Chapter 16: Nervous Tissue
Summary
- The structures that make up the nervous system are the brain, 12 pairs of cranial nerves
and their branches, the spinal cord, 31 pairs of spinal nerves and their branches, sensory
receptors
- Anatomically the nervous system consists of 2 divisions
o The central nervous system which consists of the brain and spinal cord
o The peripheral nervous system which is all nervous tissue outside the CNS
- Functionally the nervous system integrates all body activities by sensing changes
(sensory functions) interpreting them (integrative function), and reacting to them (motor
functions)
- Afferent (sensory) neurons carry sensory information from cranial and spinal nerves into
the brain and spinal cord or from a lower to a higher level in the spinal cord and brain
o Interneurons have short axons that contact nearby neurons in the brain, spinal
cord, and ganglion
- Motor (efferent) neurons carry information from the brain toward the spinal cord or out
of the brain and spinal cord into cranial or spinal nerves
- Components of the PNS includes the somatic nervous system (SMS) and autonomic
nervous system (ANS)
- The SNS consist of somatic sensory neurons that conduct impulses from somatic and
special sense receptor to the CNS (input) and somatic motor neurons from the CNS to
skeletal muscles (output)
- ANS contains autonomic sensory neurons from visceral organs (input) and autonomic
motor neurons and convey impulses from the CNS to smooth muscle tissue, cardiac
muscle tissue, and glands (output)
o 2 main subdivisions of the ANS are the sympathetic and parasympathetic
divisions
o The third subdivision is the enteric nervous system
 ENS consists of neurons in enteric plexuses in the GI tract that function
independently of the ANS and CNS to some extent
 Sensory neurons of the ENS monitor chemical changes and stretching of
the GI tract (input) and motor neurons of the ENS generate contractions
and secretions of the GI tract (output)
Introduction
- Nervous tissue is made up of neurons (nerve cells) and neuroglia (cells that support the
activities of the neurons
16.1 Overview of the Nervous System
- Structures of the Nervous System
o Neurology is the branch of science that deals with the normal and disorder
functioning of the nervous system
 Neurologist is a specialist in neurology
o Nervous system weighs only 2 kgs and is 3% of total body weight
o Smallest but most complex system in the body
o Major structures of the nervous system:
 Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses
(in small intestine), and sensory receptors
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Organization of the Nervous System
o Can be organized both anatomically and functionally
o Anatomical organization
 Cells are extremely long compared to other body systems
 Two main interconnected divisions
 The CNS and PNS
 The CNS
 Composed of the brain and spinal cord
 Brain is enclosed and protected by the skull in the cranial cavity
and contains about 85 billion neurons
 Spinal cord is enclosed and protected by the bones of the vertebral
column in the vertebral canal and contains about 100 million
neurons
 Brain and spinal cord are continuous with one another through the
foramen magnum of the occipital bone
 Processes different kinds of incoming sensory information
 Source of thoughts, emotions, and memories
 Most nerve impulses that stimulate muscles to contract and glands
to secrete originate in the CNS
 The PNS
 Composed of all nervous structures outside the CNS
o Cranial nerves and all their branches, spinal nerves and
their branches, and sensory receptors
 Link all parts of the body to the CNS
 12 pairs of cranial nerves number I through XII from right to left
emerge from the base of the brain
 Nerve: bundle of hundreds to thousands of axons (nerve cell fibers)
and associated connective tissue and blood vessels that lies outside
the brain and spinal cord
o Each nerve follows a defined path and serves a specific
region of the body
 Median nerve carries signal for motor output and
sensory input to and from the muscles and skin of
the upper limb
 31 pairs of spinal nerves emerge from the spinal cord
o Each serving a specific region on the right or left side of the
body
 Sensory receptors: are structures that monitor changes in the
internal and external environment
o Like receptors in the skin that detect touch sensations,
photoreceptors in the eyes, and olfactory (smell) receptors
in the nose
o Functional Organization
 Diverse activities of the nervous system are grouped into 3 basic functions
 Sensory = input
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o Sensory receptors detect internal stimuli, like increase in
blood pressure and external stimuli like raindrop landing on
