Neuron signaling. Electricity Principles The ECF contains primarily sodium (Na+) and chloride ions...
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Transcript of Neuron signaling. Electricity Principles The ECF contains primarily sodium (Na+) and chloride ions...
![Page 1: Neuron signaling. Electricity Principles The ECF contains primarily sodium (Na+) and chloride ions (Cl-) The ICF contains lots of potassium (K+) ions.](https://reader034.fdocuments.net/reader034/viewer/2022050709/5697bf8b1a28abf838c8b3cb/html5/thumbnails/1.jpg)
Neuron signaling
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Electricity Principles• The ECF contains primarily sodium (Na+) and chloride
ions (Cl-)• The ICF contains lots of potassium (K+) ions and other
molecules that are non-diffusable
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Electrical potential
• Separated electrical charges of opposite sign have the potential to do work if they are allowed to come together.
• AKA electrical potential. – It is determined by the the difference in the
amount of charge between the two points, – The units of electrical potential are called volts.
• We will measure in millivolts (mV).
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Because the charges attract, they line up on either side of the membrane.
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Actual membrane potential
• In an actual nerve cell at rest, – K+ concentration is greater inside– Na+ is greater outside.
• actual resting membrane potential is about -70 mV
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there is always a membrane potential
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The Na+/ K+ pump
• maintains the concentration gradients for each ion and helps create the gradients. – The pump moves 2 K+ ions
in and 3 Na+ ion out each time it works.
– Because it pumps out more + ions than it brings in, it helps make the ICF more negative.
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Action Potentials and Graded Potentials
• changes in the membrane potential from its resting level produce electrical signals. – This is the way that neurons process and transmit
information.
• There are two forms of the signals: – Graded potentials (GP) signal over short distances.
• receptors
– Action potentials (AP) signal over long distances in nerves and muscles.
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Graded potential• GP’s occur in a
small area. • When a GP occurs
– charge flows from the origin like a wave.
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What actually happens
• The membrane is depolarized by a stimulus– The area nearby becomes less negative because
• + ions will flow in
• + ions already inside the membrane will push away from those flowing in.
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Spreading of
stimulusSTIMULUS
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Size of the GP• The size of the GP is related to the size of the stimulus. • The stronger the stimulus, the bigger the GP, the farther it travels.
– Because ion channels are always open, the signal will only travel a short distance before it loses strength
– GP currents die out within a few millimeters.
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Summation
• If additional stimuli occur before the graded potential has died out, these are added to the first stimuli. – summation plays a very
important role in the senses.
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Definitions– Polarized means that the outside and inside of the cell
membrane have different net charges – Depolarized is when the potential is less negative than
at resting potential (closer to 0 mV).– Overshoot is a reversal of the resting membrane
polarity (more positive inside or between 0 mV and +50 mV).
• The inside of the cell becomes positive relative to the outside.
– Repolarizing is when the membrane potential is moving towards the resting value (between +50 mV and -70 mV).
– Hyperpolarized is when the membrane potential is more negative than the resting potential (greater than -70 mV).
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3 kinds of transport proteins
• Leak channels– Move ions from high to low concentration– Always open
• Na/ K pump– Creates the resting membrane potential
• Voltage gated channels– Ion channels that open at one voltage and close at
another. – Responsible for the graded and action potential.
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Action Potentials
• AP’s are large changes in the membrane potential. – The potential changes
from -70 mV to +30 mV and then back to resting potential.
– rapid, can occur 1000X / second.
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Excitable cells
• Excitable cells have membranes capable of producing AP’s. – neurons, muscle cells, endocrine, immune and
reproductive cells
• Their ability to produce action potentials is known as excitability. – All cells can conduct GP’s, only excitable cells
can conduct AP’s
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Start of the AP• AP begins when the
membrane depolarizes in response to a stimulus.
• This opens voltage gated Na+ channels. – This increases the number
of Na+ ions flowing into the cell and the cell becomes more and more depolarized until a threshold is reached.
– This triggers the AP.
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Threshold, depolarization, repolarization
• Once this threshold is reached more voltage gated Na+ channels open. – The membrane potential
overshoots (becomes more positive on the inside) and reaches about +30 mV.
– At the peak, voltage gated Na+ gates close and voltage gated K+ channels open.
– The membrane potential begins to rapidly repolarize to resting levels.
– Once resting potential is reached the voltage gated K+ channels close.
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Why does the action potential move?
