Chapter 3: The Mind's Machine

Action Potential

a rapid reversal of the membrane potential that momentarily makes the inside of the membrane positive with respect to the outside

Intracellular Fluid

also called cytoplasm; the watery solution found within cells

Extracellular Fluid

the fluid in spaces between cells

Cell Membrane

the lipid bilayer that ensheathes a cell

Microelectrode

an especially small electrode used to record electrical potentials inside living cells

Resting Potential

the difference in electrical potential across the membrane of a nerve cell at rest

Millivolt (mV)

a thousandth of a volt

Neurophysiology

the study of the life processes of neurons

Ion

an atom or molecule that has acquired an electrical charge by gaining or losing one or more electrons

Anion

a negatively charged ion, such as a protein or chloride ion

Cation

a positively charged ion, such as a potassium or sodium ion

Ion Channel

a pore in the cell membrane that permits the passage of certain ions through the membrane when the channels are open

Potassium Ion (K+)

an atom that carries a positive charge

Sodium Ion (Na+)

an atom that carries a positive charge

Selective Permeability

the property of a membrane that allows some substances to pass through, but not others

Diffusion

the spontaneous spread of molecules from an area of high concentration to an area of low concentration until a uniform concentration is absorbed

Electrostatic Pressure

the propensity of charged molecules or ions to move toward areas with the opposite charge

Sodium-Potassium Pump

the energetically expensive mechanism that pushes sodium out of a cell, and potassium ions in

Equilibrium Potential

the point at which the movement of ions across the cell membrane is balanced, as the electrostatic pressure pulling ions in one direction is offset by the diffusion force pushing them in the opposite direction

Axon Hillock

the cone-shaped area on the cell body from which the axon originates

Hyperpolarization

an increase in membrane potential (the interior of the neuron becomes even more negative)

Depolarization

a decrease in membrane potential (the interior of the neuron becomes less negative)

Local Potential

an electrical potential that is initiated by stimulation at a specific site, which is a graded response that spreads passively across the cell membrane, decreasing in strength with time and distance

Threshold

the stimuli intensity that is just adequate to trigger an action potential in an axon

All-or-None Property

referring to the fact that the size (amplitude) of the action potential is independent of the size of the stimulus

Afterpotential

the positive or negative change in membrane potential that may follow action potential

Voltage-Gated Na+ Channel

selective channel that opens or closes in response to changes in the voltage of the local membrane potential; mediates the action potential

Refractory

temporarily unresponsive or inactivated

Absolute Refractory Phase

a brief period of complete insensitivity to stimuli

Relative Refractory Phase

a period of reduced sensitivity during which only strong stimulation produces an action potential

Conduction Velocity

the speed of which an action potential is propagated along the length of an axon

Myelin

the fatty insulation around an axon, formed by glial cells; this sheath boosts the speed at which action potentials are conducted

Nodes of Ranvier

a gap between successive segments of the myelin sheath where the axon membrane is exposed

Saltatory Conduction

the form of conduction that is characteristic of myelinated axons, in which the action potential jumps from one node of Ranvier to the next

Multiple Sclerosis (MS)

literally "many scars"; a disorder characterized by the widespread degeneration of myelin

Neurotransmitter

the chemical released from the presynaptic axon terminal that serves as the basis of communications between neurons

Presynaptic

located on the "transmitting" side of a synapse

Postsynaptic

referring to the region of a synapse that receives and responds to a neurotransmitter

Postsynaptic Potential

a local potential that is initiated by stimulation at a synapse, which can vary in amplitude, and spreads passively across the cell membrane, decreasing in strength with time and distance

Excitatory Postsynaptic Potential (EPSP)

a depolarizing potential in the postsynaptic neuron that is normally caused by synaptic excitation; increases the probability that the postsynaptic neuron will fire an action potential

Inhibitory Postsynaptic Potential (IPSP)

a hyperpolarizing potential in the postsynaptic neuron; decreases the probability that the postsynaptic neuron will find an action potential

Chloride Ion (Cl-)

an atom that carries a negative charge

Spatial Summation

the summation of postsynaptic potentials that reach the axon hillock from different locations across the cell body; if this summation reaches threshold, an action potential is fired

Temporal Summation

the summation of postsynaptic potentials that reach the axon hillock at different times; the close in time the potentials occur, the more complete the summation is