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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_2817_Библиотеки_им_академика_М_И_Перельмана.pdf
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7. Medulla
14. Septum pellucidum
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1. Frontal lobe
2. Lateral ventricle
3. Septum pellucidum
4.
5.
6. Septal nuclei
7. Longitudinal fissure
14. Third ventricle
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1. Lateral ventricle
2. Corpus callosum
3. Thalamus
4. Hypothalamus
5. Midbrain
6. Pons
Fig. 1.7 Midsagittal section of the brain shown in a gross specimen
(top) and at the equivalent level on a magnetic resonance imaging
scan (bottom).
connects the two hemispheres and serves as a useful
landmark because it appears in all coronal sections in
which deep structures are present.
lateral ventricles, separated by the septum pellucidum.
The masses of grey matter that form the lateral walls of
the lateral ventricles are the caudate nuclei (part of the
basal ganglia). The rule for identifying them is simple:
if the lateral ventricles are visible, so is the caudate. This
applies throughout the curved extent of the lateral ventricles, as the caudate follows them the whole way.
ter to another nucleus, the putamen. These two nuclei
are almost always divided by a band of axons called the
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8. Paracentral lobule
9. Superior colliculus
10. Inferior colliculus
11. Cerebellum
12. Fourth ventricle
13. Spinal cord
Below the corpus callosum are the front ends of the two
The caudate appears connected by threads of grey mat-
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8. Cingulate gyrus.
9. Corpus callosum
Anterior limb of internal capsule
Temporal lobe
Fig. 1.8 Coronal section of the brain.
10. Caudate nucleus
11. Putamen
12. Nucleus accumbens
13. Hypothalamus
internal capsule, which is a major pathway for connections between the thalamus and the cortex. In the rostral brain, these two nuclei are continuous at the base, so
that in reality the caudate and the putamen are a single
nucleus, divided in half by the internal capsule; hence,
they are commonly called the striatum. Early anatomists
did not realize this; therefore they were named separately,
and the small ventral bridge below the internal capsule,
which connects them, was named the nucleus accumbens.
The nucleus accumbens and the septal nuclei are associated with conscious ‘reward’ and motivation and are
part of the limbic system. These structures are involved
in the mediation of the effects of addictive drugs, such as
cocaine, heroin and amphetamines.
In the next most caudal section, shown in Fig. 1.9A,
the nucleus accumbens has disappeared and the caudate
and putamen are no longer connected. The caudate
nucleus is decreased in size, and medial to the putamen,
a new set of basal ganglia nuclei emerges—the globus
pallidus external (GPe) and internal (GPi). The interventricular foramens—the diagonal openings that connect
the lateral ventricles with the midline third ventricle—are
clearly visible. The septum here is very small, and suspended from it are tracts called fornices (singular = fornix). The third ventricle is below the fornices. The fornix
connects the mammillary body (part of the hypothalamus) to the hippocampus (in the temporal lobe). Below
and lateral to the hypothalamus at the base of the brain
is the optic tract. Below, between the putamen and GPe,
and just above the temporal lobe is the anterior commissure, a white matter tract that connects the temporal lobes
of each hemisphere. In the medial part of the temporal
lobe is a circumscribed region of grey matter, the amygdala. The amygdala is a specialized part of the brain; it
1
ORGANIZATION OF THE NERVOUS SYSTEM
9THE NERVOUS SYSTEM
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1. Cingulate cortex
2. Corpus callosum
3. Lateral ventricle
4. Caudate nucleus
5. Insular cortex
6.
7.
8. Hippocampus
9. Substantia nigra
12. Red nucleus of midbrain
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1. Cingulate gyrus
2. Corpus callosum
3. Caudate nucleus
4. Internal capsule.
5. Putamen
6. Globus pallidus external
7. Globus pallidus internal
8. Optic tract
9. Longitudinal fissure
10. Lateral ventricle
22. Thalamus
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ORGANIZATION OF THE NERVOUS SYSTEM
10 SYSTEMS OF THE BODY
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Fig. 1.9 Coronal section of the brain.
11. Fornix
12. Insula cortex
13. Lateral fissure
14. Anterior commissure
15. Amygdala
16. Temporal lobe
17. Hypothalamus
18. Third ventricle
19. Claustrum
20. Frontal lobe
21. Mammillary body
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encompasses several nuclei and is part of the limbic system. It deals with the emotional significance of experiences. The insula cortex is also visible at this level, located
medial to the lateral fissure. The insula is considered part
of the limbic system and has a variety of roles including
pain and taste perception, interoception, homeostasis and
emotional and cognitive functions. It is a true integration
hub where bodily sensations, emotional processing and
autonomic functions converge. Between the insula and
putamen is a nucleus called the claustrum. The claustrum
has wide- ranging connections with the hippocampus,
amygdala, caudate nucleus and premotor, prefrontal,
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5
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10. Frontal lobe
11. Third ventricle
13. Longitudinal fissure
Thalamus
Temporal lobe
Fig. 1.10 Coronal section of the brain shown in a gross specimen
(top) and at the equivalent level on a magnetic resonance imaging
scan (bottom).
