Transcript ELECTROCHEMICAL IMPULSE

ELECTROCHEMICAL IMPULSE
Luigi Galvani, 18th century: muscle of dead frog would twitch if electricity
passed through it
These experiments lead to lots of research in the field of electrical
conductivity of muscle tissue and the body
1840: Emil Dubois-Reymond, German physiologist, made instruments that
could measure current in nerves and muscles.
1906, Willem Einthoven, Dutch physiologist, made first electrocardiogram
(ECG) that measured electrical impulses in the heart
1929: Hans Berger, German physiologist, measured electrical changes
associated with brain activity, the electroencephalaograph (EEG) was born.
Julius Bernstein suggested nerve impulses were an electrochemical
message created by the movement of ions through the nerve cell
membrane.
1939: Cole and Curtis, evidence to back up Bernstein's theory. Found rapid
change in the potential (voltage) across a squid neuron when it was excited.
RESTING POTENTIAL
Found that the resting potential of the nerve was -70 mV.
More negative charges on the inside of the nerve cell than outside.
When the nerve became excited, the potential went up to 40 mV and this
was termed the action potential.
The action potential did not last long and the nerve cell went back to its
resting potential.
It has been found that it is the movement of positive ions that causes the
potential to change in a nerve cell, not the negative ions.
The highly concentrated potassium ions want to diffuse out of the nerve cell,
while the highly concentrated sodium ions want to diffuse in...why does the
potential change if they both have the same charge?
The resting membrane is more permeable to potassium diffusion than
sodium diffusion.
This means more potassium is moving out than sodium moving in and
consequently the outside of the nerve cell is more positive than the inside.
NERVE IMPULSE
This leads to why the resting potential is -70 mV. There are fewer positive
ions inside the nerve cell than outside.
The resting membrane is said to be charged or polarized.
When the nerve cell becomes excited, it becomes more permeable to
sodium than potassium. Scientists believe that sodium and potassium gates
open and close opposite of one another. As one type of gate opens, the
other closes.
Sodium rushes into the cell which causes a reversal of charge called a
depolarization.
Once the voltage becomes positive, the sodium gates close. That is why the
max action potential under normal situations is only 40 mV.
Sodium-potassium pumps actively restore the original resting potential by
moving sodium out and potassium back in. This is called repolarization.
NERVE IMPULSE
Nerve cells cannot transport a second message until the resting potential is
reset. This is called the refractory period, the time it takes the nerve cell to
be repolarized.
Depolarization moves along the axon of the nerve cell in a wave.
The critical amount of electricity that is required from a nerve cell to fire is
known as the threshold level. Stimuli below this level do not initiate a
response.
Any amount of stimulus above the threshold level gets the same response
from the nerve cell.
Nerve firing is an all-or-none response. It fires maximally or not at all.
Homework: Handout Questions #1-15
SYNAPTIC TRANSMISSION
The spaces between neurons and adjacent neurons or effectors are known
as synapses.
Synapses usually involve many neurons.
The nerve impulse moves along the presynaptic neuron and causes
chemicals called neurotransmitters to be released into the synapse. They
diffuse across the synaptic cleft and attach to membrane receptors on the
postsynaptic neuron. This causes the depolarization to continue on.
The diffusion of neurotransmitters is a slow process, so a neural response
that involves many synapses takes a relatively longer time than a simple
reflex arc.
Acetylcholine is an example of a neurotransmitter.
It is an excitory neurotransmitter as it causes depolarization to continue
in the postsynaptic neuron by opening sodium gates.
SYNAPTIC TRANSMISSION
In order to return the postsynaptic neuron to resting potential, the sodium
gates must be closed. This is indirectly done by cholinesterase, an enzyme
that breaks down acetylcholine and thus shuts the sodium gates.
Many neurotransmitters can have an inhibitory action on a neuron by
making postsynaptic neurons more permeable to potassium. This causes
even more potassium to leave the cell and thus causes even more
potassium to leave the cell and thus causes the potential to be even more
negative or hyperpolarized.
Summation is when two or more neurons are needed to create an action
potential in a further neuron. The sum of their firing causes an action
potential in the postsynaptic neuron.
Homework Questions #16 - 24