Biology 3240 Intro Cellular Neurobiol, Key Words & Concepts

Biology 3240 Intro Cellular Neurobiol, Fall '09
Key Words & Concepts: Lectures 1-10

This page will offer some key words and ideas from lectures, provided as rough study guides.

Larry Okun, Doju Yoshikami

Lectures 1-10 Lectures 11-21 Lectures 22-32 Lectures 33-40

(to Suggested Readings instead)

Lecture 1 8/24/09

overview & terminology -- vertebrate nervous system & cells
embryonic origin:
ectodermal
neural tube, neural crest, placodes
further development of neural tube:
cell proliferation, migration, process outgrowth, formation of connections
ectodermal characteristics: tight packing, specialized junctions
main divisions of nervous system:
CNS: brain, SC
PNS: nerves to/from CNS, ganglia, plexuses of enteric system
peripheral systems: voluntary/somatic, involuntary/autonomic/visceral
peripheral ganglia:
dorsal root ganglia (DRG) -- cell bodies of sensory neurons;
carry information to CNS
sympathetic, parasympathetic ganglia -- cell bodies of output-relay neurons;
convey outputs to muscles (smooth, cardiac), glands, etc., under involuntary control
enteric plexuses in gut muscle wall -- include sensory (input) and motor (output) neurons;
monitor and control gut operations (mechanical, chemical)
voluntary/somatic system: CNS direct to/from voluntary (striated) muscles
autonomic/visceral system: sympathetic & parasympathetic gangla plus enteric plexuses
major cell types -- neurons, glia:
glial classes:
CNS: oligodendroglia, astroglia, radial glia, ependymal cells, microglia
PNS: Schwann cells
neuron structure/parts:
soma (cyton), perikaryon
dendrites, dendritic spines
axon, axon hillock, initial segment, axon collateral
myelin, nodes of Ranvier, internodes
axon terminals, synapse, pre-, post-synaptic parts
usual roles in signaling:
dendrites, soma -- receive inputs from other neurons
axon -- carry output to other neurons or effector organs (muscles, glands)
synapses -- sites of signal transmission,
usually from axon terminals to dendrites, dendritic spines, or cell soma
variation of shape, significance of shape;
exceptions to 'usual' structure, 'usual' roles of parts:
amacrine cells of retina -- no axon, DRG sensory cells -- no dendrites
axo-axonal synapses, dedro-dendritic synapses
nervous system organization -- spinal cord example:
gray matter, white matter;
dorsal horn, ventral horn, dorsal root, ventral root
Bell-Magendie Law; conceptual, experimental significance
spinal reflex circuits:
Sherrington -- analysis of spinal muscle-reflex responses
Eccles -- elextrical recording from spinal neurons involved in reflexes
some examples:
1. myotatic (muscle-stretch) reflex:
stretch-sensitive sensory endings, muscle spindle, afferent (sensory) fiber, DRG neuron
ventral-horn motoneurons; motor unit, motor pool
homonymous motoneurons, synergist motoneurons
a 'monosynaptic' reflex
2. flexor (or 'flexion') reflex:
noxious stimulus (pain; DRG neuron types different from stretch afferents)
a 'polysynaptic' reflex ('interneurons' involved)
antagonist muscle;
inhibition; interneurons can change 'sign' of signal;
reciprocal innervation
generalization of nervous-system organization; within a region, e.g., spinal cord:
inputs (afferents)
interneurons (local neurons, local-circuit neurons, internuncial neuron, associative neurons)
outputs (efferents, principal neurons, relay neurons, projection neurons)

Lecture 2 8/26/09

neuron history, Neuron Theory
early microscopy:
technical & conceptual difficulties delaying acceptance of neurons as single cells
notable early observations:
Purkinje (cerebellar Purkinje cell)
Deiters (hand-dissected motoneuron)
Golgi stain
Ramon y Cajal's studies, insights:
neurons are individual cells, communication by contact not fusio
dynamic polarization
connective (synaptic) specificity
growth cone, chemotaxis
idea that learning involves changes in strengths of connections
variety of neuron shapes, synaptic styles, synaptic specificites -- cerebellar cortex example:
general function of cerebellum (from animal ablation experiments, human injuries, mouse mutants)
highly folded (convoluted) structure, parasagittal section, folia
layers of gray matter in cerebellar cortex:
granular layer, Purkinje-cell layer, molecular layer
principal (output) neurons: Purkinje cells (define 'Purkinje layer')
one interneuron class (more in next lecture):
granule cells (billions, producing appearance of'granular layer') --
axons form 'parallel fibers' in 'molecular layer'
(defined by 'molecular' appearance of parallel fibers seen end-on in parasagittal section) --
synapse 'in passing' with spines on spiny branchlets of Purkinje cell dendrites
inputs, two types:
mossy fibers (synaptic terminals: 'rosettes'/'glomeruli' in granular layer) --
synapse with dendrites of several granule cells within each glomerulus;
climbing fibers ('climb' Purkinje-cell dendritic tree) --
synapse with smooth, mostly larger branches of Purkinje cell and stubby thorns on them
dramatic differences:
sizes/shapes of Purkinje cells vs. granule cells
locations/numbers of input or output partners --
those on Purkinje cells from parallel fibers of granule cells
vs. those on Purkinje cells from climbing fibers
synaptic styles --
mossy fiber to granule-cell dendrite ('glomerulus')
vs. granule-cell axon (parallel fiber) to Purkinje cell dendritic spine ('en passant')

