The Nervous System - A2 Flashcards
Explain how resting potential is reached.
At a potential difference of around -70. The sodium/potassium pump causes active transport of 3 Na+ out of the neurone membrane and 2 K+ into the membrane; this keeps the outside of the membrane more positive. (Polarised) The sodium ion channels are closed and only some of the potassium ion channels are open. This means that there is a sodium ion electrochemical gradient.
Describe the general structure of a motor neurone.
Cell body - contains organelles and high proportion of RER
Dendrons - branch into dendrites which carry impulses towards cell body
Axon - long unbranched fibre carries nerve impulses away from cell body
Describe the additional features of a myelinated motor neurone.
Schwann cells - wrap around axon many times
Myelin sheath - made from myelin rich membranes of Schwann cells
Nodes of Ranvier - very short gaps between neighbouring Schwann cells where there is no myelin sheath
Terminal end branch - connects neurone to effector
Name 3 processes Schwann cells are involved in.
- electrical insulation
- phagocytosis
- nerve regeneration
How does an action potential pass along an unmyelinated neuron?
- stimulus leads to influx of Na+ ions. First section of membrane depolarised
- local electrical currents cause sodium voltage-gated channels further along the membrane to open
- meanwhile, the section behind begins to re-polarise
- sequential wave of depolarisation occurs.
Explain why myelinated axons conduct impulses faster than unmyelinated axons.
Saltatory conduction - impulse jumps from one Node of Ranvier to another
- depolarisation can not occur when myelin sheath acts as an electrical insulator
- so impulse does not travel along whole axon length
How is resting potential established?
- membrane is more permeable to K+ than Na+
- sodium potassium pump actively transports 3Na+ out of cell and 2K+ into cell
- establishes an electrochemical gradient: cell contents more negative than extracellular environment
Name the stages in generating an action potential.
- depolarisation
- repolarisation
- hyperpolarisation
- return to resting potential
What happens during depolarisation?
- Stimulus - facilitated diffusion of Na+ ions into cell down electrochemical gradient
- Potential difference across membrane becomes more positive
- if membrane reaches threshold potential (-50mv) voltage-gates Na+ channels open
- Significant influx of Na+ ions reverses p.d to +40mv
What happens during repolarisation?
- voltage-gated Na+ channels close and voltage-gated K+ channels open
- facilitated diffusion of K+ ions out of cell down their electrochemical gradient
- p.d across membrane becomes more negative
What happens during hyperpolarisation?
- ‘overshoot’ - when K+ ions diffuse out = p.d becomes more negative than resting potential
- refractory period - no stimulus is large enough to raise membrane potential to threshold
- voltage-gated K+ channels close and sodium-potassium pump re-establishes resting potential
Explain the importance of the refractory period.
No action potential can be generated in hyperpolarised section of membrane.
- ensures un-directional impulse
- ensures discrete impulses
- limits frequency of impulse transmission
What is the ‘all or nothing’ principal?
Any stimulus that causes the membrane to reach threshold potential will generate an action potential.
All action potentials have the same magnitude.
Name the factors that could affect the speed of conductance.
Myelin sheath
Axon diameter
Temperature
How does axon diameter affect speed of conductance?
greater diameter = faster
- less resistance to flow of ions (depolarisation and re-polarisation)
- less leakage of ions (easier to maintain membrane potential)
How does temperature affect speed of conductance?
Higher temp = faster
- faster rate of diffusion (depolarisation and repolarisation)
- faster rate of respiration (enzyme controlled) = more ATP for active transport to re-establish resting potential
- if temperature is too high then proteins denature.
How can an organism detect the strength of a stimulus?
larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulse
What is the function of synapses?
- electrical impulses cannot travel over junction between neurons
- neurotransmitters send impulses between neurons/from neurons to effectors
- new impulses can be initiated in several different neurons for multiple simultaneous responses
Describe the structure of a synapse.
Presynaptic neurone ends in synaptic knob - contains many mitochondria, endoplasmic reticulum and vessels of neurotransmitter.
Synaptic cleft - 20/30nm gap between neurons
Postsynaptic neuron - has complementary receptors to neurotransmitter (ligand-gated Na+ channels)
Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another.
- wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels to open
- vesicles move towards and fuse with presynaptic membrane
- exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross synaptic clefts?
simple diffusion
Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another.
- neurotransmitter binds to specific receptor on postsynaptic membrane
- ligand-gated Na+ channels open
- if influx of Na+ ions raises membrane to threshold potential, action potential is generated
Explain why synaptic transmission is undirectional?
Only presynaptic neuron contains vesicles of neurotransmitter and only postsynaptic membrane has complementary receptors.
So impulse always travels from presynaptic to postsynaptic.
Define summation and name the 2 types.
Neurotransmitter from several sub-threshold impulses accumulates to generate action potential.
- temporal summation
- spatial summation
NB no summation at neuromuscular junctions
