Membrane Potentials and Impulses

1. How signals are sent through the nervous system Membrane Potentials and Impulses

2. Synapse • Synapse = Junction between two connecting neurons • Synaptic cleft-between the neurons, signal has to go across this space • Signals need to be sent from neuron to neuron, not just from neuron to muscle

4. Synapse • Presynaptic Neuron: sender of the signal; axon end (Before synapse) • Postsynaptic Neuron: receiver of the signal (After synapse); dendrite end

5. Synaptic Transmission • Transmission occurs when the message crosses the synapse • Neurotransmitters are biochemicals that complete this function • Which neurotransmitter did we learn about during the muscular unit?

7. Neurotransmitters • Distal end of axons have synaptic knobs with synaptic vesicles (store neurotransmitter)

9. Neurotransmitters • Can be: • Excitatory: increase signal transmittance • More of this type = sending of signal • Inhibitory: decrease signal transmittance • More of this type = no signal

10. Chemistry Review • Ions play an important role in the nervous system signals • What is an ion? • An atom that has lost or gained 1/more electrons • Ions are positive if electrons are lost and negative if electrons are gained • Examples: Na+ K+ Mg+2 Cl- O-2

11. Neurons at rest

12. Neurons at rest • Resting potential: inside is more negative than outside of the cell • AKA POLARIZED (think polar opposites)

13. Why?

14. Resting Neuron • To keep the cell in resting potential, a Sodium-Potassium pump restores ions to where they belong

15. Action Potential • Conditions must change in order for a signal to be sent by the neuron • This electrochemical signal = ACTION POTENTIAL • Which part of the neuron is the sender?

17. Depolarization • Environmental Stimuli (odor, touch, sound,etc.) • Receptor cell releases neurotransmitter • ONLY Na+ channels open, Na+ ions go into cell = DEPOLARIZATION

19. Depolarization • Inside of cell becomes more POSITIVE • This triggers an ACTION POTENTIAL • Will continue down rest of membrane

21. Repolarization • Quickly after the previous step, K+ is able to move across membrane through its channels; sodium can no longer move

22. Repolarization • Inside is negative again (repolarized)

24. Refractory period • Sodium – Potassium pump uses active transport to move Na+ & K+ back to where they started • During this time, the neuron cannot transmit an impulse • Known as REFRACTORY PERIOD • Membrane returns to true resting potential

26. All or None Response • Just like muscles if a nerve responds, it responds completely • Greater intensity of stimulation triggers more impulses per second • Not a greater intensity of impulse

27. Nerve Impulse • This “wave” of action potentials from one neuron to the next is known as a NERVE IMPULSE • Moves from dendrites down through axon

28. Nerve Impulses • Unmyelinated neurons conduct impulses over their entire membrane surface- SLOW • Myelinated neurons conduct impulses from node of Ranvier to node of Ranvier - FAST

29. Synaptic Transmission 1. When an impulse reaches the end of an axon, synaptic vesicles release neurotransmitters 2. The neurotransmitters react with receptors on the postsynaptic membrane to open ion channels. 3. Ions flow into the postsynaptic cell, eliciting a response.

30. Neurotransmitters • Excitatory NTs: cause depolarization • Inhibitory NTs: lessen depolarization

31. Last Step • Neurotransmitters are broken down by enzymes, or • Reabsorbed by presynaptic cell • Called re-uptake