|axon||process of a neuron, usually relatively long, of a neuron which carries efferent (outgoing)( action potentials away from the neuron cell body and towards the next cell(s) in the communication pathway;|
axons transmit impulses at greater speeds if they have larger diameters or if they are myelinated.
|axon hillock||prominence on a neuron cell body from which an axon arises; the region of the neurolemma is a trigger zone, i.e. site for summation of incoming graded potentials, EPSPs and IPSPs, which regulate whether or not the given neuron will initiate an AP|
|axoplasm||cytoplasm within a neuron's axon which is continuous with the cytoplasm of the cell body and exchanges materials by means of various axoplasmic transport mechanisms;|
Na and K ions flow in and out of this portion of the cytoplasm create the AP for nerve impulse transmission
|axolemma||portion of a neuron's lipid bi-layer cell membrane which covers its axon(s) and along which the AP (depolarization event) or nerve impulse is propagated or conducted from the soma to the axon end terminals.|
|initial segment||the first portion of an axon where it arises from the axon hillock;|
the portion is a trigger zone (site for incoming graded potentials, EPSPs and IPSPs which regulate whether or not the given neuron will initiate an AP
|trigger zone||the site on the neurolemma for summation of incoming graded potentials, EPSPs and IPSPs which regulkate whether or not the given neuron will initiate an AP.|
typically the location is at the axon hillock, less often on the initial segment of the axon.
|axon collateral||any side branch extending from the axon of a neuron;|
axon collaterals are relatively rare and usually branch at more or less right angles from the main axon;
not to be confused with the very common tuft or short "terminal branches" at the end of every axon (and axon collateral.
|axon terminal bulb|
synaptic end bulb
|swollen knob-like distal endings to the terminal branches of axons (and axon collaterals) which house the secretory components of the neuron;|
site of synthesis, packaging, storage and release of neurotransmitter molecules
|synaptic vesicle||small membrane-bound package or organelle, a secretory vesicle which contains a neurotransmitter;|
found inside the axon terminal near the presynaptic membrane and upon the arrival of an AP releases its contents into the synaptic cleft after fusing with the axon terminal membrane
|neurotransmitter||chemical substance (acetylcholine, dopamine, etc) whuich transmits a nerve impulse across a synapse by exocytosis and diffusion and binding to a receptor site on a gated ion channel; the influence on the post-synaptic cell may be excitatory or inhibitory|
|synapse||space, gap, or junction across which a nerve impulse passes from an axon terminal to the next excitable cell which may be a neuron, muscle cell, or gland cell;|
transmission may be by an electrochemical depolarization event or by release of neurotransmitter molecules into the space.
|myelination||process of forming a myelin sheath by the repeated spiral wrapping of the outer cell membrane of the myelinating cell (neurolemmocytes in PNS and oligodendrocytes in CNS) around a portion of a neuron's axon;|
this covering electrically insulates the axon and increases the speed of nerve impulse conduction along its length by saltation.
|myelin sheath||multilayered lipid and protein covering produced by neurolemmocytes in the PNS and oligodendrocytes in the CNS which surrounds the axons of most neurons;|
this covering electrically insulates the axon and increases the speed of nerve impulse along its length by saltation;
in the periphery, the neurolemmocytes aid in the regeneration of an injured axon by forming a regeneration tube which guides and stimulates regrowth of the axon.
|myelinated||term describing a neuron's axon which is insulated by a myelin sheath to permit saltatory conduction of nerve impulses (APs); it is accomplished by either oligodendrocytes in the CNS or neurolemmocytes in the PNS|
(node of Ranvier)
|the gaps in the myelin sheath which occur because the insulating neurolemmocytes in the PNS or oligodendrocytes in the CNS do not quite touch each other along the length of the myelinated axon;|
it is at these gaps that the AP is propogated in saltatory conduction
|membrane potential||electrical voltage (a charge differential with the potential to do work) across the cell membrane d/t ionic disequilibria (unequal distribution of ions), primarily the differences in Na+ and K+ ion concentrations inside and outside the cell;|
|this ionic disequilibrium is maintained by the action of the Na-K pump and consequently by the expenditure of much ATP energy|
|nerve fiber||threadlike process of a neuron, especially the prolonged axon that conducts nerve impulses.|
|nerve||a bundle of hundreds or thousands of motor axons and sensory dendrites wrapped in 3 distinct layers of CT (epi-, peri-, and endo- neurium) and being routed to or from a particular peripheral region in the body;|
|ganglion / ganglia||Small masses of nervous tissue, containing primarily cell bodies of neurons; the terms are appropriate to describe areas in either the Central or Peripheral Nervous Systems. (although the term also applies to regions of gray matter in the CNS such as the "basal ganglia", this term increasingly refers specifically to collections of cell bodies in the PNS|
|tract||a bundle (usually or mostly myelinated) nerve fibers having a common origin, termination, and function located in the white matter of the CNS|
|gray matter||nerve cell bodies, dendrites, and axon terminals or bundles of unmyelinated axons and neuroglia organized into functional groupings within the CNS;|
in the spinal cord, gray matter forms an H-shaped inner core surrounded by white matter;
in the brain, a thin outer shell of gray matter (cortex) covers the cerebral hemispheres.
