lecture 28 - transmission of AP

Question 1

two types of axon and their features

Answer 1

• unmyelinated: small diameter, slow transmission of AP, continuous - myelinated: larger diameter, fast transmission of AP, saltatory

Question 2

what are the 2 stages of action potential transmission, in both axon types?

Answer 2

• passive spread - generation of action potential

Question 3

conventional current used in diagrams show

Answer 3

flow of cations (positive ions) from +ve to -ve terminals

Question 4

3 stages in passive spread

Answer 4

1. subthreshold depolarisation at one region of the membrane 2. passive current flow (inside and outside the axon) 3. depolarisation of adjacent parts of the axon (both sides)

Question 5

why is current only able to spread over a short distance? what is this distance

Answer 5

current dissipates quickly, restricting distance to less than 1mm

Question 6

speed of unmyelinated axon AP transmission =

Answer 6

1m/sec

Question 7

speed of myelinated axon AP transmission =

Answer 7

20-100m/sec

Question 8

why is unmyelinated axon AP transmission so much slower

Answer 8

AP must be regenerated at every point in the membrane

Question 9

is passive current flow between two points rapid or slow?

Answer 9

VERY rapid

Question 10

5 steps to AP transmission in an unmyelinated axon

Answer 10

1. action potential 2. passive current flow 3. depolarisation on both adjacent parts of the membrane to THRESHOLD 4. voltage-gated Na+ channels open 5. new full sized AP generated

Question 11

glia cells function

Answer 11

wrap axon in myelin

Question 12

which cells form myelin sheath in CNS

Answer 12

oligodendrocytes

Question 13

which cells form myelin sheath in PNS

Answer 13

Schwann cells

Question 14

branch off axon called…

Answer 14

axon collateral

Question 15

gaps in myelin =

Answer 15

Nodes of Ranvier

Question 16

Myelination increases efficiency of passive spread of current because…

Answer 16

due to the insulating properties of myelin, there is less current dissipation (current can only leave via the nodes of Ranvier)

Question 17

Passive transmission occurs in one direction TRUE/FALSE

Answer 17

FALSE. Passive transmission occurs in BOTH DIRECTIONS

Question 18

Current flows ________ between nodes

Answer 18

current flows _passively_ between nodes

Question 19

in myelinated axons, where does action potential potential regeneration occur?

Answer 19

at the nodes of Ranvier

Question 20

Myelination increases AP transmission speed because

Answer 20

increasing efficiency of passive spread, so regeneration of AP only has to occur at the nodes, as opposed to every part of the membrane

Question 21

why are axons not always myelinated?

Answer 21

too large. unmyelinated can fit more per volume - important in the human brain

Question 22

AP at adjacent node will try to move passively to depolarise the ‘previous’ node, but why will it fail?

Answer 22

this node will still be in the absolute refractory period (for 1-2ms). By the time this period is over, AP has already moved on

Question 23

Can action potentials moving in opposite directions pass each other?

Answer 23

NO, like the reasons for APs moving in one direction, this is due to the refractory period

Question 24

antidromic direction =

Answer 24

towards cell body along axon

Question 25

orthodromic direction =

Answer 25

from cell body along axon

Question 26

collision =

Answer 26

process where antidromic and orthodromic APs will cancel each other out, due to their absolute refractory periods

Question 27

where are axons of: sensory neurons, motor neurons, and ‘autonomic nervous system’ found?

Answer 27

PNS

Question 28

where on the axon would a high-density of voltage-gated Na+ channels be found?

Answer 28

trigger zone of axon

Question 29

In sensory neurons, do the receptors immediately evoke an action potential?

Answer 29

NO. A receptor potential is first evoked

Question 30

receptor potential =

Answer 30

a graded depolarisation which is first evoked by sensory neurons (instead of an AP)

Question 31

once generated the receptor potential spreads _________ to the more distally located _______ ____ where APs are generated. The action potential then spreads along the axon, towards the ___. Information about the strength of the stimulus is coded into the ________ of the receptor potential, then in the ________ of the action potential.

Answer 31

once generated the receptor potential spreads _passively_ to the more distally located _trigger_ _zone_ where APs are generated. The action potential then spreads along the axon, towards the _CNS_. Information about the strength of the stimulus is coded into the _amplitude_ of the receptor potential, then in the _frequency_ of the action potential.