Lecture 8: the excitable cell

Question 1

What is the nervous system?

Answer 1

System of communication that allows an organism to react rapidly and modifiably to change in its environment

Question 2

What must neurons do?

Answer 2

Collect, integrate and output

Question 3

How fast do electric signals travel along the leg?

Answer 3

2m/s

Question 4

What provides a rapid, reliable and flexible means for neurons to receive, integrate and transmit signals?

Answer 4

Electrical activity

Question 5

What provide more flexibility?

Answer 5

Chemical messengers ( and receptors) between and within cells

Question 6

Is glutamate excitatory or inhibitory?

Answer 6

Excitatory

Question 7

Is GABA excitatory or inhibitory?

Answer 7

Inhibitory

Question 8

What are the two main types of electrical potentials?

Answer 8

Graded and Action

Question 9

What is an action potential?

Answer 9

Fixed size, all-or-nothing signals that travel along the axon. Can travel long distances in short time

Question 10

What is a graded potential?

Answer 10

Variable size, local signals not propagated over long distances. Localized in dendrites

Question 11

What are Action potentials coded by and why?

Answer 11

Frequency; are of a unit size (level of neurotransmitter, level of stimulus)

Question 12

What are graded potentials coded by and why?

Answer 12

Size; vary according to the strength of the stimulus

Question 13

Which potential, action or graded, tend to travel along one way?

Answer 13

Action

Question 14

Why do neurons have a resting potential?

Answer 14

Inevitable consequence of: selectively permeable membrane, unequal distribution of charged molecules/ions, physical factors (water, NaCl, K, Ca) and physical forces (diffusion, electrical force)

Question 15

Why are charged molecules/ions distributed unequally?

Answer 15

Channels confer selectivity (passive), pumps assist unequal charge distribution (active)

Question 16

What does the lipid bilayer provide?

Answer 16

A barrier to diffusion, so that we can end up with different concentrations if ions on either side of the membrane (a concentration gradient)

Question 17

What do electrical fields allow?

Answer 17

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

Question 18

What are the steps to measure the potential across a neuron?

Answer 18

1. connect the neuron to a voltmeter
2. 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)
3. Unequal distribution of charge across the neuronal membrane
4. Negative resting potential in an absolute requirement for a functioning nervous system

Question 19

Name two important ion pumps

Answer 19

Na/K ATPase

Ca pumps

Question 20

What does the Na/K ATPase do?

Answer 20

Exchanges internal Na for extracellular K against their concentration gradients (requires energy: uses up to 70% of ATP in the brain)

Question 21

What does the Ca pump do?

Answer 21

Transports Ca out of neurons

Question 22

Why is high intracellular Ca toxic?

Answer 22

Kills neurons

Question 23

What are four important points regarding the establishment of an equilibrium potential?

Answer 23

1. Large changes in membrane potential are caused by miniscule changes in ionic concentration
2. 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)
3. 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)
4. If the concentration difference across the membrane is known for an ion, the equilibrium potential can be calculated using the Nernst equation.

Question 24

The Nernst equation:

Answer 24

Eion=2.303(RT/zF) x log( [ion]o / [ion]i)

Question 25

Why are K channels a key determinant of the resting potential and neuronal function?

Answer 25

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

Question 26

To estimate real Vm you need the Goldman Equation:

Answer 26

Vm=61.54 x log ( (Pk [K+]o + PNa [Na+]o) / (Pk [K+]i + Pk [Na+]i))

Question 27

What is ionic driving force proportional to?

Answer 27

Membrane potential

Question 28

How often can neurons fire action potentials in theory?

Answer 28

1000 AP/s

Question 29

Where are Ca activated action potentials usually generated in?

Answer 29

Embryonic nervous system

Question 30

Which domain of Na channels is the voltage sensor?

Answer 30

S4