(Additional regions of gray matter are found deep in the brain where they are referred to as nuclei, or with diminishing frequency ganglia; AND additional regions are found within the ganglia of the PNS
|white matter||aggregations of myelinated axons spatially organized into functional tracts within the CNS or the PNS|
|nucleus||any small mass of nervous tissue gray matter, containing primarily clusters of cell bodies and dendrites of neurons with similar functions, located among the various white matter fiber tracts within the CNS|
|the difference in electrical potential = electromotive force = charge gradient between two points on a circuit.|
expressed in volts
|current||amount of electric charge flowing past a specified circuit point per unit of time expressed in amperes (amps)|
|resistance||opposition of a body or substance to current passing through it, resulting in a change of electrical energy into heat or another form of energy expressed in ohms;|
insulators have high resistance
the inverse of conductivity
|ion channel||an integral membrane protein which is a pathway for movement of charged particles through a cell membrane facilitated by integral membrane proteins which have selective water filled conductive channels that permit or exclude passage of charged particles based on size and charge;|
these pathways may be gated and regulated or nongated and always open (leaky)
|leakage (ion) channel|
nongated (ion) channel
|an integral membrane protein which is an ion channel within a cell's outer cell membrane which is always open and permits the diffusion of one or more ions in the direction which is in accord with their concentration and charge gradients|
|gated (ion) channel||an integral membrane protein which is an ion channel within an excitable cell's outer cell membrane which opens and closes in response to some stimulus (e.g., membrane potential (voltage) changes, the arrival and binding of a specific ligand or signal molecule (hormone, neurotransmitter, local hormone) or to mechanical pressure or to light energy.|
|voltage gated (ion) channel||n integral membrane protein which is an ion channel within an excitable cell's outer cell membrane which opens and closes in response to a stimulus which is a change in membrane potential (voltage) [i.e., a change in local membrane charge from negative to positive or vice versa] nearby in the membrane;|
such channels are primarily responsible for the depolarization events of nerve impulse conduction = propagation in excitable cells such as neurons, muscle cells and gland cells.
|chemically gated (ion) channel||an integral membrane protein which is an ion channel within an excitable cell's outer cell membrane which opens and closes in response to a stimulus which is the arrival and binding of a specific ligand or signal moilecule (hormone, neurotransmitter or local hormone);|
such channels are primarily responsible for impulse initiation = initial depolarization of an excitable cell such as a neuron, muscle cell or gland cell.
|mechanically gated (ion) channel||an integral membrane protein which is an ion channel within an excitable cell's outer cell membrane which opens and closes in response to a stimulus which is a mechanical pressure or vibration;|
such channels are primarily responsible for impulse initiation = initial depolarization of an excitable cell such as a mechanoreceptor (sensory cell responding to touch, vibration, compression or stretch).
|light gated (ion) channel||an integral membrane protein which is an ion channel within a photosensitive excitable cell's outer cell membrane which opens and closes in response to a stimulus which is the arrival of a photon of light energy;|
such channels are primarily responsible for impulse initiation = initial depolarization of an excitable cell such as a photoreceptor (sensory cells, rods, and cones responding to light in the retina of the eye.)
|membrane potential||an electrical voltage (charge differential with the potential to do work) across a cell membrane, due to ionic disequilibria (unequal distribution of ions), primarily the differences in Na+/K+ pump and consequently, by the expenditure of much ATP energy|
|resting membrane potential||the membrane potential of a nonconducting neuron, d/t ionic disequilibria, primarily the difference in Na+/K+ ATPase pump; a nonconducting neuron is charged positively on the outside and negatively on the inside with a typical value of -70 mV|
|Na+/K+ ATPase||an integral membrane protein active transport molecule which has the capacity to bind and hydrolyze ATP and use the energy from the ATP hydrolysis to move 3 Na+ ions from the cytoplasm to the exterior of the cell while simultaneously moving 2 K+ ions from the exterior of the cell into the cytoplasm;|
the action of this pump is the main factor in establishing a resting membrane potential for the cell.
|chemical gradient||aka concentration gradient|
a (usually graduated) difference in mass/unit volume (concentration) of a substance, a solute, through a solution or between two locations;
such a difference in solute strength promotes diffusion or osmosis or both.
|charge gradient||a (usually graduated) difference in net charge due to the sum of all charges of all ions present btw two locations such as two fluid compartments on opposite sides of a semi-permeable membrane;|
such a gradient creates a membrane voltage potential and promotes diffusion or osmosis or both.
|graded potential||a small deviation from the resting membrane potential on an excitable cell, which may make the membrane more polarized (hyperpolarization) or less polarized (depolarization) and therefore contributing to whether or not AP will be generated at that moment;|
graded potentials occur most often in the dendrites and cell body of the neuron;
the changes in potentials vary in amplitude (size) depending on the strength of the stimulus and localized to small areas of the membrane where presynaptic neurons have axon terminals attached to the cell membrane forming synapses.
|hyperpolarization||a negative change in a cell's resting membrane potential (which is normally negative) thus making the negative charge numerically larger;|
the membrane becomes more polarized;
when an excitable cell becomes more polarized it becomes less likely to depolarize and generate an AP
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