The central nervous system
Chapter 13
Anatomy 32
The Central Nervous System
I. The brain
The most amazing organ is the brain. On average it weighs 3.3lbs. It
is not yet completely understood and no type of technology has
been able to mimic it exactly. Directional term: rostrally (toward
snout ex. frontal lobe) and caudally (toward tail, ex. brainstem). The
brain is protected by meninges.
A. Embryonic development of the brain and basic
organizationduring embryonic development the brain is first formed by
several vesicles. As the size increases it folds in to
accommodate
growth resulting in the structure we know. The
vesicle known
as the telencephalon (“endbrain”) develops into the cerebral
hemispheres. The dienchephalon (interbrain)
forms into the
thalamus, hypothalamus, epithalamus, and pituitary gland. The
brain stem containing the midbrain, pons,
and medullary
oblongata arises from three vesicles and the cerebellum develops
from the metencephalon (“afterbrain”).
The wrinkles seen on
the brain allow a greater number of neurons to fit in a restricted
area.
The brain has clusters of
gray matter called brain
nuclei and sheets of gray
matter on the surface. The
remainder of the brain is
white matter or fluid filled
areas called ventricles.
The spinal cord also has a
concentration of gray
matter that resembles a
butterfly and fluid that
flows through it.
B. Ventricles of the brain- these cavities are filled with cerebrospinal fluid. It
travels around the brain, goes down the spinal cord and returns up to the brain.
The septum pellucidum separates the two lateral ventricles. The third ventricles lie
in the diencephalon and the connecting cerebral aqueduct lies in the
mesenchephalon. Finally the fourth ventricle and central canal lie within the
hindbrain (brainstem). The fluid also flows through the meninges, connecting to
the subarachnoid space is the fourth ventricle via the lateral and median
aperatures.
C. The cerebral hemisphere- This area of the brain is the
largest; it covers the diencephalon and shows deep or
shallow wrinkles. The line that divides the cerebral cortex into
a left and right half is the median longitudinal fissure.
Posteriorly, the cerebral cortex is separated from the
cerebellum by the transverse cerebral fissure. Grooves or
furrows are called sulci and the twisted brain ridges are gyri.
Deep sulci divide the brain into lobes. The central sulcus lies
on the frontal plane between the precentral and postcentral
gyrus; it separates the frontal and parietal lobes. The parietals
and occipital lobes are separated by the parieto-occipital
sulcus. The lateral sulcus (actually a fissure) separates the
parietal and temporal lobe.
In most people the left hemisphere has control of language,
math, and logic; it deals with the details of the big picture. The
right hemisphere has control of visual-spatial skills, intuition,
art and music skills, and emotions; it deals with the big
picture. The division of control may be more prevalent in
males. Both hemisphere communicate via connecting fiber
tracts called commissures.
1. Cerebral cortex-composed of six layers of gray matter (primarily
interneurons), it controls consciousness and voluntary movement.
Brodmann’s structural areas map out different regions of the brain. Studies
on brain activity have revealed functional regions of the cerebral cortex in
the areas of motor, sensory, and association.
MOTOR AREA:
a. Primary motor cortex- also called somatic motor area is along
the precentral gyrus and contains pyramidal cells. These form tracts
that reach motor neurons, they control precise voluntary motor
movements. The axons project in a contralateral path (left brain
controls right side of body). Areas that control complex skilled
movement contain a larger amount of pyramidal cells.
b. Premotor cortex- Lies anterior to precentral gyrus. It controls
even more complex movements than the primary motor cortex
because it integrates highly processed sensory information received
from other brain areas. It controls voluntary movement based on
spatial arrangement information and is involved in planning
movements.
c. Broca’s area- Lies in the left hemisphere (language dominant), it
controls speech movements, produces mental images of sounds to
be spoken, and may also story short-term memory. It’s
corresponding side on the right controls the spoken emotional
overtones.
