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Lectures 9 and 10

Terms in this set (86)

membrane potentials
Separation of positive and negative charges across the plasma membrane-- the difference in the RELATIVE NUMBER of cations and anions in ICF and ECF. Like charges repel each other, opposite charges attracted to each other.
Energy is required to separate opposite charges - When oppositely charged particles separated, electrical force of attraction between them can be harnessed to perform work when they come together again.
What are membrane potentials related to?
They are related to the uneven distribution of Na+, K+, and intracellular protein anions (A-) between the ICF and ECF. They are due to the differential permeability of the plasma membrane to these ions.
How are separate charges measured, and what can they do?
The difference in charge between thin regions of the ICF and ECF lying next to the inside and outside of the membrane has the ability to do work, and it is measured in millivolts (mV).
Describe the magnitude of the potential.
The magnitude of the potential is directly proportional to the number of positive and negative charges separated by the membrane. In this image, B has more potential than A and less potential than C.
Concentration gradient of Na+
Na+ is more concentrated in the ECF so its concentration gradient is always inward.
Concentration gradient of K+
K+ is more concentrated in the ICF so the concentration gradient is always outward.
The membrane has more K+ leak channels. The resting membrane is more permeable to K+ than Na+.
Electrical gradient for Na+ and K+
The electrical gradient for Na+ and K+ will always be toward the negatively charged side of the membrane due to the positive charge of these ions.
Na+/K+ pump
Requires ATP. Transports three Na+ ions out for every two K+ ions in to maintain membrane potential at all stages of the action potential. Unequal transport separates charges across the membrane. Outside becomes relatively more positive, inside becomes more relatively negative. The pump is important because it will help bring us back to our resting potential by stabilizing the inside and outside and keeping those charges separated.
Maintenance of membrane potentials
Active transport results in a membrane potential of 1mV to 3mV. The majority of the membrane potential is a result of passive diffusion of Na+ and K+ down their concentration gradients.
equilibrium potential
The potential that exists when the concentration gradient and opposing electrical gradient for a given ion is counterbalanced. Results in no net movement of the ion. Equilibrium potential for K+ = -90mV; equilibrium potential for Na+ = +60mV.
equilibrium potential: K+
Electrical gradient equals concentration gradient.
equilibrium potential: Na+
Electrical gradient equals concentration gradient.
Resting potential
Nernst equation
Don't forget units on exam! Equilibrium potential for a given ion with differing concentrations across a membrane. Allows you to measure the electrical potential of solutions of different concentrations. The larger the concentration gradient for a given ion, the greater the ion's equilibrium potential.
Goldman-Hodgkin-Katz Equation
Don't forget units on exam! Equilibrium potentials only exist under hypothetical/ experimental conditions. Takes into account relative permeability and concentration gradients of all permeable ions. Allows you to directly calculate the membrane potential.
Which cells display membrane potentials?
All cells display a membrane potential. Neurons and muscle cells have developed specialized use for membrane potentials. Electrical signals are produced when excited. Transient, rapid fluctuations in membrane potential produces nerve impulses in nerve cells and triggers contraction in muscle cells.
Changes in membrane potential
Occur as a result of changes in ion movement across the membrane. There is a triggering event that changes ion permeability.
Leak vs. gated channels
Water-soluble ions cannot penetrate the membrane's lipid bilayer. These ions must move via channels.
Leak channels are open all the time and permit unrestricted leakage of specific ions.
Gated channels permit ion passage via gate that can be open or closed (include voltage-gated channels, chemically gated channels, mechanically gated channels, thermally gated channels).
voltage-gated channels
Channels in the plasma membrane that open or close in response to changes in membrane potential.
chemically gated channels
Channels in the plasma membrane that open or close in response to the binding of a specific chemical messenger with a membrane receptor site that is in close association with the channel.
mechanically gated channels
Channels that open or close in response to stretching or other mechanical deformation.
thermally gated channels
Channels that respond to local changes in temperature.
Anatomy of a neuron
Graded and action potentials on a neuron
Graded potentials
Graded potentials are local changes in membrane potential that occur at varying grades of degrees of magnitude or strength. They are confined to small, specialized regions of the total membrane.
They are produced by a specific TRIGGERING EVENT that causes gated ion channels to open. The resulting ion movement produces a graded potential.
What does Na+ entering the membrane result in
A depolarization.
How strong are graded potentials?
