Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3050_Библиотеки_им_академика_М_И_Перельмана
.pdf
220 Neurological observations (I)
Measuring consciousness is another aspect of the hard problem. To try to overcome
this difculty, a unit to measure consciousness was suggested, called the phi, which ‘cap-
tures the extent of consciousness.... The larger the phi, the richer the conscious experi-
ence’ (Koch 2015: 84).
Connectivity and consciousness
It appears that no single part of the brain is responsible for consciousness, but several
parts of the brain, working together, somehow creates the phenomenon. Multiple brain
areas linked and working together is called connectivity. The main areas involved in
the connectivity of consciousness are the cerebral cortex (notably the parietal lobe and
the prefrontal cortex, PFC), the corpus striatum and the thalamus
(Figure 11.1). Meta-
consciousness is the term used to identify the brain’s ability for awareness of being aware
(Young 2021).
MOTOR
TOUCH
PARIETAL
Cingulate cortex
VISION
OCCIPITAL
Corpus striatum
Thalamus
TEMPORAL
AUDITORY
BRAIN
STEM
(RAS)
WAKEFULNESS
FRONTAL
PROCESSING
ATTENTIO
N
P
F
C
+
S
E
N
S
O
R
Y
P
R
O
C
E
S
S
I
N
G
Figure 11.1 Left side block diagram of the brain areas in connectivity of consciousness. The three
main systems are: 1. Wakefulness, i.e. the brain stem, notably the reticular activating
system (RAS), and the thalamus, that keeps the brain awake; 2. Attention and process-
ing, i.e. the frontal lobe, notably the prefrontal cortex (PFC) and motor area; 3. Sensory
processing, i.e. the parietal, occipital and temporal lobes processing touch, vision and
auditory respectively. The subcortical areas i.e. the thalamus, corpus striatum and cin
-
gulate cortex, process and integrate the stimuli with memory, emotions, anticipation
and motor activity.
https://t.me/medicina_free
Neurological observations (I) 221
The cerebral cortex
A major brain area involved in consciousness is the cerebral cortex (Figure 11.2).
The cerebral cortex is separated into two halves, the left and right hemispheres. These
are connected deeper down by the corpus callosum, through which communication
between the two hemispheres occurs. Each hemisphere is further divided into lobes. The
frontal lobe is anterior to the parietal lobe, the temporal lobes are to the side and at the
back is the occipital lobe (Figure 11.2). The surface of each lobe is folded into ridges
(called gyri) to increase the total surface area. This allows more grey matter (neuronal cell
bodies) to be packed into the limited skull cavity.
Individual parts of the cortex are involved in specic conscious experiences. The right
frontal lobe appears to house a primary centre for the concept of self, i.e. a conscious
understanding of one’s self. The parietal lobe processes the sensory information from the
body, e.g. touch, pain etc, known as somatic sensations (soma = body).
The occipital lobe processes sensations of vision, and the temporal lobe processes
sensations of hearing. All these sensations become fully conscious as the result of fur-
ther processing in other areas of the brain, notably through the prefrontal cortex (PFC).
The PFC is at the front of the brain and incorporates the orbitofrontal cortex and the
anterior cingulate cortex (Figure 11.3). These areas carry out higher executive func-
tions such as planning and attention and add some aspects of emotion to the conscious
experience.
The cells of the cerebral cortex are arranged in a strict pattern according to function;
thus, it becomes possible to map the brain surface using Brodmann numbers, as shown
in Figure 11.4 and Table 11.1.
These functional areas are all related to the activities we identied with consciousness,
i.e. control, motor function of the frontal lobe areas, awareness, sensory functions of the
parietal, temporal and occipital lobe areas and cognition, frontal lobe and other associa-
tion areas. Association areas exist between the main functional areas of the cortex and
are essential for making sense of the sensory stimuli arriving at the cortex. They contain
memories developed from previous sensory experience that are used for comparing with
Frontal lob
e
(anterior
)
Te mporal lobe
Parietal lobe
Occipital lobe
(post
erior)
Figure 11.2 The cerebral cortex from the right side showing the major lobes.
https://t.me/medicina_free
Frontal Cingulate cortex
lobe (cingulate gyrus ˜
(frontal cortex) cingulate sulcus)
POSTERIOR
ANTERIOR
C
o
r
p
Orbito-frontal
gyrus
Anterior
Cingulate
u
s
c
a
l
l
o
s
u
m
P
r
e
f
r
o
n
t
a
l
c
o
r
t
e
x
gyrus
222 Neurological observations (I)
Figure 11.3 Internal view of the brain showing areas in the brain important for processing con-
sciousness: the frontal and prefrontal cortex and the cingulate cortex.