Lecture 3 8/28/09

variety of neuron shapes & synaptic styles, synaptic specificites -- cerebellar cortex example (cont'd):
more interneurons (these all inhibitory):
Basket cells, Golgi cells, Stellate cells
two additional interneurons, recently identified as distinct:
Lugaro cells (inhibitory), Unipolar Brush Cells (excitatory)
points illustrated:
cortical sheet, higly convoluted (wrinkled) to increase surface area
cell types within sheet ordered in layers
neuron sizes & shapes widely variable; shapes related to function,
i.e., to distribution of each neuron's inputs, outputs
variety of synaptic styles; e.g., en passant, glomerulus, basket
specificity of synaptic locations, as well as partners;
e.g. different locations of synapses on Purkinje cells from
parallel fibers (granule cells), climbing fibers, Basket cells, Stellate cells
differences of convergence and divergence numbers;
e.g. convergence: Purkinje cell from parallel fibers vs. Purkinje cell from climbing fiber
divergence: mossy fiber to Purkinje cells vs. climbing fiber to Purkinje cells
functional implications of structural relations; e.g., styles of inhibition:
lateral inhibition, feedback inhibition, feedforward inhibition,
inhibitory 'sculpting' of Purkinje-cell dendritic tree, trisynaptic inhibitory loop
overview of cerebellar cortex function:
(CNS terms: brain nuclei, fiber tracts; cf. ganglia and nerves in PNS)
input sources (most relayed via various brain & brain-stem nuclei; as mossy fibers, climbing fibers):
brain motor centers (motor 'commands')
virtually all sensory sources (via brain and spinal cord ('state' of muscles, body, environment)
spinal cord motor centers (ongoing executive 'operations,' maybe 'intentions')
output targets (of Purkinje cells, principal neurons of cerebellar cortex; inhibitory on targets):
almost all to deep cerebellar nuclei,
which receive same inputs as cerebellar cortex (as collaterals of mossy fibers & climbing fibers),
and send outputs back to motor centers in brain & spinal cord (via other nuclei);
cerebellar cortex thus an elaborate inhibitory controller ('feedforward' style) of deep cerebellar nuclei;
(neurons of deep cerebellar nuclei real 'actors' of cerebellum)
climbing fiber 'training' of Purkinje cell responses to sets of parallel fibers --
'long-term depression' (LTD), an example of 'synaptic plasticity'
history of the Neuron Theory (resumed):
'neuronists' vs. 'reticularists'
neurons as independent cells vs. idea nervous system a fused, syncytial network (reticulum);
conceptual reservations about neurons as independent cells;
issues: trophic support, development, communication, existence of a 'membrane'
observations leading to change of ideas:
re: trophic issue (responsible for 'feeding' self?):
responses to axotomy --
adults: Wallerian degeneration -- axon depends on 'center'
young animals (Forel): loss of injured neurons, not neighbors
re: development issue (long processes outgrowths of single cells?)
(vs. 'cell-chain' or 'bridge' hypotheses):
His: in embryos, apparent outgrowth from cell bodies of young neurons -- 'neuroblasts'
Harrison: tissue-culture experiments; process outgrowth in absence of other cells
re: communication issue (by contact or fusion?):
Sherrington: special properties of signal propagation in reflex circuits --
e.g., rectification, delay, inhibition --
argue for communication by special contact sites, not simple fusions -- 'synapses'
re: existence of a cell membrane:
emerging hypotheses accounting for electrophysiological phenomena --
resting potential, action potential -- depended on a membrane,
(and one with special properties)
cumulative evidence, though strong, not fully compelling;
'neuronist vs. reticualrist' controversy continued until mid-20th century
1906 Nobel awards: Golgi & Cajal, on opposite sides of issue
Cajal's last paper, 1933, dealt with controversy, supported 'neuronist' position
how/when issue resolved -- in next lecture

