BIOS 252 lessons 4-7

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BIOS 252 lessons 4-7

• A cell at its resting membrane potential is

–Answer polarized

• local potentials

–Answer short distance signals of axons

• action potentials

–Answer a uniform, rapid depolarization and repolarization of the mem- brane potential of a cell. This change in the membrane potential causes a re- sponse—or action—of some sort. For a muscle fiber, the change initiates events that lead to muscle fiber contraction. Within the nervous system, signals are sent through an axon to another neuron, a muscle fiber, or a gland.

• local potential can cause

–Answer both depolarization and hyperpolarization

• local potentials are sometimes called

–Answer graded potentials because they vary greatly in size—some produce a larger change in membrane potential than others.

• local potential attributes

–Answer

• length of stimulation
• number of ion channels that open
• type(s) of ion channels that open
• reversible; when the stimulus that caused the ion channels to open stops, the

neuron quickly returns to its resting potential. 1 / 3

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• decremental in nature

–Answer The changes in membrane potential they produce are small, and the current generated is lost across the membrane over the distance of a few millimeters.

• Recall that only generate action potentials; and cell

bodies generate local potentials only.–Answer axons dendrites

• Action potentials are generated in the initial segment of the axon; for this reason,

we refer to this region as the –Answer trigger zone

• voltage-gated channels are found most abundantly in the of the neuron, which is why only axons have action potentials.

–Answer axolemma

• voltage gated potassium

–Answer the voltage-gated potassium ion channel has two possible states –Answer resting and activated. In the resting state, the channel is closed. In the activated state, the channel is open and allows potassium ions to cross the axolemma.

• voltage gated sodium

–Answer Resting state –Answer Inactivation gate opened, activation gate closed. No sodium ions cross the membrane when the channel is in the resting state. Activated state –Answer Both activation and inactivation gates opened. The channel in the activated state allows sodium ions to cross the axolemma.Inactivated state 2 / 3

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–Answer Inactivation gate closed, activation gate opened. The channel in this state no longer allows sodium ions to pass through. Notice that during this state, the activation gate remains open. When the action potential is finished, the channel returns to the resting state.

• Neuronal action potentials have three general phases

–Answer

–Answer During the depo- larization phase, the membrane potential rises toward zero and then becomes briefly positive. The membrane potential returns to a negative value during the

repolarization phase, and then becomes temporarily more negative than resting during the hyperpolarization phase.

• action potential steps

–Answer A local potential depolarizes the axolemma of the trigger zone to threshold.Voltage-gated sodium ion channels activate, sodium ions enter, and the axon section depolarizes.Sodium ion channels inactivate and voltage-gated potassium ion channels activate, and repolarization begins.Sodium ion channels return to the resting state and repolarization continues. The axolemma may hyperpolarize before potassium ion channels return to the resting state; after this, the axolemma returns to the resting membrane potential.

• Neurons are limited in how often they can fire action potentials. For a brief time

after a neuron has produced an action potential, the membrane cannot be stimulated to fire another one. This time is called the –Answer refractory period

• During the no additional stimulus, no matter how strong, is able to

produce an additional action potential.

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