BMED 2801 Lecture 16 – Histology of Nervous Tissue
Structurally, nerve tissue consists of two cell types:
1. Nerve cells, or neurons, which usually show numerous long processes - are responsible for the
reception, transmission, and processing of stimuli; the triggering of certain cell activities; and the
release of neurotransmitters and other informational molecules.
2. And several types of glial cells, which have short processes, support and protect neurons, and
participate in neural activity, neural nutrition, and the defence processes of the central nervous
system.
Parts of a Neuron
Dendrites:
- Dendrites are receptor processes which receive stimuli from other neurons or external
environment
- Dendrites can be multiple, can branch e.g. in multipolar neurons
- Appear as darker stained processes
- Has a greater diameter than axons
- Unmyelinated & tapered
- Can be smooth, or studded with small, mushroom-shaped appendages = dendritic spikes.
- They are connected to the base of the Nissl body however to distinguish between an axon
(especially when unmyelinated), the axon is not connected to the Nissl body.
Axons:
-
Are extended process projecting out of a cell body - transmit outgoing electrical signals from the
integration center of the neuron to the end of the axon (axon terminal)
Appear as clearer projections
Form bulb shaped swellings at the ends = terminal axon
Nissl granules are absent
Nerve Cell body (aka Soma, Perikaryon)
- The integration centre of the neuron
- Has an unusually large, euchromatic nucleus with a well-developed nucleolus.
- Contains Nissl bodies, which are also found in large dendrites.
Chemical and Electrical Synapses
- The synapse is responsible for the unidirectional transmission of nerve impulses.
- Synapses are sites of functional contact between neurons or between neurons and other effector
cells (e.g., muscle and gland cells).
- The function of the synapse is to convert an electrical signal (impulse) from the presynaptic cell
into a chemical signal that acts on the postsynaptic cell.
- Most synapses transmit information by releasing neurotransmitters during the signalling
process.
Electrical synapses.
- A synapse that transmits ionic signals through gap junctions (connexin channels) that cross the
pre- and postsynaptic membranes - the ion movement allows for the spread of electrical signals.
- It does not a require a neurotransmitter
- With a lumen diameter of about 1.2 to 2.0 nm, the pore of a gap junction channel is wide enough
to allow ions and even medium sized molecules like signalling molecules to flow from one cell to
the next thereby connecting the two cells' cytoplasm.
- Thus when the voltage of one cell changes, ions may move through from one cell to the next,
carrying positive charge with them and depolarizing the postsynaptic cell.
- Electrical synapses are only used for applications where a reflex must be extremely fast. They
are simple and allow for synchronized action. A benefit of electrical synapses is they will transmit
signals in both directions.
Chemical synapses.
-
The space between a chemical synapse is much larger than an electrical synapse.
-
The release of a neurotransmitter is triggered by the arrival of a nerve impulse (or action
potential) and occurs through an unusually rapid process of cellular secretion, also known as
exocytosis: Within the pre-synaptic nerve terminal, vesicles containing neurotransmitter sit
"docked" and ready at the synaptic membrane.
The arriving action potential produces an influx of calcium ions through voltage-dependent,
calcium-selective ion channels.
Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the
presynaptic-membrane and releasing their contents to the synaptic cleft.
-
-
Chemical synapses are much slower to react to stimuli. However chemical synapses transmit a
signal with constant strength or even a signal that get stronger. This is called "gain." Electrical
synapses are faster but have no "gain," the signal gets weaker as it travels along the synapse to
other neurons.
They are more complex and can vary their signal strength- their functions are influenced by
chemical outputs in the nervous system.
The Central Nervous System (CNS) Vs the Peripheral Nervous System (PNS)
Anatomically, the nervous system is divided into the
- Central nervous system, consisting of the brain (cerebrum and cerebellum) and the spinal cord.
- Peripheral nervous system composed of nerve fibres/bundles and small aggregates of nerve
cells called nerve ganglia.
