Nerve Impluses

HOW MESSAGES ARE SENT

What is a nerve impluse?

It is a message travelling down a neuron

The message comes from: Another neuron or A sensory receptor

A nerve impulse is also called an ACTION POTENTIAL.

Interesting Ions

Neurons have a rich supply of positive and negative ions both inside and outside the cell.

Sodium (Na+) ions and Potassium (K+) ions play a key role the creation of a nerve impulse.

Sodium (Na+) and Potassium (K+)

Since both are positive…. It’s all relative.▪ Positive = more positive ions▪ Negative = fewer positive ions

Sodium (Na+) and Potassium (K+)

The highly concentrated potassium ions inside the nerve cells have a tendency to diffuse outside the nerve cells.

Similarly, the highly concentrated sodium ions outside the nerve cell have a tendency to diffuse into the nerve cell.

Steps of a Nerve Impulse

Step 1: A Neuron at Rest

Not sending an impulse

The neuron is impermeable to Na+ meaning Na+ remains outside

Potassium (K+) moves freely (permeable) in and out of cell.

Step 1: A Neuron at Rest

Outside is positive All Na+ and some

K+

Inside is negative Some K+

Polarized more positive

outside cell then inside

Step 2:Action Potential

Only when polarization can occur will Na+ be able to enter. Na+ channels (gates) open, allowed Na+

to diffuse into cell… for a fraction of a second!

Charges begin to switch sides… This happens very quickly

Step 2: Action Potential

Polarity reverses: Inside positive Outside negative

“Depolarized”

Step 2: Action PotentialK+ moving out of the cell (closes

Na+ gates), trapping Na+ inside cell. Depolarization (reversal of charges)

occurs in a small area▪ Triggers a “wave” of electricity▪ Travels length of axon

Step 3: Refractory Period

Recovery period Few thousandths of a second Neuron cannot be stimulated again

Neuron must be returned to “resting potential”

Step3: Refractory Period

Sodium potassium pumps returns membrane to rest Na+ moves out K+ moves in

Repolarized Outside now + Inside now -

Saltatory Conduction

Myelinated neuronFaster transmission (100 m/s)

2m/s (unmyelinated) Jumps from one node of Ranvier to

nextUses less energy

Threshold

Minimum strength stimulus required for action potential to occur Different for each neuron

All-or-none Response

Impulses are all alike once threshold reached Strength only changes with number sent

Graph of Action Potential

WOW Website!!!

Great animation of the creation of a nerve impulse: Action Potential Demo (Harvard

University)