Chapter 2

• 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 elctrons
• anion


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


  a positively charged ion, such as a potassium or sodium ion
• intracellular fluid ("cytoplasm")


  watery solution found within cells
• extracellular fluid ("interstitial fluid")


  fluid in the spaces between cells
• cell membrane


  the lipid bilayer that encloses 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
• ion channel


  a pore in the cell membrane that permits the passage of certain ions through the membrane when the channel is open
• potassium ion (K+)


  a potassium atom that carries a positive charge
• sodium ion (Na+)


  a sodium atom that carries a positive charge
• selective permeability


  the property of a membrane that allows some substances to pass through, but now others
• diffusion


  the spontaneous spread of molecules from an area of high concentration to an area of low concentration
• 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 ions 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 or the neuron becomes less negative)
• local potential


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


  the stimulus intensity that is just adequate to trigger an action potential in an axon
• action potential ("spike")


  a rapid reversal of the membrane potential that momentarily makes the inside of a neuron positive with respect to the outside
• all-or-none property


  the condition 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 an action potential
• voltage-gated Na+ channel


  a Na+ selective channel that opens or closes in response to changes in the voltage of the local membrane potential; it 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 stimulatoin produces an action potential
• relative refractory phase


  a period of reduced sensitivity which only strong stimulation produces an action potential
• conduction velocity 


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


  the fatty insulation around an axon, formed by glial cells; the myelin sheath boosts the speed at which action potentials are conducted
• node 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) "many scars"


  a disorder characterized by widespread degeneration of myelin
• neurotransmitter 


  the chemical released from the presynaptic axon terminal that serves as the basis of communication 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, can vary in amplitude, and spreads passively across the cell membrane, decreasing in strength with time and distance