Lecture 4 8/31/09

history of the Neuron Theory (continued):
cumulative evidence around 1900 --> 'Neuron Doctrine':
neurons as anatomical units, trophic/metabolic units, developmental units, functional units
but a central issue remained unsettled:
communication between neurons by contact or fusion of processes?
-- inadequacy of light microscopy to 'resolve' issue
later, strong evidence for communication by contact:
1950's: electron microscopy: membrane, synaptic cleft
1960's: dye injections, dye coupling, gap junctions, 'electrical' synapses;
use of dye injections for Golgi-like labeling of neurons studied electrophysiologically
some exceptions to Neuron Doctrine (variations on the 'theme'):
trophic/developmental dependence on local environment, other cells
functional exceptions -- 'group' neurons, 'multiplex' neurons
some details of neuronal structure, cell biology:
soma, great variation of size
nucleus: extended chromatin:
terminally differentiated cells, birthdating, embryonic waves of neuron production/migration;
some neurogenesis in adults (stem cells) --
in mammals only seen in two places (olfactory bulb, hippocampal dentate gyrus),
not detected e.g., in cortex;
extensive transcription, classes of mRNA vs. other tissues,
alternative splicing can increase mRNA variety from 'limited' DNA genome
nucleolus -- prominent in neurons, much ribosomal production
perikaryon:
common cellular components, e.g., mitochondria, lysosomes;
particularly prominent light-microscopic features:
Nissl substance, Golgi apparartus, neurofibrils --
Nissl substance (rough ER stained by basic dyes):
synthesis of proteins destined for membranes or secretion;
chromatolysis: change in appearance of Nissl substance following axotomy
use of injury-dependent approaches for tract tracing --
chromatolysis for 'retrograde' tracing,
Wallerian degeneration for 'anterograde' tracing;
problems associated with use of these approaches: timing, errors from injury to 'axons of passage';
free (cytoplasmic, non-Nissl) polyribosomes also plentiful:
synthesis of proteins for cytoplasm, organelles (nucleus, mitochondria)
Golgi apparatus (Golgi complex):
processing/sorting of proteins for membrane or secretion;
discovery as 'intracellular net' filled by variant of Golgi stain;
prominent in secretory cells
neurofibrils: light microscopic features (with heavy-metal stains, Au, Ag, not Golgi stain);
likely clumped neurofilaments (mostly) and microtubules (more about both in next lecture)
implications of somatic features (nucleus, nucleolus, Nissl substance, Golgi complex):
much, many types of protein synthesis, particularly for membrane components --
channels, receptors for synaptic transmitters & trophic signals,
surface recognition & junction molecules, secretory vesicles
dendrites: similar in composition to perikaryon;
Nissl substance, Golgi apparatus diminish with distance from soma;
some polyribosomes found even distant, particularly at bases of spines
axons: mitochondria, smooth ER, vesicles, cytoskeletal components (e.g., light-microscopic neurofibrils);
(lots of mitochondria and vesicles at presynaptic nerve terminals)
but very rare, almost no, polyribosomes or Golgi apparatus --
implication: very little local protein synthesis in axons
(but accumulating recent evidence there is some--
e.g., a few specific proteins synthesized locally & inserted into membranes in axons, growth cones)
thus, most axonal proteins, membrane components must come from elsewhere
(as also must many/most of those in distal dendritic branches);
also no lysosomes reported in axons --
implication: proteins to be degraded must be shipped back to soma

