Lecture 4: Graded and Action Potentials

Question 1

nerve and muscle cells are _____ tissues that can change their resting potentials into _____ signals

Answer 1

EXCITABLE tissues that can change their resting potentials into ELECTRICAL signals

Question 2

Neural communication is based on ______ changes in membrane permeability to _____

Answer 2

RAPID changes in membrane permeability to ION

Question 3

why is ion permeability changes used for cell to cell signaling in the brain over a second messager system or hormones?

Answer 3

SPEED, ion permeability change is FASTER

Question 4

Graded potentials depend on the _______ changes induced by the neurotransmitter in the postsynaptic neuron

Answer 4

PERMEABILITY CHANGES

Question 5

What determines whether a neuron fires or not?

Answer 5

based on if the net input is inhibitory or excitatory

Question 6

When does temporal summation occur

Answer 6

when a single synapse receives many EPSPs (excitatory postsynaptic potentials) in a short period of time

Question 7

when does spatial summation occur

Answer 7

when a single synapse receives many EPSPs from MANY presynaptic cells

Question 8

what are the two gates in a Voltage-gated Na+ channel?

Answer 8

ACTIVATION and INACTIVATION gate

Question 9

How many gates does a Voltage-gated K+ channel have?

Answer 9

One and its either open or closed

Question 10

All voltage-gated channel gates are triggered to respond at ________ ?

Answer 10

threshold

Question 11

3 conformations of voltage-gated sodium channels

Answer 11

resting
- closed but can open (-70mV)
activated
- open (from threshold to peak potential -50 to +30 mV)
inactivated
- closed and not capable of opening (+30 to -70mV)

Question 12

2 Conformations of the voltage-gated Potassium channels

Answer 12

Closed
- at resting potential
-delayed opening occurs at threshold
- remains closed from -70 to +30 mV

Open
- from peak potential till after the hyperpolarization phase (+30 to -80mV)

Question 13

what is the Absolute Refractory Period

Answer 13

interval during which NO stimulus can elicit an action potential
- most Voltage gated Na+ channels are inactivated

Question 14

what is the Relative refractory period?

Answer 14

interval when a very large graded potential stimulus is required to elicit an action potential

due to elevated gK coupled w/ residual inactivation of Voltage gated Na+ channels

gk = membrane conductance to potassium ions

Question 15

What is the purpose of a refractory period

Answer 15

• ensures one-way propagation of the action potential
• limits the frequency of action potentials
  - energy conservation and prevents seizures

Question 16

action potentials will always be of the same _______ for a particular axon regardless of how large the stimulus is

Answer 16

will always be the same MAGNITUDE

Question 17

if a stimulus exceeds the threshold, an action potential will be ______

Answer 17

INITIATED, all or none response

Question 18

the strength of a stimulus is determined by the _____ of action potential firing

Answer 18

FREQUENCY of action potential firing

Question 19

A weak stimulus would cause ____ action potentials to be fired compared to a strong stimulus

Answer 19

FEWER

Question 20

what determines the speed of conduction

Answer 20

DIAMETER of the FIBER
- larger diameter = slower internal resistance = faster conduction
-rapid fibers (large diameter) ex: motorneurons
-slow fibers (small diameter) ex: internal organs gut, glands

DEGREE OF MYELINATION
-lipid insulator of nerve fibers increases conduction velocity

Question 21

What are the 2 types of conduction

Answer 21

CONTINUOUS CONDUCTION
- conduction in unmyelinated fibers
- action potential spreads along every portion of the membrane

SALTATORY CONDUCTION
** 50x faster
- impulse jumps over sections of fiber that is covered w/ insulating myelin

Question 22

increasing ECF K+ will cause the RMP to ??

Answer 22

RMP will decrease, causing the inside of the cell to become more + = depolarization

Question 23

Repolarization is ?

Answer 23

The return to resting potential after depolarization

Question 24

When treating critically ill patients with intravenous fluids, which two ions are most important to the neuronal membrane potential?

Answer 24

Na+ and K+

Question 25

very small shifts that can lead to action potentials IF threshold is reached

Answer 25

graded potentials

Question 26

what happens to the ion channels when threshold is reached

Answer 26

Na+ channels open and the membrane depolarizes (becomes more +)

Question 27

what is happening during the Depolarization stage of an AP

Answer 27

Na+ channels are open, Na+ is coming into the cell

Question 28

what is happening at peak AP

Answer 28

• k+ channels open and K+ begins to efflux out of the cell
• Na+ channels close

Question 29

what happens during hyperpolarization phase of an AP

Answer 29

• K+ ions continue to efflux out of cell
• cell enters into refractory period during which no more APs can fire

Question 30

What happens during an AP as the membrane returns to resting membrane potential

Answer 30

K+ channels close and RMP is restored

Question 31

what are the two types of signal conduction down an axon?

Answer 31

1. CONTIGUOUS
   
   • conduction occurs in UNmyelinated fibers
   • AP spreads along every portion of the membrane
2. SALATORY (50X FASTER)
   
   • rapid conduction along myelinated fibers
   • impulse jumps over sections of myelinated fibers

Question 32

in myelinated axons, exchange of ions across the membrane (and thus generation of an AP), can only occur where??

Answer 32

at the nodes of Ranvier where a high density of Voltage-gated Na+ channels are found

Question 33

Action Potentials are a result of ?

Answer 33

sequential openings of voltage gated ion channels

Question 34

Unlike a graded potential, APs are always of the same ______ regardless of the size of the stimulus

Answer 34

magnitude

Question 35

what determines the strength of an action potential stimulus

Answer 35

• the FREQUENCY of action potential firing

—> a weak stimulus would cause fewer APs to be fired compared to a strong stimulus

Question 36

what happens if myelin sheath gets degraded or damaged?

Answer 36

the rapid conduction in myelinated fibers would be compromised

ex: degenerative myelopathy is a progressive disease of the spinal cord that leads to paralysis

Question 37

explain what happens when muscle paralysis occurs as a result of Hyperkalemia

Answer 37

clinical signs: episodes of painful spontaneous muscle contractions followed by paralysis

cause: elevated plasma K+ levels

how it works
- hyperkalemia causes a depolarization of skeletal muscles
- at first, spontaneous APs occur since the RMP is closer to threshold
- but as depolarization becomes greater, Voltage-gated Na+ channels inactivate
- Na+ channel inactivation causes cells to be unable to fire APs, resulting in paralysis