Introduction to the levels of the Nervous System

Central nervous sytem (CNS)

\-composed of the brain and spinal cord

\-composed of nuclei and tracts

\-includes cerebrum, cerebellum, brainstem and spinal cord

Peripheral Nervous System (PNS)

\-lies outside CNS

\-composed of Ganglia and Nerves

\-can be divided into Autonomic Nervous system and Somatic nervous system

Nuclei

groups of cell bodies

Tracts

Fibers (thread like)

Ganglia

groups of cell bodies outside CNS

Nerves

fibers outside the CNS that are in the PNS

\-spinal and cranial nerves

Autonomic Nervous system

regulation of internal body function

\-self regulating system; not under our control

Somatic Nervous system

enables us to perceive sensory stimuli and carry on volitional motor activity

Basic subdivisions of the brain

\-cerebrum

\---cerebral hemisphere

\---Diencephalon

\---cerebellum

\---brainstem

The derms

\-Ectoderm (outer layer)

\-Mesoderm (middle layer)

\-Endoderm (inner layer)

Ectoderm

develops into brain and skin

Mesoderm

Becomes all the connective tissues in the body, muscles, bones, tendons, etc.

Endoderm

becomes all the inner tubes, hearts, lung, digestive tracts, etc.

Neural Ectoderm

The neural plate (flat plate folds in to create fuse)

Primary Neurulation

-Neural plate formed at week 3 of embryonic development

-at the end of week 3, the neural folds begin to fuse, forming the neural tube

-as the neural tube closes, it separates from the ectodermal surface

-during fusion, neural crest cells separate from the neural tube to form the PNS

-the cavity in the neural tube becomes the ventricular system of the brain

Sulcus limitans

Divides into the lateral wall of the neural tube into 2 halves

-Alar plate

-Basal plate

Dorsal

= back

-half forms a alar plate

-sensory neurons

Ventral

= front

-basal plate

-motor neurons

Three primary vesicles (bulges) during the fourth week

Prosenecephalon

Mesencephalon

Rhombencephalon

Two flexures that divides the vesicles

Cephalic flexure

Cervical flexure

Prosencephalon

forebrain→ cerebrum

Mesencephalon

Midbrain → mainbrain

Rhombencephalon

Hindbrain → cerebellum

Cephalic flexures

between prosencephalon & mesencephalon

Cervical flexures

Between rhombencephalon & spinal cord

five secondary vesicles during the fifth week

Telencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

Telencephalon

cerebral hemispheres

Diencephalon

thalamus, hypothalamus

Metencephalon

Pons, cerebellum

Myelencephalon

Medulla

Pontine Flexure

divides the rhombencephalon into two parts

Formation of the fourth ventricle

the walls of the neural tube spread apart (flatten out)

-the alar and basal plate come to lie on the floor of the fourth ventricle

insula

the telencephalic tissue overlying the fusion between telencephalon & diencephalon

-becomes hidden from the view later in the development

rhombic lips

-thickened lateral parts of the alar plate in the brainstem

-becomes the cerebellum

Ventricular system

the cavity of the neural tube becomes the ventricular system/ 4 ventricles and canal

-2 lateral ventricles

-3rd ventricle

-4th ventricle

-central canal

2 lateral ventricles

in telencephalon

(=one in each hemisphere)

third ventricle

-in diencephalon

fourth ventricle

in metencephalon & myelencephalon (=in pons and medulla)

central canal

in the spinal cord (in the middle of the cray matter)

Craniorachischisis

complete failure of the neural tube to close

-The CNS appears as an open furrow on the back of the head and body

Spinal Bifida

failure of the caudal neuropore to close

Myelomeningocele

a severe form of spinal bifida, in which the spinal cord and/or nerve roots protrude through the posterior defects

-often accompanied by an Arnold-Chiari malformation

Arnold-Chiari malformation

the cerebellum and caudal brainstem are elongated and pushed down into the foramen magnum

often obstruction to the flow of CSF results in hydrocephalus

Anencephaly

failure of the rostral neuropore to close

-much of each cerebral hemisphere is absent

Holoprosencephaly

partial or complete failure of the prosencephalon to separate into the diencephalon and the paired telencephalic vesicles

-the prosencephalon can develop abnormally even if neural tube closure is complete

Hydrocephalus

Characterized by enlarged ventricles of the brain

can result from:

-increased production of cerebrospinal fluid

-decreased absorption of CSF

-blockage of CSF flow

Shunt procedure may help reverse the symptoms