Action Potensials Flashcards
Nervous system is divided into 2 system:
1) Central nervous system
2) Peripheral nervous system
PNS
- Somatic nervous system
- Automatic nervous system
Functions of neurones;
- To sense changes internal + external to the body
- To process this sensory info
- To initiate a response
Neurones are excitable cells that produce…
action potentials when they receive electrical or chemical stimulation
Neurones are specialised for…
Transmission of signals and communicate with each other through chemical synapse
What is an action potensial?
- event that is specific to excitable cells like neurones or muscle fibres
- Involves road + short-lasting rise in electrical potential, immediately followed by a fall
- Result of OPENING + CLOSING of channels in the membrane
- In nerves action potentials start at the axon hillock + transmitted along nerve axon.
- Action potentials transfer info over a distance
“All or none” principle
TO REACH TRESHOLD
-When action potential begins»_space; propagates down length of the axon
- When action potential reaches end of axon»_space; neurotransmitter is relaxed into synapse
Why does resting membrane potential need to be restored before another action potential can fire?
from beginning of the action potensial to restoration of the RMP, the neuron is in a refractory period.
Why does resting membrane potential need to be restored before another action potential can fire?
from beginning of the action potensial to restoration of the RMP, the neuron is in a refractory period.
Refractory period -
Resistant to process
> either unable to fire a 2nd action potential
> or more resistant to firing a 2nd action potential
Depolarization
from -70mV up to +40mV = Na^+ ion come in to axon
Repolarization
from +30mV down to -70MV (and lower) = Na^+ channels close
Hyperpolerization
after dip under 70mV, voltage increases again - to level out
Absolute refractory period
This period the membrane is COMPLETELY resistant to further stimulation
Open/closed “voltage=gated” property of these channels means they are indifferent voltages
What happens when the cell enters relative refractory period?
Large number of channels inactive will open again
Relative refractory period
Membrane is MORE resistant to stimulation than usual
Some voltage-gated Na^+ channels are still inactivated
What is the result of membrane being more resistant to stimulation?
= 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.
Do action potentials in the body travel in one or both directions?
Axon can experimentally pass action potentials in EITHER DIRECTIONS
(normally a.p arise at only one end of nerve fibre)
ABSOLUTE REFRACTORY PERIOD
Volatge-gated Na^+ channels enter a short-lived, inactive state after they have been open + prevents nerve from being re-excited
During A.R period
Na^+ channels will not be activated by local currents and it is this that prevents bidirectional a.p propagation from occuring
Refractory period PREVENTS…
BACKWARD propagation of an action potential
Propagation of action potentials
At rest: neuronal cell membrane is polarised with then inside to the outside
During a.p polarity is briefly=
REVERSED
the change in polarity of cell membrane produces…
local currents that cause depolarisation of cell membrane in region of the a.p
What happens when threshold is reached?
Voltage Na^+ channels open and an a.p is generated at new location
A.p self-propagate along an axon
How do action potentials travel along myelinated fibres?
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
Results of MYELIN
- 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
Permeability =
the ability to store electrical energy + increases membrane resistance
Sattatory conduction
“JUMPING” between nodes of Ranvier
- conduction = fast in myelin (White - insulated)
- conduction = slow in nodes (Grey - untinsulated)
