cell excitability Flashcards
what is the nervous system?
a system of communication that allows an organism to react rapidly and modifiably to changes in its environment
how do neurons function?
- collect information
- integrate information
- produce an output
what are electrical signals that neurons produce?
- action potentials: fixed size, all or nothing signals that propagate along axon, usually unidirectional
- graded potentials: variable size, local signals not propagated along long distances, pass both ways along neuronal membrane
how do neurons encode information?
APs are coded by frequency as they are of a unit size
graded potentials are coded by size and vary according to strength of stimulus
what is resting membrane potential of neuron?
-70mv
negative membrane potential is a requirement for a functioning nervous system
why do neurons have a resting potential?
inevitable consequence of:
- selectively permeable membrane
- unequal distribution of charged molecules/ions
- physical forces
why are neurons selective and unequal?
- ion channels (selectively permeable)
- ion pumps (assist unequal charge distribution)
what are the physical forces controlling ion movements?
diffusion and electrical
examples of important ion pumps
Na+/K+ ATPase (creates high conc of Na+ outside membrane and high conc of K+ inside membrane)
Ca2+ pumps (creates gradient of Ca2+ across plasmamembrane and other membranes)
how are equilibrium potentials established?
ionic gradient influence membrane potential by determining equilibrium potential
diffusion across the membrane establishes electrical charges on both sides of the membrane, eventually the charges equillibriate n
what is equilibrium potential?
the electrical potential difference across the cell that exactly balances the conc gradient for an ion
electrostatic forces = diffusional forces
how to calculate equilibrium potential for an ion?
nernst equation
what ions is the neuronal membrane permeable to at rest?
very permeable to K+ (slightly permeable to others)
is equilibrium potential for the main ions positive or negative?
K+ = negative
Na+ = positive
Ca2+ = positive
Cl- = negative
how to calculate membrane potential?
the goldman equation
what value does membrane potential reach during an action potential?
up to 40mV
how long does an action potential last?
2 - 4 ms
limitation on the frequency of action potentials
properties of action potentials?
- transient (up and down), rapid and reversible change in membrane potential from -ve to +ve
- different types of cell may have different types of action potentials
what is membrane permeable to at rest?
mainly permeable to K+ (leaky potassium channels)
what is membrane permeable to during depolarization?
permeable to Na+ (sodium channels open)
what is membrane permeable to during repolarization?
permeable to K+ (sodium channels close, potassium channels open)
structure of voltage-gated Na+ channels
- consists of 4 subunits
- several transmembrane domains
- one subunit crosses the membrane 6 times
- loop creates distance between transmembrane domains s5 and s6
- s4 contains many positively charged amino acids, it is the voltage sensor which sense the change in membrane potential
how to voltage-gated Na+ channels open in response to depolarization?
- change in membrane potential causes the positive charges of the polypeptide chain to move from one side of the membrane to the other
- causing conformational changes of the entire protein and opens the pore
how does Na+ channel inactivation occur?
- occurs quickly ~1ms
- de-inactivation must occur before channels can be activated again
how can poisons which implicate ion channels be useful?
studying ion channels, toxins which block specific ion channels can be used to see if activity of neuron is effective
what are the factors influencing conduction velocity (speed of AP propagation)?
diameter of axon
myelination
how does diameter of axon influence conduction velocity?
the larger the diameter, the faster the speed of action potential
resistance to current flow is inversely proportional to cross-sectional area of the axon
how does myelination influence conduction velocity?
myelinated axon increases speed of action potential
prevents current loss along the axon (from leaky channels)
why are there so many unmyelinated small axons?
- the space constant is equal to the ratio of membrane resistance and resistance inside axons, so benefit of a high membrane resistance is reduced by the high internal resistance
- metabolic and volume costs of myelination
how do dendrites encode information?
graded potentials (action potentials are initiated at axon hillock)
how do action potentials encode information?
by frequency of action potentials
high stimulus intensity = high frequency of action potentials
what are the types of graded potentials?
excitatory (depolarizing) or inhibitory (hyperpolarizing)
what is GABA?
GABA in a inhibitory neurotransmitter
GABA receptors are selective for chloride which hyperpolarizes the membrane
what is glutamate?
glutamate is an excitatory neurotransmitter
glutamate receptors are selective for sodium which depolarizes the membrane
what are electrical synapses?
junctions in which the positive and negative ions can travel freely between these gaps and move to the neighboring neurons (rapid)
they can be seen using fluorescent dye
