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Chapter 14
Structure and Function of the
Neurologic System
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Overview of the Nervous System
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Central nervous system (CNS)
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Brain and spinal cord
Peripheral nervous system (PNS)
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Cranial nerves
Spinal nerves
Pathways
• Afferent (ascending; sensory to spinal column)
• Efferent (descending; innervate effector organs)
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Overview of the Nervous System
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PNS
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Somatic nervous system
• Motor and sensory pathways regulating voluntary motor
control of skeletal muscle
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Autonomic nervous system (ANS)
• Motor and sensory pathways regulating the body’s
internal environment through involuntary control of organ
systems
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
Sympathetic
Parasympathetic
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Cells of the Nervous System
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Neuron
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Variable size and structure throughout the nervous
system
Cellular constituents
• Microtubules, neurofibrils, Nissl substances
• Some neurons, such as olfactory neurons, continue to
divide
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Neuron
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Three components
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Cell body (soma)
• Located mainly in the CNS
• Densely packed cell bodies in the CNS are nuclei
• Densely packed cell bodies in the PNS are ganglia
Dendrites
• Receptive portion of the neuron (sends impulses to cell
body)
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Neuron
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Axons
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Carry nerve impulses away from cell body
Axon hillock
• Cone-shaped, Nissl-free area where the axon leaves the
cell body
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Initial segment of the axon
• Area of axon with lowest threshold for stimulation, so
action potentials begin here
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Neuron
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Axons
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Myelin
• Segmented layer of lipid material
• Insulating substance
• Formed and maintained by the Schwann cell
Endoneurium
• Delicate layer of connective tissue around each axon
Neurilemma
• Thin membrane between the myelin sheath and the
endoneurium
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Neuron
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Axons
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Nodes of Ranvier
• Regular interruptions of the myelin sheath
 Saltatory conduction
• Flow of ions between segments of myelin rather than
along the entire length of the axon
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Divergence
• Ability of branching axons to influence many neurons
Convergence
• Branches of numerous neurons converging on one or a
few neurons
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Structural Classification of
Neurons
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Based on the number of processes extending
from the cell body
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Unipolar
Pseudounipolar
Bipolar
Multipolar
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Functional Classification of
Neurons
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Sensory
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Associational
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Transmit impulses from sensory receptors to the
CNS
Transmit impulses from neuron to neuron
Motor
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Transmit impulses from the CNS to an effector
organ
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Neuroglia
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“Nerve glue”
Support the neurons of the CNS
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Astrocytes
Oligodendroglia (oligodendrocytes)
Microglia
Ependymal cells
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Neuroglia
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Nerve Injury and Regeneration
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Wallerian degeneration
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Occurs distal to the cut
• Swelling appears
• Neurofilaments hypertrophy
• Myelin sheath shrinks and disintegrates
• Axon portion degenerates and disappears
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Nerve Injury and Regeneration
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Proximal to the cut
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Swelling and dispersal of the Nissl substance
Cell increases in metabolic activity, protein
synthesis, and mitochondrial activity
New terminal sprouts project from proximal
segment
Process limited to myelinated axons
• Generally only in the PNS
Depends on location, type of injury, inflammatory
response, and scar tissue formation
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Neuroglia
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Nerve Impulse
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Neurons generate and conduct electrical and
chemical impulses by selectively changing
the electrical portion of their plasma
membranes and influencing other nearby
neurons by the release of neurotransmitters
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Synapses
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Region between adjacent neurons
Impulses are transmitted across the synapse by
chemical and electrical conduction
Presynaptic neurons and postsynaptic neurons
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Connections between pre- and postsynaptic neurons
• Axoaxonic
• Axosomatic
• Axodendritic
• Dendrodentritic
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Synapses
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Neurotransmitters
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More than 30 substances (e.g., dopamine, GABA,
endorphins)
Excitatory (excitatory postsynaptic potential)
Inhibitory (inhibitory postsynaptic potential)
Synaptic boutons
Synaptic cleft
Summation
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Temporal summation
Spatial summation
Facilitation
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Central Nervous System
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Forebrain
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Midbrain
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Cerebral hemispheres
Corpora quadrigemina, tegmentum, and cerebral
peduncles
Hindbrain (brain stem)
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Cerebellum, pons, and medulla
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Central Nervous System
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Reticular formation
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Network of connected nuclei that regulate viral
reflexes, such as cardiovascular and respiratory
function
Together with the cerebral cortex is referred to as
the reticular activating system
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Central Nervous System
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Central Nervous System
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Forebrain
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Telencephalon
• Cerebrum
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Gyri, sulci, and fissures
Gray matter and white matter
• Cerebral nuclei (basal ganglia)
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Forebrain
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Forebrain
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Central Nervous System
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Hemispheres (right, left)
Frontal lobe
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Prefrontal
• Goal-directed behavior
 Premotor
• Basal ganglia
 Primary motor area
 Broca speech area
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Central Nervous System
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Parietal lobe
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Occipital lobe
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Sensory input
Visual cortex
Temporal lobe
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Wernicke area (speech)
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Central Nervous System
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Diencephalon
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Epithalamus
Thalamus
Hypothalamus
Subthalamus
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Central Nervous System
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Midbrain
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Corpora quadrigemina (tectum)
• Superior and inferior colliculi
Tegmentum
• Red nucleus and substantia nigra
Basis pedunculi
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Central Nervous System
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Hindbrain (metencephalon)
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Cerebellum
Pons
Myelencephalon
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Medulla oblongata
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Central Nervous System
