Case 1: How is the brain organized?
Anatomy and organization of the brain
Different layers of protection
The brain is protected by bone (the skull), membranes (the meninges), and cerebrospinal fluid. The blood
brain barrier protects the brain from harmful substances in the blood.
Meninges = three connective tissue membranes that lie external to the CNS organs.
Functions:
- Cover and protect the CNS
- Protect blood vessels and enclose venous
sinuses
- Contain CSF
- Form partitions in the skull
From external to internal:
Dura mater → Strongest meninx, two layered sheet
of fibrous connective tissue.
➔ Superficial periosteal layer: attaches to the inner surface of the skull (not around spinal cord)
➔ Deeper meningeal layer: external covering of the brain and continues as spinal dura mater
They are fused together, separate to form: dural venous sinuses → collect venous blood from the brain and
direct it into the internal jugular veins of the neck.
In several places, the meningeal dura mater extends inward to form flat partitions that subdivide the cranial
cavity → dural septa: limit excessive movement of the brain within the cranium
Arachnoid mater → middle meninx, forms loose brain covering, never dips into the sulci.
Separated from the dura mater by serous cavity, the subdural space → contains a film of fluid.
Subarachnoid space: Beneath the arachnoid matter, spider-web like extensions that secure arachnoid
mater, filled with CSF and contains largest blood vessels serving the brain.
Arachnoid villi: protrude superiorly through the dura mater and into the superior sagittal sinus, absorb CSF
into the venous blood of the sinus.
Pia mater → composed of connective tissue and contains tiny blood
vessels, clings tightly to the brain. Small arteries entering the brain
tissue carry pia mater inward for short distances.
CSF → found in and around the brain and spinal cord, forms liquid
cushion, watery and similar in composition to blood plasma, from
which it is formed (contains less protein and different ion conc. →
contains more Na, Cl and H and less Ca and K)
- Gives buoyancy to CNS structures
- Reduces brain weight
- Protects and nourishes the brain
- Carries chemical signals
Choroid plexuses that hang from the roof of each ventricle form CSF
(pia mater + thin walled capillaries + layer of ependymal cells (that
also line the ventricles))
Cilia of ependymal cells keep the CSF in constant motion.
,Different structures of the forebrain
Three primary brain parts: the prosencephalon, or forebrain, the
mesencephalon, or midbrain, and rhombencephalon, or hindbrain.
- The forebrain can be divided into the telencephalon and
diencephalon.
- The hindbrain forms the metencephalon and myelencephalon.
- The midbrain is just the mesencephalon.
The four adult brain regions are: cerebral hemispheres, diencephalon, brain stem (midbrain, pons and
medulla), and cerebellum.
Cerebral hemispheres → superior part of the brain, gyri (elevated ridges) and sulci (shallow grooves)
Longitudinal fissure: separates the cerebral
hemispheres.
Transverse cerebral fissure: separates the cerebral
hemispheres from the cerebellum.
Five lobes: frontal, parietal, temporal, occipital, and
insula.
Three basic regions:
- A superficial cerebral cortex of gray mater
- Internal white matter
- Basal ganglia, islands of gray mater deep in
the white matter
Ventricles → continuous with one another and with the central canal of the
spinal cord. Filled with CSF and lined by ependymal cells.
Lateral ventricles: large C-shaped chambers deep within cerebral
hemispheres.
Third ventricle: in the diencephalon, communication to lateral ventricles via
interventricular foramen. Continuous with the fourth ventricle via the
cerebral aqueduct (runs through midbrain).
Fourth ventricle: lies in the hindbrain dorsal to the pons and superior
medulla. Continuous with the central canal of the spinal cord inferiorly. Three
openings in walls fourth ventricle: the paired lateral apertures in its side
walls and the median aperture in its roof. These connect the ventricle to the
subarachnoid space.
Cerebral white matter → deep to the cortical gray matter, responsible for
communication between cerebral areas between the cerebral cortex and lower CNS centers. Consists largely
of myelinated fibers bundled into large tracts. These fibers and tracts are classified according to the
direction:
- Association fibers: connect different parts of the same hemisphere. Short association fibers connect
adjacent gyri. Long association fibers are bundled into tracts and connect different cortical lobes.
- Commissural fibers: connect corresponding gray areas of the two hemispheres to function as a
coordinated whole. Corpus callosum, anterior and posterior commissures.
- Projection fibers: either enter the cerebral cortex from lower brain or cord centers or descend from
the cortex to lower areas.
Basal ganglia → group of subcortical nuclei. Include the caudate nucleus, putamen, and globus pallidus.
Striatum = caudate nucleus + putamen
Basal ganglia are functionally associated with the subthalamic nuclei and substantia nigra.
,They have a role in motor function, cognition and emotion. They filter out incorrect or inappropriate
responses. Important in starting, stopping and monitoring movements (inhibit especially).
Diencephalon → forming the central core of the forebrain and surrounded by the cerebral hemisphere.
Consists of grey matter areas: thalamus, hypothalamus, and epithalamus → enclose the third ventricle.
Thalamus → consist of bilateral nuclei, which form superolateral walls of third ventricle. Interthalamic
adhesion connects the nuclei.
= relay station for information coming into the cerebral cortex.
Consists of large number of nuclei, with functional specialties and each projects and receives fibers from a
specific region of the cerebral cortex.
In summary, the thalamus plays a key role in mediating sensation, motor activities, cortical arousal, learning
and memory. (gateway to the cerebral cortex).
Hypothalamus → positioned below the thalamus, caps the brain stem, forms the inferolateral walls of the
third ventricle.
= main visceral control center of the body and is
important to overall body homeostasis.
Mamillary bodies: paired pealike nuclei that bulge
anteriorly from the hypothalamus, relay stations in
olfactory pathways.
The pituitary gland is connected to the base of the
hypothalamus.
Epithalamus → most dorsal portion of the
diencephalon, forms the roof of the third ventricle.
On its posterior side is the pineal gland → secretes
the hormone melatonin and helps regulate the sleep-
wake cycle. The posterior commissure forms the
caudal border of the epithalamus.
Different structures of the midbrain
Brain stem → from superior to inferior, the regions are midbrain, pons and medulla oblongata.
- Deep gray matter surrounded by white matter, but has nuclei of gray matter embedded in the white
matter.
