Transcript neurohistology

Cellular elements
2 categories
1. Nerve cells/neurons- concerned with
information processing and signalling
2. Glial cells- supporting role
 100 billion neurons; maybe more glial
cells

If we took 1 second to count one
neuron, counting 100 billion neurons
would take more than 3000 years
 All neurons are variations on the same
theme
 Convey information by combined
electrical and chemical signaling
mechanisms

Electrical signals- rapid transmission of
information from one part of neuron to
another
 Chemical messengers carry information
between neurons
 Anatomically specialized areas for
collecting, integrating, conducting and
transmitting information

Components of neuron
Cell body= soma/perikaryon
[karyon=nucleus]
 Supports metabolic and synthetic needs
of the rest of the neuron
Processes of neurons
Dendrites –series of branching tapering
processes
 Receive information from other neurons
via synaptic contacts/synapses

Axon
One long cylindrical process
 Conducts information away from cell
body
 Gives rise to a series of terminal
branches, forming synapses on other
neurons


Neurons are anatomically and
functionally polarized, with electrical
signals travelling in only one direction
under ordinary physiologic
circumstances
Anatomical classification
Depends on number of processes
 Multipolar- vast majority-multiple
dendrites
 Bipolar- 2 dendrites
 Pseudounipolar -
Unipolar neurons are actually
pseudounipolar
 They start out as bipolar, but during
development, cell body expands
asymmetrically, leaving behind a stalk
from which both processes emerge
 Located in dorsal root and cranial nerve
ganglia

Formation of a
pseudounipolar neuron
Motor neurons and interneurons are
multipolar
 Bipolar neurons are located in retina and
CN VIII ganglia

Functional classification
Sensory neurons

Directly sensitive to various stimuli [e.g.
touch or temperature] or receive direct
connections from non-neuronal receptor
cells
Sensory neurons
Their processes are included in somatic
and visceral afferents
 Somatic afferents convey pain,
temperature, touch, pressure,
proprioception

Visceral afferents convey pain and other
sensations from mucous membrane,
glands and blood vessels
 Most sensory and motor neurons live
partly in CNS and partly in PNS


The words sensory and motor are often
used in a broader sense to refer to cells
and axons carrying information related
to sensory stimuli and the responses
generated
Motor neurons
Convey impulses from CNS/ganglia to
effector cells
 Their processes are included in efferent
nerve fibres
 Somatic efferents → skeletal muscles
 Visceral efferents → smooth muscle,
heart, glands

Interneurons
≥ 99% of all neurons
 Form a communicating and integrating
network between sensory and motor
neurons
 Local interneurons have all there
processes confined to a single area od
CNS

Probably not more than 20 million
sensory fibres in all of spinal cord and
cranial nerves combined
 No more than few million motor neurons

Projection neurons

Have long axons connecting different
areas, such as a neuron in cerebral
cortex whose axon reaches spinal cord
Strictly speaking, human nervous
system is almost entirely composed of
interneurons and projection neurons
 More than 99% are interneurons or
projection neurons

GRAY MATTER AND WHITE
MATTER
CNS is easily divisible into gray matter
and white matter
 Gray matter- preponderance of cell
bodies and dendrites. In life it is pinkish
gray due to abundant blood supply

White matter
preponderance of axons; many of whom
have myelin sheath
 Myelin sheath is mostly lipid- hence the
white appearance

Nuclei

Specific areas of gray matter in CNS
whose neurons are functionally relatedsimilar areas in PNS are called ganglia
Cortex

An area where gray matter forms a
layered surface covering some part of
CNS
Subdivisions of white
matter[collections of axons]
Variety of names in CNS- fasciculus,
funiculus, lemniscus, peduncle- most
commonly tracts
 Collections of axons in PNS are called
nerves

Fasciculus = ‘little bundle’
 Funiculus = ‘string’
 Lemniscus = ‘ribbon’- tracts flattened out
in cross section
 Peduncle= ‘little foot’- site where tracts
funnel down into a compact bundle

Features of a neuron
Synthesizes
1. neuronal enzymes,
2. structural proteins,
3. membrane components,
4. organelles and
5. some of its chemical messengers
[neurotrnsmitters]

Nucleus; large, pale staining with
dispersed chromatin
 Abundant RER, free ribosomes, stacks
of Golgi apparatus
 Many mitochondris

