neurophysiology 1.3

All parts of a neuron are covered by a membrane that is ______ thick.

8 nanometers

electrical gradient

difference in the electrical charge between the inside + outside of cell

*polarization

resting potential

electrical charge difference across a membrane when it isn’t sending signals

typical resting potential

-70 mV

selective permeable

membrane allows some chemicals to pass more freely than others

(ie uncharged molecules: H20, CO2, O2)

charged ions that cross through semi-permeable membrane via channels

• sodium

• potassium

• calcium

• chloride

sodium-potassium pump

transports 3 Na ions out of cell + 2 K ions into cell

The sodium-potassium pump is effective only bc the ______ ______ of the membrane prevents the sodium ions that were pumped out from leaking back in again.

selective permeability

concentration gradient

difference in distribution of ions across the membrane

when the membrane is at rest, are the sodium ions more concentrated inside the cell or outside? where are the potassium ions more concentrated?

• sodium ions are more con’c outside the cell

• potassium ions are more con’c inside

when the membrane is at rest, what tends to drive the potassium ions out of the cell? what tends to draw them into the cell?

the con’c gradient tends to drive potassium ions out of the cell + the electrical gradient draws them into the cell

the sodium-potassium pump also draws them into the cell

Why is a resting potential necessary?

it prepares the neuron to respond rapidly; excitation of neurons opens sodium channels, letting sodium enter cell rapidly

action potential

message sent by an axon

hyperpolarization

increasing the negative electrical charge (increased polarization)

depolarization

reduce the polarization toward zero

threshold stimulation

causes membrane to open its sodium channels + permits sodium ions to flow into the cell, driving the membrane potential upward

subthreshold stimulation

produces a small response that quickly decays

the all-or-none law

the amplitude + velocity of an AP are independent of the intensity of the stimulus that initiated it, provided that the stimulus reaches the threshold

Any ______ that reaches or passes the threshold produces an _____ ______.

depolarization; action potential

what is the difference between hyperpolarization and depolarization?

• hyperpolarization

  • exaggeration of the usual negative charge within the cell, making it more negative level than usual

• depolarization

  • decrease in amt of negatiuve charge within a cell

what happens if the depolarization does or does not reach the threshold?

• reaches or passes threshold »» cell produces an action potential

• less than threshold »» no AP arises

Does the all-or-none law apply to dendrites?

dendrites do not have APs so the law doesn’t apply to them

three basic principles behind the action potential

1. at the start, sodium ions are mostly outside the neuron, and potassium ions are mostly inside

2. depolarizing the membrane opens the sodium + potassium channels

3. at the peak of the AP, the sodium channels close

voltage-gated channels

open or close depending on the voltage across the membrane

Of all the sodium ions near the axon, less than _____ percent cross the membrane during an AP, and the sodium ions remain far more concentrated outside than inside.

one

Depolarizing the membrane causes?

potassium channels to open

the movement of sodium + potassium ions during an AP

• sodium ions cross during the peak of the AP

• potassium ions cross later in the opposite direction, returning the membrane to its original polarization

local anesthetic drugs

these drugs attach to the sodium channels of the membrane, preventing sodium ions from entering

*novocain, xylocaine

during the rise of an action potential, do sodium ions move into the cell or out of it?

• sodium ions move into the cell

• voltage-dependent sodium gates have opened letting sodium move freely

• sodium is attracted to the inside of the cell body. by both an electrical and a conc gradient

as the membrane reaches the peak of the AP, what brings the membrane down to the original resting potential?

• at the peak of an AP, potassium ions exit the cell, driving the membrane back to the resting potential

• important note: the sodium-potassium pump is too slow for this purpose

propagation of the action potential

transmission of an AP down an axon

(propagation of animal species is the production of offspring)

An AP starts in an axon and _____ w/o loss from start to finish.

propagates

6 events that occur during an AP

1. when an area of the axon membrane reaches its threshold, sodium + potassium channels open

2. opening sodium and potassium channels lets sodium ions rush into the axon. at first opening the potassium channels produces little effect

3. positive charge flows down the axon + opens the voltage-gated sodium channels at the next point

4. at the peak of the action potential, the sodium gate snaps shut

5. bc voltage-gated potassium channels remain open, potassium ions flow out of azon, returning membrane toward its original depolarization

6. a few milliseconds later, the voltage-dependent potassium channels close

absolute refractory period

time when the membrane cannot produce an AP regardless of the stimulation

at the peak of the AP, sodium channels shut tightly + remain tightly shut for approx the next millisecond

myelin

insulating material composed of fats + proteins

*covers vertebrae + axons

saltatory conduction

jumping. ofAPs from node to node

*to jump

In addition to providing rapid conduction of impulses, ______ _______ conserves energy. Instead of admitting sodium ions at every point along the axon and pumping them out, a myelinated axon admits sodium only at its ____.

saltatory conduction; nodes

In MS, the immune system attacks ____ ____.

myelin sheaths

local neurons

tiny neurons w/ no axon + communicate only w/ their immediate neighbors

*do not produce APs or follow all-or-none law; instead, they have graded potentials

graded potential

greater amounts of stimulation produce greater depolarization that spreads over the surface of the tiny neuron, declining in strength over distance

why is an action potential a better way for an axon to transmit info than electrical conduction?

an AP does not decrease its intensity over distance

when the membrane is at rest, where are the sodium and potassium ions, mostly?

sodium is mostly outside the cell, and potassium is mostly inside

what does the sodium-potassium pump do?

it pumps sodium out of the cell, and potassium into thre cell

when the membrane is at rest, which gradient or gradients tend to draw sodium ions out of the cell?

both the electrical gradient + the concentration gradient

when the membrane is at rest, which gradient or gradients tend to draw potassium ions out of the cell?

the concentration gradient

what causes the rising portion of the action potential?

sodium ions are moving into the cell

after the peak of an AP, the membrane returns to its resting level. what accounts for this recovery?

potassium ions move out bc the conc gradient

if a membrane is depolarized twice its threshold, what happens?

the action potential is the same as usual

the all-or-none law applies to what part of a neuron?

the axon

what causes the absolute refractory period of an axon?

the sodium channels are shut

what does the myelin sheath of an axon accomplish?

it increases the velocity of APs

how do local neurons differ from other neurons?

they don’t have axons

what percentage of the brain does an average person use?

100 percent