Chapter 12 part 2

Neuroglial cells protect the neurons and help them

function and outnumber neuron

• CNS - 4 types

• PNS - 2 types

Astrocytes are the most abundant

glial cells in the CNS and constitute over 90% if the tissue in some areas of the brain

Astrocytes cover the entire brain surface and most

Non synaptic regions of the neurons in the gray matter

They are named for their many branched, somewhat starshaped and they have the most

diverse functions of any glia

Astrocytes have extensions called perivascular feet, which contact the 

blood capillaries and stimulate them to form a tight, protective seal called the blood-brain barrier 

Astrocytes convert blood glucose to the lactate and supply this to the 

neurons for nourishment 

Astrocytes promote

synapse formation and fine-tune neural circuitry 

When neurons are damaged, astrocytes form a harden scar tissue and eventually fill

space occupied by the neurons - this process is called Astrocytosis (or sclerosis)

Astrocytes regulate the composition of tissue fluid. When neurons transmit signals, they release 

neurotransmitters and potassium ions 

• astrocytes absorb these and prevent them from reaching excessive levels in the tissue fluid

Astrocytes secrete nerve growth factors that 

regulate nerve development 

• use lactate for energy

Astrocytes

• gap junctions connect astrocytes

• communication

  • slow Ca2+ pulses

  • chemical messengers

• influence neuron functions 

  • information processing 

Microglia cells are small macrophages that develop from

white blood cells called monocytes

Microglia cells wander through the CNS, putting out fingerlike extensions to constantly probe the 

tissue cellular debris or other problems

Microglial cells are thought to perform a complete checkup on the 

brain tissue several times a day

Microglial cells become concentrated in areas damaged by 

infection, trauma, or stroke 

Pathologists look for clusters of microglia in brain tissue as a clue to sites of

injury or degeneration

• microglia also aid in synaptic remodeling, changing the connections between neurons

Ependymal cells produce cerebrospinal fluid (CSF), a liquid that

bathes the CNS and fills its internal cavities

• line the internal cavities of brain and spinal cord (SC)

Ependymal cells have patches of cilia on their

apical surfaces that help to circulate the CSF

• permeable barrier 

Oligodendrocytes have bulbous body with as many as 15 arms and each reaches out to an axon and 

spirals around it like the electrical tape wrapped repeatedly around a wire 

The wrapping in oligodendrocytes is called the

myelin sheath, insulates the axon from the extracellular fluid and speeds up signla conduction in the axon

PNS Neuroglial cells: Satellite cells surround the nerve cell bodies in

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

• Similar functions as astrocytes

Schwann cells wind repeatedly around an axon and produces a

myelin sheath similar to the one produced in oligodendrocytes in the CNS

Schwann cells also assist in the

regeneration of damaged axons

Myelination: Grey matter 

• unmyelinated structures

• Cell bodies, dendrites

Myelination: white matter 

• myelinated axons 

• signals being sent 

Myelin sheaths consist of the

plasma membranes of these glial cells, it is made of protein and lipids, but has some unique lipids

• lipid rich (80%)

The myelin sheath is segmented, each gap between segments is called a 

myelin sheath gap (node of Ranvier)

In the CNS, each oligodendrocyte reached out to myelinate several axons in its immediate vicinity. Since it is  

anchored to multiple axons, it can’t migrate around any one of them like Schwann cell does

In the PNS, a Schwann cell spirals repeatedly around a single axon, laying down up to

100 compact layers of its own membrane with almost no cytoplasm between the membranes

These layers constitute the myelin sheath, the Schwann cell spirals outward as it wraps the axons, finally ending with a 

thick outermost coil called the neurolemma 

Myelin sheath: PNS 

• No channel or carrier proteins 

• “Velcro” proteins, holds Schwann cells around the axon

• Nodes of Ranvier

  • collateral branches 

In the PNS even the unmyelinated axons are enveloped in Schwann cells, its membrane, doesn’t spiral repeatedly around the

axon as it does in myelin’s sheath, but folds once around each axon and may overlap itself along the edges

This wrapping is the neurolemma, several unmyelinated axons travel through individual

channels in the shared Schwann cell

• a basal laminal surround the entire Schwann cell along with its axons

Structural Class: Multipolar neurons are those, like the preceding, that have

one axon and multiple dendrites

• this is the most common type and includes most neurons of the brain and spina cords (CNS) 

Structural class: Bipolar neurons have one axon and one 

dendrite

• examples include olfactory cell of the nose, neurons of the retina, and sensory neurons of ear

Structural Class: Unipolar Neurons have only a single

process leading away from the cell body

• are represented by the neurons that carry signals to the spinal cord for such senses as touch and pain

Unipolar neurons are a short distance away from the cell body, the process branches like a T into a

peripheral process and a cental process 

In unipolar neurons the dendrites are considered to be only the short receptive ending. The rest of the process, both peripheral and central, is the 

axon, distinguished by the presence of myelin and the ability to generate action potentials 

Axon neurons have multiple

dendrites but no axon

• communicate locally through their dendrites but don’t produce action potentials 

• in the retina they help visual processes 

Functional classes: Sensory

• Stimulus info —> CNS

• Unipolar

• Bipolar

Functional classes: Interneurons

• integration

• typically, multipolar

• 99% neurons

Functional classes: Motor

• Signals to muscles and glands (effectors)

• Multipolar