SENSORY AREA:
a. Primary somatosensory cortex- lies along postcentral gyrus, it’s
involved with conscious awareness of somatic senses. It is capable
of spatial discrimination of sensory stimulus, the information travels
contralaterally.
b. Somatosensory association area- lies posterior to primary
somatosensory cortex. It integrates sensory input for comprehensive
understanding by relating it to past experience (ex. Feeling a object
and guess what it is without seeing it)
c. Primary visual cortex- the largest sensory area lies in the medial
part of occipital lobe. It receives all visual information sent by the
eye but has low level of processing, the information is sent to
opposite sides
d. Visual association area- surrouds the primary visual cortex and
continues to process information obtained by this area. Multiple
visual areas exists throughout that brain that allow the information to
travel anteriorly, one pathway process recognition and the other
spatial orientation.
e. Auditory areas- includes the primary auditory cortex
that controls conscious awareness of sound received by
receptors in the ear. The auditory association area
evaluates/ recognizes a sound and stores memory of
sound, a specialized area called Wernicke’s area
recognizes spoken words.
f. Gustatory cortex- involved in conscious awareness of
taste, located in the tongue region of the homunculus.
g. Vestibular cortex- involved in conscious awareness
of balance, it is found in posterior part of insula.
h. Olfactory cortex- involved in conscious awareness of
smell, processes by the piriform plate, olfactory tract,
and olfactory bulb. A region of the frontal cortex
recognizes and differentiates odors.
ASSOCIATION AREAS- all areas of the cortex that are not motor or
sensory, may also be called “higher order processing areas.”
a. Prefrontal cortex- frontal lobe region anterior to motor areas, its
function separates humans from other animals. It is involved in
thought, perception, and recall (cognition). It is essential for
judgment, critical thinking, planning, socializing, and some aspects
of emotion. The farther rostrally, the more complex the function.
b. General interpretation area- located in the postlateral cerebral
cortex, it integrates sensory information. Not well understood.
c. Language area- large area in left cerebral hemisphere has five
identified areas: 1- Broca’s, 2- Wernicke’s, 3-speech
comprehension, 4-coordination of auditory and visual aspects of
language, 5-word articulation and recognition
d. Insula- not well understood, apparently functions in language,
balance, visceral functions, behavior as related to cardiovascular
activity.
Body map along the Precentral and Postcental Gyrus
2. Cerebral White Matter- These areas are composed of
myelinated fibers that form large tracts and connect the
two hemispheres. This section lies beneath the gray
matter of the cerebral cortex.
a. Commisures- commissural fibers connect related
gray matter areas between the two hemispheres (run
horizontally). These include the corpus callosum.
b. Association fibers- horizontal fibers that connect
different areas of the same hemisphere.
c . Projection fibers- vertical fibers these take sensory
information to the appropriate area in the cerebral cortex
and carry away motor instructions. The form the internal
capsule and corona radiate.
3. Basal Nuclei- neural calculators that control movement by cooperating with
the cerebral cortex and can estimate time. Their exact function is still unknown
but it is know that they send their output to the motor cortex. They include the
caudate, the lentiform, the amygdala.
D. The diencephalons- surrounded by the cerebral hemisphere it forms the
central core of the forebrain. Its structures are paired and composed of gray
matter.
1. Thalamus- Greek for inner room, it specializes in the relay of signals
coming from any area to the cerebral cortex it has axons that travel to
areas of the cerebral cortex, efferent signals go to sensory areas and
afferent signal arrive from conscious senses.
2. Hypothalamus- lies below the thalamus and between the optic chiasma.
It is the main visceral control center and controls the autonomic NS,
emotional responses, regulates body temperature, regulates hunger and
thirst, controls behavior and sleep-wake cycles, as well as the endocrine
system and formation of memory.
3. Epithalamus- Located at dorsal part of the diencephalons, it includes the
pinieal body. It secretes melatonin which signals the nighttime stage of the
sleep-wake cycle.
4. Pituitary Gland- this gland lies inferior to the hypothalamus and is
connected to it by the infundibulum. It is part of the endocrine system and
releases hormones.
E. The brain stem- An area
of white matter surrounds
gray matter and nuclei
regions are located within
the white matter. Each of
the three regions of the
brainstem is about an inch
long. It has three overall
functions:
1) produce automatic
behavior needed for
survival
2) passage way for fiber
tracts to exit the brain
3) contains 10 of the 12
cranial nerves.