14. Fornix
15. Pyramidal tract
16. Pons
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auditory and visual cortices. It seems to act as the conductor of cortical function to synchronise (bind) together
the perceptual, cognitive and motor modalities relevant
to consciousness and selective attention. The claustrum
is separated from the insula by the extreme capsule and
from the putamen by the external capsule.
Moving further caudally (Fig. 1.9B), another major
nuclear structure, the diencephalon, appears, and it comprises the thalamus and hypothalamus. Both are a heterogeneous group of nuclei with specific functions: the
thalami are the gatekeepers for any information passing
to and from the cerebral cortex, while the hypothalamic
nuclei regulate homeostasis and endocrine functions.
The medial nuclei of the hypothalami and thalami form
the lateral walls of the slit- like third ventricle. This provides another anatomical rule: if the third ventricle is
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1. Longitudinal fissure
2. Cingulate cortex
3. Splenium of corpus callosum
4. Inferior horn of lateral ventricle
5. Midbrain
6. Middle cerebral peduncle
7. Cerebellum
8. Medulla
9. Parietal cortex
10. Posterior horn of lateral ventricle
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10. Posterior limb of internal capsule
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ORGANIZATION OF THE NERVOUS SYSTEM
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visible, so is the thalamus (or hypothalamus). The thalamus is located medial to the internal capsule (posterior
limb), while the putamen and globus pallidus remain lateral to it; this relationship is always preserved and is easily seen in horizontal sections. The two swellings at the
base of the midbrain are the mammillary bodies.
In Fig. 1.10, the globus pallidus and putamen have
disappeared. The caudate nuclei are very small, and the
thalami are larger. A new structure visible here in the
medial temporal lobe, shaped like a sea- horse, is the
hippocampus. At this level, more of the ventral surface
of the brainstem has been sectioned. Medial to the hippocampus on either side are diagonally running white
matter tracts that pass through the midbrain, pons and
medulla. These are the pyramidal tracts. Also visible in
the midbrain region is the substantia nigra, which has
been cut obliquely, and the red nuclei. The ventral surface of the pons is also cut, showing the transverse cerebellopontine fibres.
Fig. 1.11 shows the last section of this series and much
has changed. The posterior part of the left lateral ventricle is visible as a long diagonal slit comprising the posterior and inferior horns (and associated choroid plexus).
The lateral ventricle, like many other structures in the
brain, curves back and loops under itself like a big ‘C’.
The hippocampus, which is involved in memory formation, is clearly visible on the medial side of the lower part
of the lateral ventricle, and is connected to the fornix,
which runs inside the ventricles. This section is at the
junction of the brainstem and cerebrum. The midbrain
is identified by the cerebral aqueduct, which connects
the third and fourth ventricles, and the paired superior
11. Fornix
12. Hippocampus
13. Temporal lobe
14. Parahippocampal gyrus
15. Cerebral aqueduct
16. Pons
Fig. 1.11 Coronal section of the brain.
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1. Frontal lobe
2. Genu of corpus callosum
3. Caudate nucleus
4. Putamen
5. Fornix
6. Splenium of corpus callosum
7. Longitudinal fissure
8. Anterior horn of lateral ventricle
9. Anterior limb of internal capsule
Fig. 1.12 Horizontal section of the brain at the level of the lateral
ventricles.
11. Temporal lobe
12. Insular cortex
13. Lateral fissure
14. Thalamus
15. Posterior horn of lateral ventricle
16. Occipital lobe
17. Claustrum
18. Parietal lobe
19. Choroid plexus
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colliculi. Below the midbrain is the pons, with the two
middle cerebellar peduncles (tracts of white matter) connecting the pons to the cerebellum.
Horizontal sections
In the horizontal plane, all the major subcortical nuclear
structures are related to the positions of the ventricular
system and internal capsule. Fig. 1.12 shows a brain cut
at two different levels of the lateral ventricles—the right
superior to the left. The caudate nuclei protrude into the
anterior part of the lateral ventricles to form its lateral
wall, and on the left, the putamen is separated from the
caudate by the anterior limb of the internal capsule. The
white matter tract immediately anterior and posterior to
the ventricles is the corpus callosum, which crosses in
and out of the plane of the page. Remember that, in the
sagittal plane, it curves like a ‘C’ from front to back; in
this horizontal section, it was cut through twice, at the
front (genu) and back (splenium) end. At this level, on
the left, the true shape of the internal capsule is apparent
in the horizontal plane; it is V- shaped, with an anterior
limb and a posterior limb. The third ventricle and the
11THE NERVOUS SYSTEM
1
Association cortex areas
thalamic nuclei on either side are visible. The interventricular foramen between the lateral ventricles and the
third ventricle is visible. Also visible in the left ventricle
are the cut parts of the fornices.
