Biology 322 Human Anatomy I Cytology and Histology of the Nervous System

Brain Spinal Cord Central Nervous System (CNS) = Peripheral Nervous System (PNS) Nerves =

Recall: Nervous system begins in trilaminar embryo as a “neural plate” of the ectoderm. This develops into a “neural groove” that closes over to form a “neural tube”

One end (caudal) of this neural tube thickens but stays tube-shaped to form the spinal cord. The other (cephalic) end of this neural tube enlarges and folds to form the brain and its various divisions, which we will discuss later.

The cranial nerves and spinal nerves will form as outgrowths of this embryonic brain and spinal cord.

Recall: All organs are composed of, at most, four tissues Epithelial tissue Connective tissue Muscle tissue Nervous tissue Nervous tissue: Excitable cells (neurons) Nonexcitable (supporting) cells Connective tissue: Meninges - in central nervous system Endoneurium Perineurium in peripheral nervous system Epineurium Epithelium found only in blood vessels Muscle (smooth)

Cells of Nervous Tissue: 1) Excitable cells = Neurons Carry electrical signals from one place to another Pass these signals to other cells 2) Nonexcitable cells Glia in CNS Satellite cells Schwann cells in PNS

1) Excitable cells = Neurons Carry electrical signals from one place to another Pass these signals to other cells Found in either central nervous system, peripheral nervous system, or may have parts in both Do not divide Long-lived High metabolic rates

1) Neurons Classified by: Location

2) Neurons Classified by: Shape Bipolar Multipolar Unipolar or Pseudo-unipolar

Dendrites Axon Terminals Axon Hillock Axon Telodendria Cell Body

Exception to this classification: Unipolar or pseudo-unipolar neurons Cell body Single process Dendrites Axon branches Telodendria and axon terminals

Cells of Nervous Tissue: 1) Excitable cells = Neurons Carry electrical signals from one place to another Pass these signals to other cells 2) Nonexcitable cells Do not carry electrical information Support, nourish, protect neurons May divide

Non-excitable cells In PNS: Satellite cells surround neuron cell bodies Schwann cells surround cell processes In CNS: Four types of Glia

Glia: 4 types in central nervous system 1) Astrocytes 2) Oligodendrocytes: 3) Microglia: 4) Ependyma:

Glia: 1) Astrocytes: Regulate environment around neurons by controlling the passage of molecules into and out of capillaries, and by absorbing or secreting ions from/into extracellular matrix

Glia: 2) Oligodendrocytes: Produce myelin sheaths around axons (and some dendrites) of neurons in C.N.S.

Glia: 3) Microglia: Phagocytic. Remove degenerating cells and foreign debris from C.N.S.

Glia: 4) Ependyma: Line fluid-filled ventricles within C.N.S.

Non-excitable cells of Peripheral Nervous System 1) Satellite cells surround and protect neuron cell bodies in P.N.S. Satellite Cells

Non-excitable cells of Peripheral Nervous System 2) Schwann cells produce myelin sheaths which surround axons (and some dendrites) in P.N.S. Schwann Cells Nodes of Ranvier

Pathology of the Nonexcitable Cells Loss of oligodendrocytes causes multiple sclerosis and a group of diseases called leukodystrophies Injury to astrocytes, usually from toxic substances, leads to cerebral edema and swelling of the brain Injury to Schwann cells causes degeneration of the neuron processes and prevents their healing and regrowth. It is also responsible for Guillain-Barre syndrome

Pathology of the Nonexcitable Cells Tumors can arise from all four types of glia, and these tend to grow rapidly and destroy nearby regions of the brain or spinal cord Astrocytoma Astrocytoma Ependymoma Tumors can also arise from satellite cells or Schwann cells, compressing and destroying the neurons they surround Schwannoma