excitable cells Flashcards
what is an electrical event
a mechanism of cell-to-cell communication/sensing environmental changes/triggering intracellular events
extracellular fluid properties
similar to PLASMA in ionic composition (mostly proteins and ions) and has a high [NaCl]
predominant cation in the ecf
Na+
intracellular fluid properties
very HIGH [PROTEIN] (which give it a net negative charge), predominant salt is KCl
predominant cation of the icf
K+
membrane properties
lipid bilayer and proteins. equal amounts of protein and lipid and small amount of carbohydrates
define permeability
ability of the ion to cross the membrane
changes in permeability are (2)
- ion-specific
2. timed
simple diffusion
random diffusion down an electrical or concentration gradient (organic molecules or ions)
non-polar organic molecules undergo simple diffusion
rapidly through the membrane. no energy required. Ex. O2, CO2, fatty acids, steroid hormones
ions undergo simple diffusion
through channels that are ion-specific
define flux
amount of substance crossing a surface per unit of time
movement of ions/molecules b/w compartments is always
bidirectional
define net flux
difference b/w the two unidirectional fluxes. when = to 0, the system is at “diffusion equilibrium”
2 types of mediated transport
- facilitated diffusion
2. active transport
facilitated diffusion
no energy required. membrane protein acts as a carrier to move molecule across membrane. large/polar molecules (ex. glucose)
active transport
requires energy. molecule being transported must bind to a transporter and move up its concentration/electrical gradient
in active transport, energy can affect:
- the affinity of the transporter for the ligand on one side of the membrane
- the rate of transporter conformational change
primary active transport
energy source is hydrolysis of ATP, then the transporter gets phosphorylated, then the transporter changes its affinity for the molecule, and this increases the transport rate
secondary active transport
energy source is in ion concentration gradient across membrane (down its gradient) . the transporter has 2 binding sites.
what are the 3 factors that determine the rate of flux
- number of transporters in membrane
- extent of transporter saturation
- rate of transporter conformational change
how is transporter saturation affected
- transporter affinity for the ligand
2. ligand concentration
types of channels
- ligand-sensitive
- voltage-sensitive
- mechano-sensitive
osmosis
bulk flow of water across a membrane. water (polar) diffuses through channels down its concentration gradient.
osmolarity
total solute concentration in a solution (1 mole of NaCl=2.0 osmoles)
isotonic
no change in cell volume
hypotonic
cells swell
hypertonic
cells shrink
endocytosis
engulfment of fluid and particles from the ecm. includes pinocytosis (small particles w/ or w/o small vol of ecf) and phagocytosis (large particles/debris)
pinocytosis is performed by what cell type
all cell types
phagocytosis is performed by what cell type
phagocytes
exocytosis
export of material from a cell…secretion
voltage
charge difference b/w the inside and outside of a cell. steady in a resting cell.
resistance
plasma membrane must control the voltage and therefore must resist the ion movement
current
when R changes, ion movement occurs and this movement is current
conductance
reciprocal of resistance….g=1/r
conductance and resistance are both
membrane properties
Ohm’s law
V=IR
nernst equation
yields the equilibrium potential of a single ionic species. the equilibrium potential is the voltage across a cell membrane that exactly balances the force in the [ ] gradient of a permeable ion. theoretical membrane potential.
transient diffusion potential
asymmetric ion flow
steady diffusion potential
the electrical gradient can resist the movement of an ion down its [ ] gradient to reach an equilibrium potential
the membrane of a resting cell is most permeable to
K+….when cell is at rest, K+ ions diffuse 60x more than do Na+ ions
membrane potential vs. Ek
m.p= -70–90mV….Ek=-100mV. this is bc both Na and K are moving down their [ ] gradients. Na makes the cell more positive and keeps the m.p slightly above (more positive than) Ek
the resting membrane potential is
a STEADY diffusion potential (not transient). not due to instantaneous movement of ions but due to ions (K+) that has moved across the membrane…which causes Na to move and balance it
the Na/K ATPase pump is responsible for
moving K into the cell and Na out of the cell when it is at rest. hydrolyzes ATP for active transport of ions (up their [ ] grads). maintains [ ] grads for Na and K…replaces the Na and K that have diffused across the membrane at rest (down their [ ] grads)
poisoning the pump (slowing it down) causes
a slow decrease in the r.m.p and the m.p moves towards 0 making it more (+) charged.
depolarization
toward 0mV….getting more +
repolarization
towards resting potential (-70mV)
hyperpolarization
increase the m.p…getting more -
without the Na/K pump
r.m.p would dissipate to 0mV
what is an action potential
large transient change in membrane potential….first occurs in the INITIAL SEGMENT of neuron
steps of an a.p
- increase in P(Na)
- Na into cell via voltage-regulate channels…gNa increases
- m.p depolarizes towards 0mV
- threshhold is reached….more Na channels open (voltage regulated)
- more Na enters the cell (positive feedback cycle)…RISING PHASE
- m.p rapidly reaches peak at +40mV
- K channels (voltage gated) open/ P(K) increases and K flows out of the cell. gK increases.
- m.p rapidly/abuptly reverses to resting potential….FALLING PHASE. Na channels close here = Na inactivation
- hyperpolarization
hyperpolarization vs rising and falling phases
depolarization and repolarization are usually ~1ms where hyperpolarization can last >10ms
latent period
b/w applying the stimulus and depolarization. precedes the foot of the AP. channel gates are opening.
why does the peak of the a.p stop at ~40mV
peak of A.P approaches E(Na) which is ~58mV. P(Na) is very high during the rising phase and crosses the membrane until its ionic gradient is at equilibrium with m.p.
at the end of repolarization, m.p is close to
E(K)….P(K) is very high during the falling phase of the A.P.
a subthreshold stimulus results in
a local response in which a small depolarization/hyperpolarization occurs and is confined to the immediate region of the membrane. a GRADED RESPONSE that is proportional to the stimulus strength.
threshold/suprathreshold stimuli result in
A.Ps!…all or none property.
absolute refractory period
period when a second stimulus (threshold OR suprathreshold) cannot elicit a second AP
relative refractory period
for a longer period after an AP where a suprathreshold can elicit a second AP, but a threshold stimulus CANNOT.
refractory periods cause
a limit on the fq at which cells can fire APs.
tetrodotoxin
puffer fish toxin that binds to Na channels and blocks Na influx…no AP can occur
adaptation
property of NERVE CELLS. need “square wave” stimulus to elicit an A.P. due to the ACCOMMODATION of channels
accommodation
property of CHANNELS…gates of channels won’t open unless stimulus is quickly applied
strength/intensity vs duration
decreased intensity requires increased duration
rheobase
magnitude of the least intense stimulus that can elicit a response
utilization time
duration required to elicit a response by a stimulus with a rheobase magnitude
chronaxie
duration required to elicit a response by a stimulus with a magnitude that is twice the rheobase magnitude. used to compare the excitability of different cells.
electrotonic or local currents
occur at immediate site of stimulation. passive bc they don’t propgate. if sufficient magnitude, lead to an AP
schwann cells
produce myelin by wrapping layers of plasma membrane around an axon and creating nodes of ranvier
saltatory conduction
APs only generate at the nodes…increases the velocity of AP propogation.
breakdown of myelin causes
gross motor abnormalities
axons with larger diameter have
higher velocities of conduction
compound action potentials
peripheral nerves have many neurons. the further the recording electrodes are from the stimulating electorde, the more the individual APs are separated from each other.