Forebrain
This is the largest part of the brain (80% by volume) and
comprises the cerebral cortex, limbic system and basal
ganglia. The forebrain is involved with perception, cognition, motivation, memory, emotion and control of
higher motor functions.
The architecture of the cortex differs between the
cerebral cortex and limbic cortex. The limbic cortex is
evolutionarily older and comprises only three (archi- or
paleo- cortex) or four cell layers of grey matter, whereas
the rest of the cortex, called the neocortex, is evolutionarily more recent and comprises six layers. The cortex
ORGANIZATION OF THE NERVOUS SYSTEM
consists mostly of pyramidal cells and granule cells; simplistically, granule cells (in layer 4) receive sensory input
and pyramidal cells (layer 5) provide output. The other
layers connect to other areas of the cortex on the same
side of the brain via association fibres and to the contralateral side via commissural fibres.
The cortex is divided into areas that have a single
function such as touch, vision, hearing, taste and smell
or the production of movement. These are called primary areas (Table 1.2). Their function is to receive and
start the initial processing of information. The rest of
the cortex is association cortex, which provides higherorder processing of sensory and motor information.
Some of these regions process complex aspects of a
single sensory modality or information related to motor
function, such as premotor cortex, or areas V2–V4 in the
visual system (Chapter 7). There are four main regions
of association cortex, which carry out diverse types
of sensory integration required for purposeful movements. They provide the link between sensations and
(re)action by making connections with motor areas.
These include the posterior parietal cortex, which integrates sensory and visual stimuli; it is associated with
self- and spatial awareness, and is critical for attention to external events. The parietal–occipital–temporal
association region coordinates somatosensation with
visual and auditory cues to produce perceptual recognition or movements in response to visual or auditory
stimuli. The frontal association cortex occupies most
of the rostral part of the frontal lobe and gives us our
personality by adjusting behaviour according to moral
and social norms. It comprises several distinct regions
(described further in Chapter 9). The premotor cortex
is important in the planning of voluntary movements.
The dorsolateral prefrontal region connects with the
sensory and motor cortices and is associated with the
attentional and cognitive consequences associated
with movements. The ventromedial prefrontal cortex
is interconnected with the limbic system to encode the
emotional aspects of motor behaviour. The limbic association area is associated with the medial and inferior
surfaces of the brain and is devoted mostly to memory,
motivation and emotion. All the association areas feed
into the higher- order motor areas, which then project to
the primary motor cortex, which ultimately exerts control over the motor neurons (Fig. 1.13).
It is important to recognize the locations of the major
functional areas of the cortex, particularly the primary
somatosensory, visual, auditory and motor cortices (Figs
1.14 and 1.15). Selective damage to these regions leads to
discrete neurological deficits.
Sensory input from
peripheral receptors
(unimodal)
Primary
motor cortex
Lower
motor neurons
(output)
Fig. 1.13 Intercortical connections of primary, higher- order and association cortices used in processing a sensory stimulus to produce a
behavioural response. Inset shows the location of the various association areas.
Movement
response
12 SYSTEMS OF THE BODY
Primary
sensory area
Premotor
cortex
Parietal–temporal
occipital cortex
Parietal–temporal
occipital cortex
Posterior
parietal cortex
Prefrontal cortex
2
Premotor
cortex
3
1
4
1
Primary
motor cortex
Primary
sensory cortex
4
3
2
Premotor cortex
• Gross movement
Frontal eye field
• Conjugate eye movements
Prefrontal cortex
• Motivation
• Planning
• Judgement
• Problem solving
Broca’s area
• Expressive speech
Primary motor cortex
• Voluntary movement
Hip
Trunk
Forearm
Fingers
Thumb
Face
Lips
Jaw
Tongue
Larynx
Pharynx
Shoulder
Hand
Arm
Thumb
Face
Lips
Teeth
Tongue
Mouth
Forearm
Fingers
Eye
Shoulder
Arm
Trunk
1
ORGANIZATION OF THE NERVOUS SYSTEM
Primary somatosensory cortex
• Somatosensation
• (Location, modality, intensity)
Posterior parietal cortex
Hip
• Stereognosis
• Attention: awareness of
self & surroundings
Wernicke’s area
• Language
comprehension
Fig. 1.14 Functional anatomy of specific cortical areas on the lateral surface of the brain.
