The Brain
The brain is the most complex organ in the human body. It controls everything that
we do, say and feel. Without it we would only react without knowing or
experiencing events. The brain enables the mind and allows us to see, hear,
remember, think, feel, speak and dream. It contains a tremendous number of
neurons and neuronal pools. The interconnections are highly complex allowing for
varied responses to changing circumstances. 98% of the brain is comprised of
neural tissue. It weighs about 3lbs. 50% of our genes are needed for the brain.
Embryonic Development
The nervous system develops from ectoderm. By week 3 the neuroectoderm
develops which thickens forming a neural plate. This plate will give rise to all
neurons and glial cells of the nervous system. Soon the neural plate sinks forming a
neural groove which raised neural folds along each side. The neural folds fuse
forming a hollow tube, the neural tube which contains a fluid filled cavity-the
neurocoel. By week 4 this tube shows three primary vesicles: the prosencephalon
or forebrain, the mesencephalon or midbrain and the rhombencephalon or hind
brain. In week 5 the neural tube forms five secondary vesicles. The
prosencephalon divides into the telencephalon and the diencephalon, the
midbrain does not divide and the rhombencephalon develops into the
metencephalon & the myelencephalon. The telencephalon will become the
cerebrum and the lateral ventricles. The diencephalon develops into the thalamus,
hypothalamus, epithalamus and the third ventricle. The metenecephalon becomes
the pons, cerebellum and upper part of the fourth ventricle and the
myelencephalon becomes the medulla oblonga and lower part of the forth
ventricle. The mesencephalon gives rise to the midbrain and the cerebral aqueduct
or aqueduct of the midbrain.
The neurocoel expands forming chambers or ventricles. Each cerebral hemisphere
has a large lateral ventricle. The septum pellucidum separates the 2 lateral
ventricles. The 3rd ventricle is located in the diencephalon. The lateral ventricles
communicate with the 3rd. ventricle via the interventricular foramen or foramen of
Munro. The mesencephalic aqueduct in the mesencephalon connects the 3rd
ventricle with the 4th ventricle which extends into the medulla oblongata. The 4th
ventricle narrows to become continuous with the central canal of the spinal cord.
Ventricles are filled with cerebral spinal fluid (CSF).
Protection & Support
The brain is protected by cranial bones, cranial meninges (dura mater, arachnoid
and pia mater) and CSF. The neurons of the brain are isolated from the general
circulation by a blood-brain barrier.
The outer most meninge is the dura mater. This layer has an outer or endosteal
layer fused to the periosteum of the cranial bones and an inner fibrous layer or
meningeal. These two layers are separated by a gap. Tough, fibrous extensions of
the dura mater called dural folds hold the brain in position. There are three dural
folds. The largest is the falx cerebri found between the cerebral hemispheres in the
longitudinal fissure. The tentorium cerebelli separates the cerebellar hemispheres
from the cerebrum and the falx cerebelli divides the cerebellar hemispheres.
The arachnoid meninge is in contact with the inner layer of the dura mater. A
subarachoid space extends between the arachnoid membrane and the pia mater.
The pia mater is the inner most meninge. It sticks to the brain surface; anchored by
astrocytes. This layer extends into every fold of the brain.
Cerebral spinal fluid surrounds and bathes all exposed surfaces of the CNS. It has
several functions. It cushions the CNS from jolts and shocks, supports the brain
which actually floats in CSF and transports nutrients, chemical messengers and
waste products. Changes that occur in CSF denote changes in the function of the
CNS.
CSF forms in the choroid plexus which contains specialized ependymal cells .