arm
o Afferent neurons (sensory) carry this sensory information
into the brain and spinal cord through cranial and spinal
nerves
Integrative= control
o Nervous system processes sensory information by
analyzing and storing some of it and by making decisions
for appropriate responses
 Integration
o Perception is an integrative function
 Conscious awareness of sensory stimuli
 Occurs in the brain
o Interneurons
 Neurons that interconnect with other neurons
 Neurons that participate in integration
 Axons that only extend for short distances and
contact nearby neurons in the brain or spinal cord to
set up the complex circuit boards of the CNS
 Majority of neurons in the body are interneurons
and they make up the majority of the CNS
Motor = output
o Once sensory information is integrated the nervous system
must elicit and appropriate motor responses
 Muscle contraction or glandular secretion
o Motor or efferent neurons serve this function
 Carry information out of the brain or spinal cord to
effectors (muscles and glands) through cranial and
spinal nerves
 Stimulation of the effectors by motor neurons
causes muscles to contract and glands to secret
Blue boxes are sensory components of the PNS
Red boxes are motor components of the PNS
Green boxes are effectors (muscles and glands)
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PNS is broken down into somatic and autonomic nervous system
o Somatic nervous system
 Consist of sensory neurons called somatic sensory neurons
 Convey information to eh CNS from sensory receptors in the skin,
skeletal muscles, and joints and from the receptors for the special
senses (vision, hearing, equilibrium state, taste, and smell)
 Somatic sensory pathways are involved in the input of information to the
CNS for integration (processing)
 Somatic motor neurons convey information from the CNS to the skeletal
muscles only
 Somatic motor pathways are involved in the output of information
from the CNS that results in a controlled muscular contraction
o These contractions can be consciously controlled the
actions of these parts of the SNS are voluntary
o Autonomic nervous system
 Has sensory and motor components
 Autonomic (visceral) sensory neurons
 Convey information to the CNS from autonomic sensory receptors
located mostly in visceral organs (smooth muscle organs in the
thorax, abdomen, an pelvis)
 Autonomic motor neurons
 Convey information from the CNS to smooth muscle, cardiac
muscle, and glands and cause the muscles to contract and glands to
secrete
 Motor responses not under conscious control the actions of the ANS are
involuntary
 Two main subdivision of the ANS are the:
 Sympathetic division
o Sympathetic neurons help support exercise or emergency
actions
o Fight or flight responses
o Major regulator of the smooth muscle of the cardiovascular
system
o Wider distribution
 Blood vessels are located everywhere in the body
 Parasympathetic division
o Rest and digest activities are controlled by this division
o Major regulator of smooth muscle in the digestive and
respiratory systems
 Derived from the embryonic gut tube
 With a few exceptions the effectors receive nerves from both
division and usually the two division have opposite actions
 The third division of the ANS is the enteric nervous system (ENS)
 Brain of the gut
 Has over 100 million neurons that occur throughout most of the
length of the gastrointestinal tract
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Has both sensory and motor components and can operate
independently
o But can communicate with and are regulated by other
branches of the ANS
 Sensory neurons of the ENS monitor chemical changes within the
GI tract as well as stretching of tis walls
 Motor neurons of the ENS govern contraction of the GI tract
smooth muscle to propel food through the GI tract
 Neurons also control secretions of the GI tract organs like acid
from the stomach, and endocrine cells, which secrete hormones
 ENS is involuntary
16.2 Histology and Functions of Neurons
- Neurons and neuroglia are the types of cells that make up nervous tissue and they
combine in different ways in different origins of the nervous system
- Neurons make up the circuitry that connects all regions of the body to this central
processing unity
- Neurons provide most of the unique functions of the nervous system like sensing,
thinking, remembering, controlling muscle activity, and regulating glandular secretions
- Most neurons cannot undergo mitotic division because of how specialized they are
- Neuroglia are smaller than neurons but they outnumber by 25x
- Neuroglia nourish, support, protect, the neurons and maintain the interstitial fluid that
bathes the neurons
- Neurons
o Also known as nerve cells
o Possess electrical excitability
