Chapter 8 Function of the Nervous System

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Chapter 8 Function of the Nervous System
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Applied Learning Outcomes

• Use the terminology associated with the nervous
  system
• Learn about the following
• Different types of nervous system cells
• Nervous system development
• Nerve cell structure
• Nerve cell function
• Nerve cell physiology
• Understand the biological basis of aging and
  pathology of nerve cell function

Chapter 8 Function of the Nervous System
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Overview
The nervous system communicates critical
information between the various organ systems and
provides information about environmental
conditions to all internal organs. The nervous
system is best defined as a system of cells,
tissues, and organs that regulates the bodys
responses to internal and external stimuli.
Chapter 8 Function of the Nervous System
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The nervous system regulates the bodys response
to external stimuli- environmental factors
that influence metabolic changes in a cell or
physiological changes in tissues and
organs. internal stimuli-includes a variety of
cell secretions used to communicate information
about a cells jobs and needs.
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The Nervous System

• Starts out as a hollow neural tube of ectoderm.
• Runs lengthwise along the back of fetus.
• Forms stem cells--they form two main cell lines
• of the nervous system
• Neurons
• Neuroglia

• There is evidence of a third group of cells
  called
• Neural crest cells

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The Cells of the Nervous System

• Neurons
• Are the operational component of the nervous
  system
• They are excitable cells
• They receive, interpret, and transmit external
  and internal stimuli

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The Cells of the Nervous System

• Neuroglia
• Are not directly involved in nervous system
  communication
• They are supportive cells
• Major job is to maintain the excitability and
  health of neurons.
• Important for development and repair of nervous
  system.

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The Cells of the Nervous System

• Neural crest cells
• Are derived from embryonic neural tube
• They play a role in development of the nervous
  system.

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The Cells of the Nervous System

• The neurons and neuroglia cells start out with
  ability to communicate with each other.
• Called bidirectional communication
• Between neuroglial and neurons
• Permits the formation and maintenance of the
  nervous system

• Classification of cells----
• Neurons by their cell anatomy and mode of
  communication.
• Neuroglia classified by how they assist nerve
  cells.

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Neurons

• Have four common features
• Nerve cell body (soma) houses the nucleus and
  organelles needed to function
• Dendrite (antennae) receive a majority of the
  stimuli that communicate with the neuron
• Axon (long process) comes off the nerve cell body
  from a region called the axon hillock.
• It transmits stimuli from the nerve cell body to
  the terminus
• Terminus forms the end of the axon

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Characteristics of Neurons

• There is usually only one axon for each neuron.
• Some axons have branches or collaterals.
• Axon hillocks job- initiate the electrical
  signal that will be transmitted from axon to
  glands, muscles, and other neurons.
• Terminus releases neurotransmitters to other
  cells
• Cells must have neurotransmitter receptors to
  respond to neurotransmitters.

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How do neurons communicate with other cells?

• Neurons do not make direct contact with other
  cells.
• Ends of the terminus stop short of touching
  nearby cells.
• Form a synapse---junction where an impulse is
  transmitted from one neuron to another.

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How do neurons communicate with other cells?

• Synaptic cleft---gap between the two cells
• Presynaptic neuron--one whose terminus ends at
  the synaptic cleft.
• It produces the neurotransmitter
• Produces only one particular neurotransmitter

• Postsynaptic neuron-- is the neuron on the
  receiving end
• It receives the signal across a synapse
• Contains the neurotransmitter receptors
• Can contain a variety of neurotransmitter
  receptors
• This gives certain neurons the ability to respond
  in different
• ways based on the neurotransmitter released.

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How do neurons communicate with other cells?

