Neurons and Neuronal Excitability (Both Parts) Flashcards
Functional Unit of Nervous Sytem
Neuron
How many Neurons in a Body?
100 Billion
What part receives signals?
Dendrites
What part transmit signals?
Axons
Types of Transport (3)
Fast
Slow
Fast Retrograde
What does Fast Transport use?
Protein Kinesin
What does Fast Retrograde Transport use?
Protein Dynein
Functional Areas: What are their characteristics?
Receptor Zone
Initial Segment
Axon
Nerve Endings
RZ: Graded Electrogenesis
IS: Site of Origin of Conducted Impulse
Axon: All or None Transmission
Nerve Endings: Secretion of Synaptic Transmitter
Receptor Zones are composed of?
Somas and Dendrites
Resting Membrane Potential Value
-70mV
Is the whole cell actually charged negatively?
No, only the area just inside is negative and the area just outside is positive
Overall it is neutral
3 Mechanisms Generating RMP
- Transmembrane K+ Gradient Through Non-gated K+ Leak Channels
- Donnan Effect
- Na-K ATPase Pump
What happens to K at RMP?
Na?
Cl?
K: Tends to move out
Na: Tends to move in
Cl: Doesn’t do anything
What is mainly responsible for the RMP?
Potassium because it can freely diffuse due to potassium leak channels
Why is RMP different from the potential for K, if K’s movement is the one responsible for the RMP? [He “might” ask this]
Not a perfect system, some sodium is still able to leak in and contribute a positive charge
[Goldman Constant Field Equation]
Na
Driving Force?
Permeability?
Net Flux?
Very high driving force
Very low permeability
Small net flux
[Goldman Constant Field Equation]
K
Driving Force?
Permeability?
Net Flux?
Small driving force
Very high permeability
High net flux
[Goldman Constant Field Equation]
Cl
Driving Force?
Permeability?
Net Flux?
Least flux
How come the Nernst Equation approximates the Goldman Constant Field Equation?
Concentration and contribution of K is so high that the other factors in the equation are negligible
What happens to the RMP without ATP?
It will dissipate because it is not in a true equilibrium, it is in a steady state equilibrium
Function: Na-K ATPase Pump
Hydrolysis of ATP results in 3 Na being brought out and 2 K brought in
Define: Local Potential
Shift of membrane potential in a localized cell area
Characteristics of Local Potential (5)
- Localized within an area
- Graded Response
- Decrementally Transmitted (Magnitude decreases the further it travels)
- Potential for Summation
- Very Rapid (Faster than AP)
Formula for Time Constant
T = RC
τ = Time constant R = Resistance C = Capacitance
Formula for Space Constant
λ=√(Rm /Ra)
No need to memorize the equation, just remember
the relationships.
• Directly related to transmembrane resistance (Rm)
• Inversely related to internal axoplasmic resistance (Ra)
Define: Space Constant
Distance at which initial transmembrane voltage change has fallen to 37% of its peak value
Result of Higher Rm?
Lower Leakage -> More Ions Travel Down Axon -> Higher Space Constant
Result of Greater Diameter of Axon?
Lower Ra -> Greater Current Will Flow Farther Down Cell -> Greater Space Constant -> AP is Faster
Result of Smaller Diameter of Axon?
Does not need speed -> Lower Space Constant -> AP is slower
Define: Spatial Summation
Separate sites fire and while each firing cannot depolarize the cell, the combination of them can
Define: Temporal Summation
Firing in rapid succession can cause summation of charges causing an AP
Define: Action Potential
Fleeting, self-renewing wave of depolarization that propagates without decrement along the entire length of a nerve axon at high speed
Is the AP generated at the beginning the same as the end AP?
No it is not, it renews itself along the neuron
Impulses passing in one direction only is called?
What if it’s in the opposite direction?
Orthodromic Conduction
Antidromic Conduction
What would increase the speed of propagation of an AP?
An increase in diameter of the axon
Decrease cytoplasmic resistance
Increase flow of ions
Increase length of axon depolarized (increase space constant)
Decrease time needed for AP to travel along axon
Define: Saltatory Conduction
Phenomenon that occurs at the Nodes of Ranvier where the signals jump from one node to the next
Function: Myelin
Insulator that keeps electrical signals within the axon
Function: Oligodendrocytes
CNS Version of Myelin (Many Axons)
Function: Schwann Cells
PNS Version of Myelin (One Axon)
Presence of Myelin leads to? (5)
• Increase in effective resistance of axonal membrane (Rm): ions must flow through myelin before reaching the ECF
• Decrease in effective capacitance of axonal
resistance (Ra): greater distance between ICF and ECF
• Lowers time constant
• Increases space constant
• Increases conduction velocity
o Myelinated conduction velocity = 3- 120 m/sec
