Transcript Nerves Powerpoint

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 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
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!
Nervous System Organization
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
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)
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
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
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
Positive Ions
• Sodium and potassium are
two positively charged
particles found throughout
the body
• Neurons have sodiumpotassium 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
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
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
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
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
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?
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!
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
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
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
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
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
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!
That’s our show!
• See you Wednesday for more of the nervous
system including the brain!