12.3 - Supportive Cells

neuroglia (glial cells)

non-neuronal supportive cells that protect the neurons and help them function

six types of neuroglia

oligodendrocytes, ependymal cells, microglia, astrocytes, schwann cells, and satellite cells

oligodendrocytes

neuroglia that have a bulbous body with as many as 15 arms

oligodendrocytes (function)

insulate the axons from extracellular fluid and speed up their signal conduction through the myelin sheath

ependymal cells

neurons that resemble a cuboidal epithelium lining the internal cavities of the brain and spinal cord, with patches of cilia on their apical surfaces to help circulate CSF

cerebrospinal fluid (CSF)

liquid produced by ependymal cells that bathes the CNS and fills its internal cavities

microglia

small macrophages that develop from white blood cells called monocytes

microglia (function)

wander through the CNS, putting out fingerlike extension to constantly probe the tissue for cellular debris or other problems; also aid in synaptic remodeling

synaptic remodeling

changing the connections between neurons

astrocytes

most abundant glial cells in the CNS that cover the entire brain surface and most nonsynaptic regions of the neurons in gray matter

perivascular feet

astrocyte extensions that contact the blood capillaries and stimulate them to form the blood-brain barrier

blood-brain barrier

tight, protective seal formed by blood capillaries

astrocytosis (sclerosis)

process in which astrocytes form hardened scar tissue and eventually fill space formerly occupied by neurons

two types of peripheral glial cells

schwann cells and satellite cells

schwann cells (neurilemmocytes)

neuroglia that envelop axons of the PNS and assist in the regeneration of damaged axons

satellite cells

neuroglia that surround the nerve cell bodies in ganglia of the PNS

satellite cells (function)

provide insulation around the cell body and regulate the chemical environment of the neurons

myelin sheath

spiral layers of insulation around and axon, formed by Schwann cells in the PNS and oligodendrocytes in the CNS

myelination

production of the myelin sheath that begins in the fourteenth week of fetal development, proceeds rapidly in infancy, and isn’t completed until late adolescence

neurolemma

thick outermost coil of a Schwann cell around an axon, where the body of the Schwann cell contains its nucleus and most of its cytoplasm

endoneurium

basal lamina and thin sleeve of fibrous connective tissue, both external to the neurolemma

axons of the CNS have no…

neurolemma or endoneurium

centrifugal myelination

myelination away from the center in the PNS

centripetal myelination

myelination toward the center in the CNS

myelin sheath gap (node of Ranvier)

gap between segments in the myelin sheath

internodal segments

myelin-covered segments from each node to the next

initial segment

short section of axon between the axon hillock and the first glial cell

trigger zone

consists of the axon hillock and initial segment and plays an important role in initiating a nerve signal

in the PNS, unmyelinated axons are…

still enveloped in Schwann cells

two factors that influence the speed at which a nerve signal travels along an axon

the diameter of the axon and the presence or absence of myelin

fast nerve signals

large axons; abundant myelin

slow nerve signals

small axons; absent myelin

slow unmyelinated axons (involvement)

processes in which quick responses aren’t particularly important, such as secreting stomach acid or dilating the pupil

fast myelinated axons (involvement)

processes where speed is more important, as in motor commands to the skeletal muscles and sensory signals for vision and balance

first step of nerve regeneration

When an axon is cut, the portion distal to the injury can’t survive because it is incapable of protein synthesis. As the distal part degenerates, so do its Schwann cells, which depend on it for their maintenance. Macrophages clean up tissue debris at the point of injury and beyond.

second step of nerve regeneration

The cell body swells, the endoplasmic reticulum breaks up, and the nucleus moves off center. Some damaged neurons die at this stage, but often the axon stump sprouts multiple growth processes while the severed distal end shows continued degeneration of its axon and Schwann cells. Denervation atrophy occurs.

denervation atrophy

shrinkage of muscle fibers that are deprived of their nerve supply

third step of nerve regeneration

Near the injury, the Schwann cell neurolemma, endoneurium, and basal lamina form a regeneration tube. The Schwann cells produce cell-adhesion molecules and nerve growth factors that enable a neuron to regrow to its original destination. When one growth process finds its way into the tube, it grows rapidly, and the other growth processes are retracted.

fourth step of nerve regeneration

The regeneration tube guides the growing sprout back to the original target cells, reestablishing synaptic contact.

fifth step of nerve regeneration

When contact is established, the nerve cell body shrinks and returns to its original appearance, and the reinnervated muscle fibers regrow.

nerve injury is often followed by…

some degree of functional deficit

CNS axons (regeneration)

because it lacks Schwann cells and endoneurium, damaged CNS axons can’t regenerate at all, but they suffer less trauma than peripheral nerves because the CNS is encased in bone