Lecture 8: the excitable cell part 2 Flashcards
What two factors affect Na channel inactivation?
Time, voltage
How long does it take to inactivate a Na channel?
1 ms
What is the absolute refractory period?
The time it takes to repolarise
What is the relative refractory period?
The time while voltage-gated K channels remain open
What are useful poisons for K channels?
Tetrathylammonium, TEA
What are useful poisons for Na channels?
Lidocaine, tetradotoxin, TTX (pufferfish), saxitoxins, STXm dinoflagellates
What factors affect conduction velocity?
Diameter and leaky permeability of the membrane
What kind of axons tend to be of larger diameter?
Axons required for communication of life-threatening information i.e. motor and some sensory nerves
Why is myelination useful for accelerating conduction velocity?
It prevents current loss along the axon by increasing membrane resistance and increases the space constant. Space constant is distance from site of depolarization were it has fallen to 37%
Why are there still many unmyelinated axons?
The space constant is almost equal to Rm/Ri so the benefit of a high membrane resistance is reduced by the high internal resistance
Metabolic and volume costs of myelination are high
What is the conduction velocity of the smallest unmyelinated axons?
0.5 - 2 m/s
What is the conduction velocity of most axons (over 1.0 micrometers) which are myelinated?
5 - 120 m/s
What is the conduction velocity of the squid giant axon?
25 m/s
What is the difference between axons and dendrites?
Dendrites have voltage-sensitive channels but don’t usually produce action potentials. Dendrites operate graded potentials while axons operate action potentials.
What are the two summations found in graded potentials?
Temporal and spatial
What is the difference between temporal summation and spatial summation?
Temporal : multiple action potentials on an axon travels to one neurons
Spatial: one action potential each on multiple axons travel to one neuron
Which senses rely on spatial summation?
Visual and olfactory
What is “shunting”?
When neurons contradict each other, then there will be nullification by inhibitory inputs
What can shunting be caused by?
Opening of non-selective cation channels in the axon membrane
What kind of neurons/muscles use electrical synapses?
Retinal neurons, few other adult CNS neurons (glial junctions), cardiac muscle, smooth muscle
