosal axons to and/or from area 5 crossed the midline in the posterior, presplenial part of the corpus callosum. In the superior parietal lobule they terminated in radial patches or columns, spanning layers I-IV. These columns of various width (200-2,000 micron) were separated by gaps of similar size, free of such terminals. Callosal neurons were present not only within, but also between, the callosal terminal columns. Callosal neurons located within the callosal terminal columns were, in a statistically significant way, more numerous than those located between them. The callosal efferent zone occupied 71% of the tangential domain of area 5, whereas the callosal terminal territory occupied only 49% of it. This difference is statistically significant. The discontinuous columnar arrangement of callosal terminals and the periodic distribution of callosal neurons in the lateral part of area 5 defined three main bands of callosal connections of irregular shape which were oriented mediolaterally and ran parallel to the main architectonic borders, the border between areas 2 and 5 and that between 5 and 7.

Focal and diffuse metabolic changes in the spinal cord of the monkey elicited by microstimulation of differing motor cortical foci

The pattern of projections from identified motor cortical efferent zones onto the spinal cors was examined in the monkey using a combination of microstimulation and [ 14C]-deoxyglucose metabolic labeling. Regional differences were found in the branching patterns between the projections of proximal and distal forelimb foci of the motor cortes onto the spinal cord. Cortical efferent zones related to proximal movements project extensively within the spinal cord activating numerous motor nuclei. In contrast, the corticospinal projection of a distal movement zone was found to be significantly more restricted in overall rostrocaudal extent projecting with greatest density onto a small region of the ventrolateral horn.

Experiments on functional role of peripheral input to motor cortex during voluntary movements in the monkey.

The functional role of sensory input to the motor cortex was studied by interrupting two major input pathways. One was the dorsal column, which sends the input directly through the thalamus to the motor cortex, and the other was the sensory cortex, which transfers its input through association fibers. Removal of the sensory cortex produced very little motor disturbances and the function recovered within a week. Section of the dorsal column produced some motor deficit, but the deficit was not severe and the animals recovered nearly completely within 2 wk. Combination of dorsal column section and sensory cortex removal produced severe motor deficits. These consisted of loss of orientation within extrapersonal space and loss of dexterity of individual fingers. These deficits never recovered within the duration of observation, which lasted 4-5 wk. It is concluded that the direct sensory input from the thalamus plays an important role in the control of voluntary movements, but loss of its function can be compensated by the input from the sensory cortex. The possible neuronal basis for the observed motor deficits is discussed and it is proposed that the sensory input functions by selectively changing the excitability of cortical efferent zones before and during the execution of voluntary movements. Recovery of motor function following dorsal column section occurred in parallel with the recovery of sensory input to the motor cortex. The recovered function and sensory input disappeared again following section of the association fibers from the sensory cortex. Neuronal mechanism for this observation is also discussed.

Spatial dissociation of visual inputs alters the origin of the corpus callosum

The distribution of the origin of corpus callosum neurons was investigated in cats reared with an optically induced strabismus by applying horseradish peroxidase (HRP) to severed callosal axons. These animals demonstrated an enlargement of the region of callosal connectivity compared to normal cats. Bilaterally there was an expanded efferent zone, with callosal cell bodies widely distributed in area 17, extending down the medial bank of the lateral gyrus halfway to the fundus of the splenial sulcus. This suggests that rearing a cat with visual spatial dissociation requires additional communication between the hemispheres in the form of increased callosal connections between cortical regions representing more peripheral portions of the visual field.

Corticospinal neurones of the supplementary motor area of monkeys. A single unit study.

The direct projection from the supplementary motor area (SMA) to the spinal cord was investigated in six monkeys by means of antidromic identification of single SMA neurones. The exploration concentrated on that portion of medial area 6 from which movements were found to be elicited by stimulation at intensities of less than 30 microA in an earlier study, but also included some of medial area 4. Of 315 identified corticofugal projection neurones, 234 were found to be localized within medial area 6; of these only one third (76 cells) were corticospinal cells and the remaining two thirds were neurones which projected to the brainstem. The conduction velocities of the descending projection neurones of the SMA were slow (modal value: 10 m/s). Corticospinal cells of the SMA were found up to 6 mm rostral to the boundary between areas 4 and 6. Corticospinal neurones activated antidromically from the cervical but not from the lumbar cord ('cervico-thoracic' neurones) were concentrated in the mesial cortex; 'lumbo-sacral' neurones were found both in the dorsal cortex and the dorsal bank of the cingulate sulcus. However, there was considerable intermingling between the two types of projection neurones and there was no separation in the rostro-caudal direction. Similarly, projection neurones receiving orthodromic inputs from the somatotopical subdivisions of the precentral cortex were not segregated, but were intermingled in the entire rostro-caudal extent of the SMA. It is concluded that there is a clustering of corticospinal neurones in the SMA according to their most caudal segmental projection. However, no rostro-caudal differentiation into face, arm and leg areas was established. This observation is consistent with the results of a previous study in which corticospinal neurones in the SMA were labelled with anatomical tracers and efferent zones were investigated with intra-cortical microstimulation (Macpherson et al. 1982).