The magnitude of the triggering event influences the strength of the graded potential. The stronger the triggering event, the more voltage-gated Na+ channels open. The more positive the cell becomes (because of Na+ entering the cell), the less negative (and more depolarized) this specialized region becomes. This depolarization is the graded potential.
What is the duration of the graded potential?
The longer the duration of the triggering event, the longer the duration of the graded potential.
Spread of graded potentials
Since a graded potential occurs in a localized region, the rest of the membrane remains at the resting potential. The temporary depolarized region is the active area.
Current
Current is the flow of electrical charges. The current flows in the direction in which the positive charges are moving. The current spreads along the membrane in both directions.
Loss of current
Loss of current results in loss of magnitude progressively diminishing as it moves from the initial active area.
Why are graded potentials important?
They discriminately initiate action potentials. They are important physiologically with post-synaptic potentials, receptor potentials, end-plate potentials, pacemaker potentials, slow-wave potentials.
post-synaptic potentials
Postsynaptic potentials are graded potentials, and should not be confused with action potentials although their function is to initiate or inhibit action potentials. They are caused by the presynaptic neuron releasing neurotransmitters from the terminal bouton at the end of an axon into the synaptic cleft.
receptor potentials
A receptor potential is a graded response to a stimulus that may be DEPOLARIZING or HYPERPOLARIZING. Receptor potentials have a threshold in stimulus amplitude that must be reached before a response is generated, and their amplitude saturates in response to intense stimuli.
end-plate potentials
A chemically induced change in electric potential of the motor end plate, the portion of the muscle-cell membrane that lies opposite the terminal of a nerve fibre at the neuromuscular junction.
pacemaker potentials
SA nodal action potentials are divided into three phases. Phase 4 is the spontaneous depolarization (pacemaker potential) that triggers the action potential once the membrane potential reaches threshold between -40 and -30 mV).
slow-wave potentials
These cells cause spontaneous cycles of slow wave potentials that can cause action potentials in smooth muscle cells.
action potentials
An action potential is a brief, rapid, large change in membrane potential.
Action potential-- 100mV
Graded potential-- 15mV
They are propagated (transmitted or conducted) along the axon's membrane without loss of magnitude-- go long distances.
Describe an action potential.
A rapid, brief reversal of the axon's membrane. At peak of action potential, Na+ voltage-gated channels begins to slowly close.
An action potential is a brief reversal of membrane potential characterized by a chemical cascade of sodium rushing in, causing depolarization, and potassium rushing out of the cell membrane along the axon, which causes repolarization.
Describe membrane permeability and ion movement in relation to an action potential.
Changes in membrane permeability and ion movement lead to action potential. The resting membrane is more permeable to K+. Rapid fluxes of Na+ and K+ down their electrochemical gradient is mainly due to opening and closing of two highly sensitive voltage membrane channels (voltage-gated Na+ channels and voltage-gated K+ channels.
Voltage-gated Na+ channels
Activation gate: guards channel interior; slides open/closed.
Inactivation gate: amino acid chain opening to ICF. Ball and chain.
Three conformations of Na+ channels.
Volatage-gated K+ channels
Activation gate: can either slide open or closed. The voltage-gated K+ channels respond slowly to depolarization. they begin to open as the membrane depolarizes, but respond so slowly that they become fully activated only after the action potential reaches its peak. As K+ moves out, depolarization ends and the positive feedback loop is broken.
How much more permeable is the membrane to K+ than to Na+
25-30 times more because there are more K+ leak channels and fewer Na+ leak channels.
Resting potential
Resting membrane potential is -70mV. Na+ concentration is greater inside the axon than outside. All voltage-gated channels are closed.
Na+ voltage-gated channel activation gate closed; inactivation gate open (closed but capable of opening)
K+ voltage‐gated channel activation gate closed
Depolarization
Na+ influx. A graded potential triggers an action potential.
Membrane depolarizes toward threshold: -50 to -55 mV
Causes activation gate of the Na+ voltage‐gated channels opens.
Results in Na+ rushing into the cell.
Membrane becomes 600x more permeable to Na+. The ICF becomes less negative, reaching +30mV (near equilibrium potential of Na+).
Repolarization
K+ efflux. Inactivation gate of Na+ voltage-gated channel closes.
Membrane permeability to Na+ declines.
K+ voltage-‐gated channels open; membrane becomes more permeable to K+ (about 300x).
K+ rapidly exits the cell down its electrochemical gradient (positive charges carried out).
Inside of the cell becomes more negative.
