Histology: Nervous System
The nervous system provides rapid and precise communication between different parts of
the body via the action of specialised nerve cells neurons
CNS (central nervous system) = the brain and spinal cord
PNS (peripheral nervous system)
Nerves that run between the CNS and other tissues through nerve ‘relay systems’
termed “ganglia”
The nervous system can be divided into
Somatic nervous system (voluntary)
Autonomic nervous system (involuntary)
Sympathetic and parasympathetic
A neuron is divided into several regions, each with a different function
Cell body (nucleus and organelles responsible for maintaining the cell)
Long cell process (axon) – responsible for transmitting signals from the neuron to the
other cells
Dendrites: numerous short cell processes that increase the surface area available for
connecting with axons of other neurons
Synapses: specialised cell junctions between the neuron’s axon and other neurons to
allow direct cell communication
Perikaryon/cell body:
Nucleus: large, rounded, central nucleolus
RER = abundant rough endoplasmic reticulum (Nissi substance)
Golgi body is well developed
Mitochondria are large in number
There are numerous lysosomes
Cell inclusions are called lipofuscin
Types of neurons
Multipolar neurons – have many dendrite processes
The motor neurons have a large cell body so to provide metabolic support for the
large axon
Bipolar neurons – have 2 main processes of equivalent size arising from each end of the
body 1 dendritic, 1 axonal
They act as receptor neurons for the senses of smell, sight and balance
Pseudo unipolar neuron
A single dendrite and axon arise from a common stem of the cell body
A variation of the bipolar neuron therefore this neuron tends to be sensory neurons
, Communication between nerve cells
Synapse
A transmitter substance is secreted in a highly isolated fashion by one cell and
received uniquely by the other cell
The terminal end of an axon is swollen to form a synaptic bouton:
A small gap of 20nm wide is left (synaptic cleft)
Each side of the synaptic cleft contains special membrane proteins and receptors
involved in neurotransmission
The cell membrane contains mitochondria, microtubules, neurofilaments as well
as membrane-bound vesicles
How transmission occurs
A wave of depolarisation reaches the synaptic bouton the cell membrane opens up
protein channels
This triggers the release of a transmitter substance from the neurosecretory granules by
exocytosis
The transmitter substance then diffuses across the synaptic cleft and is able to interact
with receptors in the postsynaptic membrane of the target neuron
The membrane of the synaptic vesicle is recovered as a coated pot and recycles back
into an endosome compartment in the nerve terminal
Myelin:
Cell membranes wrapped around the axon which increases the speed of conduction
It is produced by specialised support cells (CNS = oligodendrocytes, PNS = Schwann cells)
A Schwann cell myelinates only 1 axon, but an oligodendrocyte may myelinate several
adjacent axons
Nodes of Ranvier
The gaps between the myelin sheaths
Contains most of the Na+ coated channels, creating a spread of the depolarisation
current along the axon. As the current moves along the axon the myelin sheath ensures
the speed of the current by increasing the diameter of the axon
In CNS the nodes of Ranvier are bare whereas in the PNS they are partly covered by
cytoplasm from adjacent Schwann cells
The nervous system provides rapid and precise communication between different parts of
the body via the action of specialised nerve cells neurons
CNS (central nervous system) = the brain and spinal cord
PNS (peripheral nervous system)
Nerves that run between the CNS and other tissues through nerve ‘relay systems’
termed “ganglia”
The nervous system can be divided into
Somatic nervous system (voluntary)
Autonomic nervous system (involuntary)
Sympathetic and parasympathetic
A neuron is divided into several regions, each with a different function
Cell body (nucleus and organelles responsible for maintaining the cell)
Long cell process (axon) – responsible for transmitting signals from the neuron to the
other cells
Dendrites: numerous short cell processes that increase the surface area available for
connecting with axons of other neurons
Synapses: specialised cell junctions between the neuron’s axon and other neurons to
allow direct cell communication
Perikaryon/cell body:
Nucleus: large, rounded, central nucleolus
RER = abundant rough endoplasmic reticulum (Nissi substance)
Golgi body is well developed
Mitochondria are large in number
There are numerous lysosomes
Cell inclusions are called lipofuscin
Types of neurons
Multipolar neurons – have many dendrite processes
The motor neurons have a large cell body so to provide metabolic support for the
large axon
Bipolar neurons – have 2 main processes of equivalent size arising from each end of the
body 1 dendritic, 1 axonal
They act as receptor neurons for the senses of smell, sight and balance
Pseudo unipolar neuron
A single dendrite and axon arise from a common stem of the cell body
A variation of the bipolar neuron therefore this neuron tends to be sensory neurons
, Communication between nerve cells
Synapse
A transmitter substance is secreted in a highly isolated fashion by one cell and
received uniquely by the other cell
The terminal end of an axon is swollen to form a synaptic bouton:
A small gap of 20nm wide is left (synaptic cleft)
Each side of the synaptic cleft contains special membrane proteins and receptors
involved in neurotransmission
The cell membrane contains mitochondria, microtubules, neurofilaments as well
as membrane-bound vesicles
How transmission occurs
A wave of depolarisation reaches the synaptic bouton the cell membrane opens up
protein channels
This triggers the release of a transmitter substance from the neurosecretory granules by
exocytosis
The transmitter substance then diffuses across the synaptic cleft and is able to interact
with receptors in the postsynaptic membrane of the target neuron
The membrane of the synaptic vesicle is recovered as a coated pot and recycles back
into an endosome compartment in the nerve terminal
Myelin:
Cell membranes wrapped around the axon which increases the speed of conduction
It is produced by specialised support cells (CNS = oligodendrocytes, PNS = Schwann cells)
A Schwann cell myelinates only 1 axon, but an oligodendrocyte may myelinate several
adjacent axons
Nodes of Ranvier
The gaps between the myelin sheaths
Contains most of the Na+ coated channels, creating a spread of the depolarisation
current along the axon. As the current moves along the axon the myelin sheath ensures
the speed of the current by increasing the diameter of the axon
In CNS the nodes of Ranvier are bare whereas in the PNS they are partly covered by
cytoplasm from adjacent Schwann cells
