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neural tissue part 2
martini and roth essentials of anatomy
92
Anatomy
Undergraduate 1
03/29/2012

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Term
Electrical Signals in Neurons
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.
Term
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

Definition
Generating current depends on
Term
electrons
Definition
In a wire, current is due to flow of 
Term
ions
Definition
In living cells, current is due to flow of 
Term
Ion Channels
Definition
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)
Term
high to low concentration (chemical part of gradient)
Definition
Ions move from 
Term
cations
Definition
Positively charged ions 
Term
anions (electrical part of gradient)
Definition
negatively charged ions 
Term
As ions move
Definition
they create a flow of electrical current that can change membrane potential
Term
 gates which are part of the channel protein 
Definition
Ion channels open and close due to presence of
Term
Leakage channels
Definition
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+!
Term
nLigand/chemically-gated channel
Definition
qOpen and close in response to binding of a specific chemical stimulus.
nExamples are: neurotransmitters, ions, hormones.
Term
nMechanically-gated channels
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.
Term
nVoltage-gated channels
Definition
qOpens in response to change in membrane potential (voltage)
qParticipate in generation and conduction of action potentials!
Found in axons of neurons
Term
nVoltage-gated channels
Definition
q channel Found in axons of neurons
Term
Resting Membrane Potential (RMP)
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!
Term
-70 millivolts (mV)
Definition
RMP is typically measured at 
Term
Resting Membrane Potential (RMP)
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).
Term

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)


Definition
How is RMP Created
Term

Graded potentials

Action potentials 

Definition
2 Types of Electrical Signals
Term

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.
Definition
Graded potentials 
Term

allow communication over both short and long distances within the body.

qCan transmit nerve impulse from hand to brain, and brain to muscles.
Definition
Action potentials 
Term
Graded Potentials
Definition
nArises when mechanically-gated or ligand-gated channels open or close.
Term
Graded Potentials
Definition
qLots of these channels are present in dendrites and cell bodies; so graded potentials occur most often in dendrites and cell bodies. 
Term
Graded
Definition
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.
Term
graded potential 
Definition
is a small deviation from the membrane potential that can make the inside of the cell membrane
Term
HYPERPOLARIZED
Definition
MORE NEGATIVE =
Term
DEPOLARIZED
Definition
LESS NEGATIVE = 
Term
MORE NEGATIVE = HYPERPOLARIZED 
Definition
nOccurs when mechanically or ligand-gated channels open and let in negative ions (like Cl-), or close to exclude positive ions (like Na+).
Term
LESS NEGATIVE = DEPOLARIZED
Definition
nOccurs when mechanically or ligand-gated channels open that let in positive ions (like Na+), or close to exclude negative ions (like Cl-).
Term
Hyperpolarization
Definition
nChange in mV: moves further from zero
qEx: -70 (RMP) to -75 mV
n Inside of cell membrane: MORE NEGATIVE
nOccurs with inhibitory neurotransmitters
Term
Depolarization
Definition
nChange in mV: moves closer to zero
qEx: -70 (RMP) to -60 mV
n Inside of cell membrane: LESS NEGATIVE
nOccurs with excitatory neurotransmitters
Term
Action Potentials
Definition
nArises when voltage-gatedchannels open and then close.

Most of these channels are present in the axon

Term

  DEPOLARIZATION

REPOLARIZATION 

Definition
AP takes place in two main phases
Term
DEPOLARIZATION 
Definition
qNa+ channels open.
nNa+ rushes into the cell.
nMembrane potential becomes less negative and eventually positive

“Message” is sent down axon

Term
 REPOLARIZATION 
Definition
qK+ channels open.
nK+ rushes out of the cell.

Neuron is restored to resting state; - 70mV (RMP

Term
Threshold
Definition
qlevel of depolarization  which causes opening of voltage-gated Na+ channels.
nUsually about –55mV. 
Term
Threshold
Definition
A graded potential of sufficient strength or the sum of several graded potentials causes the membrane to depolarize to 
Term
an AP is generated
Definition
If threshold is reached
Term
All-or-none principle 
Definition
AP either goes or it doesn’t,  like a row of dominoes
Term
All-or-none principle 
Definition
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

Term
Refractory period
Definition
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 

Term
Refractory period
Definition
qThe refractory period limits how many action potentials may be generated in a neuron.
Term
Propagation of Action Potential
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.
Term
nLocal anesthetics like novocaine and lidocaine block pain and other somatic sensations
Definition
qPrevent opening of voltage-gated Na+ channels

Blocks AP (pain signal) from being sent down axon and thus cannot reach the CNS

Term
NERVE IMPULSE
Definition
nA wave of action potentials that moves down the axon to the end of the nerve fiber. 
Term
NERVE IMPULSE
Definition
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
Term
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
 
 
Definition
Summary of Events in Nerve Impulse Conduction
Term
nContinuous Conduction
Definition
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.”

