Overview of viruses that specifically target the nervous system, including Herpesviruses, Rabies virus, Enteroviruses, and Flaviviruses (such as Zika and West Nile).
Viruses of the nervous system
27 December 2020 18:31
Nervous system
There are two parts of the nervous system:
The Central Nervous System (CNS)
• Brain
• Spinal cord
The Peripheral Nervous System (PNS)
• Ganglia attached to the spinal cord
• Nerves
In the peripheral nervous system there are two categories of Neurons:
• Sensory
• Motor neurons
Structure of neurons
The sensory neuron has receptors at the end of it which will detect and send a
signal up through the sensory neurons to the spinal cord which sends a signal
through the motor neuron into the effector muscle. Sensory neuron connects to
interneurons which are present in the spinal cord and connect to the cell body of
the motor neuron which sends the signal to the axon terminal present in the
muscle.
Neurons in our body can be up to one metre in length. So any virus that gain access
to a neuron would take some time to move up through the neuron and to get
further up into the nervous system.
Virus infection of the CNS
Aseptic meningitis - inflammation of meninges around brain and spinal cord
Encephalitis - inflammation of the brain
Meningoencephalitis - inflammation of the brain and meninges
Myelitis - inflammation of the spinal cord
Symptoms of infection of the CNS:
Meningitis: HSV2, Mumps, HIV
• Severe headache
• Stiff neck
• Fever
• Nausea and vomiting
• Feeling confused
• Photophobia
Encephalitis: HSV1, measles
• Confusion
• Hallucination
• Seizures
• Paralysis of body or face
• Muscle weakness
• Loss of consciousness and eventually death if not treated
Myelitis: Polio
• Long-term paralysis
Many viruses exhibit tropism for the nervous system - neurotropism
How do viruses enter the CNS?
(i) Spread to the CNS via the PNS
Viruses of the nervous system Page 1
, (i) Spread to the CNS via the PNS
The peripheral nervous system innervates the extremities through innervating
epithelial cells, skin, muscles etc. Viruses have the capacity to get through any of
these routes.
In this situation the virus will infect the cells at the periphery and transfers across
into the axon termini of the neurons. The virus then has to undergo retrograde viral
transport (retrograde = in the direction of cell body), through this route the virus
spreads to the PNS from the periphery.
Some viruses can stop at the cell bodies within the peripheral nervous system.
However, some others will undergo anterograde viral transport back to the
periphery and cause subsequent disease or it can undergo anterograde viral
transport into the CNS and cause damage to the brain and spinal cord.
(ii) Spread to the CNS via the olfactory bulb
Viruses can enter through our nose by entering the mucus layer and through the
cilia of the olfactory cells. It can gain access to the olfactory nerve and then into the
olfactory bulb located in the brain, which eventually gives access to the CNS.
Viruses that enter through the PNS
There are three types of ways in which the virus can gain access to the PNS:
Sensory, Motor, Olfactory nervous systems
Sensory Neurons:
Sensory neurons innervates for example skin cells, epithelial cells etc, viruses that
infect through this route would be for example:
• Herpes virus HSV
• VZV
• PRV
Viruses that infect our skin can potentially infect the nervous system as well.
Motor Neurons:
Viruses that enter motor neurons enter from the muscles because axons of the
motor neuron innervates the muscle. Viruses will crossover into the axons and
undergo retro viral transport to the spinal cord.
For example:
• Rabies virus (RABV)
• Poliovirus
Olfactory epithelium:
Entry through this event is rare but virus can enter through this route. There are
large number of viruses which can gain access to our CNS through our noses.
For example:
• HSV
• VSV
• BDV
• InfA
• HeV
• CHIKV
• LACV
Crossing the blood brain barrier:
The blood–brain barrier (BBB) is a highly selective semipermeable border of
endothelial cells on the basement membrane that prevents solutes in the
circulating blood from non-selectively crossing into the extracellular fluid of the
central nervous system where neurons reside.
Viruses of the nervous system Page 2
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