AP Biology

Nervous SystemsPart 2

• Important concepts from previous units:• Energy can be associated with charged particles, called ions.• Established concentration gradients can be a source of energy.• Active transport requires a transport protein, in the cell

membrane, and an energy source.

I. Membrane Potential - The ability for the membrane to potentially do work.A. This is “created” by electrical charge gradients across the membrane.

1. Ion Concentrations on each side of the membrane create the charge gradient.

Polarity exists2. (-) Anions (Inside) 3. (+) Cations (Outside)

Membrane Potential and ion concentrations

CYTOSOL EXTRACELLULARFLUID

[Na+]15 mM

[K+]150 mM

[A–]100 mM

[Na+]150 mM

[K+]5 mM

[Cl–]120 mM

[Cl–]10 mM

Plasmamembrane

Na+/K+ pumps

Cytoplasmic Na+ bonds tothe sodium-potassium pump

CYTOPLASMNa+

[Na+] low[K+] high

Na+

Na+

EXTRACELLULARFLUID

[Na+] high[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causesthe protein to change itsconformation, expelling Na+

to the outside.

P

Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.

PP

Loss of the phosphaterestores the protein’soriginal conformation.

K+ is released and Na+

sites are receptive again;the cycle repeats.

K+

K+

K+

K+

K+

Ions and Energy

150 mMKCl

Innerchamber

Outerchamber

–92 mV

Potassiumchannel

5 mMKCl

Artificialmembrane

K+

Cl–

150 mMNaCl

Innerchamber

Outerchamber

+62 mV

Sodiumchannel

15 mMNaCl

Na+

Cl–

• II. Resting Potential - This term refers to an unstimulated neuron. The electrochemical gradient is established.

• Na+ / K+ Pump - This is a cell membrane bound protein. It controls the movement of the charged particles of Na+ and K+. Remember, these molecules can also be called ligands.

1. This protein requires ATP to work. (This is an example of active transport.)2. It is responsible for generating a Nerve Impulsive. (A.K.A. Electrical Impulse.)

B. Gated Ion Channels – These proteins help the Na+/K+ pump generate a nerve impulse quickly.

1. Chemically—Gated Ion Channels - These respond to chemically charged ligands in the cellular fluids.

2. Voltage—Gated Ion Channels - These respond to the “charge” concentrations because of the ligands.

C. Depolarization1. This phase refers to the flood of Na+ into the

cell. This “destroys” the polarity.2. Graded Potentials - This term refers to “how

much” Na+ entered? (Like your tests have different levels…grades.)

a. Threshold Potential – This terms refers to the minimum that must enter to generate a nerve impulse within a neuron. It is considered an all-or-none event. It either generates a nerve impulse or it does not.

b. Action Potential “Spike” – Term refers to the electrical generated impulse in a nerve. (Action can now occur… a nerve impulse that can be relayed to the next cell.)

D. Repolarization1. This phase refers to the pumping out of K+ from the

neuron in an attempt to return to a resting potential state. (The cell is trying to re-establish polarity.)

Impulse generation

Resting potential

Threshold

Me

mb

ran

e p

ote

nti

al

(mV

)

Actionpotential

Time–100

–50

+50

0

Potassiumchannel

Extracellular fluid

Plasma membrane

Na+

Resting state

Inactivationgate

Activationgates

Sodiumchannel K+

Cytosol

Na+

Depolarization

K+

Na+

Na+

Rising phase of the action potential

K+

Na+

Na+

Falling phase of the action potential

K+

Na+

Na+

Undershoot

K+

Na+

E. Hyperpolarization1.This phase refers to the excess pumping out

of K+ from the cell; so the cell will pull some back in.2. This is necessary to re-establish a Resting

Potential; when the electrochemical concentration gradient exists.

3. No generation of a “spike” is possible until polarity is re-established.

4. Refractory Period - This term refers to the period of time when a neuron cannot make a new impulse.

Resting Potential

Hyperpolarizations

Graded potential hyperpolarizations Graded potential depolarizations

5Time (msec)

Restingpotential

43210

Threshold

–100

–50

0

Mem

bra

ne

po

ten

tial

(m

V)

Stimuli

+50

Depolarizations

5Time (msec)

Restingpotential

43210

Threshold

–100

–50

0

Mem

bra

ne

po

ten

tial

(m

V)

Stimuli

+50

Action potential

5Time (msec)

Restingpotential

43210

Threshold

–100

–50

0

Mem

bra

ne

po

ten

tial

(m

V)

Stronger depolarizing stimulus

+50Actionpotential

6

III. Propagation - This term refers to the impulsive traveling down an axon. (It is like a cup overflowing into the next empty cup. Then when it gets full, it flows over into the next empty cup in the line. This overflow of Na+ causes the voltage gates to open in the next segment, causing Na+ to flood into that segment.

Propagation

An action potential is generated as Na+ flows inward across the membrane at one location.

Na+

Action potential

Axon

Na+

Action potentialK+

The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

K+

Na+

Action potentialK+

The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

K+

IV. Saltatory Conduction – This term refers to the “hopping” of the impulse over the Schwann cells. This makes for extremely quick travel down an axon. The impulse must “jump” from node to node. This is because ion exchange can only occur at exposed membranes… where the Na+/K+ pumps are located. When you stub your toe, how long does it take for signal to get to your brain?

Saltatory Conduction

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Myelinsheath

Axon

VI. Reflex Arc (The simplest neural pathway.)A. A stimulus energy is detected by a Sensory Neuron. The neuron carries the energy signal to the spinal cord.B. An Interneuron (of the CNS – spinal cord) relays the energy back out to the motor nerve instead of to the brain. (“inter” means “go between”… between the sensory and motor neurons.)C. The energy is carried out of the CNS by the Motor Neuron. It carries the energy to an effector cell, usually a muscle.D. This is why you do not think about a reflex, it just happens. The stimulus never made it to the brain for integration.

Reflex Arc