Supplementary
motor cortex
• Gross movement
Medial prefrontal cortex
• Personality
• Emotional behaviour
• Working memory
Primary gustatory cortex
• Taste
Primary motor cortex
• Voluntary movement
Primary auditory cortex
• Bilateral hearing
Hip
Thigh
Knee
Leg
Ankle
Genitals
Toes
s
u
o
m
l
l
a
c
s
u
p
r
o
C
Thalamus
Thigh
Leg
Foot
Hip
Long-term
memory
Primary visual cortex
• Sight
Primary sensory cortex
• Location, modality, intensity
• Somatosensation
Visual primary cortex
• Contralateral visual field
Orbitofrontal cortex
• Olfaction
• Emotional behaviour
Amygdala
• Fear
• Anxiety
Hippocampus
• Memory (short term)
Fusiform gyrus
• Object/face recognition
Fig. 1.15 Functional anatomy of specific cortical areas on the medial side of the brain.
Visual association cortex
• Colour, spatial appreciation/
recognition
13THE NERVOUS SYSTEM
1
Dominant (left)
Intelligence
A
thinking
Rationalization
A
of speech
comprehension
Non-dominant (right)
drawing skills
Contralateral stereognosis
Contralateral stereognosis
Hemisphere specialization
The anatomy of the brain appears symmetrical, in that
most regions of the sensory and motor cortices are the
same on both sides of the brain, and damage to one
region leads to a contralateral deficit. However, some
of these regions, particularly in the frontal, temporal
and parietal lobes, differ in size. For example, the primary auditory cortex is larger in the right temporal
lobe than in the left; conversely, Wernicke’s area (auditory association cortex) is larger on the left than on the
right. Similarly, the left parietal lobe is larger than the
right, but the right posterior parietal region is larger
than the corresponding area on the left. In addition, the
left facial somatosensory cortex area is larger than the
right. Broca’s area on the left is also different from that
on the right. This is because there are regions in the frontal and parietal lobes that have dramatically different
ORGANIZATION OF THE NERVOUS SYSTEM
functions in the left and right brain. These are associated
with ‘higher functions’, such as language, analytical and
intuitive thinking, spatial orientation and artistic and
musical ability (Fig. 1.16). Much of what is known about
hemispheric asymmetry comes from patients with brain
lesions or who have had surgery to control diseases such
as epilepsy or cancer.
The hemisphere that contains the centres for language
production and comprehension is called the dominant
hemisphere; in most people, this is the left hemisphere.
Damage to the left hemisphere gives rise to difficulties in
speech comprehension or production that do not occur
if the lesion is in the right hemisphere. Lateralization of
language function can be determined using the Wada
test. If a patient has speech centres in the left hemisphere,
anaesthetic injected into the left carotid artery blocks
speech perception and production.
Another asymmetry occurs in writing. Most people
are either right- or left- handed; very few are ambidextrous. Handedness and cerebral dominance were
thought to be linked, since ∼90% of the population are
right- handed and the left hemisphere controls the right
hand. Conversely, in left- handed people, Broca’s area
would be in the right hemisphere. This hypothesis is easily tested by the Wada test (see above) and was found to
be false; 97% of the population, including three- quarters
of left- handers, have their language centres in the left
hemisphere.
In addition to speech production, the left hemisphere
is important in language articulation and comprehension, mathematical calculations and cognitive functions,
such as analytical and rational thinking. For example, damage to the left parietal lobe causes difficulties
in copying movements (ideomotor apraxia), naming
objects, reading (alexia), solving mathematical problems (dyscalculia) and language. The same lesion in the
right hemisphere causes difficulty in copying drawings
(agraphia), assembling puzzles (constructional apraxia)
and spatial navigation, such as finding the way to the
shops or work, because the landmarks used as a guide
are no longer familiar. In some patients, damage to the
right posterior parietal cortex results in contralateral
neglect syndrome. The patient fails to recognize the left
side of the body as theirs. They may fail to wash or dress
the left side of the body, and if presented with a stimulus such as pain, they may report it as hurting, but not as
hurting them.
Studies of lesions in the right hemisphere have shown
that it is essential for the processing of non-verbal sound
patterns such as music; for example, patients with rightsided strokes can speak but often cannot sing properly.
Lesions in the right hemisphere in regions corresponding
to Wernicke’s area produce deficits in music perception
and the appreciation of tone and the emotional nuances
of speech. Poetry, for example, may seem meaningless
after a right area lesion.
