Download …and now, for something completely different.

…and now, for something completely different.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Serratus Anterior…in Action!
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Basic Principles of Electricity:
Opposite charges attract
The coming together of opposite charges liberates
(releases) energy
Thus, situations in which there are separated (by a
membrane) electrical charges of opposite sign(+, -)
have potential energy. The potential, or
possibility, to release enegry.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Electricity Definitions
Voltage (V):
Measure of potential energy (volts, millivolts)
Measured between 2 points (called potential difference) or simply
potential
The greater the difference in charge between 2 points, the greater
the voltage
Current (I):
The flow of electrical charge between two points that can be used to
do work
The amount of charge that can travel between 2 points depends on:
Resistance (R) – the hindrance to charge flow (e.g. insulators
have high hindrance, conductors have low hindrance)
Voltage (V)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Electrical Current and the Body
Reflects the flow of ions rather than electrons
There is a potential on either side of membranes
when:
The number of ions is different across the
membrane
The membrane provides a resistance to ion flow
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Ohm’s Law
Current (I) = voltage (V)/Resistance (R)
Here:
I is proportional to V
I is inversely proportional to R
No net voltage (0V) means no net current
In the body, instead of electrons moving down a
copper wire, we have ions passing thru membranes
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Role of Membrane Ion Channels
Large proteins in membranes that allow ions to
pass
Gated channels change shape and open and close
in response to a specific signal
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Types of plasma membrane ion channels:
Passive, or leakage, channels – always open but
selective to the ion they let in
Chemically gated/ligand gated channels – open with
binding of a specific chemical (neurotransmitter)
Voltage-gated channels – open and close in
response to changes in membrane potential
Mechanically gated channels – open and close in
response to physical deformation of receptors (e.g.
touch or pressure receptors)
PLAY
InterActive Physiology ®:
Nervous System I: Ion Channels
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Operation of a Gated Channel
When gated ion channels are open, ions diffuse quickly across the
plasma membrane in the direction of their electro-chemical gradient,
creating electrical currents and voltage changes across the membrane
according to Ohms’s law: V+IxR
Thus, electro-chemical gradients underlie all electrical phenomena
in neurons
Example: Na+-K+ gated channel
Closed when a neurotransmitter is not bound to the extracellular receptor
Na+ cannot enter the cell and K+ cannot exit the cell
Open when a neurotransmitter is attached to the receptor
Na+ enters the cell and K+ exits the cell
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Operation of a Gated Channel
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.6a
Operation of a Voltage-Gated Channel
Example: Na+ channel
Closed when the intracellular environment is
negative
Na+ cannot enter the cell
Open when the intracellular environment is
positive
Na+ can enter the cell
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Operation of a Voltage-Gated Channel
Lys, Arg, His:
All positively charged amino acids
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.6b
Electrochemical Gradient
Ions flow along their chemical gradient when they
move from an area of high concentration to an area
of low concentration
Ions flow along their electrical gradient when they
move toward an area of opposite charge
Electrochemical gradient – the electrical and
chemical gradients taken together
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Measuring Membrane Potential
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Figure 11.7
Resting Membrane Potential (Vr)
The potential difference (–70 mV) across the membrane of
a resting neuron
It is generated by different concentrations of Na+, K+, Cl−,
and protein anions (A−)
Ionic differences are the consequence of:
PLAY
Differential permeability of the neurilemma to Na+ and K+
Operation of the sodium-potassium pump maintaining Na+
and K+ concentrations
InterActive Physiology ®:
Nervous System I: Membrane Potential
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
At Rest…
Membrane is impermeable to large anionic
cytoplasmic proteins
Membrane is slightly permeable to Na+
Membrane is 75x more permeable to K+
Membrane is freely permeable to Cl- (which
balances the Na+ & K+ charge)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Resting Membrane Potential (Vr)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.8
Membrane Potentials: Signals
Changes in membrane potential is used to communicate
signals and send information about environment by
neurons
Membrane potential changes are produced by:
Anything that changes membrane permeability to ions
Anything that alters ion concentrations across the
membrane
2 types of signals are produced by changes in membrane
potential:
Graded potentials (operate over short distance)
Action potentials (operate over long distance,
e.g. axons)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Changes in Membrane Potential
Changes are caused by three events
Depolarization: reduction in membrane potential, e.g. inside
of membrane side becomes less negative (moves closer to 0
mV) than the resting potential.
-45mV
Repolarization – the membrane returns to its resting
membrane potential
E.g. -70mV
E.g. -45mV
-70mV
Hyperpolarization – the inside of the membrane side
becomes more negative (moves further from 0mV) than the
resting potential
E.g. -70mV
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-90mV
Changes in Membrane Potential
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.9
Graded Potentials
Short-lived, local changes in membrane potential
Decrease in intensity with distance
Magnitude varies directly with the strength of the
stimulus
Triggered by change in environment (stimulus)
that causes gated ion channels to open
Essential for initiating action potentials
Sufficiently strong graded potentials can initiate
action potentials
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Graded Potentials
Shu
f
Inward flow of ions
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Shu
fflin
fl i n
g of
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ell
Figure 11.10
Graded Potentials
Voltage changes are decremental
Current is quickly dissipated due to the leaky
plasma membrane
Only travel over short distances
ESSENTIAL FOR INITIATING ACTION
POTENTIALS!!
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Graded Potentials
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Figure 11.11
Action Potentials (APs)
Long distance signalling by neurons
Only excitable membranes, e.g. neurons and muscle cells, can
generate action potentials.
