Action Potensials

Question 1

Nervous system is divided into 2 system:

Answer 1

1) Central nervous system
2) Peripheral nervous system

Question 2

PNS

Answer 2

• Somatic nervous system
• Automatic nervous system

Question 3

Functions of neurones;

Answer 3

1. To sense changes internal + external to the body
2. To process this sensory info
3. To initiate a response

Question 4

Neurones are excitable cells that produce…

Answer 4

action potentials when they receive electrical or chemical stimulation

Question 5

Neurones are specialised for…

Answer 5

Transmission of signals and communicate with each other through chemical synapse

Question 6

What is an action potensial?

Answer 6

1. event that is specific to excitable cells like neurones or muscle fibres
2. Involves road + short-lasting rise in electrical potential, immediately followed by a fall
3. Result of OPENING + CLOSING of channels in the membrane
4. In nerves action potentials start at the axon hillock + transmitted along nerve axon.
5. Action potentials transfer info over a distance

Question 7

“All or none” principle

Answer 7

TO REACH TRESHOLD

-When action potential begins&raquo_space; propagates down length of the axon
- When action potential reaches end of axon&raquo_space; neurotransmitter is relaxed into synapse

Question 8

Why does resting membrane potential need to be restored before another action potential can fire?

Answer 8

from beginning of the action potensial to restoration of the RMP, the neuron is in a refractory period.

Question 9

Why does resting membrane potential need to be restored before another action potential can fire?

Answer 9

from beginning of the action potensial to restoration of the RMP, the neuron is in a refractory period.

Question 10

Refractory period -

Answer 10

Resistant to process

> either unable to fire a 2nd action potential

> or more resistant to firing a 2nd action potential

Question 11

Depolarization

Answer 11

from -70mV up to +40mV = Na^+ ion come in to axon

Question 12

Repolarization

Answer 12

from +30mV down to -70MV (and lower) = Na^+ channels close

Question 13

Hyperpolerization

Answer 13

after dip under 70mV, voltage increases again - to level out

Question 14

Absolute refractory period

Answer 14

This period the membrane is COMPLETELY resistant to further stimulation
Open/closed “voltage=gated” property of these channels means they are indifferent voltages

Question 15

What happens when the cell enters relative refractory period?

Answer 15

Large number of channels inactive will open again

Question 16

Relative refractory period

Answer 16

Membrane is MORE resistant to stimulation than usual
Some voltage-gated Na^+ channels are still inactivated

Question 17

What is the result of membrane being more resistant to stimulation?

Answer 17

= Strong stimulus than usual required to open insufficient number of these channels fro ANOTHER action potential

Cell membrane is more permeable to K^+ during this period. This further causes depolarisation of membrane.

Question 18

Do action potentials in the body travel in one or both directions?

Answer 18

Axon can experimentally pass action potentials in EITHER DIRECTIONS
(normally a.p arise at only one end of nerve fibre)

Question 19

ABSOLUTE REFRACTORY PERIOD

Answer 19

Volatge-gated Na^+ channels enter a short-lived, inactive state after they have been open + prevents nerve from being re-excited

Question 20

During A.R period

Answer 20

Na^+ channels will not be activated by local currents and it is this that prevents bidirectional a.p propagation from occuring

Question 21

Refractory period PREVENTS…

Answer 21

BACKWARD propagation of an action potential

Question 22

Propagation of action potentials

Answer 22

At rest: neuronal cell membrane is polarised with then inside to the outside

Question 23

During a.p polarity is briefly=

Answer 23

REVERSED

Question 24

the change in polarity of cell membrane produces…

Answer 24

local currents that cause depolarisation of cell membrane in region of the a.p

Question 25

What happens when threshold is reached?

Answer 25

Voltage Na^+ channels open and an a.p is generated at new location

A.p self-propagate along an axon

Question 26

How do action potentials travel along myelinated fibres?

Answer 26

1) CONDUCT: along a nerve fibre is greatly enhanced by MYELIN - helps insulate the axon + reduce loss of electrical impulse

2) SHEATHS OF MYELIN that are about 20-300 layers thick surround the axon, separated every 1-2mm

3) GAPS = nodes of Ranvier

Question 27

Results of MYELIN

Answer 27

• decreases change in membrane permeability capacitance
• much of that change occurs at the nodes of Ranvier
• high concentration of Na^+ channels which allow for regeneration of a.p as it travels along axon

Question 28

Permeability =

Answer 28

the ability to store electrical energy + increases membrane resistance

Question 29

Sattatory conduction

Answer 29

“JUMPING” between nodes of Ranvier

• conduction = fast in myelin (White - insulated)
• conduction = slow in nodes (Grey - untinsulated)