Transcript Lecture_28_noquizx

Monday April 7, 2014.
Introduction to the nervous system and biological electricity
1. Pre-lecture quiz
2. A word about prelecture readings
3. Introduction to the nervous system
4. Neurons and nerves
5. Resting membrane potential
A word about the readings
• Today’s readings were fine. (section 45.1 –
Principles of electrical signaling is good
background)
• Wed: pp. 891-898 (section 45.2 – dissecting
action potentials & 45.3 The synapse).
• Fri: pp. 899-904 (section 45.4 The vertebrate
nervous system)
The ability of animals to respond RAPIDLY to the environment and to move is
due to the electrical properties of neurons and muscles.
Venus flytrap can send a signal to close
that travels 1 to 3 cm/s.
Action potentials along neurons travel
up to 100 meters per second (or 10,000
cm/s).
Mary Shelley wrote Frankenstein
in 1818 long before we knew
about neurons.
Why did she choose to use
electricity to bring Frankenstein
to life?
She knew about the work by
Galvani on frog legs.
Galvani &
frogs legs
Galvani showed the applying a current to a frog nerve could make the muscles
twitch. Previously, folks thought that nerves were pipes or tubes. Galvani
introduced the idea “biological electricity”. Many of his speculations were incorrect
but he is credited with the important insight that animals use electricity in nerve
and muscle cells.
Central vs. peripheral nervous system
The brain integrates sensory information and sends signals
to effector cells.
Sensory neuron
CNS (brain
 spinal cord)
Sensory receptor
Interneuron
Motor neuron
(part of PNS)
Effector cells
Examples of sensory receptors in vertebrates
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•
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Nocirecptors = pain stimuli
Thermorecptors = changes in temperature
Mechanoreceptors = changes in pressure
Chemoreceptors = detection of specific molecules
Photoreceptors = detection of light
Electroreceptors = detection of electric fields
Magnetoreceptors = detection of magnetic fields
Information flow through neurons
Nucleus
Dendrites Cell body
Axon
Collect
electrical
signals
Passes electrical signals
to dendrites of another
cell or to an effector cell
Integrates incoming signals
and generates outgoing
signal to axon
Nerves are bundles of
neurons surrounded by
connective tissue
Neurons form networks for information flow
An introduction to membrane
potentials
• A difference of electrical charge between any two points
creates a difference in electrical potential, or a voltage.
• Ions carry a charge, and in virtually all cells, the cytoplasm
and extracellular fluid contain unequal distributions of ions.
Therefore, there is a separation of charge across the
membrane called a membrane potential.
• Membrane potentials are a form of electrical potential and
are measured in millivolts (mV). In neurons, membrane
potentials are typically about 70–80 mV.
• A flow of charged ions is an electric current.
Electrical Properties of Cells
All cells maintain a voltage difference across their membranes (Emembrane):
Two factors a required to establish a membrane potential
1.There must be a concentration gradient for an ion
2.The membrane must be somewhat permeable to that ion
Outside of cell
Microelectrode0 mV
K channel
– 65 mV
Inside of cell
A quick lesson from physics . . .
freely permeable membrane
With a permeable membrane, it takes force to
keep the distribution of ions.
[Na+]
low
[Na+]
high
How much force (voltage) is required to
maintain the imbalance?
Answer: Nernst Equation
A quick lesson from physics . . . (see Box 45.1)
freely permeable membrane
Nernst Equation
[Na+]
low
[Na+]
high
æéX ùö
RT
ë 1û ÷
ç
E = 2.3
log
çé X ù÷
ZF
èë 2 ûø
E=voltage
R=gas constant
T=temperature in Kelvin
F=faraday’s constant (charge carried by
mole of an ion)
Z = valance (1 for Na+, -1 for Cl-)
X1 and X2 are concentrations in the
two sides.
A quick lesson from physics . . . (see Box 45.1)
freely permeable membrane
Nernst Equation
[Na+]
low
[Na+]
high
Altered under physiological conditions
Unaltered under physiological conditions
æéX ùö
RT
ë 1û ÷
ç
E = 2.3
log
çé X ù÷
ZF
èë 2 ûø
E=voltage
R=gas constant
T=temperature in Kelvin
F=faraday’s constant (charge carried by
mole of an ion)
Z = valance (1 for Na+, -1 for Cl-)
X1 and X2 are concentrations in the
two sides.
freely permeable membrane
æéX ùö
RT
ë 1û ÷
ç
E = 2.3
log
çé X ù÷
ZF
èë 2 ûø
compartment 2
[Na+]
low
[Na+]
high
compartment 1
Assume the only thing changing are the concentrations of Na+ in the two
compartments and consider the following scenarios.
Scenario 1: [Na+] in compartment 1 = 500mM,
[Na+] in compartment 2 = 50mM
Scenario 2: [Na+] in compartment 1 = 700mM,
[Na+] in compartment 2 = 50mM
Which one of the two scenarios results in a larger value for E?
freely permeable membrane
compartment 2
[Na+]
low
[Na+]
high
compartment 1
æéX ùö
RT
ë 1û ÷
ç
E = 2.3
log
çé X ù÷
ZF
èë 2 ûø
Scenario 1: [Na+] in compartment 1 = 500mM,
[Na+] in compartment 2 = 50mM
log (500 / 50) = 1
Scenario 2: [Na+] in compartment 1 = 700mM,
[Na+] in compartment 2 = 1.46
log (700 / 50) = 1.146
Outside of cell
Microelectrode
0 mV
K channel
– 65 mV
Inside of cell
Outside of cell
Increasing [K+]
outside the
neuron
Microelectrode
Equilibrium!
0 mV
K channel
– 65 mV
Increasingly
negative
charge inside
the neuron
Inside of cell
Animation of resting potential
• https://www.youtube.com/watch?v=YP_P6bY
vEjE
Homework: Calculate the membrane potential for each of the three ions below.
æéX ùö
æéX ùö
RT
ë 1û ÷
ë 1û ÷
ç
ç
E = 2.3
log
= 58mV *log
çé X ù÷
çé X ù÷
ZF
èë 2 ûø
èë 2 ûø
Inside cell
Outside cell
[K+]
400 mM
20 mM
[Na+]
50 mM
440 mM
[Cl-]
51 mM
560 mM
at 20° C
Calculating the total resting potential – the Goldman Equation
The Goldman Equation extends the Nernst Equation to consider the relative
permeabilities of the ions (P): Ions with higher P have a larger effect on Emembrane
Emembrane
 PK  [ K  ]o    PNa  [ Na  ]o    PCl  [Cl  ]i  

 58mV *log 
 PK  [ K  ]i    PNa  [ Na  ]i    PCl  [Cl  ]o  


Permeabilities change during an action potential and how this allows
neurons to “fire”.
at 20° C