Na+ voltage-gated channels are closed, but capable of opening conformation (activation gate closed; inactivation gate open).
Hyperpolarization
K+ voltage-gated channels are slow to close-- resulting in prolonged K+ permeability.
More K+ leaves than is necessary to bring cell back to resting potential.
The inside of the cell briefly becomes more negative than normal resting potential (about 80 mV).
Once K+ voltage-gated channels close, membrane returns to resting potential until Na+/K+ ATPase pump restores ions to original location.
Na+/K+ ATPase pump maintains ion concentration long-term. Is not directly involved in ion fluxes or changes that occur during an action potential .
Signal conduction
Once an AP has been initiated, the impulse is automatically conducted throughout the neuron without additional stimulation. Two types: contiguous conduction and saltatory conduction.
contiguous conduction
Action potential physically propagates down the length of an unmyelinated axon - AP initiated in axon hillock with current spreading.
Like the 'wave' at a stadium.
saltatory conduction
Current only flows along the nodes of Ranvier to produce an AP. Current exposed to ECF.
Concentrated voltage‐gated Na+ and K+ channels between Nodes of Ranvier.
Impulse 'jumps' from node to node - Skipping myelinated regions.
Faster (50x faster than contiguous conduction).
Conserves energy since Na+/K+-‐ATPase pump active only at nodal regions.
Why are refractory periods important?
They ensure one-way propagation of action potential
Absolute refractory period
Under no circumstances can a membrane undergo another AP until it has recovered from the depolarization event.
Once Na+ voltage-gated channels open at threshold, they cannot open again in response to another depolarizing triggering event until they reset at 'closed, but capable of opening' conformation.
Relative refractory period
Following the absolute refractory period is the relative refractory period-- the period of time when the membrane can be stimulated prior to full recovery of previous action potential by a stronger than normal stimulus
Describe refractory periods
All-or-None Law
Action potentials occur maximally in response to stimulation - Or not at all.
Variable strengths of stimuli (e.g., warm vs. hot) are coded by differing frequency of AP - Not a larger action potential.
A stronger stimulus causes more neurons to reach threshold - Increases information sent to CNS
What determines an action potential's speed?
Once an action potential has been initiated, the speed at which an action potential travels down the axon depends on whether the fiber is myelinated and the diameter of the fiber.
What is myelination?
Axons are covered in myelin: a thick layer of lipids that wrap around axon along the length of the axon in intervals. Acts as an insulator since ions are water-soluble.
How does fiber diameter affect current?
Conduction velocity is the speed with which an action potential is propagated. It depends on the diameter of the axon and how well the axon is insulated with myelin. Increased fiber diameter decreases current resistance. The larger the fiber diameter, the faster propagation of the action potential.
Large myelinated fibers (like the ones found in skeletal muscles) conduct up to 120m/sec (268mph).
Small unmyelinated fibers (like the ones found in the digestive tract) conduct up to 0.7m/sec (2mph).
What happens at the axon terminal?
When the action potential reaches the axon terminal, a chemical messenger is released that alters the activity of the cell.
Neurons INNERVATE when they terminate on a muscle (contraction) or a gland (secretion).
Neurons SYNAPSE when they terminate on another neuron (convey electrical signal along nerve pathways).
What is a synapse?
A synapse is a junction between two neurons. Two types: electrical synapses and chemical synapses.
Electrical synapses
Via gap junctions. Allow ions to pass. They are unregulated and rare. Ex: retina, pulp of tooth.
Chemical synapses
There is a presynaptic neuron and a postsynaptic neuron. A single neurotransmitter is released (acetylcholine, norepinephrine, epinephrine, histamine, dopamine).
Postsynaptic potentials
Presynaptic neuron releases only one neurotransmitter - acetylcholine, norepinephrine, epinephrine, histamine, dopamine.
Different neurotransmitters cause different ion permeability changes.
Kinds of synapses
Two types of synapses: excitatory synapses and inhibitory synapses.
EPSP
If NT opens chemically-gated Na+ and K+ channels, there will be a simultaneous permeability of both ions that leads to a small depolarization. The excitatory post-synaptic potential increases the likelihood that threshold will be reached.
IPSP
If NT opens chemically-gated K+ or Cl- channels, the permeability of K+ or Cl- will be increased making ICF more negative relative to ECG leading to a small hyperpolarization. The inhibitory post-synaptic potential decreases the likelihood that threshold will be reached.
