Nerve Impulses

Syllabus 3.5.2Toole: Pages 164-170

Aims:

1. Explain how resting potentials occur.

2. Explain how an action potential is generated, in terms of:

• membrane permeability• all-or-nothing principle• the refractory period.

Plasma Membrane

Membrane Potential

• All living cells have an electrical charge difference across their plasma membrane.

• The inside of the cell is more negative than the outside.

• This difference in charge gives rise to an electrical voltage gradient across the membrane, which can be measured with ultra fine microelectrodes.

• The voltage measured across the plasma membrane is called the membrane potential, and is typically in the range of -50mV to -100mV in an animal cell. In this condition the axon is said to be polarised.

• The voltage outside the cell is said to be zero, so the minus sign indicates that the inside of the cell is negative with respect to the outside.

• The resting potential arises from two things:

1. Differences in the ionic composition of the intracellular and extracellular fluids.

– List the differences in cation (positively charged ions) and anion (negatively charged ions) concentrations within the intracellular and extracellular fluid of an axon.

2. The selective permeability of the plasma membrane.

– Ions being electrically charged, cannot dissolve in lipid, and thus cannot directly diffuse across the lipid of the plasma membrane.

– In order to cross the membrane, ions must either be carried by transport proteins or move through ion channels.

INTRACELLULAR FLUID

EXTRACELLULAR FLUID

CATIONS (+)Mostly Potassium Some Potassium

Some Sodium Mostly Sodium

ANIONS (-)Protein Mostly Chloride

Amino acids

Sulphate

Phosphate

Some Chloride

Sodium-Potassium Pump

• Watch the following animation and explain the features of the sodium-potassium pump.

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Resting potential animation

• http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf

Resting Potential• Use the text book, the principle of the sodium-

potassium pump, sodium ion channels, potassium ion channels, diffusion and the resting potential animation to describe and explain:

1. What a resting potential is.2. How ion concentration gradients and electrical concentration

gradients maintain a resting potential of –65mV.

• This information should be displayed in the form of an annotated diagram.

• These diagrams should then be presented to the group.

Changes in membrane potentials• Certain cells have the ability to generate changes in their

membrane potentials.

• These cells include neurons and muscles.

• Neurons have gated ion channels that allow a cell to change its membrane potential in response to stimuli the cell receives.

• The effect of the stimulus on the neuron depends on the type of gated ion channel opened by the stimulus.

• There are three main types of response:1. Opening of potassium gated channels2. Opening of a few sodium gated channels3. Opening of many sodium gated channels

Opening of potassium gated channels

• An increased efflux of potassium will occur.

• The membrane potential will become more negative.

• Such an increase in the electrical gradient across the membrane is called hyperpolarisation.

Opening of a few sodium gated channels

• An increased influx of sodium will occur.

• The membrane potential will become less negative.

• Such a decrease in the electrical gradient across the membrane is called depolarisation.

Graded Potentials

• Voltage changes produced by stimulation of this type are said to be graded potentials.

• The magnitude of the change (either hyperpolarisation or depolarisation) depends on the strength of the stimulus.

• Therefore a larger stimulus will open more channels and produce a larger change in permeability.

Opening of many sodium gated channels

• A much increased influx of sodium will occur.

• The membrane potential will become less negative.

• When a particular level of depolarisation is reached, known as the threshold potential an action potential will be triggered.

Action Potentials• The action potential is the

nerve impulse.

• It is a non-graded or all-or-nothing event.

• This means that the magnitude of the action potential is independent of the strength of the depolarising stimulus that produced it, provided that the depolarisation is sufficiently large to reach threshold.

• Once the action potential is triggered, the membrane potential goes through a stereotypical sequence of changes.

Animation

• http://www.sinauer.com/neuroscience4e/animations2.3.html

Action Potential Explained

1. Na+ & K+ channels are closed.

2. Na+ are opened & K+ channels remain closed.

3. Na+ are closed & K+ channels open.

4. Na+ remain closed & K+ channels remain open.

The Refractory Period• During the undershoot, the NA+ channels are closed.

• If a second depolarising stimulus arrives during this period, it will be unable to trigger an action potential, as the Na+ gates have not had time to recover after the preceding action potential.

• This period when the neuron is insensitive to depolarisation is called the refractory period.

• The advantages of the refractory period are:1. It ensures that an action potential is propagated in one direction.2. It produces a discrete impulse.

Plenary

• http://bioactive.mrkirkscience.com/39/swfs/ch39quiz.swf