Nerves. By the end of this class you should understand: The divisions of the nervous system and how...
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Transcript of Nerves. By the end of this class you should understand: The divisions of the nervous system and how...
![Page 1: Nerves. By the end of this class you should understand: The divisions of the nervous system and how they interconnect The general role of glial cells.](https://reader036.fdocuments.us/reader036/viewer/2022062407/56649cdc5503460f949a7528/html5/thumbnails/1.jpg)
Nerves
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By the end of this class you should understand:
• The divisions of the nervous system and how they interconnect
• The general role of glial cells and the specific function of selected glial cells
• The structure and function of a neuron• The mechanism of an action potential and
neuron signaling• The function of myelin in the nervous system
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Nervous System
• The nervous system is one of the two control systems of the body– The other being the
endocrine system– These two systems have a
lot of interaction!• Made of neurons and
neuroglia– Nervous tissue!
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Nervous System Organization
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Neuroglia
• Also known as glial cells• Serve many functions
related to protecting the neurons– Physical protection– Myelination (more on that
later)– Chemical protection– Antimicrobial protection
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Neurons
• There are three classifications of neurons– Sensory Neurons– Interneurons– Motor Neurons
• Interneurons form the central nervous system (CNS)
• Sensory and motor neurons form the peripheral nervous system (PNS)
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Neuron Structure
• Neurons have three general structures:– Soma (cell body)– Axon (signal
transmission)– Dendrite (signal
reception)• Some neurons look
rather different but all have these three parts in some way
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Electrical Activity
• Recall that muscle and nervous tissue are electrically active
• They maintain a potential across their cell membrane (outside has different electrical charge than inside)– This is also called a
voltage
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Membrane Potential
• There are many negatively charged particles both inside and outside the cell– Outside: chloride, inside:
proteins• Whichever side of the
membrane has more positively charged particles will be positive– The other side will be
negative even though it clearly has many positively charged particles as well
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Positive Ions• Sodium and potassium are
two positively charged particles found throughout the body
• Neurons have sodium-potassium pumps that pump three sodiums out of the cell for every two potassiums they pump into the cell– Creates a net positive charge
outside the cell
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Sodium/Potassium Balance• Our diet must include both
sodium and potassium on a regular basis– This is why sodium tastes good– The kidneys can help adjust this
balance• Cramping and nervous problems
can result from chronic or acute imbalances of ions– Sweating profusely can lose ions,
which is why it is recommended to rehydrate with salt pills or gatorade
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Resting Membrane Potential• When a neuron is at rest, it is
constantly pumping sodium out and potassium in to create a net positive charge on the outside of the cell– Typically the inside is -70mV
compared to the outside• This requires a lot of daily
ATP– Neurons can only use glucose,
so blood sugar must be maintained at a constant level
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Graded potentials• Neuron potentials can
be altered by many different structures– Sensory neurons can
have their potentials altered by the stimulus they are supposed to detect (pressure, heat, stretch, light, etc)
• These stimuli may cause the membrane potential to be reduced to -55 mV
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Threshold Voltage
• -55mV is usually the threshold voltage, at which point the behavior of certain membrane proteins changes
• These membrane channels are voltage-gated which means they open or close depending on the cell voltage
• When the voltage hits -55mV, the voltage-gated sodium channels open
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THINK FAST!
• When the voltage-gated sodium channels open, what happens to the:– Sodium?– Potassium?– Membrane Voltage?
• Bonus thought: what would happen if these voltage-gated sodium channels were blocked?
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Voltage-Gated Ion Channels• The sodium will
rush into the cell– Potassium ions
cannot fit through the sodium channel and so remain concentrated inside the cell
• Net effect: the voltage is now positive inside the cell!
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Voltage-gated Potassium Channels
• The voltage change also opens voltage-gated potassium channels– These open more slowly
than the sodium channels• When they open,
potassium rushes out of the cell– Sodium channels inactivate
quickly, whereas potassium channels inactivate slowly
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Voltage Changes
• The potassium rushing out of the cell counterbalances the sodium rushing in– Cell voltage drops back to -
70mV or more and the voltage-gated channels close
• This brief flicker back and forth of voltage is called an action potential– All-or-none due to the
voltage-gated ion channels
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Action Potential• The action potential can
happen again only once the sodium and potassium have been replaced back to the outside and inside of the cell– Sodium-potassium pump
required for this– The delay before firing
again is called the refractory period
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Action Potential• So what is the purpose of
the action potential??– Must send a signal
somehow!• Each voltage flip (action
potential) on the membrane triggers the neighboring membrane proteins to flip their voltage– Sends a rapid wave down
the cell membrane
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Speed of Conduction
• Action Potentials may be sped up if there is myelin on the axon– In peripheral nervous system,
Schwann cells provide the myelin and can also regrow the axon if it is damaged
• Myelin conducts the voltage from one ion-channel node to the next much more quickly than if they were continuous
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Neurotransmitter
• Recall from muscles that a neuron will release a neurotransmitter called acetylcholine onto a muscle
• All neurons release a neurotransmitter at the end of the axon!– Acetylcholine is most common
and usually stimulating– Dopamine and serotonin are
commonly used in the brain and may be stimulating or inhibiting
– There are many others!
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That’s our show!
• See you Wednesday for more of the nervous system including the brain!