Transcript Cell Biology of the Nervous System

Cell Biology of the Nervous
System
• D. Allen, Ph.D.
Types of Cells in the Nervous
System
• Glial
– Supporting cells
• Neurons
• Neural Stem Cells
Neural Stem Cells
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Relatively new discovery
Both developing and adult brains
Precursors to both neurons and glial cells
Role in mature brains unclear
– Hippocampus – neurons that have divided
Glial cells – Supporting cells
• Macroglia
– Astrocytes – CNS
– Oligodendrocytes – CNS
– Schwann Cells – PNS
• Microglia
– Microglia - CNS
CNS - Macroglia
Astrocytes
• Star-shaped
• Functions
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Cell signaling
Scavengers
Blood-brain barrier
Development
Support for neurons
Other functions of astrocytes
• Clean up cellular debris
• Component of blood brain barrier (restrict
substances that enter the brain from the blood)
• Astrocytes produce trophic substances that help
keep neurons alive and promote neuron process
outgrowth
• Radial glia – form a framework for migration of
neurons during development
Myelin-producing cells
• Schwann cells – PNS
• Oligodendrocytes – CNS
• Functions of myelin
– Insulate axons and soma of neurons
– Speed up rate of transmission of action
potentials
• Schwann cells also can be phagocytic
Schwann cells
• Promote regeneration
in the peripheral
nervous system
• Surround unmyelinated
axons, but do not form
a myelin sheath (p 25)
Microglia
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Phagocytic cells
Macrophages that migrate into brain
Normally at rest in brain
Become activated after injury, infection or
disease
• Ingest and destroy bacteria
• Destroy injured or aging neurons
• Associated with some diseases
– Alzheimer’s disease
– AIDS
Neurons
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Dendrites
Soma (cell body)
Axon
Presynaptic terminal (bouton)
Soma
• Contains nucleus
• Site of protein synthesis
• Receives information from other neurons
Dendrites
• Project from soma
– Branchlike extensions
• Specialized for receiving information
• One or many dendrites
Axon
• Specialized for the transmission of
information
• Single axon
• Arises from axon hillock
• Action potentials occur only at the axon
• Axon can branch at their distal end (axon
collaterals)
Presynaptic terminal
• At ends of axon branches
• Specialized for transmission of information to
other cells at the synapse
• Usually use neurotransmitters to transmit the
information
Axoplasmic transport
• The distance from the soma (site of protein
synthesis) to the presynaptic terminal can be
1-2 meters. This is too long for diffusion to
be effective.
• Axoplasmic transport is the mechanism for
the relatively rapid transport of organelles
and molecules to and from the presynaptic
terminal
Axoplasmic transport
• Anterograde transport
• Retrograde transport
• Slow axoplasmic transport (1 mm/day)
Types of Neurons
• Bipolar
– Dendrite root
– Axon
• Pseudounipolar
– Specialized bipolar
neurons
– Both processes are
axons
• Multipolar
– Multiple dendrite
– Single axon
Functions of neurons
• Bipolar
– Transmitting information from one neuron
to another
– Many sensory neurons
• Multipolar
– Receive and integrate
large amounts of
information
– Spinal motor neurons
• 8000 synapses on
dendrites
• 2000 synapses on soma
– Purkinje cells in
cerebellum
• 150,000 synapses on
dendrites
Interactions between Neurons
• Two main overall patterns
– Divergence
– Convergence
Convergence and Divergence
Divergence
• A single axon has several branches that
terminate on many different cells
• Single sensory neuron - proprioception
– Interneurons that mediate reflexes
– Cerebral cortex for consciousness of sensation
– Cerebellum – unconscious proprioception
Convergence
• Multiple inputs from different neurons
terminate on a single neuron
• Neurons may:
– Respond to 2 or more stimuli
• Texture and temperature
– Respond to complex stimuli
• Lines at a specific angle
Convergence in visual system
• Rods and cones – points of light
• Retina – On center / Off surround
• Visual cortex – bars of light
• Most neurons require excitatory input from
more than one neuron in order to fire an
action potential
– Summation
• Spinal motor neurons – 10,000 synapses
Diseases of Myelination
• With demyelination, there is a slowing or
even blocking of action potentials
• Peripheral demyelination
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Metabolic abnormalities
Viruses
Toxic chemicals
Immune system
• Central demyelination
Demyelination
• 2 diseases
• Guillan-Barre
– Peripheral demyelination
– Schwann cells
• Multiple Sclerosis
– Central demyelination
– Oligodendrocytes
Guillain-Barre
• Acute inflammation and demyelination
• Patient Hx
– Mild infection
– Symptoms begin 2-3 weeks later
– Thought to be autoimmune attack on myelin in
peripheral nervous system
G-B – Signs and Symptoms
• Decreased sensation
• Motor paralysis
• Cranial nerves often affected
– Difficulty chewing, swallowing, speaking,
facial expressions
• Weakness in respiratory muscles
• Altered sensation/pain
• Autonomic dysfunction
G-B Prognosis
• Symptoms worsen over 2-3 weeks, then
plateau
• Patient then usually gradually gets better
(weeks to years)
• If demyelination is severe, there may be
scarring and a permanent loss of axons
• ~3% mortality – cardiac or respiratory
problems
Multiple Sclerosis
• Central nervous system demyelination
• Incidence
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High in this region (ND 2nd highest state)
2-3X in women
Highest in whites, Scandinavian descent
Living in northern climates before age of 15
Highest prevalence in young adults
• Increases up to age 35, then gradually declines
MS
• Autoimmune disease
• Will see patches of demyelination, called
plaques, due to the degeneration of
oligodendrocytes
• Variable course of the disease
– Periods of exacerbation and remission
• In chronic MS, the myelin sheath is
replaced by glial scarring
MS – Initial S & S
• Sensory symptoms usually first complaint
– Paresthesia – abnormal non-painful sensations
– Dysesthesia – abnormal painful sensations
– In one extremity or head and face
• Visual blurring
• Diplopia – double vision