Ependymal cells are interconnected by tight junctions which surround the
capillaries of the choroid plexus. Ependymal cells secrete CSF into the ventricles
and remove waste products from the CSF and adjust its composition. 500ml of
CSF are produced each day. The entire volume is replaced every 8 hours. CSF
circulates from the choroid plexus through the ventricles and central canal of the
spinal cord. It reaches the subarachnoid space through 2 lateral apertures and a
median aperture in the roof of the 4th ventricle. CSF flows through the subarchnoid
place to surround the brain, spinal cord and cauda equine. Extensions of the
arachnoid membrane called arachnoid villi penetrate the dura mater. Clusters of
villi form arachnoid granulations from where CSF is absorbed into the venous
circulation.
Blood-Brain Barrier
Neurons are very active. They have a high demand for energy. They have no
energy reserves and no oxygen. To meet their high demands there is a very
extensive circulatory supply. The brain receives about 15% of the blood and
consumes 20% of the oxygen and glucose. Arterial blood reaches the brain through
the internal carotid and the vertebal arteries. Venous flow leaves via internal
jugular veins. Despite its importance to the brain, blood carries substances that
might be harmful to the brain such as antibodies, macrophages, etc. The bloodbrain barrier (BBB) isolates the CNS from the general circulation. This barrier is
the result of tight junctions between endothelial cells lining capillaries of the CNS.
These junctions prevent diffusion of materials between adjacent cells. Only lipid
soluble compounds can diffuse across membranes into the interstitial fluid of the
brain and spinal cord. Waste products and ions must pass through channels. Larger
water soluble compounds pass by active or passive transport. This restricted
permeability depends on chemicals secreted by astrocytes. The outer surface of
endothelial cells are covered by the processes of astrocytes which secrete
chemicals that control the permeability of the endothelium.
Brain Structure
The internal parts of the brain are rolled up to fit into the skull. The biological
history of the brain is much like an archeological dig; the deeper you gothe older
structures are found. The older parts are more apt to include basic mechanisms for
survival. Newer brain structures are built onto older ones.
There are four major parts: brainstem, cerebellum, diencephalon and the cerebrum.
The brainstem is the oldest, innermost, and deepest part of the brain. It consists of
the medulla oblongata, the pons and the midbrain. It begins where the spinal cord
enters the skull and swells-the medulla oblongata and projects to the
diencephalon. The medulla regulates autonomic functions of the body such as
breathing, heartbeat, blood pressure & digestion or vegetative processes. The
medulla is also the cross over point where nerves come in and cross to the opposite
side-decussation. There are several groups of nuclei in the medulla. The
cardiovascular center adjusts heart rate and blood flow through peripheral tissues.
Respiratory rhythmicity centers set the basic pace for respiratory movements.
Centers for vomiting, deglutition (swallowing), sneezing, coughting and
hiccupping can be found in the brain stem. Sensory or motor nuclei of cranial
nerves, VIII, IX, X, XI, XII provide motor commands to muscles of the neck,
pharynx and back and provide the sensations of taste, hearing and equilibrium.
The pons connects the cerebellum to the brain stem. It has nuclei for somatic and
visceral motor control-cranial nerves V, VI, VII, VIII. It innervates the jaw and
face muscles, eye muscles and organs of vestibular sense. It also contains nuclei
for control of respiration, the apneustic and pneumotaxic centers. These centers
modify activity of respiratory rhythmicity centers of the medulla.
The midbrain or mesencephalon extends from the pons to the diencephalon. It
connects the hindbrain and the forebrain The cerebral aqueduct passes through
here connecting the third and fourth ventricles. In the posterior part is the tectum
tectum or roof which houses 2 pair of sensory nuclei: the superior and inferior
colliculi which comprise the corpora quadrigemina. Superior colliculi receive
visual information and control reflexes related to vision (bright light in eyes).
Inferior colliculi receive auditory sensations and control reflexes in relation to
sound. On each side of the mesencephalon one can find the red nucleus and the
substantia nigra. The red nucleus contains numerous blood vessels and receives
information from the cerebrum and cerebellum. It issues subconscious motor
commands affecting background muscle tone and limb position. The substantia
nigra inhibits activity of the cerebral nuclei by releasing dopamine. Nerve fiber
bundles on the ventrolateral surface of the mesencephalon, the cerebral peduncles
are descending fibers which project to the cerebellum via the pons. The peduncles
carry voluntary motor commands from the primary motor cortex.
