Nervous Impulses action potential How does this work

Nervous Impulses “action potential”

How does this work? • Nerves need to be able to: – Send a message from the nerve cell ALONG the nerve AXON – Send the message from one nerve cell to ANOTER nerve cell • Mechanisms: – Impulse – Synapse

IMPULSE • RECEPTION: – A nerve receives a STIMULUS • • • PROCESSING: – • Sensory nerve? Stimulus from an external or internal trigger Association or Motor nerve? Stimulus from an adjoining nerve cell Either INDUCTION or SUPRESSION of the impulse TRANSMISSION: – Impulse moves along the nerve AXON

RESTING AXON • Axons are LEAKY to ions • Na+ (sodium) ions diffuse INTO the axon from the extra-cellular space • K+ (potassium) ions diffuse OUT of the axon from the cytoplasm • The membrane contains a PUMP that pumps leaky ions back…. . BUT unequal pumping!

• Na+/K+ PUMP – 3 Na+ pumped OUT – 2 K+ pumped IN • Net result? MORE positive ions OUTSIDE • Outside = relatively POSITIVE • Inside= relatively NEGATIVE (compared to outside) • RESULT: POLAR (charged)

Check it out! • http: //highered. mcgrawhill. com/sites/0072495855/student_view 0/ chapter 2/animation__how_the_sodium_po tassium_pump_works. html • The charge is -70 m. V inside the membrane • This “resting” axon is now ready for an impulse

Nervous Impulse • STIMULUS – Membrane is sensitive – Stimulation affects membrane proteins – Specialized protein channel for Na+ ions OPENS – Na+ ions FLOOD into the axon – This DEPOLARIZES the membrane (now positive inside!)

• Depolarization is localized – Happens near the site of stimulus • Depolarization acts as a new STIMULUS for the NEXT region along the axon • This in turn triggers a NEW section of axon to respond (Na+ channels open) etc. • This continues along the ENTIRE axon!

• The axon needs to recover RESTING potential so that it can receive another stimulus • Recovery: – Special K+ channels open flooding out – This helps to return the axon membrane to resting potential (-ve inside, +ve outside) – Meanwhile, the Na+/K+ pump keeps on working returning the axon to original ion balance – Called REFRACTORY PERIOD (very fast!)

Check it out! • http: //highered. mcgrawhill. com/sites/0072495855/student_view 0/ chapter 14/animation__the_nerve_impulse. html

• The strength of a stimulus to trigger a nerve impulse is called ACTION POTENTIAL • Impulses are ALL or NOTHING…this means a nerve either fires or not. • The “strength” of an impulse comes from the number of times a nerve fires • How does this happen so fast? ? ?

The role of Myelin • Nerve cells are surrounded by Schwann cells (wrapped around the axon) that contain myelin • This waxy coating acts as a INSULATOR • Allows for nerve impulses to “SKIP” along the axon • Depolarization takes place at the NODES between Schwann cells only. This speeds things up CONSIDERABLY!

Check it out! • http: //www. blackwellpublishing. com/matth ews/actionp. html • http: //www. emc. maricopa. edu/faculty/farab ee/biobk/Bio. Book. NERV. html#Peripheral% 20 Nervous%20 System