Download Basic Organization of the Nervous system

Basic Organization of the
Nervous System
Dr. G.R. Leichnetz
The central nervous system
(CNS) consists of the:
brain and spinal cord
The peripheral nervous
system (PNS) consists of:
12 pairs of cranial nerves,
31 pairs of spinal nerves,
nerve plexuses, ganglia,
and emergent peripheral
nerves
Neural tissue of the CNS and PNS contains two general
categories of cells: neurons and supportive cells.
The central nervous system (CNS) consists of about
10% neurons and 90% neuroglia (supportive cells).
The CNS contains approximately 10 billion neurons.
While the PNS contains all three morphological types of
neurons (unipolar, bipolar, multipolar), the CNS only
contains multipolar neurons. These multipolar neurons,
however, are of a wide variety of size and shape.
The neuroglia (supportive cells) of the CNS are
astrocytes, oligodendrocytes, and microglia.
The supportive cells of the PNS are Schwann cells and
satellite cells.
Central Nervous System vs. Peripheral Nervous System
Gray Matter vs.
White Matter
Typical Neuron
Gray matter contains
cell bodies of neurons.
CNS gray matter is
organized in two ways:
nucleus (cluster) or
cortex (layers).
Cell Body
Note: A ganglion is a cluster of
neurons in the PNS.
Myelinated axon
White matter contains
myelinated axons.
Both gray matter and
white matter also
contain neuroglia
(astrocytes, oligodendrocytes).
From: Noback & Demarest, The Human Nervous System
Cytoarchitecture of the Brain
Nuclei contain
the cell bodies
of neurons
Nissl-stained (cresyl-violet) cross section of medulla
(see “nuclei,” containing cell bodies, stained darker purple)
Myeloarchitecture of the Brain
Tracts contain
myelinated axons
of neurons
Compare to C.V.stained section
Myelin-stained section of medulla
(see tracts, containing myelinated axons, stained darker black; nuclei are pale)
A “nucleus” is a
cluster of cell bodies
of neurons in the
CNS.
Myelin-stained section
through the midbrain,
showing the
trochlear nucleus
(pale round structure just
above small dark tract)
Trochlear nucleus
Darkfield photomicrograph
showing HRP-containing cell
bodies of neurons
The neuron cell bodies which make up the
trochlear nucleus can be seen by injecting
HRP into its target muscle (superior oblique
muscle of the eye). The enzyme is retrogradely
transported back to parent cell body.
The trochlear nucleus seen here contains the
cell bodies of many multipolar motor
neurons.
A “cortex” is laminated gray matter.
The surface of the cerebrum is covered
with gray matter, the cerebral cortex,
which contains layers of neuronal cell
bodies. The cerebrum and cerebellum
each have a cortex.
In a coronal section of
the brain, the laminated
gray matter (cerebral
cortex) covers the
surface of the cerebrum
Cortex
Nuclei
(Note: subcortical
nuclei- black arrows).
Cresyl-violet stained section
of the cerebral cortex.
In a “cortex” the neurons
are in layers on the surface
of the cerebrum.
The cerebral cortex has six
cellular layers.
CORTEX
Layer III Pyramidal neurons
Subcortical white matter
Brain Development:
Basic Neuroembryology
The entire CNS develops
from the embryonic
neural tube.
Neural ectoderm on the
dorsal aspect of the
developing embyro gives
rise to neural groove,
neural folds, and then
fuses to form the neural
tube.
The neural crest pinches
off dorsolateral to the
neural tube, and gives
rise to neural structures
in the PNS.
Neural Tube: gives rise
to CNS
Neural Crest: gives rise
to PNS
Both are “neural ectoderm”
and contain primordial cells
which give rise to neurons
and supportive elements
(ie. neuroglia in CNS; Schwann
cells & satellite cells in PNS)
Mesencephalon Rhombencephalon
Prosencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Telencephalon
The rostral end of the neural tube undergoes cephalization
(developing brain) giving rise to three primary brain
vesicles (prosen-, mesen- and rhomben- cephalon); then
five secondary vesicles (telen-, dien-, mesen-, meten-,
myelen- cephalon).
Cowan
The three primary brain vesicles (prosen-, mesen-, and
rhomben-cephalon) differentiate into five secondary brain
vesicles (telen-, dien-, mesen-, meten-, myelen-cephalon).
The terms for the five embryonic secondary brain vesicles
are used in the adult for the five subdivisions of the brain.
Five subdivisions
of adult brain:
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
(Metencephalon)
(Mesencephalon)
(Myelencephalon)
Brain Development:
Differentiation of the
Neural Tube
The mantle layer of the neural tube (presumptine gray matter)
differentiates into neuroblasts (neurons) and glioblasts (neuroglia).
The marginal layer (presumptive white matter) contains processes of
developing neurons. The ependymal layer, which is a germinal layer
in the embryonic neural tube, lines the neurocoel (ventricles).
Mantle layer
Ependymal layer
PNS
The neural tube
has three layers.
