Lecture 3

Question 1

Basic strucutre of a neuron

Answer 1

• Cell body Dendrites Axon

Question 2

Cell body

Answer 2

• Cell body houses the nucleus and other typical cell organelles
• plasma membrane around the cell body is characterized by local potentials
• Voltage-gated ion channels are not characteristic of the cell body membrane

Question 3

Dendrites

Answer 3

• Dendrites are cellular extensions of the neuron
• Although variable, the number of dendrites is typically a few to many
• dendrites are characterized by the presence of ligand(neurotransmitter)-gated ion channels
• Dendrites conduct local potentials

Question 4

Axon

Answer 4

• neuron charecterized by a single axon that is variable in length
• Axon is an etension of the cell body and is typically opposite the side of the cell body where the dendrites are located
• Distal end of the axon is cahrecterized by the presence of membrane bound vesicles filled with neurotransmitter molecules

Question 5

Axolemma

Answer 5

• Characterized by the presence of voltage -gated ion channels and the ability to conduct an action potential
• The axon is an extension of the cell and is covered by the plasma membrane (referred to as the axolemma)

Question 6

Concentration gradient

Answer 6

• Cell membrane or plasmalemma,functions to maintain separate intracellular and extracellular environmnts
• Difference in ion concentrations(dichotomous distribution of ions) can change depending on whether or not the plasmalemma is permeable to specific ion at a given periods of time

Question 7

What are the concentration of ions inside and outside the cell?

Answer 7

• Concentration of sodium and chloride ions are much more highly concentrated outside the cell
• potasium ions are considerably more concentrated inside the cell

Question 8

• Diffusion Potential

Answer 8

Diffusion potential is caused by an ion concentration difference on either side of a membrane

Question 9

Nernst potential

Answer 9

• Nernst potential is the idffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane

Question 10

Nernst equation

Answer 10

E=2.3RT/F logCo/Ci

• E=difference in the electrical potential between inside and outside the neuron=Nernst potential
• R=niversal gas constant
• T=absolute temperature
• F=electric charge per gram equivalent of univalent ions

Used to determine the diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane. It measures the potential for one type of ion at a time

To measure the combined potential for more than one type of ion, the goldman equation may be used

Assumptions:

• Equation can only be used for one ion at a time
• Membrane must be completely permeable to that ion
• Ion must be at equilibrium

Question 11

Electrial Dipole Layers

Answer 11

• Positive and negtaive charges are distributed somewhat evenly within both the extracellular and intracellular fluids.When both the indifferent electrode and the rerdoign electrode are in the same fluid, the voltage will read as “0”
• However ,the distribution of ions immediately on either side of the membrane is mostly positive or mostly negative. This dichotomous distribution of ions on either side of the membrane represents a voltage change area called the elctrical dipole layer
• The potential electrical difference across the cell membrane is recorded as negative, because the recording electrode is inisde the membrane

a. Recording electrode is in the extracellular fluid

B.Recording electrode peirces cell membrane

C.Recording electrode pierces other side of membrane and is in the extracellular fluid again

Question 13

Simplified Nernst Equation

Answer 13

• By combinign the constnats ,converting to log base 10 instead of natural log, and considering the potential at body temperature , the above equation can be simpliefied to:
• Eion=z(61.5)xlog([ion]outside/[ion]inside)
• z=valence of ion , and nomrally valence is 1 unless doing with calcium(and have to be concerned with if its negatvie or positive)

Question 14

Diffusion Potential

Answer 14

• Going to reach an equilbirium where ions are equal on both side and our voltage shoud be zero as we reach zero
• Sodium ions drag larger clouds of water molecules that chloride ions,so chlordie is more mobile
• The left side will be negative until concentration equilibrium is reached
  
  • Describes a transient situation
• This scenario does not describe an equilibrium condition but rather transient situation that exists as long as there is net diffusion

Question 15

Equilibrium potential

Answer 15

• Equilbiruim will be reached when the electrical force driving chloride ions out of hte left hand compoartment exactly balances the concentraional force driving chloride ions out of hte right hand compartment
• Equilibrium for an ion is determined not only by concentraional forces but also by electrical forces
• Chlordie is more concentrated on outside then insdie , and expecte it to move insdie but as we accumulate more negative charges , then some are going to repel and stay on the outside
• two forces working here , electrical and chemical
• Use Nernst equation to calcualte electromotive force for chloride ion
  
