File Name: SLS1_12.DOC

EXPERIMENT SLS1 BSP12

CEREBRAL CORTEX ULTRASTRUCTURE

[PIs:]{.underline}

[Russia:]{.underline}

L. N. Dyachkova, Institute of Evolutionary Morphology and Ecology of
Animals, Russian Academy of Sciences, Moscow

[US:]{.underline}

O. Lowry, Washington University, Saint-Louis, Missouri

[Introduction]{.underline}

Study of mammalian adaptation to spaceflight effects is closely related
to an investigation of adaptive changes of the nervous system and its
highly organized structure - cerebral cortex. Earlier examinations of
the ultrastructure of the cerebral cortex of rats flown for 7 or 14 days
on Cosmos-1667, Cosmos-1887 or Cosmos-2044 allowed the conclusion that
the system of interneuronal contacts of the neocortex showed the highest
level of adaptive changes in microgravity \[2, 8, 9\]. In order to
follow the time course of adaptive changes in the ultrastructure of the
cerebral cortex, we examined the motor, somatosensory, visual and
olfactory cortex of the rats flown for 9 days on U.S. SLS-1.

[Materials and Methods]{.underline}

Fragments of the motor, somatosensory, visual and olfactory cortex of
the right hemisphere of male rats (Sprague-Dowley) flown in space and
dissected immediately after recovery (R+0), vivarium control rats (V+0),
flight rats allowed to survive for 9 days after flight (R+9), and their
vivarium controls (V+9) were fixed within 3 min after animal
decapitation by placing the biosamples into 2 ml of 2.5% glutaraldehyde
in 0.1 M cacodylate buffer, pH 7.3, at +4 o C. The rats were decapitated
2-3 hours after recovery. Brain dissection and fixation in
glutaraldehyde were performed by a U.S. investigator at the Columbia
recovery site. The fixed samples were shipped (at +4 o C) to NASA Ames
Research Center, where a Russian scientist performed all subsequent
procedures, including post-fixation, dehydration and embedding in
araldite. The scientist cut cortical fragments into 0.3-0.5 mm sections,
post-fixed with 2% OsO4 in 0.1 M cacodylate buffer, pH 7.3, at +4 o C,
dehydrated in ethanol solutions of increasing concentrations and acetone
that was changed twice, and embedded in araldite. While embedding in
araldite, brain sections were oriented in such a way as to have all
cortical layers sectioned in frontal ultrathin sections. Ultrathin
frontal sections of 3-5 blocks of each cortical structure were placed
onto blends and grids, contrasted and examined in a JEM-5U electron
microscope at X 5,000-15,000. Biosamples from 5 rats of each group were
investigated.

[Results]{.underline}

Electron microscopy of fragments of the cerebral cortex of vivarium
controls (V+0) (Figures 1, 2) as well as other animal groups showed that
the cortex ultrastructure was well preserved. This evidenced a high
reliability of the method, which had been previously developed in
relation to gravitational neuromorphology studies \[7\] and was
successfully used for this experiment: rapid fixation of rat brain
fragments by placing them into a fixative, and their subsequent
dehydration and embedding in epoxide resin.

Comparative analysis of the ultrastructure of the cerebral cortex of
flight (R+0) and vivarium control (V+0) rats revealed changes in
neurons, interneuronal contact, glia and capillaries that developed
during space flight. Structural features of the changes were similar in
all brain structures taken under study, the differences being limited to
the degree and localization of the changes.