Lecture 5 9/2/09

cytoskeletal elements (all proteins): actin, neurofilaments, microtubules
actin (5-9 nm diameter):
structure -- globular subunits, fibrous 'double-stranded' polymer
location -- meshwork at cell cortex (near memabrane)
roles -- with 'teams' of other proteins, connect through membrane to outside;
anchor, localize membrane proteins;
mediate some responses to extracellular signals;
involved in cell movements, e.g., of growth cones, dendritic spines
neurofilaments (~10nm diam.):
members of 'intermediat filament' (IF) family; among toughest, least soluble of cellular proteins
structure -- rope-like polymer (rod-like coiled monomers, helical, 'coiled-coils' pairs (dimers),
helical tetramers (staggered dimers), staggered bundles of 8 tetramers)
neurofilaments: mixes of three monomer classes, 2 with high molecular weights;
non-coiled ends of high-MW monomers yield 'side arms' of filaments --
may link neurofilaments to each other or other skeletal elements,
or may serve to separate ('space') neurofilaments from one another
location -- throughout neuron, especially rich in large axons
roles -- assumed to confer structural strength, e.g., to axons,
and may help determine axon diameter
microtubules (~25nm diam.):
structure -- globular subunits, 13 'protofilament' polymers arranged in ring (tube);
polarized (like actin filaments) - fast-growing (+) end, slow-growing (-) end;
'+' ends usually oriented away from cell center, all '+' ends oriented away from soma in axons;
orientations mixed in proximal dendrites, reported to be polarized (as in axons) in distal dendrites
location -- throughout neuron; present even in smallest dendrites, axons (why thought necessary?)
microtubule-associated proteins:
two classes -- MAPs, tau proteins
bind to micrtubules, may promote/maintain polymerized state, link microtubules to other elements;
some MAPs known to be transport motors;
classes of MAPs, tau proteins differ
- among cell types (e.g., different MAPs in neurons vs. glia)
- in embryonic vs. adult cells
- in parts of neuron (dendrites & soma vs. axon)
microtubule role -- 'tracks' for fast axonal transport
axonal transport:
early axon-ligation experiment; 'axoplasmic flow'
later radio-tracer studies: transport categories -- approximante rates of transport, nature of transported elements:
fast anterograde (somatofugal) transport, ~400mm/day: vesicular elements, membrane components
slow anterograde transport, ~1-2mm/day: actin, soluble enzymes
very slow anterograde transport, ~0.2-1mm/day (sometimes reported to be faster, within 'slow' range): neurofilaments, microtubules, MAPs, tau protein
retrograde (somatopetal) transport, ~200mm/day: vesicles with materials for degradation by lysosomes in cell body
or materials endocytosed from environment of axon terminals, carried back to soma --
some good: trophic signals, some bad: viruses (e.g., polio, rabies), toxins (tetanus)
use of fast anterograde & retrograde transport for tract tracing,
replacing older methods based on Wallerian degeneration, chromatolysis
mechanism of fast anterograde and retrograde transport:
requires ATP and microtubules, not ongoing protein synthesis or even attached cell body;
computer-enhanced video microscopy, isolated microtubules in vitro --
transport of vesicles, small plastic beads in presence of ATP and specific MAPs;
identified MAP 'motor proteins':
kinesin family (usually toward + ends of microtubules)
dyneins (usually toward - ends of microtubules)
mechanism of slow transport -- unclear
some other current questions:
how different materials are sorted, 'targeted' to axons or dendrites;
how dynein motors are transported anterograde in axons (to carry things back);
how kinesin motors are returned to cell body for another 'trip' (if they are);
whether slow transport 'good enough' to supply needed new neurofilaments, microtubules, MAPs

Nature of the nerve signal - some history:
early speculation about relation to another mysterious phenomenon -- electricity
state of knowledge ca. 1750:
electrical technology -
sources: static machines, lightning, Leyden jars or Franklin magic squares (for 'storage')
detectors: sensation, sparks, electroscopes
known facts about electricity and nerves -
animal tissues conduct electricity
electricity (from static machines, Leyden jars) a good stimulus of nerve and muscle (Caldani)
doubts about electricity as a nerve signal:
other agents also effective nerve stimuli (initiators of 'nerve signal') --
e.g., mechanical insults (cuts, pinches), some salts
nerves poor conductors -- high resistance (compared to metal wires), leaky
crush of nerve stops nerve signal, not flow of electric current along whole nerve
questions about whether animals can generate electricity, maintain charges separate in conductive body salines;
convincing demonstration (sparks) of electricity generated by some eels,
but persistent question of whether there is electricity in 'ordinary' animals

Lecture 6 9/3/09

Invertebrate nervous systems
Cnidaria (e.g., jellyfish, hydra) have amorphous nerve net.
Annelida (e.g., earthworm, leech) and Arthopoda (e.g., insects) have ventral nerve cord with segmental ganglia (~"CNS"); Mollusca (e.g., squid) have distributed ganglia.
Some of these organisms provide tissues that can be thought of as 'model preparations.' Invertebrates and lower vertebrates are cold-blooded so isolated tissues (e.g., dissected out and maintained as "whole mount") function well at ambient (room) temperature. Also, metabolic activity is relatively low, so no need to take special steps to provide oxygen (exposure to air suffices).

Whole mount of a live ganglion from leech (Fig. 15.2 on p. 295, see also Fig. 15.1). Use of the leech was championed by J. G. Nicholls (1st author of NMWF4). Its ganglia are transparent with ~400 neurons/ganglion. Specific neurons can be reproducibly identified.

Other model preparations (particularly amenable to experiments because large cells allow ready recording with intracellular microelectrodes.
Squid giant axon
Squid giant synapse (a model synapse)
Neuromuscular junction (another model synapse)

Model (genetic) organisms
Three outstanding bacteriophage geneticists turned their attention to neurobiology in the 1960's and fathered the following model systems. C. elegans (nematode or round worm): Sydney Brenner (pursued simplest system, entire nervous system of worm has ~300 neurons). Drosophila (fruit fly): Seymour Benzer (pursued small part of complex, but genetically tractable system). Danio rerio (zebra fish): George Streisinger (kept fish as a hobby, exploited optical transparency of this vertebrate).
Other model vertebrates: frog -- Xenopus laevis, Xenopus tropicalis; mouse -- Mus musculus ('knockout' mice available, thanks to Mario Capecchi)

Brain imaging
CT scan uses x-rays. Computed tomography. Aka CAT (computerized axial tomography). 'tom' = 'cut' as in microtome (or atom = uncuttable, indivisible). (EMI, Ltd = Electric and Musical Industries, Ltd, for whom The Beatles made the money that funded the research of Hounsfeld, one of the developers of CT.)

