Chapter 9 and 11 Membrane potentials

Graded or local potentials

1. A graded potential is a small change in the resting membrane potential that is confined to a small area of the plasma membrane 2. an increase in membrane permeability to Na+ can cause graded depolarization and 3.an increase in membrane permeability to K+ or Cl- can result in graded hyperpolarization 4. the term graded potential is used because a stronger stimulus produces a greater potential change than a weaker stimulus 5. graded potentials can summate or add together 6.a graded potential decreases in magnitude as the distance from the stimulation increases 7. a depolarizing graded or loacal potential could lead to an action potential

Depolarization Phase 2

1. Ligand gated Na+ channels open leading to depolarization 2. Voltage gated Na+ channels open 3. When local potential reaches threshold the cell reaches action potential 4. Voltage gated K+ channels begin to open slowly

Differences in concentration are due to

1. Permeability character of membrane: membrane is selectively permeable, K+ ions can diffuse through, but Na+ ions cannot as readily 2. Presence of negatively charged proteins and other anions within cells: negative proteins are large so cannot diffuse out of cell, negative Cl- are repelled so they move out of cell, negative charge within cell attracts K+ and Na+. Only K+ is permeable so high concentration of K+ within cell 3. The sodium- potassium pump: moves ions by active transport, moves 3 Na+ out of cell and 2 K+ into the cell ( does not happen until after an action potential)

Resting Membrane potential 6

1. Voltage gated K+ channels close 2. Na+ K+ pump brings cell back to RMP 3. Voltage gated Na+ channels are closed but capable of opening 4. Decreased permeability to Na+

At Peak 3

1. Voltage gated Na+ channels close and are incapable of opening 2.Decrease permeability to Na+ 3. Voltage gated K+ channels begin to open slowly

strenght of stimuli affects the frequency of action potentials

1. a subthreshold stimulus produces only a local potential 2. a threshold stimulus casuses a graded potential that reaches threshold and results in a single action potential 3. a submaximal stimulus is greater than a threshold stimulus and weaker than a maximal stimulus. the action potential frequency increases as the strength of the submaximal stimulus increases 4. a maximal or supramaximalstimulus produces a maximum frequency of action potentials

Propagation of action potentials

1. an action potential generates local currents, which stimulate voltage gated Na+ channels in adjacent regions of the plasma membrane to open, producing a new action potential 2. in an unmyelinated axon, action potentails are generated immediately adjacent to previous action potentials 3. In a myelinated axon, action potentials are generated at successive nodes of ranvier 4. reversal of the direction of action potential propagation is prevented by the absolute refractory period 5. action potentials propagate most rapidly in myelinated, large diameter axons

Action Potentials

1. an action potential is a larger change in the resting membrane potential that spreads over the entire surface of the cell 2. threshold 3. occur in an all-or-none fashion 4. depolarization occurs as the inside of the membrane becomes more positive because Na+ ions diffuse into the cell through voltage-gated ion channels 5. Repolarization 6. Afterpotential

Chemical Synapses

1. anatomically a chemical synapse has three components 2. an action potential arriving at the presynaptic terminal causes the release of a neurotransmitter, which diffuses across the synpatic cleft and binds to the receptors of the desynaptic membrane. this can result in depolarization if Na+ channels open or hyperpolarization if K+ or Cl- channels open

Electrical Synapses

1. are gap junctions in which tubular proteins called connexons allow local currents to move between cells 2. at an electrical synapse, an action potential in one generates a local current that causes an action potential in an adjacent cell

three anatomical components of a chemical synapse

1. the enlarged ends of the axon are presynaptic terminals containing synaptic vesicles 2. the postsynaptic membranes contain receptors for the neurotransmitter 3. the synaptic cleft is a space separating the presynaptic and postsynaptic membranes.