Growth and reshaping of axons in the establishment of visual callosal connections.

The visual cortical areas in the two hemispheres are interconnected by axons running through the corpus callosum. In adult cats, these axons originate from, and terminate in, tangentially restricted portions of each area. In young kittens, however, callosal axons originate from the entire extent of each area, although they apparently enter the gray matter only in the restricted regions where they will also be found in adults. In kittens, but not in adults, callosal axons also reach other regions, but there they appear to be confined to the lowest part of layer VI. During the first two postnatal months, the callosal efferent zones become progressively restricted to their adult locations. During this process, many neurons eliminate the axons (or axon collaterals) that they had formerly sent through the corpus callosum and form permanent connection ipsilaterally.

Postnatal shaping of callosal connections from sensory areas.

Horseradish peroxidase (HRP) was injected unilaterally into the first and second visual areas (V1 and V2; areas 17 and 18) of 20 kittens aged between 2 and 90 days and into the second somatosensory area (S2) of 16 kittens aged between 1 and 52 days. The radial and tangential (normal and parallel to the pial surface, respectively) distributions of neurones giving origin to callosal axons (callosal neurones) were studied. In adult cats, callosal efferent zones (CZs) are defined by the distribution of callosal neurones. CZs occupy in the visual cortices, tangentially and radially restricted parts of areas 17, 18, 19 of the lateral suprasylvian gyrus and in the somatosensory cortices, parts of S1 and S2. At birth, callosal neurones are distributed throughout the tangential extent of visual and somatosensory areas; they are also more widespread in depth than in the adult. During the first postnatal month, as a result of the gradual disappearance of callosal neurones from parts of the visual and somatosensory areas, the adult CZs emerge. The CZ in areas 17 and 18 undergoes a further tangential reduction during the second and third postnatal months.

Spontaneous electrical activity of the claustrum

The dynamics of spontaneous electrical activity of the rostral, dorsal, ventral, and caudal parts of the claustrum were studied in chronic experiments on 12 dogs with implanted electrodes during extinction of orienting reactions and during biologically meaningful changes in the animals' functional state. In the animal at rest, electrical activity of the claustrum was similar to that of subcortical structures, but in a state of attention, its activity began to resemble that of the electrocorticogram, in agreement with the view that this nucleus occupies an intermediate position between cortical and subcortical structures. Enhancement of fast activity in the claustrum with the appearance of orienting reactions was expressed more intensively, and during extinction it was preserved for a longer time, in the dorsal and ventral parts of the claustrum than in the rostral and caudal parts, evidence of structural heterogeneity of the nucleus. Under conditions of increased food excitability the amplitude and frequency of the fast waves recorded from the rostral and caudal parts of the claustrum were selectively increased, whereas during reflexes to a rejected stimulus (acids) and in defensive reflexes they were increased in recordings from the dorsal and ventral parts, on which basis the existence of specialized efferent zones participating in biologically different types of activity can be postulated. Enhancement of fast activity in the claustrum during reflexes to food and acid differed in its genesis: The switch from the first to the second took place through a phase of suppression of the first. Differentiation of a food-conditioned stimulus was accompanied by the development of active inhibition in the "food zones" of the claustrum and by enhancement of activity in the "zones of orienting reactions."

Two Types of Brain Plasticity?

Brain plasticity has two aspects: on one hand, abnormal distribution of axon terminals in their target structures can take place (afferent plasticity). It can be based on collateral sprouting, rerouting of axons, or on the stabilization of transitorily widespread axonal terminations at early developmental stages. This kind of plasticity can still occur, to some extent, in the adult brain. On the other hand, deviations from the normal selection of projection neurons from transitorily exuberant populations can occur (efferent plasticity). In the cortex, they result in abnormal (i.e., for size, location, distribution of neurons) efferent zones possibly accompanied by changes in the terminal territories to which the normal efferent zones would have projected. This kind of plasticity is limited to the immature brain. In both the visual and somatosensory systems, the set of connections established through the corpus callosum shows one main feature: the neurons of origin of the corpus callosum (callosal neurons) are unevenly distributed within limited portions (callosal zones) of the main visual and somatosensory representations. The main consequence of this distribution is that, in the messages that sensory areas send through the corpus callosum, the cortical sensory maps are incomplete.