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A comprehensive overview of cell membrane transport mechanisms, including diffusion, mediated transport, and epithelial transport. It delves into the intricacies of cardiac electrophysiology, covering the electrical pathway, action potentials in pacemaker and contractile cells, and excitation-contraction coupling. The document also explores the cardiac cycle, cardiac output, and the autonomic nervous system's role in regulating heart function. It is a valuable resource for students studying cell biology and physiology.
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What do cell membranes consist of?
composed of phospholipid bilayer, proteins, cholesterol
What is the purpose of cholesterol?
contributes to fluidity of membrane
Define Diffusion
passive moment from high to low concentration
factors that influence the rate of diffusion through the membrane
what kinds of molecules pass through the membrane easily
nonpolar, small uncharged, hydrophobic
permeability includes...
solubility in membrane, temperature, membrane thickness
rate of diffusion
factors are proportional to each other (C x A x P)
can the permeability of the membrane be changed?
yes, temporarily by channels or permanently by leak channels
movement of ions...
is subject to diffusional and electrical forces
how does membrane potential change?
net movements of ions
SGLT transporter
secondary active transport; substance is moved against gradient using a secondary source of energy, usually a Na+ concentration gradient (lumen to cell)
working together!
transcellular transport
transport through both cell membranes
how does transcellular transport work
apical membrane
faces fluid cavity (lumen), body surfaces
basolateral membrane
later surfaces face other epithelial cells; basal cells face connective tissue
tight junctions
barrier preventing proteins from moving into basolateral membrane; keeps fluid separate in each membrane
what determines the composition of intra and extracellular fluids?
transporters/leak channels (Na/K ATPase and PMCA)
the diffusion gradient/leak channels
how can you make Ek+ more negative?
increase intracellular or decrease extracellular K+, which will increase the chemical driving force and result in more charge separation
equilibrium potential for an ion is the membrane potential at which...
an equilibrium is achieved between its chemical diffusion and electrical attraction/repulsions
to know the equilibrium potential for a particular ion, you need....
the ion's charge sign and its concentration inside and outside the membrane
GHK equation
predicts membrane potential by combining each ion's concentration and permeability
what ways does the ATPase pump contribute to RMP...
gives asymmetry of ion concentrations, creating the membrane potential
can we temporarily change the membrane potential?
yes, by opening gated channels which increases permeability
why do ions move through the membrane?
the electrochemical driving force (the force at which a particular type of ion is driven through a channel)
electrochemical driving force
(mathematically) the equilibrium potential minus RMP (150mV)
electrical path
~internodal pathway~
how are cells of the heart different?
fewer channels mean less action potentials
T-type Ca+ channels
helps further depolariation
L-type Ca+ channels
long lasting; much more permeable to Ca+. somewhat permeable to Na+
VG K+ channels
huge electrochemical driving force that pushes K+ out cell for repolarization
plateau phase (atria and ventricles)
no net ion flux
leak channels and Na/K ATPase pump account for RMP as usual
Ca+ influx
L-type Ca+ channels open causing plateau
K+ efflux
K+ channels open and offset Ca+ influx
excitation-contraction coupling
connects action potentials and contraction (crossbridge cycle and generating tension)
skeletal muscle cell
relaxation
3 pumps for relaxation
what turns these pumps on to begin relaxation?
the availability of Ca+, turned on by contraction
cardiac muscle
the action potentials and tension occur over a much longer period of time
skeletal muscle
action potentials that are close in time can lead to summation of tension
what explains the comparatively long period of tension in cardiac muscle?
sustained release of Ca+ during the plateu phase
why is summation not possible in cardiac muscle cells? why would it harmful?
there is no ability to generate another AP until the refraction period. it would be harmful because there would be no time for ejection or filling
open before ejection and close after ejection
right side of heart
sends deoxygenated blood to the lungs and pulmonary capillaries
left side of heart
receives oxygenated blood from the lungs and pumps it to systemic capillaries and rest of the body
venous return
amount of blood returned to the heart (L/min)
cardiac output is calculated...
HR x SV
autonomic NS controls...
HR, SV, and conduction velocity
in response to...
reflex stimulation (baroreceptors, proprioceptors), higher brain activity (emotions)
hormones control...
HR, SV, and conduction velocity
in response to...
fight or flight response, need to elevate metabolism/temperature
ANS effect are:
Types of adrenergic receptors (bind to NE)
b1 adrenergic receptors
responsible to positive chronotropic effects on sympathetic NS (through Gs system)
what does PKA do?
channel phosphorylation increases Ca+ permeability
what does cAMP do?
increases HCN permeability
big picture
sympathetic stimulation increases permeability of channels by second messenger system (GCPR)
role of hormones stimulated b
adrenal medulla releases epinephrine which binds to b1 receptor
thyroid hormones
T3 can pass through the cell to find an intracellular receptor (THR), which transcribes more b1 receptors
M2 receptor
responsible for negative chronotropic effects on parasympathetic NS
when ACh binds to M2...
alpha inhibatory subunit is released, inhibits adenylyl cyclase, cAMP gets cut off
remaining beta gamma binds to K+ leak channels (increasing permeability would slow pacemaker)
