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| Electrical Signals in Neurons |
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Definition
nLike muscle fibers, neurons are electrically excitable.
nThey communicate with each other by signals that generate electrical current.
nElectrical current is a flow of charged particles.
qIn a wire, current is due to flow of electrons.
qIn living cells, current is due to flow of ions. |
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qExistence of a resting membrane potential: electrical voltage difference across the plasma membrane of a neuron.
Presence of specific kinds of ion channels in the neuron’s plasma membrane |
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| Generating current depends on |
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| In a wire, current is due to flow of |
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| In living cells, current is due to flow of |
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nWhen ion channels are open, they allow specific ions to move across plasma membrane, down their electrochemical gradient
qIons move from high to low concentration (chemical part of gradient)
qPositively charged ions (cations) move toward negatively charged ions (anions) (electrical part of gradient) |
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| high to low concentration (chemical part of gradient) |
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| anions (electrical part of gradient) |
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| they create a flow of electrical current that can change membrane potential |
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gates which are part of the channel protein |
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| Ion channels open and close due to presence of |
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nopen and close at random.
qFound in nearly all cells
qPlasma membranes have many more K+ leakage channels than Na+ leakage channels
qK+ leakage channels more leaky than Na+ leakage channels
nCell membrane is more permeable to K+ than to Na+! |
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nLigand/chemically-gated channel |
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Definition
qOpen and close in response to binding of a specific chemical stimulus.
nExamples are: neurotransmitters, ions, hormones. |
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nMechanically-gated channels |
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Definition
qOpens in response to mechanical stimulation such as vibration, tissue stretching, touch, pressure, etc.
nFound in dendrites of some sensory receptors (touch, pressure, pain), auditory receptors in ear. |
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qOpens in response to change in membrane potential (voltage)
qParticipate in generation and conduction of action potentials!
Found in axons of neurons |
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q channel Found in axons of neurons |
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Term
| Resting Membrane Potential (RMP) |
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Definition
nis due to a buildup of positively charged particles on the outside of the membrane, and an equal buildup of negatively charged particles on the inside of the membrane.
nBuildup of charge occurs only very close to membrane! |
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| RMP is typically measured at |
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| Resting Membrane Potential (RMP) |
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Definition
qMinus sign indicates that the inside of the membrane is negative relative to the outside.
qA cell that has a resting membrane potential (positive charges on one side of membrane equal to negative charges on other side) is at rest or POLARIZED (not conducting an impulse). |
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1) Unequal distribution of ions in ECF and ICF (cytosol
n2) Inability of most anions to leave the cell
n3) Action of sodium-potassium pump (Na+/K+/ATPase)
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Graded potentials
Action potentials |
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Definition
| 2 Types of Electrical Signals |
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or short-distance communication only.
qResults from change or stimuli that affects membrane potential in the region of the membrane exposed to stimulus; it is “localized”
qA graded potential will “die out” within a few hundred micrometers, so it is useful only for short-distance communication within the neuron. |
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allow communication over both short and long distances within the body.
qCan transmit nerve impulse from hand to brain, and brain to muscles. |
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Definition
nArises when mechanically-gated or ligand-gated channels open or close. |
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qLots of these channels are present in dendrites and cell bodies; so graded potentials occur most often in dendrites and cell bodies. |
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nmeans that the potentials vary in amplitude/size, depending on:
qStrength of the stimulus.
qHow many channels have opened (or closed).
qHow long the (opened) channels stay open. |
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| is a small deviation from the membrane potential that can make the inside of the cell membrane |
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| MORE NEGATIVE = HYPERPOLARIZED |
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Definition
nOccurs when mechanically or ligand-gated channels open and let in negative ions (like Cl-), or close to exclude positive ions (like Na+). |
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| LESS NEGATIVE = DEPOLARIZED |
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Definition
nOccurs when mechanically or ligand-gated channels open that let in positive ions (like Na+), or close to exclude negative ions (like Cl-). |
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nChange in mV: moves further from zero
qEx: -70 (RMP) to -75 mV
n Inside of cell membrane: MORE NEGATIVE
nOccurs with inhibitory neurotransmitters |
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nChange in mV: moves closer to zero
qEx: -70 (RMP) to -60 mV
n Inside of cell membrane: LESS NEGATIVE
nOccurs with excitatory neurotransmitters |
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nArises when voltage-gatedchannels open and then close.
Most of these channels are present in the axon |
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DEPOLARIZATION
REPOLARIZATION |
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Definition
| AP takes place in two main phases |
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qNa+ channels open.
nNa+ rushes into the cell.
nMembrane potential becomes less negative and eventually positive
“Message” is sent down axon |
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qK+ channels open.
nK+ rushes out of the cell.
