(Transcribed from Dr. Glassers lecture, 22 Feb 2000 by Brian Buschman)
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The CNS stems from the neural tube and the PNS from the neural crest cells. The neural tube begins to close from the middle with the cranial neurophore closing first at about day 25 and the caudal neurophore closing about day 27.
The spinal cord forms around a ring of pseudostratified columnar epithelia that proliferate and give three regions:
1) Marginal zone has neuroblasts that form the white matter.
2) Intermediate zone has neuroblasts that form the gray matter.
3) Ventricular zone has neuroblasts that form the supporting cells of the spinal column.
(Remember that microglia come from the blood therefore being of mesodermal origin.)
During development of the neural tube two basal plates and two alar plates are formed. The alar plates are located dorsally and will give rise to sensory neurons. The basal plates are located ventrally and give rise to motor neurons. On each side, the basal and alar plates are separated by a sulcus limitans and they are separated one side to the other by a roof and a floor plate. The roof and floor plates do not have any neuroblasts cells, only represent the path of a few contralateral connections.
Myelination begins around the 4th month of development and continues until about one year after birth. When a nerve is myelinated only then does it begin to function. This is the reason that a baby will not be able to walk until it is at least a year old.
During the development of the spinal column, the spinal cord and vertebral column are the same length about the 8th week of development. The vertebral column and dura mater will grow faster then the spinal cord producing a space known as the lumbar cistern. The lumbar cistern is the space between the end of the conus medularis and then end of the dura matter. Running from the conus medularis to the sacral hiatus is the filum terminalie that is a sleeve of pia to hold the spinal cord in place. The lumbar cistern contains the cauda equina that are the roots of the spinal nerves to exit below the conus medularis.
When you need to analyze CSF, the place to draw it is from the lumbar cistern as it is the only location where CSF can be drawn without damaging important neural structures. The conus medularis is located around L2-L3 in the newborn and L1-L2 in the adult.
Spina bifida is a set of congenital spinal defects that technically result from lack of fusion of vertebral lamina.
1) Spina bifida occulta is a common non-fusion of L5 or S1, occurring in about 10% of the population, who demonstrate no clinical symptoms or neurological signs other then a small dimple with a tuft of hair.
2) Spina bifida cysterna is where a small sac is herniated through the non-fused area.
a) With meningeocyle involves the meninges and CSF.
b) With meninamyelocele involves the meninges, CSF and nervous tissue.
3) Spina bifida with myeloschisis The spinal cord is open and flat due to non-fusion of the posterior (caudal) neurophore. It is thought that this is increased when the pregnant mother has a deficiency of folic acid.
The brain develops from the rostral end of the neural tube and is divided by two flexures that develop due to unequal differentiation of neuroblasts cells. The cephalic flexure separates the prosencephalon from the mesencephalon. The cervical flexure separates the rhombdencephalon from the spinal cord. The primary brain flexures are:
1) Prosencephalon (forebrain)
2) Mesencephalon (midbrain)
3) Rhombdencephalon (hindbrain)
The pontine flexure develops to separate the rhombdencephalon into the metencephalon and mylencephalon. The prosencephalon also differentiates into the telencephalon, which grows laterally, and the diencephalon that stays medially. The most rostral part of the neural tube is the lamina terminalis. The diencephalon will grow under and then grow forward and out, up and around. It will enclose the majority of the other parts of the brain including the telencephalon that will give rise to the connections between the hemispheres of the diencephalon.
The hole from the middle of the neural tube will persist as the central canal in the spinal cord and as the ventricular system in the brain. The ependymal cells of the ventricles in combination with pia covered arteries form the choroid plexus to produce CSF.
Each half of the mylencephalon has two alar nucli and a pyramid.
It contains the fourth ventricle and has various derivatives of the basal and alar plates. The alar plate gives:
1) GVA sensory of viscera
2) SVA taste
3) GSA general sensory of head
4) SSA hearing and balance (VIII)
The basal plate gives:
1) SVE branchial motor fibers
2) GVE hypoglossal
3) GSE IX, X contributions of the gut.
The rostral medulla also gives the inferior olivary nucleus.
The metencephalon gives the pontine nuclei that is a major relay nuclei to the cerebellum. The rhombic lips (rhombdencephalon) form from the alar plate and will grow up to form the cerebellum.
The alar plates give:
1) GVA
2) SVA
3) Somatic afferents
The basal plates give:
1) GVE salivary glands
2) SVE V, VIII motor fibers
3) GSE abducens
The CSF at this level is in the 4th ventricle and exits through three openings.
1) One medial F. of Magendie
2) Two lateral F. of Luschka
(M for medial, L for lateral)
The remnants of the neural tube for the cerebral aqueduct that closes off before it connects to the central canal of the spinal cord (CSF does not enter the spinal cord). The basal plate gives:
1) GSE III, IV
2) GVE III (from the Edinger-Westphal nucleus)
The alar plate gives two colliculi:
1) Superior colliculi mediate the visual reflex.
2) Inferior colliculi mediate the auditory reflex.
The midbrain also gives the red nucleus, the substancia nigra and the cerebral peduncles (fibers of passage through the midbrain).
The diencephalon is made of a roof plate and two alar plates on the sides. The roof plate gives the epithalamus and the rest comes from the alar plates. That includes the thalamus, hypothalamus and subthalamus. The diencephalon gives:
1) The pineal gland that senses light via the optic tract and regulates diurnal cycles.
2) The eyes are a lateral outgrowth of the diencephalon. The optic nerve should really be called the optic tract.
3) The neurohypophysis forms from the diencephalon as the infundibulum. The adenohypophysis forms as Rathkeys pouch from the roof plate of the mouth.
The diencephalon forms the lisencephalic cerebral cortex that is a smooth cerebral cortex precursor. The gyri and sulci begin to form about the 25th week of development. When the diencephalon forms it starts in the middle then grows lateral, forward, down, around and under. It makes a C so many diencephalic structures are C shaped.
The telencephalon gives the lamina terminalis.
Brain anomalies occur in about 1 in 3000. They usually have something to do with rostral neurophore closure and often effect overlying structures like the calveria or meninges.
Anencephaly is when the vault of the skull does not develop and you get a mass of necrotic tissue.
Microencephaly involves a normal face but the brain is exceptionally small.
Defects in closure of the cranium occur in the medial plane of the calveria and often in part of the occipital bone. They are:
1) Cranial memingocele that involves the herniation of meninges and CSF.
2) Meningoencephalocele involves the herniation of meninges, CSF and part of the brain.
3) Meningohydroencephalocele involves the meninges, CSF, part of the brain and ventricular system.
|
Ventricle |
Level |
|
Lateral ventricle |
Cerebral cortex |
|
3rd ventricle |
Diencephalon |
|
Cerebral aqueduct |
Between 3rd and 4th ventricles |
|
4th ventricle |
Met/mylencephalon |
Hydrocephalus is the obstruction of CSF flow resulting in pressure against the brain in one of two forms:
1) Communicating The CSF has an open path to get into the subarachnoid space but is unable to drain. It accumulates in the subarachnoid space and crushes the brain from the outside.
2) Non-communicating The CSF does not make it to the subarachnoid space usually due to closure of the cerebral aqueduct. It is congenital and usually happens before the calveria closes resulting in a big head. It can now be treated in the uterus by inserting a shunt to drain it into the peritoneal cavity.
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