Lecture 21 – Excitable cells in plants and microbes Flashcards
Characteristics of excitable cells:
- Resting membrane potential
- Regenerative (‘all-or-none’) action potentials
- Fast (short-lasting) signals
- Fast signal transmission
- Transduction: e.g. touch/chemical → electrical (interactions)
Used for:
- Synchronising cells in a population
- Fast, purposeful and adaptive responses (due to short-lasting)
- Decision-making
Evolutionary origin of ion channels:
- Ion channels have their origins in prokaryotes
- The earliest ion channels were probably K+ channels
- Excitability depends on V-gated cation channels (Ca2+ and Na+)
- Gene sequencing allows us to establish evolutionary relationships of channels
- Na+ channels evolved from Ca2+ channels in prokaryotes
- Microbes and plants have their place in the story of excitability
Some evolutionary relationships in the ion channel superfamily: origin of V-gated Na+ channels:
- Go back to prokaryotes, potassium channels are present
Ion channels in prokaryotes:
- No evidence that prokaryotic cells are excitable
- Bacteria possess a wide range of ion channels (e.g. Na+, Cl-, Ca-gated K+, ionotropic glutamate receptors) with function largely unknown
- Evidence that oscillations in membrane potential due to K+ flux regulate waves of metabolic activity in bacterial populations (biofilms)
- (2016) Genetic engineering to express bacterial Na+ channels in mammalian cells in vitro enhanced and restored excitability – potential therapy for loss of function in nerve and muscle??
single celled ciliate protozoan
No nervous system
Paramecium – behaviour:
Single celled organism – 100-300 µm long
- Purposeful swimming locomotion – can swim in all directions
- Swims by coordinated beating of cilia
- Rapidly changes direction to avoid obstacles and predators – due to depolarisation of action potentials which travels all across the cell membrane and if they bump into something then hyperpolarisation will occur
- Behavioural mutants – help us explore the mechanisms underlying locomotory behaviour
Excitability and locomotory behaviour in Paramecium:
- Resting membrane potential -40 mV
- Stimulus = chemical, heat, touch, light
- Ca2+-linked mechanoreceptors at front end → backwards swim; K+-linked mechanoreceptors at back → faster forwards swim
- Stimulus → receptor potential → Ca2+-based action potential → increased intracellular Ca2+ → reversal of ciliary beat
- Receptor potential graded to stimulus intensity – allows for decision-making
- Repolarisation → return to forward swimming
- Mutants without action potentials can move but show impaired responses to stimuli – locomotion no longer purposeful
Falling phase
- Falling phase in an action potential is because of inactivation of calcium channels as there is an accumulation of calcium within the cell and potassium gates open
- Cell swims backwards when depolarised
How cilia move:
- Whip-like movements of cilia coordinated into a wave
- ‘9 + 2’ arrangement of microtubules to create axoneme – crosslinked of the protein dynein of adjacent microtubules
- Protein crosslinks stabilise the microtubules in the axoneme
- Bending caused by crosslinks of dynein ‘walking’ along the microtubule – cf. muscle sliding filament
- Increased intracellular Ca2+ causes reversal of ciliary beat – depends on action potential which depends on calcium voltage channels
Behavioural mutants of Paramecium:
- Single gene mutations show specific deficits in locomotory responses
- Examples:-
- Pawn: little or no V-gated Ca current – cannot generate APs and cannot reverse direction of locomotion
- Dancer: enhanced Ca current – reverses in response to much weaker stimulation
- Pantophobiac: reduced V-gated K current – prolonged depolarisation and therefore swims backwards for longer
Didinium nasutum - ciliate protozoan:
¥ A voracious predator of Paramecium
¥ Both predator and prey show fast, directed movements using beating cilia
¥ Didinium ‘captures’ much larger Paramecium with mouth and engulfs it
Rapid movements in plants:
¥ Protection from damage (response to external stimulus)
¥ Prey capture (response to external stimulus)
¥ Spreading pollen and seeds – for reproduction
¬ Sensory structure
¬ Fast signal transmission
¬ Movement – transducing
Mimosa pudica – the ‘sensitive plant’:
¥ Rapid response to touch, light, vibration, temperature
¥ Leaflets fold up to avoid damage and at night
¥ Apparent ‘wilting’ exposes thorny stems and deters herbivores and pests
¥ Cells respond to touch by generating overshooting action potentials that propagate from cell to cell to the base of the leaflet – causes it to fold up
¥ APs have fast rising phase and prolonged plateau
¥ Excitable cells located in vascular bundle - RMP (resting membrane potential) -150 mV
¥ Leaflet rapidly bends downwards
Cl- ion-based action potential causes cell shrinkage:
“excitation-turgor loss coupling” Action potential: ¥ Fast rising phase - Cl- efflux (NOT CATIONS LIKE K+ & NA+) ¥ Slower repolarising phase - K+ efflux ¥ H2O follows by osmosis - Sudden loss of turgor - Ions and H2O pumped back in
Movement of mimosa leaflet:
¥ Pulvinus (ring) attaches leaflet to stem
¥ Cells on upper surface have thick walls and cannot shrink (turgor)
¥ Cell on lower surface shrink causing bending of pulvinus
¥ The thick cells stay same length but the thin layers’ cause bending and moves downwards in response to touch
Dionea muscipula - Venus flytrap:
¥ Sensitive hairs within trap – deflects it (electrical signals travel by gap tight junctions – heart)
¥ Trap snaps shut to catch prey
rest
Dionea muscipula - Venus flytrap:
¥ Resting membrane potential of sensory cells and other tissue cells is -150 mV
¥ All cells can generate APs, which are 1-3 sec duration and 150 mV amplitude
APs are Ca2+-based (chloride based in the other plant)
action
Dionea muscipula - Venus flytrap:
¥ Cell to cell AP transmission is electrotonic via ‘plasmodesmata’ = cytoplasmic links between cells (cf. electrical synapses)
Mechanism of trap closure is uncertain, but probably involves changes in turgor of cells in midrib and trap lobes. Lobes ‘flip’ from convex to concave – so snaps shut very quickly
