Gated Ion Channels
A. Voltage-gated Na+ channels
5. generation of AP dependent only on Na+
repolarization is required before another AP can occur
K+ efflux
Gated Ion Channels
A. Voltage-gated Na+ channels
6. positive feedback in upslope
a. countered by reduced emf for Na+ as Vm approaches ENa
b. Na+ channels close very quickly
after opening (independent of Vm)
Gated Ion Channels
B. Voltage-gated K+ channels
1. slower response to voltage changes than Na+ channels
2. gK increases at peak of AP
Gated Ion Channels
B. Voltage-gated K+ channels
3. high gK during falling phase
decreases as Vm returns to normal
channels close as repolarization
progresses
Gated Ion Channels
B. Voltage-gated K+ channels
4. hastens repolarization for generation of more action potentials
Does [Ion] Change During AP?
A. Relatively few ions needed to alter Vm
B. Large axons show negligible change in Na+ and K+
concentrations after an AP.
Potential Transmission
A. Electrotonic
1. graded
2. receptor (generator) potentials
Potential Transmission
a.  stimulus, then  ∆ Vm
b. electrical signal spreads from source of stimulus
c. problem: no voltage-gated channels here
d. signal decay
“passive electrotonic transmission”
Potential Transmission
A. Electrotonic
3. good for only short distances
4. might reach axon hillock
- that’s where voltage-gated channels are
- where action potentials may be triggered
Potential Transmission
B. Action potential
1. propagation without decrement
2. to axon terminal
Synaptic Transmission
Synaptic Transmission
A. Presynaptic neuron
1. neurotransmitter (usually)
2. synaptic cleft
Synaptic Transmission
B. Postsynaptic neuron
1. bind neurotransmitter
2. postsynaptic potential (∆ Vm)
3. may trigger action potential on postsynaptic effector
Synaptic Transmission
C. Alternation of graded and action potentials
Intraneuron Transmission
A. All neurons have electrotonic conduction (passive)
B. Cable properties
1. determine conduction down the axon process
2. some cytoplasmic resistance to longitudinal flow
3. high resistance of membrane to current
“but membrane is leaky”
Intraneuron Transmission
C. Nonspiking neurons
1. no APs
2. local-circuit neurons
3. still release neurotransmitter
4. vertebrate CNS, retina, insect CNS
5. are very short with
increased Rm
Intraneuron Transmission
A. All neurons have electrotonic conduction (passive)
B. Cable properties
1. determine conduction down the axon process
2. some cytoplasmic resistance to longitudinal flow
3. high resistance of membrane to current
“but membrane is leaky”
Intraneuron Transmission
C. Nonspiking neurons
1. no APs
2. local-circuit neurons
3. still release neurotransmitter
4. vertebrate CNS, retina, insect CNS
5. are very short with
increased Rm
Intraneuron Transmission
D. Propagation of action potentials
1. ∆ Vm much larger than threshold
- safety factor
Intraneuron Transmission
D. Propagation of action potentials
2. spreads to nearby areas
- depends on cable properties
- inactive membrane depolarized by electrotonically
conducted current
Intraneuron Transmission
D. Propagation of action potentials
- K+ efflux behind region of Na+ influx
Intraneuron Transmission
D. Propagation of action potentials
3. unidirectional
a. refractory period
b. K+ channels still open
Intraneuron Transmission
D. Propagation of action potentials
4. speed
a. relates to axon diameter and presence of myelin
b.  axon diameter,  speed of conduction
Intraneuron Transmission
E. Saltatory conduction
1. myelination
a.  Rm ,  Cm
b. the more layering, the greater the resistance
between ICF and ECF
Intraneuron Transmission
E. Saltatory conduction
c. charge flows more easily down the axon than
across the membrane
Intraneuron Transmission
E. Saltatory conduction
2. nodes of Ranvier
a. internodes (beneath Schwann cells or oligodendrocytes)
b. nodes are only exit for current
c. only location along axon where APs are generated