Sex differences also exist in the CNS. Sexual dimorphism of the hypothalamic preoptic nucleus, corpus
callosum and cerebral cortical regions has been documented. These are likely related to sex- specific behaviours associated with hormonal levels of testosterone and
oestrogen that relate to not only reproductive function
but also to non-reproductive (e.g. visuospatial processing or phonological language) functions. In addition,
hormones are related to brain development. In the male
foetus androgens are converted to oestrogen, and this is
related to masculinization of the brain; in the female foetus it is the lack of oestrogen that leads to feminization.
After puberty, androgen and oestrogen secretion leads to
14 SYSTEMS OF THE BODY
nalytical
rticulation
Handwriting
and reading
Language
Fig. 1.16 Hemispheric specialization of function.
Simple language comprehension
(intonation and gesture)
Spatial
Mental
arithmetic
awareness
Art and
Intuition
Geometric
analysis
Music
appreciation
1
the development of secondary sexual characteristics, as
well as sexually dimorphic behaviour.
Although these studies tell us that there are differences between the two hemispheres, it is unclear what
this means. Cortical areas are more similar than they are
different. However, if the hemispheres process information differently, this implies that they ‘think’ differently.
There are various speculative theories as to why the
brain evolved like this. One is that the left hemisphere is
important in the control of fine movements, and this is
important in controlling the production of speech, which
involves fine control of the muscles of the larynx, tongue
and oral cavity. The left hemisphere is also involved in
the production of actions; damage leads to an inability to
copy movements. Finally, another link between language
and movement occurs in the representation of language
areas. Verbs are ‘doing’ words that describe actions or
states, whereas nouns are names of things. Verbs appear
to be processed only in the left hemisphere, while nouns
are processed in both hemispheres. Thus, the left hemisphere has a role in the production of both actions and
mental representations of actions in the form of words.
Box
‘Split brain’ syndrome
1.2
If the left hemisphere is involved in fine motor control, what about the right hemisphere? One idea is that
the right hemisphere is specialized for spatial movement
relative to the surroundings, so that at a higher level, it
can produce mental images of such movements. Damage
to this lobe would impair such abilities. This is indeed
the case.
One last controversial idea about asymmetry is that
the left hemisphere is critical for language interpretation,
and this is what sets humans apart from other animals.
Evidence for this comes from ‘split- brain’ studies (Box
1.2), in which the corpus callosum was surgically cut in
order to reduce the severity of seizures in patients with
severe epilepsy. This meant that the two hemispheres
could no longer communicate with each other. Patients
undergoing this procedure were shown two pictures of
related objects, to both hemispheres. Then several more
pictures were shown, and patients were asked to select
a picture that had an inferred relationship with the first
two objects. For example, if the first two pictures were
of rain and clouds, the third might be an umbrella. The
right brain cannot make the connection, but the left can.
ORGANIZATION OF THE NERVOUS SYSTEM
The brain houses two minds, not one, but they only orchestrate into a single personality if the two cerebral hemispheres communicate. Under normal conditions, both
halves receive nearly identical information on the world,
and life proceeds as though nothing is different about
perception. Both hemispheres share the same knowledge
base and reactive inclinations. Thus consciousness is controlled by both hemispheres, and there is a crossing over
of functions so that one never normally experiences a dissociation of information. However, ‘splitting’ the brain by
sectioning the corpus callosum and thereby disconnecting
the hemispheres reveals separate functions. In the early
1960s, researchers showed that when a cat had its optic chiasm and corpus callosum severed, two independent learning centres were established, one in each hemisphere. The
same effect was seen in humans. It was concluded that
the brain had ‘two separate realms of conscious awareness; two sensing, perceiving, thinking and remembering
systems’.
Severing the corpus callosum was used by surgeons to
treat chronic intractable forms of epilepsy. To the casual
observer, split- brain patients appeared normal, and their
seizures disappeared. However, psychological testing
revealed that if the patient held up an object like a comb
in the left hand, they could not say what it was. When it
was transferred to the right hand, the patient had no trouble at all in communicating its identity. The same happened
with words. If a card with a printed word like ‘ring’ was
visible only in the patient’s left visual field, they could not
read it, yet vision in the left eye was fine. When the word
was in the right field, the patient immediately recognized
it. In order to explain these observations, it was necessary
to understand certain basic rules of perception. Right and
left worlds of touch and sight project to opposite cerebral
hemispheres. Sounds project to both hemispheres simultaneously, so the patient could be cued by the doctor’s
voice. Many things that people learn and think about are
non-verbal: music, art, spatial relationships and geometry.
To test the functions of the right hemisphere of split- brain
patients, psychologists constructed a screen with a slot
under which a patient could reach and touch objects, but
not see them. Then they focused a picture of one object in
the patient’s left field of view (signalling the right hemisphere) and asked them to match the picture to the objects
that they could feel behind the screen with the left hand.
