1. ELECTROCHEMICAL IMPULSE
SBI 4U: Metablic Processes
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.
Section 1.3

2. SBI 4U: Metablic Processes
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.
Section 1.3

3. SBI 4U: Metablic Processes
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.
Section 1.3

4. SBI 4U: Metablic Processes
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
Section 1.3

5. SYNAPTIC TRANSMISSION
SBI 4U: Metablic Processes
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.
Section 1.3

6. SYNAPTIC TRANSMISSION
SBI 4U: Metablic Processes
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 #
Section 1.3