• Sensitivity of a postsynaptic neuron to a
  presynaptic neuron depends on the number of
  neurotransmitter receptors located around the
  synaptic cleft.
• Neurons with less receptors are less sensitive.
• Neurons can adjust the number of neurotransmitter
  receptors

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Morphine Addiction

• Morphine mimics natural neurotransmitters
• It alters the receptors in the brain
• High levels of morphine stimulate pleasure
  receptors
• Long term morphine use --- the brain reduces the
  number of receptors to desensitize the neurons
  from the continuous stimulation.
• Person develops an addiction to morphine because
  the body now needs it for the new level of normal
  receptor function.
• Person needs more morphine to receive that same
  feeling.

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Types of Nervous System Cells
The cells of the nervous system are neuroglia,
neurons, and neural crest cells. Neuroglia The
most common cells in the nervous system assist,
protect, and support neurons Neurons Excitable
cells that communicate information about the body
and the environment Neural Crest Cells Involved
in nervous system maintenance and healing
Chapter 8 Function of the Nervous System
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Neurons have three primary shapes

• Unipolar--have a long axon that directly connects
  the dendrites to the terminus (dendrite and axon
  emerging from same process)
• The nerve cell body is positioned to the side of
  the axon
• Usually carry sensory information (sensory
  neurons)
• Bipolar--one dendrite and one axon on opposite
  ends of cell body
• Called interneurons
• Multipolar--many dendrites that attach directly
  to the nerve cell body
• Most numerous type of neuron in the body
• Usually communicate information from the brain to
  glands and muscles
• Called motor neurons

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Unipolar or Sensory neuron
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Multipolar or Motor neuron
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Three primary shapes of neurons
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Neuroglia and Stem Cells

• Neuroglia cells
• make up the bulk of the cell types in the
  nervous system
• Most are found in the brain
• Carry out crucial roles in regulating the
  environment of neurons
• Have very high lipid content
• This makes them shiny and white in appearance

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Major types of neuroglia cells

• Astrocytes or macroglia--- largest class of
  neuroglia
• Attach to neurons or form covers over small blood
  vessels
• Astrocytes associated with blood vessels control
  types of materials that pass from the blood to
  the neurons
• Blood brain barrier- barrier between brain blood
  vessels and brain tissue that restricts the
  passage of materials from blood into brain.
• Ependymal cells
• Mainly secretory cells that line cavities of
  brain and spinal cords.
• They produce cerebrospinal fluid
• Microglia
• Many carry out phagocytosis for protection
• Migrate through CNS removing foreign matter
• Other microglia produce secretions that maintain
  the health of neurons and assist neuron healing.

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Major types of neuroglia cells

• 4. Oligodendrites --large cells with numerous
  branching processes
• that wrap around the axons of neurons
  to form an
• insulating cover called the myelin
  sheath.
• Myelin sheath made by oligodendrites limited to
  neurons in brain and spinal cord.
• Facilitates transmission of stimul along axon
• Radial glia --found in developing nervous system
• Provides framework that organizes
  interconnections of neurons
• Brain and eyes of adults --the radial cells carry
  on bidirectional communication with neurons. This
  means they communicate the needs of certain
  neurons.
• Satellite cells-- very numerous small cells.
• Cover surface of neurons outside of brain and
  spinal cord support neurons in PNS
• Help maintain chemical environment of neurons and
  may assist with nerve cell repair.
• Schwann cells-- form myelin sheath around PNC.
  neurons located outside of brain
• and spinal cord.
• Flattened cells almost completely cover axon
• Slight gap between cells are called Nodes of
  Ranvier

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Neural crest cells

• A type of stem cells in adult humans
• These cells spread out from brain of embryo
• They migrate to various parts of the body
• Embryos these cells assist with formation of
  nervous system
• They can produce neuroglia, neurons, and other
  stem cells
• Hoped these cells will be found around components
  of the nervous system in adults
• Then they could be used to regenerate neuroglia
  and neurons destroyed by disease or injury