Magnetic resonance imaging (MRI), images the NMR signal of protons (in water).

Functional MRI (fMRI) The blood-oxygen-level-dependent (BOLD) magnetic resonance signal arises from the loss of oxygen from hemoglobin, which influences the magnetic field experienced by protons in surrounding water molecules. Increased neural activity --> increase O₂ use --> disproportionate increase in local blood flow & blood volume --> net decrease in the amount of deoxygenated hemoglobin present, i.e., [Hb]/[HbO₂] ratio decrease --> detected by BOLD (blood oxygen level dependent) MRI signal.

Image on cover of NMWF4 is fMRI superimposed on MRI. fMRI is a difference image; i.e., subject looking at flashing checker board pattern vs. eyes closed. (Compare with Fig. 1.1 from Cajal.)

PET Another way to monitor increased metabolic activity is by glucose utilization. Cellular uptake of 2-deoxyglucose (2-DG), like that of glucose, increases with increased metabolic activity. It is phosphorylated to 2-deoxyglucose-6-phosphate (2-DG-6-P), but can't be further processed via glycolysis since the 2-hydroxyl group (present in normal glucose) is missing. 2-DG-6-P cannot leave the cell and accumulates.
--> [³H]-2-deoxyglucose uptake detected by autoradiography (requires frozen sections, therefore experimental subjects have to be sacrificed).
--> 2-[¹⁸F]-2-deoxyglucose uptake detected by PET (positron emission tomography), as follows: when ¹⁸F decays, a positron is emitted, which upon collision with an electron annihilate each other and energy is released in the form of two gamma photons. Imaging can be done on living subjects.

SPECT (not presented in lecture due to lack of time) Single-photon emission computed tomography, SPECT, uses isotopes that release a single gamma photon upon radioactive decay; e.g., ¹²³I. Binding sites in the brain for molecules labeled with such isotopes can be imaged.

Lecture 7 9/9/09

Nature of the nerve signal, possible electrical character (early history cont'd);
initial investigations of the 'nerve signal' (and the birth of electrical technology):
1770's-1790's, Galvani: apparent stimulation by inherent 'animal electricity' conducted via metal
contacts ('arcs') between muscle and nerve; but 'bimetallic arcs' worked best
1790's, Volta: bimetallic electricity; detection by sensations and electroscope
controversy -- animal electricity or bimetallic electricity?
1797, Galvani: 'contraction without metals' --
stimulation of a nerve/muscle preparation when its nerve dropped on injured muscle of another;
detection of 'animal electricity' in the injured muscle, or stimulation by some other aspect of it?

advances in electrical technology:
1799, 'Voltaic pile' (electrical battery), source of steady current
1819-20, Oersted: relation between magnestism and electricity --
influence of an electric current on nearby magnetic needle
1820's, Ampere (and others): galvanometers --
'multipliers,' 'astatic' galvanometer, 'counterwound astatic' galvanometer

investigations using galvanometers (more sensitive physical detectors of electricity):
1820's-1840's, Nobili, Matteucci: injury current from muscle
decrease of muscle injury current with strychnine treatment of spinal cord to induce muscle 'tetany'
1850's, duBois Reymond:
injury current from nerve (injured site negative with respect to uninjured site) --
clear demonstration of electricity produced by ordinary nerve
negative variation (of nerve injury current) during tetanic stimulation of nerve --
an electrical chage accompanying activity in nerves
experiment arguing that each nerve signal of tetanic train involves a separate negative variation event
1850, von Helmholtz: propagation velocity of nerve impulse:
experimental design dealing with issue of 'utilization times' and able to measure short times;
velocity of nerve impulse vs. velocity of electrical propagation in metal wire --
nerve signal, if electric, is different from electric signals in wires
1868, Bernstein: propagation velocity of negative variation:
'sampling rheotome' to measure size, and timing after nerve stimulation, of brief negative-variation events,
even though galvanometer could only display effects of a tetanic series;
velocities of negative variation events and nerve signals same;
plot of negative variation with time after stimulus -- first record of an 'action potential'

by ca. 1900, 3 key facts about nerve signals had been discovered:
1. electricity is a good stimulus for initiating nerve signals (or directly causing contractions in muscles)
2. a current, detectable by galvanometer, flows between uninjured and injured parts of a nerve (or muscle)
3. nerve signals travel with velocity slow by comparison to that of electrical effects in wires
and are accompanied* by negative variation events (decreases in electrical difference of uninjured nerve from injured site)
*inferred from similarity of velocities
a clear model of the nerve signal was built (by Bernstein) from these facts --
to be discussed soon, along with questions it posed guiding further research,
but next two lectures will first expand on the facts themselves, providing some basic background
in electricity and illustrating several important aspects of nerve physiology

