Potentials and Synapses

Ch. 12-4

Graded Potentials

• When a stimulus causes a channel to open or close in an excitable cell

• Causes more polarization (more - inside)• Causes less polarization (less – inside)• Hyperpolarized (much more – inside)• Depolarized (much less – inside)• Graded signals – vary in size– Spread out along plasma membrane and die out– Only used for short distance communication

•

Generation of Action Potentials

• Action potential (AP) or impulse – sequence of rapidly occurring events that take place in two phases– Depolarizing phase – negative membrane potential

becomes• Less negative• Reaches 0• positive

– Repolarizing phase – membrane potential is• Restored to the resting state of -70 mV

•

Action Potential

Depol. vs. Repol.

• Na+ channels open• Na+ rushes into cell

• All or none principle– when depolarization reaches the threshold, the AP

occurs– AP is always the same size– Like dominos – no matter how hard you push the first

domino they will all fall or they won’t – same goes for the AP

• K+ channels open• K+ flows out of cell

Depolarizing Phase

• Na+ channels open rapidly – Na+ rushes into the cell

• Potential goes from -55 to +30 (inside is 30 more positive than the outside)

• The more Na+ there is, the more channels open – this is a ____________-feedback mechanism

Repolarizing Phase

• K+ channels open as the Na+ channels are closing

• Na+ inflow slows, K+ outflow increases• Potential goes from +30 to -70

Depol. and Repol.

Painkillers

• Local anesthetics – block pain and other body sensations

• Novacaine or Lidocaine• Block the opening of Na+ channels• The nerve impulse can’t go past the blocked

region – CNS never gets the message that there is pain

Synaptic vocabulary

• Presynaptic neuron – the one sending the signal

• Postsynaptic neuron – the one receiving the signal

• 3 types– Axodendritic – axon to dendrite– Axosomatic – axon to body cell– Axoaxonic – axon to axon

Electrical Synapse

• Action potentials conduct between cells through gap junctions– Tunnels connect the two cells together and the electrical

impulse can pass• places:– Smooth muscle, cardiac muscle, developing embryo

• advantages:– Faster – don’t have to “jump the gap”– Synchronization – allows groups of cell’s to work

together – heart beating

Electrical Synapse

Chemical Synapse

• Pre- and post-synapses do not touch – separated by synaptic cleft

• Transfer of signal must occur– Pre-synaptic neuron turns electrical signal into

chemical signal (neurotransmitter)– Post-synaptic neuron turns chemical signal into

electrical signal• this transfer takes more time than electrical

signals

Chemical Synapse

1. Nerve impulse arrives2. Depolarizing phase opens

Ca++ channels, Ca++ flows in

3. Ca++ causes release of NT from vesicles

4. NT binds to receptors on postsynaptic neuron

5. Action potential continues (Na+, K+, Cl-)