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Chapter 11 Part 2
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25
Anatomy
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01/27/2011

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Term
Neurophysiology
Definition
-processing of sensory information and communication between neurons and other sensory cells
-"Messages" of the nervous system are conveyed as action potentials, propogated changes in the transmembrane potential.
Term
Membrane potential:
Definition
-potential difference measured across cell membranes that results from the uneven distribution of (+) and (-) ions across the cell membrane.
Term
Sodium and Potassium are important to membrane potential:
Definition
-Sodium: Na+ and Potassium: K
Term
Ionic concentration differences in the ECF and cytosol:
Definition
-ECF has a higher sodium ion concentration
-Cytosol has a higher potassium ion concentration
-Result: Slightly negative charge on inside of cell, slightly positive charge outside
Term
Forces that affect ion movement:
Definition
-A cell's membrane potential is the result of passive and active forces:
•Chemical gradient: concentration gradient across the membrane
•Electrical gradient: potential difference across the membrane, slightly negative charge on inner surface and a slightly postitive charge on the outer surface
•Electrochemical gradient: sum of chemical and electrical forces acting across the cell membrane
Term
Ion channels:
Definition
-Passageway for ions through the cell membrane
-They are specific and show a level of saturation, so many ions that can move through the
-channel at a certain time.
Term
Types of ion channels:
Definition
•Leak channels: always open
•Gated ion channels: open and close in response to stimulus
Term
Three classes of gated channels:
Definition
•Ligand-gated ion channels: (chemically regulated) chemicals bind to specific receptors associated with the ion channel causing it to open or close
•Voltage-gated ion channels: found in areas of excitable membrane; capable of generating and conducting AP; respond to changes in membrane potential, open and close in response to changes in membrane potential
•Other gated ion channels: open or close in response to physical distortion of membrane (touch receptors) or temperature changes
Term
Resting potential :
Definition
-the membrane potential of an undisturbed cell (-70mV for an average neuron)
-The membrane potential of a cell can be changed by opening and closing channels, -resulting in a current
-Neurons are highly permeable to K+, less so to Na+:
-Potassium is the primary determinant of the resting potential of a cell and sodium
determines disruptions of TMP.
Term
Changes in the resting potential:
Definition
-Depolarization: decrease in membrane potential in which the charge difference decreases, membrane becomes less polar.
-Hyperpolarization: increase in membrane potential in which the charge difference increases, increases polarity, it is related to an inhibition or harder to stimulate the cell
-Repolarization: taking it back to resting, membrane potential returns to resting potential
Term
Ions involved in altering resting potential:
Definition
-K+: changes in permeability to K+ results in hyperpolarization
-Na+: changes in permeability to Na+ results in depolarization
-Ca++: changes in permeability to Ca++ results in depolarization
-Cl-: changes in permeability to Cl- results in hyperpolarization
Term
Opening ion gates change the TMP (tran membrane potential):
Definition
-When we open them we will see one of two affects:
-At resting, most gates are closed. Opening gates changes the TMP leading to two possible types of potentials:
•Graded potentials: local potentials, affects a patch of a cell but doesn’t spread very far, aggrivating one cell, can graduate to an action potential.
•Action potentials: propogated potentials, will spread across the entire membrane
Term
Graded potentials (local potentials):
Definition
-only affect a small patch of the cell.