[Ultrastructure of the somatosensory and motor cortex of the flight R+0
rats.]{.underline} In the somatosensory and motor structures of the
cerebral cortex of the flight R+0 rats, ultrastructural changes were
most distinct in the neuropile of surface layers, particularly in
structural components of interneuronal contacts. In the cerebral cortex
of the flight rats, degenerating axonal terminals were frequently
detected. Most terminals underwent changes of the light type
degeneration (Figure 3). In this case, the electron density of the
matrix was decreased, the number of synaptic vesicles was reduced, and
some mitochondria were destroyed. The majority of those terminals showed
an insignificant amount (5 to 10) of intact synaptic vesicles located
near the presynaptic membrane. The light type degenerating terminals, as
a rule, had intact outer and synaptic membranes. It should be noted that
some terminals contained not only a small number of synaptic vesicles
but also vesicles which were 2-2.5 times larger than synaptic vesicles
and were similar in their appearance to growth cone vesicles.
Occasionally, in the area, where the vesicles accumulated, the surface
of the terminals was swollen. It can therefore be postulated that in
microgravity the process of light type degeneration did not lead to a
complete destruction of terminals, and indicated only a decrease or
arrest of functional activity of one or several synapses formed by the
terminal. The terminal could be involved in the formation of a growth
cone and new synapses, the localization of which was evidently
determined by the pattern of functional effects in space flight.
Terminals that underwent light type degeneration were mainly presynaptic
structures of axo-dendrite synapses located in the II - IV layers of the
motor and somatosensory cortex. They were concentrated in the lower
third of the III layer and IV layer of the somatosensory cortex,
primarily, on thin dendrite branches and spines, occurring occasionally
on dendrites of

![](media/image1.pct){width="5.875in" height="4.041666666666667in"}

[Figure 1.]{.underline} Motor cortex of a vivarium control rat (C+0)

Neruonal body and meuropile portion

N - neuronal nucleus

C - neuronal cytoplasm

NP - neuropile

x 18,000

![](media/image2.pct){width="5.791666666666667in"
height="4.194444444444445in"}

[Figure 2.]{.underline} Motor cortex of a vivarium control rat (C+0)

Neruonal body and meuropile portion

N - neuronal nucleus

C - neuronal cytoplasm

NP - neuropile

x 15,000

![](media/image3.pct){width="4.875in" height="3.9583333333333335in"}

[Figure 3.]{.underline} Somatosensory cortex of a flight rat (R+0)

Neruopile

LD - light-type degeneration of axomal terminals

I - inclusions in dendrite dendroplasma

GC - growth cone

x 20,000

![](media/image4.pct){width="5.0in" height="3.4444444444444446in"}

[Figure 4.]{.underline} Motor cortex of a flight rat (R+0)

Neruopile

DD - dark-type degeneration of axomal terminals

I - inclusions in dendrite dendroplasma

x 20,000

large and medium diameters and on cellular bodies. Light type
degeneration also developed in presynaptic structures of fibers that
formed de passage synapses on pyramidal cells of deeply located layers
of the motor cortex. The number of terminals that developed dark type
degeneration was significantly less than that of terminals showing light
type degeneration. The majority of terminals which underwent dark type
degeneration was at an early stage of the process (Figure 4). The
terminals of dark type degeneration were diffusely distributed in the
II - IV layers (they frequently occurred in the lower third of the III
layer) and were mainly localized on dendrites and spines. Axonal growth
cones were regularly observed in the motor and somatosensory cortex of
the flight R+0 rats (Figure 3). They were diffused in all layers of the
cortex and mainly

in the neuropile, being rarely in contact with the surface membrane of
neurons. Growth cones of dendrites were, as a rule, located in the same
structures of the cerebral cortex under examination as growth cones of
axons. In addition to changes in synaptic contacts, which suggested a
decrease (in the case of light type degeneration) or termination ( in
the case of dark type degeneration) of synaptic transmission, the
cerebral structures of the flight rats (R+0) exhibited highly active
synaptic contacts more often than those of vivarium controls (V+0).
Also, ultrastructural changes of dendrites of the motor and
somatosensory cortex were seen in the flight rats. These changes were
mainly located

in post-synaptic structures of dendrite branches and spines. Those
latter displayed destruction and disorientation of components of the
spine apparatus. Thin branches and stems of dendrites showed
vacuolization of tubules, destruction of mitochondria, various
inclusions of the cytolysome type and various membrane formations
(Figures 3 and 4). In some cases dendrites developed autophagocytosis,
mainly in the areas where degenerating synapses were located. The
layer-by-layer localization of dendrites, the cytoplasm of which showed
the above changes, was identical to that of axonal terminals, which
underwent light type degeneration.