Running through the brainstem is the reticular formation, a loosely organized area
of gray and white matter arranged in a net-like fashion. It runs vertically through
all levels of the brainstem from the superior part of the spinal cord through the
brainstem and into the inferior part of the diencephalon. The ascending part is
called the reticular activating system (RAS). It consists of sensory axons that
project to the cerebral cortex and is responsible for consciousness. It is also active
during arousal from sleep and helps maintain attention and alertness. It filters out
insignificant information which is called habituation-a process by which the brain
learns to ignore repetitive stimuli while remaining sensitive to others. When the
RAS is inactivated the result is sleep. Damage to the area results in coma.
Cerebellum
The cerebellum or little brain is the largest part of the hindbrain. It extends from
the rear of the brainstem and consists of right and left cerebellar hemispheres
connected by a vermis. Each hemisphere has slender folds or pleating called folia
which increase the surface area. Each hemisphere consists of an anterior and a
posterior lobe and a flocculonodular lobe. The anterior and posterior lobes govern
subconscious aspects of skeletal muscle movement. The flocculonodular lobe
contributes to equilibrium and balance. In cross section one can see the arbor
viking or tree of life. This is the white matter of the cerebellum. Deeper there are
cerebellar nuclei. Three paired cerebral peducles attach the cerebellum to the brain
stem.
The cerebellum is an automatic processing center responsible for adjusting
ongoing movements by comparing arriving sensations and sensations experienced
previously. The cerebellum adjusts postural muscles and coordinates rapid,
automatic adjustments that maintain balance and equilibrium. That is it is
responsible for the coordination of voluntary movements. It allows you to walk
straight by monitoring proprioceptive, visual, tactile, balance and auditory
sensations. It keeps you from jerking. The cerebellum is also responsible for
programing and fine tuning movements controlled at both the conscious and
subconscious levels. It refines learned movement patterns or learned motor
responses such as driving a car, playing a musical instrument or any activity
performed nearly unconsciously.
The diencephalon forms a central core of brain tissue just superior to the midbrain.
It is almost completely surrounded by the cerebral hemispheres. It extends from the
brain stem to the cerebrum surrounding the third ventricle. It consists of three
major parts: thalamus, hypothalamus and epithalamus. Projecting from the
hypothalamus is the pituitary gland.
The thalamus is an egg shaped structure located at the top of the brainstem.
Thalamus is Greek for couch. Early anatomists thought that it looked as if the
cerebral hemispheres rested comfortably on the thalamus. It comprises 80% of the
diencephalon. The intermediate mass joins the right and left halves of the
thalamus. Axons that connect the thalamus and cerebrum pass through the internal
capsule, a thick band of white tissue. The thalamus serves as a relay station. It
receives information from all the senses, except smell, and routes them to higher
brain regions dealing with seeing, hearing, tasting & touching. The thalamus is also
important in motor control by relaying signals from the cerebellum to the
cerebrum.
The thalamus contains 7 major groups of thalamic nuclei. The anterior nuclei are
part of the limbic system, a primitive brain area effecting motivation and emotion.
Medial nuclei deal with conscious awareness of emotional states and connects
emotional centers in the hypothalamus with the frontal lobes. The ventral nuclei
relay information from basal nuclei of the cerebrum and cerebellum to somatic
motor areas of the cerebral cortex. They relay information regarding touch,
pressure, pain and temperature to sensory areas of the cerebral cortex. There are
two posterior nuclei. The lateral geniculate nucleus receives visual information
from the optic tract and the medial geniculate nucleus relays auditory information
to the cerebral cortex. Lateral nuclei form a feedback loop with the limbic system
and parietal lobes, important in emotional states.
