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-)