Peripheral Nervous System
- Neuronal tissue outside the brain and spinal cord – i.e. neuronal tissue projecting off the spinal
cord = spinal cord nerves
- It consists of a feltwork of nerve fibres = plexuses and bundles of nerve fibres forming nerve and
nerve roots.
- Connective tissue is a main component of the peripheral nerve structure e.g. the endoneurium,
perineurium and epineurium are connective tissue structures that provide structural support.
- 2 specialised supporting cells =
o Satellite cells- located around the ganglion cells of sensory and autonomic ganglia.
o Schwann cells – associated with peripheral nerve axons- form the myelin surrounding PN
axons.
The Central Nervous System
- Neuronal tissue within the brain and spinal cord
- Does not contain connective tissue other than that in the meninges – hence collagenous fibers
and fibrocytes will not be observed.
Contains several types of non-neuronal supporting cells = Neuroglia
4 different types of Neuroglia
1. Astrocytes – fibrous astrocytes and protoplasmic astrocytes – provide physical support and
forms part of the blood brain barrier
2. Oligodendrocytes – form the myelin in the CNS
3. Microglia – phagocytic cells
4. Ependymal cells –line the ventricles of the brain and the central canal of the spinal cord –
theses cells absorb cerebrospinal fluid.
The Central Nervous System
- Consists of
Grey matter
Cell bodies
Dendrites and axons
Neuroglial cells
= Site of synaptic communication
White matter
Axons
Neuroglial cells
Blood vessels
= Site of transmission
Support cells of the CNS – Neuroglial cells
The are 4 different types:
ASTROCYTES
2 subclasses
Protoplasmic Astrocytes
• Mostly found in grey matter
• Short, branched processes
Fibrous Astrocytes
• Mostly found in white matter
• Long, unbranched processes
• expanded ends of processes
• cover vessels and neurons
• Blood-Brain Barrier
OLIGODENDROCYTES
• Most common
• produce myelin - have processes that wrap around axons, producing a myelin sheath.
• Small
• Few processes
MICROGLIA
• Phagocytic cells
• Small
EPENDYMA
• Ciliated, simple columnar cells
• Microvilli
• Line spinal canal and ventricles
• form the choroid plexus
• produce cerebrospinal fluid
(CSF)
Connective tissue of the CNS
MENINGES = brain and spinal cord covered by connective tissue membranes
1. DURA MATER
• Dense connective tissue continuous with the periosteum of the skull.
• Is covered by simple squamous epithelium.
• attaches to skull bone
• Spinal nerves pass through it
• The dura mater is separated from the periosteum of the vertebrae by the epidural space, which contains
thin-walled veins, loose connective tissue, and adipose tissue.
• The dura mater is always separated from the arachnoid by the thin subdural space.
2. ARACHNOID
• Delicate connective tissue devoid of blood vessels
• Covered with simple squamous epithelium
• sends trabelculae into pia mater
• The arachnoid has two components: a layer in contact with the dura mater and a system of trabeculae
connecting the layer with the pia mater.
• The cavities between the trabeculae form the subarachnoid space, which is filled with cerebrospinal
fluid
3. PIA MATER
• Delicate/loose connective tissue containing many blood vessels
Squamous cells of mesenchymal origin cover pia mater.
• attached to brain and spinal cord surface
Between the pia mater and the neural elements is a thin layer of neuroglial processes, adhering firmly to
the pia mater and forming a physical barrier at the periphery of the central nervous system. - This barrier
separates the central nervous system from the cerebrospinal fluid
ARACHNOID SPACE
• Between arachnoid and pia mater
• contains cerebrospinal fluid and blood vessels
Astrocytes form a three-dimensional net
around the neurons (not shown). Note that the
footlike processes of the astrocytes form a
continuous layer that involves the blood
vessels that contribute to the blood–brain
barrier.