A brief reversal of membrane potential with a total amplitude of
100 mV
Unlike graded potentials, APs do not decrease in strength over
distance
They are the principal means of neural communication
An action potential in the axon of a neuron is a nerve impulse
PLAY
InterActive Physiology ®:
Nervous System I: The Action Potential
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potentials (APs)
In neurons, AP is generated in the axon
In summary, stimulus changes the permeability of
the neuron’s membrane by opening specific
voltage gated ion channels on the axon
Thus, VGICs are activated by local currents
(Graded Potentials) that spread toward the axon
along the dendritic cell body membranes
Often, the transition from GP to AP occurs at the
axon hillock
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potential: Resting State
Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Each Na+ channel has two voltage-regulated gates
Activation gates –
closed in the resting
state
Inactivation gates –
open in the resting
state
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12.1
Action Potential: Resting State
Maintaining -70mV
Depolarization opens and then inactivates Na+
channels
Both must be open for Na+ to enter
Only one must be closed
to close Na+ channels
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potential: Depolarization Phase
Na+ permeability increases; membrane potential reverses
Na+ gates are opened; K+ gates are closed
Threshold – a critical level of depolarization
(-55 to -50 mV)
At threshold, depolarization becomes self-generating and
progresses by positive feedback
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12.2
Action Potential: Repolarization Phase
Slow Na+inactivation gates close
A) Membrane permeability to Na+ declines to
resting levels
AP spike stops rising
B) As sodium gates close, slow voltage-sensitive
K+ gates open
K+ exits the cell and
internal negativity
of the resting neuron
is restored (-70mV)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12.3
Action Potential: Hyperpolarization
Potassium gates remain open, causing an excessive efflux of K+
before K+ channels close
This efflux causes hyperpolarization of the membrane
(undershoot)
The neuron is insensitive to stimulus and depolarization
during this time
Na+ channels reset
Na/K pump redistributes ions
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12.4
Action Potential:
Role of the Sodium-Potassium Pump
Repolarization
Restores the resting electrical conditions of the
neuron
Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is
restored by the sodium-potassium pump
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Phases of the Action Potential
1 – resting state
2 – depolarization
phase
3 – repolarization
phase
4 – hyperpolarization
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.12
Propagation of an Action Potential
When Na+ channels close, the AP must propagate
away from that portion of membrane. How??
AP is initiated at one end of the axon and is
conducted away toward the axons terminus. How?
When Na+ channels close (and K+ channels open),
we see a repolarization event “chase” the
depolarization event.
This only occurs in unmyelinated axons.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Propagation of an Action Potential
(Time = 0ms)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.13a
Propagation of an Action Potential
(Time = 2ms)
Ions of the extracellular fluid move toward the area
of greatest negative charge
A current is created that depolarizes the adjacent
membrane in a forward direction
The impulse propagates away from its point of
origin (the origin is still repolarizing!)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Propagation of an Action Potential
(Time = 2ms)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.13b
Propagation of an Action Potential
(Time = 4ms)
The action potential moves away from the stimulus
Where sodium gates are closing, potassium gates
are open and create a current flow
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Propagation of an Action Potential
(Time = 4ms)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.13c
Threshold and Action Potentials
Threshold – membrane is depolarized by 15 to 20 mV
Established by the total amount of current flowing through
the membrane
Weak (subthreshold) stimuli are not relayed into action
potentials
Strong (threshold) stimuli are relayed into action potentials
All-or-none phenomenon – action potentials either happen
completely, or not at all
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Coding for Stimulus Intensity
All action potentials are alike and are independent
of stimulus intensity
Strong stimuli can generate an action potential
more often than weaker stimuli,
E.g. rate or frequency of stimuli
Amplitude does not increase (always constant)
The CNS determines stimulus intensity by the
frequency of impulse transmission
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Stimulus Strength and AP Frequency
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.14
Absolute Refractory Period
Time from the opening of the Na+ activation gates
until the closing of inactivation gates
The absolute refractory period:
Prevents the neuron from generating an action
potential
Ensures that each action potential is separate
Enforces one-way transmission of nerve impulses
PLAY
InterActive Physiology ®:
Nervous System I: The Action Potential, page 14
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Absolute and Relative Refractory Periods
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.15
Relative Refractory Period
The interval following the absolute refractory period
when:
Sodium gates are closed
Potassium gates are open
Repolarization is occurring
The threshold level is elevated, allowing strong stimuli
to intrude into the relative refractory period and
increase the frequency of action potential events
PLAY
InterActive Physiology ®:
Nervous System I: The Action Potential, page 15
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Conduction Velocities of Axons
Conduction velocities vary widely among neurons
Rate of impulse propagation is determined by:
Axon diameter – the larger the diameter, the faster
the impulse
Presence of a myelin sheath – myelination
dramatically increases impulse speed
PLAY
InterActive Physiology ®:
Nervous System I: Action Potential, page 17
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Saltatory Conduction
Current passes through a myelinated axon only at
the nodes of Ranvier
Voltage-gated Na+ channels are concentrated at
these nodes
Action potentials are triggered only at the nodes
and jump from one node to the next (about 1mm)
30x faster than conduction along unmyelinated
axons
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Saltatory Conduction
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.16
Multiple Sclerosis (MS)
An autoimmune disease that mainly affects young
adults
Symptoms: visual disturbances, weakness, loss of
muscular control, and urinary incontinence
Nerve fibers are severed and myelin sheaths in the
CNS become nonfunctional scleroses
Shunting and short-circuiting of nerve impulses
occurs
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Multiple Sclerosis: Treatment
The advent of disease-modifying drugs including
interferon beta-1a and -1b, Avonex, Betaseran, and
Copazone:
Hold symptoms at bay
Reduce complications
Reduce disability
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings