Lesson 3 - Action Potential Conduction Flashcards
what is impulse conduction?
- When a patch of excitable membrane generates an action potential, this causes an influx of Na+ and depolarizes the membrane from -70 mV to +30 mV
- Temporarily the membrane goes from “-” on the inside to “+” on the inside (influx of Na+)
- This local reversal in potential serves as the source of depolarizing current for the adjacent excitable patch in the membrane
- the electromagnetism allows for Na+ channels to be opened in adjacent membrane
- Therefore, once started, an AP will propagate from its origin across the rest of the cell till the axon terminal (or it will die before that in some scenarios)
which cells are excitable cells and why? do most cells want to carry a signal/be excitable?
where does the AP start, travel, and end on a neuron?
do non-excitable cells also conduct currents?
- Most cells are not ‘excitable’, (i.e. they do not generate APs) for the simple reason that they lack voltage- gated Na+ channels. do be excitable a neuron needs: voltage gated Na+ channels, long axons, and muscle cells
- Most cells are not interested in carrying a signal any distance, they do not have an ‘axon’
- An axon is a long extension of the cell body (like a wire) that carry AP away to some other location. the AP starts right at the start of the axon (axon hillock) and ends at the presynaptic axon terminal
- These cells will however conduct passive currents, but will not generate APs
why or why not are excitable cells in biological tissue good conductors
In biological tissue if we put a voltage across membrane on one location (i.e. step change in voltage) and measure the voltage across the membrane some distance away > It doesn’t look anything like what we started with
- this is because biological tissue, resistors and capacitors are not a good conductor
- in contrary, a copper wire will be much better at preserving the potential
what does lambda measure in cable properties for signals
before answering we should know:
- lambda = length constant
* lambda measures how quickly a potential difference disappears (decays to zero) as a function of distance
* Thus, the conduction velocity of an AP along an axon depends on the membrane length constant, lambda
- in other words, lambda measures how far the signal/potential difference will travel before it dies
how do we prevent the loss of a signal along a membrane (2)
- we have to improve and increase lambda
1. lambda is increased by increasing diameter (thicker straw = more water travelled up it) (The larger the diameter > less internal resistance > less voltage is lost across that resistance as the currents travel down the membrane)
- lambda is increased by increasing membrane resistance (less holes or bends in a straw will be better for the water to travel up it) (The higher the membrane resistance > less current is leaked out > current is forced down the membrane)
what is the equation of the length constant lambda. what are the variable and the modified version of the equation
- depends on:
Ri = internal resistance
Ro = extracellular fluid resistance
Rm = membrane resistance
initial eq:
lambda = sqrt(Rm/(Ro+Ri))
Since the extracellular fluid resistance is not adjustable and is relatively low (Ri»>Ro), it drops from the equation and we’re left with internal resistance and membrane resistance
modified eq:
lambda = sqrt(Rm/Ri)
what percent voltage dropped is lambda defined until
lambda is defined as the distance you can travel, to the point where the voltage drops to about 37% of its original value
Ideally, you want to increase lambda as much as possible so that the depolarizing current will spread a great distance
what is myelination
- why is this method better for increasing membrane resistance?
- what are glial cells
- what are the two types of specialized glial cells and where are they found
- how do glial cells work
- myelination: when myelin, a fatty substance, forms a sheath (composed of glial cells) around the axons of nerve cells
- this is the most efficient means of increasing conduction velocity and membrane resistance (not by inc. diameter - harder physiologically)
- ‘Glial’ cells are cells that assist the nervous system, they are required for nutrition and increased membrane resistance
- Specialized ‘glial’ cells (Schwann cells of the PNS or oligodendrocytes within the CNS) wrap around successive sections of an axon
how do glial cells work - myelination in action:
- 50-100 layers wrapping around the axon making myelin sheaths > this greatly increases the membrane resistance > reduces the leakage of current out of the membrane
what percent of axons are myelinated and why?
- about 20% of the axons are myelinated
- this is because it takes space for the glial cells to make the sheath
- only done to the signals that need to go very quickly or far
what is the difference between schwann cells and Oligodendrocyte for myelination
schwann cells:
- located at the PNS
- wraps around a single segment of one axon
- They wrap tightly around the axon, and during this process, most of their cytoplasm is squeezed out, leaving layers of myelin insulation (increases speed of transmission)
Oligodendrocytes:
- located at CNS
- these cells extend multiple processes that wrap around segments of several different axons at once, similar to the arms of an octopus.
- This efficient structure allows oligodendrocytes to myelinate multiple axons simultaneously in the CNS.
what are the regions between the portions of the myelin sheath? why are they important?
- There are small gaps left between adjacent portions of the myelin sheath (a glial cell will wrap one section and next glial cell will wrap another section)
- This small gap left between adjacent glial cells > the ‘Node of Ranvier’
- important because this space is where the AP is and can be generated
-> only place where we can find the voltage gated sodium channels which is required for an AP
-> NoR allows for currents to cross and be depolarized at the membrane - if we didn’t have the gaps the sodium wouldn’t be able to enter the cell –> def not through the myelination
what disease would we acquire if we were to lose the myelinations on our axons
- Multiple sclerosis (MS)
- nervous system disease which effects brain and spinal cord
- the damaged myelin leads to blockage and slowing down of signals
- may lead to vision disturbances, muscle weakness, incoordination and imbalance etc.
what is saltatory conduction?
- In myelinated axons, only the membrane exposed (NoR) at the nodes is excitable
- Because the APs are only generated at these nodes, it means that the AP will ‘jump’ from one place to the next — in between you’re not generating any AP (Only the current is going through the myelin sheaths)
- This ‘jumping’ mode of conduction is known as ‘saltatory conduction’
how many nodes can an AP’s depolarizing current travel?
what are the two processes that occur when an AP is travelling along the axon?
what is the role of the myelin here in one of the processes?
how does the current travel beyond the max number of nodes?
- If we have an AP on one node, the depolarizing current that is generated at the site is strong enough and will travel down that axon for many nodes (5-10 nodes)
- If there is sufficient strength in the current, it will bring all the following nodes to threshold potential
- Therefore, as the AP travels along the axon there is saltatory conduction where the current jumps from one node to the next but also passive spread of the depolarizing current through the myelin sheaths
- Myelin prevents leakage of current across membrane between nodes
- note that even though the next ten nodes will be triggered for an AP, since the triggering happens sequentially, at a moment in time all ten nodes will be at different phases at the AP. however, it’s important to note that it is the tenth node, that when passed above threshold potential, will be the one activating the next 10 nodes.
what is the safety factor that is encoded in saltatory conduction?
You could poison some of the nodes and the depolarizing current will just skip past that and move on to the next healthy patch of membrane (i.e. you have to destroy a fair length of the membrane to stop AP in its track)