but even before that...
some conventions and terms:
charge: 'types' classified by electroscope experiments -- attraction, repulsion, neutralization;
only two found -- named 'positive' and 'negative' by Franklin
amount of charge -- symbol: 'q'; units: coulombs
current: present when there is net charge moving;
intensity (amount of charge passing a point per second) -- symbol: 'i'; units: amperes
(1 ampere = 1 coulomb/sec)
convention on direction: that in which positive charges would be moving
if they were the only carriers of the current
electrode/ion nomenclature (introduced by Faraday, words supplied by Whewall),
for currents introduced into solutions:
electrodes ('electro-doors'):
anode (by which current enters solution; '+' electrode)
cathode (by which current leaves solution; '-' electrode)
ions ('goers'):
cations (go toward cathode; '+' ions)
anions (go toward anode; '-' ions)

Lecture 8 9/11/09

elaboration on the 3 key facts known ca. 1900:

1. electricity as stimulus --
phenomena associated with electrical stimulation of nerve
(with extracellular stimulating electrodes):
impulse usually initiated at stimulating cathode
threshold, strength-duration curve
(threshold current strength, i_T, vs. pulse duration, t);
empirical strength-duration equations, rheobasic current;
for small t, i_T x t about constant (implying what?)
effects of subthreshold currents -- summation;
latent addition (persistence of effects after subthreshold current turned off)
Pflüger's rules, 'electrotonus'
anode block
accommodation
(stimulation by AC currents: effectiveness depends on frequency)
anode-break excitation
refractory period -- absolute, relative

2. injury current
current in galvanometer implies existence of potential difference ('injury potential') in preparation;
potential difference in preparation implies current in extracellular medium (a 'conductor')
definition of terms just used and some basic background --

potential difference:
definition: work vs. electrical forces per unit positive charge moved between points;
measured in volts (joules/coulomb)
electrical force is 'conservative' -- net work vs. (unchanging) electrical force around a closed path = 0
work done going one way 'gotten back' returning -- idea of electrical potential energy;
potential difference between two points same/any path (Kirchoff's 1st Rule)

electrical conductors:
have charges free to move; electrical forces (producing potential differences) cause movement;
fixed electrical force establishes charge movement at an average 'drift' velocity against
opposing 'friction-like force' (of 'bumping into' other things in the conductor),
thus produces current (and heat), maintained only while electrical force applied;
so potential difference in a conductor implies a current, and vice versa;
size of current, i, (amount of charge passing a point per unit time) depends on:
strength of electrical force (thus on potential difference)
and on two properties of conductor --
amount (concentration) of charge free to move
and how easy it is for charge to move --
both determining its 'conductance'
Ohm's Law --
relation between potential difference, ΔV, across a simple conductor and current, i, through it,
expressed either of two ways:
i = g⋅ΔV, 'g' = conductance (property of conductor)
ΔV = i⋅R, 'R' = resistance = 1/conductance;
('resistor': pessimist's name for a conductor)
units of resistance: ohm (volt/ampere), of conductance: siemens (ampere/volt)

measuring potential differences:
principle -- let small bit of charge 'fall' through difference, measure work it can do;
ideals -- use very small amount of charge, accurately reflect changes with time
(galvanometers poor at both -- require steady current and respond slowly)
amplifiers: small amount of charge 'falling' onto control element (work)
controls large current from separate supply, able to 'drive' an insensitve display
voltmeters: measure potential differences;
electrometers: very sensitive voltmeters, requiring very small amounts of charge

Lecture 9 9/14/09

elaboration on the 3 key facts known ca. 1900 (cont'd):

2. injury current (cont'd)
as noted in Lect. 8
injury current in galvanometer implies potential difference in extracellualr medium ('injury potential')
injury potential in extracellular medium (a conductor) implies current in it --
experimental study of that current (in extracellular medium) & what it can tell about nerve:
mapping extracellular injury current by mapping potential differences in the medium,
with assumption about resistances -- simple case: uniform, thin film of medium;
assumed equal resistance along equal-length stretches along medium;
steps of potential change over equal-length stretches; steps greatest near injured site;
thus extracellular injury current strongest near injury, decays rapidly further way;
Kirchoff's 2nd Law: current going into a point in a conductor must equal current leaving,
therefore, current must be coming from nerve (uninjured parts) into medium;
same general result with injured single nerve fiber --
therefore, current must leak through 'membrane' of nerve

