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A.D.A.M. Benjamin/Cummings Interactive Physiology

Nervous System
The Neuron: Action Potential
Glossary
Absolute refractory period - The period of time following an action potential when an excitable cell cannot generate another action potential no matter how large a stimulus it receives.
Action potential - A long-distance regenerative electrical signal transmitted along an axon. The action potential is an all-or-none event. Also called a nerve impulse, spike, or discharge.
Active channels - Ion channels that can be opened or closed by gates.
All-or-none event - An event, such as an action potential, that occurs in its full form or not at all.
Anion - A negatively charged ion.
Axon - A long, thin cytoplasm process that extends from a neuron's cell body. The axon may be insulated with myelin. Each neuron has a single axon, which transmits action potentials.
Axon collateral - A branch of an axon.
Axon hillock - The first portion, or initial segment, of the axon.
Axon terminal - The bulbous ending of a branch of an axon. Also called synaptic ending or synaptic bouton.
Cation - A positively charged ion.
Cell body - The portion of the neuron containing the nucleus and organelles; also called the soma.
Central nervous system - The brain and spinal cord.
Chemically-gated channels - Ion channels with gates that open or close when a neurotransmitter binds to them.
Concentration gradient - The difference in the concentration of a particular substance between two adjacent areas.
Conduction velocity - The speed with which an action potential is propagated along an axon.
Dendrite - A cytoplasmic process that extends from a neuron's cell body. Neurons typically have many dendrites, which are usually shorter and thicker than axons and highly branched. Dendrites receive signals from other neurons.
Depolarization - The process by which the membrane potential of a cell changes to become more positive than its resting membrane potential.
Diffusion - The movement of molecules from one location to another because of random thermal motion. Net movement is from a region of higher concentration to a region of lower concentration.
ECF - Extracellular fluid. Located outside cells, the ECF includes interstitial fluid and plasma.
Electrical potential - An electrical force across a cell membrane that results from the separation of charged particles across that membrane.
Electrochemical gradient - The combined electrical and chemical forces on an ion. This force determines the net movement of charged particles.
Equilibrium - A state of chemical stability in which no net change occurs. For an ion in equilibrium across a cell membrane, the amount moving out is equal to the amount moving in.
Equilibrium potential - For a given ion, the membrane potential at which the electrical and chemical forces that drive that ion across the cell membrane are equal and opposite.
Excitable - Refers to a cell that responds to stimuli by generating an electrical signal at the cell membrane. Neurons and muscle cells are excitable.
GABA - Gamma-aminobutyric acid, a neurotransmitter.
Gated channels - Ion channels that are opened or closed by gates. The gates may be controlled by chemicals or by membrane voltage.
Hyperpolarization - The process by which the membrane potential of a cell changes to become more negative than its resting membrane potential.
Integral proteins - Proteins that are embedded in the lipid bilayer of the cell membrane.
Ion - An atom or small molecule with a positive or negative electric charge.
Ion channels - Integral proteins containing watery pores through which ions pass to cross the cell membrane.
Membrane potential - The electrical potential, or voltage, across a cell membrane that results from the separation of charged particles across the membrane.
millivolt (mV) - One millivolt is equal to 0.001 volt.
mM (millimole) - One millimole is equal to 0.001 mole. One liter of a solution having a concentration of one mole contains an amount of solute equal to its molecular weight in grams.
Multipolar neuron - A neuron with a central cell body that gives rise to multiple dendrites and a single axon. The most common type of neuron in humans.
Myelin sheath - The insulation surrounding axons. The myelin sheath is formed by support cells that wrap repeatedly around an axon, forming a thick layer of cell membrane.
Myelination - The insulation of an axon with myelin.
Na-K pump - A membrane enzyme that uses the energy of ATP to transport Na+ and K+ ions against their respective electrochemical gradients.
Nerve impulse - Another term for action potential.
Neurotransmitter - A chemical that is released at synapses.
Nodes of Ranvier - Tiny areas of bare axon between neighboring segments of myelin sheath. In a myelinated axon, charge flows across the membrane only at the nodes of Ranvier so the action potential appears to jump along the axon.
Passive channels - Ion channels that are always open. Also called leakage channels.
Peripheral nervous system - That portion of the nervous system that lies outside the brain and spinal cord; includes nerves and ganglia.
Permeability - The characteristic of a cell membrane that permits particles such as ions to pass from one side of the membrane to the other.
Positive feedback loop - A regenerative process that amplifies an original signal. For example, at threshold, depolarization leads to greater influx of Na+, which leads to further depolarization.
Propagated - Transmitted or conducted.
Regenerative - Self-generating. Regenerative processes produce positive feedback loops.
Relative refractory period - The period of time following an action potential when an excitable cell can generate another action potential only if it receives a stimulus stronger than normal (i.e., more positive than its normal threshold).
Repolarization - The process whereby the membrane potential of a cell moves back toward its resting value.
Resting membrane potential - The membrane potential when a cell is at rest.
Saltatory conduction - The process by which an action potential appears to jump from one node of Ranvier to the next along a myelinated axon.
Schwann cell - One type of support cell in the nervous system. Found in the peripheral nervous system, Schwann cells form myelin sheaths around axons.
Selective permeability - The characteristic of a cell membrane that permits some particles to cross it, but prevents other particles from crossing.
Soma - Another word for body. In the nervous system, the soma refers to the cell body of a neuron.
Support cells - Nonexcitable cells that provide structural support or that serve important physiological and immunological functions in the nervous system. Also called glial cells.
Synapse - A junction between a neuron and its target cell (another neuron, muscle, or gland). Signals between neurons and other cells are communicated across synapses.
Synaptic potential - A short-distance electrical signal that can vary in amplitude. Synaptic potentials are the incoming signals to neurons.
Target cell - A cell that receives signals from a neuron or a hormone.
Threshold - A critical level of membrane potential at which the depolarization process becomes regenerative and the cell generates an action potential.
Voltage - Another term for electrical potential. A force capable of moving charged particles toward regions having an opposite charge and away from regions having a like charge.
Voltage-gated channels - Ion channels with gates that are opened or closed by changes in membrane voltage