- Brain stem centers produce the rigidly programmed,
automatic behaviors necessary for survival.
- Pathway for fiber tracts running between higher and
lower neural centers.
Midbrain → located between the diencephalon and the pons,
running through it is the cerebral aqueduct
Amygdaloid body: is fear perceiving.
On the dorsal midbrain, are the superior colliculi, visual reflex
centers that coordinate head and eye movements, and the
inferior colliculi, part of the auditory relay from the hearing
receptors of the ear to the sensory cortex.
,In each side of the midbrain are the substantia nigra and the red nucleus. The substantia nigra has a dark
color because of high melanin pigment (functionally linked to basal ganglia).
The red nucleus lies deep to the substantia nigra, red nuclei are relay nuclei in some descending motor
pathways that effect limb flexion, and they are embedded in the reticular formation, a system of small
nuclei scattered through the core of the brain stem.
Different structures of the hindbrain
Pons → brain stem region between the midbrain and
medulla. The pons is composed of conduction tracts that
are oriented in two different ways:
- The deep projection fibers run longitudinally as part
of the pathway between higher brain centers and
the spinal cord.
- The ventral fibers are oriented transversally and
dorsally. They form the middle cerebellar
peduncles and connect the pons bilaterally with the
two sides of the cerebellum dorsally. These fibers
issue form numerous pontine nuclei, relay
informatio between motor cortex and cerebellum.
Medulla oblongata→ most inferior part of the brain stem. It blends with the spinal cord. Together with the
pons, it forms the ventral wall of the fourth ventricle.
Flanking the midline (ventral side) are two longitudinal ridges, called pyramids formed by the pyramidal
tracts descending form the motor cortex.
Decussation of the pyramids = at medulla-spinal cord junction the fibers cross over to the opposite side.
The inferior cerebellar peduncles are fiber tracts that connect the medulla to the cerebellum dorsally.
Cerebellum
➔ Processes inputs received from the cerebral motor cortex,
various brain stem nuclei, and sensory receptors, the cerebellum
provides the precise timing and appropriate patterns of skeletal
muscle contraction for smooth, coordinated movements needed
for daily living.
Bilaterally symmetrical. The vermis connects its two cerebellar
hemispheres medially.
Surface is convoluted → gyri known as folia
Deep fissures divide each hemisphere into anterior, posterior and
flocculonodular lobes.
Several types of neurons in cerebellar cortex → Purkinje cells: large
cells with extensively branched dendrites, are the only cotrcical neurons that send axons through the white
matter to synapse with the central nuclei of the cerebellum. → arbor vitae = resembles branching tree
The anterior and posterior lobes, which coordinate body movements, have three sensory maps of the entire
body. The part of the cerebellar cortex that receives sensory input from a body region influences motor
output to that region.
- The medial portions influence the motor activities of the trunk and girdle muscles.
- The intermediate part of each hemisphere influences the distal parts of the limbs and skilled
movements.
, - The lateral most parts of each hemisphere integrate information from the association areas of the
cerebral cortex and appear to play a role in planning movements rather than executing them.
- The flocculonodular lobes receive inputs from the equilibrium apparatus of the inner ears, and
adjust posture to maintain balance.
-
Three paired fiber tracts- the cerebellar peduncles – connect the cerebellum to the brain stem. (virtually all
fibers entering and leaving the cerebellum are ipsilateral = same side of the body.)
- The superior cerebellar peduncles connecting cerebellum and midbrain carry instructions from
neurons in the deep cerebellar nuclei to the cerebral motor cortex via thalamic relays. Like the basal
nuclei, the cerebellum has no direct connections to the cerebral cortex.
- The middle cerebellar peduncles carry one-way communications from the pons to the cerebellum,
advising the cerebellum of voluntary motor activities initiated by the motor cortex (via relays in the
pontine nuclei).
- The inferior cerebellar peduncles connect medulla and cerebellum. These peduncles convert
sensory information to the cerebellum from muscle proprioceptors throughout the body, and the
vestibular nuclei of the brain stem, which are concerned with equilibrium and balance.
Spinal cord
= the major pathway for information flowing back and forth between the brain and the skin, joints, and
muscles of the body. It contains neural networks responsible for locomotion.
Divided into four regions: cervical, thoracic, lumbar and sacral → correspond to the adjacent vertebrae.
Each spinal region is subdivided into segments, and each segment gives rise to a bilateral pair of spinal
nerves. Just before a spinal nerve joins the spinal cord, it divides into two branches called roots. The spinal
cord has a butterfly or H-shaped core of gray matter and a surrounding of white matter.
- The dorsal root of each spinal nerve is specialized to carry incoming sensory information. Dorsal
root ganglia on the dorsal roots contain cell bodies of sensory neurons. Sensory fibers from the
dorsal roots synapse with interneurons in the dorsal horns of the gray matter. The dorsal horn cell
bodies are organized into two distinct nuclei, one for somatic information and one for visceral
information.
- The ventral root carries information from the CNS to the muscles and glands. The ventral horns of
the gray matter contain cell bodies of the motor neurons that carry efferent signals to muscles and
glands. The ventral horns are organized into somatic motor and automatic nuclei. Efferent fibers
leave the spinal cord via the ventral root.
The white matter of the spinal cord can be divided into a number of columns composed of tracts of axons
that transfer information up and down the cord:
- Ascending tracts take sensory information to the brain. They occupy the dorsal and external lateral
portions of the spinal cord.
- Descending tracts carry mostly efferent (motor) signals from the brain to the
cord. They occupy the ventral and inferior lateral portions of the white matter.
- Propriospinal tracts are those that remain within the cord.
The spinal cord can function as a self-contained integrating center for simple spinal
reflexes, with signals passing from a sensory neuron through the gray matter to an
efferent neuron.
Brain vascularization
Two pair of arteries supply blood to the brain → the vertebral arteries and the internal
carotid arteries.
There is a ring of connected arteries at the brain’s base → circle of Willis → if one artery
is blocked by an obstruction, blood can still go to that brain part via the circle.
,Neurons and glia
Neuron tissue is made up of just two principal types of cells:
- Supporting cells called neuroglia, small cells that surround and wrap
the more delicate neurons.
