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Chapter 12 - Neurophysiology 1 Chapter 12 Neurophysiology pp. 388 – 389; 398 - 412 2 SECTION 12-2, Page 388 - 389 Neurons are nerve cells specialized for intercellular communication
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Chapter 12!
Neurophysiology! pp. 388 – 389; 398 - 412!
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SECTION 12-2, Page 388 - 389!Neurons are nerve cells specialized for intercellular communication!
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Neuron Anatomy Figure 12-2!
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Parts of a Neuron (1 of 3)!
Cell body (soma)!Cell membrane!• Has different types of membrane channels!
Cytoplasm!• Perikaryon = cytoplasm surrounding nucleus!• Neurofilaments, neurotubules = cytoskeleton!• Nissl bodies!
RER and free ribosomes → Gray matter!
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Parts of a Neuron (2 of 3)!
Dendrites!• Receive information from other neurons!• Carry information towards cell body!• Transmit graded potentials, not action
potentials (usually)!Axon!• Axolemma, axoplasm!• Connects to soma at axon hillock!v First part = initial segment
Initial segment generates action potentials
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Parts of a Neuron (3 of 3)!
Axon collaterals!• Major branches of an axon!
Telodendria!• Small branches at the end of an axon!
Synaptic terminals - ends of the telodendria!• a.k.a. boutons, synaptic end bulbs, synaptic
knobs!• Store neurotransmitter in synaptic vesicles!• Release neurotransmitter in response to
electrical activity!
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Structure of a Synapse Figure 12-3!
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SECTION 12-4, page 398 !The transmembrane potential is the electrical potential of the cell’s interior relative to its surroundings!
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Transmembrane potential = Electrochemical gradient!• “Potential” = voltage difference across a
membrane!• Arises from the sum of all chemical and
electrical forces acting across the cell membrane!
• Usually reported in millivolts (mV)!• Inside is negative, outside positive!
The Transmembrane Potential (1 of 2)!
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Factors determining transmembrane potential!Flux = P • ΔC
1. Ion concentration differences (ΔC)!2. Sodium-potassium pump (maintains ΔC)!3. Membrane permeability differences for ions!
Membrane channel types:!a. Leak channels!b. Gated channels!
4. Fixed anions (non-diffusible anions; P ≈ 0)!• Mostly negatively-charged proteins and phosphate!
The Transmembrane Potential (2 of 2)!
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1. Resting (membrane) potential!• Voltage difference across the cell membrane
for an unstimulated (“resting”) cell!2. Graded potentials!• Local changes in membrane potential due to
chemical or physical changes in the membrane!
• Do not self-regenerate or spread over long distances!
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Types of Membrane Potentials (1 of 3)!
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Types of Membrane Potentials (2 of 3)!!3. Action potentials!• Self-regenerating changes in membrane
potential due to chemical or physical changes in the membrane!
• Spread over long distances!
In order to understand why the transmembrane potential changes during graded and action potentials, you must understand the equilibrium potential. Next slide.!
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Types of Membrane Potentials (3 of 3)!
4. Equilibrium potential (for a particular ion)!
• The equilibrium potential is the membrane voltage at which the electrical forces and the concentration difference forces acting on an ion are equal.!
• No net diffusion of the ion occurs at this membrane potential.!
• Understanding this concept is REALLY, REALLY important.
Discussion begins on Slide 20.!
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Resting Potential Spotlight Figure 12-9!
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Resting Membrane Potential (1 of 5)!
• Concentration differences!• Na+/K+ ATPase pump!• Permeability differences!• Electrical potential difference ! across membrane!• Overall charges!
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Resting Membrane Potential (2 of 5)!
• Concentration differences!• Na+/K+ ATPase pump!• Permeability differences!• Electrical potential difference ! across membrane!• Overall charges!
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Resting Membrane Potential (3 of 5)!
• Concentration differences!• Na+/K+ ATPase pump!• Permeability differences!• Electrical potential difference ! across membrane!• Overall charges!
