HUMAN ANATOMY II LECTURE NOTES 03/14/2026
BRAIN ANATOMY
The brain is the control center for registering sensations, correlating them with one another
and with stored information, making decisions, and taking actions. It also is the center for the
intellect, emotions, behavior, and memory. But the brain encompasses yet a larger domain: It
directs our behavior toward others. With ideas that excite, artistry that dazzles, or rhetoric that
mesmerizes, one person’s thoughts and actions may influence and shape the lives of many
others.
In order to understand the terminology used for the principal parts of the adult brain, it will be
helpful to know how the brain develops. The brain and spinal cord develop from the
ectodermal neural tube. The anterior part of the neural tube expands, along with the
associated neural crest tissue. Constrictions in this expanded tube soon appear, creating three
regions called primary brain vesicles: prosencephalon, mesencephalon, and
rhombencephalon. Both the prosencephalon and rhombencephalon subdivide further, forming
secondary brain vesicles. The prosencephalon, or forebrain, gives rise to the telencephalon
and diencephalon, and the rhombencephalon, or hindbrain, develops into the metencephalon
and myelencephalon. The various brain vesicles give rise to the following adult structures:
• The telencephalon develops into the cerebrum and lateral ventricles.
• The diencephalon forms the thalamus, hypothalamus, epithalamus, and third ventricle.
• The mesencephalon, or midbrain, gives rise to the midbrain and aqueduct of the midbrain
(cerebral aqueduct).
• The metencephalon becomes the pons, cerebellum, and upper part of the fourth ventricle.
• The myelencephalon forms the medulla oblongata and lower part of the fourth ventricle.
The walls of these brain regions develop into nervous tissue, while the hollow interior of the
tube is transformed into its various ventricles (fluid-filled spaces). The expanded neural crest
tissue becomes prominent in head development. Most of the protective structures of the
brain—that is, most of the bones of the skull, associated connective tissues, and meningeal
membranes—arise from this expanded neural crest tissue.
Major Parts of the Brain
The adult brain consists of four major parts: brainstem, cerebellum, diencephalon, and
cerebrum. The brainstem is continuous with the spinal cord and consists of the medulla
oblongata, pons, and midbrain. Posterior to the brainstem is the cerebellum. Superior to the
brainstem is the diencephalon, which consists of the thalamus, hypothalamus, and
epithalamus. Supported on the diencephalon and brainstem is the cerebrum, the largest part
of the brain.
Protective Coverings of the Brain
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, HUMAN ANATOMY II LECTURE NOTES 03/14/2026
The cranium and the cranial meninges surround and protect the brain. The cranial meninges
are continuous with the spinal meninges, have the same basic structure, and bear the same
names: the outer dura mater, the middle arachnoid mater, and the inner pia mater.
However, the cranial dura mater has two layers; the spinal dura mater has only one. The two
dural layers are called the periosteal layer (which is external) and the meningeal layer (which
is internal). The dural layers around the brain are fused together except where they separate to
enclose the dural venous sinuses (endothelial-lined venous channels) that drain venous blood
from the brain and deliver it into the internal jugular veins. Also, there is no epidural space
around the brain. Blood vessels that enter brain tissue pass along the surface of the brain, and
as they penetrate inward they are sheathed by a loose-fitting sleeve of pia mater. Three
extensions of the dura mater separate parts of the brain: (1) The falx cerebri separates the
two hemispheres (sides) of the cerebrum. (2) The falx cerebelli separates the two
hemispheres of the cerebellum. (3) The tentorium cerebelli separates the cerebrum from the
cerebellum.
Brain Blood Flow and the Blood-Brain Barrier
In an adult, the brain represents only 2% of total body weight, but it consumes about 20% of
the oxygen and glucose used by the body, even when you are resting. Neurons synthesize
ATP almost exclusively from glucose via reactions that use oxygen. When the activity of
neurons and neuroglia increases in a particular region of the brain, blood flow to that area
also increases. Even a brief slowing of brain blood flow may cause disorientation or a lack of
consciousness, such as when you stand up too quickly after sitting for a long period of time.
