ELECTROCHEMICAL IMPULSE

Luigi Galvani, 18 th 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