L19 Membrane Potential And Nervous Signals Flashcards
Where can neurons repair?
- More successful in PNS than in CNS
- Neuronal recovery only in axons damaged, not cell body
Why is recovery so limited in the CNS?
- Higher density of axons in brain & spinal cord
- Astrocytes & oligodendrocytes can block regrowth during repair
- Microglia cant remove all cell debris from damaged area
Why is neuronal repair in the CNS prevented?
- Improper removal of cell debris prevents recovery
- Astrocytes proliferate to try and repair the axon
- Oligodendrocytes reestablish connections with severed axon
- These conflicting events prevent neuronal repair
What name do we give the process of tissue repair in the PNS?
Wallerian degeneration
What are the steps of Wallerian degeneration? in the PNS
1) Axon distal to injury degenerates (dies)
2) Macrophages clean up the area
3) Schwann cells proliferate to form a solid cord
4) Recovering axon fuses with long cord of schwann cells
5) Axon continues to grow inside the cord towards the distal cord
What are the nervous system pathologies?
- Destruction of neurons = loss of sensory perception and motor control
What is demyelination?
Destruction of the myelin sheat in the CNS (oligodendrocytes) or PNS (Scwann cells)
What are examples of neuropathologies?
1) Heavy metal poisoning
2) Multiple sclerosis
3) Guillain-Barre syndrome
What does the neuropathology of heavy metal poisoning do to your nerves and neurons?
Slowing or disruption of nerve impulses by oxidative stress, denaturing proteins, and damaging organelles of neurons
What does the neuropathology of multiple sclerosis do to you?
- Progressive autoimmune disease, causing myelin destruction
What does the neuropathology of Guillain-Barre syndrome do to you?
Temporary, virally-triggered autoimmune disease causing, myelin destruction
How is the membrane potential created?
- The Na+/K+ ion pump moves sodium out and potassium into the cell, creating an electrochemical gradient.
- Inside the membrane = negative because of ion pump & negatively charged proteins.
- This difference is called the membrane potential.
What is a neuron at rest called?
- Resting membrane potential (RMP)
What is the charge of the resting membrane potential and what is its value?
- In neurons -70
- Negatively charged at rest (in cytosol)
How do we establish the resting membrane potential?
- With three membrane proteins
- Na+/k+ Ion pump
- NA+ leak channels
- K+ leak channels
What does the Na+/k+ ion pump do?
Active transporter that pumps three Na+ out and two K+ in
What does the Na+ leak channels do?
- Passive transporters
- Always open
- lets Na+ into the cell down its concentration gradient
What do K+ leak channels do?
- Passive transporters
- Always open
- Let K+ out of cell down its concentration gradient
How can cells change the resting membrane potential?
By altering the permeability of the membrane to Na+ or K+
Why do cells alter the permeability of the membrane to Na+ or K+?
- Perform exocytosis
- Contract muscle cells
- Send action potentials along neurons
How does the nervous system use membrane potential?
- Sensory signals stimulate neurons by changing their membrane potential
- Motor signals cause changes in membrane potential in effectors, causing muscle contraction or exocytosis of glandular cells
What are the different types of membrane channels?
1) Leak channels
2) Gated channels
How do leak channels control how ions move across the cell membrane?
- Always open
- Passively allowing ions to pass freely
Gated channels need to be stimulated to open. What are the type of gated channels?
1) Chemically gated channels
2) Mechanically gated channels
3) voltage gated channels
How do chemically gated channels work?
- Opened once molecule bounds
Ex: neurostransmitters
How do mechanically gated channels work?
- Opened by physical forces
Ex: Merkel cells, which sense ligh touch
How do voltage gated channels work?
- Opened when membrane potential reaches a certain value
What is a move away from the resting membrane potential called?
Graded potential
How do we define a graded potential?
A local chance in membrane potential
How do we generate graded potentials?
- By opening chemically or mechanically gated ion channels
- Ions diffuse into or out, changing the resting membrane potential
- In neurons, the str of the graded potential determines if it triggers an action potential
In which cells can graded potentials occur?
