Topic 9.5 Nervous transmission Flashcards
What is resting potential?
Potential difference (voltage) across neurone membrane when not stimulated (50 to -90 mV, usually about -70mV in humans).
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 electrochemical gradient: cell contents more negative than extra cellular enviroment.
Name the stages in generating an action potential
- Depolarisation
- Replolarisation
- Hyperpolarisation
- Return to resting potential
What happens during depolarisation?
- Stimulus -> facilitated diffusion of Na+ into cell down electrochemical gradient
- p.d. across membrane become more positive
- If membrane reaches threshold potential (-50mV), voltage-gated 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
- Refactory 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 refactory period
No action potential can be generated in hyperpolarised sections of membrane
- Ensures unidirectional impulse
- Ensures discrete impulse
- Limits frequency of impulse transmission
How is action potential propagated along an unmyelinated neuron?
- Stimulus leads to an influx of Na+ ions. First section of membrane depolarises
- Local electrical currents cause sodium voltage-gated channels further along membrane to open. Meanwhile, the section behind begins to repolarise
- Sequential wave of depolarisation
Describe the structure of a motor neuron
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 impulsed away from cell body
Describe the additional features of a myelinated motor neuron
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
Explain why myelinated axons conduct impulses faster than unmyelinated axons
Saltatory conduction: Impulse ‘jumps’ from one node of ranvier to another. Depolarisation cannot occur where myelin sheath acts as an insulator.
So impulse does not travel along the axon length.
What is the function of synapses?
- Electrical impulse cannot cross junction
- Neurotransmitters send impulses between neurons/ from neurons to effectors for excitatory or inhibitory response
- Summation of sub-threshold impulses
- New impulses can be initiated in several different neurons for multiple simultaneous responses
Describe the structure of a synapse
- Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter
- Synaptic cleft: 20-30 nm gap between neurons
- Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels)
Explain the role of acetylcholine
- Causes muscle contraction at motor end plate.
- Causes excitation at preganglionic neurons. Causes inhibition at parasympathetic postganglionic neurons (e.g. of heart and breathing rate).
What happens to acetylcholine from the synaptic cleft?
- Hydrolysis into acetyl and choline by acetylcholine esterase (AChE).
- Acetyl and choline diffuse back into the presynaptic membrane
- ATP us used to reform acetylcholine for storage in vesicles
