Transcript Ionic basis of Resting and Action potentials

Welcome to 632
Nerve Muscle and Movement
Chris Elliott - [email protected]
Sean Sweeney [email protected]
John Sparrow - [email protected]
Web page:
http://biolpc22.york.ac.uk/632/
Course Overview
 Lectures
 Chris
2 : Nerve and Synapse
 Sean 2: Synapse development
 Chris 2: Channels
 John 4: Muscle
 Chris 4: Movement
Nerve & brain lectures
 In B006
 Nerve
1 Ionic basis of Resting and Action potentials
2 Mechanism of synaptic actions and
neuromodulation
3 The Patch clamp approach to Neurobiology
4 Effect of Insecticides on Neural function
Movement lectures
 Neural Control of singing and hearing in
insects
 Locomotion
 Types
& Principles of locomotion
 Walking running & jumping
 Swimming
 floating
 Flying – birds, bats & insects
Not only lectures…
 Practicals - No
 Group Case Study 30%
 Exam
 70%
paragraph answers; paper criticism
Case Study
 group
of 4 - 7
 work on problem together
 submit single report
 choice of 4 Studies
 Group list: Wednesday 3 May 1115
 e-mail appointment; or come Wed 31 May
 deadline : Friday 2 June
Books, etc
 Purves, D (et al) (2001)
Neuroscience Sinauer
 Simmons PJ and Young D
(1999) Nerve Cells and
Animal Behaviour CUP
 McNeill - Alexander R.
How Animals Move [CD
Rom borrow in teaching]
Other books on nerve
 Shepherd, G. M. (1994) Neurobiology. OUP
An excellent text
 Nicholls, J et al (2002) From Neuron to Brain
(4th ed)
 Robinson, D. Neurobiology (ISBN 3-540-637788): (1998)
What needs explaining?
 what are nerve cells like?
 what happens at rest ?
 Resting
potentials
 dynamic equilibrium
 what happens when activated?
 Action
potentials
 All-or-none
 speed
 comparative differences
Mammalian cells
 Brain has
neurons 109
 glia 3 • 109
 blood vessels

 Parts of a
neuron
dendrite
 soma
 axon

Identifying cells
 silver staining
 fluorescent dyes
 antisera
Invertebrate cells
 Ganglion

400 to 106 cells
 nerve or neuron?
Summary so Far
 Brains made of neurons and glia
Squid neurobiology
 Contract mantle as fast
as possible
Big axon (250µM)
 insert electrodes
 replace contents

Resting potential
 Cells are all
negative
 contain K+
 outside Na+
 anions e.g. Cl have semipermeable
membranes
 Squid giant axon
Animations of resting
potential
 Bezanilla

http://pb010.anes.ucla.edu/
Resting potential
 Balance between
diffusion and
electrical force?
 Use Nernst
Equation to test
this out
 Conclusion:
passive balance
is OK for squids
Ediff

K in 
RT

ln
zF K out 
Ediff
440
 56 log
mV
20
Ediff  75mV
Summary so Far
 Brains made of neurons and glia
 All cells have resting potentials
 Normally maintained passively by balance of
diffusion and electrical forces
Action potential
 membrane
becomes
permeable to
Na+
 Na+ floods in
diffusion
 electrical

 K+ still goes
out
 Squid giant axon
Action potential
 Two crucial properties
of the Na+ current
starts at a voltage
threshold
 stops itself

 Arise from
Na+
channel
channel is voltage
sensitive and opens
 closes with a second
mechanism
-30mV
closed
open 1ms
inactivated

-70mV
How do we know ? (i)
 Hodgkin &
Katz
replaced Na+
in the
seawater
How do we know ? (ii)
 Hodgkin & Huxley
devised the voltage
clamp experiment
separates the ionic and
capacitative currents
 use replace ions to determine
role of each

Interlude
 What is resistance ?

Write it down now
 What are current and
voltage?

Write it down now
 V 
R
I
 Use V for voltage
 use I for current
 Rule (Ohm’s law)
V
= IR
Interlude
 What is capacitance?

Write it down now
Resistance Rule
(Ohm’s law)
V
= IR
- +
 V 
R
I
 Rule
C
Q=CV
dQ/dt
= CdV/dt
I = dQ/dt = CdV/dt
H&H Experiment
Voltage
 Step the
clamp from
-70mV to
different
voltages
Current
H &
H (ii)
 Add
tetrodotoxin
and block
Na+ current
 tetra-ethylammonium
and block K+
current

H&H reconstruction
 H&H measured the kinetics of the currents
 used
this to postulated the kinetics of channels
 used this to build a mathematical model
 Animations of H&H model
 Bezanilla
 see
http://biolpc22.york.ac.uk/632
Summary so Far
 Brains made of neurons and glia
 All cells have resting potentials
 Normally maintained passively by balance of
diffusion and electrical forces
 Properties of Na and K channels determine
action potential
How does it spread?
 electrostatically
How fast is the action
potential?
 Up to 100m/s
 major component of latency to respond
 for
2m high human, 2/100*1000 = 20ms
 for a 40m dinosaur...
 slowed by capacitance
How do we
know?
 Myelinated axons
run faster,

capacitance is
reduced
 channels only at
Nodes of Ranvier
Myelination
 Schwann cell
(blue) grows
round axon
(orange)
 In Multiple
sclerosis (MS)
myelin sheath
is disrupted
Comparative
neurobiology
 Action potentials are not all the same
 in
vertebrates K+ current is very small
 in molluscs, Ca++ current supplements
the Na+
 only vertebrates have myelination, but
all animals have glia
 protozoa have action potentials too
A word of caution
 students often write
conductance when they mean
conduction
 conductance is a measure of
permeability
 how
easy it is for ions to cross
the membrane
 conduction is the process of
movement along the axon
 e.g.
conduction velocity
Final Summary
 Brains made of neurons and glia
 All cells have resting potentials
 Normally maintained passively by balance of
diffusion and electrical forces
 Properties of Na and K channels determine
action potential
 Capacitance (myelination) determines speed
 Web page: http://biolpc22.york.ac.uk/632/