The ultrastructure of neuronal bodies of the motor and somatosensory
cortex of the flight R+0 rats was very similar to that of vivarium
controls V+0. However, the ultrastructure of stellate and large
pyramidal cells of R+0 and V+0 rats was different. Bodies of some
stellate cells of the II-IV layers of the above structure of the flight
R+0 rats showed a higher activity. This was evidenced by redistribution
of nuclear chromatin, increase in the number of bound and free ribosomes
in the cytoplasm, increase in the number of elements of the Golgi
complex, particularly its vesicular component. It should be noted that
neurons with an enhanced functional activity normally formed groups
consisting of 3-5 cells. These groups were alternating with groups of
cells, whose ultrastructure did not differ from that of stellate neurons
of vivarium controls. The

ultrastructure of large pyramidal neurons of the V layer of the motor
cortex suggested that the functional activity was, as a rule, not high.

The motor and somatosensory cortex of the flight R+0 rats showed, when
compared to those of vivarium controls, a slightly greater density of
distribution of glial cell processes in the neuropile. The increased
distribution density of elements of the oligodendroglia was very
distinct near the neuronal surface. Also, more frequently than controls
the R+0 rats showed subsurface membrane structures in the neuronal
cytoplasm near the outer membrane of the cell contacting the glia; those
structures were, as a rule, arranged in 2-3 layers (Figure 5).

The motor and somatosensory cortex of the flight rats exhibited a
slightly greater than that of the controls density of distribution of
capillaries, mainly in the I-IV layers. In terms of their structure,
those capillaries remained intact and similar to that of capillaries of
the same brain structures of control animals.

[The ultrastructure of the visual cortex of flight F+0
rats.]{.underline} The visual cortex of the R+0 rats exhibited, mainly,
the same changes as the somatosensory cortex of those animals (Figures
6, 7). Terminals that underwent light type degeneration were primarily
located in the area of synaptic zones of the neuropile of the I-IV
layers. Terminals that underwent dark type degeneration were few, being
detected in all cortical layers. The distribution density of light type
degenerating terminals was also significantly lower than that in the
motor and somatosensory areas. Growth cones of dendrites and axons were,
as a rule, located in the neuropile of the I-IV layers of the visual
cortex. The ultrastructure of a substantial amount of synapses on thin
dendrite branches and spines pointed to a high functional activity of
those interneuronal contacts. The distribution density of those contacts
was particularly high in the synaptic zones of the neuropile of the IV
layer of the visual cortex. The dendroplasm of wide and medium-size
dendrites of the II-V layers typically contained various inclusions and
occasionally autophagocytotic changes. The ultrastructure of a large
portion of neurons demonstrated their high functional activity.

The visual cortex of the flight R+0 rats, as was also the case with the
motor and somatosensory cortex, showed a slight enlargement of the area
occupied by glial elements; however, in the visual cortex the
enlargement was significantly less expressed. The most distinct increase
was seen in the area taken by elements of the oligodendroglia located
near cellular surfaces and wide dendrites, mainly, in the II-V layers of
the cortex.

In the visual cortex of the R+0 rats, as compared to the vivarium
controls (V+0) the number of capillaries was slightly increased; the
increase was more marked in the IV

![](media/image5.pct){width="4.916666666666667in"
height="3.9166666666666665in"}

[Figure 5.]{.underline} Motor cortex of a flight rat (R+0)

Neruon contacting with a glial cell

MS - subsurface membrane structure in the neuronal cytoplasm in the area
of the neuroglial contact

N - neuronal nucleus

C - neuronal cytoplasm

NG - nucleus of a glial cell

CG - cytoplasm of a glial cell

x 20,000

![](media/image6.pct){width="5.125in" height="2.1944444444444446in"}

[Figure 6.]{.underline} Visual cortex of a flight rat (R+0)

Neruopile

I - Inclusion in dendrite dendroplasma

x 23,000

![](media/image7.pct){width="5.111111111111111in"
height="2.4305555555555554in"}

[Figure 7.]{.underline} Visual cortex of a flight rat (R+0)

Neruopile

I - inclusions in dendrite dendroplasma

GC - growth cone

x 23,000

layer, where capillaries formed focal aggregates. The capillary density
distribution decreased toward the II and V layers.

