CNS Neuroglial Cells

• Greatly outnumber neurons in the CNS (think worker ants vs. Queen ant)

1. Microglial cells– Scattered throughout CNS– Support neurons and phagocytize

bacterial cells and cellular debris

2. Oligodendrocytes– Occur in rows along nerve fibers– Provide layers of myelin around axons

within brain and spinal cord

CNS Neuroglial Cells, continued….

3. Astrocytes– Found between neurons and blood vessels– Provide structural support, help regulate

nutrients and ions in tissues– Form scar tissue to fill spaces after CNS

injuries

4. Ependymal cells– Form epithelial-like membrane in parts of the

brain (choroid plexuses)– Form inner linings that enclose ventricles in

the brain and central canal in the spinal cord

CNS Neuroglial Cells

PNS Neuroglial cells

1. Schwann cells: form myelin sheath around axons

Neurons

• Vary in size and structure, but have common features:

1. Cell Body2. Dendrites3. Axon

• Mature neurons do not divide, but neural stem cells can divide and form neurons or neuroglial cells.

1. Cell Body• Contains normal cellular structures

(golgi apparatus, mitochondria, cytoplasm, cell membrane, etc.)

• Neurofibrils – fine threads that extend into the axon

• Nissl bodies (chromatophilic substances)–Membranous sacs in the cytoplasm – Similar to rough ER– Ribosomes on Nissl bodies synthesize

______

2. Dendrites

• Usually short and highly branched (dendr = ?)

• The main receptive surfaces for receiving communication from axons of other neurons

3. Axons

• Arise from a slight elevation of the cell body, called the axonal hillock.

• Conduct nerve impulses away from the cell body

• Contains many mitochondria, microtubules, and neurofibrils

• Originates as a single structure, but may have branches, especially at the end to interact with receptive surfaces of other cells

PNS Axons

• Enclosed in myelin sheaths composed of many Schwann cells

• Myelin is a lipoprotein.

• Neurilemma sheath surrounds the myelin sheath

• Nodes of Ranvier – narrow gaps in the myelin sheath between the Schwann cells

Classification of Neurons

• Classification based on Structural differences:

• Bipolar neurons• Unipolar neurons• Multipolar neurons

• Classification based on Functional differences:

• Sensory neurons (afferent neurons)• Interneurons (association or internuncial neurons)• Motor neurons (efferent neurons)

Structural Differences• Sketch the neurons below. Notes on

the next 3 slides:

Structural Differences, cont…..

1. Bipolar:– 2 processes• Axon• Dendrite

– Found in specialized parts of the eyes, nose, and ears

Structural Differences, cont…..

2. Unipolar:– 1 process divides into 2 branches, which

function as a single axon• 1 branch (peripheral process) associated with

dendrites• Other branch (central process) enters brain or

spinal cord

Structural Differences, cont…..

3. Multipolar:– Many processes arising from cell body:• 1 axon• Many dendrites

– Most neurons whose cell bodies lie in the brain or spinal cord are multipolar.

• Direction of impulse is ALWAYS from dendrites to axon.

Functional Differences

1. Sensory (afferent) neurons– From peripheral body parts to the brain or

spinal cord– Have specialized receptor ends at the tips

of their dendrites OR– Dendrites closely associated with receptor

cells in the skin or sensory organs.–Most are unipolar, but some are bipolar.

Functional Differences, cont…..

2. Interneurons (association or internuncial neurons)– Lie entirely in the brain or spinal cord– Multipolar and link other neurons– Transmit impulses from one part of the brain

or spinal cord to another

3. Motor (efferent) neurons– Multipolar– Carry nerve impulses from brain or spinal

cord to effectors– Stimulate muscles or glands