Cell membranes + excitation Flashcards
What are the 2 main types of excitable cells? + what do they do when they’re excited?
Muscle = smooth, skeletal, cardiac- contract
Nerve = neurones- send constant electrical signals over long/short distances = conduction
Describe the structure of a neuron
How many dendrites are there on each neuron?
5-7
Where does an action potential begin?
Axon hillock
What is a transmembrane resting potential?
The potential difference between the extracellular fluid and intracellular fluid
What is the transmembrane resting potential of a neuron?
-70mv
What can be utilised to generate action potentials?
All living cells have a chemical disequilibrium that can be used by excitable cells to generate action potentials
What is the main creation of a neuron transmembrane resting potential?
Diffusion of + charge potassium ions (K+) out of cell via potassium leak channels
- so + charge is lost = inside of cell becomes more negative
= - 70mv
Describe how the -70mv transmembrane resting potential is created.
- Transport protein acts as potassium channel
- Potassium leak channels always open
- More potassium inside cell
- K+ moves by facilitated diffusion through potassium leak channels down conc gradient, from inside to outside of cell
- K+ lost = inside of cell becomes more negative = -70mv
What are potassium leak channels aided by?
The sodium-potassium pump = electrogenic pump
Describe how the sodium-potassium pump works.
- Very few sodium leak channels
- 3 Na+ pumped out of cell and 2 K+ pumped inside of cell via active transport- both against conc gradient
How does the sodium-potassium pump help to maintain the -70mv transmembrane resting potential?
Intracellular conc of K+ high and Na+ low as less sodium leak channels in comparison K+ so more K+ can leak out of cell = more negative
S-P pump = net loss of one + ion from inside of cell
Other than S-P pump and potassium leak channels, what else helps to maintain the -70mv TMR potential?
Negatively charged proteins
Chloride ions
Describe how proteins help to maintain -70mv TMR potential
Negatively charged proteins more inside of cell- unable to move freely out of cell as too large
= more negative inside
Describe how chloride ions help to maintain the TMR potential
Negative chloride ions in higher conc outside than inside- membrane is permeable to Cl- so they move done their conc and electrical gradient until equilibrium
What channels are involved in the generation of an action potential?
Voltage gated potassium and sodium channels
Name some similarities + differences in the way the different voltage-gated channels work.
Differences:
- Potassium ions move down conc gradient outside of cell, but sodium moves into cell down conc gradient
- Potassium voltage gated channels are slower to open + close, but sodium VG channels are faster
Similarities:
- Both are normally closed
- Voltage change = protein changes shape
- Both take time to reset so action potential travels one-way down axon
Do nerve cells have a high or low threshold for excitation?
Low threshold
What are the 2 types of response to neuron excitation?
- Local non-propagated
- Propagated
What is a non-propagated response?
+ where do they occur on a neuron?
= Graded response- magnitude of response is relative to the strength of stimulus
- usually occurs at dendrites or cell body
Define: Depolarisation
Membrane potential gets less negative
Describe how a neuron non-propagated response works
- Small strength stimulus applied
- Results in small depolarisation
- This loses strength through the cytoplasm
Describe how a neuron non-propagated response works
- Small strength stimulus applied
- Results in small depolarisation
- This loses strength through the cytoplasm
What is a neuron propagated response?
+ where does is begin and end?
= ACTION POTENTIAL = magnitude of response is always the same if there is a sufficient stimulus = ALL OR NOTHING RESPONSE:
- Begins at axon hillock where is travels in a one-way direction down axon
- Ends at terminal boutons where synapse releases neurotransmitter
Define: Hyperpolarisation
+ what is it caused by?
= membrane potential becomes more negative than -70mv resting potential
- caused by VG potassium channels slowly closing so too many K+ ions leave = more negative
Describe how a propagated response in a neuron works.
- Large enough stimulus applied = threshold reached
- VG sodium channels open = influx of sodium down conc gradient, which brings + charge into cell
- = Depolarisation of cell membrane reaches threshold = becomes more + on inside compared to outside
- VG sodium channels close + VG potassium channels open
- Potassium ions leave cell = net loss of + charge which makes membrane more negative
- Hyperpolarisation = membrane goes below -70mv
- Repolarisation = recovery period- goes back to -70mv
What is a refractory period?
When neurons reset + recover after firing an action potential
What are the 2 types of refractory periods?
Absolute and relative
What is an absolute refractory period and when does it occur?
= cells cannot fire anymore action potentials to ensure their one-way travel
- occurs immediately after an action potential is fired
What is a relative refractory period and when does it occur?
Another action potential could be intiated if neuron recieves a large enough stimulus- stronger than previous action potential
- occurs after the absolute refractory period
If a larger stimulus is applied does that mean a bigger action potential?
No, they just occur more frequently
What is the difference between a myelinated and unmyelinated axon?
Myelinated = has myelin sheath produced by schwann cells
How does an AP travel down an unmyelinated axon?
- AP generated = impermanent reversal of - charge in membrane potential
- negative charge from outside of membrane pulls charge from membrane ahead
- New action potential created in adjacent active zone and zone behind becomes refractory = no AP
How do APs travel down a myelinated sheath?
- AP jumps between nodes of ranvier
- AP pulls + charge from node ahead to create new AP in node- then becomes refractory
= saltatory conduction
Which type of axon is faster at conducting APs?
Myelinated