[The ultrastructure of the olfactory cortex of the flight R+0
rats.]{.underline} The olfactory, i.e., paleocortex of the R+0 rats
developed changes in the ultrastructural elements of the neuropile which
were similar to those in the neocortex (Figures 8, 9). Light type
degeneration of presynaptic terminals was observed, predominantly, in
the area of thin dendrite branches and axo-spine contacts. The
distribution density of dark type degenerating terminals was low. Growth
cones of axons and dendrites were located in the areas of the finely
arranged neuropile. The number of growing elements was, as a rule,
greater than that of degenerating elements. The distribution density of
synapses in the R+0 group was similar to that in the controls.
Destructive changes were seen in the post-synaptic structures of small
and medium dendrite branches. In some axo-spine synapses the integrity
of the spine apparatus was disturbed. In contrast to the neocortex,
autophagocytosis of dendrites occurred very rarely. The ultrastructure
of some neurons pointed to their enhanced functional activity. Also, the
number and size of glial processes in the neuropile and near cellular
elements were increased. In the olfactory cortex of the R+0 rats the
distribution density of capillaries increased. Changes in the capillary
structure of the olfactory cortex were more distinct than in the
neocortical structures under study. In addition to well preserved
capillaries, abnormal capillaries were seen: they showed fiber
separation and vacuolization of the basal membrane and swelling of
pericapillary processes of astrocytes.

[The ultrastructure of the cerebral cortex of the flight rats sacrificed
at R+9.]{.underline} In the somatosensory and motor structures of the
brain cortex of the R+9 rats, the ultrastructure of a significant number
of axo-dendrite synapses (III-IV layers) showed signs of their high
functional activity; in this case the number of axo-dendrite synapses
with a high functional activity was significantly greater than in the
R+0 rats. The neuropile of the II-IV layers showed terminals that
underwent dark type degeneration, their majority being located in the
cytoplasm of glial cells. Those layers also contained an insignificant
number of terminals which underwent light type degeneration and single
growth cones. The distribution density of glial elements in the
neuropile of the II-IV layers was similar to that of the R+0 rats. Those
brain structures typically contained groups of 3-5 stellate cells, whose
ultrastructure demonstrated their high functional activity. As a rule,
these cells were alternating with the cells, whose ultrastructure was
indicative of moderate or low functional activity. The ultrastructure of
some large pyramidal cells of deeply located layers of the motor cortex
pointed to their high functional activity. The surfaces

![](media/image8.pct){width="4.833333333333333in" height="3.0in"}

[Figure 8.]{.underline} Olfactory cortex of a flight rat (R+0)

Neruopile

I - membrane inclusion in dendrite dendroplasma

x 24,000

![](media/image9.pct){width="4.833333333333333in"
height="3.4444444444444446in"}

[Figure 9.]{.underline} Olfactory cortex of a flight rat (R+0)

Neruopile

I - membrane inclusion in dendrite dendroplasma

GC - growth cone

x 18,000

of those cells showed synapses. The glia covered an insignificant
portion of the neuronal surface or was absent at all. In addition, there
were pyramidal neurons, whose ultrastructure was very close to that seen
in the R+0 rats (low functional activity and distinct glial elements).

The ultrastructure of the visual cortex of the R+9 rats was similar to
that of V+9 rats. In the neuropile of the II-IV layers there was an
insignificant amount of terminals undergoing light type degeneration,
which were uniformly distributed across the cortex. Dark type terminals
and growth cones occurred occasionally and did not exceed their number
in the V+9 rats. The distribution density of axo-dendrite synapses,
which had a high functional activity (which was in agreement with their
structural features), was slightly higher than in the V+9 rats. However,
the predominance of functionally active synapses was less distinct than
in the visual cortex of the R+0 rats. Structural characteristics of
neurons and the distribution density of glial elements were very close

to those parameters of the V+9 rats.

In the olfactory cortex of the R+9 rats ultrastructural changes were
primarily discerned in the finely arranged neuropile. Many axo-dendrite
synapses had the structure which suggested their high activity.
Terminals undergoing dark type degeneration were diffused, and their
distribution density was slightly higher than in the olfactory cortex of
the R+0 rats. The distribution density of terminals undergoing light
type generation and that of growth cones were sufficiently low but
higher than in the olfactory cortex of the V+0 rats. When compared to
the V+9 rats, the R+9 rats also showed an enlargement of the area
occupied by processes of glial cells.

