Nerve impulses Flashcards
Structure of a myelinated motor neurone
Cell body: contains organelles- proteins + neurotransmitters are made here
Dendrites: carry action potentials to surrounding cells
Axon: conductive, long fibre that carries nervous impulses around the motor neurone
Schwann cells: wrap around the axon to form the myelin sheath which is a lipid so doesn’t allow charged ions to pass through it (gaps between myelin sheath are nodes of Ranvier)
What is resting potential?
When a neurone is not conducting an impulse, theres a difference between the electrical charge inside + outside the neurone - resting potential
Theres more positive ions (Na+ and K+) outside than inside so inside is more negative at -65mV
Establishing a resting potential
Resting potential is maintained by sodium-potassium pump, involving active transport + so ATP.
The pump moves 2 K+ ions in and 3Na+ ions out. This creates an electrochemical gradient causing K+ to diffuse out and Na+ to diffuse in but because the membrane is more permeable to K+, more are moved out resulting in -65mV.
What is an action potential?
When the neurones voltage increases beyond the resting potential (-65mV) - this generates an action potential
An increase in voltage (depolarisation) is due to membrane becoming more permeable to Na+
Depolarisation
-Stimulus provides energy causing some sodium voltage-gated channels to open so Na+ can diffuse into axon so theres a slight increase in voltage
-If threshold value is met (-55mV) then this provides more energy to open more sodium-voltage gated channels, so more Na+ diffuse in (voltage increases to +40mV)
Repolarisation
-Sodium voltage-gated channels close (preventing further influx of Na+) + potassium voltage-gated channels open
-More K+ ions diffuse out, causing a decrease in voltage
Hyperpolarisation
-Overshoot beyond resting potential (refractory period), inside of axon is more negative than outside
-potassium voltage-gated channels close + sodium-potassium pump re-establishes resting potential
Passage of an action potential along a myelinated axon
Fatty sheath of myelin around axon acts an an electrical insulator, preventing action potentials from forming
Action potentials can occur at nodes of Ranvier + jump from node to node in process called saltatory conduction
Action potential passes along a myelinated neurone faster than along the axon of an unmyelinated one of the same diameter
All-or-nothing principle
-Any stimulus below the threshold value will fail to generate an action potential so no impulse produced
-Any stimulus above threshold value will generate an action potential so nerve impulses will travel
-All action potentials are around the same size so strength of stimulus can’t be determined by size but instead by frequency
- Bigger stimuli increase the frequency of action potentials
Importance of refractory period
No action potential can be generated as sodium voltage gated channels are recovering + can’t be opened
Importance:
-Ensures discrete impulses are produced: an action potential can’t be generated immediately after another one so makes sure each is separated
-Ensures action potentials travel in one direction: stops action potential from spreading out in both directions which would prevent a response
-Limits the number of impulse transmission: prevents over reaction to a stimulus
Factors affecting speed of conductance
-Myelination + saltatory conduction
-Axon diameter
-Temperature
Myelination + saltatory conduction
Action potential jumps from node to node (satlatory conduction) meaning the action potential travels along the axon faster as it doesn’t have to generate an action potential along the entire length, just at nodes of Ranvier
Axon diameter
The wider the diameter, the faster the speed of conductance as less leakage of ions so action potentials travel faster as threshold value is met quicker
Temperature
Higher temp increases speed of conductance for 2 reasons:
-Ions diffuse faster so nerve impulse in faster
-Energy for active transport comes from respiration, enzymes in respiration work faster when temp is increased. So there’s more ATP for active transport in Na+/k+ pump