  • -61.5mV

Question 16

Principle of electrical Neutrality

What are the concentrations of the ions indicated by the question marks?

use the nernst equation to calculate the elctromotive force for the chloride ion

Answer 16

• Under biological conditions the sum of the concentrations of cations within any compartment must be equal to the sum of the concentrations of anions in that compartment.
• in this model both osmotic and electrical factors must be considered in equilibrium
• Begin on left side, balance the negative with the positive so chloride would need to be 50 total concentraion will be 200
• Cl- will be 100 on left side and 100m for Na+
• Electrive motive force would be -18.4mV

Question 17

Donnan(Gibbs-Donnan)Equilibrium

Answer 17

• If equilibrium is to be reached with two permeant ions(ex. chloride and potassium), the electrical potential acoss the cell membrane must exactly balance the concentration gradient for both ions
• Because the membrane potential can only have one value , his equilibrium condition will be satisfied only when equilibrium potentials for both ions are equal
• Concentration of chloride ions will be 125
  
  • Then multiply that by 5 to et 625 , and take the square roote to get 25 which means total ion concentration is 50
  • protein concentration is 200
• Donnan equilbirum specifies conditions that must be met if two ions that can cross a cell membrane are simultaneously to be at equilibrium across that membrane

Question 18

Action potential Charecteristics

Answer 18

• Ot os all or none:it will either occur or not occur
• It is self-propagating:each region of depolarization serves to generate action potentials on either side
• It is non-decremental: it doesn ot decrease in strength
• Once you start,won’t stop till you reach the end ( end of the axon)
• Non-decremental:does not decrease in strength

Question 19

Ion channels related to Action potential propagation

Answer 19

• Ion channels are channesl that allow the passage of ions from one side of the membrane to the other. They are typically very selective ,allowing only one kind of ions to pass through
  
  • They may always be open(such as slow-leak channels)
  • They may be gated, only opening when certain conditions are met
    
    • Ligand gated:involves the attachment of a chemical messenger such as a neurotransmitter or hormone to a receptor
    • Voltage gated:involves a change in the membrane potential

Question 20

Molecular strucutre of voltage gated sodium channels

Answer 20

• Channel consists of four domains
• Four domains are thoguht to be arranged in a cylindrical configuration
• Each domain has six hydrophobic transmembrane segments(s1-S6)
• The S4 segment within each domain has a high positive charge
• Inactivation gate is associated with an intracellular hydrophilic linkage between domains three and four

Question 21

Voltage gated sodium channels

Answer 21

• Have two gates
  
  • Activation gate
  • inactiviation gate
• Activation gate is closed and the inactivation gate is opened at -90mV(with the inside of the axon membrane negative relative to the outside)
• Both gates are opened between -90mV to +35mV
• Activation gate opens as voltage reaches -70mV to -50mV
• Activation gate is opened and the inactivation gate is closed at +35mV to -90mV

Question 22

Voltage Gated Potassium Channels

Answer 22

• Potassium Channels have channel diamters that are too small for hydrated potassium and hydrated sodium ions
• Near the entrance to the channel there are loop from the pore helix that are bound to carbonyl oxygens
  
  • This forms a selectivity filter
• Smaller hydrated ions such as sdoium are not affected by the selectivity filter but are too large to pass through the potassium channel
• Larger hydrated ions such as potassium will be dehyrated by the selectivity filter, allowing the smaller”naked” potassium ion to pass through the channel
• Single gate closed at a resting potential of -90mV
• slow activation opens the gate from +35mV to-90mV
• No ions are passing through gated channls during resting potential

Question 23

Steps in an action potential on a neuron axon membrane

Answer 23

• Resting Stage
  
  • -90mV
• Depolarization stage
  
  • Membrane suddenly becomes permeable to sodium ions
  • Membrane potential may overshoot for large axons
• Repolrazation stage
  
  • Sodium channels close within a few 10,000ths of a second
  • Potassium channels open more than normal
• Sodium and potassium conductance
  