[Discussion]{.underline}

The data obtained allow the conclusion that most distinct
ultrastructural changes in the cerebral cortex of rats during space
flight developed in the neuropile of surface layers of the neocortex,
primarily, in the system of interneuronal synapses. During this process
morphofunctional rearrangement of interneuronal interactions and,
consequently, rearrangement of neuronal functional systems took place in
the cerebral cortex in microgravity. Structural characteristics of the
process were destructive changes of pre- and post-synaptic structures of
some synapses, as well as emergence of growth cones of axons and
dendrites and de novo formation of synapses. Gradually developing
degeneration of axonal terminals of the light type resulted from the
reduced flow of afferent signals to the cortex in microgravity. At the
same time, in spite of the lysis of some of their internal structures
and, consequently, decline or termination of synaptic

function, these terminals maintained their potential for the functional
recovery. This was evidenced by an involvement of axonal terminals
undergoing light type degeneration in the emergence of growth cones and
de novo synapse formation. This was also supported by earlier
electrophysiological and morphological examinations, which showed that
synaptic function remained intact in the course of gradually developing
light type degeneration of presynaptic terminals in the central nervous
system of vertebrates \[4\].

On the Earth the brain of an adult mammalian is the site of continuous
processes of growth and degeneration of axonal terminals and de novo
formation of synapses required for constant rejuvenation of synapses of
central neurons \[6\]. Thus, structural transformation of the synaptic
structure of the cerebral cortex that emerged in microgravity was
qualitatively identical to that occurring in the normal function of the
central nervous system. The corresponding structures of the cerebral
cortex of the V+0 rats also showed degenerating axonal terminals and
growth cones; however, their number was significantly lower than that in
the R+0 rats. The lower third of the III layer and IV layer of the
somatosensory cortex, where light type degeneration of axonal terminals
was most distinct, was the area in which efferents of neurons of the
thalamic ventral nuclei ended, which in their turn had an input (through
spinal ganglia and Goll\'s and Burdach\'s nuclei) from extero- and
propriomechanoreceptors of the body and extremities. Due to this, a
significant increase in the number of axonal terminals of light type
degeneration in these structures gave evidence for a drastic decrease of
stimulation of mechanoreceptors of the body and extremities in
microgravity. This conclusion is supported by the conclusions derived
from previous morphological investigations of the cerebral cortex and
spinal ganglia of rats flown for 5, 7 or 14 days in space \[8, 9, 10,
11\].

In contrast to light type degeneration, rapidly developing dark type
degeneration is a result of a drastic increase of afferent signals at
the input to the cerebral cortex after recovery. This conclusion finds
support in the predominance of dark terminals at early stages (early
hours) of dark degeneration, as well as in the large number of axonal
terminals that underwent dark type degeneration in the neuropile of the
flown rats, which were exposed to the Earth\'s gravity for a longer time
than the SLS-1 rats (those flown on Cosmos-1667, -1887 or -2044) \[2, 9,
10\].

An increase in the number of highly active synapses, including synapses
formed by presynaptic terminals undergoing light type degeneration can
probably be attributed to an increase of afferent signals after
recovery. It can be assumed that in certain cases this increase in the
afferent flow can induce destructive changes in axonal terminals in the
state of light degeneration and cause dark degeneration in these
terminals.

In space flight distinct changes in the post-synaptic components of
synapses of all brain structures examined were also observed. In
addition to growth cones of dendrites and newly formed post-synaptic
structures, destructive changes in the cytoplasm of spine and dendrite
stems were detected. The concurrent process of autophagocytosis causing
changes in the outline of the outer membrane of dendrites was, probably,
one of the factors responsible for dendrite reorientation during
adaptive rearrangement of interneuronal connections in microgravity. The
formation of growth cones of dendrites, as well as post-synaptic
structures in the developing synapses evidently occurred as a result of
the effect from presynaptic structures. This was in agreement with the
concept postulating that the development and integrity of post-synaptic
components of synapses in various structures of the nervous system was
determined by the stimulation from the presynaptic structures \[6\].
Conversely, the formation of growth cones also involved stellate cells
of the II-IV layers of the neocortex, where the amount of components of
the Golgi complex contributing to the formation of growth cones was also
increased \[3\].

