Lecture 11 Flashcards
Are mature neurons capable of cell division?
No, this is why damage to nervous tissue can be permanent. PNS has a little capacity to heal itself, but CNS does not.
Can nerve fibres be repaired?
Yes, if the damage is not extensive and the neurilemma (Schwann cell cytoplasm around the nerve fibre) are intact, and scarring has not occurred.
Can spinal nerve fibres with long axons repair?
No, this is why a spinal injury can result in paralysis.
Neurons can exhibit…
Conductivity and excitability; they can conduct electrical impulses, as well as generate them.
Describe a nerve impulse.
A wave of electrical fluctuation that travels along the plasma membrane.
Resting membrane potential.
Difference in ion concentration across the membrane; -70 mV.
Ion distribution inside and outside of the cell.
More potassium inside the cell; more sodium outside the ell.
Movement across a membrane os regulated by…
Permeability of the cell membrane.
Is a cell at resting membrane potential polarized?
Yes, as the inside is more negative than the outside.
2 principle ions that drive membrane potential.
Sodium and potassium.
Potassium is the primary contributor to resting membrane potential.
Active transport of K+ outside of the cell creates a gradient that makes the inside of the cell more negative. Changing the permeability of an ion causes a change in resting membrane potential.
Which pump contributes to the membrane potential?
Na+K+ pump” 3 Na+ pout for every 2 K+ in; this maintains the electrochemical gradient.
Depolarization.
Membrane potential becomes less negative.
Repolarization.
A return to the resting membrane potential.
Hyperpolarization.
Resting membrane potential becomes even more negative than -70 mV. This makes it difficult for a neuron to fire, it would need a stronger signal.
Mechanically gated channels.
Sensory neurons, physical trigger.
Chemically gated channels.
Respond to ligands.
Voltage gated channels.
Respond to changes inc ell membrane potential.
Channelopathies.
Mutations in a channel. Example: cystic fibrosis.
How is resting membrane potential maintained?
Transport channels are selective in what they allow across.
Can anionic proteins exit the cell through a channel?
No, there are no channels for anionic proteins, which dominate the intracellular fluid. They are trapped inside, which helps maintain the membrane potential.
Membrane channels dictate…
Permeability to ions.
Active transport also helps maintain resting membrane potential.
Maintains the imbalance of ions (example: sodium and potassium).
Sodium potassium ATPase dependent pump is also responsible for maintaining the resting membrane potential.
Maintains the difference in ionic balance. selective permeability.
Stimulating a neuron: local potentials.
A slight shift away from the resting membrane potential in response to certain stimuli. Example: excitation of a neuron occurs when a stimulus triggers the opening of Na+ channels on the dendrites, allowing the potential to move to 0 (depolarization); this is also an action potentials.
Summation zone.
Sum must be enough to reach the threshold value of -59 mV for the neuron to fire.
Inhibition of a neuron.
Occurs when a stimulus triggers the opening of additional K+ channels at the synapse, increasing the membrane potential by allowing K+ to diffuse out of the cell.
What happens to a neuron when potassium channels are opened?
K+ leaks out, preventing the membrane potential from reaching the threshold value (moves below -70 mV), thereby inhibiting the neuron.
Action potential (nerve impulse).
it is the membrane potential of an active neuron. Adequate stimulus triggers Na+ channels to open, allowing Na+ oto diffuse into the cell, thereby causing a local depolarization. It is the nerve impulse that conducts an electrical signal down the axon and that causes the release of a neurotransmitter.
Where are Na+ and K+ channels found in the neuron?
There are many in the membrane of the neurons summation zone and conduction zone.
Where do local depolarizations occur?
On the dendrites.
What charge does the neuron local depolarization reach when Na+ is at its maximum entry into the cell?
+30 mV.
How long do Na+ channels stay open?
1 millisecond; this allows for the same magnitude of the action potential to always be reached.
Na+ channels have 2 gates.
Activation gates and inactivation gates. At the resting membrane potential, the inactivation gate closes the channel. Depolarization stimulus: activation gates open and sodium goes in; after 1 millisecond, the inactivation gate is put in, so Na+ entry stops. During the depolarization caused by K+ leaving the cell, the 2 gates reset to their original positions.
Depolarization can be triggered by 3 types of signals.
Chemical, mechanical, and electrical.
Refractory period.
Cell is hyper polarized because the potassium channels are still open.
Absolute refractory period.
An area of the neurones membrane resists restimulation; will not respond to stimulus, no matter how strong. Time is required before a neuron can fire again (usually about half of a millisecond).
Relative refractory period.
The membrane is repolarizing; will only respond to a very strong stimulus.
At what charge is the neuron when it is firing?
0 mV.
Conduction of the action potentials.
Depolarization during the action potential causes electrical current to flow between the site of the action potential and the adjacent regions of the membrane, triggering voltage-gated Na+ channels to open in that next segment; this area now exhibits the action potential.
Positive feedback loop.
Positive charge allows more sodium ions to enter until the signal is strong enough for an action potential.
Why does an action potential never move backwards?
The refractory period prevents restimulation.
In myelinated fibres, where fo action potentials occur?
Only at the Nodes of Ranvier; this is called saltatory conduction. Impulse regeneration leaps from node to node, as only the nodes have Na+ voltage gated channels.
Myelinated fibres insulate the axon and allow for what?
Optimal conductance of the electrical signal.
Advantages of the myelin sheath.
You need less sodium to propagate the action potential because the sodium and electrical signal is conserved. Results in an increase in conduction velocity.
Damage to myelinated axons.
Conduction slows and leaks out; the signal may never get to where it needs to go. Example: multiple sclerosis; sodium moves down the axon, but leaks out due to damaged myelin sheath, so you lose the strength of the signal.
