3.6.2.1 Nerve impulses (A-level only) Flashcards
Components of structure of mylinated neurone
Cell body Dendrons/dendrites Axon Myelin sheath Schwann cells Nodes of Ranvier
Structure of a myelinated motor neurone
Cell body
└structure: contains organelles (e.g endoplasmic reticulum), and the nucleus
└function: associated with the production of proteins and neurotransmitters
Structure of a myelinated motor neurone
Dendrons/dendrites
└structure: extensions of the cell body which subdivide into smaller dendrites
└function: carry neve impulses towards the cell body
Structure of a myelinated motor neurone
Axon
└structure: single long fibre
└function: carries never impulses away from the bell body
Structure of a myelinated motor neurone
Myelin sheath
└structure: covers the axon. Is made up from membranes of schwann cells, which are rich in the lipid myelin
└function: provides electrical insulation
Structure of a myelinated motor neurone
Schwann cells
└structure: surround the axon
└function: protection, electrical insulation, nerve regeneration, phagocytosis (to get rid of cell debris)
Structure of a myelinated motor neurone
Nodes of Ranvier
└structure: gaps between ajacent schwann cells where there is no myelin sheath
└function: saltatory conduction
Cell membrane at resting potential
└the membrane is polarised as there is a difference in charge
└the inside of the membrane is negatively charged/more negative than the outside of the membrane (surrounding tissue fluid)
└voltage across the membrane= -70mV (millivolts)
How resting potential is maintained/created
sodium potassium pump
└actively transports (using ATP)
└3 sodium ions (Na+) OUT neurone/axon
└2 potassium ions (K+) IN neurone
└membranes are impermeable to sodium
└can’t diffuse back in
└created a sodium ion electrochemical gradient
└as outside is more positive than inside (polarisation)
└membrane is permeable to potassium
└can diffuse back out through K+ ion channels, down their concentration gradient
└this creates a potential difference
└an equilibrium is established
└chemical and electrochemical gradients are balanced
└no net movement of ions
Why sodium ions and potassium ions can only cross the axon membrane through proteins
└can’t pass through the lipid bilayer
└as charged/ hydrophilic/ not lipid soluble/ water soluble
Transport of ions through membrane in resting potential
└potassium ions
└sodium potassium pump- active transport
└potassium ion channels- facilitated diffusion
└sodium ions
└sodium potassium pump- active transport
Action potential definition
The temporary reversal of polarity
└inside of axon becomes more positive
└depolarised
Action potential process (full)
Resting potential
└some K+ voltage gated channels are open
└all Na+ voltage gated channels are closed
Energy from a stimulus excites axon cell membrane
└Na+ channels open
└=membrane more permeable to Na+ ions
└Na+ ions diffuse into neurone down Na+ ion electrochemical gradient via facilitated diffusion
└inside of neurone is less negative
Depolarisation (potential difference= -55mV)
└IF threshold is reached
└more Na+ channels open
└more Na+ diffuse rapidly into neurone
Repolarisation (potential difference= +30mV)
└Na+ channels close
└K+ channels open
└=membrane more permeable to K+
└K+ ions diffuse out of neurone down their concentration gradient
└to get resting potential back
Hyperpolarisation (potential difference= <70mV)
└K+ channels are slow to close
└=slight overshoot of electrochemical gradient
└(too many K+ ions diffuse out of neurone)
└potential difference becomes more negative than resting potential (<70mV)
└during this time, the membrane cannot be re-exited
└(refractory period)
Resting potential
└ion channels are reset
└sodium-potassium pump returns to resting potential
└maintained until the next stimulus
Action potential stages
Resting potential (stimulus) Depolarisation Repolarisation Hyperpolarisation Resting potential
How does a bigger stimulus affect action potentials
↑size of stimulus
↑frequency action potentials are fired
How an organism can perceive the size of an impulse
└number of impulses passing in a given time
└having neurones with different threshold values
All or nothing principle
A stimulus needs to reach the threshold (a certain level of stimulus) in order to generate an action potential
└if the stimulus is below the threshold, no action potential will be fired
What happens once the threshold has been reached
└an action potential will fire with the same change in voltage regardless of how big the stimulus is
└anything above the threshold will only generate one action potential
What happens when energy from a stimulus excites axon cell membrane
└Na+ channels open
└=membrane more permeable to Na+ ions
└Na+ ions diffuse into neurone down Na+ ion electrochemical gradient via facilitated diffusion
└inside of neurone is less negative
Depolarisation process
(potential difference= -55mV)
└IF threshold is reached
└more Na+ channels open
└more Na+ diffuse rapidly into neurone
Repolarisation process
(potential difference= +30mV)
└Na+ channels close
└K+ channels open
└=membrane more permeable to K+
└K+ ions diffuse out of neurone down their concentration gradient
└to get resting potential backHyperpolarisation process
(potential difference= <70mV)
└K+ channels are slow to close
└=slight overshoot of electrochemical gradient
└(too many K+ ions diffuse out of neurone)
└potential difference becomes more negative than resting potential (<70mV)
└during this time, the membrane cannot be re-exited
└(refractory period)
The passage of an action potential
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The passage of an action potential along myelinated axons
Saltatory conduction
└he process of an action potential jumping from node to node
└faster
The passage of an action potential along non-myelinated axons
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Nerve impulse definition
└transmission of an action potential along the axon of a neurone
Refractory period definition
└the short period of time after an action potential when the neurone can’t be exited and no action potential occurs
└as sodium ion channels are closed
Importance of the refractory period
roduces discrete (separate) impulses- don’t overlap
└
Limits the frequency of impulse transmission
└so no overload, limits the strength of stimulus
Ensures action potentials are unidirectional (travel in one direction)
└
Factors affecting the speed of conductance
└ myelination and saltatory conduction
└ axon diameter
└ temperature
Factors affecting the speed of conductance
myelination and saltatory conduction
└myelin sheath is an electrical insulator
└so action potentials cant form in parts of the axon covered in it
└depolarisation only occurs at nodes of ranvier
└saltatory conduction
└increases speed of transmission
└depolarisation only occurs at nodes of ranvier
└so action potentials jump from one node of ranvier to the other
Factors affecting the speed of conductance
axon diameter
↑diameter of axon
└↓leakage of ions
└so action potential easier to maintain
└so ↑speed of conductance
Factors affecting the speed of conductance
temperature
Increase in temperature up to optimum ↑temperature ↑speed of ion diffusion ↑speed of conductance ↑enzyme activity in respiration ↑ATP production ↑speed of active transport Increase in temperature after optimum └proteins /enzymes (in respiration) denature └↓speed of conductance
MS 0.2 Students could use appropriate units when calculating the maximum frequency of impulse conduction given the refractory period of a neurone.
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