CNS = