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BIOELECTRICITY AND EXCITABLE TISSUE
THE ORIGIN OF BIOELECTRICITY AND HOW NERVES WORK
D. C. Mikulecky Department of Physiology andFaculty Mentoring Program
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THE RESTING CELL
HIGH POTASSIUMLOW SODIUMNA/K ATPASE PUMP RESTING POTENTIAL ABOUT 90
- 120 MVOSMOTICALLY BALANCED
(CONSTANT VOLUME)
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BIOELECTRICITY
THE ORIGIN OF THE MEMBRANE POTENTIAL
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MOBILITY OF IONS DEPENDS ON HYDRATED SIZE
IONS WITH SMALLER CRYSTAL RADIUS HAVE A HIGHER CHARGE DENSITY
THE HIGHER CHARGE DENSITY ATTRACTS MORE WATER OF HYDRATION
THUS THE SMALLER THE CRYSTAL RADIUS, THE LOWER THE MOBILITY IN WATER
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IONS MOVE WITH THEIR HYDRATION SHELLS
- +- +- +
- +
Hydration Shells
- +
- +
-
+
- +
- +- +
-
+
-
+
- +
- +-
+
-
+
- +
- +
- +
- +
- +
- +
- + -
+
- +
- +
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ELECTRONEUTRAL DIFFUSSION
HIGH SALT CONCEMTRATION
LOW SALT CONCEMTRATION
+
-
BARRIER SEPARATES THE TWO SOLUTIONS
+
-+
-
+
-+
-+
-
+
-
+
-
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ELECTRONEUTRAL DIFFUSSION
HIGH SALT CONCEMTRATION
LOW SALT CONCEMTRATION
+
-
BARRIER REMOVED
+
-
+
-
+
-+
-+
-+
-
+-
+ -
CHARGE SEPARATION = ELECTRICAL POTENTIAL
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ELECTRICAL POTENTIAL=CHARGE SEPARATION
In water, without a membrane hydrated Chloride is smaller than hydrated Sodium, therefore faster:
Cl-
Na+
The resulting separation of charge is called an ELECTRICAL POTENTIAL
+ -
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THE MEMBRANE POTENTIAL
MEMBRANE
ExtracellularFluid
IntracellularFluid
Na+
K+
Sodium channel is less open causing sodium to be slower
Potassium channel is more open causing potassium to be faster + - MEMRANE POTENTIAL
(ABOUT 90 -120 mv)
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THE ORIGIN OF BIOELECTRICITY
POTASSIUM CHANNELS ALLOW HIGH MOBILITY
SODIUM CHANNELS LESS OPENCHARGE SEPARATION OCCURS
UNTIL BOTH MOVE AT SAME SPEED STEADY STEADY IS ACHIEVED
WITH A CONSTANT MEMBRANE POTENTIAL
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THE RESTING CELL
HIGH POTASSIUMLOW SODIUMNA/K ATPASE PUMP RESTING POTENTIAL ABOUT 90
- 120 MVOSMOTICALLY BALANCED
(CONSTANT VOLUME)
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ACTIVE TRANSPORT
ADP
ATP
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ACTIVE TRANSPORT REQUIRES AN INPUT OF ENERGY
USUALLY IN THE FORM OF ATPATPase IS INVOLVEDSOME ASYMMETRY IS
NECESSARYCAN PUMP UPHILL
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EXCITABLE TISSUES
NERVE AND MUSCLEVOLTAGE GATED CHANNELSDEPOLARIZATION LESS THAN
THRESHOLD IS GRADEDDEPOLARIZATION BEYOND
THRESHOLD LEADS TO ACTION POTENTIAL
ACTION POTENTIAL IS ALL OR NONE
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THE NERVE CELL
CELLBODY
DENDRITES
AXON
AXONHILLOCK
AXONTERMINALS
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EXCITABLE TISSUES:THE ACTION POTENTIAL
THE MEMBRANE USES VOLTAGE GATED CHANNELS TO SWITCH FROM A POTASSIUM DOMINATED TO A SODIUM DOMINATED POTENTIAL
IT THEN INACTIVATES AND RETURNS TO THE RESTING STATE
THE RESPONSE IS “ALL OR NONE”
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FOR EACH CONCENTRATION DIFFERENCE ACROSS THE MEMBRANE THERE IS AN ELECTRIC POTENTIAL DIFFERENCE WHICH WILL PRODUCE EQUILIBRIUM.
AT EQUILIBRIUM NO NET ION FLOW OCCURS
EQUILIBRIUM POTENTIALS FOR IONS
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THE EQUILIBRIUM MEMBRANE POTENTIAL FOR POTASSIUM IS -90 mV
+ -CONCENTRATION
POTENTIALK+ K+
IN
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THE EQUILIBRIUM MEMBRANE POTENTIAL FOR SODIUM IS + 60 mV
Na+Na+
+-CONCENTRATION
POTENTIAL
INOUT
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THE RESTING POTENTIAL IS NEAR THE POTASSIUM EQUILIBRIUM POTENTIAL
AT REST THE POTASSIUM CHANNELS ARE MORE OPEN AND THE POTASSIUM IONS MAKE THE INSIDE OF THE CELL NEGATIVE
THE SODIUM CHANNELS ARE MORE CLOSED AND THE SODIUM MOVES SLOWER
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EVENTS DURING EXCITATION
DEPOLARIZATION EXCEEDS THRESHOLDSODIUM CHANNELS OPENMEMBRANE POTENTIAL SHIFTS FROM
POTASSIUM CONTROLLED (-90 MV) TO SODIUM CONTROLLED (+60 MV)
AS MEMBRANE POTENTIAL REACHES THE SODIUM POTENTIAL, THE SODIUM CHANNELS CLOSE AND ARE INACTIVATED
POTASSIUM CHANNELS OPEN TO
REPOLARIZE THE MEMBRANE
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OPENING THE SODIUM CHANNELS ALLOWS SODIUM TO RUSH IN
THE MEMBRANE DEPOLARIZES AND THEN THE MEMBRANE POTENTIAL APPROACHES THE SODIUM EQUILIBRIUM POTENTIAL
THIS RADICAL CHANGE IN MEMBRANE POTENTIAL CAUSES THE SODIUM CHANNELS TO CLOSE (INACTIVATION) AND THE POTASSIUM CHANNELS TO OPEN REPOLARIZING THE MEMBRANE
THERE IS A SLIGHT OVERSHOOT (HYPERPOLARIZATION) DUE TO THE POTASSIUM CHANNELS BEING MORE OPEN
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GRADED VS ALL OR NONE
A RECEPTOR’S RESPONSE TO A STIMULUS IS GRADED
IF THRESHOLD IS EXCEEDED, THE ACTION POTENTIAL RESULTING IS ALL OR NONE
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PROPAGATION OF THE ACTION POTENTIAL
+++++
--------
---------------------
+++++++++++++AXON MEMBRANE
INSIDE
OUTSIDEACTION POTENTIAL
DEPOLARIZINGCURRENT
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PROPAGATION OF THE ACTION POTENTIAL
+++++
--------
---------------------
+++++++++++++AXON MEMBRANE
INSIDE
OUTSIDEACTION POTENTIAL
DEPOLARIZINGCURRENT
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PROPAGATION OF THE ACTION POTENTIAL
--+++
++------
+++------------------
---++++++++++AXON MEMBRANE
INSIDE
OUTSIDEACTION POTENTIAL
DEPOLARIZINGCURRENT
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PROPAGATION OF THE ACTION POTENTIAL
--------
+++++
++++++-------
-----------++++AXON MEMBRANE
INSIDE
OUTSIDEACTION POTENTIAL
DEPOLARIZINGCURRENT
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SALTATORY CONDUCTION
+++++
--------
--------
+++++AXON MEMBRANE
INSIDE
OUTSIDEACTION POTENTIAL
DEPOLARIZINGCURRENT
MYELINNODE OFRANVIER
NODE OFRANVIER
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NORMALLY A NERVE IS EXCITED BY A SYNAPSE OR BY A RECEPTOR
MANY NERVES SYNAPSE ON ANY GIVEN NERVE
RECEPTORS HAVE GENERATOR POTENTIALS WHICH ARE GRADED
IN EITHER CASE WHEN THE NERVE IS DEPOLARIZED BEYOND THRESHOLD IT FIRE AN ALL-OR-NONE ACTION POTENTIAL AT THE FIRST NODE OF RANVIER
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THE SYNAPSE
JUNCTION BETWEEN TWO NEURONSCHEMICAL TRANSMITTERMAY BE 100,000 ON A SINGLE CNS
NEURONSPATIAL AND TEMPORAL
SUMMATIONCAN BE EXCITATORY OR INHIBITORY
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THE SYNAPSE
SYNAPTICVESSICLES
•••
•••
••• •••
•••
•••
•••
•••
•••
INCOMINGACTION POTENTIAL
CALCIUM CHANNEL
IONCHANNEL
RECEPTOR
ENZYME
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THE SYNAPSE
SYNAPTICVESSICLES
•••
•••
••• •••
•••
•••
•••
•••
•••
INCOMINGACTION POTENTIAL
CALCIUM CHANNEL
IONCHANNEL
RECEPTOR
ENZYME
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THE SYNAPSE
SYNAPTICVESSICLES
•••
•••
••• •••
•••
•••
•••
•••
•••
INCOMINGACTION POTENTIAL
CALCIUM CHANNEL
IONCHANNEL
RECEPTOR
ENZYME
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THE SYNAPSE
SYNAPTICVESSICLES
•••
•••
•••
•••
•••
•••
•••
•••
•••
CALCIUM CHANNEL
IONCHANNEL
RECEPTOR
ENZYME
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THE SYNAPSE
SYNAPTICVESSICLES
•••
••• •••
•••
•••
•••
•••
•••
•••
CALCIUM CHANNEL
IONCHANNEL
RECEPTOR
ENZYME
•••
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THE SYNAPSE
SYNAPTICVESSICLES
•••
•••
••• •••
•••
•••
•••
•••
•••
CALCIUM CHANNEL
IONCHANNEL
RECEPTOR
ENZYME
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THE SYNAPSE
SYNAPTICVESSICLES
•••
••• ••• •••
•••
•••
•••
•••
•••
CALCIUM CHANNEL
IONCHANNEL
RECEPTOR
ENZYME
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POSTSYNAPTIC POTENTIALS
RESTINGPOTENTIAL
IPSP EPSP
TIME
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TEMPORAL SUMMATION
TIME
TOO FAR APART IN TIME:NO SUMMATION
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TEMPORAL SUMMATION
TIME
CLOSER IN TIME: SUMMATION BUT BELOW THRESHOLD
THRESHOLD
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TEMPORAL SUMMATION
TIME
STILL CLOSER IN TIME: ABOVETHRESHOLD
THRESHOLD
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SPATIAL SUMMATION
TIME
SIMULTANEOUSINPUT FROM TWOSYNAPSES: ABOVETHRESHOLD
THRESHOLD
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EPSP-IPSP CANCELLATION
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NEURO TRANSMITTERS
ACETYL CHOLINEDOPAMINENOREPINEPHRIN
EEPINEPHRINESEROTONIN
HISTAMINEGLYCINEGLUTAMINEGAMMA-
AMINOBUTYRIC ACID (GABA)