- Neurons, nerve cells that are excitable (responsive to stimuli) and
transmit electrical signals.
Neuroglia
6 types – four in the CNS and two in the PNS:
CNS: most neuroglia have branching processes and a central cell body. They can
be distinguished by their smaller size and the dark staining nuclei. They out-
number neurons in the CNS by about 10 to 1.
Astrocytes → most abundant and versatile glial cells. Their numerous processes
cling to neurons and their synaptic endings and cover nearby capillaries.
- They support and anchor neurons to their nutrient supply lines.
- Play a role in making exchanges between capillaries and neurons,
helping determine capillary permeability.
- They guide the migration of young neurons and formation of synapses
between neurons.
- Control the chemical environment around the neurons (recapturing
released neurotransmitter and leaked potassium)
- Respond to nearby nerve impulses and neurotransmitters. Connected
by gap junctions, astrocytes signal each other both by taking in calcium,
creating slow-paced intracellular calcium pulses, and by releasing
extracellular chemical messengers.
- Influence neuronal functioning and therefore participate in information
processing in the brain.
Microglial cells → small and ovoid with relatively long processes. Their
processes touch nearby neurons, monitoring their health, and when they sense
that neurons are injured, they migrate toward them.
➔ The microglial cells transform into a special type of macrophage that
phagocytized the microorganisms or neuronal debris.
Important because immune system has limited access to the CNS.
Ependymal cells → range in shape from squamous to columnar and many are ciliated. They line the central
cavities of the brain and the spinal cord.
➔ Form permeable barrier between the CSF and the tissue fluid bathing the cells of the CNS. Their cilia
helps to circulate the CSF.
Oligodendrocytes → they have fewer processes than astrocytes. Line up along the thicker nerve fibers in the
CNS and wrap their processes tightly around the fibers, producing a myelin sheath.
PNS: the two kinds of PNS neuroglia differ mainly in location.
Satellite cells → surround neuron cell bodies located in the peripheral nervous system, and are thought to
have many of the same functions in the PNS as astrocytes do in the CNS.
Schwann cells → surround all nerve fibers in the PNS and form myelin sheaths around the thicker nerve
fibers. They are functionally similar to oligodendrocytes. Schwann cells are vital to regeneration of damaged
peripheral nerve fibers.
,Neurons
Are the structural units of the NS. Typically, large, highly specialized cells that conduct messages in the form
of nerve impulses from one part of the body to another.
➔ Neurons have extreme longevity. They can function for a lifetime
➔ Neurons are amitotic. They lose their ability to divide once they assume their role
➔ Neurons have exceptionally high metabolic rate and require continuous supplies of oxygen and glucose.
Although neurons vary in structure, they all have a cell body and
one or more processes:
Neuron cell body → also called perikaryon or soma, contains the
usual organelles. Most neuron cell bodies are located in the CNS.
Clusters of cell bodies in the CNS are called nuclei, whereas those
that lie along the nerves in the PNS are called ganglia.
Neuron processes → the CNS contains both neuron cell bodies and
their processes. The PNS consists chiefly of neuron processes.
Bundles of neuron processes are called tracts in the CNS and nerves
in the PNS.
There are two types of neuron processes, dendrites and axons, who
differ in structure and function of their plasma membranes.
Dendrites: (of motor neurons) are short, diffusely branching extensions. Typically, motor neurons have
hundreds of dendrites clustering close to the cell body. All organelles present in the cell body also occur in
dendrites.
➔ Main receptive or input regions, provide huge surface for receiving signals from other neurons.
Dendrites convey incoming messages toward the cell body. These electrical signals are usually not AP
but graded potentials (short-distance signals).
Axon: each neuron has a single axon. The initial region of the axon arises from an area of the cell body called
the axon hillock. In some neurons, the axon is very short or absent, but in others it accounts for nearly the
entire length of the neuron (long axon = nerve fiber).
➔ Each neuron has only one axon, but axons may have occasional branches along their length → axon
collaterals. An axon usually branches profusely at its end → terminal branches. The knoblike distal
endings of the terminal branches are called axon terminals or terminal boutons.
➔ The axon is the conducting region of the neuron. It generates nerve impulses and transmits them away
from the cell body along the axolemma, or plasma membrane. In motor neurons, the nerve impulse is
generated at the junction of the axon hillock and axon and conducted along the axon to the axon
terminals, which are the secretory region of the neuron.
➔ Because each neuron both receives signals from and sends signals to other neurons, it carries on
“conservations” with many different neurons at the same time.
➔ An axon contains the same organelles found in the dendrites and cell body with the exception of the ER
and golgi. So the axon depends on the cell body to renew the necessary proteins and membrane
components and on efficient transport mechanisms to distribute them.
Myelination → many nerve fibers that are long or large in diameter are covered with a whitish, fatty
(protein-lipoid), segmented myelin sheath.
➔ Myelin protects and electrically insulates fibers, and it increases the transmission speed of nerve
impulses. (only with axons, never dendrites)
➔ In PNS: formed by Schwann cells, which do not touch one another, so there are gaps in the sheath →
Nodes of Ranvier (axon collaterals can emerge from these gaps)
➔ In CNS: oligodendrocytes form myelin sheaths, it has multiple flat processes that can coil around many
axons. White matter contains myelinated fibers. Gray matter contains mostly nerve cell bodies and
nonmyelinated fibers.
,Functions of the brain
Functions of the hypothalamus
- Control the autonomic nervous system. The ANS is a system of peripheral nerves that regulate
cardiac and smooth muscle and secretion by the glands. Hypothalamus regulates ANS activity by
controlling the activity of centers in the brain stem and spinal cord.
- Initiate physical responses to emotions. It contains nuclei involved in perceiving pleasure, fear, and
rage as well those involved in biological rhythms and drives.
- Regulate body temperature. Hypothalamic neurons monitor blood temperature and receive input
from other thermoreceptors in the brain and body periphery. Initiates cooling (sweating) or heat-
generating (shivering).
- Regulate food intake. In response to changing blood levels of certain nutrients (glucose and amino
acids or hormones, the hypothalamus regulates feelings of hunger and satiety.
- Regulate water balance and thirst. Osmoreceptors (hypothalamic neurons) are activated when body
fluids become too concentrated. Also stimulate the thirst center.