Neuron is restored to resting state; - 70mV (RMP |
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qlevel of depolarization which causes opening of voltage-gated Na+ channels.
nUsually about –55mV. |
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| A graded potential of sufficient strength or the sum of several graded potentials causes the membrane to depolarize to |
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| AP either goes or it doesn’t, like a row of dominoes |
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nAn action potential is only generated if the stimulus reaches threshold response; if it doesn’t, NO GO!
nUnlike graded potentials, AP’s are all the same size/amplitude.
A larger stimulus will not cause a stronger AP (but it will increase the frequency e.g. APs/second |
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nPeriod of time during which neuron can not generate another action potential in response to a normal threshold stimulus
nUntil you get enough Na+ out of the cell and K+ back into the cell, neuron cannot respond to normal threshold stimulus and send another AP.
It’s like trying to flush your toilet again |
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qThe refractory period limits how many action potentials may be generated in a neuron. |
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| Propagation of Action Potential |
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Definition
q1) At depolarization, Na+ flows into the cell and affects voltage of adjacent areas
q2) Their voltage-gated Na+ channels open
q3) AP is “self-propagated” along membrane. |
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nLocal anesthetics like novocaine and lidocaine block pain and other somatic sensations |
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qPrevent opening of voltage-gated Na+ channels
Blocks AP (pain signal) from being sent down axon and thus cannot reach the CNS |
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nA wave of action potentials that moves down the axon to the end of the nerve fiber. |
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n(a) An action potential in one region stimulates the adjacent region, and (b and c) a wave of action potentials (a nerve impulse) moves along the axon |
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n1) Cell membrane of neuron maintains RMP by diffusion of Na+ and K+ down their concentration gradients as the Na+/K+ pump “pumps” them up the gradients.
n2) Neuron receives stimulation, causing localized, graded potentials which may sum to reach threshold.
n3) Sodium channels in a local region of the membrane open
n4) Sodium ions diffuse inward, depolarizing the membrane
n5) Potassium channels in membrane open
n6) Potassium ions diffuse outward, repolarizing the membrane
n7) The resulting action potential arises at the trigger zone of the axon and causes an electric current that stimulates adjacent portions of the membrane
n8) Action potentials occur sequentially along the entire length of the axon as a nerve impulse
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| Summary of Events in Nerve Impulse Conduction |
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nOccurs in unmyelinated axons.
nAP is propagated by step-by-step depolarization and repolarization of each portion of the length of the axon membrane.
“Domino effect.” |
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nOccurs in myelinated axons.
nDepolarization/repolari-zation occurs only at nodes of Ranvier (where ion channels are concentrated).
qCurrent is carried through ECF and “jumps” from node to node. |
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| Factors Affecting Speed of APs/Nerve Impulse Conduction |
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qMyelinated fibers conduct APs faster than unmyelinated fibers
nIn myelinated axons, ions only have to diffuse in/out at nodes of Ranvier, not across the entire membrane as in unmyelinated axons. This allows for more rapid conduction of action potentials. |
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qAxons with larger diameters conduct impulses faster than those with smaller diameters (due to larger surface area) |
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qAxons propagate AP’s at lower speeds when cooled |
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qAP’s conduct directly between adjacent cells through gap junctions (contain fluid-filled tunnels that connect the cytosol of neighboring cells)
qAP spreads from cell to cell via cytosol.
qCommon in visceral smooth muscle and cardiac muscle.
qConduction is faster than chemical synapse.
qAllows for synchronization; a large number of neurons or muscle fibers can produce AP’s in unison to produce a coordinated contraction.
nCardiac muscle: Heartbeat
nSmooth muscle in GI tract: Peristalsis |
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| gap junctions (contain fluid-filled tunnels that connect the cytosol of neighboring cells |
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| AP’s conduct directly between adjacent cells through |
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| AP spreads from cell to cell via |
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a large number of neurons or muscle fibers can produce AP’s in unison to produce a coordinated contraction.
nCardiac muscle: Heartbeat
nSmooth muscle in GI tract: Peristalsis
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| in cardiac muscle and visceral smooth muscle allow the action potential itself to quickly spread from cell to cell, enabling coordinated contraction of muscle fibers. |
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nan AP is transferred from the presynaptic (”sending”) neuron to the postsynaptic (”receiving”) neuron across a synaptic cleft.
