Nerve Impulses Flashcards
State the function of sensory neurons
Carriers nerve impulses from receptor towards intermediate neurons within CNS
State the function of motor neurons
Connect sensory to motor neurons
Describe the structure and function of the axon
Single long fiber that carriers nerve impulses
Describe the structure and function of the cell body
Contains a nucleus and large number of endoplasmic reticulum
Produces neurotransmitters
Describe the structure of the myelin sheath
Covers the axon
Made up of membranes of Schwann cells
Membranes are rich in the lipid myelin
Describe the structure and function of Schwann cells
Surround and wrap around axon, providing protection and electrical insulation
Describe the structure of Nodes of Ranvier
Small gaps between adjacent Schwann cell
Sodium ion channels are concentrated at the nodes
When is the sympathetic system activated?
In times of stress
What is the sympathetic system responsible for?
For increasing heart rate and ventilation and pupil dilation
When is the parasympathetic system active?
Most active in relaxed states
What is the parasympathetic system responsible for?
Responsible for decreasing heart rate and ventilation rate and pupil constriction
What does the parasympathetic system enable?
Enables everyday tasks to be completed (digest food, fight infections, etc.)
Describe and explain the charge of a neuron’s resting state
In neurone’s resting state, outside of membrane is positively charged compared to inside
∵ more positive ions outside cell than inside
(resting potential = about -70 mv)
∴ membrane = polarised
Difference in charge across it
State how resting potential is created and maintained
By sodium-potassium pumps & potassium ion channels
Describe how the resting potential is created and maintained by sodium-potassium pumps and potassium ion channels
-Sodium-potassium pump uses active transport to move 3 Na+ out of neurone for every 2 K+ ions moved in
-ATP needed to do this
-Membrane isn’t permeable to Na+ = can’t diffuse back
-Creates sodium ion electrochemical gradient
∵ more Na+ outside cell than inside
-Membrane is permeable to K+ = diffuse back out though K+ channels, down their concentration gradient
facilitated diffusion
-Makes outside of cell positively charged compared to inside
Neurone cell membranes become ________ when they’re stimulated
Depolarised
If a stimulus is big enough, it triggers rapid change in ___
p.d.
Name the 5 stages of how an action potential occurs i.e. how neurone cell membranes become depolarised when they’re stimulated
-Stimulus
-Depolarisation
-Repolarisation
-Hyperpolarisation
-Resting potential
Action Potentials
Describe the stage stimulus
Stimulus excites neurones cell membrane = Na+ channels to open, making membrane more permeable to Na+
Na+ then diffuse (down electrochemical gradient) into neurone, making it less negative
Action Potentials
Describe the stage depolarisation
Once threshold has been met (around -55mv), more Na+ channels open = more Na+ to diffuse in rapidly
Action Potentials
Describe the stage repolarisation
-(At around +30mV) Na+ channels close and the K+ channels open
-(Na+ channels have to close or membrane will remain depolarised)
-Membrane is now more permeable to K+
= K+ diffuse out of neurone down K+ conc. gradient
-Gets membrane back to its resting potential
Action Potentials
Describe the stage hyperpolarisation
-K+ channels are slow to close so there’s a slight overshoot where too many K+ diffuse out of neurone
-Causes p.d. to become more negative than resting potential
Action Potentials
Describe the stage resting potential
-Ion channels are reset
-Sodium-potassium pump returns membrane to its resting potential
& maintains until membrane’s excited by another stimulus
Explain why after an action potential, the neurone cell membrane can’t be excited again straight away
-∵ ion channels are recovering & can’t be made to open
-Na+ channels are closed during repolarisation and K+ are closed during hyperpolarisation
Describe how an action potential moves along a neurone
-When action potential occurs, some Na+ that enter neurone diffuse sideways
-Causes Na+ channels in next region of neurone to open and Na+ ions diffuse into that part
-Causes wave of depolarisation to travel along neurone
-Wave move away from parts of membrane in refractory period ∵ these parts can’t fire an action potential
What is the refractory period?
When ion channels are recovering and can’t be opened
What does the refractory period act as?
Acts as a time delay between 1 action potential and the next
Name 3 things the refractory period ensures
-Action potentials don’t overlap
-But pass along discrete (separate) impulses
-Limit to frequency of which nerve impulses can be transmitted
-Action potentials are unidirectional (only travel in 1 direction)
Describe the all-or-nothing nature of action potentials
-Once threshold is reached, action potential will always fire with same change in voltage
-No matter how big stimulus is
-If threshold isn’t reached, action potential won’t fire
What does a bigger stimulus cause?
Causes action potentials to fire more frequently
DOESN’T = bigger action potential
Name 3 factors that affect the speed of conduction of action potentials
Temperature
Axon Diameter
Myelination
Describe and explain how temperature affects the speed of
conduction of action potentials
-Speed of conduction increases as temp. increase
-∵ ions diffuse faster
-Speed only increases up to 40°C
-After proteins, denature & speed decreases
-Active transport is used for sodium-potassium pump & enzymes are used
Describe how axon diameter affects the speed of conduction of action potentials
Bigger diameter = faster the conduction of action potentials
Explain why action potentials are conducted quicker along axons with bigger diameters
-∵ there’s less resistance to flow of ions in cytoplasm of bigger axon
-∵ less leakage of ions from axon
-With less resistance, depolarisation reaches other parts of neurone cell membrane quicker
In myelinated neurone, where does depolarisation only occur?
At nodes of Ranvier
Describe and explain how myelination increases the speed of
conduction of action potentials
Neurone’s cytoplasm conducts enough electrical charge to depolarise the next node ∴ impulse jumps from node to node
Called saltatory conduction & it’s very fast
In non-myelinated neurone, impulse travels as wave along whole length of axon membrane
(Get depolarisation along whole length of membrane)
Slower than saltatory conduction
