10.6: Cell Membrane Potential
• A cell membrane is usually electrically charged, or polarized, so that the inside
of the membrane is negatively charged with respect to the outside of the
membrane (which is then positively charged).
• This is as a result of unequal distribution of ions on the inside and the outside
of the membrane.
1
Distribution of Ions
• Potassium (K+) ions are the major intracellular positive ions (cations).
• Sodium (Na+) ions are the major extracellular positive ions (cations).
• This distribution is largely created by the Sodium/Potassium Pump
(Na+/K+ pump).
• This pump actively transports 3 sodium ions out of the cell and 2
potassium ions into the cell.
2
Resting Potential
• Resting Membrane Potential (RMP):
• 70 mV difference from inside to
outside of cell
• It is a polarized membrane
• Inside of cell is negative relative
to the outside of the cell
• RMP = -70 mV
• Due to distribution of ions inside
vs. outside
• Na+/K+ pump restores
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High Na+
Low K+
Low Na+
Impermeant
anions
High K+
Cell body
Axon Axon terminal
+
–
(a)
+
–
+
–
+–
– –
+ +
– –
+ +
+
–
–
+
+
–
+
–
–
–
+
+
–70 mV
+
–
–
+
(b)
+
+
– + –
+ +
– Na
– Low
+
Na+ Pump
–
–
K+
+
–
Low K++ – High K+–
+ +
High Na+
+
–
–
–
+
+
–70 mV
+
–
–
+
+
–
–
+
(c)
3
+
–
–
+
Local Potential Changes
• Caused by various stimuli:
• Temperature changes
Gate-like mechanism
• Light
• Pressure
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Protein
Cell
membrane
(a) Channel closed
Fatty acid
tail
Phosphate
head
(b) Channel open
• Environmental changes affect the membrane potential by opening a
gated ion channel
• Channels are 1) chemically gated, 2) voltage gated, or 3) mechanically
gated
4
Local Potential Changes
• If membrane potential becomes more negative, it has hyperpolarized
• If membrane potential becomes less negative, it has depolarized
• Graded (or proportional) to intensity of stimulation reaching threshold potential
• Reaching threshold potential results in a nerve impulse, starting an action potential
5
Local Potential Changes
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Na+
Na+
–62 mV
Neurotransmitter
(a)
Chemically-gated
Na+ channel
Presynaptic
neuron
Voltage-gated
Na+ channel
Trigger zone (axon hillock)
Na+
Na+
Na+
Na+ Na+
–55 mV
(b)
6
Action Potentials
• At rest, the membrane
is polarized (RMP = -70)
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Na+
• Threshold stimulus
reached (-55)
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
Na+
Na+
Na+
Na+
–0
–70
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
(a)
• Sodium channels
open and membrane
depolarizes (toward 0)
K+
Na+
Na+
K+
Na+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
–0
K+
Threshold
stimulus K+
K+
Na+
Na+ channels open
K+ channels closed
K+
Na+
Na+
–70
Na+
• Potassium leaves
cytoplasm and
membrane repolarizes
(+30)
• Brief period of
hyperpolarization (-90)
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Region of depolarization
(b)
K+
K+
Na+
K+
Na+
K+
Na+
Na+
Na+
K+
K+
K+
K+
K+
K+
Na+
Na+
Na+
K+
K+
K+
K+
K+
K+
K+
K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
–0
K+ channels open
Na+ channels closed
–70
Na+
Region of repolarization
(c)
7
Action Potentials
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+40
Membrane potential (millivolts)
Action potential
+20
0
–20
Resting potential
reestablished
–40
Resting
potential
–60
–80
Hyperpolarization
0
1
2
3
4
5
Milliseconds
6
7
8
8
Action Potentials
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Region of
action potential
+
+
+
+
+
+
+
+
+
+
+
–
–
–
–
–
–
–
–
–
+
+
–
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
–
–
–
–
–
–
–
–
–
–
–
–
+
+
+
(a)
+
–
+
+
–
–
+
+
Direction of nerve impulse
–
–
–
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
(b)
–
–
–
–
–
–
–
+
+
–
–
–
–
–
–
–
–
–
+
+
–
–
+
(c)
+
+
+
+
+
+
+
+
9
All-or-None Response
• If a neuron axon responds at all, it responds completely – with an action
potential (nerve impulse)
• A nerve impulse is conducted whenever a stimulus of threshold intensity or
above is applied to an axon
• All impulses carried on an axon are the same strength
10
Refractory Period
• Absolute Refractory Period
• Time when threshold stimulus does not start another action potential
• Relative Refractory Period
• Time when stronger threshold stimulus can start another action potential
11
Impulse Conduction
12
10.7: Synaptic Transmission
• This is where released neurotransmitters cross the synaptic cleft and react
with specific molecules called receptors in the postsynaptic neuron
membrane.
• Effects of neurotransmitters vary.
• Some neurotransmitters may open ion channels and others may close ion
channels.
13
Synaptic Potentials
• EPSP
• Excitatory postsynaptic potential
• Graded
• Depolarizes membrane of postsynaptic neuron
• Action potential of postsynaptic neuron becomes more likely
• IPSP
• Inhibitory postsynaptic potential
• Graded
• Hyperpolarizes membrane of postsynaptic neuron
• Action potential of postsynaptic neuron becomes less likely
14
Summation of
EPSPs and IPSPs
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• EPSPs and IPSPs are added together in
a process called summation
• More EPSPs lead to greater probability
of an action potential
Neuron
cell body
Nucleus
Presynaptic
knob
Presynaptic
axon
15
Neurotransmitters
16
Neurotransmitters
17
Neuropeptides
• Neurons in the brain or spinal cord synthesize neuropeptides.
• These neuropeptides act as neurotransmitters.
• Examples include:
• Enkephalins
• Beta endorphin
• Substance P
18
10.8: Impulse Processing
• Way the nervous system processes nerve impulses and acts upon them
• Neuronal Pools
• Interneurons
• Work together to perform a common function
• May excite or inhibit
• Convergence
• Various sensory receptors
• Can allow for summation of impulses
• Divergence
• Branching axon
• Stimulation of many neurons ultimately
19
Neuronal Pools
• Groups of interneurons that make synaptic connections with each other
• Interneurons work together to perform a common function
• Each pool receives input from other neurons
• Each pool generates output to other neurons
20
Convergence
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• Neuron receives input from several neurons
• Incoming impulses represent information from
different types of sensory receptors
• Allows nervous system to collect, process, and
respond to information
1
2
• Makes it possible for a neuron to sum impulses
from different sources
3
(a)
21
Divergence
• One neuron sends impulses to several
neurons
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• Can amplify an impulse
• Impulse from a single neuron in CNS
may be amplified to activate enough
motor units needed for muscle
contraction
4
6
5
(b)
22