Degeneration and regeneration of the nervous system

The necessity of interhemispheric connections, and the nature of this necessity, are demonstrated by the following hypothetical event. An intelligent being from outer space lands on earth and is asked to design the brain of a cat. The being is intrigued to find that the body of a cat is bilaterally symmetric (he looks himself rather like a multieyed and multiwhiskered octopus).

General Organization of Callosal Connections in the Cerebral Cortex

As part of a long-term study designed to examine the ontogeny of visual memory in monkeys and its underlying neural circuitry, we have examined the connections between inferior temporal cortex and medial temporal-lobe structures in infant and adult monkeys. Inferior temporal cortical areas TEO and TE were injected with WGA conjugated to HRP and tritiated amino acids, respectively, or vice versa, in 1-week-old and 3-4-yr-old Macaca mulatta, and the distributions of labeled cells and terminals were examined in both limbic structures and temporal-lobe cortical areas. In adult monkeys, inferior temporal-limbic connections included projections from area TEO to the dorsal portion of the lateral nucleus of the amygdala and from area TE to the lateral and lateral basal nuclei; inputs to both areas TEO and TE included those from the lateral, lateral basal, and medial basal nuclei of the amygdala and to area TE from the accessory basal nucleus. Additional limbic inputs to both areas TEO and TE arose from the posterior portion of the presubiculum. In infant monkeys, we found, in addition to these adultlike connections, a projection from area TEO to the lateral basal nucleus of the amygdala. Inferior temporal cortical connections in adult monkeys included projections from area TEO to area TE and, in turn, from area TE to area TG and perirhinal area 36, as well as from area TE back to area TEO; inputs to both areas TEO and TE included those from area TG, perirhinal areas 35 and 36, and parahippocampal areas TF and TH. All of these adultlike connections were also observed in infant monkeys, but, in addition, the infants showed projections from area TE to perirhinal area 35 as well as to parahippocampal areas TF and TH, and from area TEO to area TF. Moreover, in infants, the projection from area TE to perirhinal area 36 was considerably more widespread than in adults, both in areal extent and in laminar distribution. The results therefore indicate the existence of projections in infant monkeys from inferior temporal areas to the amygdala, perirhinal cortex, and parahippocampal cortex that are either totally eliminated in adults or more refined in their distribution. Both elimination and refinement of projections thus appear to characterize the maturation of axonal pathways between the inferior temporal cortex and medial temporal-lobe structures in monkeys.

https://www.jneurosci.org/content/jneuro/11/4/1095.full.pdf

Connections of inferior temporal areas TE and TEO with medial temporal- lobe structures in infant and adult monkeys

Some of the first electrophysiological studies on somatosensory cortex of cats found that more than one cortical region could be activated by somatosensory stimuli (Adrian, 1941; Marshall et al., 1941). Adrian (1941) initially recognized that the “second” somatic receiving zone was a distinctly separate entity. He suggested, however, that this second representation may have been a special adaptation “for sheathing and unsheathing” claws in cats as a comparable second representation was not found by him in dogs or monkeys. Woolsey and coworkers (Woolsey, 1943, 1946, 1947, 1958; Woolsey and Fairman, 1946; Woolsey and Wang, 1945) were able to demonstrate, with evoked potential recording techniques, that a “second” somatic area existed in a variety of animals including many that did not have claws and that clawless parts of the body, like the face (Woolsey and Wang, 1945), were also represented in SII. Subsequent studies have repeatedly shown the presence of this second cortical region in a vast array of animal species (Table I). This “duality” of somatosensory representations, which appears to be a “general principle of cortical organization” (Woolsey, 1947), has been extended to include the idea that multiple cortical areas exist for each of the major sensory systems. In general, the terminology adopted to describe these multiple areas has been to assign numbers to each region according to the historical order of its discovery; this numbering does not imply primacy. Indeed, one of the important unresolved issues remains the determination of the unique contributions of each of the somatosensory cortical areas to sensory-motor behavior.

Second Somatosensory Cortex and Related Areas

The sections in this article are:


1
Organization of Prethalamic Components of Somatic Afferent System
1.1
Dorsal Column System

1.2
Ascending Systems Originating in Dorsal Horn

1.3
Dorsal Column Nuclear Complex and Its Efferent Projections

1.4
Somesthetic Defects Produced by Lesions of Spinal Pathways

1.5
Summary: Prethalamic Components




2
Functional Organization of Thalamocortical Components of Somatic Afferent System
2.1
Thalamic Targets of Ascending Somesthetic Pathways

2.2
Organization of Thalamic Somesthetic Nuclei




3
Functional Organization of Somatic Sensory Areas of Cerebral Cortex
3.1
Introduction and Background

3.2
Structure and Connectivity of Somatic Sensory Areas

3.3
Functional Organization of Postcentral Gyrus, SI

3.4
Functional Organization of Second Somatic Area, SII




4
Dynamic Neural Operations in Somesthesis
4.1
Introduction

4.2
Central Neural Mechanisms in Tactile Sensibility

4.3
Central Neural Mechanisms in Flutter-Vibration




5
Modifiability of Microconnectivity in Somatic Afferent System

Central Nervous Mechanisms in Mechanoreceptive Sensibility

The distribution of S1 (first somatosensory area) and S2 (second somatosensory area) neurons projecting to the contralateral S2 was studied with horseradish peroxidase in normal adult cats and in cats aged between 129 and 248 days in which the injected S2 area had been deprived of some of its input by an earlier lesion (on postnatal days 3 to 30; day of birth = day 1) of ipsilateral S1, alone or combined with a lesion of contralateral S2. In animals with S1 lesions, as in the normal controls, labeled neurons were selectively distributed to the regions of the trunk representation and to parts of the forelimb and hindlimb representations; however, the normally acallosal region in the forepaw representation contained scattered labeled neurons in three of the four animals whose S1 had been lesioned during the first postnatal week. In these animals, the distribution of labeled neurons in the contralateral S2 was apparently normal. Furthermore, the additional lesion of this area during the first postnatal week (one animal) did not increase the degree of filling-in of the normally acallosal parts of S1. The partial filling-in of the acallosal parts of S1 is probably due to the preservation to adulthood of some of the callosal neurons which are present in these regions during the early postnatal life. Possibly, these neurons did not disappear (or lose their callosal axons) because the neonatal lesion (i) allowed their successful competition for terminal space in contralateral S2 or (ii) induced a reorganization of the peripheral input to this area.

R. Caminiti

Papers

The postnatal development of somatosensory callosal connections after partial lesions of somatosensory areas.