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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.

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slide1

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

slide2

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)

slide3

Gated Ion Channels

B. Voltage-gated K+ channels

1. slower response to voltage changes than Na+ channels

2. gK increases at peak of AP

slide4

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

slide5

Gated Ion Channels

B. Voltage-gated K+ channels

4. hastens repolarization for generation of more action potentials

slide6

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.

slide7

Potential Transmission

A. Electrotonic

1. graded

2. receptor (generator) potentials

slide8

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”

slide9

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

slide10

Potential Transmission

B. Action potential

1. propagation without decrement

2. to axon terminal

slide12

Synaptic Transmission

A. Presynaptic neuron

1. neurotransmitter (usually)

2. synaptic cleft

slide13

Synaptic Transmission

B. Postsynaptic neuron

1. bind neurotransmitter

2. postsynaptic potential (∆ Vm)

3. may trigger action potential on postsynaptic effector

slide14

Synaptic Transmission

C. Alternation of graded and action potentials

slide15

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”

slide16

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

slide17

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”

slide18

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

slide19

Intraneuron Transmission

D. Propagation of action potentials

1. ∆ Vm much larger than threshold

- safety factor

slide20

Intraneuron Transmission

D. Propagation of action potentials

2. spreads to nearby areas

- depends on cable properties

- inactive membrane depolarized by electrotonically conducted current

slide21

Intraneuron Transmission

D. Propagation of action potentials

- K+ efflux behind region of Na+ influx

slide22

Intraneuron Transmission

D. Propagation of action potentials

3. unidirectional

a. refractory period

b. K+ channels still open

slide23

Intraneuron Transmission

D. Propagation of action potentials

4. speed

a. relates to axon diameter and presence of myelin

b.  axon diameter,  speed of conduction

slide24

Intraneuron Transmission

E. Saltatory conduction

1. myelination

a.  Rm ,  Cm

b. the more layering, the greater the resistance between ICF and ECF

slide25

Intraneuron Transmission

E. Saltatory conduction

c. charge flows more easily down the axon than across the membrane

slide26

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