Nerve Impulses (15) Flashcards
What are the general features of a motor neuron’s structure?
cell body
dendrons
axon
Describe the cell body.
contains organelles and a high proportion of RER
Describe dendrons.
branch into dendrites which carry impulses towards cell body
Describe the axon.
long, unbranched fibre which carries nerve impulses away from cell body
What are the general additional features of a motor neuron’s structure when it is myelinated?
schwann cells
myelin sheath
nodes of ranvier
Describe schwann cells.
made out of lipids and wrap around axon many times
Describe myelin sheath.
made of schwann cells
Describe nodes of ranvier.
very short gaps between neighbouring schwann cells where there’s no myelin sheath
What 3 processes are schwann cells involved in?
1) electrical insulation
2) phagocytosis
3) nerve regeneration
How does an action potential pass along an unmyelinated neuron?
1) stimulus leads to influx of sodium ions and the 1st section of membrane depolarises
2) local electrical currents cause sodium voltage-gated channels further along membrane to open
3) sequential wave of depolarisation
Why do myelinated axons conduct impulses faster than unmyelinated axons?
- saltatory conduction
- depolarisation only occurs at the nodes
- impulse does not travel along the whole length
What is saltatory conduction?
impulse jumps from 1 node of ranvier to another
What is resting potential?
the potential difference across neuron membrane when not stimulated
What is the resting potential in humans?
usually about -70mV
How is resting potential established?
- membrane is more permeable to potassium ions than sodium ions
- sodium-potassium pump actively transports 3 sodium ions out of axon and 2 potassium ions into axon
- this established an electrochemical gradient
What is meant by an electrochemical gradient?
when cell contents is more negative than extracellular environment
What are the 4 stages in generating an action potential?
1) depolarisation
2) repolarisation
3) hyperpolarisation
4) return to resting potential
What happens in depolarisation?
1) stimulus causes facilitated diffusion of sodium ions into cell down concentration gradient
2) potential difference across membrane becomes more positive
3) if membrane reaches threshold potential (-50mV), voltage-gated sodium ion channels open
4) significant influx of sodium ions reverses potential difference to +40mV
What happens in repolarisation?
1) voltage-gated sodium ion channels close and voltage-gated potassium ion channels open
2) facilitated diffusion of potassium ions out of cell down their electrochemical gradient
3) potential difference across membrane becomes more negative
What happens in hyperpolarisation?
1) there is an ‘overshoot’ when potassium ions diffuse out so potential difference becomes more negative than resting potential
2) causes refractory period
3) voltage-gated potassium ion channels close and sodium-potassium pump re-establishes resting potential
What is the refractory period?
where no stimulus is large enough to raise membrane potential to threshold so no action potential can be generated in hyperpolarised sections of membrane
What is the importance of the refractory period?
1) ensures unidirectional impulses
2) ensures discrete impulses
3) limits frequency of impulse transmission
What are the units for max frequency of impulse conduction?
Hz
What is meant by the ‘all or nothing’ principle?
any stimulus that causes the membrane to reach threshold potential will generate an action potential and all action potentials have the same magnitude
What 3 factors affect the speed of conduction?
myelination
axon diameter
temperature (don’t mention if only 2 factors are asked for)
How does the axon diameter affect speed of conduction?
- greater diameter = faster
- less resistance to flow of ions in depolarisation and repolarisation
- less ‘leakage’ of ions so easier to maintain membrane potential
How does temperature affect speed of conduction?
- higher = faster
- too high = membrane proteins become denatured
- faster rate of diffusion for depolarisation and repolarisation
- faster rate of respiration in enzyme-controlled steps, so more ATP for active transport to re-establish resting potential
How do organisms detect the strength of a stimulus?
larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation so there’s a greater frequency of impulses
