Membrane Potentials - Ch. 5

download Membrane Potentials - Ch. 5

of 23

  • date post

    11-Jan-2016
  • Category

    Documents

  • view

    221
  • download

    5

Embed Size (px)

description

Physio

Transcript of Membrane Potentials - Ch. 5

Introduction to Physiology: The Cell and General PhysiologyTextbook of Medical Physiology, 11th Edition
GUYTON & HALL
Chapter 5:
Slides by Thomas H. Adair, PhD
Copyright © 2006 by Elsevier, Inc.
Molecular Gradients
ATP
K+
Na+
Na+
K+
inside
outside
Remember: sodium is pumped out of the cell, potassium is pumped in...
Copyright © 2006 by Elsevier, Inc.
Simple Diffusion
Membrane Potential (Vm):
K+
Na+
K+
Na+
inside
outside
…how can passive diffusion of potassium and sodium lead to development of negative membrane potential?
Copyright © 2006 by Elsevier, Inc.
Simplest Case Scenario:
inside
outside
K+
K+
K+ would diffuse down its concentration gradient until the electrical potential across the membrane countered diffusion.
Copyright © 2006 by Elsevier, Inc.
Simplest Case Scenario:
If a membrane were permeable to only K+ then…
The electrical potential that counters net diffusion of K+ is called the K+ equilibrium potential (EK).
inside
outside
The Potassium Nernst Potential
EK = -61 log(140/4)
EK = -61 log(35)
EK = -94 mV
So, if the membrane were permeable only to K+, Vm would be -94 mV
…also called the equilibrium potential
Ki
Ko
Simplest Case Scenario:
If a membrane were permeable to only Na+ then…
The electrical potential that counters net diffusion of Na+ is called the Na+ equilibrium potential (ENa).
inside
outside
Na+ would diffuse down its concentration gradient until potential across the membrane countered diffusion.
Copyright © 2006 by Elsevier, Inc.
The Sodium Nernst Potential
EK = -61 log(14/142)
EK = -61 log(0.1)
EK = +61 mV
EK = 61 log
 
So, if the membrane were permeable only to Na+, Vm would be +61 mV
Nai
Nao
Resting Membrane Potential
Why is Vm so close to EK?
Ans. The membrane is far more permeable to K than Na..
EK -94
ENa +61
Vm -90 to -70
Vm for warm blooded animals in skeletal muscle and nerve is between -55 and -100 mV. Vm is between -55 and -30 for smooth muscle fibers. Most textbooks give a value of -70 mV for nerve and skm.
Copyright © 2006 by Elsevier, Inc.
The Goldman-Hodgkin-Katz Equation
Calculates Vm when more than one ion is involved.
NOTE:
-61
or
Cl ions are distributed across the membrane in accordance with their equilibrium potential – when Vm changes, Cl ions redistribute themselves in accordance to the new Vm, i.e., Eq Cl = Vm. Cell membranes of muscles have Cl permeability equal to or greater than that of K.
Copyright © 2006 by Elsevier, Inc.
The Goldman-Hodgkin-Katz Equation
The resting membrane potential is closest to the equilibrium potential for the ion with the highest permeability!
Take home message…
Resting Membrane Potential Summary
Figure 5-5; Guyton & Hall
Resting and action potentials
have membranes that are essentially permeable to K+ at rest
Membrane electrical potential difference (membrane potential) is generated by diffusion of K+ ions and charge separation
measured in mV (=1/1000th of 1V)

Properties of action potentials
have constant conduction velocity
True for given fiber. Fibers with large diameter conduct faster than small fibers. As a general rule:
myelinated fiber diameter (in mm) x 4.5 = velocity in m/s.
Square root of unmyelinated fiber diameter = velocity in m/s
are initiated by depolarization
action potentials can be induced in nerve and muscle by extrinsic (percutaneous) stimulation
have constant amplitude
APs do not summate - information is coded by frequency not amplitude.
threshold
-70
+60
mV
Stimulus
0
non-myelinated
(squid)
0
800
400
The AP - membrane permeability
Na permeability increases
memb. potential approaches ENa
mem. potential approaches EK
not always seen!
due to delayed closure of K+ channels
During the downstroke of an action potential:
Na permeability decreases
Copyright © 2006 by Elsevier, Inc.
Number of open channels
Ion channels
have water filled channel that runs through protein
Ion channel - properties
transition between states = ‘gating’
changes in membrane potential (usually depolarization)
voltage-gated channels. Action potential propagation relies on voltage-gated channels
occupation of receptor
mechanical forces
open
inactivation
inactivated
closed
depolarization
repolarization
Propagation:
Opening of Na+ channels generates local current circuit that depolarizes adjacent membrane, opening more Na+ channels…
Rest
Stimulated
Signal Transmission:
Schwann cells surround the nerve axon forming a myelin sheath
Sphingomyelin decreases membrane capacitance and ion flow 5,000-fold
Sheath is interrupted every 1-3 mm : node of Ranvier
Figure 5-16; Guyton & Hall
Saltatory Conduction
channels concentrated here!)
Conduction velocity
Multiple Sclerosis
- MS is an immune-mediated inflammatory demyelinating disease of the CNS -
- About 1 person per 1000 in US is thought to have the disease - The female-to-male ratio is 2:1 - whites of northern European descent have the highest incidence
Patients have a difficult time describing their symptoms. Patients may present with paresthesias of a hand that resolves, followed in a couple of months by weakness in a leg or visual disturbances. Patients frequently do not bring these complaints to their doctors because they resolve. Eventually, the resolution of the neurologic deficits is incomplete or their occurrence is too frequent, and the diagnostic dilemma begins.
http://www.emedicine.com/pmr/topic82.htm