Graded and action potentials

important to remember about gradient

It is never destroyed!! In the overall scheme of things, it is just a really little change

myelin forming cells

Schwann cells in PNS and oligodendrocytes in the CNS.

myelin

a substance composed of multiple concentric layers of phospholipid membrane. Speeds up signal transmission

saltatory conduction

action potential jumps from cell to cell. Nodes of Ranvier are only cells that have lots of voltage gated Na channels. Conduction not slowed by channel openings so can fly down mylenated cells to next node

action potentials

brief, large depolarizations that travel for long distances through a neuron without losing strength.

why can graded potentials lost strength?

current leak: membrane of neuron cell body has open leak channels that allow + charge to leak out into extracellular fluid. This diminishes depolarization. Also can have cytoplasmic resistance.

excitatory vs. inhibitory

excitatory: depolarization and inhibitory: hyperpolarization

Graded potentials depend on

how much sodium enters the cell

key difference between graded and action potentials

if two stimuli arrive at dendrites, they can be added together. However, not for action potentials. If two suprathreshold graded potentials reach the action potential trigger zone within the absolute refractory period, the secon dgraded potential has no effect because the Na+ channels are inactivated and cannot open yet.

trigger zone

integrating center for neuron. High concentration of voltage-gated Na_ channel. If graded potential depolarizes the membrane to threshold voltage, then channels open and AP begins.

more about graded potentials

occur in dendrites/cell body. Size is directly proportional to strength of triggering event. All three kinds of channels are involved and can be depolarizing or hyperpolarizing. Can be summed.

why are myelin cells faster

the cells are insulated from leaking charges.

graded potentials

variable-strength signals that travel over short distances and lose strength as they travel through the cell.