Lecture 8: the excitable cell Flashcards
What is the nervous system?
System of communication that allows an organism to react rapidly and modifiably to change in its environment
What must neurons do?
Collect, integrate and output
How fast do electric signals travel along the leg?
2m/s
What provides a rapid, reliable and flexible means for neurons to receive, integrate and transmit signals?
Electrical activity
What provide more flexibility?
Chemical messengers ( and receptors) between and within cells
Is glutamate excitatory or inhibitory?
Excitatory
Is GABA excitatory or inhibitory?
Inhibitory
What are the two main types of electrical potentials?
Graded and Action
What is an action potential?
Fixed size, all-or-nothing signals that travel along the axon. Can travel long distances in short time
What is a graded potential?
Variable size, local signals not propagated over long distances. Localized in dendrites
What are Action potentials coded by and why?
Frequency; are of a unit size (level of neurotransmitter, level of stimulus)
What are graded potentials coded by and why?
Size; vary according to the strength of the stimulus
Which potential, action or graded, tend to travel along one way?
Action
Why do neurons have a resting potential?
Inevitable consequence of: selectively permeable membrane, unequal distribution of charged molecules/ions, physical factors (water, NaCl, K, Ca) and physical forces (diffusion, electrical force)
Why are charged molecules/ions distributed unequally?
Channels confer selectivity (passive), pumps assist unequal charge distribution (active)
What does the lipid bilayer provide?
A barrier to diffusion, so that we can end up with different concentrations if ions on either side of the membrane (a concentration gradient)
What do electrical fields allow?
Cause ions to move; opposite charges attract and like charges repel. Movement of ions give rise to an electrical current, but how much depends on electrical potential (voltage= force exerted upon a charged particle), electrical conductance and Ohms Law
What are the steps to measure the potential across a neuron?
- connect the neuron to a voltmeter
- insert a glass microelectrode into the neuron and another electrode into the solution surrounding the neuron
(no potential difference within the extracellular solution)
(valve changes between -65 mV and - 90 mV) - Unequal distribution of charge across the neuronal membrane
- Negative resting potential in an absolute requirement for a functioning nervous system
Name two important ion pumps
Na/K ATPase
Ca pumps
What does the Na/K ATPase do?
Exchanges internal Na for extracellular K against their concentration gradients (requires energy: uses up to 70% of ATP in the brain)
What does the Ca pump do?
Transports Ca out of neurons
Why is high intracellular Ca toxic?
Kills neurons
What are four important points regarding the establishment of an equilibrium potential?
- Large changes in membrane potential are caused by miniscule changes in ionic concentration
- Net difference in electrical charge occurs at the inside and outside surfaces of the membrane (the bilayer is so thin (to 5nm) that ions on either side of it interact electrostatically (the net negative charges in the cell is localized to the inner surface of the plasma membrane)
- Ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential (if the membrane potential is 0 and the equilibrium potential is -80 mV, ions will flow more quickly than if the membrane potential is at +60mV)
- If the concentration difference across the membrane is known for an ion, the equilibrium potential can be calculated using the Nernst equation.
The Nernst equation:
Eion=2.303(RT/zF) x log( [ion]o / [ion]i)
Why are K channels a key determinant of the resting potential and neuronal function?
The membrane is highly permeable to K at rest, so changes in K concentration can have big effects. Increasing extracellular K causes a shift in Ek
To estimate real Vm you need the Goldman Equation:
Vm=61.54 x log ( (Pk [K+]o + PNa [Na+]o) / (Pk [K+]i + Pk [Na+]i))
What is ionic driving force proportional to?
Membrane potential
How often can neurons fire action potentials in theory?
1000 AP/s
Where are Ca activated action potentials usually generated in?
Embryonic nervous system
Which domain of Na channels is the voltage sensor?
S4