Neuroblasts that
give rise to neurons
do not undergo
mitosis
Marginal layer
CNS
From: Noback & Demarest, The Human Nervous System
Glioblasts give rise to
neuroglia which retain
the capacity to divide
throughout life
Derivatives of Neural Tube vs. Neural Crest
Differentiation of Neural Tube Into Spinal Cord
The mantle layer of the neural tube (presumptive gray matter) gives
rise to alar and basal plates, separated by the sulcus limitans.
The alar plate differentiates into sensory nuclei of the CNS (dorsal
horn of the spinal cord gray); the basal plate into motor
nuclei (ventral horn of spinal gray).
Differentiation of Neural Tube Into Brainstem (Medulla)
At the level of the developing medulla, the alar plate of
the neural tube (which gives rise to sensory nuclei) is
displaced laterally, separated from the basal plate (motor
nuclei) by the sulcus limitans.
Further differentiation of the alar and basal plates,
dividing into cell columns that ultimately break up
into sensory and motor nuclei.
Cell columns in
developing medulla
The longitudinal cell columns (derived from alar and basal plates)
break up into sensory and motor nuclei, corresponding to
functional components of cranial nerves
(sensory: GSA, GVA, SSA, SVA; motor: GSE, GVE, SVE).
Dorsal view of the adult brainstem, showing location of sensory
(blue) and motor (red) nuclei, which are derived from sensory
and motor cell columns. The longitudinal cell columns break up
into separate nuclei.
Sensory nuclei
derived from
cell columns
that originated
from alar plate
Motor nuclei
derived from
cell columns
that originated
from basal
plate
Dorsal view of brainstem
with cerebellum removed
The sulcus limitans
remains as a
landmark in the
floor of the fourth
ventricle,
separating motor
(medial) and
sensory (lateral)
nuclei.
Basic LM Neurohistology:
Neurons
Morphological Classification of Neurons
Unipolar- General sensory- found in cranial and
dorsal root ganglia (PNS)
Bipolar-
Special sensory- found in cochlear &
vestibular ganglia, olfactory epithelium,
and retina (PNS)
Multipolar- All CNS neurons are multipolar.
Multipolar neurons in the PNS are found
in autonomic ganglia
Dorsal root ganglion
containing (pseudo-)
unipolar neurons
Dorsal root ganglia and cranial
sensory ganglia contain unipolar
neuron cell bodies.
Origin of
single
process
Unipolar neurons in the
dorsal root ganglion
(derived from neural crest)
are associated with general
sensation (eg. pain, temp.,
touch, and visceral sense).
The small nuclei between
neuronal cell bodies are
satellite cells and
fibroblasts.
Unipolar
neuron
cell body
Bipolar neurons are found in ganglia associated with
special senses (eg. cochlear & vestibular ganglia).
The retina also contains bipolar neurons.
Bipolar neuron
cell body
Central process
Peripheral
process
Multipolar motor neurons of the
ventral horn of the spinal cord
stained with a silver stain.
Neuron
cell body
Origin of axon
(axon hillock)
Neuron
cell body
Dendrites
Origin of axon
(axon hillock)
Motor neurons of the ventral horn of the spinal
cord stained with cresyl violet or H & E
Neuron
cell body
Pyramidal Neurons of the Cerebral Cortex- are multipolar
Cresyl Violet
(Nissl) Stain
Apical
dendrite
Oil 100 X
Silver Stain
Basal
dendrites
H & E Stain
Axon
Oil 100 X
Purkinje cells of the
cerebellar cortex are
multipolar neurons.
Dendritic
arborization
H&E
Primary
dendrite
Silver Stain
Basic LM Neurohistology:
Neuroglia
Neuroglia of the Central Nervous System:
Astrocytes, Oligodendrocytes, Microglia
Astrocyte
Neuron
Astrocyte
Haines
Neuroglia support the function
of neurons in the CNS
Astrocyte
Astrocytes:
Oligodendrocyte
1. Carry nutrients from
capillaries to neurons
2. Maintain optimal
ionic conditions around
neurons (remove excess
neurotransmitter, ions)
3. Neural developmentaxon guidance, enhance
synapse formation
Astrocyte
4. Enhance synaptic
transmission (uptake
calcium, release ATP)
Oligodendrocytes:
myelination of CNS
axons
From: Noback & Demarest, The Human Nervous System
Protoplasmic astrocytes are found in gray matter.
Pyramidal
neuron
Protoplasmic
astrocyte
Fibrous
astrocytes are
found in CNS
white matter.
Astrocytes have long processes that extend to brain
capillaries, ending in perivascular end feet.
Capillary
Astrocyte
Astrocyte
Capillary
Astrocyte
Capillary
Blood-Brain Barrier
Selective permeability
of the capillary
endothelium, excludes
certain substances.
In the strictest sense,
the blood-brain
barrier is formed by
the tight junctions
between capillary
endothelial cells.
However, some people
refer to the bloodbrain barrier more
inclusively as
including the
endothelium, basal
lamina, and astrocytic
end feet.
Myelination of
CNS and PNS
Axons
Oligodendrocytes
CNS axons are
myelinated by
oligodendocytes
CNS
PNS
Schwann cells
Oligodendrocytes can myelinate as many
as 50 internodal segments, whereas
Schwann cell myelinate only 1 internode.
PNS axons are
myelinated by
Schwann cells