SPINAL CORD
In cross sections of the spinal cord, white matter is peripheral and gray matter is central, assuming the
shape of an H. In the horizontal bar of this H is an opening, the central canal, which is a remnant of the
lumen of the embryonic neural tube. It is lined with ependymal cells. The gray matter of the legs of the H
forms the anterior horns. These contain motor neurons whose axons make up the ventral roots of the
spinal nerves. Gray matter also forms the posterior horns (the arms of the H), which receive sensory
fibres from neurons in the spinal ganglia (dorsal roots).
Features to identify the grey matter:
 White matter: blue stained, no cell bodies, all fibres, blood vessels associated with ventral fissure, but
no dorsal sulcus
 Grey matter: parts of dorsal horn and ventral horn, a lot of cell bodies within it and surrounds central
canal
GREY MATTER
• In the core
• appears as the profile of an H
WHITE MATTER
• Outer layer
• Axon bundles = TRACTS
TRACTS
- Can be visualized with specific stains
Dorsal medial
sulcus
CEREBRUM and CEREBELLUM
GREY MATTER
• Outer region = cortex
• Islands = nuclei
• Variety of cell body types
Neuropil
= meshwork of axonal, dendritic and neuroglial processes
WHITE MATTER
• Inner core
CEREBELLUM
Histological characteristics of GREY MATTER
(1) MOLECULAR LAYER
• Outermost layer of cortex
• Basket cells
• Few cell bodies
• Stains pale pink (Eosin)
(2) PURKINJE CELL LAYER
• Large cells
• Flask shaped
• Dendrites extend into molecular layer
• Axons extend into granular layer
(3) GRANULAR LAYER
Granule Cells
• Small nuclei
• send axons to molecular layer
Golgi type II Cells
• Larger nuclei
Stain blue (Haematoxylin)
Histological characteristics of WHITE MATTER
• Internal layer
• Axons, neuroglial cells, blood vessels
• Stain black (Silver)
Photomicrograph of the cerebellum. The staining
procedure used (H&E) does not reveal the
unusually large dendritic arborzization of the
Purkinje cell. Low magnification.
The cerebellar cortex has three layers:
-
an outer molecular layer
a central layer of large Purkinje cells
And an inner granule layer.
The Purkinje cells have a conspicuous cell body
and their dendrites are highly developed, assuming
the aspect of a fan (Figure 9–3). These dendrites
occupy most of the molecular layer and are the
reason for the sparseness of nuclei. The granule
layer is formed by very small neurons (the smallest
in the body), which are compactly disposed, in
contrast to the less cell-dense molecular layer.
 Axons synapse on purkinje cells (big nerve
cells)
 Purkinje cells extend their fibres into the
molecular layer
 Granular layer synapse with purkinje cells and
myelinate the fibres found in the white matter
Myelinated Vs Unmyelinated Axons
-
-
the axon is lighter/clear compared to the Schwann cell cytoplasm
Myelination begins when a schwann cell surround the axon and its cell membrane becomes
polarised
The thickness of myelin sheaths are determined by the axonal diameter – some axons are very
large, large means further travel of action potentials = greater demand for insulation = greater
demand for myelination.
unmyelinated nerves are still enveloped by the Schwann cell cytoplasm
The Peripheral Nervous System
Individual nerve fibres (axons) are associated with Schwann cells and myelin sheath, if present. The
individual fibres are separated from one another by a delicate connective tissue layer called the
endoneurium.
Groups of fibres are surrounded by a specialised sheath consisting perineural cells and collagen and this
layer is called the Perineurium. The Perineurium creates bundles of fibres which are referred to as
fascicle.
The fascicle configuration creates space between the fascicles and these spaces are filled wtih
epineurium connective tissue. It is continuous with the connective tissue of the fascia. The smallest
peripheral nerves may be made up of only one fascicle therefore they will lack Epineurium.
The nuclei of Schwann cells are curved around the axons. The fibroblasts of the endoneurium are
elongated and straight.