The hypothalamus lies below the thalamus in the floor of the diencephalon. It
contains 12 or so nuclei located in 4 different areas. The mammillary region is
adjacent to the midbain. The most posterior part contains the mammilary bodies
that serve as relay stations for reflexes related to smell. The tuberal region contains
the infundibulum which connects the hypothalamus to the pituitary gland. The
supraoptic region lies superior to the optic chiasm and contains the
suprachiasmatic nucleus. This nucleus is the body’s internal clock by establishing
the circadian rhythm. The preoptic region is found anterior to the supraoptic
region and regulates autonomic activities.
The hypothalamus has maintenance functions. It is a major regulator of
homeostasis. It regulates hunger, thirst, hormone production, body temperature &
sexual behavior. It controls the pituitary gland and the ANS. Hypothalamic centers
are stimulated by sensory information from the cerebrum, brainstem and spinal
cord, by changes in CSF composition and by chemicals circulating in the blood.
The hypothalamus directs somatic motor patterns associated with rage, pleasure,
pain and sexual arousal. It directs facial expressions associated with emotions. The
hypothalamus controls autonomic functions. It adjusts & coordinates centers in the
pons and medulla which regulate heart rate, blood pressure, respiration and
digestive functions. The hypothalamus also functions to coordinate activities of
neurons and endocrine cells in the pituitary by making regulatory hormones. The
hypothalamus is also an endocrine gland because it secretes hormones. ADH or
antidiuretic hormone is produced by supraoptic nucleus. This hormone restricts
water loss at the kidneys. Oxytoxin made by the paraventricular nucleus stimulates
smooth muscle contractions in the uterus. The hypohthalamus produces emotions
and behavior drives such as hunger, thirst, and sexual desire. It coordinates
voluntary and autonomic functions, preparing the body for emergencies. It
regulates body temperature via the preoptic area and it controls circadian rhythms
by the suprachiasmatic nucleus.
The epithalamus or roof is a small region, superior and posterior to the thalamus.
It contains the pineal gland and the habenular nuclei. The pineal gland is a part of
the endocrine system. It secretes melatonin, a hormone involved in day/night
cycles and regulation of reproduction. Melatonin is believed to be responsible for
sleep. The habenular nuclei are involved in olfaction, especially one’s emotional
response to odors.
Cerebrum
The cerebrum is the most visible part of the brain. It is the seat of intelligence
allowing us to read, remember, write, speak, compose music and make plans.
Elaboration of the cerebral cortex releases a species from genetic control and
increases its adaptability. It is the ultimate control and information processing
center. The cerebrum is the largest brain area, comprising 80% of the brain’s
weight. The cerebrum is covered by a neural or cerebral cortex, a superficial layer
of gray matter composed of interconnected neural cells, pyramidal cells, 6 layers
thick (1/8th inch). There are 30 billion nerve cells. There are two main neurons:
stellate and pyramidal cells. Stellate cells receive and process information on a
local level. Pyramidal cells are output neurons of the cerebrum and transmits
signals to other parts of the CNS. The cortex forms elongated ridges, convolutions
or gyri separated by deep depressions, sulci or deeper grooves, fissures. The most
prominent fissure is the longitudinal fissure which separates the cerebrum into a
right and left cerebral hemisphere. Each lobe has specific functions. This is
termed hemisphere specialization.
The hemispheres are connected by a band of white tissue containing axon of
neurons called the corpus callosum. Each hemisphere can be further divided into 4
lobes made distinct by convolutions or gyri. The lobes are named for the bone each
covers. The central sulcus separates the frontal lobe from the parietal lobe. The
lateral sulcus separates the frontal lobe from the temporal lobe. The parietooccipital sulcus separates the parietal lobe from the occipital lobe. Frontal lobes,
behind the forehead are the executive centers. Parietal lobes at the top and to the
rear function in spatial relationships. The pre and post central gyri are located here
which are involved in movement and sensations. Occipital lobes located at the
back of the brain are for sight and temporal lobes found just above ears are
involved in hearing.
