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Nerve Impulses
Syllabus 3.5.2
Toole: 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
•
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.
Membrane
Potential
•
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
Amino acids
Sulphate
Phosphate
Some Chloride
Mostly Chloride
Sodium-Potassium Pump
• Watch the following animation and explain
the features of the sodium-potassium
pump.
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter2/animation__how_the_sodium_po
tassium_pump_works.html
Resting potential animation
• http://bcs.whfreeman.com/thelifewire/conte
nt/chp44/4401s.swf
Resting Potential
•
Use the text book, the principle of the sodiumpotassium pump, sodium ion channels, potassium ion
channels, diffusion and the resting potential animation
to describe and explain:
1.
2.
What a resting potential is.
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 channels
2. Opening of a few sodium gated channels
3. 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-ornothing 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/a
nimations2.3.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