Virginia Tech BMSP 2135 RAT #16

what two properties do neurons share with skeletal muscle fibers?

excitability, conductivity

excitability

- All neurons are excitable (responsive) in the presence of various stimuli, including chemical signals, local electrical signals, and mechanical deformation

- These stimuli generate ____________ ____________ across the plasma membrane of the neuron

electrical changes

conductivity

- electrical changes across the plasma membrane don't stay in one place

- instead, they are rapidly conducted along the entire length of the ________________, similar to how an electrical impulse is conducted through a copper wire

membrane

separation of charges across the plasma membrane; type of electrical gradient

voltage

voltage

Thin layer of (positive/negative) charges in the cytosol lining the inside of the membrane

negative

voltage

Thin layer of (positive/negative) charges in the ECF lining the outside of the membrane

positive

membrane potential of a neuron at rest; -70mV

resting membrane potential

the resting membrane potential is negative because the cell constantly loses small numbers of positively charged _______________ _________ due to leak channels (always open)

potassium ions

_____________ = the membrane potential is at the negative or positive side; the state of a cell at its resting membrane potential

polarized

The closer the membrane potential comes to ____mV, the less polarized the membrane potential becomes

0

channels are needed for ion movement across a plasma membrane because charged particles cannot pass through the what?

hydrophobic portion of the phospholipid bilayer

____________ channels are always open and require no stimulus for opening/closing

leak

channels that open/close in response to a specific stimulus

gated channels

gated channel that binding of a ligand to a receptor associated with the channel is the stimulus for opening/closing

ligand-gated channel

gated channel that voltage changes across the plasma membrane is the stimulus for opening/closing

voltage-gated channel

gated channel that mechanical deformations of the channel (by pressure, stretch, etc.) is the stimulus for opening/closing

mechanically gated channel

Sodium ion concentration higher in (cytosol/ECF)

Potassium ion concentration higher in (cytosol/ECF)

ECF, cytosol

the sodium potassium pump maintains the gradient of Na+ and K+ by moving ____ Na+ ions out of the cell and ____ K+ ions into the cell

3,2

A temporary increase in a cell's membrane potential caused by influx of positive charges; cell becomes less polarized as its membrane potential approaches 0mV

depolarization

Movement of a cell's membrane potential back toward resting level after a depolarization has taken place

repolarization

A change in the membrane potential of an excitable cell to a value more negative than its resting membrane potential; caused by the cell losing positive charges; can also occur from the opening of channels for anions (example = Cl- ions) which would allow these negatively charged ions to flow into the cell

hyperpolarization

a small change in the membrane potential in a specific region of a cell's plasma membrane; occurs when a neuron is stimulated just once; occurs instead of an AP

local potential

effects of local potentials

1. ________________: positive charges enter the cytosol and make the membrane potential less negative

2. _____________________: either positive charges exit or negative charges enter the cytosol to make the membrane potential more negative

depolarization, hyperpolarization

Local potentials are sometimes called ____________ potentials because they vary greatly in size—some produce a larger change in membrane potential than others.

graded

Another feature of local potentials is that they are _______________; when the stimulus that caused the ion channels to open stops, the neuron quickly returns to its resting potential.

reversible

Local potentials are also __________________ in nature: The changes in membrane potential they produce are small, and the current generated is lost across the membrane over the distance of a few millimeters

decremental

since local potentials are decremental, local potentials cannot send signals over great distances, and are useful for short distance signaling only (which is why they're called local potentials).

despite this, local potentials are vital triggers for what?

action potentials

uniform, rapid depolarization and repolarization of the membrane potential of a cell; causes a response (action) of some sort

action potential

Where in a neuron does an action potential occur?

axon

Where does an action potential start?

trigger zone (initial segment of the axon)

what are the 2 states for a voltage-gated potassium ion channel?

resting, activated

what are the 3 states for a voltage-gated sodium ion channel?

resting, activated, inactivated

for a resting voltage-gated sodium ion channel, what gates are open and/or closed?

inactivation gate open, activation gate closed

for an activated voltage-gated sodium ion channel, what gates are open and/or closed?

inactivation and activated gates are open

for a inactivated voltage-gated sodium ion channel, what gates are open and/or closed?