1. Midbrain- it is divided by the cerebral aquaduct into the superior cerebral
peduncles (little feet of the cerebrum) and corpora quadrigemina
(superior/inferior colliculi) It relays tracts that connect the midbrain to the
cerebellum and another set to the spinal cord.
It also had a gray matter area called periaqueductal gray matter that send
signals involved in the “fight-or-flight” response as well as mediate visceral
pain response. This region also has cell bodies for the oculomotor and
trochlear nerve.
The corpora quadrigemina are involved in visual and auditory reflexes. They
contain the sustantia nigra (related to basal nuclei function) and red nucleus
(flexion of joints).
2. Pons- Lies inbetween the midbrain and medulla oblongata, it forms a
ventral bridge between the halves of the cerebellum. It contains the motor
and sensory nuclei of several cranial nerves (trigeminal, abducens, facial).
3. Medulla oblongata- It connects directly to the spinal cord at the foramen
magnum. It has the decussation of pyramids = the area where most of the
pyramidal tracts cross over. Also forms the inferior cerebellar peduncles
that connect the medulla to the cerebellum. It has some cranial nerves
attached (vestibulocochlear,glossophyrngeal, vagus, accessory, and
hypoglossal.
The medulla oblongata holds sensory nuclei and nuclei for control of autonomic
visceral function such as: 1) heartbeat rate,
2)blood pressure,
3) breathing rate
4) actions like coughing, sneezing, hiccups, swallowing.
F. The cerebellum- Has two lobed hemispheres that connect via the vermis. It
has outer gray matter, imbedded white matter and deep gray matter (nuclei).
Its surface ridges are called folia and they are separated by fissures. It
coordinates movement and makes it smooth, it helps in maintaining posture
and equilibrium and is involved in cognition.
• 1. cerebellar
pedunclessuperior,
middle, and
inferior
peduncles are
thick nerve fiber
tracts that
connect the
cerebellum to
the brainstem
(ipsilateral).
This is how
information is
linked between
thecerebellum
and the cerebral
cortex.
G. Functional brain systems- These are system that spread throughout the brain and
may be far from each other but still function as one.
1. Limbic system- Found in each cerebral hemisphere (medial area) and
diencephalons its output is relayed by the hypothalamus. It is the emotional brain.
The amygdala processes fear, threatening facial expression, and direction of a gaze.
The cingulated gyrus controls thought shifts, pain interpretation, and conflict
resolution. It is also involved in processing of memories. The stress we experience
causes physical conditions like high blood pressure.
2. Reticular formationThese are clusters of
neurons (white matter)
that run through the
core of the brainstem.
They project as far
from the brainstem and
some parts of it are
involved in maintaining
the brain alert. It also
arouses the brain from
sleep. There is also a
motor tract that goes
down the spinal cord
and is involved in
movement.
H. Protection of the brain- The brain is fragile and easily damaged. It is
protected by the bones of the skull, the meninges (connective tissue layers),
cushion by cerebral spinal fluid, and protected by the blood brain barrier.
1. Meninges- Three membranes of connective tissues that line the brain
and spinal cord. They enclose blood vessels, contain cerebral spinal fluid,
and cover and protect the brain and spinal cord. The three layers are called:
dura matter -outermost and toughest
arachnoid matter- middle layer, has subarachnoid space filled with cerebral
spinal fluid.
pia matter-has fine blood vessel, closest to the brain,
2. Cerebrospinal
fluid- serves as a
cushion, gives
buoyancy to the brain,
it nourishes the brain,
and removes waste. It
is produced in the
choroid plexuses
(within ventricles) from
blood plasma.
3. Blood brain
barrier- this is a
barrier that does not
allow toxins to cross
from the capillaries into
the brain. It is
endothelial cells in the
capillaries that reduce
its permeability.
However, anything that
is fat soluble may
cross this barrier.
• II. Spinal cord - Spinal cord is protected by three layers of
meninges. The only difference from the brain is that the dural matter
does not attach to bone. The dural matter is surrounded externally
by a layer of cushioning fat called epidural space.
