PSY 340 Brain and Behavior Class 06 Neural Impulse (part II)

January 23, 2024

PSY/BSC 340 Brain and Behavior

Class 06: Neural Impulse (Part II)

The Action Potential
(this page is a continuation of PowerPoint presentation)

The Na+ and K+ channels are called voltage-activated gates because they act only when the surrounding area reaches a certain voltage.

Remember that the resting potential (when nothing is happening) is -70 mV.
The point at which the Na+ channel opens (#1 on diagram) is called the threshold of excitation (about -40 mV). The K+ channel responds more slowly, but does begin to allow K+ ions to exit.

NOTE: Different researchers report different values for the threshold of excitation: I use the value of -40 mV, but others suggest a value of -50, -55, or even -65 mV. Don't be too concerned about these differences other than to remember that the threshold of excitation is a value somewhat less negative (i.e., somewhat more positive) than the resting potential of -70 mV.

When the sodium ions pour into the neuron (#1 to #2 on diagram), this phase of the action potential is called depolarization.
With the Na+ gate closed at +50 mV, K+ ions pour outside the neuron (between #2 and #3 on the diagram). Thus, the neuron winds up "hyperpolarized" at #3, i.e., at -90mV the neuron is much more polarized than at its resting state of -70mV.
During the time (#4) the Na-K Pump is restoring the area to its resting potential, the neuron has entered into a "refractory period", i.e., it cannot fire for about 1 mSec.
Hence, an action potential equals the rapid depolarization followed by brief hyperpolarization at a location along the axon membrane.

NOTE: all of the specific voltage values cited above may be different for specific neurons. So, the threshold of excitation may actually range between -55mV and -40mV; the Na+ gate and the end of depolarization may occur at +40mV or +45mV; and, the resting potential of a neuron might be -67mV, etc. The values of +50mV and -90mV represent the voltage equilibrium points for Na & K voltage-gated channels respectively. And -70mV is the standard general value that is used in neuroscience.

What are some real-world implications of this?

Nerve poisons (e.g., scorpion or sea anemone venom; "Red tide"; Puffer fish [Fugu rubripes]) often have a variety of effects on the Na+ and K+ channels. Some open Na+ channels and shut K+ channels & this leads to the disruption of any action potentials.

The puffer fish, considered in Japan to be a real delicacy (call "Fugu"), contains the poison, tetrodotoxin (CDC), which binds to the Na+ channel for 10-40 seconds. During this time, no action potential can move because the channel is completely blocked. Exposed to as small an amount as 1 milligram, a human being will die of respiratory paralysis. The fish has developed a mutated type of Na+ channel which is not affected by its own poison.

“To humans, tetrodotoxin is deadly, up to 1,200 times more poisonous than cyanide. There is enough toxin in one pufferfish to kill 30 adult humans, and there is no known antidote.” (https://www.nationalgeographic.com/animals/fish/group/pufferfish/)

Local anesthetic drugs (Lidocaine [brand name: Xylocaine®]; Prilocaine; Novocaine® is almost never used any more) used by dentists and doctors block the Na+ channels and prevent action potentials along sensory neurons.

General anesthetics used in hospitals (ether, chloroform) open some K+ channels in the brain a bit wider than usual. This counter-acts the effects of Na+ channels being opened and prevents action potentials from propagating, too.

All or None Law

Axons either "fire" or they don't "fire" to use the metaphor of a gun. If they do "fire," then the size and speed (amplitude and velocity) of the resulting action potential along an axon is relatively invariant. Stronger stimuli do not increase either the size or the speed. This is what the "All or None Law" states. (Exception: see below "local neurons")

How, then, does a neuron signal a stimulus of stronger intensity? By firing more frequently. Frequency (i.e., the number of action potentials per second) conveys the strength of the original stimulus while each action potential moves at the same speed and has the same "size".

Myelin: Two Sources

What is myelin? Here’s one description: "“Myelin is an insulating layer, or sheath that forms around nerves, including those in the brain and spinal cord. It is made up of protein and fatty substances. This myelin sheath allows electrical impulses to transmit quickly and efficiently along the nerve cells. If myelin is damaged, these impulses slow down.” [from https://medlineplus.gov/ency/article/002261.htm]”

Where does the myelin come from? Within the Central Nervous System (CNS, that is, the brain and the spinal cord), the myelin comes from glial cells, specifically oligodendrocytes. Outside the CNS, that is, in the Peripheral Nervous System (PNS), the myelin comes from Schwann cells which wrap themselves around the axon of neurons in the PNS. See the diagram on the right.

Propagating an Action Potential: Method and Speed
So far, we've looked at an action potential as it is generated at one place: the axon hillock. How does it propagate or "move" down the axon?

Look at the diagram to the left. An action potential propagates along an axon by a process in which, once triggered, the influx of positive ions causes the adjacent Na+ gate to open and, in turn, this causes the next Na+ gate to open, and so on. Hence, an action potential is actually self-propagating.

How fast does an action potential move along an axon?

the thinnest axons propagate an action potential at less than 1 meter per second (1 m/s).


Thick axons propagate action potentials at about 10 m/s.

However, myelin sheaths permit speeds up to 100 m/s. How?

Triple Spiral YouTube Video link

Saltatory conduction: When the Na+ ions enter the inside of the axon, they quickly spread. The insulation of the myelin allows the ions to move quickly to the next Na+ gate at a node of Ranvier. The movement of ions from one node to the next is known as local current flow. And, so, the action potential jumps from one node to the next.

In the peripheral nervous system (e.g., your leg or arm), this allows the signal to more fare more speedily than if there was no myelin insulation, i.e., 100 meters per second.

By the way, the Latin word, saltus, which is the origin of "saltatory," means "a jump".

Local Neurons

Local neurons contain very short dendrites and short (or absent) axons. They occur most often in the olfactory bulb (smell), retina, and frontal orbital cortices (pl. of cortex).

They exchange information with only with neighboring neurons.

They do not generate action potentials, but rather depolarizations (becoming less negatively charged) or hyperpolarizations (more negatively charged). These are called graded potentials and do not follow the All-or-None Law. These charges are conveyed by contact with neighboring neurons.

As we mentioned earlier, astrocytes (a type of glial cell) also act a bit like local neurons by exchanging chemicals back and forth with nearby neurons. They therefore have some control on whether neurons will fire or not.


MISCONCEPTION: The 10% Brain Use Myth

"We only use 10% of our brains". How often have you heard this?

The origin of the myth is unclear: Some have attributed it to Albert Einstein, William James (late 19th century psychologist & philosopher), or Karl Lashley (neuropsychologist of the 1930s & 1940s)

The myth takes different forms:

Most of the brain is inactive most of the time (only 10% working & 90% inactive). This is simply not true.

Only 10% of the brain is used for consciousness; the rest is part of the unconscious. Actually, we may only use 1% of our brain for consciousness while 99% of our brain carries out important processes outside of our awareness.

Only 10% of the brain's areas have been mapped and found to have a role/function. While there is still a huge amount to learn about the brain, year by year we are finding more and more functions & roles for different brain areas. We do NOT find areas of the brain without some type of function.

The myth is a myth: It is UNTRUE!!! There is no evidence whatsoever to support the myth in any of its forms.

Functional neuroimaging which can detect activity in the brain shows that the entire brain is always doing something.

It doesn't make evolutionary sense. The brain has high needs for energy (food & oxygen). Organisms in nature only develop functions that are advantageous to it. Having an energy hungry brain that isn't functioning for important purposes makes no sense.

This page was first posted January 27, 2005