COMP 266 Lecture Notes - Lecture 14: Cranial Nerves, Neural Crest, Fourth Ventricle
29 Apr 2018
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Biology 235 The Brain and Cranial Nerves
Unit 14
Neural tube
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
-The walls of these brain regions develop into nervous tissue, while the hollow interior of the
tube is transformed into its various ventricles
- 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.
Brain developement
The neural tube forms three primary brain vesicles. The primary brain vesicles give rise to five
secondary brain vesicles, which give rise to various adult structures
> primary brain vesicles: prosencephalon, mesencephalon, and rhombencephalon
1. prosencephalon (forebrain)
>The telencephalon (tel′-en-SEF-a-lon; tel- = distant; -encephalon = brain) develops into the
cerebrum and lateral ventricles.
>The diencephalon (d′-en-SEF-a-lon) forms the thalamus, hypothalamus, epithalamus, and third
ventricle.
2. Midbrain
>The mesencephalon (mes′-en-SEF-a-lon (mes- = middle)), or midbrain, gives rise to the
midbrain and aqueduct of the midbrain (cerebral aqueduct).
3. rhombencephalon (hindbrain)
>The metencephalon (met′-en-SEF-a-lon; met- = after) becomes the pons, cerebellum, and upper
part of the fourth ventricle.
>The myelencephalon (m-el-en-SEF-a-lon; myel- = marrow) forms the medulla oblongata and
lower part of the fourth ventricle.
Major Parts of the Brain
four major parts: brain stem, cerebellum, diencephalon, and cerebrum
Sagittal section of the brain (Medial View)
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Brain stem
The portion of the brain immediately superior to the spinal cord, continuous with the spinal cord,
made up of the medulla oblongata, pons, and midbrain.
Cerebellum
The part of the brain lying posterior to the medulla oblongata and pons; governs balance and
coordinates skilled movements.
Diencephalon
Superior to the brain stem, which consists of the thalamus, hypothalamus, and epithalamus
Cerebrum
The two hemispheres of the forebrain (derived from the telencephalon), making up the largest
part of the brain.
cranial meninges
along with the cranium protect the brain
- are continuous with the spinal meninges, have the same basic structure, and bear the same
names
- the outer dura mater (DOO-ra MĀ-ter), the middle arachnoid mater (a-RAK-noyd), and the
inner pia mater
dura mater (outermost layer)
- cranial dura mater has two layers
- 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
- Blood vessels that enter brain tissue pass along 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.
Pia mater (innermost layer)
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
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.
- Astrocytes are one type of neuroglia; the processes of many astrocytes press up against the
capillaries and secrete chemicals that maintain the permeability characteristics of the tight
junctions. A few water-soluble substances, such as glucose, cross the BBB by active transport.
- Other substances, such as creatinine, urea, and most ions, cross the BBB very slowly.
- Still other substances—proteins and most antibiotic drugs—do not pass at all from the blood
into brain tissue. However, lipid-soluble substances, such as oxygen, carbon dioxide, alcohol,
and most anesthetic agents, are able to access brain tissue freely
Cerebrospinal (CSF)
A fluid (clear, colorless liquid composed primarily of water produced by ependymal cells that
cover choroid plexuses in the ventricles of the brain; the fluid circulates in the ventricles
continuously, the central canal, and the subarachnoid space around the brain and spinal cord.
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- contains small amounts of glucose, proteins, lactic acid, urea, cations , and anions (Cl− and
HCO3−); it also contains some white blood cells.
Ventricle
Cavity in the brain filled with cerebrospinal fluid
4 CSF ventricles
> One lateral ventricle in each hemisphere of the cerebrum. (1 and 2.)
- Septum pellucidum - Anteriorly seperates the lateral ventricles by a thin membrane
- Third ventricle is a narrow slitlike cavity along the midline superior to the hypothalamus and
between the right and left halves of the thalamus.
- Fourth ventricle lies between the brain stem and the cerebellum.
Functions of CSF
1. Mechanical protection.
CSF serves as a shock-absorbing medium that protects the delicate tissues of the brain and spinal
cord from jolts that would otherwise cause them to hit the bony walls of the cranial cavity and
vertebral canal. The fluid also buoys the brain so that it "floats" in the cranial cavity.
2. Homeostatic function.
The pH of the CSF affects pulmonary ventilation and cerebral blood flow, which is important in
maintaining homeostatic controls for brain tissue. CSF also serves as a transport system for
polypeptide hormones secreted by hypothalamic neurons that act at remote sites in the brain.
3. Circulation.
CSF is a medium for minor exchange of nutrients and waste products between the blood and
adjacent nervous tissue.
Choroid plexus (PLEK-sus)
A network of capillaries located in the roof of each of the four ventricles of the brain; ependymal
cells around choroid plexuses produce cerebrospinal fluid.
Blood-cerebrospinal fluid barrier
- permits certain substances to enter the CSF but excludes others, protecting the brain and spinal
cord from potentially harmful blood-borne substances
- Selected substances (mostly water) from the blood plasma, which are filtered from the
capillaries, are secreted by the ependymal cells to produce the cerebrospinal fluid.
- This secretory capacity is bidirectional and accounts for continuous production of CSF and
transport of metabolites from the nervous tissue back to the blood.
- Because of the tight junctions between ependymal cells, materials entering CSF from choroid
capillaries cannot leak between these cells; instead, they must pass through the ependymal cells
Circulation of CSF
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), 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,
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Document Summary
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. The walls of these brain regions develop into nervous tissue, while the hollow interior of the tube is transformed into its various ventricles. 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. The neural tube forms three primary brain vesicles. The primary brain vesicles give rise to five secondary brain vesicles, which give rise to various adult structures. > primary brain vesicles: prosencephalon, mesencephalon, and rhombencephalon: prosencephalon (forebrain) >the telencephalon (tel -en-sef-a-lon; tel- = distant; -encephalon = brain) develops into the cerebrum and lateral ventricles.