3. negative variation:
implies site of impulse is 'like' (less differerent from) injured site,
therefore should be negative with respect to uninjured site;
it is -- this is the extracellular action potential, a travelling extracellular negativity
with respect to a 'reference' (also extracellular) at an uninjured, inactive site
brief history of attempts to record action potentials:
improvment of detectors & displays: increased sensitivity, reduced mass (quicker response)
1870's Lippmann: capillary electrometer (Nobel prize 1908, but for a first color-photographic process)
1888 Gotch & Horsley: first measure of single APs, from whole nerve with cap. electrometer
1882 d'Arsonval galvanometer: light-weight coil in field of permanent magnet
1901 Einthoven: string galvanometer (first accurate EKGs) (Nobel prize 1924)
1880s-1912 Edison, Fleming, deForest: currents in vacuum tubes, vacuum-tube amplifiers
1920's Adrian: single APs from single fibers
with vacuum-tube amps, capillary electrometers or light d'Arsonval galvanometers;
frequency encoding of sensory intensity (e.g., in muscle stretch receptive fibers);
(Nobel prize 1932, shared with Sherrington)
1897 Braun: cathode-ray tube -- display element (electron beam) with verylow mass;
(Nobel prize 1909, shared with Marconi)
1922 Erlanger & Gasser: vacuum-tube amplifier and CRO display; accurate time course of APs;
'compound APs' from whole nerve -- different fiber- (axon-) groups in nerve
have different AP propagation velocities and thresholds;
(Nobel prize 1944)

Bernstein ca. 1900: the 'membrane hypothesis' (or 'local current hypothesis' or 'local circuit hypothesis')
(based on the '3 key facts'):
injury current suggests (hypothesis) pre-existing 'resting potential' --
created and maintained by a special 'membrane' (hypothesis)
negative variation suggests (hypothesis) site of impulse involves
temporary (transient) injury-like event -- 'membrane breakdown'
site of 'membrane breakdown' should give local 'injury-like' currents,
with pattern as found in 'experiment' discussed above --
same as pattern of currrents expected near an extracellular stimulating cathode
(known to excite nerve, initiate impulses),
suggesting (hypothesis) that local currents near 'membrane breakdown' could/do excite
next bit of nerve, propagating impulse (like spark in fuse)
Bernstein model: nerve impulse a succession of 'membrane breakdown' events,
in successive stretches of nerve, each producing 'local currents'
causing 'membrane breakdown' in next stretch --
thus propagating as the spark does along a fuse

questions posed by Bernstein's hypothesis (guiding later research, to be outlined in following lectures):
1. Are there local currents near (ahead) of impulse?
Are they strong enough to excite next bit of nerve?
2. Is there a pre-existing resting potential (RP)?
If so, how is it produced? (Is there a membrane?)
3. Does active site (mechanism of AP) involve membrane breakdown?
If so, what about local currents produces breakdown? And how is breakdown repaired?
and a 'puzzle' from what's been discussed so far:
Bernstein's proposed 'membrane' keeps inside and outside charges separate
in resting nerve, but (from experiment discussed today) similar resting,
uninjured membrane must 'leak' charge if there is an injury nearby.
How can the same (resting) membrane keep charge separate under one
circumstance and leak charge in another?
another puzzle (not mentioned in class):
The charges inside and out in Bernstein's model account for the direction of
currents observed near an injured site, but would be quickly dissipated.
The injury current lasts a long time (and is enough to drive a galvanometer!).
What keeps it going?
-- answers to both should become clear in later lectures

Experiment answering the first set of key question raised by Bernstein's hypothesis --
Are there local currents near (ahead) of impulse?
If so, are they strong enough to excite next bit of nerve?
Hodgkin 1937:
examined currents & excitability of nerve ahead of 'blocked' impulse
(by measue of extracellular potential differences, as in injury-current experiment described above,
and measure of thresholds with a 'testing' cathode, as described in previous lecture);
results: there are local currents ahead of impulse, associated with increased excitablity;
change of these currents (and of excitability increase) with distance was determined,
then extrapolated back to region of block --
indicated currents near blocked impulse are more than strong enough to excite next bit of nerve: 'safety factor';
anode-block demonstration of 'safety factor': current required from an anode to block impulse
much greater than current (threshold) required from a cathode to initiate impulse,
so (cathode-like) currents ahead of impulse must be much greater than threshold level

Next lecture: before consideration of the two other sets of questions raised by Bernstein hypothesis --
those about RP and AP mechanisms, we'll discuss two aspects of the (essentially correct) 'local-current'
model for impulse propagation:
prediction of extracellular-AP shapes --
from current patterns around impulse site
passive spread of local currents ahead of impulse
(those responsible for 'exciting' next stretch) --
how far?, how much? what properties of nerve determine extent of spread?: 'cable theory'

