Neuron Membranes & Neuron Membranes & the Action Potentialthe Action Potential

Chapter 9: Nervous SystemChapter 9: Nervous System

Unit 3: Integration and Unit 3: Integration and CoordinationCoordination

Cell Membrane Cell Membrane PotentialPotential The surface of the cell membrane The surface of the cell membrane

is usually polarized (charged), with is usually polarized (charged), with respect to the inside.respect to the inside.

– This polarization arises from an This polarization arises from an unequal distribution of positive and unequal distribution of positive and negative ions between sides of the negative ions between sides of the membrane.membrane.

This polarization is particularly important This polarization is particularly important in the conduction of muscle and nerve in the conduction of muscle and nerve impulses.impulses.

Distribution of IonsDistribution of Ions

The distribution of ions inside and The distribution of ions inside and outside cell membranes is outside cell membranes is determined in part by pores or determined in part by pores or channels in those membranes.channels in those membranes.

– Some channels are always open, and Some channels are always open, and others can be opened and closed.others can be opened and closed.

– Most channels are Most channels are selective selective and and only allow one type of ion/molecule only allow one type of ion/molecule through.through.

Resting Membrane Resting Membrane PotentialPotential Active transport creates a Active transport creates a

concentration gradient across the concentration gradient across the cell membrane of sodium and cell membrane of sodium and potassium ions.potassium ions.– Na+/K+ pumps work to move Na+ out Na+/K+ pumps work to move Na+ out

of the neuron and K+ into the neuron.of the neuron and K+ into the neuron.– Uses ATP as energy sourceUses ATP as energy source

ATPase breaks down ATP into ADP + PATPase breaks down ATP into ADP + P Similar effects as those we saw in the Similar effects as those we saw in the

ATPase of myosin heads in muscle fibersATPase of myosin heads in muscle fibers

Resting Membrane Resting Membrane PotentialPotential The Na+/K+ pump creates:The Na+/K+ pump creates:

– High concentration of sodium outsideHigh concentration of sodium outside– High concentration of potassium insideHigh concentration of potassium inside

Negative amino acids are found in Negative amino acids are found in abundance inside the cellabundance inside the cell– Would make the intracellular fluid more Would make the intracellular fluid more

negative, but…negative, but… Chlorine ions (Cl-) are found in Chlorine ions (Cl-) are found in

abundance outside the cellabundance outside the cell– Counteract the negative of the amino acidsCounteract the negative of the amino acids

Resting Membrane Resting Membrane PotentialPotential If these were the only factors involved, If these were the only factors involved,

the neuron would be neutralthe neuron would be neutral However, K+ leaks out of the cell However, K+ leaks out of the cell

through through leakage channelsleakage channels that are that are always openalways open

Positives leaving the intracellular Positives leaving the intracellular space means the overall charge is space means the overall charge is negative insidenegative inside– (-70mV)(-70mV)

Potential ChangesPotential Changes

When neurons are excited When neurons are excited (stimulated)(stimulated)

– Affect the resting potential in a Affect the resting potential in a particular region of a nerve cell particular region of a nerve cell membrane.membrane.

– If the membrane’s resting potential If the membrane’s resting potential decreases (as the inside of the decreases (as the inside of the membrane becomes less negative membrane becomes less negative when compared to the outside), the when compared to the outside), the membrane is said to be membrane is said to be DepolarizingDepolarizing..

Potential ChangesPotential Changes

Changes in potential are directly Changes in potential are directly proportional to the proportional to the intensityintensity of of the stimulation.the stimulation.

– If additional stimulation arrives If additional stimulation arrives before the effect of previous before the effect of previous stimulation subsides, summation stimulation subsides, summation takes place.takes place.

– As a result of summated potentials, As a result of summated potentials, a level called a level called Threshold PotentialThreshold Potential may be reached.may be reached.

Action Potential Action Potential

An An action potentialaction potential can be can be thought of as the “firing” of the thought of as the “firing” of the neuron.neuron.

Action potentials will Action potentials will propagate propagate down the length of a neuron’s down the length of a neuron’s axonaxon

Action potentials are the electrical Action potentials are the electrical signals that move down a neuronsignals that move down a neuron

Action Potential Action Potential

Many subthreshold potential changes Many subthreshold potential changes must combine to reach threshold, and must combine to reach threshold, and once threshold is achieved, an event once threshold is achieved, an event called called Action PotentialAction Potential occurs. occurs.

– At the threshold potential, permeability to At the threshold potential, permeability to ions changes suddenly at the region of the ions changes suddenly at the region of the cell membrane being stimulated.cell membrane being stimulated.

