Title: RMP, GP, AP
URL Source: blob://pdf/c6aa3cf6-ddea-4d93-89b6-177b816734e9
Markdown Content: MEMBRANE POTENTIAL
Explain the generation of the resting membrane potential, graded
potential, and the action potential
Describe the factors that maintain a resting membrane potential
Describe the phases of an action potential
Describe how action potentials are propagated THE PRODUCTION OF
a ligand stimulus
between open and closed position
plasma membrane?
This channel opens in response to a change in
membrane potential
What type of ion channel responds to stimulation such
as vibration, touch, pressure or tse stretching? ION CHANNELS ION CHANNELS RESTING MEMBRANE
Exists d/t unequal distribution of ions across the cell membrane
AT REST:
Cytosol: negative ions (e.g., proteins, phosphate groups, Cl-)
ECF: positive ions (Na+ and Ca2+)
The slight electrical imbalance across the membrane creates a
localized charged separation:
INSIDE: SLIGHTLY NEGATIVE
OUTSIDE: SLIGHTLY POSITIVE RESTING MEMBRANE POTENTIAL
The buildup of charge occurs only very close to the membrane . The
cytosol or extracellular fluid elsewhere in the cell contains equal
numbers of positive and negative charges and is electrically neutral.
The voltage difference we measure as the membrane potential
is not a bulk property of the entire ICF or ECF - CONCENTRATED
AT THE MEMBRANE ONLY
To allow fast responsiveness - allowing rapid
depolarization and repolarization
In neurons: -70 mV
In muscles: -90 mV THREE FACTORS THAT
ECF: Na+ and Cl-
Cytosol: K+ and 2 dominant anions - Phosphates (ATP) and
amino acids
Leak channels for K+ are more numerous than Na+ leak
channels
Number of K+ ions > Na+ that diffuse down
More and more K+ exit Inside cell: inc (-), outside cell: inc
(+) RMP : THREE FACTORS 1. 2. Inability of most anions to leave the cell
Most anions are attached to nondiffusable molecules such as
ATP and large proteins, hence, cannot follow K+ out of the cell RMP : THREE FACTORS 1. 2. 3. Electrogenic nature of the Na+-K+ ATPases
3 Na+ ions are pumped outside for each 2 K+ ions to the inside
net deficit of positive ions on the inside = negative potential
inside the cell
The pump removes more (+) charges from the cell than they
bring into the cell
Total contribution is very small: -3 mV of the total -70 mV RMP
of a typical neuron RECITATION GRADED POTENTIAL GRADED POTENTIAL
Small deviation f rom the resting membrane potential that makes
the membrane either
More polarized (inside more negative)
Hyperpolarizing graded potential
Less polarized (inside less negative)
Depolarizing graded potential
Occurs when a stimulus causes mechanically-gated or ligand-
gated to open or close
Happens mainly in the dendrites and cell body of a neuron GRADED POTENTIAL
Happens mainly in the dendrites and cell body of a neuron
Mechanically-gated channels and ligand-gated channels:
dendrites of sensory neurons
Ligand-gated channels: dendrites and cell bodies of
interneurons and motor neurons
GRADED Potential
Electrical signals vary in amplitude (small or large) - depending
on the strength of the stimulus
How many gated channels have opened
How long each remains open GRADED POTENTIAL
DECREMENTAL CONDUCTION
Graded potentials die out as they spread along the membrane
Because of this, GPs are useful for short distance
communication only RECITATION SPATIAL AND TEMPORAL
DECREMENTAL CONDUCTION
Although an individual GP dies out, it can become stronger and
last longer by summating with other graded potentials
Summation
Process by which graded potentials add together
The greater the summation of EPSPs (excitatory postsynaptic
potential), the greater the chance that threshold will be reached SPATIAL SUMMATION
Summation of postsynaptic potentials in response to stimuli that
occur at different loc ation s in the membrane of postsynaptic cell at
the same time
Buildup of neurotransmitter released simultaneously by several
presynaptic end bulbs
Many people voting YES or NO sa poll at the same time TEMPORAL SUMMATION
Summation of postsynaptic potentials in response to stimuli that
occur at the same loc ation in the membrane of postsynaptic cell but
at different times
Buildup of neurotransmitter released by a single presynaptic end
bulb two or more times in rapid succession
One person voting repeatedly and rapidly EPSPs vs IPSPs
Excitatory postsynaptic potential
Inh ib itory postsynaptic potential THE NET SUMMATION OF EPSPs
a. Total excitatory effects > total inhibitory effects BUT less than
threshold = EPSP that does not reach threshold
b. Subsequent stimuli can more easily generate a nn impulse
through summation since the neuron is partially depolarized
a. Total excitatory effects > total inhibitory effects
b. Impulses continue to be generated as long as the EPSP is at or
above the threshold level EPSPs vs IPSPs 1. as
a
IPSP
a. Total inhibitory effects > excitatory effects
b. Membrane hyperpolarizes (more negative) = inhibition of the
post synaptic neuron + inability to generate nerve impulse ACTION POTENTIAL NEURONS
Axon hillock
Helps to differentiate the axon from a dendrite of a neuron on
microscopy
Serves as the origin from which the axon extends
Serves as a neurons primary integrative zone, receiving various
excitatory and inhibitory stimuli
Nerve impulses arise at the junction of the axon hillock and the
initial segment, an area called the trigger zone , from which they
travel along the axon to their destination ACTION POTENTIALS
Electrical signal that propagates (travels) along the surface of the
membrane of a neuron
Aka nerve impulse
Sequence of rapidly occurring events that decrease and reverse the
membrane potential and then eventually restore it to the resting
state ACTION POTENTIALS
2 MAIN PHASES
(1) depolarizing phase
Negative membrane potential becomes less negative -
reaches 0 - then becomes positive
(2) repolarizing phase
Restored to -70 mV
After-hyperpolarizing phase
MP becomes more negative than the resting level ACTION POTENTIALS
2 TYPES OF VOLTAGE GATED CHANNELS OPEN AND CLOSE
DURING AN AP
Voltage-gated Na+ channels
First to open
Allow Na+ to rush into the cell - causes depolarization
Voltage-gated K+ channels
Allow s K+ to flow out - produces repolarization
After-hyperpolarizing phase - if voltage-gated K+ channels
remain open after repolarization ends ACTION POTENTIALS
AP IN NEURONS HAPPENS WHEN DEPOLATIZATION REACHES
THRESHOLD OF ABOUT -55 mV RECITATION THE GENERATION OF AN ACTION POTENTIAL
DEPENDS ON WHETHER A PARTICULAR STIMULUS IS ABLE TO BRING THE MEMBRANE TO THRESHOLD .
Will not occur in response to subthreshold stimulus
Weak depolarization that cannot bring the membrane potential to
threshold
Will occur in response to threshold stimulus
Stimulus just strong enough to depolarize the membrane to threshold
Several will form in response to suprathreshold stimulus
Stimulus that is strong enough to depolarize the membrane above
threshold ACTION POTENTIALS
CAUSED BY SUPRATHRESHOLD / THRESHOLD STIMULI
= amplitude
different frequency
frequency increases with stimulus strength until limited by
absolute refractory period
Threshold stimulus = one action potential
Suprathreshold stimulus = multiple action potentials
Keeps membrane depolarized / repeatedly stimulates it ALL -OR -NONE / ALL -OR -NOTHING
An action pot ential occurs completely (t hreshold stimulus) or it
does not occur at all (subthreshold stimulus) SUMMATION IS NEEDED WHEN A
(1) DEPOLARIZING PHASE / DEPOLARIZATION
Membrane depolarizes to threshold
Na+ channels open rapidly
Influx of Na+ (inward movement of Na+ d/t electrochemical
gradients)
-55 mV to +30 mV
Voltage-gated Na+ channel
Activation gate and inactivation gate
Resting state: activation gate: close; inactivation gate: open
Activated stage: activation and inactivation gates: open ACTION POTENTIALS
(1) DEPOLARIZING PHASE / DEPOLARIZATION
Na+ channels open > Na+ inflow increases > membrane depolarizes
further > more Na+ channels open
(2) REPOLARIZING PHASE / REPOLARIZATION
Inactivation gates of voltage-gated Na+ channels close
Voltage-gated K+ channels also open at threshold-level
depolarization but slowly
Hence, the opening of these channels occurs about the same
time the voltage-gated Na+ channels are closing ACTION POTENTIALS
(2) REPOLARIZING PHASE / REPOLARIZATION
Slower opening of voltage-gated K+ channels and the closing of
previously open voltage-gated Na+ channels
(3) AFTER-HYPERPOLARIZING PHASE / HYPERPOLARIZATION
Outflow of K+ may be large enough to cause an after-
hyperpolarizing phase of the AP
Voltage-gated K+ channels remain open
Voltage-gated K+ channels
Alternate between closed (resting) and open (activated) states ACTION POTENTIALS
(3) AFTER-HYPERPOLARIZING PHASE / HYPERPOLARIZATION
Voltage-gated K+ channels
Alternate between closed (resting) and open (activated) states
Do not exhibit inactivated state
Membrane potential becomes even more negative (about -90 mV)
As voltage-gated K+ channels close, membrane potential returns to
resting level of -70 mV REFRACTORY PERIOD REFRACTORY PERIOD
Period of time after an action potential begins during which an
excitable cell cannot generate another action potential in response
to a normal threshold stimulus
2 TYPES:
Absolute
Relative REFRACTORY PERIOD
ABSOLUTE REFRACTORY PERIOD
Even a very strong stimuli cannot initiate a second AP
Coincides with the period of Na+ channel activation and
inactivation
RELATIVE REFRACTORY PERIOD
Period of time during which a second AP can be initiated, but
only by a larger-than-normal stimulus
Coincides with the period when the voltage-gated K+ channels
are still open RECITATION PROPAGATION OF ACTION
Action potentials
Allows communication between cells
Needed to travel long distances: propagation
Graded potentials die out but APs do not.