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Resting Membrane Potential (4 of 5)!
• Concentration differences!• Na+/K+ ATPase pump!• Permeability differences!• Electrical potential difference ! across membrane!• Overall charges!
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Resting Membrane Potential (5 of 5)!
• Concentration differences!• Na+/K+ ATPase pump!• Permeability differences!• Electrical potential difference ! across membrane!• Overall charges!
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Resting Potential Spotlight Figure 12-9!
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Equilibrium Potential!
Definition: The membrane potential at which the electrical and concentration difference forces acting on a particular ion are equal.!
ENa+ = (-58mV) log [Na+]inside = +66 mV! [Na+]outside!
EK+ = (-58mV) log [K+]inside = -90 mV! [K+]outside!
Eion= (-58mV) log [ion]inside = ? mV! [ion]outside!
This results in NO NET MOVEMENT of the ion across the membrane.!
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Equilibrium Potential for K+ Figure 12-10a,b!
Looking only at K+; ignore other ions.!
Resting membrane !Imagine that the
membrane suddenly becomes freely permeable to
K+…!
Equilibrium potential about -90 mV;!
no net diffusion of !K+. Electrical and
concentration forces equal.!
Net electrochemical gradient when
membrane suddenly
becomes leaky to K+!
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Equilibrium Potential for Na+ Figure 12-10c,d!
Looking only at Na+; ignore other ions.!
Resting membrane!Imagine that
membrane suddenly becomes freely
permeable to Na+…!
Equilibrium potential about +66 mV;!
no net diffusion of Na+. Electrical and
concentration forces equal.!
Net electrochemical gradient when
membrane suddenly
becomes leaky to Na+!
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Membrane at rest is “polarized”!• Ion flow can cause changes in potential.!• Inside of membrane becomes more
positive = “depolarized”!• Inside of membrane becomes more
negative = “hyperpolarized”!
Changes in the Transmembrane Potential!
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Membrane Channel Types!
Ions cross the membrane through:!1. Leak channels - always open!2. Gated channels - open or closed!
a. Voltage-gated channels!b. Chemically-gated (ligand-gated) channels!c. Mechanically-gated channels!
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Leak Channels (Passive Channels)!
Always open • Important for establishing resting potential!• Ions “leak” down their electrochemical
gradients!e.g. K+ leak channels, Na+ leak channels!
• Size, charge, etc. determine which ion(s) can pass through a channel!
• Determine resting permeabilities for membrane!E.g. PK+ at rest, 50–100x greater than PNa+!
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Gated Channels!
A.K.A. active channels (does not refer to ATP use)!Can exist in three states:!• Open (activated) !• Closed and cannot be opened (inactivated) !• Closed, but can be opened!
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Chemically-gated (Ligand-gated) Channels!
• Open after binding a specific chemical (ligand)!
• Most abundant on cell body, dendrites and motor end plate!
Binding of ACh changes shape of receptor.!• Channel becomes permeable to small ions
like Na+ and K+.!!Which ion will move most readily through this channel?!What will be the effect on the membrane potential?!
Acetylcholine receptor!
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Voltage-gated Channels Figure 12-11b!
Channel opens in response to changes in membrane potential - threshold!
• Important in action potential conduction, neurotransmitter release from end bulbs!
E.g. voltage-gated K+, Na+ and Ca2+ channels!
Resting membrane;!Closed, but can open!
Open! Closed and inactivated;!cannot be opened!
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Mechanically-gated Channels Figure 12-11c!
Open or close in response to physical distortion.!e.g. touch, pressure receptors!
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Gated Channels - Summary Figure 12-11!
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A change in membrane potential that decreases with distance!
Caused by ions entering cell through channels!• Local depolarization or hyperpolarization!• Does not spread very far from site of stimulus
(unlike action potential)!• Does not involve voltage-gated channels
Why don’t graded potentials travel very far?!• Cytoplasm resists ion flow!• The cell membrane is LEAKY TO IONS!
