The Neuron & Action PotentialModule 9: Biological Psychology & Neurotransmission

The basic building block of our nervous system and how it sends

messages.

Cell Body & Nucleus

The Cell Body

– round, centrally located structure

– contains DNA

– controls protein manufacturing

– directs metabolism

– no role in neural signaling

Contains the cell’s nucleus

Dendrites

Dendrites

• RECEIVE info/signals from other neurons

• Inputs/receptor sites may number in thousands

• If enough inputs received it may cause the cell’s axon to generate an electrical impulse (action potential)

Dendritic Growth• As you learn, new dendrites can grow, creating more

connections to other neurons• New connections are basis for learning• People with higher education have more dendritic

connections than a high school dropout.

Axon

Axon

Axon• SENDS an electrical message

when signaled by the dendrites

• Where all the action is• Action Potential takes place

– electrical charge is sent down the axon.

• One axon per cell, 2 distinct parts– tube-like structure – branches at end (axon

terminals) that branch out to dendrites of other cells

Myelin Sheath & Nodes of Ranvier

Myelin Sheath• White fatty casing on axon • Acts as an electrical insulator • Not present on all cells• When present, increases the speed of neural

signals down the axon allowing the action potential to “jump” to each Node of Ranvier - like a paved highway (see video below to compare mylenated axons vs. non-mylenated axons

• If this degenerates (dirt road), you have multiple sclerosis and can’t control your muscles.

Axon Terminal or Buttons

Axon Terminals

Axon Terminal or Buttons

• End of Axon where the electrical impulse triggers synaptic transmission sending message to the dendrites of a receiving neuron.

• Let’s Review with this Quick Video.

One Direction

Communication

Glial Cells

•They are the janitors of the neuron.

•Support cells that provide neurons with structural support and nutrition.

•They also remove cell wastes and enhance the speed of the neuron

Action PotentialHow neurons send an electrical message

How Neurons Communicate• Neurons communicate by means of an electrical signal

called the Action Potential• Action Potentials are based on movements of ions

between the outside and inside of the axon• When an Action Potential occurs, a molecular message is

sent to neighboring neurons• Action Potential is an All or Nothing Process

(like a gun firing)

Threshold: Triggering Action Potential

•When a neuron is resting = balance of excitatory & inhibitory signals.

•If one of these exceeds the other stimulus threshold is reached

•Triggering the neuron to transmit an electrical impulse down its axon (action potential)

•How do you feel something that is intense?•More neurons fire, the intensity of their electric impulse always stays the same.

•Lou Gehrig’s Disease - too many inhibitory stimuli cause the muscles to freeze up.•Parkinson’s Disease - too many excitatory stimuli cause the muscles to move without control.

Steps to Action Potential

Resting Potential

• At rest, the inside of the cell is at -70 microvolts• With inputs to dendrites inside becomes more positive • If resting potential rises above threshold, an action potential

starts to travel from cell body down the axon• Figure shows resting axon being approached by an AP

Axon at Resting Potential - fluid inside the axon is mostly negatively charged with positive on the outside (polarized)

Step 1: Threshold is Reached

• An impulse is triggered in the neuron’s dendrite when stimulated by:– Pressure– Heat– Light – NT - chemical messenger from another neuron

(synaptic transmission)

• Minimal level of stimulation that causes the axon to fire is called Stimulus Threshold

Step 2: Action Potential Begins

• When neuron fires, its axon membrane is selectively permeable.

• Gates in the axon called ion channels open allowing positive sodium ions to enter the axon while potassium ions leave causing a brief positive electrical charge in the axon (depolarized).

• The brief positive charge is action potential.

Depolarization Ahead of AP

• AP opens cell membrane to allow sodium (Na+) in

• Inside of cell rapidly becomes more positive than outside

• This depolarization travels down the axon

Step 3: Refractory Period• As the next gates open allowing positive sodium ions in,

the previous gates close and begin to pump the positively charged sodium ions out of the axon and potassium ions back inside. (repolarized).

• This step is called the refractory period and the axon cannot fire again until it returns to resting potential (negative polarized state).

• The entire process is like falling dominoes all the way down the axon except these dominoes can set themselves back up as soon as they fall over.

• Why do you think the axon has to set itself back to a resting state so quickly (3 milliseconds)?

• So the neuron can fire again and send another message immediately after the last one.

Repolarization follows

• After depolarization potassium (K+) moves out restoring the inside to a negative voltage

• This is called repolarization• The rapid depolarization and repolarization produce a

pattern called a spike discharge

Finally, Hyperpolarization

• Repolarization leads to a voltage below the resting potential, called hyperpolarization

• Now neuron cannot produce a new action potential• It must return a resting state• This is the refractory period

Action Potential Within a Neuron

1. Threshold is reached2. +Na ions enter

beginning of axon3. this triggers the next Na

gates to open. 4. As they open & allow

in Na+, 5. previous gates begin

pumping the Na+ out.6. Before the action

potential has reached the end, the beginning of the axon is back at resting potential & ready for another firing.

A Review Action Potential

DAILY

DOUBLE

How can a toilet represent Action Potential?

• Full Toilet – Resting Potential• Push Flush Lever – Threshold Stimulus

triggering Action Potential.• Toilet Refilling/Can’t Flush –

Repolarization/Refractory Period• Sewer Pipes – One-way communication

like action potential only goes from dendrite end to axon terminal end.