AP maintain their full strength throughout their travel d/t
regeneration of signals at each segment (sequential opening of
voltage-gated sodium channels) PROPAGATION OF AP
Two types of propagation
Continuous conduction
Saltatory conduction CHARACTERISTIC GRADED POTENTIALS ACTION POTENTIALS
Origin Mainly in dendrites and cell body Tirgeer zones and propagate along axon
Type of channels Ligand-gated or mechanically-gated Voltage-gated channels for Na+ and K+
Conduction Decremental; over short distances Propagate; over longer distances
Amplitude Depending on the strength of stimulus All or none
Polarity May be hyperpolarizing or depolarizing Always has depolarizing phase followed by
repolarizing phase then return to RMP
Refractory period None; (+) summation Present; (-) summation MYELINATION MYELINATION
Myelin sheath
Multilayer lipid and protein covering
Insulates axons and increas es speed of nerve conduction
When an axon is surrounded by a myelin sheath , it is said to be
myelinated
Axons without covering are unmyelinated
2 types of neuroglia that produce myelin sheaths
(1) Schwann cells - PNS
(2) Oligodendrocytes - CNS MYELINATION
Nodes of Ranvier
Gaps in the myelin sheath (unmyelinated)
Appear at interval s along t he axon
Amount of myelin inc from birth to maturity PROPAGATION OF AP
CONTINUOUS CONDUCTION
Involves step-by-step depolarization and repolarization of each
adjacent segment of the PM
Occurs in unmyelinated axons and in mm fibers
SALTATORY CONDUCTION
Occurs along myelinated axons
Myelin sheath: few voltage-gated channels are present
Nodes of Ranvier: numerous voltage-gated channels
Current carried by Na+ and K+ flows across the membrane mainly at
the nodes PROPAGATION OF AP
FLOW OF CURRENT AT THE NOTES OF RANVIER
jump)
Current flows from one node to next > travels much faster than
it would in an unmyelinated axon of the same axon
Smaller number of channels at the nodes rather than many
channels in each adjacent segment of the membrane
Minimal inflow of Na+ and outflow of K+
Less ATP used by Na+-K+ ATPase FACTORS THAT AFFECT THE
THREE MAJOR FACTORS THAT AFFECT SPEED OF PROPAGATION
a. More rapidly along myelinated axons
a. Large diameter > smaller diameter axons d/t surface area
a. Slows when cooled and vice versa CLASSIFICATION OF NERVE
a. Large, myelinated
a. Small, myelinated
a. Sm all, unmyelinated RECITATION SIGNAL TRANSMISSION AT
SYNAPSE
Region where communication occurs between 2 neurons or
between a neuron and an effector cell (mm cell / glandular cell)
PRESYNAPTIC NEURON
Nerve cell that carries a nerve impulse toward a synapse
Cell that sends a signal
POSTSYNAPTIC NEURON
Carries a nn impulse away from a synapse or an effector cell
that responds to the impulse TYPES OF SYNAPSES SYNAPSES
AP conduct directly between PM of adjacent neurons thru gap
junctions
Common in visceral smooth mm, cardiac mm, developing
embryo, brain
2 main advantages
(1) Faster communication
(2) Synchronization SYNAPSES 1. 2. CHEMICAL SYNAPSE
Synaptic cleft
Space filled with interstitial fluid in between presynaptic and
postsynaptic neurons
An indirect form of communication
Presynaptic neurons releases neurotransmitter that diffuses
through the fluid in the synaptic cleft and binds to receptors in
the PM of the postsynaptic neuron
Pre-synaptic neuron:
Postsynaptic neuron: SYNAPSES 1. 2. CHEMICAL SYNAPSE
Synaptic delay
0.5 msec
Time required for the aforementioned processes to occur
Reason why chemical synapses relay signals more slowly
than electrical synaps es NEUROTRANSMITTERS SYNAPSES
Most common neurotransmitters in the PNS: Acetylcholine and
Norepinehprine
Most common inhibitory neurotransmitter in the CNS: GABA
(gamma-aminobutyric acid)
Most common excitatory neurotransmitter in the CNS: Glutamate
Dopamine:
INC dopamine - schizophrenia
DEC dopamine - Parkinsons dse RECITATION Thank