ch. 3 - development of the nervous system

Number of Chromosomes in Typical Person

23 pairs, 46 individual

22 pairs of autosomes and 1 pair of sex chromosomes (XX women, XY Men)

Mitosis

Division of somatic (body cells)

Mitosis and Meiosis errors

Whole chromosome deletion: Turner's syndrome

Partial Chromosome deletion: Cri du Chat

Chromosome addition: down syndrome

Meiosis

division of sex cells

Germinal stage

the 2-week period of prenatal development that begins at conception

- cleavage: zygote begins to divide to form a morula (ball of cells)

- Blastocyst: when zygote enters uterus and differentiates into outer and inner cells

Embryonic Stage

weeks 2-7; blastocyst implants into wall of uterus

Beginning layers of embryo:

1. endoderm

2. mesoderm

3. ectoderm

endoderm

most inner layer; forms gut, liver, and lungs

mesoderm

middle layer; form skeleton, muscles, kidneys, blood, and heart

ectoderm

will form skin and nervous system

Fetal Stage

weeks 9-40

Six Phases of Nueral Development in Order

1. Dorsal Induction

2. Ventral Induction

3. Neural Proliferation

4. Neuronal Migration

5. Cortical Organization and Synapse formation

6. Myelination

Dorsal Induction

GA: 3-7 weeks

3rd week: neural plate is formed

4th week: neural plate wraps around to form neural tube (neurulation)

6th week: ends of nueral tube close

brain and spinal cord develop out of neural tube

Neural Tube Defects

Involves failure of neural tube ends to close

- encephalocele

- anencephaly

- spina bifida

Encephalocele

when part of the brain protrudes from the skull

anterior neuropore does not close

Anencephaly

Brain development ceases at brainstem; child without cerebral hemispheres

Anterior neuropore does not close

Spina Bifida

Cyst on back that may or may not involve spinal cord; posterior neuropore does not close

3 severity types:

1. Most severe - myelomeningocele

2. medium severe - meningocele

3. Least severe - spina bifida occulta

myelomeningocele

cyst involves spinal cord

• most severe form of spina bifida

meningocele

cyst but spinal cord not inside

• mid-severe form of spina bifida

spina bifida occulta

vertebrae are malformed

• least severe form of spina bifida

Ventral Induction

(GA:2-3 months) Neurodevelopmental period when face and brain develop out of superior end of the neural tube.

Nueral tube closes and bends ventrally, and the superior end forms the major components of the brain

developmental divisions of the nervous system structures

Prosencephalon (forebrain): telencephalon and diencephalon

• Telencephalon → cerebral hemispheres

• Diencephalon** → thalamus, hypothalamus, epithalamus, subthalamus

**all are subcortical structures

Mesencephalon → midbrain

Rhombencephalon (hindbrain): metencephalon and myelencephalon

• Metencephalon → pons and cerebellum

• Myelencephalon → medulla

Ventral Induction Defects

Holoprosencephaly: failure of brain cleavage (dividing)

3 forms of Holoprosencephaly:

1. Lobar form

2. Semilobar

3. Alobar

lobar form of holoprosencephaly

brain looks normal, but abnormal connections between hemispheres

• least severe form of holoprosencephaly

semilobar form of holoprosencephaly

some cleavage and development of right and left hemispheres, but incomplete; no corpus callosum

• mid-severe form of holoprosencephaly

alobar form of holoprosencephaly

no cleavage at all resulting in no distinct cerebral hemispheres; no corpus callous

• most severe form of holoprosencephaly

Neural Proliferation

GA 3-4 months

Neurogenesis: The birth of new neurons

Neurons born out of spinal cord and brainstem

Neural Proliferation Defects

Microcephaly

Abnormally small brain and head

Neural Migration

GA: 3-5 months

New neurons move in wavelike fashion to their correct position in the cerebral hemispheres

A chemical, reeling, signals neurons where to stop

At about 20 weeks GA, the process stops and 6 layers of the cerebral cortex are established

Neuronal Migration Defects

Schizencephaly: clefts in the cerebral hemispheres

Lissencephaly: smooth brain that lacks characteristic sulci (valleys) and gyri (hills)

Both conditions due to lack of reelin leading to a failure of neurons to migrate to proper places

Cortical Organizaiton and Synapse Formation

GA: 5 months to years

Once neurons arrive at intended spot, they sprout dendrites and axons

Synapses begin to form between neurons (synaptogensis)

- synaptic pruning eliminates unneeded connections

Failure in this stage can lead to polymicrogyria: too many gyri in the cerebral hemispheres (epilepsy, developmental delay, intellectual disability, speech and swallowing issues)

- synapses not being pruned

Myelination

GA: birth to years after birth

Neuron axons are coated with myelin at this stage

- peaks at about 1 year after birth

Failure: hypomyelination: reduced ability to form myelin resulting in intellectual disability

nuerogenesis

birth of new nuerons