Typically, an interruption in blood flow for 1 or 2 minutes impairs neuronal function, and
total deprivation of oxygen for about 4 minutes causes permanent injury. Because virtually no
glucose is stored in the brain, the supply of glucose also must be continuous. If blood entering
the brain has a low level of glucose, mental confusion, dizziness, convulsions, and loss of
consciousness may occur. People with diabetes must be vigilant about their blood sugar levels
because these levels can drop quickly, leading to diabetic shock, which is characterized by
seizure, coma, and possibly death.
The blood–brain barrier (BBB) consists mainly of tight junctions that seal together the
endothelial cells of brain blood capillaries and a thick basement membrane that surrounds the
capillaries. The processes of many astrocytes press up against the capillaries and secrete
chemicals that maintain the “tightness” of the tight junctions. The BBB allows certain
substances in blood to enter brain tissue and prevents passage to others. Lipid-soluble
substances (including O2, CO2), steroid hormones, alcohol, barbiturates, nicotine, and
caffeine) and water molecules easily cross the BBB by diffusing across the lipid bilayer of
endothelial cell plasma membranes. A few water-soluble substances, such as glucose, quickly
cross the BBB by facilitated transport. Other water-soluble substances, such as most ions, are
transported across the BBB very slowly. Still other substances— proteins and most antibiotic
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, HUMAN ANATOMY II LECTURE NOTES 03/14/2026
drugs—do not pass at all from the blood into brain tissue. Trauma, certain toxins, and
inflammation can cause a breakdown of the BBB.
Cerebrospinal Fluid
Cerebrospinal fluid (CSF) is a clear, colorless liquid composed primarily of water that
protects the brain and spinal cord from chemical and physical injuries. It also carries small
amounts of oxygen, glucose, and other needed chemicals from the blood to neurons and
neuroglia. CSF continuously circulates through cavities in the brain and spinal cord and
around the brain and spinal cord in the subarachnoid space (the space between the arachnoid
mater and pia mater). The total volume of CSF is 80 to 150 mL in an adult. CSF contains
small amounts of glucose, proteins, lactic acid, urea, cations (Na+, K+, Ca2+, Mg2+), and
anions (Cl- and HCO3-); it also contains some white blood cells.
The CSF formed in the choroid plexuses of each lateral ventricle flows into the third ventricle
through two narrow, oval openings, the interventricular foramina. More CSF is added by
the choroid plexus in the roof of the third ventricle. The fluid then flows through the
aqueduct of the midbrain (cerebral aqueduct), which passes through the midbrain, into the
fourth ventricle. The choroid plexus of the fourth ventricle contributes more fluid. CSF enters
the subarachnoid space through three openings in the roof of the fourth ventricle: a single
median aperture and paired lateral apertures, one on each side. CSF then circulates in the
central canal of the spinal cord and in the subarachnoid space around the surface of the brain
and spinal cord.
CSF is gradually reabsorbed into the blood through arachnoid villi, finger-like extensions of
the arachnoid mater that project into the dural venous sinuses, especially the superior
sagittal sinus. (A cluster of arachnoid villi is called an arachnoid granulation.) Normally,
CSF is reabsorbed as rapidly as it is formed by the choroid plexuses, at a rate of about 20
mL/hr (480 mL/day). Because the rates of formation and reabsorption are the same, the
pressure of CSF normally is constant. For the same reason, the volume of CSF remains
constant.
The Brainstem and Reticular Formation
The brainstem is the part of the brain between the spinal cord and the diencephalon. It
consists of three structures: (1) medulla oblongata, (2) pons, and (3) midbrain. Extending
through the brainstem is the reticular formation, a netlike region of interspersed gray and
white matter.
Medulla Oblongata
The medulla oblongata or more simply the medulla, is continuous with the superior part of
the spinal cord; it forms the inferior part of the brainstem. The medulla begins at the foramen
magnum and extends to the inferior border of the pons, a distance of about 3 cm.
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BRAIN ANATOMY
The brain is the control center for registering sensations, correlating them with one another
and with stored information, making decisions, and taking actions. It also is the center for the
intellect, emotions, behavior, and memory. But the brain encompasses yet a larger domain: It
directs our behavior toward others. With ideas that excite, artistry that dazzles, or rhetoric that
mesmerizes, one person’s thoughts and actions may influence and shape the lives of many
others.
In order to understand the terminology used for the principal parts of the adult brain, it will be
helpful to know how the brain develops. The brain and spinal cord develop from the
ectodermal neural tube. The anterior part of the neural tube expands, along with the
associated neural crest tissue. Constrictions in this expanded tube soon appear, creating three
regions called primary brain vesicles: prosencephalon, mesencephalon, and
rhombencephalon. Both the prosencephalon and rhombencephalon subdivide further, forming
secondary brain vesicles. The prosencephalon, or forebrain, gives rise to the telencephalon
and diencephalon, and the rhombencephalon, or hindbrain, develops into the metencephalon
and myelencephalon. The various brain vesicles give rise to the following adult structures:
• The telencephalon develops into the cerebrum and lateral ventricles.
• The diencephalon forms the thalamus, hypothalamus, epithalamus, and third ventricle.
• The mesencephalon, or midbrain, gives rise to the midbrain and aqueduct of the midbrain
(cerebral aqueduct).
• The metencephalon becomes the pons, cerebellum, and upper part of the fourth ventricle.
• The myelencephalon forms the medulla oblongata and lower part of the fourth ventricle.
The walls of these brain regions develop into nervous tissue, while the hollow interior of the
tube is transformed into its various ventricles (fluid-filled spaces). The expanded neural crest
tissue becomes prominent in head development. Most of the protective structures of the
brain—that is, most of the bones of the skull, associated connective tissues, and meningeal
membranes—arise from this expanded neural crest tissue.
Major Parts of the Brain
The adult brain consists of four major parts: brainstem, cerebellum, diencephalon, and
cerebrum. The brainstem is continuous with the spinal cord and consists of the medulla
oblongata, pons, and midbrain. Posterior to the brainstem is the cerebellum. Superior to the
brainstem is the diencephalon, which consists of the thalamus, hypothalamus, and
epithalamus. Supported on the diencephalon and brainstem is the cerebrum, the largest part
of the brain.
Protective Coverings of the Brain
EDWIN NYAKUNDI 1
, HUMAN ANATOMY II LECTURE NOTES 03/14/2026
The cranium and the cranial meninges surround and protect the brain. The cranial meninges
are continuous with the spinal meninges, have the same basic structure, and bear the same
names: the outer dura mater, the middle arachnoid mater, and the inner pia mater.
However, the cranial dura mater has two layers; the spinal dura mater has only one. The two
dural layers are called the periosteal layer (which is external) and the meningeal layer (which
is internal). The dural layers around the brain are fused together except where they separate to
enclose the dural venous sinuses (endothelial-lined venous channels) that drain venous blood
from the brain and deliver it into the internal jugular veins. Also, there is no epidural space
around the brain. Blood vessels that enter brain tissue pass along the surface of the brain, and
as they penetrate inward they are sheathed by a loose-fitting sleeve of pia mater. Three
extensions of the dura mater separate parts of the brain: (1) The falx cerebri separates the
two hemispheres (sides) of the cerebrum. (2) The falx cerebelli separates the two
hemispheres of the cerebellum. (3) The tentorium cerebelli separates the cerebrum from the
cerebellum.
Brain Blood Flow and the Blood-Brain Barrier
In an adult, the brain represents only 2% of total body weight, but it consumes about 20% of
the oxygen and glucose used by the body, even when you are resting. Neurons synthesize
ATP almost exclusively from glucose via reactions that use oxygen. When the activity of
neurons and neuroglia increases in a particular region of the brain, blood flow to that area
also increases. Even a brief slowing of brain blood flow may cause disorientation or a lack of
consciousness, such as when you stand up too quickly after sitting for a long period of time.
Typically, an interruption in blood flow for 1 or 2 minutes impairs neuronal function, and
total deprivation of oxygen for about 4 minutes causes permanent injury. Because virtually no
glucose is stored in the brain, the supply of glucose also must be continuous. If blood entering
the brain has a low level of glucose, mental confusion, dizziness, convulsions, and loss of
consciousness may occur. People with diabetes must be vigilant about their blood sugar levels
because these levels can drop quickly, leading to diabetic shock, which is characterized by
seizure, coma, and possibly death.
The blood–brain barrier (BBB) consists mainly of tight junctions that seal together the
endothelial cells of brain blood capillaries and a thick basement membrane that surrounds the
capillaries. The processes of many astrocytes press up against the capillaries and secrete
chemicals that maintain the “tightness” of the tight junctions. The BBB allows certain
substances in blood to enter brain tissue and prevents passage to others. Lipid-soluble
substances (including O2, CO2), steroid hormones, alcohol, barbiturates, nicotine, and
caffeine) and water molecules easily cross the BBB by diffusing across the lipid bilayer of
endothelial cell plasma membranes. A few water-soluble substances, such as glucose, quickly
cross the BBB by facilitated transport. Other water-soluble substances, such as most ions, are
transported across the BBB very slowly. Still other substances— proteins and most antibiotic
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, HUMAN ANATOMY II LECTURE NOTES 03/14/2026
drugs—do not pass at all from the blood into brain tissue. Trauma, certain toxins, and
inflammation can cause a breakdown of the BBB.
Cerebrospinal Fluid
Cerebrospinal fluid (CSF) is a clear, colorless liquid composed primarily of water that
protects the brain and spinal cord from chemical and physical injuries. It also carries small
amounts of oxygen, glucose, and other needed chemicals from the blood to neurons and
neuroglia. CSF continuously circulates through cavities in the brain and spinal cord and
around the brain and spinal cord in the subarachnoid space (the space between the arachnoid
mater and pia mater). The total volume of CSF is 80 to 150 mL in an adult. CSF contains
small amounts of glucose, proteins, lactic acid, urea, cations (Na+, K+, Ca2+, Mg2+), and
anions (Cl- and HCO3-); it also contains some white blood cells.
The CSF formed in the choroid plexuses of each lateral ventricle flows into the third ventricle
through two narrow, oval openings, the interventricular foramina. More CSF is added by
the choroid plexus in the roof of the third ventricle. The fluid then flows through the
aqueduct of the midbrain (cerebral aqueduct), which passes through the midbrain, into the
fourth ventricle. The choroid plexus of the fourth ventricle contributes more fluid. CSF enters
the subarachnoid space through three openings in the roof of the fourth ventricle: a single
median aperture and paired lateral apertures, one on each side. CSF then circulates in the
central canal of the spinal cord and in the subarachnoid space around the surface of the brain
and spinal cord.
CSF is gradually reabsorbed into the blood through arachnoid villi, finger-like extensions of
the arachnoid mater that project into the dural venous sinuses, especially the superior
sagittal sinus. (A cluster of arachnoid villi is called an arachnoid granulation.) Normally,
CSF is reabsorbed as rapidly as it is formed by the choroid plexuses, at a rate of about 20
mL/hr (480 mL/day). Because the rates of formation and reabsorption are the same, the
pressure of CSF normally is constant. For the same reason, the volume of CSF remains
constant.
The Brainstem and Reticular Formation
The brainstem is the part of the brain between the spinal cord and the diencephalon. It
consists of three structures: (1) medulla oblongata, (2) pons, and (3) midbrain. Extending
through the brainstem is the reticular formation, a netlike region of interspersed gray and
white matter.
Medulla Oblongata
The medulla oblongata or more simply the medulla, is continuous with the superior part of
the spinal cord; it forms the inferior part of the brainstem. The medulla begins at the foramen
magnum and extends to the inferior border of the pons, a distance of about 3 cm.
EDWIN NYAKUNDI 3