White Matter of Cerebrum
White matter consists primarily of myelinated axons. Most of the volume of the
cerebrum is white matter. These axons or fibers form bundles or tracts: projection,
commissural and association. Association fibers interconnect gyri in the same
hemisphere. Shorter fibers are termed arcuate fibers. These fibers curve in an arc
from one gyrus to another. Longer fibers are organized into fasciculi.
Commissural fibers connect gyri in one hemisphere to gyri in the other
hemisphere. Two of the main commissural fibers are the corpus callosum and
anterior commissure. Projection fibers link the cerebral cortex to lower parts of
the CNS such as the diencephalon, brainstem, cerebellum and spinal cord. The
entire collection of projective fibers is named the internal capsule.
Gray Matter
Gray matter is found in three places: cerebral cortex, basal nuclei and limbic
system. Basal nuclei are located deep within each cerebral hemisphere. Two lie
side by side, just lateral to the thalamus. These are the globus pallidus and the
putamen. Together these are termed the lentiform nucleus. The caudate nucleus
has a large head and a slender curved tail. These three together are called the
corpus striatum. They function to monitor activities that occur at the subconscious
level. They help initiate and terminate movements and cognitive functions. Once
motion is underway the basal nuclei provide the basic pattern and rhythm for
movement. They may work with the limbic system for emotional control. There is
a thin sheet of gray matter laying lateral to the putamen called the claustrum which
may be involved in visual attention.
Limbic System
The limbic system is sometimes called the emotional brain. It plays a role in a
range of emotions including pain, pleasure, docility, affection, anger and memory.
consists of a series of structures that encircle the upper part of the brain stem and
the corpus callosum. The limbic lobe is a rim of cerebral cortex on the medial
surface of each hemisphere. It includes the cingulate gyrus and the
parahippocampal gyrus. The hippocampus is part of the parahippocampal gyrus.
It functions in making memories and retrieving memories. The dentate gyrus lies
between the hippocampus and the parahippocampal gyrus. The amydgala is
composed of several groups of neurons located near the tail of the caudate nucleus.
When a certain area is stimulated in a cat, the cat displays anger. When damaged in
humans the person cannot determine angry expressions. Septal nuclei are found in
the septal area and mammillary bodies of the hypothalamus are found close the
midline near the cerebral peduncles. The anterior nucleus and the medial nucleus
of the thalamus also participate in limbic circuits. The fornix is also part of the this
area. All parts are interconnected.
Functional Organization of the Cerebral Cortex
The cerebral cortex covers the hemispheres of the brain. It consists of 6 layers and
is called the neocortex.The cerebral cortex is responsible for higher brain functions
such as: sleep, memory, cognition, emotion, sensation, motor control and
language. Most of these are not the responsibility of just one brain area. The
cortex is arranged according to function. There are sensory areas, motor areas and
association areas. Sensory areas receive sensory information and are involved in
perception, the conscious awareness of sensation. Motor areas control the
execution of voluntary movements. Association areas deal with complex,
integrative functions such as memory, emotions, reasons, personality and
intelligence.
Sensory Areas
Primary sensory areas that receive sensory impulses are found mainly in the
posterior half of both cerebral hemispheres. Sensory association areas are often
found adjacent to the primary sensory areas. These areas integrate sensory
experiences to generate meaningful patterns of recognition and awareness.
The primary somatosensory area is the area of the cortex that receives incoming
information from the skin and from movement of body parts. It is located directly
posterior to the central sulcus on the post central gyrus at the front of the parietal
lobes. Stimulation here produces sensations of being touched on specific parts of
the body. The more sensitive a body part, the more area of the sensory cortex is
allotted to it. Body parts that need precise control such as fingers and mouths had a
greater amount of cortical space allotted to it. This can be seen by viewing a
homunculus. Other sensory functions are decoded in other parts of the brain. The
primary visual area is found in the occipital lobes. Damage here could cause
blindness. The primary auditory area is located in the temporal lobe. Also on the
medial surface of the temporal lobe and inferior surface of the frontal lobe is the
primary olfactory area. The primary gustatory area which receives information
from taste receptors is located at the base of the post central gyrus.