Frontal Parietal
Occipital
Frontal Temporal
8
6
4
3
1
2
7
40
39
19
18
17
37
21
20
22
38
42
41
43
44
45
10
47
11
46
9
Figure 11.4 Map of the left cerebr al cortex according to cell function. Each area has a Brodmann
number (Table 11.1).
https://t.me/medicina_free
Neurological observations (I) 223
Table 11.1 Brodmann numbers as shown in Figure 11.3
Brodmann Notes
number
1, 2, 3 Primary somatosensory cortex in the parietal lobe
4 Primary motor cortex in the frontal lobe
6 Premotor cortex in the frontal lobe
7 Somatosensory association cortex (i.e. the visual-motor area, e.g. location of
objects in space) in the parietal lobe
8 Part of the frontal lobe, including the frontal eye elds
9 Part of the dorsolateral prefrontal cortex (along with area 46)
10 Anterior most part of the prefrontal cortex
11 Orbitofrontal cortex in the frontal lobe
17 Primary visual cortex in the occipital lobe
18 Secondary visual area in the occipital lobe
19 Associated visual area in the occipital lobe
20 Inferior temporal gyrus in the temporal lobe
21 Middle temporal cortex in the temporal lobe
22 Superior temporal gyrus in the temporal lobe (part of Wernicke’s area, processing
language)
24 Anterior cingulate cortex (not shown in Figure 11.4, see 11.3)
29 Posterior cingulate cortex (not shown in Figure 11.4, see 11.3)
37 Fusiform gyrus in the temporal lobe (visual and language processing)
38 Temporopolar area in the anterior temporal lobe
39 Angular gyrus (part of Wernicke’s area, processing language)
40 Supramarginal gyrus (part of Wernicke’s area, processing language and many
other functions)
41 Auditory area in temporal lobe
42 Primary auditory area in temporal lobe
43 Area subcentralis, includes part of the primary gustatory area (taste)
44 Part of Broca’s area (with area 45) in the frontal lobe (verbal speech area)
45 Part of Broca’s area (with area 44) in the frontal lobe (verbal speech area)
46 Part of the dorsolateral prefrontal cortex (along with area 9)
47 Inferior prefrontal cortex of the frontal lobe (language processing)
new stimuli. Association areas must learn sensory stimulations and store them as memo-
ries for future use.
There are important association areas in the following lobes.
1 The parietal lobe sensory association area, which processes somatic sensations
2 The occipital lobe sensory association area, which processes visual sensations from the
visual cortex
3 The temporal lobe sensory association areas, which process sensations of hearing from
the auditory cortex (Figure 11.4).
Even within the main specialised functional areas of the motor cortex (frontal lobe) and
sensory cortex (parietal lobe), the cells are carefully arranged in a layout that matches the
body plan (Figure 11.5).
https://t.me/medicina_free
Neck
Shoulder
Medial
Lateral
3
3
1
2
2
1
Posterior
Tr unk
Hand
To es
Foot
Arm
Elbow
Forearm
Wrist
Head
Leg
Upper lip
Lips
Lower lip
Te eth, gums, jaw
To ngue
Pharynx
Face
Nose
Eyes
Little
Ring
Middle
Index
Thumb
Digits
Anterior
Intra-abdominal
224 Neurological observations (I)
Figure 11.5 T he sensory cortex (Brodmann areas 1, 2 and 3), showing the layout of cells accord-
ing to the areas of the body from which they receive transmissions. Notice the large
areas involved in sensation of the lips (a very sensitive area) and the large areas for the
ngers and hands compared with the feet and toes.
Subcortical areas affecting consciousness
Connectivity with some subcortical regions of the brain, i.e. below the cortex, is required
for a complete conscious experience, e.g. the corpus striatum and the thalamus, which
adds further emotional processing (Figure 11.1).
The brainstem supplies the cerebral cortex with essential information that makes con-
sciousness possible and maintains the state of awareness (Koch 2017). The brainstem has
around 40 separate nuclei, each with a different role. Some of these roles relate to con-
sciousness. One such nucleus (the rostral dorsolateral pontine tegmentum) has a pathway
that passes to two higher centres: the left ventral anterior insula (AI) and the pregenual
anterior cingulate cortex (pACC), thus forming a network (Figure 11.6; Fischer et al. 2016).
