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PSL300H1 Lecture Notes - Peripheral Nervous System, Axoplasmic Transport, Myelin

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Michelle French

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Consists of the brain and the spinal chord
Neurons outside the CNS make up the peripheral nervous system
The PNS includes neurons entirely outside the CNS, and also parts of neurons
that project into or out of the CNS
The PNS comprises the somatic nervous system, for controlling voluntary action
via the skeletal muscle
Part of the autonomic nervous system is the enteric nervous system, which
controls digestion and movements of the intestines
oReceives lots of input from the spinal chord, but can also operate
independently of the CNS
Most of the body's neurons are in the CNS
Dominate for information processing
Brain has ~86 billion neurons and spinal chord has about 1 Billion
Estimates for the enteric nervous system range from 100-600 million and the
total for the PNS is probably not much more
Both CNS and PNS also have cells called glia, which support and protect
neurons, and are likely about as numerous
Most of the neurons in the brain are in the cerebellum
only 10% in cerebrum
The brain is protected by bone, meninges and cerebrospinal fluid
Between the skull |(cranium) and the brain lie 3 tissue layers called meninges:
the dura mater, the arachnoid mater and the pia mater
The subarachnoid space contains cerebrospinal fluid (CSF)
The same layers protect the spinal chord
CSF is produced in fluid filled spaces called ventricles
From top to bottom: lateral -> third -> fourth -> central canal of spinal chord
CSF is secreted into the ventricles and flows throughout the subarachnoid space
Secreted by choroid plexus in ventricle walls
Then reabsorbed by arachnoid villi
Cushions the brain and maintains a chemical environment with lower pH and [K+]
than plasma, no blood cells and little protein
CSF is secreted by the choroid plexus
Goes from capillary to ependymal cells into ventricle
The choroid plexus consists of specialized regions in the walls of the ventricles
It's ependymal cells transport ions, vitamins, nutrients and water from the blood
into the ventricles
CSF is reabsorbed into the blood by arachnoid villi
The arachnoid villi are projections of the arachnoid membrane through the dura,
into the venous sinus
By this circulation, from choroid plexus to arachnoid villi, the entire volume of
CSF is recycled ~3 times a day
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Glial cells called astrocytes help create myelin sheaths and the blood brain barrier
Extensions of the astrocyte wrap around axons to form myelin sheaths, which
speed up nerve conduction
Other extension called foot processes contact capillaries to form the blood brain
The blood brain barrier helps keep harmful substances out of the interstitial fluid
Astrocytes secrete paracrines that promote tight junction between the cells of the
capillary endothelium
Patients with the motor disorder Parkinsons disease have too little of the
neurotransmitter dopamine. Dopamine can't cross the blood brain barrier, so dopamine
pills don't help. But the smaller precursor L-dopa can cross
The CNS is fueled mainly with oxygen and glucose
The brain receives 15% of the blood pumped by the heart. Oxygen from the
membrane crosses the blood brain barrier
Membrane transporters move glucose from the plasma into the interstitial fluid of
the CNS
The brain is responsible for half the body's glucose consumption
Efficient energy consumption because heat is not released
Whole body uses 100W, brain uses 20% = 20% 20W
The CNS includes gray and white matter
Gray matter consists of unmyelinated nerve cell bodies, dendrites, and axon
terminals. The cell bodies are arranged either in layer (in parts of the brain) or in clusters
called nuclei
White matter consists of myelinated axons running in bundles called tracts
In the peripheral nervous system, clusters of neurons are called ganglia, and
bundles of axons are called nerves
Scientists can trace neural pathways in the CNS by various methods
One method is to inject and enzyme, horseradish peroxidase, near axon
terminals. The HRP is brought into the neurons by endocytosis and carried, by fast axonal
transport, to the cell body and dendrites
Throughout the cell, the HRP takes part in reaction yielding coloured or
fluorescent products that let scientists see the whole neuron
Another method is to modify mice genetically so their neurons produce novel
proteins which can then be tagged by fluorescent anti-bodies, making neurons light up.
Spinal chord
It is arranged in 31 segments, each with a pair of spinal nerves
The dorsal root of each spinal nerve carries incoming sensory signals
The dorsal root ganglion contains cell bodies of sensory neurons
The ventral root carries signals from the central nervous system to muscles and
glands (ventral = near belly)
Gray matter in the spinal chord consists of sensory and motor nuclei
Visceral sensory nuclei
Somatic sensory nuclei
Somatic motor nuclei
Autonomic efferent nuclei
White matter consists of axon tracts
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