nOne-way information transfer ! |
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qFrom axon to dendrite
qFrom axon to cell body |
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| Nerve impulses cannot cross the |
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| In response to a nerve impulse or AP this converts the electrical signal into a chemical signal (releases neurotransmitter into synaptic cleft). |
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| Neurotransmitter (chemical signal) |
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| binds to receptors on post-synaptic neuron and turns it into an electrical signal (AP) |
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| Neurotransmitters can be either |
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| If the neurotransmitter is excitatory |
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Definition
nit causes an
qEPSP: depolarizing postsynaptic potential
nResults from the opening of Na+ channels
nPostsynaptic cell is more likely to reach threshold and trigger an action potential/nerve impulse |
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| If the neurotransmitter is inhibitory |
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Definition
nit causes an
qIPSP: hyperpolarizing postsynaptic potential
nResults from the opening of Cl- or K+ channels
nCauses the postsynaptic cell to become more negative inside or hyperpolarized
nPostsynaptic cell is less likely to reach threshold |
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nEach neuron in brain and spinal cord may receive neurotransmitters from thousands or more synaptic knobs of other axons!!!!
nSome neurotransmitters may be excitatory→EPSP; some may be inhibitory →IPSP.
nThe integrated sum of EPSPs and IPSPs determines whether an AP/nerve impulse will be generated in the postsynaptic neuron.
qIf the net effect is more excitatory than inhibitory, threshold may be reached, and an action potential/nerve impulse reached
qIf the net effect is inhibitory, no action potential/nerve impulse is generated |
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Definition
| of the excitatory and inhibitory effects of the postsynaptic potentials commonly takes place at the trigger zone in the proximal region of the axon. |
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nNeurotransmitter can be removed by:
q1) Diffusion
nNT simply diffuses out of synaptic cleft, losing contact with postsynaptic neuron and NT receptors.
q2) Enzymatic degradation
qEnzyme from postsynaptic neuron breaks down NT after AP has been generated.
nAcetylcholine & acetylcholinesterase.
nEpinephrine and monoamine oxidase (MAO).
q3) Uptake into cells
nNT may be actively transported into neighboring neuroglial cells or back into presynaptic neuron (re-uptake). |
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nAfter AP is generated in postsynaptic neuron, the neurotransmitter must be removed. Why? |
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nblocks re-uptake of dopamine.
Produces continuous stimulation/pleasure |
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| SSRI’s (selective serotonin re-uptake inhibitors) |
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nblocks reuptake of serotonin.
qKeeps serotonin in synaptic cleft as long as possible.
nElevates mood, relieves depression, e.g. Prozac, Paxil, Zoloft |
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nMAO (monoamine oxidase) inhibitors. |
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qPrevent MAO from breaking down epinephrine and norepinphrine.
nEPI and NE do a lot of stuff, including elevating mood.
Preventing breakdown may help treat depression |
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| are chemical substances that neurons use to communicate with other neurons, muscle fibers, and glands |
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nBoth excitatory and inhibitory neurotransmitters are present in CNS and PNS
qThe same neurotransmitter may be excitatory in some locations and inhibitory in others. |
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| Representative Neurotransmitters |
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Definition
nAcetylcholine (ACh) and nAmino Acids |
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qReleased by many PNS neurons & some CNS
qExcitatory in skeletal muscle; inhibitory in cardiac muscle. |
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| is an inhibitory NT for 1/3 of all brain synapses and ½ of inhibitory synapses in spinal cord. |
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| enhances binding of GABA (agonist); enhances its inhibitory effect à decreases anxiety. |
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| is an inhibitory NT for other ½ of inhibitory synapses in spinal cord |
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| blocks glycine receptors (antagonist) on skeletal muscles à no inhibition of muscle contraction à can’t relax muscles, inc. diaphragm à can’t inhale; die! |
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| affects mood, dreaming, awakening from deep sleep. |
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| affects skeletal muscle tone and some aspects of movement, emotion, pleasure, addictive behavior. |
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| degeneration of dopamine-releasing neurons which causes tremors and muscular stiffness |
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ninvolved in control of mood, appetite, temperature regulation; induces sleep.
qDepression is thought to be linked to imbalance of serotonin, dopamine and norepinephrine in the brain |
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qEndorphins and enkephalins and substance P are |
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| released by sensory neurons, transmits pain-related information to CNS; enhances perception of pain |
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| Endorphins and enkephalins suppress release of |
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| Endorphins and enkephalins |
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qpain relievers (200X stronger than morphine); released in response to stress and sexual activity; feelings of pleasure and euphoria
nMorphine, heroin, opium fit same receptors.
nAcupuncture increases their release (pain relief) |
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