Axonal transport along peripheral nerves can convey certain viruses and toxins. For instance, the herpes zoster virus, which causes chickenpox, can invade dorsal root ganglion cells and remain dormant for many years. However, a change in immunestatus can activate the virus, causing it to be transported along sensory axons to the skin and resulting in the painful disease known as shingles. Another example is the axonal transport of tetanus toxin. If a person is not vaccinated against tetanus toxin and Clostridium tetani bacteria grow in a dirty wound, the toxin can be transported retrogradely in the axons of motor neurons. This toxin can then escape into the extracellular space of the spinal cord ventral horn and block the synaptic receptors for inhibitory amino acids, leading to tetanic convulsions.

Glia, the major nonneuronal cellular elements of the nervous system, outnumber neurons in the human central nervous system (CNS) by an order of magnitude. There are approximately 10^13 glia and 10^12 neurons. Glial cells in the CNS include astrocytes, oligodendrocytes, microglia, and ependymal cells, while Schwann cells and satellite cells are the glial cells in the peripheral nervous system (PNS). Traditionally considered supportive cells, glial cells regulate the microenvironment and play a role in myelination of axons. However, they are now recognized as important determinants of signal flow through neuronal circuits. Glial cells modulate synaptic and nonsynaptic transmission, contribute to synaptogenesis and maintenance, and have a crucial role in the overall functioning of neuronal circuits.

Astrocytes, named for their star shape, are involved in regulating the microenvironment of the CNS both under normal conditions and in response to nervous system damage. They possess a cell body from which several main branches extend, and these branches give rise to numerous branchlets through repeated branching. The processes of astrocytes make contact with neurons and surround synaptic endings, isolating them from adjacent synapses and the general extracellular space. The foot processes of astrocytes also contact the capillaries and connective tissue at the surface of the CNS, known as the pia mater. These foot processes may play a role in facilitating the entry of substances into the CNS. Astrocytes actively uptake K+ ions and neurotransmitter substances, which they metabolize, biodegrade, or recycle. Thus, astrocytes serve as buffers for the extracellular environment of neurons by regulating the levels of ions and neurotransmitters. Glial filaments within the cytoplasm of astrocytes provide mechanical support for CNS tissue. Following an injury, astrocytes undergo various changes and become reactive astrocytes. One type of reactive astrocytes forms a glial scar around an area of focal damage, segregating the damaged tissue and allowing inflammatory processes to selectively act at the site of damage, minimizing the impact on surrounding normal tissue. Astrocytes also have the ability to influence the properties of synaptic transmission.
Certainvirusesandtoxinscanbeconveyedbyaxonaltransportalongperipheralnerves.Forexample,herpeszoster,thevirusofchickenpox,invadesdorsalrootganglioncells.Thevirusmaybeharboredbytheseneuronsformanyyears.However,eventuallythevirusmaybecomeactivebecauseofachangeinimmunestatus.Thevirusmaythenbetransportedalongthesensoryaxonstotheskin,causingshingles,averypainfuldisease.Anotherexampleistheaxonaltransportoftetanustoxin.Clostridium tetani bacteriamaygrowinadirtywound,andifthepersonhadnotbeenvaccinatedagainsttetanustoxin,thetoxincanbetransportedretrogradelyintheaxonsofmotorneurons.Thetoxincanescapeintotheextracellularspaceofthespinalcordventralhornandblockthesynapticreceptorsforinhibitoryaminoacids.Thisprocesscanresultintetanicconvulsions.
The major nonneuronal cellular elements of the nervous system are the glia (
Fig. 4.4 ). Glial cells in the human CNS outnumber neurons by an order of magnitude: there are about 1013 glia and 1012 neurons. Glial cells in the CNS include astrocytes, oligodendrocytes, microglia, and ependymal cells (see
Fig. 4.4 ); in the PNS the glial cells are Schwann cells and satellite cells. Traditionally, glial cells were thought of as supportive cells, and consistent with that conception, their functions include regulation of the microenvironment and myelination of axons. Glial cells are now also recognized to be important determinants of the flow of signals through neuronal circuits, and related to that they act to modulate synaptic and nonsynaptic transmission and have important roles in synaptogenesis and maintenance.
Astrocytes (named for their star shape) help regulate the microenvironment of the CNS, both under normal conditions and in response to damage to the nervous system. Astrocytes have a cell body from which several main branches arise. Through repeated branching these main processes give rise to hundreds to thousands of branchlets. Astrocyte processes contact neurons and surround synaptic endings, isolating them from adjacent synapses and the general extracellular space. Astrocytes also have foot •Fig. 4.4 SchematicrepresentationofcellularelementsintheCNS.Twoastrocytesareshownendingonasomaanddendritesofaneuron.Astrocytesalsocontactthepialsurfaceorcapillariesorboth.Anoligodendrocyteprovidesthemyelinsheathsforaxons.Alsoshownaremicrogliaandependymalcells. N,neuron.(RedrawnfromWilliamsPL,WarwickR.Functional Neuroanatomy of Man. Edinburgh:ChurchillLivingstone;1975.) processes that contact the capillaries and connective tissue at the surface of the CNS, the pia mater (see
Fig. 4.4 ). These foot processes may help mediate the entry of substances into the CNS. Astrocytes can actively take up K+ ions and neurotransmitter substances, which they metabolize, biodegrade, or recycle. Thus astrocytes serve to buffer the extracellular environment of neurons with respect to both ions and neurotransmitters. The cytoplasm of astrocytes contains glial filaments that provide mechanical support for CNS tissue. After injury the astrocytes undergo a variety of changes to become reactive astrocytes. One example is a class of reactive astrocytes that act to form a glial scar around an area of focal damage, which segregates the damaged tissue and thereby allows inflammatory processes to act selectively at the site of damage, minimizing the impact on surrounding normal tissue. Astrocytes can also affect the properties of synaptic transmission, which is discussed in