- Regulate sleep-wake cycle. Acting with other brain regions, the hypothalamus helps regulate sleep.
- Control endocrine system function. Its releasing and inhibiting hormones control the secretion of
hormones by the anterior pituitary gland and it supraoptic and paraventricular nuclei produce the
hormones ADH and oxytocin.
Functions of the medulla oblongata
The medulla has a crucial rile as an autonomic reflex center involved in maintaining body homeostasis. It
contains several important functional groups of visceral motor nuclei:
- Cardiovascular center. This includes the cardiac center, which adjusts the force and rate of heart
contraction, and the vasomotor center, which changes blood vessel diameter to regulate BP.
- Respiratory centers. These generate the respiratory rhythm and control the rate and depth of
breathing (together with pontine centers).
- Various other centers. Additional centers regulate such activities as vomiting, hiccupping,
swallowing, coughing and sneezing.
The hypothalamus controls many visceral functions by relaying its instructions through medullary reticular
centers, which carry them out.
Functions of the cerebellum
Cerebellar processing fine-tunes motor activity as follows:
- The motor areas of the cerebral cortex, via relay nuclei in the brain stem, notify the cerebellum of
their intent to initiate voluntary muscle contractions.
- At the same time, the cerebellum receives information from proprioceptors throughout the body
and from visual and equilibrium pathways. This info enables the cerebellum to evaluate body
position and momentum.
- The cerebellar cortex calculates the best way to coordinate the force, direction and extent of muscle
contraction to prevent overshoot, maintain posture, and ensure smooth coordinated movements.
- Then via the superior peduncles, the cerebellum dispatches to the cerebral motor cortex for
coordinating - movement. Cerebellar fibers also send info to brain stem nuclei, which in turn
influence motor neurons of the spinal cord.
The cerebellum continually compares the body’s performance with the higher brain’s intention and sends
out messages to initiate appropriate corrective measures.
Functional brain systems
= networks of neurons that work together but span relatively large distances in the brain, so they cannot be
localized to specific regions.
,The limbic system → includes the amygdaloid body, nucleus that sits on the tail of the caudate nucleus, and
the cingulate gyrus, septal nuclei, hippocampus, dentate gyrus and parahippocampal gyrus. In the
diencephalon, the main limbic structures are: hypothalamus and the anterior thalamic nuclei. The fornix and
other fiber tracts link these limbic system regions together.
➔ Our emotional or affective brain. (especially important are amygdaloid body and the anterior part of the
cingulate gyrus)
Amygdaloid body → critical for responding to perceived threats with fear or aggression.
Cingulate gyrus → plays a role in expressing our emotions through gestures and in resolving mental conflicts
when we are frustrated.
Extensive connections between the limbic system and lower and higher brain regions allow the system to
integrate and respond to a variety of environmental stimuli.
➔ Most limbic system output is relayed through the hypothalamus. The hypothalamus is the neural house
for both autonomic (visceral) function and emotional response.
Because the limbic system interacts with the prefrontal lobes, there is an intimate relationship between our
feelings and our thoughts. As a result, we react emotionally to things we consciously understand to be
happening and are consciously aware of the emotional richness of our lives.
Hippocampus & amygdaloid body → play role in memory
The reticular formation → extends through the central core of the medulla, pons and midbrain.
Composed of loosely clustered neurons in white matter. They project to the hypothalamus, thalamus,
cerebral cortex, cerebellum and spinal cord. → govern the arousal of the brain as a whole.
➔ unless inhibited by other brain areas, the neurons of the part of the reticular formation known as the
reticular activating system (RAS) send a continuous stream of impulses to the cerebral cortex, keeping
the cortex alert and conscious and enhancing its excitability.
➔ Impulses from all the great ascending sensory tracts synapse with RAS neurons. They filter the sensory
input. Repetitive, familiar or weak signals are filtered out, but unusual, significant or strong impulses do
reach consciousness.
Lateralization of cortical functioning
We use both cerebral hemispheres for almost every activity, and the hemispheres appear nearly identical.
Nonetheless, there is a division of labor, and each hemisphere has abilities not completely shared by the
other → lateralization.
Although one cerebral hemisphere or the other dominates each task, the term cerebral dominance
designates the hemisphere that is dominant for language. In most people:
- left hemisphere: has greater control over language abilities, math and logic.
- The other hemisphere (usually the right): is more free-spirited, more involved in visual-spatial skills,
intuition, emotion, and artistic and musical skills.
The two cerebral hemispheres have almost instantaneous communication with each other via connecting
fiber tracts, as well as complete functional integration. Furthermore, although lateralization means that each
hemisphere is better that the other at certain functions, neither hemisphere is better at everything.
Summary functions
Cingulate gyrus = detection of emotional/cognitive conflicts
Superior/inferior colliculi – visual/auditory pathways
Hippocampus = learning/memory
Cerebellum = dexterity/movement
Basal ganglia = regulate motor activity
Globus pallidus = high level of activity under Parkinson’s, constantly inhibits thalamus
Amygdala = detection of fear/threatening
Thalamus = sensory relay station except for smell
Medulla oblongata = vital functions (breathing, wake-sleep etc.)
, Hypothalamus = temperature, hunger, thirst, endocrine, sex
Mamillary bodies = (recollective) memory
Limbic system = Adapting to environment, emotional brain
Brain lobes:
Frontal lobe = higher cognitive functions, motor behavior
Parietal lobe = plays an important role in somatic sensation/senses (smell, taste, touch)
Temporal lobe = plays an important role in emotions and memory, smelling, tasting, perception, aggression
and sexual behavior
Occipital lobe = controls sight and recognition
Insula = adds emotional context to sensory input
Brain areas:
Premotor cortex, primary motor cortex = motor
Sensory cortex = sensory
Visual cortex in the occipital lobe = seeing
Taste center in the post central sulcus of the parietal cortex = tasting
Prefrontal cortex =
important for intellect,
complex learning, mood and
personality (cognition)
Limbic system = emotions
Broca’s area = formulate
speech, motor instruction to
motor cortex
Primary auditory area =
detect sounds
Auditory association area =
recognize sounds
Wernicke’s area, auditory
association area = recognize
and understand words
Brain stem: pons and
medulla = vital functions
and reflexes (autonomic)
Anatomy and organization of the brain
Different layers of protection
The brain is protected by bone (the skull), membranes (the meninges), and cerebrospinal fluid. The blood
brain barrier protects the brain from harmful substances in the blood.