•cannot spread far from point of stimulus
•cause local currents: movement of ions parallel to membrane
Term
Action Potential:
Definition
-propogated change in TMP that spreads across the entire membrane
-Opening of voltage-regulated channels necessary for AP
-stimulus that initiates AP is a depolarization great enough to open voltage-regulated Na+ channels
Threshold is membrane potential when voltage gated sodium channels open
Threshold: degree of depolarization necessary to cause an aP
Review process 11.18 for AP
For an action potential to occur membrane potential must reach threshold
•occurs at threshold potential (-60 - -55 mV)
•demonstrate all-or-none principle: stimulus will either reach threshold and trigger an AP or it will not
•once initiated, AP will spread across the entire membrane and cannot be stopped
Term
Steps involved in AP generation (see figure 11.18)
Definition
Step 1. Resting potential:
-Voltage-gated Na+ channels are closed and voltage-gated K+ channels are closed

Step 2. Depolarization to threshold:
•depolarization is due to local currents changing the distribution of Na+ and K+ ions along the cell membrane
•gates of voltage-regulated Na+ channels open at threshold
•Na+ rush into the cell due to the high electrochemical gradient
•rapid depolarization occurs
Step 3. Repolarization:
•at +30mV the Na+ channels become inactivated (close)
•K+ channels open and K+ ions move out of the cell
•TMP approached resting potential (begins repolarization)
•during this phase of the repolarization the Na+ are closed and not able to open
Step 4. End of repolarization and afterpotential:
•voltage-regulated Na+ channels regain resting condition but remain closed
•K+ continues to diffuse out of the cell causing the afterpotential (period of hyperpolarization following an action potential
Step 5. Resting membrane ptoential:
•voltage-regulated K+ channels close
•membrane potential returns to normal resting levels
Refractory period:
- time following an AP that a cell membrane cannot respond to another AP
•time when the Na+ are either all open or when Na+ are closed but are inactivated (cannot open)
Term
Action Potential Propogation:
Definition
1. Continuous propogation:
•occurs in unmyelinated axons
•graded potentials depolarize a section of the axon to threshold, local currents then depolarize the adjacent section
•propogation is on direction: graded potentials initiated in the soma or at axon hillock
2. Saltatory Propogation:
•occurs in myelinated axons
•myelin sheets cover areas of axon; these areas are not able to propogate AP
•due to local currents, the AP jumps from one node (area of exposed axon) to the next
-allows for faster propogation
Term
Relationship between axon diameter and propogation speed:
Definition
•larger diameter: faster propogation more room
Term
Types of neurons based on AP propogation
Definition
•Type A: largest axon, myelinated
•Type B: myelinated, smaller axon
•Type C: unmyelinated we don’t care about the diameter
Myelination of axons is not completed until adolescences
Term
Synapse:
Definition
-Communication junction btwn a neuron and another cell
-Presynaptic cell: sends message
-Poststynaptic cell: recieves message
-Commmunicationis commonly by the release of chem. messages reffered to as neurotransmitters
Term
Synaptic transmission:
Definition
1. Electrical synapse
•rare
•found in CNS and PNS
•neuron-neuron synapes
•membranes are connected by gap junctions
2. Chemical synapse
•more common
•neuron-cell and neuron-neuron synapses
•release of neurotransmitters by presynaptic cell
•neurotransmistters can be excitatory (cause depolarization) or inhibitory (cause hyperpolarization)
-Typical example of a chemical synapse is a cholinergic synapse, the neurotransmitter is acetylcholine (ACh) figure 11.24
Term
AP Propogation Delay
Definition
-Synaptic Delay: Time from the release of neurotransmitter by the presynaptic cell and change in activity in the postsynaptic cell
-Synaptic Fatigue: Neurotransmitter is depleted
Term
Steps of AP propogation at a cholinergic synapse (figure 11.24)
Definition
Step 1.
Arrival of AP and depolarization of synaptic terminal (end of presynaptic cell).
Step 2.
Extracellular calcium ions enter the cell causing the release of ACh by exocytosis
Step 3.
ACh moves across the synaptic cleft
Step 4.
ACh binds to chemically-regulated ion channels, causing an increase permeability to Na+. Sodium ions rush into the cell and the postsynaptic cell reaches threshold, triggering an AP
Term
Neurotransmitters function in one of 3 ways
Definition
1. directly opening or closing ion channels (as described with the cholinergic synapse)
2. indirectly by initiating the production or release of second messengers that in turn alter ion channel activity, begining molecule starts a cascade of events called Second messengers are other substances within the cell that will alter cellular activity (ex. cAMP, Ca++)
3. diffusing across the membrane and binding to enzymes within the cell
Term
*Effect of a Neurotransmitter on Postsynaptic Cell Depends on the Nature of the Receptor
Definition
Term
Postsynaptic Potentials
Definition
-Excitatory postsynaptic potentials (EPSPs): graded potentials that open channels in portions of the membrane (if the signal causes a depolorization in the post synaptic cell it will be referred to as an EPSP)
-Inhibitory postsynaptic potentials (IPSPs): transient hyperpolarization (The result of hyperpolorization)
-Postsynaptic potentials are integrated to determine the overall effect on the cell
•summation: integrate the arriving EPSPs and IPSPs both in time (temporal) and area of the neuron (spatial)
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