week 1. Neurons Part One Flashcards
Understand the basic characteristics of the neuronal membrane.
Comprises a bi-layer of phospholipids with protein channels spanning them intermittently.
Distinguish between electrical and concentration gradients that operate on the movement of ions across the cell membrane.
An electrical gradient occurs where there is a difference in charge. A concentration gradient occurs where there is a difference of concentrations of molecules.
Distinguish between resting potentials, action potentials, and graded potentials.
Resting potential is when the cell is capable of producing an action potential if there is a stimulus which can trigger above threshold.
An action potential occurs when the cell is triggered above threshold, such that is passes a message along its axon. Irrespective of how strong the stimulus, the action potential has the same magnitude, it just needs threshold to be reached.
A graded potential is a graded response (in the membrane potential) to a stimulus. ie the magnitude of the potential varies with variation in magnitude of the stimulus. These occur in Local Neurons which are small, have no axon, and pass the potential to other adjacent cells but the potential gradually decays over distance.
Understand events that occur during the resting potential (resting state
At resting potential, all gated channels are closed.
Na+ has a lower concentration inside cell, and higher outside cell.
K+ has a higher concentration inside cell and lower concentration outside cell.
Na+/K+ pump works putting 3Na+ out and bringing 2K+ in. Cell is more negatively charged inside than outside (although the difference is very small).
Many K+ leak channels remain open.
K+ ions have an electrical gradient trying to get it in but the concentration gradient is stronger and so potassium leaks out of the cell. This is the main cause of the resting membrane potential.
Na+ would like to follow both electrical and concentration gradients to move inside the cell but there are very few sodium channels open, so it cannot move in in any significance.
Resting membrane potential differs for each neuron but is usually -90 to -60 mv.
Understand the electrical changes observed during an action potential and understand how these are related to the movement of ions across the membrane.
In the prequel to an Action Potential, Na+ ions arrive due to some stimulation, at the axon Hillock. If enough arrive, the membrane reaches Threshold, and an Action Potential ensues.
During an AP, briefly, the inside of the cell becomes more +ve and the outside -ve, which causes another AP further along, or if reaches the terminal buds, results in release of neurotransmitter at the synapse.
Parts of the AP:
a)Rising Phase=Depolarisation;both Na+ and K+ gated channels begin to open. The main effect is influx of Na+ into cell. When Threshold is reached, the Sodium channels are at maximum opening.
b)Falling Phase=Repolarisation:Right at the AP peak, where repolarisation begins, the Na+ gates abrubtly close. The cell is at positive charge and K+ concentration gradient and electrical gradient drive K+ out of cell, and the K+ gates are still open.
c)Undershoot phase= Hyperpolarisation;enough K+ leaves cell to cause membrane hyperpolarisation. At some point, voltage-gated K+ channels close.The Na+/K+ pump then works to restore from hyperpolarisation back to resting membrane potential.
(overshoot phase is where membrane potential is above zero (ie is positive).
Understand the changes in sodium channels during an action potential. Show which parts of the action potential are associated with each of these changes.
6
Understand the changes in the potassium channels during an action potential. Indicate which parts of the action potential are associated with each of these changes.
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Understand the process whereby an action potential is propagated down the axon.
Propogation occurs via movement of Na+ ions within an AP triggering another AP further along the axon.
Myelin sheaths insulate some nerves and consist of fat and protein. Where there is myelin, there are very few Na+ voltage gated channels. But the na+ gates occur regularly where there are Nodes of Ranvier between the myelin, along the axon. Thus the AP is propogated along these nodes, and at greater speed over distance, cf non myelinated nerves. Saltatory Conduction occurs where there is myelin, and the AP jumps from 1 node to the next. Where dz’s reduce myelin, the AP dies out as it cannot transmit across the distance with paucity of Sodium channels without the myelin to insulate.
Understand how the absolute and relative refractory periods work.
The Absolute refractory period means irrespective of stimulus, no AP can be generated. This is largely because the sodium channels are closed, and unable to be re-opened for a period. This period usually lasts around 1 millisecond.
Following this, there is a Relative Refractory Period where an AP is possible, but requires a stronger stimulation (because the membrane is hyperpolarised, ie more negative than usual). This lasts around 2-4milli seconds.
Understand how the all-or-none law relates to the action potential.
The all or none law means a neuron will have an AP of the same magnitude, irrespective of the magnitude of the stimulus. This applies to most neurons, especially those with axons.
Some tiny neurons however have no axon, but simply pass any AP they have to surrounding neurons. Their AP’s do NOT obey the all or nothing rule and are graded in response to the magnitude of the stimulus. Their AP’s however tend to fade very quickly over a small area and cannot be passed along too far.
Local anaesthetic
blocks Na+ channels, so even though receptors receiving signals of pain, they are not transmitted or perceived.