Lesions in the rostral dorsolateral pontine tegmentum are associated with coma, so this
brainstem centre is essential for arousal and maintenance of consciousness, and if this
network is disrupted the patient lapses into a coma. The two higher centres (i.e. AI and
pACC) contain cells called von Economo neurons (VENs),
1
which are also known as spin-
dle neurons because of their characteristic long spindle shape. Unlike other neurons, they
have one dendritic extension at each end (Figure 11.7). They also vary in size between
50% and 200% larger than other neurons.
Spindle neurons are also found in the dorsolateral prefrontal cortex (Figure 11.6,
Table 11.1). These cells appear to be fast-acting and form part of the emotional pathway
between the amygdala, i.e. part of the limbic system that processes basic emotions and
the frontal cortex, which provides sophisticated responses to emotions. The AI and pACC
areas are involved in responding to social cues, the sense of self and emotions such as
love, grief and anger. The reason for their distribution within a pathway that maintains
https://t.me/medicina_free
Neurological observations (I) 225
DLPC
pACC
AI
RDPT
AI
RDPT
AB
RIGHT LEFT
POSTERIOR
Cerebellum
ANTERIOR
insula
Figure 11.6 The rostral dorsolateral pontine tegmentum (RDPT) has connections with both the left
ventral anterior insula (AI) and the pregenual anterior cingulate cortex (pACC) in rela-
tion to maintaining consciousness. The dorsolateral prefrontal cortex (DLPC) is also
shown. The DLPC, AI and pACC all contain spindle neurons (Figure 11.7). (A) Left
lateral view (shown in section). (B) Anterior view (in section).
A B
Figure 11.7 Different types of neuron. (A) von Economo (or spindle) neuron. (B) Normal neuron.
https://t.me/medicina_free
226 Neurological observations (I)
consciousness has not been established. They may have a function directly linking con-
scious awareness of social cues with rapid changes in behaviour (Williams 2015).
Another brainstem area involved in consciousness and the sleep-wake cycle is the reticular
formation (RF).
2
These small nuclei inhibit any sensory stimuli that are irrelevant or repetitive
from proceeding to the higher conscious areas. Important, strong or unusual sensory stimuli
are allowed to pass and will help to maintain a state of alertness. The RF contains the reticular
activating system (RAS), which also has an alerting effect as part of the sleep-wake cycle.
Awareness requires the delivery of sensations coming into the brain from the body to
the relevant sensory areas. Such stimuli must rst pass to the thalamus, a sensory relay
station above the brainstem that directs sensations the correct cerebral area. Information
must go to the correct designated area. The thalamus is therefore the brain’s server (as on
a computer network), directing the emails (stimuli) to the correct terminal computer (sen-
sory cell). What is amazing is that nature designed human computers with server systems
some 4 million years or so before the rst computer was invented.
3
Neurons
How the neurons in these areas generate a concept of reality is the real hard problem.
What is known is that processing takes just thousandths of a second, which makes con-
sciousness virtually instantaneous.
Neurons have a cell body and a long process, the axon, extending for varying
distances away from the cell body (Figure 11.8). Dendrites are shorter cell body
Nucleus
Cell
body
Dendrites
Schwann cell
nucleus
Axon
Layers
of
myelin
Axon
Nodes of
Ranvier
Myelin
sheath
(Schwann cells)
Synaptic
endings
Axon
hillock
Nissl
bodies
Figure 11.8 The neuron. The cell body shows a nucleus surrounded by Nissl bodies. The axon starts
at the axon hillock and terminates at the synaptic endings. Myelin covers the axon in
segments with gaps between, called the nodes of Ranvier. An enlarged myelin segment
is shown. Myelin is laid down in layers by cells called Schwann cells.
https://t.me/medicina_free
Grey
matter
Ganglia
White
matter
Grey matter
(cell bodies)
White matter
(myelinated axons)
Neurological observations (I) 227
processes connecting cells together. Dendrites are afferent (= towards) because they
convey impulses towards the cell body; axons are efferent (= away from) because
they convey impulses away from the cell body. The axons are mostly myelinated,
i.e. covered by a fat (lipid) layer, the myelin sheath, the purpose of which is to speed
up nerve impulse transmission from about 2 metres per second (unmyelinated) to
as much as 120 metres per second (myelinated). Neuroglia (often shortened to glial
cells) do not convey nerve impulses, but they provide other structural or chemical
functions that are essential for brain activity.
