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Chapter 15:Coordination
Part 2
Transmission of Nerve Impulses
• Neurons transmit electrical impulses
• Impulses travel rapidly along the cell
surface membrane from one end of the cell
to the other
• NOT a flow of electrons (electric current is
flow of electrons)
• Nerve impulses
• Brief changes in the distribution of electrical
charge ACROSS the cell surface membrane
• Called ACTION POTENTIALS
• Caused by rapid movement of sodium ions and
potassium ions INTO and OUT of an axon
Terms to Know
• Electric potential or “Potential”: when separated electrical charges of opposite signs
have potential of doing work if they are allowed to come together
• Resting Potential: when the inside of the cell membrane has a slightly more negative
electric potential than the outside of the cell membrane; usually -60mV to -70mV
• Action Potential: brief change in distribution of electrical charge across the cell
membrane caused by rapid movement of ions in & out of the cell membrane
• Depolarization: when the resting potential is LESS negative (more positive) than 70mV; caused by influx of positive ions INTO the cell; positive feedback mechanism.
• Repolarization: Event that occurs when potassium ions move OUT of the cell,
returning the resting membrane potential to -70mV; Resting potential briefly becomes
more negative than normal resting potential
• Refractory Period: Recovery period in an axon when the axon is unresponsive to
another stimulus; when an axon is recovering from an action potential and voltagegated Na+ ion channels are tightly shut and cannot be stimulated to open
• Threshold Potential : the value at which an action potential can be generated; when
the membrane potential difference is -60mV to -50mv.
Resting Potential
• Resting axon  always has slightly
NEGATIVE electrical potential compared
with the outside of the axon
• Potential Difference
• Difference between the electrical potential
inside the cell compared to the outside of
the cell
• Usually between -60mV and -70mV
(DIFFERENCE)
• Electrical potential of INSIDE of axon is
between 60 to 70 mV LOWER than the
outside (MORE NEGATIVE)
• RESTING POTENTIAL
• Difference between the inside electrical
potential and the outside electrical potential
of an axon
Important transmembrane proteins
All of these neuron transmembrane proteins are found only in the UNMYELINATED portions of the axon
(Nodes of Ranvier)
• Na-K Protein Pumps
• Globular
• Requires ATP
• Maintains resting potential
• K+ ion protein channel
•
•
•
•
Much ore numerous than Na+ channels
Open all the time
“leaky potassium channels”
K+ ions flow through here according to concentration gradient
• Na+ ion protein channel
• Not very effective
• Not very numerous in number
• Not an efficient way for Na+ ions to go through
• Voltage-gated K+ channels
•
•
•
•
Closed during resting potential
Slow opening
Slow closing
Only active when membrane depolarizes
• Voltage-gated Na+ channels
•
•
•
•
Closed during resting potential
Fast opening
Fast closing
Inactivate spontaneously
Role of Sodium-Potassium Pumps
• Produces and maintains the resting potential of a neuron
• Na-K pumps constantly move 3 Na+ ions out of axon and pump 2 K+
ions into axon
• Review Na-K Pump
• Membrane proteins
• Globular, transmembrane
• Require ATP
• Use energy from hydrolysis of ATP
• Has a SPECIFIC binding site for ATP
• Use active transport
• Moves ions AGAINST their gradient
• Rely on electrochemical gradient
• Why does cell membrane stay more negative?
• Cell membrane has potassium and sodium channels that are ALWAYS open
• More POTASSIUM CHANNELS than sodium channels
• K+ ions diffuse out of cell very quickly (faster than Na+ ions)
• Many large, negatively charge molecules inside the cell that attract K+ ions
• Decreases chance that K+ will diffuse out
• Inside of cell is left with overall EXCESS of negative ions inside the cell
membrane compared to the outside
What influences movement of Na+ ions into the
cell during action potential (nerve impulse)?
• Two things:
1. Steep concentration gradient
2. Inside of cell membrane is negatively
charged
• These two things create a double
gradient we call an
ELECTROCHEMICAL GRADIENT
Two Things that influence the INWARD movement
of Na+ ions into a cell during an ACTION
POTENTIAL
1. Steep concentration gradient
• During action potential, lots of Na+ ions outside, les inside….so Na+ ions want
to go down concertation gradient, back INTO cell
2. Electrical gradient
• More negative potential inside the cell than outside the ccell…negative
interior attracts the positive ions from outside
• This double gradient is called the ELECTROCHEMICAL GRADIENT
Things to remember about charges…
• Like charges REPEL each other
• + charge will try and move farther way from other positive charges
• This is important in transmission of action potentials
• Creates LOCAL CIRCUIT
• Created by temporary depolarization of cell membrane at site of action potential
• Created between the depolarized region and resting region of a specific area of an axon
cell membrane
• Unlike charges are attracted to each other
• + charge is attracted to – charge
How an Action Potential is Generated: Step 1
• Electric current stimulates axon
• This comes from neurotransmitters being released
• Receptor cells are stimulated
• These are cells that respond to stimulus by initiating action potential
• They are transducers convert energy from one form into energy in an electrical impulse in a neuron)
• Found in sense organs
• Two types of receptor cells:
• Can be the end of a sensory neuron
• Can be a specialized cell (such as light receptor or chemical receptor cells) that detect a specific stimulus and influence the activity of a sensory neuron
How an Action Potential is Generated: Step 2
• Stimulation of axon causes
opening of voltage-gated Na+
channels
• Lots of Na+ flow into cell
• This causes DEPOLARIZATION
(membrane potential inside
the cell is less negative/more
positive than outside)
How an Action Potential is Generated: Step 3
• Depolarization at one region of
membrane creates local circuit
(positive ions move away from
area where they are all rushing
in)
• Creates further depolarization at
an adjacent region of membrane
= more voltage-gated Na+
channels opening up
• POSITIVE FEEDBACK MECHANISM
• Potential difference must reach 50 mV (from -70 mV, resting
potential) for more channels to
open
• This is the THRESHOLD POTENIAL
• Action potential is only generated
when THRESHOLD VALUE is
reached (-60mV to -50mV)
How an Action Potential is Generated: Step 4
• After 1 millisecond…
• ALL voltage-gated Na+ channels close
(tightly shut)
• Na+ STOPS diffusing into the cell
• Voltage-gated K+ ion channels OPEN
• K+ ions diffuse OUT of cell, down the
concentration gradient
• When K+ ions leave, the inside of the cell
becomes LESS POSITIVE/more negative (getting
closer to resting potential)
• This is called REPOLARIZATION
• Membrane potential actually becomes slightly
more negative than -70mV and then returns
back to -70MV
• The time it takes the membrane to be
returning to resting potential is called
REFRACTORY PERIOD
• Axon cannot generate another action potential
until this is complete
How an Action Potential is Generated: Step 5
• Voltage-gated K+ channels CLOSE
• Voltage-gated Na+ channels become
responsive again
• Na-K pump continues pumping out
Na+ ions and in K+ ions, maintaining
resting potential (awaiting next
action potential)
Significance of Refractory Period
• Action potentials NEVER merge
together
• Discrete, individual events
• Minimum time between action
potentials occurring at any one place
on the neuron
• Length of refractory period
determines maximum frequency at
which impulses are transmitted on
neuron