The Excitable Cell Flashcards
Neurones must..?
- Collect
- Integrate
- Output
What does the nervous system do?
System of communication that allows an organism to react rapidly and modifiably to changes in its environment
Electrical activity provides a ____, _____, and _____ means for neurons to receive, integrate and transmit signals
- Rapid
- Reliable
- Flexible
Electrical signals may be divided into…
- Action potentials
- Graded potentials
Action potential
- Fixed size
- All-or-nothing
- Propagate the axon
- Can pass both ways but tend to go only one way
Graded potential
- Variable size
- Local signals not propagated over long distances
- Pass both ways along neuronal membrane
Action potentials are coded by…
Frequency
Graded potentials are coded by…
Size
Why do neurons have a resting potential?
Consequence of..
- Selectively permeable membrane
- Unequal distribution of charged molecules/ions
- Physical forces
Channels….
Confer selectivity
Pumps assist…
Unequal charge distribution
What is the resting potential of a membrane?
Between -65mV and -90mV
What is an absolute requirement for a functioning nervous system?
A negative Vm
Important ion pumps
- Na+/K+ ATPase
- Ca2+ pumps
Na+/K+ ATPase
- Exchanges extracellular K+ for intracellular Na+
- Against concentration gradient - uses ATP
- Uses about 70% brain ATP
Ca2+ pump
- Transports Ca2+ out of neurons
- Maintains low intracellular Ca2+
- Ca2+ is toxic at high intracellular levels
Intracellular K+
100 mM
Extracellular K+
5 mM
Intracellular Na+
15 mM
Extracellular Na+
150 mM
Intracellular Ca2+
0.0002 mM
Extracellular Ca2+
2 mM
Intracellular Cl-
13
Extracellular Cl-
150
Increasing extracellular K+ causes the membrane to become…
Depolarised (more positive)
The Nernst Equation
Used to calculate the equilibrium potential (Eion) for an ion
The Goldman Equation
Used to estimate real Vm (combined potentials)
Properties of the action potential
- Transient, rapid and reversible change to membrane potential
- -ve to +ve
- Different types of excitable cell have different types of AP
- Neuron AP triggered by Na+ permeability increase
- All same size and duration
- Don’t decrease down the axon
Factors influencing conduction velocity of an axon
- Diameter (larger diameter = faster)
- Permeability of membrane (leaky)
- Insulation (myelin vs non-myelin)
Why does diameter affect Cv?
Resistance to current flow is inversely proportional to cross-sectional area of the axon
Why does myelination affect Cv?
It prevents current loss along the axon by increasing membrane resistance (Rm) and the Space Constant
Why do we have so many unmyelinated, small axons?
- Benefit of high membrane resistance is reduced by the high internal resistance
- Metabolic and volume costs of myelination
The Space Constant
The distance from the site of depolarisation where it has fallen to 37%
Cv of smallest unmyelinated axons? (0.2 - 1.5 micrometers)
0.5 - 2 m/s
Cv of most axons > 1.0 micrometers?
5 - 120 m/s
Cv of squid giant axon?
25 m/s
What does lidocaine act on?
Poisons Na+ channels
What does tetraethylammonium (TEA) act on?
K+ channels
What does tetrodotoxin (TTX) act on?
Na+ channels
What do saxitoxins (STX) act on?
Na+ channels
What do dinoflagellates act on?
Na+ channels
What type of conduction does myelination result in?
Saltatory conduction
Absolute refractory period
Neuron is incapable of generating another action potential no matter what
Relative refractory period
A stronger stimulus is able to cause an action potential as the threshold is raised
Graded potentials can be ____ or _____
Excitatory
Inhibitory
What are graded potentials caused by?
Opening of neurotransmitter-gated ion channels
OR
Opening/closing of K+ channels
Electrical synapses
- Retinal neuronal
- Cardiac muscle
- Smooth muscle
- Rapid
- Two way?