– This is due to the presence of This is due to the presence of voltage-voltage-gated ion channelsgated ion channels

Channels that respond to changes in membrane Channels that respond to changes in membrane potential (voltage)potential (voltage)

There are VGICs that are permeable to only K+ There are VGICs that are permeable to only K+ and others that are permeable to only Na+and others that are permeable to only Na+

Action PotentialAction Potential

When When threshold threshold is reached:is reached:– 1. VGICs that are permeable to Na+ open1. VGICs that are permeable to Na+ open

Na+ diffuses into neuronNa+ diffuses into neuron Neuron’s membrane potential rises from -Neuron’s membrane potential rises from -

70mV to +40mV (70mV to +40mV (depolarizesdepolarizes))

– 2. Na+ channels close as K+ VGICs open2. Na+ channels close as K+ VGICs open K+ diffuses out of neuronK+ diffuses out of neuron Neuron’s membrane potential Neuron’s membrane potential repolarizesrepolarizes, ,

going from +40mV to nearly -85mVgoing from +40mV to nearly -85mV

Action PotentialAction Potential

When When threshold threshold is reached:is reached:– 3. The cell is now in what is called the 3. The cell is now in what is called the

refractory periodrefractory period (it’s too negative) (it’s too negative) Neuron will not respond to further Neuron will not respond to further

stimulation at this timestimulation at this time This is due to K+ channels being open a This is due to K+ channels being open a

bit too longbit too long

– 4. All VGICs return to normal and the 4. All VGICs return to normal and the neuron is ready to “fire” againneuron is ready to “fire” again

Nerve ImpulseNerve Impulse

When an action potential occurs in one When an action potential occurs in one region of a Neuron membrane, it region of a Neuron membrane, it causes a bioelectric current to flow to causes a bioelectric current to flow to adjacent portions of the membrane.adjacent portions of the membrane.

– This This Local CurrentLocal Current stimulates the stimulates the adjacent membrane to its threshold level adjacent membrane to its threshold level and triggers another action potential.and triggers another action potential.

A wave of action potentials move down the axon A wave of action potentials move down the axon to the end.to the end.

– This propagation of action potentials along a This propagation of action potentials along a nerve axon constitutes a nerve axon constitutes a Nerve ImpulseNerve Impulse..

– AnimationAnimation

Events Leading to the Events Leading to the Conduction of a Nerve Conduction of a Nerve ImpulseImpulse

1.1.Neuron membrane maintains resting Neuron membrane maintains resting potential.potential.

2.2.Threshold stimulus is received.Threshold stimulus is received.

3.3.Sodium channels in a local region of Sodium channels in a local region of the membrane open.the membrane open.

4.4.Sodium ions diffuse inward, Sodium ions diffuse inward, depolarizing the membrane.depolarizing the membrane.

Events ContinuedEvents Continued

5.5.Potassium channels in the membrane Potassium channels in the membrane open.open.

6. Potassium ions diffuse outward, 6. Potassium ions diffuse outward, repolarizing the membrane.repolarizing the membrane.

7.7.The resulting action potential causes a The resulting action potential causes a local bioelectric current that stimulates local bioelectric current that stimulates adjacent portions of the membrane.adjacent portions of the membrane.

8.8.Wave of action potentials travels the Wave of action potentials travels the length of the axon as a nerve impulse.length of the axon as a nerve impulse.

Impulse ConductionImpulse Conduction

A myelinated axon functions as an A myelinated axon functions as an insulator and prevents almost all ion insulator and prevents almost all ion flow through the membrane it encloses.flow through the membrane it encloses.

– Nodes of Ranvier between adjacent Nodes of Ranvier between adjacent Schwann cells interrupt the sheath.Schwann cells interrupt the sheath.

Action potentials occur at these nodes, where the Action potentials occur at these nodes, where the exposed axon membrane contains sodium and exposed axon membrane contains sodium and potassium channels.potassium channels.

– Nerve impulses jump from node to node, and are Nerve impulses jump from node to node, and are many times faster than conduction on an many times faster than conduction on an unmyelinated axon.unmyelinated axon.

Speed of Nerve Speed of Nerve ImpulsesImpulses The speed of nerve impulse The speed of nerve impulse

conduction is proportional to the conduction is proportional to the diameter of the axon.diameter of the axon.

– The greater the diameter, the faster The greater the diameter, the faster the impulse.the impulse.

All-or-None ResponseAll-or-None Response

Nerve impulse conduction is an all-Nerve impulse conduction is an all-or-none response.or-none response.

– If a neuron responds at all, it responds If a neuron responds at all, it responds completely.completely.

A nerve impulse is conducted whenever a A nerve impulse is conducted whenever a stimulus of threshold intensity or above is stimulus of threshold intensity or above is applied to an axon, and all impulses carried applied to an axon, and all impulses carried on that axon are of the same strength.on that axon are of the same strength.

– A greater intensity of stimulation does not A greater intensity of stimulation does not produce a stronger impulse, but more impulses produce a stronger impulse, but more impulses per second.per second.