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Spinal Cord
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Lies within the vertebral canal and is
protected by the vertebral column
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Connects the brain and the body
Conducts somatic and autonomic reflexes
Provides motor pattern control centers
Modulates sensory and motor function
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Spinal Cord
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Sections
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Cervical, thoracic, lumbar, and sacral
Horns
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Posterior
Lateral
Anterior
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Spinal Cord
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Spinal Cord
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Spinal Cord
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Reflex Arc
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Receptor
Afferent (sensory) neuron
Efferent neuron
Effector
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Reflex Arc
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Upper and Lower Motor Neurons
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Upper motor neurons
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Efferent pathways primarily relaying information
from the cerebrum to the brainstem or spinal cord
Synapse with interneurons
Destruction = partial recovery
Lower motor neurons
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Neurons having direct influence on muscles
Cell bodies originate in gray matter of spinal cord,
but their axons extend into the PNS
Destruction = permanent paralysis
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Neuromuscular Junction
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Motor Pathways
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Lateral corticospinal
Corticobulbar
Basal ganglia
Vestibulospinal
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Sensory Pathways
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Anterior spinothalamic
Lateral spinothalamic
Posterior (dorsal)
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Three neuron chain
Ipsilateral transmission
Contralateral transmission
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Protective Structures
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Cranium
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Eight bones
• Frontal
• Occipital
• Temporal (2)
• Parietal (2)
• Sphenoid
• Ethmoid
Galea aponeurotica
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Protective Structures
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Meninges
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Protective membranes surrounding the brain and
spinal cord
• Dura mater
• Arachnoid
• Pia mater
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Meninges
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Protective Structures
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Cerebrospinal fluid (CSF) and the ventricular
system
CSF is a clear, colorless fluid similar to blood
plasma and interstitial fluid
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125 to 150 ml
Produced by the choroid plexuses in the lateral,
third, and fourth ventricles
Reabsorbed through the arachnoid villi
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Protective Structures
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Vertebral column
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33 vertebrae
• 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4
fused coccygeal
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Intervertebral disks
• Nucleus pulposus
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Vertebral Column
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Vertebral Column
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Blood Supply to the Brain
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800 to 1000 ml per minute
CO2 is the primary regulator for CNS blood
flow
Internal carotid and vertebral arteries
Arterial circle (circle of Willis)
Blood-brain barrier
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Blood Supply to the Brain
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Blood Supply to the Brain
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Blood Supply to the Brain
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Blood Supply to the Brain
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Blood Supply to the Spinal Cord
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Vertebral arteries
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Anterior spinal arteries
Posterior spinal arteries
Aorta
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Blood Supply to the Spinal Cord
INSERT Figure 14-22
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Peripheral Nervous System
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31 pairs of spinal nerves
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Names correlate with the vertebral level from
which they exit
Mixed nerves
Arise from the anterior and posterior horn cells of
the spinal cord
12 pairs of cranial nerves
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Sensory, motor, and mixed
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Peripheral Nervous System
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Fascicles
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Rami
Plexuses
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Bundles of myelinated axons in the PNS
Brachial plexus
Lumbar plexus
Sacral plexus
Dermatomes
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Peripheral Nervous System
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Autonomic Nervous System
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Located in both the CNS and PNS
Coordinates and maintains a steady state
among the visceral (internal) organs
Neurons
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Preganglionic (myelinated)
Postganglionic (unmyelinated)
Two divisions
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Sympathetic
Parasympathetic
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Sympathetic Nervous System
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Mobilizes energy stores in times of need
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Receives innervation from cell bodies located
from the first thoracic through the second
lumbar
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“Fight or flight response”
Thoracolumbar division
Sympathetic (paravertebral) ganglia
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Sympathetic Nervous System
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Parasympathetic Nervous
System
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Functions to conserve and restore energy
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Receives innervation from cell bodies located
in the cranial nerve nuclei and sacral region
of the spinal cord
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“Rest or repose response”
Craniosacral division
Preganglionic neurons travel to ganglia close
to the organs they innervate
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Parasympathetic Nervous
System
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Neurotransmitters and
Neuroreceptors of the ANS
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Sympathetic preganglionic fibers
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Sympathetic postganglionic fibers
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Acetylcholine and cholinergic receptors
Norepinephrine and adrenergic receptors
Parasympathetic pre- and postganglionic fibers
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Acetylcholine and cholinergic receptors
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Neurotransmitters and
Neuroreceptors of the ANS
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Aging and the Nervous System
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Structural changes
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Decrease in the number of neurons
Decreased brain weight and size
Increased adherence of the dura mater to the skull
Fibrosis and thickening of the meninges
Narrowed gyri and widened sulci with a
corresponding increase in the size of the
subarachnoid space
Basal ganglia and ventricular system
• Aberrations in vascular structures
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Aging and the Nervous System
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Cellular changes
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Deposition of lipofuscin
Presence of senile plaques
Multiple neurofibrillary tangles
Lewy bodies
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Aging and the Nervous System
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Functional changes
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Slowed response to neural signals
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Tests of Nervous System
Function
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Skull and spine roentgenograms
Computed tomography
Magnetic resonance imaging
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Magnetic resonance angiography
Positron emission tomography scan
Brain scan
Cerebral angiography
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Tests of Nervous System
Function
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Myelography
Echoencephalography (ultrasound)
Electroencephalography
Evoked potentials
Cerebrospinal fluid analysis
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