Action Potential

refractory period

"downtime" when a second action potential cannot be initiated. limits maximum frequency at which action potentials can be generated and ensures all signals travel down axon in one direction (cell body to axon terminal)

hyperpolarization

Caused by opening of potassium channels in a resting neuron. It is the change in a cell's membrane potential such that the membrane becomes more negative relative to the outside, reducing the change a neuron will transmit a nerve impulse

Undershoot

Membrane permeability to K+ is higher than at rest, membrane potential closer to E(k) than at resting potential

Nodes of Ranvier

gaps in the myelin sheath where voltage gated sodium channels are restricted to. Extracellular fluid contacts axon membrane only at these pointed. Action potentials are not generated in regions between these.

Epilepsy

groups of nerve cells fire simultaneously and excessively to produce seizure when there are mutations affecting sodium channels in brain

threshold

particular value of voltage that when reached causes an action potential during depolarization (~55mV in mammals)

Myotonia

periodic spasming of skeletal muscles caused by mutations affecting voltage-gated sodium channels in skeletal muscles

Voltage Gated Ion Channels

cause action potentials to arise when ion channels in neurons open/close when membrane potential passes a particular level. Depolarization opens voltage gated sodium channels, which results in further depolarization and opening of more sodium gated channels.

Effect of Rate of Action Potential

conveys information about strength of input signal. Example: Hearing, louder, more frequent action potentials

Myelin Sheath

electrical insulation surrounding vertebrate axons. produced by glia cells (oligodendrocytes in CNS, Schwann cells in PNS). Wraps axon in layers of lipid membrane to insulate

graded potential

response to hyperpolarization/depolarization is a shift in membrane potential. Magnitude varies with strength of stimulus (large stimulus = greater change). Induces small electrical current that leaks out of neuron when flowing along membrane which decays with time/distance from source

space efficiency

selective advantage of myelination

depolarization

the reduction in the magnitude of the membrane potential. Involves gated sodium channels in neuron. Stimulus causes gated channels to open, membrane permeability to sodium increases causing sodium to diffuse along concentration gradient

Conduction of Action Potentials

1. Action potential initiated at axon hillock 2. Sodium inflow during rising phase creates electrical current, depolarizes neighboring region of axon membrane 3. depolarization reaches threshold, causing action potential in neighboring region 4. process repeats along axon (magnitude/duration same at each position on axon) 5. Behind traveling zone of depolarization due to Na+ inflow, there is a zone of repolarization caused by K+ outflow (inactivates Na+ channels)

5 Steps of Voltage-Gated Channels shaping Action Potentials

1. Membrane of axon is at resting potential, voltage gated sodium channels are closed and some potassium open but voltage-gated potassium channels closed. 2. Stimulus depolarizes membrane, some gated sodium channels open - causes further depolarization and more sodium gates to open and sodium diffuses into cell. 3. Threshold is crossed, positive feedback of sodium gates opening rapidly brings membrane potential close to E(Na) 4. Two events prevent E(Na) achievement: First, voltage gated sodium channel inactivated after opening to stop Na flow. Second, most voltage gated potassium channels open to cause rapid outflow of K+ and bring membrane potential back to E(K) 5. Final phase of action potential - undershoot - membrane's permeability to K+ is higher than at resting potential, gated potassium channels eventually close and membrane potential reaches resting potential.

Sodium and Potassium Channel functions in depolarization

Sodium open first, as action potential proceeds they close Potassium channels open slowly but remain open and function till end

Falling Phase

When sodium inflow is reduced to prevent membrane potential from achieving E(Na). Two events: 1) voltage gated sodium channels inactivated 2) voltage gated potassium channels open to bring membrane potential back to E(k)

Action Potential

a great enough shift in membrane potential that results in massive change in membrane voltage. Have constant magnitude. Can regenerate in adjacent regions of membrane and spread along axons. Magnitude is independent of strength of triggering stimulus (all or none stimulus)

Saltatory Conduction

action potentials jumping along axon from node to node

electrical insulation

allows vertebrates to have narrow diameter axons but conduct action potentials at high speed. Caused depolarization current to travel further along axon interior.