Action potential frequency

1. the strength of stimuli affects the frequency of action potentials 2. a low frequency of action potentials represents a weaker stimulus than a high frequency

Characteristics responsible for resting membrane potential

1. there are equal number of charge molecules and ions inside and outside of cell 2. higher concentration of K+ in than out of cell, and higher concentration of Na+ outside than inside of cell 3.Plasma membrane is 50 to 100 times more permeable to K+ than to Na+ B/C of passive K+ channels 4.Plasma membrane is impermeable to negatively charged protein 5. K+ ions diffuse from inside to outside of cell through concentration gradient to maintain equilibrium 6. Since negative protein cannot leave cell small negative charge develops just inside the plasma membrane 7.Negative charge inside attracts K+ and Na+ ions 8.Only K+ can go into cell, electrical gradient, passive K+ channels 9. when negative charge inside of cell is great enough to prevent additional K+ from diffusing out of cell, equilibrium is established 10.The negative charge inside cell membrane at equilibrium is called resting membrane potential 11. the resting membrane potential is a charge difference across the plasma membrane when the cell is in an unstimulated condition. the inside of the cell is negatively charged compared to the outside of the cell 12. the resting membrane potential is due mainly to the tendency of positively charged K+ ions to diffuse out of the cell, which is opposed by the negative charge that develops inside the plasma membrane

end of repolarization and after-potential 5

1. voltage gated Na+ channels are closed but capable of opening 2. Decreased permeability to Na+ 3. K+ channels begin to close however are slow 4. More K+ leaving cell produces afterpotential or temporary hyperpolarization

Resting Membrane Potential 1

1. voltage gated Na+ channels are closed but capable of opening 2. voltage gated K+ channels are closed

Repolarization 4

1. voltage gated Na+ channels close and are incapable of opening 2. decreased permeability to Na+ 3. voltage gated K+ channels open

The synapse

Electrical and Chemical

Ion channels

Passive: always open Active or gated: open and closed in response to stimuli, 3 states. -open: activated- -closed but capable of opening -closded and incapable of opening

Resting membrane potential

Slight negative develops on inside of membrane The transmembrane potentisal in an undisturbed cell varies by cell type. Skeletal muscle: -85 mV cardiac muscle: -90mV neuron: -70mV

Afterpotential

a breif period of hyperpolarization following repolarization

depolarization

a decrease in the resting membrane potential

Gates open

activation and inactivation are open

Gates closed and capable of opening

activation gate is open or closed and inactivation is open

Na+

always comes into cell when channel is open

K+

always goes out of cell when channel is open

hyperpolarization

an increase in resting membrane potential

Charge across membrane

cell membrane has excess positive charge outside and negative charge inside

factors affecting resting membrane potential

concentration difference of K+ across membrane Change in permeability to K+ and Na+ ions Stimuli - temperature, pressure, chemicals etc

Relative refractory period

follows the absolute refractory period and is the time during which a stronger than threshold stimulus can evoke another action potential

Gates closed and incapable of opening

inactivation gate is closed

Concentration differences across membrane

intracellular and extracellular fluid have different concentraton of cations and anions but are electrically neutral

movement of ions through membrane

ion channels Na+ and K+ pump

Repolarization

is a return of the membrane potential toward the resting state. It occurs because voltage-gated Na+ channels close and Na+ diffusion into the cell slows to resting levels and because voltage-gated K+ channels continue to open and K+ diffuse out of the cell

Threshold

is the membrane potential at which a graded potential depolarizes the plasma membrane sufficiently to produce an action potential

absolute refractory period

is the time during an action potential when a second stimulus, no matter how strong, cannot initiate another action potential

two types of active or gated channels

ligand gated channels: chemicals can bind to receptors on gating proteins causing the channels to open or close. Ex: Arch Voltage-sensitive channels: gating proteins open or close channels in response to the charge across the membrane

Na+ Ka+ exchange pump

maintains Na+ and K+ concnetration across the plasma membrane -it moves 3 Na+ out of cell and brings 2 K+ into cell -moves ions against their concentration gradient (needs ATP)

Electrical properties of cells

result from the ionic concentration differences across the plasma membrane and from the permeability characteristics of the plasma membrane

Potential difference

when negative and positive ions are kept apart, potential difference exists. unit of measurement is volts. since charges are seperated by membrane it is know as trans-membrane potential