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Neuron Physiology
Neurons communicate to other cells via
neurotransmitters Neurotransmitters can be
excitatory or inhibitory
Excitatory Stimulates a neuron Inhibitory
Hinders a neuron
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The following events characterize the
transmission of a nerve impulse 1-Resting
potential- describes the unstimulated,
polarized state of a neuron. (charge inside the
cell is about -70millivolts) More Na on the
outside of the cell then the inside. More K
on the inside of the cell then the outside.
Cell is polarized.
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• 2- Action potential - In response to a stimulus
• gated ion channels in the membrane open
• This permits Na on the outside to rush into the
  cell.
• Cell membrane becomes depolarized, or more
• positive on the inside.
• If stimulus is strong enough (-55 mV above
  threshold level) more Nagates open, causing an
  action potential, or complete depolarization (now
  it is 30mV on the inside of the cell)
• This causes Nagates to open down the length of
  the axon

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3- Following depolarization across the length of
the neuron Na channels close and the K
channels open. Outflow of positive potassium
causes the negative value inside to increase
and neurons begin to repolarize back to resting
potential. 4-Repolarization-when Na is flowing
in another gated channel opens and allows K
to flow out of the cell. This restores the
original membrane polarization. Unlike the
resting potential, the Kare now on the
outside and the Na is on the inside of the cell.
A. Resting potential
B. C. Action potential
D. E. Repolarization
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• 5- Refractory Period-during this time the neuron
  
• will not respond to a new stimulus.
• the membrane is polarized but Naand K
• are on the wrong side of the membrane.

6- Recovery phase a.To reestablish the original
distribution of these ions, Na and Kreturn to
their original location by the Na/K pumps in
the cell membrane. b. Once these ions are in
original place the neuron is ready to receive
another stimulus.
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Speed of an action potential
Propagation of an action potential is fairly
slow. Speed accelerated by presence of myelin
sheath. Stops the action potential along the
axon except where axon is exposed. Action
potential is restricted to the nodes of Ranvier
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Neurotransmitters
Are chemical signals that transfer the action
potential from an affector (sensory neuron
receptor) to a motor neuron of the effector
(muscle or gland) Neurotransmitters can have
either an excitatory or inhibitory effect on
the postsynaptic neuron An excitatory
neurotransmitter helps a postsynaptic neuron
reach threshold, thereby making it more likely
to produce an action potential. Excitatory
neurotransmitters cause the sodium channels to
open Inhibitory neurotransmitters make it more
difficult for the neuron to achieve an action
potential.
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Sequence of events when Neurotransmitters
communicate with other cells

1. Synthesis and storage of neurotransmitters
2. Neurotransmitter release occurs as calcium rushes
   into terminus
3. Neurotransmitter binding to postsynaptic
   receptors
4. Inactivation of neurotransmitters

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Omit
Types of Neuron Communication p. 305-307
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Reflexes
Reflexes are involuntary responses associated
with survival.
The path of information the reflex arc is as
follows affector ? sensory neuron ? interneuron
? motor neuron ? effector
Chapter 8 Function of the Nervous System
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Reflexes
Reflexes --involuntary responses to a
stimulus --links a stimulus to a response
without requiring the intervention of
conscious control --example putting hands out
to catch yourself if you are about to
fall Reflex arc --neuron arrangement --involvin
g an affector (sensory neuron receptor), a
sensory neuron, an interneuron, a motor neuron,
and an effector (a gland or muscle).
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Pathology of Nervous System Function

• How can the nervous system be affected?
• Infectious diseases caused by microorganisms
• Degenerative progressive deterioration of a cell
  or tissue over time
• Congential born with it, diseases caused by
  embryological and
• maturation errors that affect the nervous
  system.
• Toxicological damage caused by poisons that
  affect cell
• metabolism or communication.
• 5. Traumatic injuries resulting from a wound
  that was caused by an
• external force or violence.