The fact that the above ultrastructural changes were well visible in all
brain structures of the flight animals suggests that the cerebral cortex
was actively involved in adaptive transformations in microgravity.

On the contrary, the observation that the structure of a significant
portion of capillaries was well preserved and that their number
increased in the cerebral cortex indicates that the autoregulation
mechanisms of cerebral circulation in the areas of adaptive changes of
synaptic structures remained intact.

The data accumulated suggest that structural components of the cerebral
cortex constitute a single system, whose nervous, glial and vascular
elements are closely interrelated to support adaptive responses of the
brain.

Comparative analysis of ultrastructural findings in the cerebral cortex
of animals exposed to microgravity for varying periods of time
demonstrated a certain sequence of developments in the structural
components of the brain.

After a 7-day flight (Cosmos-1667) \[10\] ultrastructural changes in the
cerebral cortex were evidently at an early, highly reactive stage of
adaptation, which was structurally characterized by: a) destructive
changes of presynaptic, axonal terminals including both light type and
dark type degeneration; those changes resulted from, on the one hand,
decrease of afferent flow in microgravity and, on the other, its
increase postflight, i.e. at the Earth\'s gravity; b) formation of
growth cones of axons and dendrites as the foundation for a new pattern
of synaptic arrangement to meet the microgravity requirements. An
increase in the number of capillaries in the cortical structures
examined was in conformity with active adaptive developments.

After 14-day flights (Cosmos-1887 and Cosmos-2044) \[9\] signs of
further development of adaptive responses in the brain were seen.
Together with ongoing degeneration of terminals and formation of growth
cones, changes occurred in dendrites and glial cells. Some spines showed
destructive changes of the spine apparatus, whereas other spines
(probably, some of them were newly formed) developed a new apparatus.
Besides, the number of dendrites with autophagocytosis increased
drastically in the flight animals (primarily, in the area of
degenerating synapses). This process led to a modification of the outer
outline of the dendrite and thus facilitated dendrite reorientation,
which took place as the system of interneuronal contacts was
transforming in the course of adaptation to microgravity. Thus, at that
stage more specific (as related to the altered afferents) changes,
including the formation of new postsynaptic structures occurred. An
enlargement of the area occupied by glial cell processes facilitated to
a certain extent the delineation of not only structural but also
functional units in the synaptic zones to meet the new cerebral
requirements. An increase in the number of capillaries, as was the case
at the earlier stage, enhanced active adaptive responses of the brain.

The ultrastructure of the cerebral cortex of rats after a 9-day SLS-1
flight can be viewed as an intermediate stage between the above two
stages. The ultrastructure was characterized by a milder light type
degeneration, as compared to the 7-day flight. Lysis of internal
structures of degenerating terminals was less significant and
cytolysome-like inclusions were less frequent. Most synapses in
degenerating terminals were functionally active. Some terminals
undergoing light type degeneration were involved in the formation of
growth cones. Thus, it can be assumed that it was the stage at which the
potential capability of light type degenerating terminals to recover
began to work. As compared to the findings after a 7-day flight, the
number of dendrite growth cones increased and new synapses and
post-synaptic structures evolved. However, the formation of the spine
apparatuses was less distinct than observed after 14-day flights. The
number of capillaries was increased to a lesser degree than after a
7-day flight and was close to what was observed after 14-day flights. In
summary, ultrastructural changes in the cerebral cortex under study can
be used to identify sequential stages of rat brain adaptation to
microgravity. A more detailed analysis helped detect a host of
additional components that gave a better idea of adaptive stages in
microgravity. In the first place, this is the interrelationship and
interdependence of changes in various structural components of the
neuropile and, also the impact which structural features of different
cortical structures have on these changes.

The level of adaptive restructuring of synaptic components of the
neuropile of the cerebral cortex in microgravity was the greatest in the
upper layer of the neocortex, which developed at a later stage of
evolution.