inactivation gate closed, activation gate open

When the ___________ _______________ is finished, the channel returns to the resting state

action potential

steps of action potential

1. a local potential depolarizes the axolemma of the trigger zone to _______________

threshold

steps of action potential

1. a local potential depolarizes the axolemma of the trigger zone to threshold

2. voltage-gated Na+ channels activate, Na+ enter, and the axon section ________________

depolarizes

steps of action potential

1. a local potential depolarizes the axolemma of the trigger zone to threshold

2. voltage-gated Na+ channels activate, Na+ enter, and the axon section depolarizes

3. Na+ channels inactivate and voltage-gated K+ channels activate, and ____________________ begins

repolarization

steps of action potential

1. a local potential depolarizes the axolemma of the trigger zone to threshold

2. voltage-gated Na+ channels activate, Na+ enter, and the axon section depolarizes

3. Na+ channels inactivate and voltage-gated K+ channels activate, and repolarization begins

4. Na+ channels return to the ___________ state and repolarization continues

resting

steps of action potential

1. a local potential depolarizes the axolemma of the trigger zone to threshold

2. voltage-gated Na+ channels activate, Na+ enter, and the axon section depolarizes

3. Na+ channels inactivate and voltage-gated K+ channels activate, and repolarization begins

4. Na+ channels return to the resting state and repolarization continues

5. the axolemma may _________________ before K+ channels return to the resting state; after this, the axolemma returns to the resting me

hyperpolarize

What must be reached in order for voltage-gated sodium ion channels to open?

threshold (about -55mV)

The period during which an excitable cell either cannot respond to another stimulus (absolute) or requires a stronger stimulus to respond (relative)

refractory period

absolute refractory period

- no additional stimulus, no matter how strong, is able to produce an additional action potential.

- period coincides with the voltage-gated sodium ion channels being in their what state(s)?

- sodium ion channels may not be activated until they return to their resting states with their activation gates closed and their inactivation gates open.

activated and inactivated states

relative refractory period

- Immediately following the absolute refractory period is the relative refractory period, during which only a strong stimulus will produce an action potential.

- The relative refractory period is marked by a return of voltage-gated sodium ion channels to their ___________ state while some potassium ion channels remain activated.

resting

relative refractory period

- It's difficult to depolarize the membrane to threshold during this period because the _____________ ________ channels are activated, and the membrane is repolarizing or even hyperpolarizing.

- However, if a greater than normal stimulus is applied, the membrane may depolarize to threshold, and the axon may fire off another action potential.

potassium ion

local v. action potentials

local potentials

- Graded (produce changes in membrane potential of varying degree

- _______________ (once the stimulus stops, the ion channels close and the resting membrane potential is restored)

- Decremental and decrease over short distances

Reversible

local v. action potentials

action potentials

- Causes a ____________ __________________ of the same amount (to about 30mV) = all-or-nothing principle

- Size of AP is not determined by the strength, frequency, or length of the stimulus, and therefore is not graded like a local potential

- Irreversible (once threshold is reached, it cannot be stopped and will proceed to completion)

- Nondecremental (their strength does not diminish)

maximum depolarization

action potentials are _________________ = each action potential triggers another one in a neighboring section of the axon

self-propagating

propagation occurs from the trigger zone to the axon terminal in one direction because the membrane in the previous section (behind the AP) is still in its _____________ ______________

refractory period

AP propagation steps

1. axolemma depolarizes to threshold due to ____________ _________________

local potentials

AP propagation steps

1. axolemma depolarizes to threshold due to local potentials

2. as ________ channels activate, an AP is triggered and spreads down the axon

Na+

AP propagation steps

1. axolemma depolarizes to threshold due to local potentials

2. as Na+ channels activate, an AP is triggered and spreads down the axon

3. the next section of the axolemma depolarizes to threshold and does what? as the previous section of the axolemma repolarizes

4. the current continues to move down the axon, and the process repeats

fires an AP

what 2 factors influence conduction speed?

diameter of the axon, presence or absence of myelin sheath

____________ ____________________ = rapid type of neuronal AP conduction that occurs in myelinated axons in which only the nodes of Ranvier are depolarized

Saltatory conduction

______________ _______________ = disease in which certain cells of the immune system attack the myelin sheath around axons of the CNS

multiple sclerosis

multiple sclerosis

- a type of autoimmune disorder (one in which the patient's own immune system attacks a certain part of the body).

- in most cases, MS causes a progressive loss of the myelin sheath, which in turn produces loss of current from the neurons.

- symptoms of the disease result from gradual slowing of action potential propagation as _____________ ________________ becomes less efficient.

saltatory conduction