Satellite cells, similar to astrocytes, surround dorsal root and cranial nerve ganglion cells and regulate their microenvironment (Fig. 4.5C). Microglia, derived from erythromyeloid stem cells, migrate into the CNS during early development and play a crucial role in immune responses within the CNS. In cases of CNS damage, microglia aid in the removal of cellular debris through phagocytosis, assisted by other glia and circulating phagocytes. Recent evidence suggests that microglia also have important roles in normal brain development and function, including synaptic pruning and plasticity.

Ependymal cells line the ventricular spaces of the brain and secrete cerebrospinal fluid (CSF). The choroid plexuses, located in the ventricular system, contribute significantly to CSF production. The ependyma, situated between the brain's extracellular space and the CSF, allows for the diffusion of various substances.

While most neurons in the adult nervous system are postmitotic, some stem cells may persist in certain brain regions. Glial precursor cells in the adult brain retain the ability to divide and differentiate. Consequently, intrinsic brain tumors in adults commonly arise from glial cells such as astrocytes (ranging from slow-growing astrocytomas to rapidly fatal glioblastoma multiforme), oligodendroglia (oligodendroglioma), or ependymal cells (ependymoma). Meningeal cells can also give rise to slow-growing tumors (meningiomas) that compress brain tissue, as can Schwann cells (e.g., acoustic schwannomas formed by the eighth cranial nerve's Schwann cells). In infants' brains, neuroblastomas (e.g., in the roof of the fourth ventricle) or retinoblastomas (in the eye) can arise from dividing neurons.

The Peripheral Nervous System (PNS) serves as the interface between the CNS and the external environment, transmitting sensory information to the CNS and motor commands from the CNS. It comprises sensory (primary afferent) neurons, somatic motor neurons, and autonomic motor neurons.

Sensory pathways into the nervous system begin with receptors, which can be specialized parts of axons in the PNS or include additional cells. Each sensory receptor converts specific types of energy into electrical signals, categorized based on the type of energy transduction (e.g., photoreceptors for light, mechanoreceptors for displacement and force). They may also be classified by the source of the input (e.g., exteroceptors for external events, proprioceptors for body part states like elbow angle, and interoceptors for gut distension).

Transduction processes lead to receptor potentials, electrical responses in primary afferents that trigger action potentials in the fibers innervating the receptors. These action potentials convey sensory stimulus information to the CNS through the primary afferents.
Fig. 4.5C
Satellite cells encapsulate dorsal root and cranial nerve ganglion cells and regulate their microenvironment in a fashion similar to that of astrocytes.
Microglia are derived from erythromyeloid stem cells that migrate into the CNS early in development. They play an important role in immune responses within the CNS. When the CNS is damaged, microglia help remove the cellular products of the damage by phagocytosis. They are assisted by other glia and by other phagocytes that invade the CNS from the circulation. In addition to their role in immune responses, recent evidence suggests they are also active in healthy brain tissue and may have important roles in normal brain development and function, including pruning of excess synapses that are formed during development and synaptic plasticity.
Ependymal cells form the epithelium lining the ventricular spaces of the brain, which contain (cerebrospinal fluid) CSF. CSF is secreted in large part by specialized ependymal cells of the choroid plexuses located in the ventricular system. Many substances diffuse readily across the ependyma, which lies between the extracellular space of the brain and the CSF.
Mostneuronsintheadultnervoussystemarepostmitoticcells(althoughsomestemcellsmayalsoremainincertainsitesinthebrain).Manyglialprecursorcellsarepresentintheadultbrain,andtheycanstilldivideanddifferentiate.Thusthecellularelementsthatgiverisetomostintrinsicbraintumorsintheadultbrainaretheglialcells.Forexample,braintumorscanbederivedfromastrocytes(whichvaryinmalignancyfromtheslowlygrowingastrocytomatotherapidlyfatalglioblastomamultiforme),fromoligodendroglia(oligodendroglioma),orfromependymalcells(ependymoma).Meningealcellscanalsogiverisetoslowlygrowingtumors(meningiomas)thatcompressbraintissue,ascanSchwanncells(e.g.,acousticschwannomas,whicharetumorsformedbySchwanncellsoftheeighthcranialnerve).Inthebrainofinfants,neuronsthatarestilldividingcansometimesgiverisetoneuroblastomas(e.g.,oftheroofofthefourthventricle)orretinoblastomas(intheeye).
The Peripheral Nervous System
The PNS provides an interface between the environment and the CNS, both for sensory information flowing to the CNS and for motor commands issued from the CNS. It includes sensory (or primary afferent) neurons, somatic motor neurons, and autonomic motor neurons.
Sensory pathways into the nervous system start with a receptor, which may simply be a specialized part of an axon in the PNS or may include additional cells. Each sensory receptor is organized so that it transduces a specific type of energy into an electrical signal. Thus they can be classified in terms of the type of energy they transduce (e.g., photoreceptors transduce light, mechanoreceptors transduce displacement and force). They may also be classified according to the source of the input (e.g., exteroceptors signal external events, proprioceptors signal the state of a body part such as the angle of elbow, and interoceptors signal the distension of the gut).
The transduction process leads to an electrical response in the primary afferent called a receptor potential, which triggers action potentials in the primary afferent fibers innervating the receptor. These action potentials contain information about the sensory stimulus that is conveyed to the CNS via the primary afferent.