Meninges = three connective tissue membranes that lie external to the CNS organs.
Functions:
- Cover and protect the CNS
- Protect blood vessels and enclose venous
sinuses
- Contain CSF
- Form partitions in the skull
From external to internal:
Dura mater → Strongest meninx, two layered sheet
of fibrous connective tissue.
➔ Superficial periosteal layer: attaches to the inner surface of the skull (not around spinal cord)
➔ Deeper meningeal layer: external covering of the brain and continues as spinal dura mater
They are fused together, separate to form: dural venous sinuses → collect venous blood from the brain and
direct it into the internal jugular veins of the neck.
In several places, the meningeal dura mater extends inward to form flat partitions that subdivide the cranial
cavity → dural septa: limit excessive movement of the brain within the cranium
Arachnoid mater → middle meninx, forms loose brain covering, never dips into the sulci.
Separated from the dura mater by serous cavity, the subdural space → contains a film of fluid.
Subarachnoid space: Beneath the arachnoid matter, spider-web like extensions that secure arachnoid
mater, filled with CSF and contains largest blood vessels serving the brain.
Arachnoid villi: protrude superiorly through the dura mater and into the superior sagittal sinus, absorb CSF
into the venous blood of the sinus.
Pia mater → composed of connective tissue and contains tiny blood
vessels, clings tightly to the brain. Small arteries entering the brain
tissue carry pia mater inward for short distances.
CSF → found in and around the brain and spinal cord, forms liquid
cushion, watery and similar in composition to blood plasma, from
which it is formed (contains less protein and different ion conc. →
contains more Na, Cl and H and less Ca and K)
- Gives buoyancy to CNS structures
- Reduces brain weight
- Protects and nourishes the brain
- Carries chemical signals
Choroid plexuses that hang from the roof of each ventricle form CSF
(pia mater + thin walled capillaries + layer of ependymal cells (that
also line the ventricles))
Cilia of ependymal cells keep the CSF in constant motion.
,Different structures of the forebrain
Three primary brain parts: the prosencephalon, or forebrain, the
mesencephalon, or midbrain, and rhombencephalon, or hindbrain.
- The forebrain can be divided into the telencephalon and
diencephalon.
- The hindbrain forms the metencephalon and myelencephalon.
- The midbrain is just the mesencephalon.
The four adult brain regions are: cerebral hemispheres, diencephalon, brain stem (midbrain, pons and
medulla), and cerebellum.
Cerebral hemispheres → superior part of the brain, gyri (elevated ridges) and sulci (shallow grooves)
Longitudinal fissure: separates the cerebral
hemispheres.
Transverse cerebral fissure: separates the cerebral
hemispheres from the cerebellum.
Five lobes: frontal, parietal, temporal, occipital, and
insula.
Three basic regions:
- A superficial cerebral cortex of gray mater
- Internal white matter
- Basal ganglia, islands of gray mater deep in
the white matter
Ventricles → continuous with one another and with the central canal of the
spinal cord. Filled with CSF and lined by ependymal cells.
Lateral ventricles: large C-shaped chambers deep within cerebral
hemispheres.
Third ventricle: in the diencephalon, communication to lateral ventricles via
interventricular foramen. Continuous with the fourth ventricle via the
cerebral aqueduct (runs through midbrain).
Fourth ventricle: lies in the hindbrain dorsal to the pons and superior
medulla. Continuous with the central canal of the spinal cord inferiorly. Three
openings in walls fourth ventricle: the paired lateral apertures in its side
walls and the median aperture in its roof. These connect the ventricle to the
subarachnoid space.
Cerebral white matter → deep to the cortical gray matter, responsible for
communication between cerebral areas between the cerebral cortex and lower CNS centers. Consists largely
of myelinated fibers bundled into large tracts. These fibers and tracts are classified according to the
direction:
- Association fibers: connect different parts of the same hemisphere. Short association fibers connect
adjacent gyri. Long association fibers are bundled into tracts and connect different cortical lobes.
- Commissural fibers: connect corresponding gray areas of the two hemispheres to function as a
coordinated whole. Corpus callosum, anterior and posterior commissures.
- Projection fibers: either enter the cerebral cortex from lower brain or cord centers or descend from
the cortex to lower areas.
Basal ganglia → group of subcortical nuclei. Include the caudate nucleus, putamen, and globus pallidus.
Striatum = caudate nucleus + putamen
Basal ganglia are functionally associated with the subthalamic nuclei and substantia nigra.
,They have a role in motor function, cognition and emotion. They filter out incorrect or inappropriate
responses. Important in starting, stopping and monitoring movements (inhibit especially).
Diencephalon → forming the central core of the forebrain and surrounded by the cerebral hemisphere.
Consists of grey matter areas: thalamus, hypothalamus, and epithalamus → enclose the third ventricle.
Thalamus → consist of bilateral nuclei, which form superolateral walls of third ventricle. Interthalamic
adhesion connects the nuclei.
= relay station for information coming into the cerebral cortex.
Consists of large number of nuclei, with functional specialties and each projects and receives fibers from a
specific region of the cerebral cortex.
In summary, the thalamus plays a key role in mediating sensation, motor activities, cortical arousal, learning
and memory. (gateway to the cerebral cortex).
Hypothalamus → positioned below the thalamus, caps the brain stem, forms the inferolateral walls of the
third ventricle.
= main visceral control center of the body and is
important to overall body homeostasis.
Mamillary bodies: paired pealike nuclei that bulge
anteriorly from the hypothalamus, relay stations in
olfactory pathways.
The pituitary gland is connected to the base of the
hypothalamus.
Epithalamus → most dorsal portion of the
diencephalon, forms the roof of the third ventricle.
On its posterior side is the pineal gland → secretes
the hormone melatonin and helps regulate the sleep-
wake cycle. The posterior commissure forms the
caudal border of the epithalamus.
Different structures of the midbrain
Brain stem → from superior to inferior, the regions are midbrain, pons and medulla oblongata.
- Deep gray matter surrounded by white matter, but has nuclei of gray matter embedded in the white
matter.
