BI0005 - Lecture 3 - nerves 3
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What are rapid communication pathways?
Rapid communication pathways comprise nerves which transmit information rapidly by way of nerve impulses or action potentials to or from the central nervous system
What are the approximate concentrations of Na+, K+ and Cl- across a mammalian nerve cell membrane?
- Na+: 15 mM inside, 150 mM outside
- K+: 150 mM inside, 5 mM outside
- Cl-: 10mM inside, 110 mM outside
mM is millimoles per liter
What is a membrane potential?
A voltage (difference in electrical charge) across their plasma membrane.
What is a resting potential?
What is it typically?
- It is the membrane potential of a resting neuron - one that is not sending signals.
- Typically it is between -60 and -80 mV (millivolts). The minus sign indicates that the inside of a neuron at rest is negative relative to the outside.
How are the Na+ and K+ gradients maintained?
How does this work?
- By sodium-potassium pumps in the plasma membrane.
- These ion pumps use the energy of ATP hydrolysis to actively transport Na+ out of the cell and K+ into the cell.
What are ion channels, and what do they do?
- Ion channels are pores formed by clusters of specialized proteins that span the membrane. Ion channels allow ions to diffuse back and forth across the membrane.
- As ions diffuse through channels, they carry with them units of electrical charge.
- Any resulting net movement of positive or negative charge will generate a voltage, or potential, across the membrane.
What does it mean to say that the ion channels have selective permeability?
It means they only allow certain ions to pass. For example a potassium channel allows K+ to diffuse freely across the membrane, but not other ions, such as Na+
Comment on the number of open potassium channels and sodium channels in a resting neuron.
A resting neuron has many open potassium channels, but very few open sodium channels.
How is the diffusion of K+ through open potassium channels critical for formation of the resting potentials?
In a resting mammalian neuron, these channels allow K+
to pass in either direction across the membrane.
- Because the concentration of K+ is much higher inside the cell, the chemical concentration gradient favors a net outflow of K+.
- However, since the potassium channels allow only K+ to pass, Cl- and other anions inside the cell cannot accompany the K+ across the membrane.
As a result, the outflow of K+ leads to an excess of negative charge inside the cell.
This buildup of negative charge within the neuron is the source of the membrane potential.
Substance X moves from a ... concentration to a ... concentration
Ion X+ moves towards ....
...opposite (negative) charge...
During the steady state of a resting cell there is a...
- Net passive efflux of K+
- Net passive influx of Na+
- Cell is not loosing Na+, K+. Cl- or A-
What does an electrophysiologist do?
- Uses intracellular recording to measure the membrane potential of neurons and other cells.
- A microelectrode is put in the cell, and another is put in the extracellular fluid. A voltage recorder then measures the voltage between the microelectrode tip inside the cell and a reference electrode place in the solution outside the cell.
What is an action potential?
- A sudden rapid alteration in the potential of a portion of the membrane from its resting value of about -70 mV to a value of about +30 mV and then back again to its resting value.
- Typically occurs in the space of about 1-4 msec.
When neurons are active, membrane permeability and membrane potential change. Why?
- The change occurs because neurons contain gated ion channels, ion channels that open or close in response to stimuli.
- The opening or closing of ion channels alters the membrane's permeability to particular ions, which in turn alters the membrane potential.
What is an excitable cell?
A cell that can fire action potentials when stimulated appropriately
What happens when gated potassium channels that are closed in a resting neuron open?
Opening more potassium channels increases the membrane's permeability to K+, increasing the net diffusion of K+ out of the neuron. In other words, the inside of the membrane becomes more negative
What is hyperpolarization?
- The increase in magnitude of the membrane potential.
- In general, hyperpolarization results from any stimulus that increases either the outflow of positive ions or the inflow of negative ions.
What is depolarization?
- The reduction in the magnitude of the membrane potential.
- Depolarization in neurons often involves gated sodium channels.
What are graded potentials?
- Potentials where the magnitude of the change in membrane potential varies with the strength of the stimulus.
- A larger stimulus causes a greater change in permeability, and thus a greater change in the membrane potential.
- Graded potentials are not the actual nerve signals that travel along axons, but they have a major effect on the generation of nerve signals
What are voltage gated ion channels?
They are ion channels which open or close in response to a change in the membrane potential.
What happens if depolarization opens voltage-gated sodium channels?
- If depolarization opens voltage-gated sodium channels, the resulting flow of Na+ into the neuron results in further depolarization.
- Because the sodium channels are voltage gated, an increased depolarization in turn causes more sodium channels to open, leading to an even greater flow of current.
- The result is a very rapid opening of all the voltage-gated sodium channels.
- Such a series of events triggers a massive change in membrane voltage called an action potential.
When do action potentials occur?