Synaptic summation
EPSPs and IPSPs are graded potentials. unlike the action potential all-or-none response, graded potentials vary in magnitude and have no refractory period. Graded potentials can be summed. Grand postsynaptic potential takes into account all EPSP and IPSP.
Temporal summation
EPSP from a single presynaptic input that is repetitively firing so close together they summate.
Spatial summation
EPSP from different presynaptic inputs summate.
IPSPs and summation
IPSPs can undergo both temporal and spatial summation-- progressively moving potential further from threshold.
Cancellation
If EPSP and IPSP simultaneously activates, they cancel one another out. In most cases, this results in postsynaptic membrane remaining at resting potential.
Summary of action potentials
A stimulus greater than threshold will send an action potential down the axon by depolarizing the membrane. Sodium rushes in and potassium rushes out. This all or none event acts like a domino effect along the length of the axon to the synaptic terminal. Once it reaches the synaptic terminal, a change in membrane permeability allows calcium to rush into the axon terminal, causing vesicles of acetylcholine to line up along and fuse with the presynaptic membrane and burst. This flood of acetylcholine across the synapse causes the continuation of the message. In the meantime, the axon has to prepare for the next stimulus, so it re-establishes resting membrane potential by pumping the sodium back out and the potassium back into the axon. Then it is ready to send the next impulse.
Location of chemically gated channels
For the most part, chemically gated channels are located on the dendrites and the cell body.
Location of nongated channels
Nongated channels are located in the cell membrane on the dendrites, the cell body, and the axon.
Location of voltage-gated channels
For the most part, voltage-gated channels are found on the axon hillock, all along unmyelinated axons, and at the nodes of Ranvier in myelinated axons.
Function of nongated channels
Nongated channels are responsible for the resting membrane potential (K+ is the ion that most affects resting membrane potential).
Function of chemically gated channels
Chemically-gated channels are responsible for synaptic potentials, the incoming signals to the neuron.
Function of voltage-gated channels
Voltage-gated channels are responsible for generation and propagation of the action potential, the outgoing signal from the neuron.
Function of cell body of neuron
The cell body is the main nutritional and metabolic region of the neuron. Like the dendrites, it receives signals from other cells and sends them toward the axon. Also sums up or integrates the incoming signals.
Function of the dendrites of neuron
The branched dendrites rece3ive signals coming in from other cells and send them toward the axon. Also sums up or integrates the incoming signals.
Function of the axon
The axon generates an action potential and conducts it to the next cell.
What force opposes the diffusion of K+ out of the cell?
The electrical potential. There is less K+ outside the cell, but K+ will not diffuse out because of its electrochemical gradient. Chemical gradient wants K+ to go out; electrical gradient pulls K+ into the cell. The electrical potential across the cell membrane is the membrane potential. There is a small leak of K+ to the exterior of the cell because of the concentration gradient, but the Na+/K+ pump counterbalances the rate of passive leakage.
How is Cl- transported across the membrane?
The membrane potential is responsible for driving the distribution of Cl- across the membrane. Most cells are highly permeable to Cl- but have no active-transport mechanisms for this ion. With no active forces acting on it, Cl- passively distributes itself to achieve an individual state of equilibrium. In most cells, Cl- does not influence resting membrane potential; instead, membrane potential passively influences the Cl- distribution.
At resting potential, how do Na+ and K+ move?
At -70mV, K+ leaks out of the neuron, and Na+ leaks into the neuron. If the ion leaks continue, the concentration gradients for Na+ and K+ will decrease and the membrane potential will move toward zero. Neurons can prevent the K+ and Na+ gradients from running down by transporting K+ back into the cell and Na+ back out of the cell. Na+/K+ pump does the transporting. Keep in mind that the Na+/K+pump does not create the membrane potential; its job is to maintain it.
Threshold
If the stimulus to the axon hillock is great enough, the neuron depolarizes by about 15mV and reaches a trigger point called threshold. An action potential is generated at threshold. Weak stimuli that do not reach threshold do not produce an action potential. When, and only when, a neuron reaches threshold, a positive feedback loop is established. At threshold, depolarization opens more voltage-gated Na+ channels. This causes more sodium to flow into the cell, which in tun causes the cell to depolarize further and opens still more voltage-gated sodium channels.
What breaks or interrupts the positive feedback loop of the action potential?
Two processes:
1. The inactivation of the voltage-gated Na+ channels.
2. The opening of the voltage-gated K+ channels.
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