- Brain stem centers produce the rigidly programmed,
automatic behaviors necessary for survival.
- Pathway for fiber tracts running between higher and
lower neural centers.
Midbrain → located between the diencephalon and the pons,
running through it is the cerebral aqueduct
Amygdaloid body: is fear perceiving.
On the dorsal midbrain, are the superior colliculi, visual reflex
centers that coordinate head and eye movements, and the
inferior colliculi, part of the auditory relay from the hearing
receptors of the ear to the sensory cortex.
,In each side of the midbrain are the substantia nigra and the red nucleus. The substantia nigra has a dark
color because of high melanin pigment (functionally linked to basal ganglia).
The red nucleus lies deep to the substantia nigra, red nuclei are relay nuclei in some descending motor
pathways that effect limb flexion, and they are embedded in the reticular formation, a system of small
nuclei scattered through the core of the brain stem.
Different structures of the hindbrain
Pons → brain stem region between the midbrain and
medulla. The pons is composed of conduction tracts that
are oriented in two different ways:
- The deep projection fibers run longitudinally as part
of the pathway between higher brain centers and
the spinal cord.
- The ventral fibers are oriented transversally and
dorsally. They form the middle cerebellar
peduncles and connect the pons bilaterally with the
two sides of the cerebellum dorsally. These fibers
issue form numerous pontine nuclei, relay
informatio between motor cortex and cerebellum.
Medulla oblongata→ most inferior part of the brain stem. It blends with the spinal cord. Together with the
pons, it forms the ventral wall of the fourth ventricle.
Flanking the midline (ventral side) are two longitudinal ridges, called pyramids formed by the pyramidal
tracts descending form the motor cortex.
Decussation of the pyramids = at medulla-spinal cord junction the fibers cross over to the opposite side.
The inferior cerebellar peduncles are fiber tracts that connect the medulla to the cerebellum dorsally.
Cerebellum
➔ Processes inputs received from the cerebral motor cortex,
various brain stem nuclei, and sensory receptors, the cerebellum
provides the precise timing and appropriate patterns of skeletal
muscle contraction for smooth, coordinated movements needed
for daily living.
Bilaterally symmetrical. The vermis connects its two cerebellar
hemispheres medially.
Surface is convoluted → gyri known as folia
Deep fissures divide each hemisphere into anterior, posterior and
flocculonodular lobes.
Several types of neurons in cerebellar cortex → Purkinje cells: large
cells with extensively branched dendrites, are the only cotrcical neurons that send axons through the white
matter to synapse with the central nuclei of the cerebellum. → arbor vitae = resembles branching tree
The anterior and posterior lobes, which coordinate body movements, have three sensory maps of the entire
body. The part of the cerebellar cortex that receives sensory input from a body region influences motor
output to that region.
- The medial portions influence the motor activities of the trunk and girdle muscles.
- The intermediate part of each hemisphere influences the distal parts of the limbs and skilled
movements.
, - The lateral most parts of each hemisphere integrate information from the association areas of the
cerebral cortex and appear to play a role in planning movements rather than executing them.
- The flocculonodular lobes receive inputs from the equilibrium apparatus of the inner ears, and
adjust posture to maintain balance.
-
Three paired fiber tracts- the cerebellar peduncles – connect the cerebellum to the brain stem. (virtually all
fibers entering and leaving the cerebellum are ipsilateral = same side of the body.)
- The superior cerebellar peduncles connecting cerebellum and midbrain carry instructions from
neurons in the deep cerebellar nuclei to the cerebral motor cortex via thalamic relays. Like the basal
nuclei, the cerebellum has no direct connections to the cerebral cortex.
- The middle cerebellar peduncles carry one-way communications from the pons to the cerebellum,
advising the cerebellum of voluntary motor activities initiated by the motor cortex (via relays in the
pontine nuclei).
- The inferior cerebellar peduncles connect medulla and cerebellum. These peduncles convert
sensory information to the cerebellum from muscle proprioceptors throughout the body, and the
vestibular nuclei of the brain stem, which are concerned with equilibrium and balance.
Spinal cord
= the major pathway for information flowing back and forth between the brain and the skin, joints, and
muscles of the body. It contains neural networks responsible for locomotion.
Divided into four regions: cervical, thoracic, lumbar and sacral → correspond to the adjacent vertebrae.
Each spinal region is subdivided into segments, and each segment gives rise to a bilateral pair of spinal
nerves. Just before a spinal nerve joins the spinal cord, it divides into two branches called roots. The spinal
cord has a butterfly or H-shaped core of gray matter and a surrounding of white matter.
- The dorsal root of each spinal nerve is specialized to carry incoming sensory information. Dorsal
root ganglia on the dorsal roots contain cell bodies of sensory neurons. Sensory fibers from the
dorsal roots synapse with interneurons in the dorsal horns of the gray matter. The dorsal horn cell
bodies are organized into two distinct nuclei, one for somatic information and one for visceral
information.
- The ventral root carries information from the CNS to the muscles and glands. The ventral horns of
the gray matter contain cell bodies of the motor neurons that carry efferent signals to muscles and
glands. The ventral horns are organized into somatic motor and automatic nuclei. Efferent fibers
leave the spinal cord via the ventral root.
The white matter of the spinal cord can be divided into a number of columns composed of tracts of axons
that transfer information up and down the cord:
- Ascending tracts take sensory information to the brain. They occupy the dorsal and external lateral
portions of the spinal cord.
- Descending tracts carry mostly efferent (motor) signals from the brain to the
cord. They occupy the ventral and inferior lateral portions of the white matter.
- Propriospinal tracts are those that remain within the cord.
The spinal cord can function as a self-contained integrating center for simple spinal
reflexes, with signals passing from a sensory neuron through the gray matter to an
efferent neuron.
Brain vascularization
Two pair of arteries supply blood to the brain → the vertebral arteries and the internal
carotid arteries.
There is a ring of connected arteries at the brain’s base → circle of Willis → if one artery
is blocked by an obstruction, blood can still go to that brain part via the circle.
,Neurons and glia
Neuron tissue is made up of just two principal types of cells:
- Supporting cells called neuroglia, small cells that surround and wrap
the more delicate neurons.