Motor Areas
In 1870, Fritsch & Hilzig stimulated dog cortices and found they could move body
parts by stimulating an area in an arch shaped region at the back of the frontal
lobes running from one ear to the other across the top of the brain. This area is the
primary motor cortex. When an area was stimulated specific muscles contracted
on the opposite side of the stimulation site. The primary motor cortex controls
voluntary movements. The motor homunculus shows that the more precise the
movements a muscle can make the more area in the brain is devoted to it.
In 1865, Broca noticed that damage in the left frontal lobe left patients struggling
to form words but they were able to understand speech. Brocas area is another
motor area devoted to speech. It is located in the frontal lobe and allows us to
speak and to understand language.
Association Areas
Identification of the cortical functions for these sensory and motor areas of the
brain still leaves ¾ of the cerebral cortex with seemingly nothing to do. When
these areas are stimulated there are no observable responses. These areas are not
dormant. We do not use 10% of our brains. These areas represent association
areas of the cortex. Association areas are connected to the sensory and motor
regions of the cortex and are responsible for integrating information. They
associate sensory inputs with stored memories and are important in cognition or
thinking. The Somatosensory association area located just posterior to the
primary somatosensory area receives information from the primary somatosensory
area, the thalamus and other parts of the brain. It lets us determine the exact shape
and texture of an object just by feeling it. It also stores memory and lets us
recognize objects by touching them. Visual association areas found in the occipital
lobe monitor patterns of activity in the visual cortex and interpret the results. It
helps us recognize and made conclusions on what we have seen. The facial
recognition area in the inferior temporal lobe receives information from the visual
association area allowing us to store information about faces and recognize
individuals by their faces. This is primary isolated to the right hemisphere.
Auditory association areas located posterior to the primary auditory area in the
temporal lobe monitor sensory activity in the auditory cortex and help us recognize
music, speech and noise. The orbitofrontal cortex found in the lateral part of the
frontal lobe receives information from the primary olfactory area and allows us to
identify odors. Wernicke’s area or the posterior language area found in the left
temporal and parietal lobes interprets the meaning of speech by recognizing the
spoken word . This center regulates patterns of breathing and vocalizations needed
for speech; damaged a patient makes sounds but not words. A common
integrative area receives information from the somatosensory, visual, auditory
association areas and from the thalamus, parts of the brain stem as well as the
primary gustatory and the primary olfactory areas. This area integrates sensory
interpretation from the association areas allowing one to form thoughts based on
sensory information. The prefrontal cortex or the frontal lobe association area
forms an extensive area in the anterior frontal lobe. This area makes numerous
connections with other areas of the plan. This area is connected with our
personality, intellect and complex learning. Damage here can alter personality and
remove inhibitions. The premotor cortex located immediately anterior to the
primary motor area is important in coordination of learned movements. The frontal
eye field area located in the frontal lobe controls voluntary scanning movements
with the eyes which allows you to read this page.
Hemispheric Lateralization
The right and left cerebral hemispheres are not paired like the eyes and kidneys.
They share certain functions but each is specialized to perform certain functions.
From stroke and accident data we know that the left hemisphere is where most
people process speech. Damage to the left hemisphere causes problems with
reading, writing, speaking, arithmetic and reasoning impairments. The left
hemisphere is also important for reasoning, numerical and scientific skills.
The right hemisphere is specialized for musical an artistic awareness, for spatial
and pattern perception and for recognition of faces and emotional content of
language. It is important for the ability to discriminate between different smells
and allows us to make mental images of sensory stimuli.