Download Neural Pathways and Transmission

Neural Pathways and
Transmission
Neural Circuits
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There are two basic responses to
stimuli coming in from the
environment
– The Reflex Arc
– The Neural Circuit
The Reflex Arc
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The reflex arc is an automatic response which
cuts out the pathway to the brain
The body responds without thinking about it
 5 basic steps to the reflex arc
Reflex Arc
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Stimulus: The receptors receive information from
the environment big enough to illicit a response
from the nervous system
Sensory Neurons: Carry information from the
receptors to the interneurons in the spinal cord
Interneurons: transmit the message through the
spinal cord immediately to the motor neurons
Motor Neurons: carry the message away from the
CNS to the peripheral nervous system to illicit a
muscle response
Muscle Response: When the effectors (muscles)
respond to the stimuli received from the motor
neurons
Ex. Hand pulls away in response to a hot burner on a
stove
REFLEX ARC
http://www.nelson.com/ABbio20-30/teacher/protect/otr/Bio2030OTR/attachments/i_AnimationSimulation/stretch_reflex_v2.html
The Neural Circuit
The neural circuit is very similar to the reflex arc,
however, the information is carried all the way
to the brain to allow for decision making and
analysis of the stimuli
 Once the information is relayed to the CNS via
the interneurons, the information continues onto
the brain
 Brain decides response and carries the message
back to the spinal cord via interneurons to be
carried to the motor neurons and illicit response
Ex. After hand has been pulled away, respond
“ouch”
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Electrochemical Impulses
But how does information travel within
the neuron to reach the CNS?
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Each neuron is polarized
This means that charged particles, called ions, line
the membrane of the neuron, causing a difference
in charge across the membrane
Potassium and sodium ions line the membrane in
unequal distribution to cause a positive exterior
and a negatively charged interior
Potassium (K+) is concentrated on the interior of
the cell, sodium (Na+) on the outside
When the neuron is stimulated, the charges
reverse, causing the message to be carried down
the neuron in one direction only
Neuron Ion Movement
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Neuron membrane has more K+
channels than Na+ channels
– Channels – facilitated diffusion
– More potassium out than sodium in
Na/K Pump
– Active transport
– Moves K+ back into the neuron
– Moves Na+ out of neuron
Resting State of the
Neuron
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At resting, sodium lines the exterior of the cell membrane,
potassium lines the interior of the cell, although not in equal
proportions
This creates an overall charge that is negative within the cell at rest
The internal environment is generally at –70 mV
Depolarization
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When the stimuli elicits a response big
enough to break the neuron’s threshold, the
neuron will transmit the message down the
axon to other neurons
This is an “all or nothing” response, firing
regardless of the strength in stimuli, or not
firing at all
If threshold is reached, the neuron will
have a response
During this period, the membrane becomes
more permeable to sodium than potassium
http://www.youtube.com/watch?v=pbg5E9
GCNVE&feature=related
Depolarization
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Ion channels specific for sodium open within the
cell membrane, allowing sodium to move into the
neuron
This causes a very temporary reversal in charges, in
which the interior is now positively charged, and
the exterior is negatively charged
Internal environment is, on average, 30 mV at this
state
This depolarization can also be referred to as the
action potential, which shows the differences in
charges from resting to depolarization
Once the maximum amount of sodium has diffused
in, the cell begins to repolarize
Repolarization
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During repolarization, the cell must
remove the sodium from the internal
environment within the cell
This is accomplished using
sodium/potassium pumps, a form of active
transport, requiring ATP
Repolarization only takes about 0.001s, in
which the cell returns to it’s resting state
During this process, the cell cannot be
stimulated again, until the resting state has
been re-established
Hyperpolarization
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For an instant after repolarization, the cell overcompensates by transporting more sodium than necessary
across the membrane, called hyperpolerization
The cell becomes slightly more negative than normal
Sodium will move back across the membrane until resting
state is returned (-70 mV)
Again, no new impulses can be fired at this point
The refractory period is the time after depolarization,
through repolarization, and to hyperpolarization
http://www.nelson.com/ABbio20-30/teacher/protect/otr/Bio2030OTR/attachments/i_AnimationSimulation/action_potential_v2.html
Action Potential of a
Myelinated Neuron
(saltatory conduction)
In myelinated axons, depolarization happens only at the nodes (A) and an
action potential jumps to the next node (B).
The red arrows show the direction of the nerve impulse and the
black arrows show the flow of ions.
http://www.nelson.com/ABbio20-30/teacher/protect/otr/Bio2030OTR/attachments/i_AnimationSimulation/saltatory_conduction.html
Neuron threshold and the All-orNone Response
The threshold level is the minimum level of a stimulus required to produce
a response – each neuron has its own threshold level - once threshold has
been met the neuron will fire
All-or-none response:
The intensity of the nerve impulse and speed of transmission remain the
same
How do we
determine
intensity of a
response???