HOW MESSAGES ARE SENT

 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.

 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.

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

 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.

 Not sending an impulse  The neuron is impermeable to Na+  meaning Na+ remains outside  Potassium (K+) moves freely (permeable) in and out of cell.

 Outside is positive  All Na+ and some K+  Inside is negative  Some K+  Polarized  more positive outside cell then inside

 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

 Polarity reverses:  Inside positive  Outside negative  “Depolarized”

 K+ 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

 Recovery period  Few thousandths of a second  Neuron cannot be stimulated again  Neuron must be returned to “resting potential”

 Sodium potassium pumps returns membrane to rest  Na+ moves out  K+ moves in  Repolarized  Outside now +  Inside now -

 Myelinated neuron  Faster transmission (100 m/s)  2m/s (unmyelinated)  Jumps from one node of Ranvier to next  Uses less energy

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

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

 Great animation of the creation of a nerve impulse:  Action Potential Demo (Harvard University) Action Potential Demo