Term
nSaltatory Conduction
Definition
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.
Term
nAmount of myelination
nAxon diameter
nTemperature


Definition
Factors Affecting Speed of APs/Nerve Impulse Conduction
Term
nAmount of myelination
Definition
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.
Term
nAxon diameter
Definition
qAxons with larger diameters conduct impulses faster than those with smaller diameters (due to larger surface area)
Term
nTemperature
Definition
qAxons propagate AP’s at lower speeds when cooled
Term
nElectrical Synapse
Definition
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
Term
gap junctions (contain fluid-filled tunnels that connect the cytosol of neighboring cells
Definition
AP’s conduct directly between adjacent cells through 
Term
cytosol
Definition
AP spreads from cell to cell via 
Term
synchronization
Definition
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
 

Term
Gap junctions 
Definition
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. 
Term
chemical synapse
Definition
nan AP is transferred from the presynaptic (”sending”) neuron to the postsynaptic (”receiving”) neuron across a synaptic cleft.
nOne-way information transfer !
Term
qFrom axon to dendrite
qFrom axon to cell body
Definition
nSynapses can be 
Term
synaptic cleft
Definition
Nerve impulses cannot cross the
Term
presynaptic neuron
Definition
In response to a nerve impulse or AP this converts the electrical signal into a chemical signal (releases neurotransmitter into synaptic cleft).
Term
Neurotransmitter (chemical signal) 
Definition
binds to receptors on post-synaptic neuron and turns it into an electrical signal (AP)
Term
excitatory or inhibitory
Definition
Neurotransmitters can be either 
Term
If the neurotransmitter is excitatory
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
Term
If the neurotransmitter is inhibitory
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
Term
Summation
Definition
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
Term
Summation
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.
Term
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).
Definition
nAfter AP is generated in postsynaptic neuron, the neurotransmitter must be removed. Why?
Term
Cocaine 
Definition
nblocks re-uptake of dopamine.

Produces continuous stimulation/pleasure

Term
SSRI’s (selective serotonin re-uptake inhibitors) 
Definition
nblocks reuptake of serotonin.
qKeeps serotonin in synaptic cleft as long as possible.
nElevates mood, relieves depression, e.g. Prozac, Paxil, Zoloft
Term
nMAO (monoamine oxidase) inhibitors.
Definition
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

Term
Neurotransmitters
Definition
are chemical substances that neurons use to communicate with other neurons, muscle fibers, and glands
Term
Neurotransmitters
Definition
nBoth excitatory and inhibitory neurotransmitters are present in CNS and PNS
qThe same neurotransmitter may be excitatory in some locations and inhibitory in others.
Term
Representative Neurotransmitters
Definition
nAcetylcholine (ACh) and nAmino Acids
Term
nAcetylcholine (ACh)
Definition
qReleased by many PNS neurons & some CNS
qExcitatory in skeletal muscle; inhibitory in cardiac muscle.
Term
nAmino Acids
Definition
GABA and Glycine
Term
GABA
Definition
is an inhibitory NT for 1/3 of all brain synapses and ½ of inhibitory synapses in spinal cord.     
Term
Valium
Definition
enhances binding of GABA (agonist); enhances its inhibitory effect à decreases anxiety.
Term
Glycine
Definition
is an inhibitory NT for other ½ of inhibitory synapses in spinal cord
Term
Strychnine
Definition
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!
Term
Norepinephrine
Definition
affects mood, dreaming, awakening from deep sleep.
Term
Dopamine
Definition
affects skeletal muscle tone and some aspects of movement, emotion, pleasure, addictive behavior.
Term
Parkinson’s disease
Definition
degeneration of dopamine-releasing neurons which causes tremors and muscular stiffness
Term
Serotonin
Definition
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
Term
nNeuropeptides
Definition
qEndorphins and enkephalins and substance P are
Term
Substance P
Definition
released by sensory neurons, transmits pain-related information to CNS; enhances perception of pain
Term
Substance P
Definition
Endorphins and enkephalins suppress release of 
Term
Endorphins and enkephalins
Definition
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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