L3: Membrane Potentials & Action Potentials

diffusion potentials

the membrane voltage where the concentration gradient and electrical gradient for an ion are equal and opposite, resulting in no net ion movement

resting membrane potential

every cell is electrically active: all pump ions across the cell membranes and maintain an electrical potential difference across the membrane

potential across the membrane at homeostasis, or a cell at rest

reflects a difference in charge on either side of the membrane

normally, inside is negative with respect to the outside

unequal ion distribution, selective membrane permeability, diffusion potentials, sodium potassium pump

how is the resting membrane potential maintained?

unequal ion distribution

cells maintain different ion concentrations inside vs outside the membrane

selective membrane permeability

the membrane is more permeable to some ions than others

much more permeable to K+

mainly because of potassium leak channels (passive ungated)

so K+ diffused out more easily than Na+ diffuses in

diffusion potentials

the membrane voltage at which there is no net movement of a specific ion across the membrane

sodium potassium pump

against the concentration gradient

nerve (neuron) action potentials

rapidly changing electrochemical impulses that quickly spread along the nerve fiber membrane

can be described as rapid, temporary change in a cells resting membrane potential

polarized

at rest, cells are more ______, meaning they are more negative inside the cell than outside

depolarization

action potential begins with a sudden change from the normal RMP (negative value) to a more positive value

this is due to increased Na+ entering the cell through voltage gated channels

repolarization

Na+ channels quickly close, and membrane potential returns to its resting value

the K+ activation gate opens due to depolarization, and K+ exits the cell

how are action potentials elicited?

mechanical disturbances of the membrane, chemical effects on membrane, passage of electricity through the membrane

mechanical disturbances of the membrane

pain and sensory fibers

chemical effects on the membrane

neurotransmitter binding to receptors, open Na+ channels

passage of electricity through the membrane

in heart and intestinal cells, gap junctions

myelin

many large nerve fibers are insulated by ________, formed by Schwann cells wrapped around the axon

they reduce ion movement across the membrane, so action potentials occur only at the Nodes of Ranvier, where ions move between the extracellular fluid and the axon

saltatory conduction

instead of occurring continuously along the entire membrane, the action potential “jumps” node to node

why having myelin on nerves is important

myelin insulated most of the axons and prevents ion movement

greatly increases conduction speed, allowing for rapid signaling and conservation of energy

actin fibers

skeleton of your cell

myosin

grabs on to the actin on both sides and pulls it

this is how your muscle contracts

sarcomere

part of your muscle that has the actin and myosin reacting

t-tubule

invagination of cell membrane (extra cellular fluid inside cell)

sarcoplasmic reticulum

stores calcium (Ca2+)

muscle contractions

they begin when calcium channels in the SR open and release calcium into the cytoplasm

neuromuscular junction

an action potential arrives here

ACh is released, binds to receptors, and opens sodium ion channels, leading to an action potential in sarcolema

action potential travels along the t-tubules

muscle shortens and produces tension

troponin

has a binding site to calcium

this opens it up and the myosin is able to grab on to it

how muscles contract on a molecular level

an action potential occurs

calcium is released from the SR

Ca2+ reveals the binding site for myosin in actin filaments

myosin attaches to actin

frequency summation

occurs when a skeletal muscle is stimulated repeatedly before it has completely relaxed from the previous twitch