Neuronal Physiology (Action Potential)

resting potential

The difference in electric charge between the inside and outside of an excitable neuron's cell membrane. Cell membrane is said to be polarized.

absolute refractory period

A period of time following an action potential during which no additional action potential can be evoked regardless of the level of stimulation. (usually because Na+ channel closed whle K+ efflux). Once Na channels have been open already, they cannot be stimulated to be open again with further stimulation.

action potential (6)

With the voltage gated K+ channels open, the membrane begins to repolarize back towards its resting potential. K moves from ICF to ECF

nerve impulse (action potential)

a self-propagating (electrical) wave of electro-negativity that sweeps over the surface of a neuron's membrane

cause of membrane potential (pump)

activity of the Na/K pump which contributes approximately 20%; other electrogenic pumps

repolarization

after Na+ ions have rushed into the cell, K+ ions rush out of the cell to restore the balance and the original polarity. Follows behind depolarization. This would be termed a

cardiac arrhythmias

formation of these is caused by mutations in the sodium channels

effect of scorpion venom

holds Na channels open and closes K channels. This is an effect of...

Type B- Lightly myelinated (rate action potential conduction)

intermediate diameter. capable of conduction velocities of 15m/s

action potential (1)

resting membrane potential -70mV

action potential (2)

stimulus is received by dendrites and/or cell body of a nerve. Stimulates depolarization

refractory period

that time period when a new action potential cannot be started. These time periods limit the number of action potentials that can be produced in a given time period. Neuron cannot have another impulse.

all or none law

the membrane responds to stimulus with a maximal action potential which spreads without reduction throughout the entire membrane or it does not respond at all.

one way propagation

the refractory period that ensures that the action potential travels down the axon AWAY from the original site of activation

if threshold potential is not reached at -55mV

there is no action potential

unmyelinated neuron conduction velocity

travels at 0.5m/sec. Cerebral cortex is an example.

myelinated neuron conduction velocity

travels at 110m/sec (ex: football field)

cause of membrane potential (unequal)

unequal distribution of ions between the intracellular fluid (ICF) and extra cellular fluid (ECF) compartments

action potential (8)

voltage gated K channels close

action potential (3)

voltage gated Na+ channels open. In multipolar neurons at axon hillock. If the opening is sufficient to drive interior potential from -70mV up to -55mV (threshold) process continues.

action potential (5)

At 30mV, the Na+ channels close and the voltage gated K+ channels continue to open. Since the K+ channels are much slower to open. Signals repolarization

action potential (9)

Membrane returns to resting state of -70mV. The Na/K pumps continue to pump sodium out of the cell and K in to help maintain the resting membrane potential.

action potential (4)

Once threshold voltage is reached, more voltage gated Na channels open. The Na influx of ions drives the interior of the cell membrane up to +30mV. Slow K channels start to open. Now called depolarization.

saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane Depolarization occurs at nodes and is very fast (110m/sec). Uses less ATP than contiguous conduction since the Na/K pump does not have to work as hard in the insulated (myelinated) areas

action potential (7)

Repolarization overshoots (goes below) the resting potential to about -90mV called hyperpolarization. Hyperpolaization prevents the neuron from receiving another stimulus. Raises the threshold for any new stimulus. Helps insure that the impulse is unidirectional.

Type C - Unmyelinated

Smallest diameter with a conduction velocity from 0.5-1.0 m/s

Weak stimulus and graded potential

Starts above threshold, but decreases in strength as it travels through cell body. At the axon hillock (trigger zone), it is below threshold and does not initiate an action potential.

Strong stimulus and graded potential

Stimulates at the same point on cell body and creates a graded potential that is above threshold by the time it reaches the trigger zone. An action potential is a result.

sodium channel info

The actual number of Na+ ions is 1/100,000th of the total number of Na+ ions. Approximately 2000 ions move through each channel. Approximately 20 sodium channels per square micrometer in non-myelinated neurons and 10,000 per square micrometer at the nodes of myelinated neurons.

hyperpolarization

A change in the axon membrane potential from -70 mV to -90 mV would be termed a(n)

relative refractory period

The period of time following an action potential when it is possible, but difficult, for the neuron to fire a second (If the triggering event is much stronger than normal.) action potential due to the fact that membrane is further from threshold potential (hyperpolarized).

depolarization

The process during the action potential when sodium is rushing into the cell causing the interior to become less negative (more positive). This would be termed as

Type A - Somatic Sensory and Motor Neurons (rate action potential conduction)

largest diameter with thick myelin sheaths. Capable of conduction velocities up to 120m/s and high rate of impulse conduction

graded potentials

local changes in membrane potentials that are short-lived. Confined to small local regions of the plasma membrane and die out within 1-2mm of their site of origin.

effect of local anesthetics

novocaine and lidocaine prevent openings of the Na channels which inhibit action potentials (dentists use). This is an effect of...

effect of general anesthetics

open K channels too much. K exists as fast as Na enters therefore no action potential occurs. This is an effect of...

cause of membrane potential (permeability)

permeability of sodium, potassium (cell membrane is 50-100 x more permeable to potassium) and large non-penetrating intracellular proteins (anions) that are negatively charged

effect of alcohol (intoxication)

produces behavioral (biomechanical) side effects. Metabolism of this _______ produces fatty acid ethyl esters which keep K channels open. Allows K ions to leave the neuron when it is at rest. LOWERS membrane potential, which makes it harder for the potential to reach the threshold for impulse conduction. Creates difficult speech and motor function. This is an effect of...