The patients passed the matching test, with scores similar
to those of any normal person. However, when the same
task was performed with the right hand, the patient failed
to choose the objects correctly because the right hand is
controlled by the left hemisphere, with which the patient
cannot see the object, as it is not in the right visual field.
It is now recognized that cutting only the anterior threequarters of the corpus callosum and the anterior commissure and leaving the splenium (posterior part) intact
is sufficient to stop seizures completely or render them
responsive to drugs. At the same time, psychological tests
show results identical to those of normal subjects, suggesting that the cerebral hemispheres totally integrate if just a
small fraction of the corpus callosum remains intact.
15THE NERVOUS SYSTEM
1
(heart rate/breathing/hormone release)
Hippocampus
Fornix
thalamic nucleus
gyrus
cortex
Septal nucleus
Hypothalamus
Olfactory
cortex
Diagonal band of Broca
Amygdala
Association
cortices
Fig. 1.17 The limbic system: neuroanatomical connections of the amygdala.
ORGANIZATION OF THE NERVOUS SYSTEM
The same is true if words, rather than pictures, are used.
The ability to infer leads to the ability to believe, and this
amygdalofugal
Temporal
lobe cortex
sets humans apart from other animals. Parrots can be
trained to talk, but they cannot infer or believe things.
Limbic system
The limbic lobe, situated on the medial side of the brain,
surrounds the rim of the ventricles (see Figs 1.3C and
1.15). The cingulate and parahippocampal gyri, together
with their associated nuclei—the hippocampus, amygdala, septal nucleus and insula—comprise the limbic system. The limbic system is evolutionarily old; it is found
in fish, amphibians, reptiles and mammals. It controls
emotional behaviour and the internal factors that motivate animals and people to adapt to a constantly changing external environment.
Emotions have three components: a visceral sensory
component caused by endocrine and autonomic stimuli
(a ‘gut feeling’), a motor component involving the facial
muscles to communicate the pleasantness or unpleasantness of the situation to others, and a cognitive–evaluative
component to assess the situation. Pathology involving damage to the limbic system can elicit inappropriate
behavioural patterns. These patterns include motivational behaviour relating to nutrition and fluid balance,
sexual courtship behaviours and expressions of mood
and affect. Memory is an important component of these
patterns, and lesions within several limbic areas affect
memory.
Input to the limbic system comes from the association cortex areas, olfactory cortex and medial temporal
lobe regions, and is ultimately passed on to the hypothalamus, which controls the endocrine system and
autonomic nervous system. The connections of the limbic system and the functions of the various nuclei are
detailed in Figs 1.17 and 1.18 and Table 1.3.
The uncus of the medial temporal lobe houses the hippocampus and amygdala. The amygdala is a collection
Ventro-
pathway
DLF
Stria terminalis
Periaqueductal greyPAG =
Locus coeruleusLC =
DLF = Dorsal longitudinal fasciculus
Entorhinal
Fig. 1.18 The limbic system: the Papez circuit for emotions. Neuroanatomist James Papez demonstrated that emotions are not a function of any specific brain centre but involve a circuit comprising the
hypothalamic mammillary bodies, thalamus, cingulate gyrus and
hippocampus. Damage to any of these structures affects emotions
and the ability to create memories.
Table 1.3 Functions of limbic system structures
Structure Function
Hippocampus Memory acquisition,
Amygdala Emotional content of stimuli;
Septal nucleus Pleasure and reward
Cingulate cortex Affective significance
of subnuclei and necessary for self- preservation. Clinical
studies have shown that when stimulated, it gives rise to
fear and anxiety or euphoria, depending on which part
is stimulated. This structure is responsible for the feelings of fear or dread when you are walking home late at
night and hear footsteps behind you. Another important
Perforant
pathway
Habenula
Interpeduncular
nucleus of midbrain
Reticular
formation
PAG —fear–freezing response
pain–antinocioception
LC —arousal
Cingulum
consolidation and recall
fear, anxiety and danger
Sleep–wake cycle
Mammillary
body
Mammillo–thalamic
tract
Anterior
Internal
capsule
Cingulate
16 SYSTEMS OF THE BODY
1
Box
Klüver–Bucy syndrome
1.3
In the 1930s, Heinrich Klüver and Paul Bucy found that
bilateral temporal lobectomy in monkeys produced a
dramatic effect on the animals’ responses to fearful situations. The bizarre behavioural abnormalities could be
placed into five categories: visual agnosia, oral exploration of all objects, hyper- metamorphosis (a compulsion to
touch everything, and place each found object into the
mouth), altered and increased sexual behaviour and emotional changes, including fearlessness and decreased facial
expressions usually associated with emotion. This constellation of symptoms is called Klüver–Bucy syndrome. Nearly all
the symptoms of Klüver–Bucy syndrome reported in monkeys have also been found in humans with temporal lobe
lesions. In addition to visual recognition problems, oral
tendencies and hypersexuality, people appear to have ‘flattened’ emotions.