- Neurons, nerve cells that are excitable (responsive to stimuli) and
transmit electrical signals.
Neuroglia
6 types – four in the CNS and two in the PNS:
CNS: most neuroglia have branching processes and a central cell body. They can
be distinguished by their smaller size and the dark staining nuclei. They out-
number neurons in the CNS by about 10 to 1.
Astrocytes → most abundant and versatile glial cells. Their numerous processes
cling to neurons and their synaptic endings and cover nearby capillaries.
- They support and anchor neurons to their nutrient supply lines.
- Play a role in making exchanges between capillaries and neurons,
helping determine capillary permeability.
- They guide the migration of young neurons and formation of synapses
between neurons.
- Control the chemical environment around the neurons (recapturing
released neurotransmitter and leaked potassium)
- Respond to nearby nerve impulses and neurotransmitters. Connected
by gap junctions, astrocytes signal each other both by taking in calcium,
creating slow-paced intracellular calcium pulses, and by releasing
extracellular chemical messengers.
- Influence neuronal functioning and therefore participate in information
processing in the brain.
Microglial cells → small and ovoid with relatively long processes. Their
processes touch nearby neurons, monitoring their health, and when they sense
that neurons are injured, they migrate toward them.
➔ The microglial cells transform into a special type of macrophage that
phagocytized the microorganisms or neuronal debris.
Important because immune system has limited access to the CNS.
Ependymal cells → range in shape from squamous to columnar and many are ciliated. They line the central
cavities of the brain and the spinal cord.
➔ Form permeable barrier between the CSF and the tissue fluid bathing the cells of the CNS. Their cilia
helps to circulate the CSF.
Oligodendrocytes → they have fewer processes than astrocytes. Line up along the thicker nerve fibers in the
CNS and wrap their processes tightly around the fibers, producing a myelin sheath.
PNS: the two kinds of PNS neuroglia differ mainly in location.
Satellite cells → surround neuron cell bodies located in the peripheral nervous system, and are thought to
have many of the same functions in the PNS as astrocytes do in the CNS.
Schwann cells → surround all nerve fibers in the PNS and form myelin sheaths around the thicker nerve
fibers. They are functionally similar to oligodendrocytes. Schwann cells are vital to regeneration of damaged
peripheral nerve fibers.
,Neurons
Are the structural units of the NS. Typically, large, highly specialized cells that conduct messages in the form
of nerve impulses from one part of the body to another.
➔ Neurons have extreme longevity. They can function for a lifetime
➔ Neurons are amitotic. They lose their ability to divide once they assume their role
➔ Neurons have exceptionally high metabolic rate and require continuous supplies of oxygen and glucose.
Although neurons vary in structure, they all have a cell body and
one or more processes:
Neuron cell body → also called perikaryon or soma, contains the
usual organelles. Most neuron cell bodies are located in the CNS.
Clusters of cell bodies in the CNS are called nuclei, whereas those
that lie along the nerves in the PNS are called ganglia.
Neuron processes → the CNS contains both neuron cell bodies and
their processes. The PNS consists chiefly of neuron processes.
Bundles of neuron processes are called tracts in the CNS and nerves
in the PNS.
There are two types of neuron processes, dendrites and axons, who
differ in structure and function of their plasma membranes.
Dendrites: (of motor neurons) are short, diffusely branching extensions. Typically, motor neurons have
hundreds of dendrites clustering close to the cell body. All organelles present in the cell body also occur in
dendrites.
➔ Main receptive or input regions, provide huge surface for receiving signals from other neurons.
Dendrites convey incoming messages toward the cell body. These electrical signals are usually not AP
but graded potentials (short-distance signals).
Axon: each neuron has a single axon. The initial region of the axon arises from an area of the cell body called
the axon hillock. In some neurons, the axon is very short or absent, but in others it accounts for nearly the
entire length of the neuron (long axon = nerve fiber).
➔ Each neuron has only one axon, but axons may have occasional branches along their length → axon
collaterals. An axon usually branches profusely at its end → terminal branches. The knoblike distal
endings of the terminal branches are called axon terminals or terminal boutons.
➔ The axon is the conducting region of the neuron. It generates nerve impulses and transmits them away
from the cell body along the axolemma, or plasma membrane. In motor neurons, the nerve impulse is
generated at the junction of the axon hillock and axon and conducted along the axon to the axon
terminals, which are the secretory region of the neuron.
➔ Because each neuron both receives signals from and sends signals to other neurons, it carries on
“conservations” with many different neurons at the same time.
➔ An axon contains the same organelles found in the dendrites and cell body with the exception of the ER
and golgi. So the axon depends on the cell body to renew the necessary proteins and membrane
components and on efficient transport mechanisms to distribute them.
Myelination → many nerve fibers that are long or large in diameter are covered with a whitish, fatty
(protein-lipoid), segmented myelin sheath.
➔ Myelin protects and electrically insulates fibers, and it increases the transmission speed of nerve
impulses. (only with axons, never dendrites)
➔ In PNS: formed by Schwann cells, which do not touch one another, so there are gaps in the sheath →
Nodes of Ranvier (axon collaterals can emerge from these gaps)
➔ In CNS: oligodendrocytes form myelin sheaths, it has multiple flat processes that can coil around many
axons. White matter contains myelinated fibers. Gray matter contains mostly nerve cell bodies and
nonmyelinated fibers.
,Functions of the brain
Functions of the hypothalamus
- Control the autonomic nervous system. The ANS is a system of peripheral nerves that regulate
cardiac and smooth muscle and secretion by the glands. Hypothalamus regulates ANS activity by
controlling the activity of centers in the brain stem and spinal cord.
- Initiate physical responses to emotions. It contains nuclei involved in perceiving pleasure, fear, and
rage as well those involved in biological rhythms and drives.
- Regulate body temperature. Hypothalamic neurons monitor blood temperature and receive input
from other thermoreceptors in the brain and body periphery. Initiates cooling (sweating) or heat-
generating (shivering).
- Regulate food intake. In response to changing blood levels of certain nutrients (glucose and amino
acids or hormones, the hypothalamus regulates feelings of hunger and satiety.
- Regulate water balance and thirst. Osmoreceptors (hypothalamic neurons) are activated when body
fluids become too concentrated. Also stimulate the thirst center.