• A. Cross-section view of spinal cord- wider laterllay than
anteroposteriorly. In the middle on the dorsal side is a shallow
groove called the posterior median sulcus and on the ventral side is
the anterior median fissure (deeper).
• 1. The center consist of gray matter shaped like a butterfly and there
is an opening at the center. These include: gray commisure (cross
bar), central canal (right in the middle), two posterior horns (dorsal
side) and two anterior horns (ventral side) Because they run the
entire length of the spinal cord they may be called columns rather
than horns. Each horn can be divided into somatic sensory (SS),
visceral sensory (VS), visceral motor (VM) and somatic motor (SM)
• 2. Around the gray matter lies white mater which can also be
divided into anterior, lateral, and posterior columns (funiculi).
Ascending and descending motor tracts run along the white
columns.
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III. Spinal tracts- These are known as sensory and motor pathways consisting of
multineuron pathways connecting the CNS to the PNS. At some point most
pathways crossover (decussate), there are at least two or three neurons in the
chain, they are spatially arranged, and are paired (left & right).
A. Ascending (sensory) Pathways- Four main ascending tracts that conduct
afferent signals two send it to the cerebral cortex and the other two to the
cerebellum.
1. Dorsal column pathway- carries signal of fine touch, pressure, and
proprioception, ascends up dorsal white column in fasciculus gracilis or cutaneatus
to medulla oblongata to the thalamus to primary somatosensory cortex (post
central gyrus).
2. Spinothalamic pathway- carries signals of pain, temperature, deep pressure,
and course touch. From psterior gray horn decussate into lateral and anterior
funiculi up to the thalamus to primary somatosensory cortex (postcentral gyrus).
3,4- Posterior and anterior spinocerebellar pathways- carry subsconcious
proprioception. Dorsal gray horn- to lateral column- to medulla oblongata- to pons
– to cerebellum.
3,4- Posterior and anterior spinocerebellar pathways- carry subsconcious
proprioception. Dorsal gray horn- to lateral column- to medulla oblongata- to pons
– to cerebellum.
1.
•
Dorsal column
pathway- carries
signal of fine touch,
pressure, and
proprioception,
ascends up dorsal
white column in
fasciculus gracilis
or cutaneatus to
medulla oblongata
to the thalamus to
primary
somatosensory
cortex (post central
gyrus).
3,4- Posterior and
anterior
spinocerebellar
pathways- carry
subsconcious
proprioception.
Dorsal gray horn- to
lateral column- to
medulla oblongatato pons – to
cerebellum.
• 2.
Spinothalamic
pathwaycarries signals of
pain,
temperature,
deep pressure,
and course
touch. From
psterior gray
horn decussate
into lateral and
anterior funiculi
up to the
thalamus to
primary
somatosensory
cortex
(postcentral
gyrus).
• B. Descending (motor) Pathways- these are divided into
pyramidal (corticospinal) tracts or all others. All of these
types of tracts cross over at some point.
• 1. Pyramidal (corticospinal) tracts- control skill motor
(precise movement) from the brain the tracts first
descend from the primary motor area of cerebral cortex
to midbrain, to pons, to medulla oblongata, to anterior or
lateral column to ventral root. Decussation happens in
the spinal cord.
• 2. All other tracts may originate from the midbrain pass
through the pons, to medulla oblongata, to anterior horn
and out through ventral root. Decussation happens in
brain stem.
• 1. Pyramidal
(corticospinal)
tracts- control skill
motor (precise
movement) from
the brain the tracts
first descend from
the primary motor
area of cerebral
cortex to midbrain,
to pons, to medulla
oblongata, to
anterior or lateral
column to ventral
root.
• 2. All other tracts
may originate from
the midbrain pass
through the pons,
to medulla
oblongata, to
anterior horn and
out through
ventral root.
Decussation
happens in brain
stem.
PET scan of the brain of an Alzheimer’s patient. Blue and purple
areas show parts of the brain with affected neural activity. Tissue is
degenerated as it is invaded by plaques. Patients suffer from
dementia and other mental ailments.
Infant with spina bifida cystica showing lumbar
myelomeningocele (cyst of the meninges)