role is in recognizing the emotions indicated by other
peoples’ facial expressions. The amygdala receives input
from the hippocampus, olfactory cortex and temporal
lobe association cortex, and aminergic input from septal
and brainstem nuclei. Its main outputs are to the septal
nucleus and hypothalamus, via the stria terminalis and
diagonal band of Broca (septal nucleus only), temporal
cortex and other autonomic and brainstem reticular formation centres, and the nucleus accumbens (via the ventral amygdalofugal pathways). Damage to the amygdala
is rare, but experimental studies show that bilateral damage may produce an inability to perceive situations as
dangerous, which can have disastrous consequences for
people and animals. Such lesions produce the Klüver–
Bucy syndrome (Box 1.3).
The hippocampus is involved in memories and the
recall of such information. It stamps the place, time
and date on the memory and, in conjunction with other
regions of the medial temporal lobe, medial thalamic
nuclei and medial prefrontal cortex, is involved in the
consolidation of information. Consolidation is sometimes
termed ‘long- term memory’, and is stored as a modification of activity- dependent connections between neurons,
and is analogous to information stored on the hard drive
of a computer. The hippocampus allows humans to compare the conditions of a present situation, such as danger,
with similar past experiences in order to decide what the
best option is to guarantee survival. The hippocampus
is part of the hippocampal formation, which includes
the dentate gyrus and subiculum. The dentate gyrus is
at the tip of the hippocampus, and the subiculum is at
the base and is continuous with the entorhinal cortex
of the parahippocampal gyrus. There is a one- way flow
of information through the hippocampus. Information
arrives via the perforant pathway from the entorhinal
cortex to the dentate gyrus, which passes information to
the CA3 region (CA stands for cornu Ammonis or ‘ram’s
horn’ because of its shape). From CA3, it goes to the CA1
region and on to the subiculum, which is responsible for
the output projection to either the nuclei of the mammillary bodies (part of the hypothalamus) via the fornix,
or back to the sensory cortex via the entorhinal cortex.
Somewhere along these pathways, memories are created.
Bilateral damage to the hippocampi impairs the formation of new memories; nothing is retained, and information is soon forgotten. The hippocampus is also damaged
in disease states such as dementia and epilepsy (see
Chapters 13 and 14) and rabies.
The septal nuclei are the anterior thickenings of the
septum pellucidum and are located immediately anterior
to the anterior commissure. This area is a sexually dimorphic nucleus, and its stimulation evokes pleasurable
sensations, particularly sexual sensations. It has been
suggested that this is the location of the orgasm centre,
and that women have four centres, while men only have
one! Inputs are from the amygdala, hippocampus, mesolimbic dopaminergic system and olfactory tract. Outputs
are to the hypothalamus (via the medial forebrain bundle), hippocampus and habenula (via the stria terminalis) and reticular formation.
The cingulate cortex evaluates the affective significance of events, that is, whether they are harmful or
beneficial. Anatomical studies have revealed prominent
afferent input to the cingulate motor areas from the limbic structures and the prefrontal cortex, which can send
information about motivation and the internal state of
subjects, as well as cognitive evaluation of the environment. The anterior cingulate cortex is also involved
in pain perception, receiving input from the posterior
insula cortex. Other important inputs are from the anterior thalamic nucleus, which receives its input from the
mammillary bodies, forming the Papez circuit, involved
in the cortical control of emotion (see Fig. 1.18). The
anterior cingulate gyrus communicates between the prefrontal cortex and subcortical areas of the limbic system.
Bilateral destruction releases the ‘rage centres’ of the
amygdala and hypothalamus from any prefrontal inhibitory influence.
The limbic system is tightly connected to the prefrontal cortex, and together, they funnel emotional input
to the hypothalamus. Frontal lobe asymmetry exists in
regard to emotional processing. Activation in the left
prefrontal regions may be part of a mechanism that
inhibits ‘negative’ affects (e.g. sadness and disgust);
conversely, the right prefrontal regions may inhibit positive emotions (e.g. happiness). People with increased
left prefrontal activity are described as more ‘optimistic’ and more adept at minimizing negative emotions.
Lesions of the left prefrontal neocortex are more likely
to be associated with depression than lesions in the
homologous location in the right hemisphere. During
the Wada test, when the left hemisphere is temporarily anaesthetized, patients report negative changes in
mood (e.g. sadness). Positron emission tomography
(PET) studies have indicated increased left- side orbitofrontal blood flow during self- generated sadness.
