Lunatics and Princesses

NEUROPHYSIOLOGY...
...Electrical Properties of Nerve cells (neurons)

electrophysiology of neurons lies in their... Membrane Physiology
the model organism is : Squid Giant Axon [Loligo pealei]
there is a a great diversity* of Nervous Systems

FUNCTIONS of NERVOUS SYSTEM -
[a la SIGNAL TRANSDUCTION : reception - transduction - response ]
1. gathers sensory input (sense organs via Peripheral NS) --> info in
2. integrates information (CNS - brain & spinal cord)
3. responds with motor output (effector organs via PNS - muscles)

STRUCTURAL PARTS of Nervous System -
central nervous system - brain and spinal cord...
(neural stem cell = origin of brain)
peripheral nervous system - outside the CNS- carries signals in/out of CNS
PNS = sensory (affernet) and motor (efferent) neurons:
- somatic nervous system - carries signal to skeletal muscle - under conscious control
- autonomic nervous system - signals regulate internal systems - involuntary control
2 antagonistic Parasympathetic (cranial & cholenergic)* - calming: HR-, energy storage

systems: Sympathetic (spinal & noradrenaline)* - energy & arousal: HR+, glycogen--> glu


FUNCTIONAL TERMINOLOGY of Neurons -
Nerve - bundle of neurons wrapped in connective tissue
Ganglia - cluster of cell bodies of individual neurons
Sensory neurons... (afferent neurons) - external stimuli from receptors toward CNS
Interneurons... integrate & relay sensory input to motor neuron
Motor Neurons... (efferent neurons) - convert signals to effector cells = response
Pathway through Nervous system*

STRUCTURE of a NERVE CELL
Cell Body- is main part of neuron cell with cytoplasm & organelles
Dendrites*- short cylindrical outgrowths of cell Body
- carry signals (electrical impulses) into cell body
Axon**- long outgrowth of cell body - carry signals to next neuron
Schwann cell* - cells surrounding axon in vertebrates -
- produce myelin (sheath) membrane (components)
protein + lipid-like membrane insulation surrounding axon
Nodes of Ranvier* - space between successive Schwann cells
inter-node area is a non-myelinated area
speed of conduction - w/o myelin speed is less (5 m/sec)
w/myelin (100 m/sec or 200 mi/hr)

Multiple Sclerosis - degeneration of myelin sheaths
Synaptic Knob* - enlarged ends of neuron
- holds neurotransmitters in synaptic vesicles

Reflex Arc - hard wired, unconscious rapid response to external stimulus
involving spinal nerves & effector cell electrical impulses


The electrical properties of cells: resting & action potentials

RESTING POTENTIAL - the characteristic electric charge
exhibited by a cell at rest... most often negative (-)

potential - (in electrical terms) is amount of electrical charge
at one point in an electric circuit compared to some other
point in the same circuit measured with a volt-meter (multi-meter*)

How to measure membrane potentials* in cells - microelectrode* pic*
inside vs. outside of cells
SGA - 65 to - 70 mVi
Frog muscle fibers - 90 mVi
Nitella (algae) - 150 mVi
Valonia (algae) + 15 mVi


Causes of Resting Potential... -65 mV inside for SGA ...
ion distributions* - all of which make inside of cell negative (-) HOW ?
ENa = +62mVi 150/15 = 10
Nernst Emv = +/- 62 log10 [Co]/[Ci] EK = -92mVi 5/150 = 0.033
ECl = -65mVi 120/10 = 12

1. active transport of Na & K...* high Na outside [3] & high K inside [2]
2. differential permeability* of K (faster out) & Na (slower in) = inside (-)
3. lots of protein anions* (-) at cell pH, thus inside more (-) then outside
4. normal diffusion of* Cl- into the cell



ACTION POTENTIAL -
- a self-propagating change in the RP voltage across membrane of nerve cells...
- name given to changes in electrical charges that occur during the
stimulation of a nerve cell,
- usually visualized graphically from an oscilloscope* recording [graph]

PROPERTIES of an AP...
... requires a living cell, i.e., requires O2 for metabolism
eliminated by metabolic poisons, such as cyanide
... is measured using microelectrodes impaled into cells
... has a threshold - minimal amount of stimulus needed to "fire" an AP
... is an "all-or-none" phenomena, either yes or no, no in-between
... is very rapid - time course = 2-3 msec

EVENTS DURING an AP oscilloscope trace* graph [Gasser & Erlanger]
depolarization* - cell goes from inside negative (-) to inside positive (+)
Na channel opens - Na floods in* --> -70mV to +58mV
repolarization* - Na channels close & K channels open [returns to inside (-)]
K follows diffusive gradient & K floods out* of cell
hyperpolarization* - "undershoot" of resting potential (-75mv)
refractory period - time before another AP can 'fire'
current changes during an AP* and overall* mechanism*


CONDUCTION of an AP along an axon
local spreading of electric charge depolarizes* adjacent membranes
change in membrane permeability of adjacent non-myleinated region
leads to an autocatalytic - "domino effect"............ figure*

Synaptic Transmission...
synapse - functional connection between neurons allowing transmission of
AP's between cells: can be electrical or chemical
synaptic cleft - open space between neurons [ 10 to 20 nm across ]
across which a chemical neurotransmitter may diffuse
synaptic knob - site of vesicles holding neurotransmitter at end of axon
synaptic vesicle - holds neurotransmitters (ex: acetylcholine)
action potentials trigger release of neurotransmitters*
pre-synaptic side - releases neurotransmitter
post-synaptic side - has a receptor which binds transmitter and ...
... ion channels open - leads to change of potential charge
on the post-synaptic membrane ----> new AP
removal of stimulus - an enzyme destroys neurotransmitter: ex. "ACH-ase"



Post-synaptic responses:

EPSP - excitatory post-synaptic potential (-15mVi)
excitatory PSM neurons --> open Na channels --> inside + --may--> AP

IPSP - inhibitory post-synaptic potential (-75mVi) figure*
inhibitory PSM neurons --> opens Cl channels - in -> more - --> no AP
--> opens K channels - out -> more - --> no AP

AP - all or none 120mv depolaorization/repolarization (-65 to +62 mVi)

MEPP - minature end-plate potential (-2 to 5 mVi)
small membrane depolarizations due to vesicle fusions with pre-SM
integration of impulses* & multiple innervations*
summation of impulses* & temporal/spatial summations*