The human brain has about 100 billion neurons, with about 50 to 80 times more neu-
roglia. Most glial are astrocytes, named after their star shape, which have vital chemical
and nutritional roles to perform in the brain.
Neuronal myelin sheaths are layers of fat, making the axons appear white. Neuronal
cell bodies, being free from myelin, appear grey. Grey matter consists of cell bodies
packed together, e.g. on the surface of the cortex, whereas white matter consists of axons
packed together as they extend deeper into the brain (Figure 11.9).
Neurons connect to each other through synapses, microscopic gaps between one
axon and what lies beyond, which could be a cell body or dendrites or another axon
(Figure 11.10).
Figure 11.9 Neurons in clusters form grey matter (cell bodies) and w hite matter (axons). Gan-
glia are cell bodies (patches of grey matter) separated from the main group of cell
bodies.
https://t.me/medicina_free
228 Neurological observations (I)
Action
potential
Na
+
K
+
Ca
2+
Released
neurotransmitter
in cleft
Vesicles with
neurotransmitter
Postsynaptic
membrane
Cleft
Presynaptic
bulb
Action
potential
sweeps
over bulb
Axon
Postsynaptic Postsynaptic
receptors with receptors with
no bound bound
neurotransmitter neurotransmitter
Figure 11.10 Action potentials and the synapse. An action potential (or impulse) consists of a
sodium (Na
+
) inow into the axon (depolarisation), followed by a potassium (K
+
)
out ow from the axon (repolarisation), followed by a return of the ions (refractory).
This happens all the way down the axon to the presynaptic bulb. At the bulb the
inow of ions changes from sodium (Na
+
) to calcium (Ca
2+
). This causes vesicles
to release a neurotransmitter into the cleft, i.e. the gap between the presynaptic
and postsynaptic membranes. The neurotransmitter binds to receptor sites on the
postsynaptic membrane. Neurotransmitters effect the cell beyond the postsynaptic
membrane.
Neurotransmitters
Some neurons may have up to 100,000 synaptic connections each with other neurons.
Synapses require a neurotransmitter to ll the gaps during the passage of an impulse. The
neurotransmitter binds to receptors on the other side of the synapse causing a change
in the post-synaptic membrane. In many cases this change is a new impulse in the next
neuron. There are many different neurotransmitters, the main one in the cerebral cortex
being glutamate. Glutamate is excitatory in almost every site it is used, i.e. it generates a
new increase in neuronal activity when released at the synapse. Glutamate is produced
https://t.me/medicina_free
Neurological observations (I) 229
as part of a chemical cycle that also generates another neurotransmitter called gamma-
aminobutyric acid (GABA; Figure 11.11).
GABA is largely inhibitory, i.e. it binds to receptors that prevent any changes in the
postsynaptic membrane, so it reduces (inhibits) brain activity. The brain has both excita-
tory and inhibitory abilities because this allows the brain to operate at a moderate level
most of the time, with the option of increasing or decreasing brain activity as the need
arises. By carefully regulating the chemical cycle that produces both the excitatory and
inhibitory neurotransmitters, the brain can ne-tune glutamate or GABA production to
meet its activity needs (Carlson and Birkett 2017).
The 40 or so brainstem nuclei use various neurotransmitters at their synapses. There are
a range of receptors for each neurotransmitter, and so it is not currently possible to assign
consciousness to any specic neurotransmitter. It all depends on which receptor it binds
to, and it is likely that a combination of neurotransmitters and receptors are involved.
The rostral dorsolateral pontine tegmentum projects acetylcholine-producing bres to the
higher centres, and acetylcholine is a neurotransmitter known to have a general alerting
(excitatory) effect.
Pyridoxine (vitamin B
6
)
Pyridoxal phosphate (PLP)
GAD GABA-T
Alpha-
ketoglutarate
Tr icarboxylic
(Krebs) cycle
Succinate
GABA
GABA
Glutamate Glutamate
GABA-T
Figure 11.11 The gamma-aminobutyric acid (GABA) and glutamate (glutamic acid) cycle. The
Krebs cycle (Chapter 1, Figure 1.3, page 4) provides alpha-ketoglutarate as the start-
ing point for glutamate, which returns to the Krebs cycle as GABA. The enzymes are
GABA transaminase (GABA-T) and glutamic acid decarboxylase (GAD). Vitamin B6
(pyridoxine) is important for these enzymes to function.
https://t.me/medicina_free
Соседние файлы в папке Библиотека им академика М.И. Перельмана