Lecture 10 9/16/09

two aspects of the 'local-current' model and impulse propagation:
prediction of extracellular-AP shapes --
from current patterns around impulse site
passive spread of local currents ahead of impulse
(those responsible for 'exciting' next stretch) --
how far?, how much? what determines extent of spread?: 'cable theory'

predicting qualitative extracellular AP shapes:
principle: use extracellular current pattern around active site of Bernstein model
(and assumption about resistances) to predict potential differences --
inverse of current mapping from potential-difference measurements
as used for injury-current 'experiment' above
current pattern around active impulse site -- similar ahead of/behind;
pattern 'moves' as successive bits of nerve become excited, with recovery behind
why is propagation generally unidirectional (forward)?;
case for impulse initiated midway along nerve fiber (bi-directional propagation);
case for impulses initiated at opposite ends of fiber -- 'collision'
predict (roughly) potential difference between electrodes as sum of differences in steps going from one to other;
direction of potential change in a step given by direction of current (if any) in that step
some common recording situations and AP shapes expected for them --
1. thin film of extracellular medium;
extracellular currents constrained to run along (parallel to) nerve
a. electrodes widely separated (with respect to active current pattern)
& impulse blocked before reaching second electrode:
AP monophasic and asymmetric
(asymmetric because distribution of current pattern ahead somewhat different from distribution behind)
b. electrodes widely separated (as in a),
impulse allowed to pass second electrode: biphasic (or 'diphasic') AP, phases separated
c. electrodes closely spaced (allowing only a small part of active curent pattern between them,
impulse passes both: biphasic AP, phases not separated, smaller than in b (why?)
2. volume conductor (bath of extracellular medium);
extracellular currents 'spread out' in medium away from nerve,
one electrode very near nerve fiber, one distant in medium;
only step(s) near nerve have signifcicant current and add significant potential changes;
result ~same as for second electrode just beyond (radially) first --
potential differences associated with current out from or into nerve radially: triphasic AP

passive spread of local currents ('electrotonic' currents) in axons -- cable theory
axon as simple electrical cable, like underwater cable --
conductive core, insulating sheath (leaky), conductive outside environment;
current injected at one point, travels in both directions along inside;
some leaks out through membrane at each stretch,
successively reducing internal current with distance away from site of injection
how pattern, rate of decay depend on axon properties -- simple case, assumptions:
resistive only (capacitance ignored),
one-dimensional (currents only axially along inside & outside, with membrane leaks),
passive/'linear' (currents proportional to potential differences, no 'active currents,' resistances constant),
uniform (properties same everywhere along axon),
infinite (local currents 'see' effectively infinite cable beyond)
fraction of current leaking at each stretch (i_leaking)
relative to current continuing on down inside (i_continuing)
is determined by relative resistances
r_l - resistance of leakage path through membrane around that stretch and
r_∞ - effective resistance of path continuing into rest of infinite (half) cable,
leaking out & returning somewhere beyond;
from Kirchoff's 1st Rule and Ohm's Law: i_leaking/i_continuing = r_∞/r_l
for uniform, infinite cable, this ratio of 'continuing-path' and 'leakage-path' resistances
is same at each equal-length stretch (why?);
therefore fraction of internal current leaking is same at each equal-length stretch;
thus decay of internal current with distance is exponential:
the same (constant) fraction being lost at each equal-length stretch,
and one can write for i(x),
the amount of injected current left continuing along inside at distance, x, from injection site:
i(x) = i₀e^-kx, where i₀ is the current at x=0, and k is some constant

notes and comments on cable theory:

1. Why is change by a constant fraction per equal-length interval 'exponential' change?
(and why is 'e' used so often in equations describing it?) --
first, why such change is 'exponential':
successive, same fractions remaining are multiplied:
for fraction, f, of self lost at each successive interval,
fraction of self remaining after each interval is (1-f);
thus, if start with amount i₀, what remains after n such intervals is i₀(1 - f)ⁿ;
i.e., the number, n, of equal-length intervals appears as exponent,
so change is 'exponential' with respect to n
for continuous curve passing through same points, can write: i(x) = i₀(1-f)^x/L,
where i₀ is the initial amount at x = 0,
and L is the interval length over which fraction f is lost
(so x/L = 1, 2, 3,...n at x = 1L, 2L, 3L,...nL),
and the same fraction is lost over length L starting anywhere on the curve;
this can be written as a more 'typical' exponential equation:
i(x) = i₀b^k_bx,
where the base, b = (1-f), and the constant k_b = 1/L,
and this same equation, describing the same curve, can easily be converted to use another base
(involving some different fraction lost),
with an appropriate change of the constant k_b
(involving the length over which that different fraction is lost);
thus an equation for the change can be written if actual fraction, f, lost is known for some particular length interval, L;
and the equation, describing the same curve, could be rewritten to use any other base,
with an appropriate change of of the constant 1/L (the k_b for that base);

next, given that any base can be used --
why is 'e' is so often used as the base in such equations?
what's 'natural' about logs to the base e?
and why is it called 'e'?
answers in next lecture... with additional cable-theory comments

Biology 3240 Intro Cellular Neurobiol, Fall '09 Key Words & Concepts: Lectures 1-10