Lecture 12, AP (part 2) Flashcards
Action Potentials
action potential propagation occurs due to a positive feedback loop (related to Na+ channels)
membrane repolarization occurs due to a negative feedback loop (related to K+ channels)
Feedback System (positive feedback loop)
the depolarizing stimulus opens Na+ ion channels, which begins the positive feedback loop
- depolarization = the fluctuation in membrane potential from the set point
depolarization is sensed by adjacent Na+ ion channels, which elicits a response
- response = the opening of more Na+ channels
the response pushes the variable further from the baseline
- depolarizes the next segment of the membrane
Feedback (negative feedback loop)
the depolarizing opens K+ ions channels, which begins the negative feedback loop
- depolarization = fluctuation in membrane potential from the set point
depolarization is sensed by adjacent K+ ions channels, which elicits a response
- response = opening of more K+ ion channels, allow for the membrane to repolarize
the responses returns the variable back to baselines
- repolarizes the membrane back to resting potential
Refractory Period
if a second depolarizing stimulus is received during an action potential, a second action potential will not occur
- this is the absolute refractory period
- voltage-gated Na+ ion channels are already open, so a second depolarizing stimulus will have no effect
- additionally, inactivation fates are in place that prevent Na+ ion channels from opening too soon after an action potential
◦ the membrane needs to be fully repolarized for
these inactivation gates to be removed, and the
Na+ channels can be reopened
follow the absolute refractory period is the relative refractory period
- a second action potential can be initiated if the second depolarizing stimulus is stronger than usual
- some, but not all, Na+ ion channels have reopened to their resting states (no inactivation gates)
- a stimulus needs to be large enough to open as many Na+ ion channels as possible
◦ and large enough such that Na+ influx exceeds K+
efflux (as some K+ ion channels are open)
Refractory Periods as a Regulatory Mechanism
a second depolarization stimulus can initiate an action potential during relative refractory period if:
- it is large enough to achieve threshold potential
◦ strong enough to open all the available Na+ ion
channels
◦ allow for enough Na+ influx to overcome K+
efflux
- it outlasts the relative refractory period
◦ stimuli remains present as more Na+ ion
channels become available
refractory periods serve to limit the number of action potential that can be transmitted down an excitable membrane in a given time frame
- most neurons are limited to 100 action potentials per second
- some may produce higher frequencies for brief periods, depending on the depolarizing stimulus
refractory periods can also separate action potentials, such that individual signals are kept separate
- also help guide the direction of action potential propagation
Action Potential Propagation
an action potential is only initiated if the membrane reaches the threshold potential
- depolarization below threshold = graded potential
if threshold potential is reached (1), it triggers the opening of Na+ channels in the next segment (2)
- this allows the next segment (2) to be depolarized, and so on
-> the depolarization of the action potential (+30 mV) is sufficient to easily depolarize the next segment of the membrane to threshold potential
]Action Potential Propagation (3 Steps)
first step:
- depolarization signal causes Na+ ion channels to open
- Na+ influx into the cell
- depolarization of the membrane to threshold potential
- initiation of the action potential
second step:
- action potential reaches the next segment of the membrane, causing Na+ ion channels to open
- Na+ influx into the cell in the next segment
- in the first segment, Na+ ion channels close and K+ ion channels open
- the first segment is in a refractory period, and is undergoing repolarization
third step:
- action potential reaches the third segment of the membrane, causing Na+ ion channels to open
- Na+ influx into the cell in the third segment
- in the second segment, Na+ ion channels close and K+ ion channels open
- the second segment is in a refractory period, and is undergoing repolarization
- the first segment has been repolarized and is at rest
- the first segment is ready for another action potential
Myelin
the myelin sheath is formed by neuroglial cells, and is made up of lipids and proteins
- provides electrical insulation
in the central nervous system, one oligodendrocyte can form the myelin for up to 40 different axons
in the peripheral nervous system, one schwann cell forms the myelin for only one segment of axon (~1mm in length)
- one axon has several schwann cells associated with it, forming the myelin sheath
Myelin (2)
myelin is an insulator; makes it more difficult for charged molecules (i.e., Na+, K+) to flow between compartments
- in myelinated segments of the axon, there is less “leakage” of charge across the membrane (K+ leak channels)
- an action potential at one node can spread farther along the axon, and reach the next node faster
◦ the transmission of action potentials from node to node = Saltatory conduction
depolarization of the axon can only occur at the nodes of Ranvier; the myelinated segments cannot depolarize
Myelin (3)
myelination of the axon conserves energy
- after an action potential, energy is used by Na+/K+ pumps to repolarize the membrane back to resting potential
- myelination allows for the signal to be transmitted down the axon, with less overall ion flux
saltatory conduction: the regeneration of action potentials only at the nodes of Ranvier along a myelinated axon
- the action potential appears to ”jump” from node to node, but does not actually jump and is instead just regenerated at each node
depolarization at one node spreads quickly to the next node, where the action potential is regenerated
sodium ions come in and spread (can quickly regenerate) - very quick to spread to next node
the velocity of action potential propagation depends on:
- myelination
- axon diameter (larger diameter = less resistance to current = faster velocity)
Multiple Sclerosis
Multiple Sclerosis (MS) is a chronic, progressive autoimmune condition that involves demyelination of axons in the brain, spinal cord, or optic nerves
- the loss of myelin can occur in one or multiple places in the nervous system
in MS, immune cells mistakenly attack the myelin sheath
- disrupts saltatory conduction and causes inflammation
MS symptoms and disease severity
symptoms and disease severity depends on the extent of myelin damage:
- if damage is minor, transmission of nerve impulses is only slightly disrupted
- if damage is significant, scar tissue forms in replacement of the damaged myelin, which can halt action potential transmission or even cause damage to the neuron itself
symptoms of MS may include: numbness, weakness/paralysis, tingling sensations, impaired vision, fatigue, vertigo, impaired speech, neurocognitive decline, and more
MS severity and treatment
the cause of MS is unknown, and there is no known cure
severity and presentation of MS varies significantly between individuals
- some patients experience rare, isolated flare-ups with complete recovery in between
- other patients experience a continuous, debilitating progression of the disease over time
current treatment options include anti-inflammatory drugs and immunosuppressants
- slow the progression of MS
- reduce severity by suppressing the immune system (inhibiting their attack on the myelin)