[Conclusion]{.underline}

Electron microscopic examinations of the motor, somatosensory, visual
and olfactory cortex of the rats flown for 9 days on U.S. SLS-1
demonstrated ultrastructural changes in neuronal and glial cells, which
suggested functional changes of the cortical structures that occurred
both during and after flight. In the somatosensory and motor cortex,
ultrastructural changes pointed to the following developments in
microgravity:

a\) drastic decrease of the afferent flow to the cortex; b) reduction of
the afferent flow to large pyramidal neurons of the V layer, and c)
diminished function of large pyramidal neurons of the V layer,
evidently, responsible for the hypofunction of spinal motoneurons, which
was previously detected morphologically by other authors. At the same
time, the ultrastructure of some axonal terminals, axo-dendrite synapses
and stellate cells gave evidence that the synapses and stellate cells
were in the excitation state, which was associated with an increased
afferent flow to the cortex during 2 - 3 hours after recovery. The
ultrastructure of the somatosensory and motor cortex 9 days after
recovery indicated both an enhanced afferent flow to the cortex on the
Earth, as well as an increased functional activity of large pyramidal
neurons of the V layer. In the visual cortex of the flight rats,
ultrastructural changes were similar to those in the somatosensory
cortex but were less significant, which also evidenced a slight decrease
of the afferent flow to the visual cortex in microgravity. At the same
time, the high functional activity of synapses of the IV layer of the
visual cortex and that of neurons gave evidence that the visual flow
evidently increased after recovery. However, 9-day exposure of the
animals to the Earth\'s gravity normalized the functional state of
structural elements of the visual cortex. Ultrastructural changes of the
olfactory cortex suggested a slight decrease of the afferent flow and an
increase of the functional activity of neurons postflight. The
ultrastructure of axonal terminals, dendrites, synaptic contacts and
post-synaptic structures, increased number of axonal and dendrite growth
cones, enlarged area occupied by glial cell processes, and a greater
number of capillaries in the cortical structures of the rats flown for 9
days pointed to an active restructuring in the cortical connections,
which formed the structural foundation for the adaptation of the
cerebral cortex to microgravity.

[References]{.underline}

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[Legend to Figures]{.underline}

[Fig. 1.]{.underline} Motor cortex of the brain of a vivarium control
rat (V+0). Neuronal body and neuropile

N - nucleus of the neuron C - cytoplasm of the neuron NP - neuropile
X18,000

[Fig. 2.]{.underline} Motor cortex of the brain of a vivarium control
rat (V+0). Neuronal body and neuropile

N - nucleus of the neuron C - cytoplasm of the neuron NP - neuropile
X15,000

[Fig. 3.]{.underline} Somatosensory cortex of the brain of a flight rat
(R+0). Neuropile

LD - light type degeneration of axonal terminals I - inclusions into the
dendroplasm of dendrites GC - growth cone X20,000

[Fig. 4.]{.underline} Motor cortex of the brain of a flight rat (R+0).
Neuropile

DD - dark type degeneration of axonal terminals I - inclusions into the
dendroplasm of dendrites X20,000

[Fig. 5.]{.underline} Motor cortex of the brain of a flight rat (R+0).
Neuron contacting a glial cell

MS - subsurface membrane structure in the neuronal cytoplasm in
neuro-glial contact area N - nucleus of the neuron C - cytoplasm of the
neuron NG - nucleus of the glial cell CG - cytoplasm of the glial cell
X20,000

[Fig. 6.]{.underline} Visual cortex of the brain of a flight rat (R+0).
Neuropile

I - inclusions into the dendroplasm of dendrites X23,000

[Fig. 7.]{.underline} Visual cortex of the brain of a flight rat (R+0).
Neuropile

I - inclusions into the dendroplasm of dendrites GC - growth cone
X23,000

[Fig. 8.]{.underline} Olfactory cortex of the brain of a flight rat
(R+0). Neuropile

I - membrane inclusions into the dendroplasm of dendrites X24,000

[Fig. 9.]{.underline} Olfactory cortex of the brain of a flight rat
(R+0). Neuropile

I - membrane inclusions into the dendroplasm of dendrites GC - growth
cone

X18,000