 The ability to respond to a stimulus and convert it into a nerve impulse
o Stimulus is any change in the environment that is strong enough to initiate a nerve
impulse
o Nerve impulse or action potential is an electrical signal that propagates (travels)
along the surface of the membrane of a neuron
 Begins its travels due to the movement of ions between interstitial fluid
and the inside of a neuron through specific ion channels in the neurons
plasma membrane
o Some neurons are small and propagate impulses over a short distance within the
CNS
 Others are the longest cells in the body
 Sensory neurons that allow you to feel sensations in your toes
stretch from the foot to the lower position of the brain
o Parts of a Neuron
 Differ in shape and sizes a general design exists
 2 basic parts
 The cell body
o Contains a nucleus surrounded by cytoplasm
o Nissl bodies: clusters of rough endoplasmic reticulum
 Responsible for protein synthesis
o Cytoskeleton has neurofibrils and microtubules
o Lipofuscin: yellowish brown pigment in the cytoplasm
o Ganglion: a collection of neuron cell bodies outside the
CNS
o Somatic gemmules or somatic spines increase the surface
area available for interactions with other nerve cells
 Variable number of processes called nerve fibers
o Great variation in size and length and are classified based
on distinct structural and functional difference
 Dendrites or axons
o General term for any neuronal process (extension) that
emerges from the cell body of a neuron
o Dendrite: are the receiving or input portions of a neuron
 Usually short, tapering, and highly branches
 Greater the branching the greater the surface
area of the neuron for receiving synaptic
communication form other neurons
 Tree shaped array of extensions from the
cell body most the time
 Have cell receptor sites on the plasma membrane
 More than the cell body
 Receptor sites are increased by small bumps and
projections on the membrane called dendritic
gemmules or dendritic spines
o Axon:
 Vary in length from a mm to a meter
 Single axon of a neuron carries nerve impulses
toward another neuron, muscle fiber, or gland cell
 Long, thin, cylindrical projection that often joins the
cell body at the axon hillock (cone shaped
elevation)
 Initial segment is the part of the axon closest to the
axon hillock
 Trigger zone is junction between the axon
hillock and the initial segment where
impulses arise and then travel along the
axon
 Axon terminal arborizations
 The fine processes that axons end in by
dividing
o Neurotoxins are found in certain shellfish and organisms
 Substances that produce their poisonous effect by acting on the nervous
system
 TTX is lethal neurotoxin present in Japanese’s pufferfish and block
nerve impulses
o Local anesthetics are drugs that block pain and other somatic sensations
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These drugs act by blocking nerve impulses so pain signals do not reach
the CNS
Cooling of a nerve can have similar effects as anaesthetic because when
cooled axons transmit signals at lower speeds
Synapses:
o Site of communication between 2 neurons or between a neuron and an effector
cell
o Presynaptic neuron
 Refers to a nerve cell that carries a nerve impulse toward a synapse
 The cell that sends the signal
o Postsynaptic cell
 Carries a nerve impulse away from a synapse or an effector that response
to the impulse of at the synapse
o Tips of some axons swell and are synaptic end bulbs and others have swollen
bumps called varicosities
o Synaptic vesicles: tiny membrane enclosed sacs on the ends of the axon that store
a chemical called a neurotransmitter
 The bulb or the varicosities
o Neurotransmitter
 Molecule released from a synaptic vesicle that excites or inhibits
postsynaptic neurons, muscle fibers, or gland cells
 Most neurons contain 2-3 neurotransmitters
Neuromuscular Junction
o Neuromuscular junction is synapse between a motor neuron and a muscle fiber
o Most synapses have a small gap between cells called a synaptic cleft
o Nerve impulse cannot jump the gap to excite the next cell it must rely on
neurotransmitters
o Synaptic vesicles release the neurotransmitters Ach which is picked up by Ach
receptors in the cell membrane and triggers an action potential
Synapse Between Neurons
o The neuron sending the signal is called the presynaptic neuron and the one
receiving the message is the postsynaptic neuron
o Most synapses between neurons are axodentritic
 From presynaptic axon to postsynaptic dendrite
o Axosomatic
 From presynaptic axon to postsynaptic cell body (soma)
o Axoaxonic
 Axon to axon
o Can be electric or chemical
 Electrical synapse
 The plasma membranes of the presynaptic and postsynaptic
neurons are tightly bound by gap junctions that contain connexions