Nissl bodies/Nissl substanceribosomes, stained intensely with basic
dyes, appear as clumps- prominent in
large neurons
 Many mitochondria


Cytoskeleton composed of microtubules,
neurofilaments [aggregates of these are
called neurofibrils] and microfilaments
Dendrites
Tapered extensions of neuronal body
 Collectively provide a great increase in
surface area available for synaptic
inputs
 In spinal cord, dendritic surface area
may be 30 or more times that of cell
body

Dendrites of many neurons are studded
with small protuberances called
dendritic spines
 These are preferred sites of some
synaptic contacts

Dendrites and dendritic
spines
Axons
Different from dendrites
 Cylindrical process- arises abruptly from
an axon hillock on one side of neuronal
body
 Initial segment has bundles of
microtubules, neurofilaments and
mitochondria- no Nissl substance

It is most electrically excitable part of a
neuron
 Beyond initial segment, many axons are
encased in spiral wrapping of a
membrane called myelin sheathgreatly increases speed of propagation
of electrical impulses

Transport of macromolecules and
organelles synthesized by cell body
occurs away from soma[anterograde]
and towards it [retrograde]
 It can be slow or fast
 Microtubules act as ‘railroad tracks’ for
fast transport

Many peripheral nerves are myelinatedresemble a string of sausages
 Each link of sausage corresponds to a
length of axon wrapped in myelin with
adjacent links separated by a gap in
myelin


At this site axon is separated from
extracellular space only by fingerlike
processes from Schwann cells
Myelin sheath between 2 nodes is called
internode- formed by s single Schwann
cell
 Most of smaller axons in peripheral
nerves are unmyelinated- slow
conductors of electrical signals

Glial cells
Glia =Gr. Glue
 Cells are so named because they fill up
most of the spaces between neuronsappear to hold them in place
 Some do provide structural support
 Play a wide variety of additional roles

Neuroglial cells, collectively known as
the neuroglia or simply as glia, have
important ancillary functions.
 The neuroglial cells of the normal CNS
are astrocytes, oligodendrocytes,
ependymal cells (derived from neural
tube ectoderm), and microglia (derived
from mesoderm)

Astrocytes occur throughout the brain
and spinal cord
 Oligodendrocytes produce myelin and
are also found next to the cell bodies of
some neurons.

Microglial cells become phagocytes
when local injury or inflammation is
present.
 The neuroglial cells of the peripheral
nervous system are Schwann cells in
nerves and satellite cells in ganglia.

Synapse
Neurons, neuropil, and the
common glial cells of the CNS
Neurons, neuropil, and the
common glial cells of the CNS
Supporting cells
Glia/neuroglia in CNS
 Schwann cells and supporting cells in
PNS
 Schwann cells surround neurites,
isolating them from adjacent cells and
extracellular matrix

In PNS ganglia, supporting cells are
Satellite cells- surround nerve cell
bodies [nucleus containing part]analogous to Schwann cells
 In development, glial cells serve as
scaffolding that directs neuronal
migration to appropriate sites

Functions
Physical support for neurites
 Electrical insulation for nerve cell bodies
and processes
 Metabolic exchange pathway between
vascular system and neuronsS

Oligodendrocytes
Processes extend around several axons
in CNS
 Predominant glial cell in white matter

Astrocytes
Form a network of cells within CNS
 Communicate with neurons- modulate
and support their activities
 Fibrous astrocytes- few, long processesfound in white matter
 Protoplasmic astrocytes- many short
branched processes- found in gray
matter

Important role in proper formation of
CNS in fetal and embryonic
development
 Control ionic environment of neurons
 Form scar tissue in CNS damage

Perivascular feet
 contribute to blood- brain barrier
 Regulate vasodilatation
 Regulate transfer of oxygen, ions and
other substances from blood to neurons
Ependymal cells
Low cuboidal/columnar
 Line ventricles of brain and central canal
of spinal cord
 In some places they are ciliated to
facilitate movement of CSF- in others
they have long microvilli

Microglia
5% of all glial cells
 Part of MPS- monocyte precursors
 Proliferate and become actively
phagocytic in regions of injury and
disease
 Remove debris of apoptotic cells during
CNS development

Synapse
Types of synapses
Nerve injury/regeneration

Nerve cells, unlike neuroglial cells,
cannot proliferate but can regenerate
their axons, located in the PNS