- Action potentials occur whenever a depolarization increases the membrane voltage to a particular value, called the threshold.
- For mammalian neurons, the threshold is a membrane potential of about -55 mV.
Once initiated, what determines the magnitude of an action potential?
What does it mean that action potentials are an all-or-none response?
- Once initiated, the action potential has a magnitude that is independent of the strength of the triggering stimulus.
- They are an all-or-none response because action potentials occur fully or not at all.
- This all-or-none property reflects the fact that the depolarization opens voltage-gated sodium channels, and the opening of sodium channels causes further depolarization.
Comment on the frequency of action potentials.
- In most neurons, an action potential lasts only 1-2 milliseconds.
- Because action potentials are so brief, a neuron can produce hundreds of them per second.
- Furthermore, the frequency with which a neuron generates action potentials can vary in response to input.
- Such differences in action potential frequency convey information about signal strength.
- In hearing for example, louder sounds are reflected by more frequent action potentials in neurons connecting the ear to the brain.
Describe the 5 stages of firing an action potential.
- 1. At the resting potential, most voltage-gated sodium channels are closed. Some potassium channels are open, but most voltage gated potassium channels are closed.
- 2. When a stimulus depolarizes the membrane, some gated sodium channels open, allowing more Na+ to diffuse into the cell. The Na+ inflow causes further depolarization, which opens still more gated sodium channels, allowing even more Na+ to diffuse into the cell.
- 3. Once the threshold is crossed, this positive-feedback cycle rapidly brings the membrane potential close to ENa. Na+ influx makes the inside of the membrane positive with respect to the outside. This phase is called the rising phase.
- 4. However, two events prevent the membrane potential from actually reaching ENa: Voltage-gated sodium channels inactivate soon after opening, halting Na+ inflow; and most voltage-gated potassium channels open, causing a rapid outflow of K+, which makes the inside of the cell negative again. This stage is called the falling phase - or repolarisation.
- 5. In the final phase of an action potential, called the undershoot, the membrane's permeability to K+ is higher than at rest, so the membrane potential is closer to EK than it is at the resting potential. The gated potassium channels eventually close, and the membrane potential returns to resting potential
What is the refractory period?
What is the difference between the absolute and relative refractory periods?
- The channels remain inactivated during the falling phase and the early part of the undershoot. As a result, if a second depolarizing stimulus occurs during this period, it will be unable to trigger an action potential.
- The "downtime" following an action potential when a second action potential cannot be initiated is called the refractory period.
- An action potential absolutely cannot be fired during the absolute refractory period: it can be fired during the relative refractory period even though it is still recovering
How do action potentials travel along the neuron?
- It regenerates itself as it travels from the cell body to the synaptic terminals, much like a flame traveling along a lit fuse.
- At the site where the action potential is initiated (usually the axon hillock), Na+ inflow during the rising phase creates an electrical current that depolarizes the neighboring region of the axon membrane.
- The depolarization in the neighboring region is large enough to reach the threshold, causing the action potential to be reinitiated there.
- This process is repeated over and over again as the action potential travels the length of the axon.
- At each position along the axon, the process is identical, such that the shape and magnitude of the action potential remain constant.
Why can't action potentials travel back towards the body?
- Immediately behind the traveling zone of depolarization due to Na+ inflow is a zone of repolarization due to K+ outflow.
- In the repolarized zone, the sodium channels remain inactivated. Consequently, the inward current that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it. Thus, an action potential that starts at the axon hillock moves only in one direction - towards the synaptic terminus.
Vertebrate axons have narrow diameters but can still conduct action potentials at high speed. How is this possible?
- The adaptation that enables fast conduction in narrow axons is a myelin sheath, a layer of electrical insulation that surrounds vertebrate axons.
- The insulation provided by the myelin sheath has the same effect as increasing the axon's diameter: It causes the depolarizing current associated with an action potential to spread farther along the interior of the axon, bringing more distant regions of the membrane to the threshold sooner.
What is the great advantage of myelination?
- Its space efficiency. A myelinated axon 20μm in diameter has a conduction speed faster than that of a squid giant axon that has a diameter 40 times greater.
- Furthermore, more than 2,000 of those myelinated axons can be packed into the space occupied by just one giant axon.
What are Nodes of Ranvier?
What is saltatory conduction?
- In a myelinated axon, voltage-gated sodium channels are restricted to gaps in the myelin sheath called nodes of Ranvier.
- The extracellular fluid is in contact with the axon membrane only at the nodes.
- As a result, action potentials are not generated in the regions between the nodes. Rather, the inward current produced during the rising phase of the action potential at a node travels all the way to the next node, where it depolarized the membrane and regenerates the action potential.
- The mechanism is called saltatory conduction because the action potential appears to jump along the axon from node to node.
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