ORGANIZATION OF THE NERVOUS SYSTEM
17THE NERVOUS SYSTEM
1
Orbitofrontal cortex
The orbitofrontal cortex is part of the prefrontal cortex and anatomically linked with the limbic system
and anterior temporal lobe cortex. It mediates the conscious perception of smell. The orbitofrontal cortex
interacts with the limbic system in support of higherorder functions such as association, integration and
regulation of central autonomic processes, mood and
affect, and those motor patterns that are under limbic
control. Orbitofrontal lesions interfere with motivation
and arousal. Prefrontal leucotomy patients are typically unmotivated and lethargic. Orbitofrontal lesions
reduce the sensation of chronic and intractable pain and
sometimes reduce the expression of anger and frustration when expected rewards are not received. In the
past, such findings gave impetus to the use of prefrontal leucotomy as a treatment for intractable emotional
problems and psychosis. Orbitofrontal lesions interfere
ORGANIZATION OF THE NERVOUS SYSTEM
with the prediction of reward. Humans with orbitofrontal lesions are unable to anticipate the future positive or negative consequences of their actions, although
immediately available rewards and punishments do
influence their behaviour. A classic case of damage to
the prefrontal and orbitofrontal lobes is that of Phineas
Gage (see Box 15.7).
• acognitiveloop,concernedwithmotorintentions
and advanced planning for later movements
• alimbicloop,concernedwiththeemotiveaspects
of movement
• anoculomotorloop,concernedwithvoluntary
saccadic eye movements.
Damage to basal ganglia structures results in move-
ment disorders (see Chapter 10).
Diencephalon
The diencephalon connects the midbrain to the forebrain.
It is located deep within the brain and comprises the epithalamus, thalamus, subthalamus and hypothalamus.
The epithalamus forms the roof of the diencephalon and
consists of the pineal gland (an endocrine gland involved
in circadian rhythms and the onset of puberty) and the
habenular nuclei, whose functions are associated with
the limbic system, as they connect to the septal nuclei via
a tract called the stria terminalis thalami. The subthalamus is located dorsolateral to the hypothalamus and has
two notable cell groups: the subthalamic nucleus and the
zona incerta. The former is part of the basal ganglia circuitry (see Chapter 10), while the latter is a rostral extension of the brainstem reticular formation.
Basal ganglia
The basal ganglia are a group of large subcortical nuclei
found in the forebrain. They are located above and anterior to the thalamus (see Figs 1.8, 1.9 and 1.12). They
comprise the caudate nucleus, putamen and globus
pallidus. The caudate nucleus fuses at its anterior end
with the putamen to form the corpus striatum, which is
named after the strands of grey matter that can be seen
connecting the two structures as they become anatomically separated by the anterior limb of the internal capsule (see Fig. 1.8). At the bottom of the anterior part of
the striatum is the nucleus accumbens. This structure,
although anatomically part of the basal ganglia, is considered part of the limbic system and is involved in the
reward systems of addiction mechanisms, as mentioned
above. The body of the caudate runs over the thalamus,
and then curves to pass into the temporal lobe, where
it becomes the caudate tail and ends at the amygdala,
which is another nucleus that is functionally part of the
limbic system. The subthalamic nucleus and substantia
nigra (in the midbrain) are functionally associated with
the striatum and the globus pallidum. The basal ganglia
are concerned with the initiation and maintenance of
actions and are involved in decision- making about what
the body is going to do next. The basal ganglia work via
four circuit loops that start and end in the cortex:
• amotorloop,concernedwithlearnedmovements
and involved in the correct sequencing of actions
for the execution of learned motor programmes
Thalamus
This bullet- shaped structure is the largest nuclear mass
in the body and, together with the hypothalamus, forms
the lateral wall of the third ventricle. The two thalami
face each other medially across the third ventricle and
touch at the inter- thalamic adhesion. The thalamus comprises 12 subnuclei that have reciprocal connections with
the cortex (except for the inhibitory reticular nucleus) via
four thalamic peduncles that are incorporated into the
corona radiata, which is a white matter tract in the brain.
The thalamic input to the cerebral cortex is the first step
in generating sensory perception. The thalamus is the
gateway to the cortex and functions to coordinate and
integrate sensory, motor and autonomic information to
initiate appropriate responses.
The thalamus contains a sheet of fibres called the
internal medullary lamina that divides the thalamus into
three nuclear groups: the anterior, medial and lateral
groups. The anterior and medial groups are associated
with the limbic system. The lateral part of the thalamus
receives restricted sensory or motor input, and is further
subdivided into dorsal and ventral nuclei, based on function and projection.
Additionally, thalamic nuclei are classified into three
functional relay groups: specific, association and diffuse
(Table 1.4). Specific nuclei process either a single sen-
sory modality or input from a motor region, and project to a specific (primary) cortical region. Each receives
18 SYSTEMS OF THE BODY