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All or None principle
• If a stimulus causes depolarization to reach threshold, then an AP will always be generated. – Size of the AP is always the same regardless of
the stimulus strength. – AP is an all or none response to the stimulus.
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Speed of the AP… FAST
• Unmyelinated neuron: .5 m/sec
• Large Myelinated neuron: 100 m/sec– At 100 meters/ sec, an AP will travel from the
big toe to the brain in 0.02 seconds.
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Myelinated axon cause saltatory conduction
• Myelin is mostly lipids. – So no charge will flow
through this tissue. – When less ions leak, the AP
spreads farther.
• It doesn’t cover the entire cell length. – where there is no myelin,
there are large numbers of voltage gated Na+ channels.
– AP’s only occur here (nodes of Ranvier).
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What is the “stimuli” that starts an action potential?
• In afferent neurons (going towards the brain), the initial depolarization is done by a GP (aka receptor potential) generated by sensory receptors
• In all other neurons, – Synaptic potential- GP from
the synaptic input to the neuron (occurs at the synapse).
– Pacemaker potential- a spontaneous changes in the neuron’s membrane potential.
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Smell, chemical stimulus
Action Potential
Graded
Potential
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Hair, mechano receptors in ear
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Current
• The movement of electrical charge is called a current. – The electrical potential between charges tend to
make them flow, producing a current.
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Resistance
• The hindrance to electrical charge movement is known as resistance. – If resistance is high, then the current flow will
be low.
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Conductors/insulators• Conductors- materials that have a low resistance to
current flow.– Water is a good conductor. (ICF, ECF)– Current flows easily through water.
• Insulators- Materials that have a high electrical resistance and reduce current flow.– Membranes are non-polar and are regions of high resistance. – They separate the ICF and the ECF.
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Resting membrane potential
• All cells at rest have a potential difference across their cell membrane. – The inside of the cell is - charged compared to
the outside. – ECF has more + ions than negative ions– the ICF has more negative ions, so the membrane
potential has a negative voltage
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Ions present• Ions that matter must be diffusable.
– include: Na+, K+ and Cl-• Na+ and Cl- highest outside cell
• K+ is highest inside cell.
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Size of the resting potential
• The size is determined by two factors:– Differences in ion concentrations between the
intracellular and ECF.– the number of open ion channels
• More ion channels open, more permeability
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How a membrane potential changes
• The changes in membrane potential occur because of changes in membrane permeability to ions. – Some ion channels are gated, open or closed by
electrical, chemical or mechanical stimuli. – i.e. when a cell receives a chemical signal from a
neighboring cell, some channels will open allowing ions to flow into the cell (chemical messengers, ion channel).
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Pacemaker Potential
• Pacemaker potential is a spontaneous generation of an action potential.
• Pacemaker potentials are found in the neurons that control the heartbeat, breathing and peristalsis
• a graded potential is caused by the behavior of some ion channels in the membrane.
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Pacemaker Potential
– When the threshold is reached an AP is generated.
– The membrane then repolarizes and again begins to depolarize.
– There is no stable, resting membrane potential.
– The rate at which the membrane depolarizes to threshold determines the AP frequency.
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Drugs modify synapses
• Most drugs that act on the nervous system do so by altering the mechanism by which synapse work and changing the strength of the synaptic potential.
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Synaptic Mechanisms vulnerable to drug influence
• A- Increase leakage of neurotransmitter from vesicle to cytoplasm, this causes it to be broken down by enzymes.
• B- Increase neurotransmitter release into the synapse.
• C- Blocking neurotransmitter release • D- Inhibit transmitter synthesis, • E block transmitter reuptake• F Block enzymes in the synapse that
break down the neurotransmitter. • G Bind to receptors on the post
synaptic neuron to block (antagonist) or mimic (agonist) transmitter action.
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What long term effects can drugs have?
• It is difficult to predict because body adapts to imbalances created by the drugs– It does this through feedback mechanisms that
regulate the affected process. • For example, if a drug interferes with the action of a
neurotransmitter by blocking its synthesis, the cells may respond by producing more of the enzymes involved in synthesis.
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Diseases can also affect synapses
• A toxin that cause Tetanus is made by a bacteria. – The toxin destroys the proteins that move the
vesicles containing neurotransmitters into the synapse.
– No neurotransmitter is released. – The neurons that depend on the transmitter are
inhibitory • (without the neurotransmitter, they will not send a signal
to relax).
– the muscles that are affected do not relax and a person can become paralyzed.