- Regulate sleep-wake cycle. Acting with other brain regions, the hypothalamus helps regulate sleep.
- Control endocrine system function. Its releasing and inhibiting hormones control the secretion of
hormones by the anterior pituitary gland and it supraoptic and paraventricular nuclei produce the
hormones ADH and oxytocin.
Functions of the medulla oblongata
The medulla has a crucial rile as an autonomic reflex center involved in maintaining body homeostasis. It
contains several important functional groups of visceral motor nuclei:
- Cardiovascular center. This includes the cardiac center, which adjusts the force and rate of heart
contraction, and the vasomotor center, which changes blood vessel diameter to regulate BP.
- Respiratory centers. These generate the respiratory rhythm and control the rate and depth of
breathing (together with pontine centers).
- Various other centers. Additional centers regulate such activities as vomiting, hiccupping,
swallowing, coughing and sneezing.
The hypothalamus controls many visceral functions by relaying its instructions through medullary reticular
centers, which carry them out.
Functions of the cerebellum
Cerebellar processing fine-tunes motor activity as follows:
- The motor areas of the cerebral cortex, via relay nuclei in the brain stem, notify the cerebellum of
their intent to initiate voluntary muscle contractions.
- At the same time, the cerebellum receives information from proprioceptors throughout the body
and from visual and equilibrium pathways. This info enables the cerebellum to evaluate body
position and momentum.
- The cerebellar cortex calculates the best way to coordinate the force, direction and extent of muscle
contraction to prevent overshoot, maintain posture, and ensure smooth coordinated movements.
- Then via the superior peduncles, the cerebellum dispatches to the cerebral motor cortex for
coordinating - movement. Cerebellar fibers also send info to brain stem nuclei, which in turn
influence motor neurons of the spinal cord.
The cerebellum continually compares the body’s performance with the higher brain’s intention and sends
out messages to initiate appropriate corrective measures.
Functional brain systems
= networks of neurons that work together but span relatively large distances in the brain, so they cannot be
localized to specific regions.
,The limbic system → includes the amygdaloid body, nucleus that sits on the tail of the caudate nucleus, and
the cingulate gyrus, septal nuclei, hippocampus, dentate gyrus and parahippocampal gyrus. In the
diencephalon, the main limbic structures are: hypothalamus and the anterior thalamic nuclei. The fornix and
other fiber tracts link these limbic system regions together.
➔ Our emotional or affective brain. (especially important are amygdaloid body and the anterior part of the
cingulate gyrus)
Amygdaloid body → critical for responding to perceived threats with fear or aggression.
Cingulate gyrus → plays a role in expressing our emotions through gestures and in resolving mental conflicts
when we are frustrated.
Extensive connections between the limbic system and lower and higher brain regions allow the system to
integrate and respond to a variety of environmental stimuli.
➔ Most limbic system output is relayed through the hypothalamus. The hypothalamus is the neural house
for both autonomic (visceral) function and emotional response.
Because the limbic system interacts with the prefrontal lobes, there is an intimate relationship between our
feelings and our thoughts. As a result, we react emotionally to things we consciously understand to be
happening and are consciously aware of the emotional richness of our lives.
Hippocampus & amygdaloid body → play role in memory
The reticular formation → extends through the central core of the medulla, pons and midbrain.
Composed of loosely clustered neurons in white matter. They project to the hypothalamus, thalamus,
cerebral cortex, cerebellum and spinal cord. → govern the arousal of the brain as a whole.
➔ unless inhibited by other brain areas, the neurons of the part of the reticular formation known as the
reticular activating system (RAS) send a continuous stream of impulses to the cerebral cortex, keeping
the cortex alert and conscious and enhancing its excitability.
➔ Impulses from all the great ascending sensory tracts synapse with RAS neurons. They filter the sensory
input. Repetitive, familiar or weak signals are filtered out, but unusual, significant or strong impulses do
reach consciousness.
Lateralization of cortical functioning
We use both cerebral hemispheres for almost every activity, and the hemispheres appear nearly identical.
Nonetheless, there is a division of labor, and each hemisphere has abilities not completely shared by the
other → lateralization.
Although one cerebral hemisphere or the other dominates each task, the term cerebral dominance
designates the hemisphere that is dominant for language. In most people:
- left hemisphere: has greater control over language abilities, math and logic.
- The other hemisphere (usually the right): is more free-spirited, more involved in visual-spatial skills,
intuition, emotion, and artistic and musical skills.
The two cerebral hemispheres have almost instantaneous communication with each other via connecting
fiber tracts, as well as complete functional integration. Furthermore, although lateralization means that each
hemisphere is better that the other at certain functions, neither hemisphere is better at everything.
Summary functions
Cingulate gyrus = detection of emotional/cognitive conflicts
Superior/inferior colliculi – visual/auditory pathways
Hippocampus = learning/memory
Cerebellum = dexterity/movement
Basal ganglia = regulate motor activity
Globus pallidus = high level of activity under Parkinson’s, constantly inhibits thalamus
Amygdala = detection of fear/threatening
Thalamus = sensory relay station except for smell
Medulla oblongata = vital functions (breathing, wake-sleep etc.)
, Hypothalamus = temperature, hunger, thirst, endocrine, sex
Mamillary bodies = (recollective) memory
Limbic system = Adapting to environment, emotional brain
Brain lobes:
Frontal lobe = higher cognitive functions, motor behavior
Parietal lobe = plays an important role in somatic sensation/senses (smell, taste, touch)
Temporal lobe = plays an important role in emotions and memory, smelling, tasting, perception, aggression
and sexual behavior
Occipital lobe = controls sight and recognition
Insula = adds emotional context to sensory input
Brain areas:
Premotor cortex, primary motor cortex = motor
Sensory cortex = sensory
Visual cortex in the occipital lobe = seeing
Taste center in the post central sulcus of the parietal cortex = tasting
Prefrontal cortex =
important for intellect,
complex learning, mood and
personality (cognition)
Limbic system = emotions
Broca’s area = formulate
speech, motor instruction to
motor cortex
Primary auditory area =
detect sounds
Auditory association area =
recognize sounds
Wernicke’s area, auditory
association area = recognize
and understand words
Brain stem: pons and
medulla = vital functions
and reflexes (autonomic)