Graded Potentials!
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Graded Potentials Figure 12-12!
Neurotransmitter
Leak channels present, but not shown!!Chemically-gated Na+ channels present!!Voltage-gated Na+ channels absent
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Depolarization and Hyperpolarization Fig. 12-13!
DepolarizationInside more positive than at rest!e.g. Na+ enters cell!
HyperpolarizationInside more negative than at rest!e.g. K+ leaves cell!e.g. Cl- enters cell!
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Graded Potentials – Summary Table 12-3!
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SECTION 12-5 !An action potential is a nerve impulse
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A sudden major change in membrane potential !• An all-or-none phenomenon:!
Either happens or does not happen!• Occurs when membrane reaches a specific
membrane voltage called threshold !• Does not degrade over long distances
(unlike graded potentials)!• Depends upon the presence of voltage-
gated Na+ and K+ channels!• At threshold, voltage-gated Na+ channels
open!
Action Potentials - Introduction!
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Action Potential Recording (Table 12-3: 8th edition)!
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Action Potential Recording Figure 12-14!
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Key to Action Potential Figures Figure 12-14!
Voltage-gated channels!
K+ channelActivation
gate!
K+
Na+
Na+ channel• Activation
gate!!• Inactivation
gate!
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The Generation of an Action Potential!
Summary Figure! Figure 12-14!
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Voltage-gated Channels - Resting Membrane!
Resting membrane!Voltage-gated Na+ channels• Activation gate closed!• Inactivation gate open!!Voltage-gated K+ channels• Closed!!Leak channels for both ions are open and are unaffected by the voltage changes to come.!
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Voltage-gated Channels - “Step 1”!
Local depolarization begins !e.g. chemically-gated Na+ channels have opened nearby.!!Local current flow depolarizes membrane towards threshold, but threshold has not been reached. (Note that the number on my voltmeter is -65, not -60, as in text.)!Voltage-gated Na+ channels:• Activation gates closed• Inactivation gates open!Voltage-gated K+ channels:• closed
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Voltage-gated Channels - “Step 2”!
Threshold was reached at-60 mV.!Voltage-gated Na+ channels:• Activation gate open• Inactivation gate open• Na+ entry is depolarizing! cell membrane!!Voltage-gated K+ channels:• closed
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Voltage-gated Channels - “Step 3”!
Membrane depolarized to +30mV!Voltage-gated Na+ channels:• Activation gate open• Inactivation gate closed• No further influx of Na+!!Voltage-gated K+ channels:• Change in voltage causes ! these to open• K+ efflux begins repolarization!
Step 3
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Voltage-gated Channel - “Step 4”!Re- and Hyperpolarization!Membrane potential greater (more negative) than at rest!!Voltage-gated Na+ channels:• Activation gates closed• Inactivation gates back open!Voltage-gated K+ channels:• Open until voltage reaches
about -90 mV!• Continued efflux of K+ until
gates close causes hyperpolarization!
Step 4
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Voltage-gated Channels - Back to Rest!
Back to resting conditions!Voltage-gated Na+ channels:• Activation gate closed• Inactivation gate open!Voltage-gated K+ channels:!• closed!Note that leak channels have remained open throughout this process!
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Action Potential–Continuous Propagation!Figure!12-15!
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Action Potential–Saltatory Conduction!Figure!12-15!
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Graded vs. Action Potentials Table 12-3!
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Action Potentials and Muscle Cells!
Motor end plate = muscle cell membrane at neuromuscular junction!• Contains few voltage-gated Na+ channels!• Does not generate action potentials (graded
potentials only)!• Local current flow spreads to adjacent
sarcolemma where action potentials are produced.!
• About 100 vesicles, each containing 100,000 ACh molecules, are released into synapse to produce muscle action potential.!
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Cholinergic Synaptic Activity Figure 12-16!