 Ions flow from one neuron to another through connexons
o Nerve impulse is generated and passes from one cell to
another
 Not common in brain
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Common in visceral smooth muscles, cardiac muscle tissue, and
developing embryo
 Permit readily communication and uniform, coordinated
movements, such as required to make a heart beat
 Chemical Synapses
 Involve the release of a neurotransmitter from presynaptic neuron
 Occur between most neurons and between all neurons and
effectors (muscle cells and glandular cells
 Nerve impulses arrives at synaptic end bulb of presynaptic axon
 Nerve impulse opens voltage channels and allow calcium to travel
into syntactic end bulb
 Increase in Ca causes the release of neurotransmitters into the
synaptic cleft
 Neurotransmitter molecules diffuse across the synaptic cleft and
bind to neurotransmitter receptors in the postsynaptic neurons
plasma membrane
o At most synapse only one-way information transfer can occur
o Only synaptic end bulbs of presynaptic neurons can release neurotransmitters
o Only postsynaptic neurons membranes have the correct receptor proteins to
recognize and bind that neurotransmitter
o A neurotransmitter affects the postsynaptic neuron, muscle fiber, or gland cell as
long as it remains bound to its receptor
 Removal of the neurotransmitter is essential for normal synaptic function
o Neurotransmitters are removed in 1 of 3 ways
 Some of the released neurotransmitter molecules diffuse away from the
synaptic cleft
 Once it’s out of reach it an no longer exert its effect
 Some neurotransmitters are destroyed by enzymes
 Many neurotransmitters are actively transported back into the neuron that
released them (reuptake), others are transported into neighboring
neuroglial (uptake)
Neurotransmitters:
o Ach is an excitatory neurotransmitter at the neuromuscular junction
 Can also be inhibitory at other synapses
 Ex, Parasympathetic neurons slow heart rate by releasing Ach at
inhibitory synapses
o Several amino acids are neurotransmitters in the CNS
 Glutamate and aspartate have powerful excitatory effects
 GABA is an inhibitory neurotransmitter
 Antianxiety drugs enhance the action of GABA
o Norepinephrine plays roles in arousal, dreaming, and regulating mood
o Dopamine are active during emotional responses, addictive behaviour, and
pleasurable experiences
 Affect skeletal muscle tome and some aspects of movement due to
contraction
 One way to get schizophrenia is due to accumulation of excess dopamine
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o Serotonin is involved in sensory perception, temperature regulation, control of
mood, appetite, and onset of sleep
o Depression
 Linked to the imbalance of serotonin, norepinephrine, and dopamine in the
brain
 Drugs to treat depression may inhibit the reuptake of serotonin by
serotonin receptors and prolong the activity of the neurotransmitter at
synapses in the brain
o Endorphins are neuropeptides (neurotransmitters linked by peptide bonds)
 Serve as the body’s natural pain killer
 Linked to improved memory and learning and to feelings of pleasure or
euphoria
o Nitric oxide
 Not synthesized in advance and packing into synaptic vesicles like the
other neurotransmitters
 Formed on demand and doesn’t react with receptors
 Acts only on cells close to its point of synthesis because its consumed so
quickly
 An excitatory neurotransmitter
 Learning and memory role
o Carbon monoxide
 Not produced in advance
 Formed as needed and diffuses out of cells that produce it into adjacent
cells and doesn’t react with receptors
 Excitatory neurotransmitter produced in the brain and in response to some
neuromuscular and neuroglandular functions
 Dilation of blood vessels, memory, olfaction, vision, thermos regulation,
insulin release, anti-inflammatory activities
Structural Diversity in Neurons
o A few small neurons lack axons and other have short axons
o Longest axon is almost as long as you are tall from toes to lower part of brain
o Multipolar neurons
 Have several dendrites and one axon
 Most neurons in the brain and spinal cord are multipolar
 All are motor (efferent) neurons
o Bipolar neurons
 Have one main dendrite and one axon
 Found in the retina of the ye, inner ear, and the olfactory area of the brain
o Unipolar or pseudo unipolar neurons
 Sensory neurons that being in the embryo as bipolar neurons
 Axon and dendrite are brought together into a single process and divides
into 2 branches a short distance from the cell body
 Both branches have the characteristics and function of an axon
 Long, cylindrical processes that propagate action potentials
 Axon branch that extends into the peripheral are peripheral process
 Has dendritic branches at its distal tip
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Axon branch that extends into the CNS is the central process and ends in a
synaptic bulb
 Dendrites monitor a sensory stimulus such as touch or stretching
 Trigger zone for nerve impulses in a unipolar neuron is at the junction of
the dendrites and axon
 Propagate toward the synaptic end bulbs
o Purkinje cells in the cerebellum and pyramidal cells found in the cerebral cortex
o Distinctive pattern of dendritic branching allows identification of a particular type
of neuron in the CNS
16.3 Histology and Function of the Neuroglia
- Make up half the volume of the CNS
- The glue that holds the nervous tissue together
- Ability to divide and are smaller than neurons but greater in number
- In the case of injury, the neuroglia multiply to fill the space formerly occupied by
neurons
- Brain tumours derived from glia are gliomas and tend to be highly malignant and rapidly
growing
- Six types of neuroglia
o 4 in the CNS: astrocytes, oligodendrocytes, microglia, and ependymal
o 2 in the PNS: Schwann cells, satellite cells
- Neuroglia of the CNS
o Can be distinguished based on their size, cytoplasmic membrane and intracellular
organization into the 4 types
o Astrocytes: largest and most numerous neuralgia
 Star shaped and armlike processes that make contact with blood
capillaries, neurons, and the pia mater
 2 types one found in the white matter one in the grey matter
 They contain microfilament that provide them with considerable strength
 Neurons of the CNS must be isolated from harmful substances
 In embryo they secrete chemicals that appear to regulate the growth,
migration and interconnection among neurons in the brain
 Help to maintain the appropriate chemical environment for the generation
of nerve impulses
 May also play a role in learning and memory by influencing the formation
of neural synapses
o Oligodendrocytes
 Resemble astrocytes but are smaller and contain fewer processes
 Processes are responsible for forming and maintaining the protective
covering around CNS axons
 The myelin sheath covering around some axons that insulates the axon and
increases the speed of nerve impulse conduction
o Microglial
 Small cells with slender processes that give off numerous spine like
projections
 Originate in red blood marrow and migrate to the CNS
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Remove cellular debris formed during normal development of the nervous
system and damaged nervous tissue
o Ependymal cells
 Cuboidal to columnar cells arranged in a single layer that possess cilia
 Line the ventricles of the brain and central canal of the spinal cord
 Produce, monitor and assist in the circulation of cerebrospinal fluid
 Form the blood-cerebrospinal fluid barrier
Neuroglia of the PNS
o Completely surrounds axons and cell bodies
o Schwann cells
 Flat cells that encircle PNS axons
 Form a myelin sheath around axons like oligodendrocytes in the CNS
 Schwann cell myelinates a single axon (OG one does several axons)
 Can enclose 20 unmyelinated axons
 Participate in axon regeneration
o Satellite cells
 Flat cells that surround cell bodies of neurons of PNS ganglia
 Ganglia are connection of neuronal cell bodies outside the CNS
 Provide structural support, regulate the exchange of materials between
neuronal cell bodies and interstitial fluid
Myelination
o Axons that are surrounded by a multilayered lipid and protein covering are said
to be myelinated
o Sheath electrically insolates the axon of a neuron and increases the speed of nerve
impulse conduction
o Schwann cells and oligodendrocytes produce myelin sheaths
o Neurolemma (sheath of Schwann)
 When axon is injured the neurolemma aids regernation by guiding and
stimulating regeneration
o Nodes of Ranvier
 Appear at intervals along the axon
 Impulses are formed more quickly at nodes of Ranvier and appear to leap
from node to node as opposed to being conducted more slowly through
every part of the membrane in unmyelinated axons
o CNS has nodes of Ranvier but there are less
 Little injury and regrowth in CNS axons
o Amount of myelin increases from birth to maturity
 Infant’s responses to stimuli not as coordinated as an older person or as
fast
o Demyelination
 Loss or destruction of myelin sheaths around axons
 Can happen from MS or Tay-Sachs, radiation or chemotherapy
Gray and White Matter
o White matter is aggregations of myelinated and unmyelinated axons of many
neurons
 White color of myelin makes it white
o Gray matter contains neuronal cell bodies, dendrites, unmyelinated axons, axon
terminal and neuroglia
 No myelin in these areas
 Nissl bodies make a grey color
o Blood vessels are in both white and gray matter
o Spinal cord
 White matter surrounds an inner core of gray matter in H shape
o Brain
 Thin shell of gray matter covers the surface of the largest portions of the
brain
 Cerebrum and cerebellum
o Nucleus is a cluster of neuronal cell bodies within the CNS
 When used to describe nervous tissue
o Many nuclei of gray matter also lie deep within the brain
o Much of CNS white matter consists of tracks
 Bundles of axons in the CNS that extend for some distance up or down the
spinal cord or connect part of the brain with each other and with the spinal
cord
o CNS nucleus: collection of nerve cell bodies
 Ex, red nucleus
o PNS ganglion: collection of nerve cell bodies
 Ex, vestibular ganglion
o CNS tract: collection of nerve fibers
 Ex, posterior column tract
o PNS nerve: collection of nerve fibers
 Ex, vagus (X) nerve
16.4 Neural Circuits
- CNS has billions of neurons organized into complicated networks called neural circuits
o Each is a functional group of neurons that processes a specific kind of information
- Simple series circuit
o A presynaptic neuron stimulates a single postsynaptic neuron
 That neuron then stimulates another and so on
 Most neural circuits are more complex though
- MS a disease that causes a progressive destruction of myelin sheaths surrounding neurons
in the CNS
o Auto-immune disease
 The body’s own immune system spearheads the attack
o Myelin sheaths are deteriorated to scleroses which are hardened scars or plaques
in the white matter of the brain and spinal cord
- A single presynaptic cell may synapse with several postsynaptic neurons
o Divergence
- In a diverging circuit the nerve impulses from a single presynaptic neuron causes the
stimulation of increasing numbers of cells along the circuit
o A small number of neurons in the brain that govern a particular body movement
stimulate a much larger number of neurons in the spinal cord
- Sensory signals often sent through diverging circuits
o Effect of amplifying the signal
- Convergence
o Several presynaptic neurons synapse with a single postsynaptic neuron
o Permit more effective stimulation or inhibition of the postsynaptic neuron
- In a converging circuit the postsynaptic neuron receives nerve impulses from several
different sources
o Ex, a single motor neuron that synapses with skeletal muscle fibers at
neuromuscular junctions received input from several pathways that originate in
different brain regions
- Some circuits are constructed so that stimulation of the presynaptic cell causes the
postsynaptic cell to transmit a series of nerve impulses
o Reverberating circuit
 Incoming impulse stimulates the first neuron which stimulates the second
and so on
 Branches from later neurons synapse with earlier ones
 Muscular activates, waking up, sleeping (reverberating stops), short-term
memory
16.5 Regeneration and Neurogenesis
- Plasticity: the capability to constantly change, grow, and remap itself over the course of
lifetime
o Nervous system does this
- Neurons have limited powers of regernation
o The capability to replicate or repair themselves
- In PNS damage to dendrites and myelinated axons may be repaired if the cell body
remains intact and if the Schwann cells that make the myelination are active
o Severed axon can’t be repaired or regrow
- Neurogenesis: the birth of new neurons from undifferentiated stem cell
o Happens in some animals like song birds
- Epidemical growth factor (EGF) stimulated cells taken from the brains of adult mice
- Neurogenesis occurs in the olfactory bulb, caudate nucleus and cerebellum of some
mammals
- Complete lack of neurogenesis in other regions of the brain and spinal cord seem to result
from
o Inhibitory influences from neuroglia (oligodendrocytes)
o Absence of growth-stimulating cues that were present during fetal development
- Acupuncture: the use of needles inserted into specific exterior body location an
manipulated to relieve pain an provide therapy for various conditions
o May cause a release of neurotransmitters like endorphins
- Excitotoxicity
o The destruction of neurons through prolonged activation of excitatory synaptic
transmission caused by a high level of glutamate in the interstitial fluid of the
CNS
o Most common cause is oxygen deprivation of the brain during a stroke
- GBS
o An acute demyelination disorder in which macrophages strip myelin from axons
in the PNS
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Neuroblastoma
o A malignant tumor that consists of immature nerve cells
o Most common in abdomen in the adrenal glands
Neuropathy
o Any disorder that affects the nervous system
o Disorder of cranial or spinal nerve
o Ex, facial neuropathy
Rabies
o Fatal disease caused by virus that reaches the CNS via an axon usually
transmitted by the bite of a dog or other meat eating animal