Exam 3 Review:  Chapter 11:  Histology of the Neuron's Axon  

axon - A 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 - The prominence on a neuron cell body from which an axon arises; the region of the neurolemma is a trigger zone, i.e., a site for summation of incoming graded potentials, EPSPs and IPSPs, which regulate whether or not the given neuron will initiate an action potential.

axoplasm - The 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; sodium and potassium ion flows in and out of this portion of the cytoplasm create the action potential for nerve impulse transmission.

axolemma - The portion of a neuron's lipid bi-layer cell membrane which covers its axon(s) and along which the action potential (depolarization event) = 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, i.e., a site for summation of incoming graded potentials, EPSPs and IPSPs, which regulate whether or not the given neuron will initiate an action potential.

trigger zone - The site on the neurolemma for summation of incoming graded potentials, EPSPs and IPSPs, which regulate whether or not the given neuron will initiate an action potential; 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 of short "terminal branches" at the end of every axon (and axon collateral).

axon terminal bulb = synaptic end bulb - The swollen or knob-like distal endings to the terminal branches of axons (and axon collaterals) which house the secretory components of the neuron; the site of synthesis, packaging, storage and release of neurotransmitter molecules.

synaptic vesicle - A small membrane-bound package or organelle, a secretory vesicle, which contains a neurotransmitter; it is found inside the axon terminal near the presynaptic membrane, and upon the arrival of an action potential (nerve impulse) releases its contents into the synaptic cleft after fusing with the axon terminal membrane.

neurotransmitter - A chemical substance, e.g., acetylcholine, dopamine, etc., which transmits a nerve impulses across a synapse by exocytosis and diffusion and binding to a receptor site on a gated ion channel; the influence on the postsynaptic cell may be excitatory or inhibitory.

synapse - The 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.