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