  • inside is going to become more positive and need potential to repolarize

Question 24

Action potentials

Answer 24

• Current flowign down the insdie of an axon at a particular point can continue down the interior of the fiber or cross the membrane at that point. Usinv either of these two alternatives,the speed at which hte action potential travels(is propagated) can be icnreased
  
  • Increasing the diamter of the axon: utilized by invertebrates(especially cephalopods such as squids) but also plays a factor in vertebrate axons of various diamters
  • Large diamter axons offer larger cross-sectional area to internal flow of current
  • Current has many alternative paths to follow and resistance is lowered and the action potential speed is increased
• Membrane permeability and the cross sectional diamter ways to increase speed and true of any conductive cell
• not trying to force current through narrow gauge wire

Question 25

Increaseing the membrane resistance of the axon

Answer 25

* accomplised by wrapping axon with layers of insulation(myelin sheaths) * Creates a capacitor effect: * A capacitor consists of two conducting plates separated by an insulating barrier * Two conducting plates for an axon are the intracellular and the extracellular fluids * Insulating barier of an axon is the lipid layer of the cell membrane * A capcitor's capacitance is: * Directly proportional to the area of the plates(bigger plates can store more charge) * Inversely proportional to the distance separating the plates * All axons have schwann cell,and not all axons ahve a myleniation * Capcitor consists of two conducting plates speerated by an insulating barrier ,can hold a charge and release once it gets to certain level * Myelinated axons current can only flow across the axon membrane at the points of least ressitance, the myelin-free junctions between the Schwann cells that form the myelin sheath * This causes the potential flow to jump from one node to the next greatly increasing the speed of conduction * Jumping of current flow is referred to as **Saltatory conduction** * Vertebrate axons come in a variety of diamters and their conduction speeds vary accordingly * However even though the axons of human neurons do not come close in diameter to the largest axons of invertebrates, the fastest conducting vertebrate axons(large diamter and myelinated) are faster than the fastest invertebrate axons

Question 26

Threshold of the Action potential

Answer 26

* Threshold is the point at which a local potential will elicit an action potential * -65mV * Direction of propagation * Action potential travels in all directions from the point of stimulation * Orthodromic direction * Direction normally taken(towards the distal en of the axon) * Antidromic direction * Opposite direction(toward the axon hillock/neuron cell body)

Question 27

Factors Affecting propagation Speed

Answer 27

* Myelination * Sphingomyelin is hte principle lipid found in myelin sheaths * Schwann cells are cells that form the myelin sheaths in peripheral axons * Nodes of Ranvier are the unmyelinated junctions between Schwann cells * increases velocity of nerve transmission * Allows 100x les loss of ions and requries little energy for repolarization * Fiber Diamter: * Smaller diamter fibers conduct action potential slower than large diamter fibers * What would be the characterisitcis of the fastest conducting axons verus the slowest conducting axons in the human nervous system * Increase velocity of nerve transmission with less resistance there so potential can jump there at the nodes of ranvier * Classical small diameter nonmyleninated fibers are the pain fibers * While other fibers are bigger and slighly myelinated and go faster then the pain fibers

Question 28

Answer 28

* Gap between still covered by schwann cell, but not cover by myelin sheath(node of ranvier) *

Question 29

Absolute Refractory Period

Answer 29

* Period during which a second action potential canot be elicited even with a strong stimulus * Energy is derived directly from breakdown of ATP * During this time most fast sodium channels are either open or in the inactivated state during absolute refractory period * Unable to transfer sodium ions until they return back to the closed state * Length of this period determines the maximum frequency of action potentials

Question 30

Relative refractory period

Answer 30

* Stronger than normal stimulus can cause action potential * Mechanism for this is the elevated potassium conductance

Question 31

Resting potential for a typical neuron , such as a motor neuron is best represented by which fo the following potentials?

Answer 31

* -90mV

Question 32

Selectivity filter with carbonxyl oxygens is associated with which fo the following kinds of ion channels?

Answer 32

Potassium

Question 33

Saltatory conduction is chareceristic of which part of a typical neuron?

Answer 33

Axon

Question 34

Which of the following types of neurons would transmit an action potential the fastest? * Small diameter,non-myelinated * small diamter,myelinated * large diameter,myelinated * large diamter,non-myelinated

Answer 34

Large diamter ,myelinated