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Neurological Diseases
Infectious Bacteria that release their toxins
into the blood cause the most common type of
damage. Bacteria that enter through food or
wounds also can affect the nervous
system. Botulism a bacterium that sometimes
grows in spoiled food ---This bacteria
blocks the action of acetylcholine.
Causes flaccid paralysis, or inability of
muscles to contract. Tetanus a
bacterium that produces a secretion that
enhances the effects of acetylcholine.
---This disease prevents the muscles from
relaxing following contractions
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Neurological Diseases
Infectious diseases Endotoxins certain
bacteria release these poisons. They can cause
immediate death to the neuroglia and
neurons. Diseases they can cause Encephaliti
s (inflammation of the brain) Meningitis
(inflammation of the membranes
surrounding the brain and spinal
and spinal cord)
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Neurological Diseases
Genetic degenerative disease of the nervous
system Amylotrophic lateral sclerosis (ALS)
also known as Lou Gehrigs disease ---caused
by faulty mitochondria passed down by the egg
affects the motor neurons, causing gradual
cessation of muscle function. Multiple
Sclerosis (MS) --- Demyelination disease
results from loss of neuroglia around the axons
and bodies of neurons results in
slower neural impulses and eventual
degeneration of the axon.
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Neurological Diseases
Congenital disorders Krabbes disease--lacks an
important enzyme that prevents the accumulation
of toxic wastes in nerve cells. Usually starts
out in the embryo and causes a build up of
harmful fats in the nervous system
cells. Hirschsprungs disease--nerve cells
normally grow in the intestines of a
developing fetus. With this disease the nerve
cells stop growing, causing a loss of
large intestines function.
Toxicological Arsenic and cynaide
poisoning--block cell respiration and diable
neurons Lead poisoning---slow neuron
functioning Tetrodotoxin---is a neuron poison
found in tropical frogs and puffer
fish. Inhibits the flow of sodium ions into the
nerve cells stopping the action potential.
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Neurological Diseases
Traumatic Athletic injuries, car accidents,
work-related falls Neurons cannot be replaced
once they die. Injured neurons can be repaired
as long as intact neuroglia are nearby.
Undamaged neuroglial cells carry out neuron
repair
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Aging of the Nervous System
Neurons stop going through cell division as they
begin to form mature nervous system
structures. A person has the same nerve cells in
maturity that are present in childhood. Neuroglia
and neurons accumulate a significant amount of
injury over a persons lifespan. Nerve cells
under higher rate of metabolic activity. Puts
them at a higher risk for cellular damage. This
damage can alter the DNA of the nerve cells. This
can metabolic errors may be fatal to nerve
cells. Regular use of alcohol, drugs,
cigarettes, and exposure To pollution also
contribute to nerve cell damage.
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Aging of the Nervous System
With age Most people lose consistent blood flow
to various tissues. Neurons -- slow down due
to inadequate nutrients --may run low on raw
materials needed to build
neurotransmitters. Neuron receptors also lose
responsiveness. Production of cytokines declines
with age. --this is a protein that stimulates
or inhibits the growth and activity of
particular cells. --this is essential form
maintaining the activity and health of
cells.
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Aging of the Nervous System
Reduction of tonic control. --tonic control is
the result of regular nerve cell
communication to certain glands and muscles.
--helps to keep muscles slightly tense,
which assists with movement and posture. --
as one ages, they tend to lose some
mobility and have difficulty with balance
and posture.
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Wellness and Illness over the Life Span
Much of the nervous systems aging is due to
reduced neuroglia and neuron function. Neurons
naturally age by accumulating plaques and
tangles. Neurons accumulate damage from oxidation
and can be hastened by toxins. Neurons cannot be
replaced when lost. Nervous system pathology is
degenerative, congenital, traumatic, or
toxicological.
Chapter 8 Function of the Nervous System
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Summary
The nervous system communicates internal and
external stimuli using electrical impulses called
action potentials. It takes stimuli from the
environment and converts it to useful information
needed for appropriate body responses. It allows
the body to quickly adapt to environmental
changes and stresses. The nervous systems
ability to communicate internal stimuli helps
regulate the bodys internal conditions. This
communication is responsible for movement and all
mental activities.
Chapter 8 Function of the Nervous System