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Nervous System I
Neurons, ion channels, the membrane potential, and the action potential
61
Anatomy
Undergraduate 2
04/26/2007

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Term
Support Cells
Definition
Nonexcitable cells that provide structural support or that serve important physiological and immunological functions in the nervous system. Also called glial cells.
Term
Ion
Definition
An atom or small molecule with a positive or negative electrical charge
Term
Central Nervous System
Definition
The brain and spinal cord
Term
Peripheral Nervous System
Definition
That portion of the nervous system that lies outside the brain and spinal cord; includes nerves and ganglia
Term
Excitable
Definition
Refers to a cell that responds to stimuli by generating an electrical signal at the cell membrane. Neurons and muscle cells are excitable.
Term
Target Cell
Definition
A cell that recieves signals from a neuron or a hormone
Term
Multipolar Neuron
Definition
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.
Term
All Neurons have these three characteristic structural features:
Definition
Cell body, receptive portion, and a transmitting portion.
Term
Cell Body
Definition
The portion of the neuron containing the nucleus and organelles; also called the soma[image]
Term
Dendrite
Definition
A cytoplasmic process that extends from a neuron's cell body, Neurons typcially have many dendrites, which are usually shorter and thicker than axons and highly branched. Dendrites recieve signals from other neurons.[image]
Term
Axon
Definition
A long, thin cytoplasmic 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.[image]
Term
Action Potential
Definition
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.
Term
Synapse
Definition
A junction between a neuron and its target cell (another neuron, muscle, or gland). Signals between neurons and other cells are communicated across synapsese.
Term
Axon Hillock
Definition
The first portion, or initial segment, of the axon
Term
Axon Collateral
Definition
A branch of an axon
Term
Axon Terminal
Definition
The bulbous ending of a branch of an axon. Also called synaptic ending or synaptic bouton.
Term
Anterograde
Definition
Movement toward axonal terminal
Term
Retrograde
Definition
Movement away from axonal terminal
Term
Myelin Sheath
Definition
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.
Term
Schwann Cell
Definition
One type of support cell in the nervous system. Found in the peripheral nervous system, Schwann cells form myelin sheaths around axons.
Term
Myelination
Definition
The insulation of an axon with myelin
Term
Nodes of Ranvier
Definition
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.
Term
Integral Proteins
Definition
Proteins that are embedded in the lipid bilayer of the cell membrane
Term
Ion Channels
Definition
Integral proteins containing watery pores through which ions pass to cross the cell membrane
Term
Ion Channels are:
Definition
Selective, Passive or active, Regionally located, and Functionally unique
Term
Active Channels
Definition
Ion channels that can be opened or closed by gates
Term
Passive Channels
Definition
Ion channels that are always open. Also called leakage channels.
Term
Voltage-gated Channels
Definition
Ion channels that are opened or closed by changes in membrane voltage
Term
Membrane Potential
Definition
The electrical potential, or voltage, across a cell membrane that results from the separation of charged particles across the membrane.
Term
Passive Channels are located:
Definition
In the cell membrane on the dendrites, the cell body, and the axon
Term
Chemically gated channels for the most part are located:
Definition
In the dendrites and the cell body of the neuron
Term
Voltage gated channels for the most part are located:
Definition
On the axon hillock, all along unmyelinated axons, and at the nodes of Ranvier in myelinated axons.
Term
Passive channels are responsibel for:
Definition
The resting membrane potential
Term
Chemically gated channels are responsibel for:
Definition
Synaptic potentials, the incoming signals to the neuron.
Term
Voltage-gated channels are responsible for:
Definition
Generation and propagation of the action potential, the outgoing signal from the neuron.
Term
Resting Membrane Potential
Definition
The membrane potential when a cell is at rest.
Term
Synaptic Potential
Definition
A short-distance electrical signal that can vary in amplitude. Synaptic potentials are the imcoming signals to neurons.
Term
Diffusion
Definition
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.
Term
Anion
Definition
A negatively charged ion.
Term
Cation
Definition
A positvely charged ion.
Term
Selective Permeability
Definition
The characteristic of a cell membrane that permits some particles to cross it, but prevents other particles from crossing.
Term
Permeability
Definition
The characteristic of a cell membrane that permits particles such as ions to pass from one side of the membrane to the other
Term
Electrical Potential
Definition
An electrical force across a cell membrane that results from the separation of charged particles across that membrane.
Term
Electrochemical Gradient
Definition
The combined electrical and chemical forces on an ion. This force determines the net movement of charged particles.
Term
Equilibrium
Definition
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.
Term
Equilibrium Potential
Definition
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.
Term
Threshold
Definition
A critical level of membrane potential at which the depolarization process becomes regenerative and the cell generates an action potential.
Term
Regenerative
Definition
Self-generating. Regenerative processes produce positive feedback loops.
Term
Positive Feedback Loop
Definition
A regenerative process that amplifies an original signal. For example, at threshold, depolarization leads to greater influx of Na+, which leads to further depolarization.
Term
Repolarization
Definition
The process whereby the membrane potential of a cell moves back toward its resting value.
Term
Hyperpolarization
Definition
The process by which the membrane potential of a cell changes to become more negative than its resting membrane potential.
Term
Concentration (mM)/L of NA+ in intracellular and extracellular fluid.
Definition
Inside cell=15. Outside cell=150.
Term
Concentration (mM)/L of K+ in intracellular and extracellular fluid.
Definition
Inside cell=150. Outside cell=5.
Term
Concentration (mM)/L of Ci- in intracellular and extracellular fluid.
Definition
Inside cell=10. Outside cell=125.
Term
Absolute Refractory Period
Definition
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.

Many Na+ channels are inactive and will not open no matter what voltage is applied to the membrane. Most K+ channels are open.
Term
Relative Refractory Period
Definition
The period of time following an action potential when an excitable cell can generate another action potential, but only if it is depolarized to a value more positive than normal threshold. This is true because some Na+ channels are still inactive and some K+ channels are still open.
Term
Conduction Velocity
Definition
The speed with which an action potential is propagated along an axon.
Term
Conduction velocity depends on:
Definition
the diameter of the axon and how well the axon is insulated with myelin.
Term
Ions flow along their chemical gradient when:
Definition
They move from an area of high concentration to an area of low concentration
Term
Ions flow along their electrical gradient when...
Definition
they move toward an area of opposite charge
Term
Electrochemical gradient
Definition
The electrical and chemical gradients taken together.
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