All your cells, because they all generate a membrane potention
A graded potential can cause something special in neurons what is it?
- Neurons and muscle cells have EXCITABLE membranes
- Graded potential can trigger an action potential, perpetuating electric signals along their membrane
What special property causes a membrane to be excitable?
Voltage gated ion channels
How is an action potential produced?
- A graded potential strong enough (triggers the opening of voltage gated channels on the axon hilock)
What happens if the graded potential was not strong enough?
change in the membrane potential does not reach axon hilock, therefore no action potential is produced
What happens if a large change in the membrane potential occurs?
An action potential is produced
What happens if a strong graded potential reaches the axon hilock?
voltage gated ion channels open
What happens when volate gated ion channels open and the graded potential reaches the axon hilock and sends a aciton potential through the axon?
- Channels open, causing voltage gated channels along the membrane to open
- Perpetuates signal across the surface of the axon
What is the threshhold of a graded potential to trigger an action potential?
-55mV
What happens in a neuron at rest?
- Membrane potential -70mV (maintained by ion pump and leak channels)
- All voltage gated ion channels are closed
What happens in a neuron that is stimulated?
- graded potential occurs
- if threshold -55mV reached at the axon hilock, VG Na+ channels open and neuron depolarizes
- If not reached returns to resting membrane potential -70mV
What happens in a neuron that is depolarizing
- Na+ enters cell, causing more voltage gated channels to open on neighboring axolemma
- causing depolarization signal to creep along the axon
- Single patch, membrane potential increases from -55mV to zero and rises
- this continues until it reaches +30mV
What happens in a neuron with a membrane potential of +30mV
- Voltage gated na+ channel inactivate
- Voltage gated K+ channels open (slowly), but open fully at +30mV causing k+ to leave the cell
- Repolarization occurs
What happens in a neuron that is repolarizing back to resting membrane potential?
- Inactivated VG Na+ channels close at -55mV
- membrane potentials overshoots to -90mV
- (-70 to -90 is called hyperpolarization)
- VG K+ channels fully closed during hyperpolarization at -90mV
How do we reach resting membrane potential when a neuron is hyperpolarized?
Na+/k+ ion pump
How is RMP maintained?
Ion pump
leak channels
maintain membrane potential of -70mV
How does a neuron get ready for the next action potentials?
when both voltage gated channels are closed
How are action potentials prevented from moving backwards?
Voltage gated Na+ channels have 3 settings
1) Closed settings
2) open settings
3) inactivated
What does the closed setting prevent in voltage gated Na+ channels?
- ions from moving through the channel
How do we open a closed setting in a voltage gated Na+ channel?
if the threshhold is reach, it will cause it them to open
What does the open setting allow in voltage gated Na+ channels?
- Ions to move through the channel DOWN their concentration gradient
- Flow of Na+ causes depolarization
What does the inactivated setting do in voltage gated Na+ channels?
- Prevents ions from moving through the channel
- Channel cannot be re-opened
- Inactivation occurs at +30mV, while reseting to a closed position at -55mV (during repolarization)
What settings to volated gated K+ channels have?
1) Open
2) Closed
When does the closed setting happen in a voltage gated K+ channel
- Neuron at rest
- at the peak of hyperpolarization (approx -90mV)
When does the open setting happen in voltage gated K+ channels?
- When neuron depolarizes to +30mV
- causing efflux of k+ repolarization and then hyperpolarization
How does an action potential move along the membrane?
Down the axon in a wave of depolarization from the axon hilock
What does the inactivation of voltage gated Na+ channels prevent?
A new signal from triggering a new action potential, this creates refractory periods
How does the absolute refractory period occur? (ARP)
- Channels are inactive and cannot open again until reset
- No new action potential can occur during this time
What is the relative refractory period? (RRP)?
- During repolarization and hyperpolarization
- Na+ channels are closed but can reopen if threshold is reached again
- Membrane potential is lower than usual
- Stronger stimulus is needed to trigger another action potential
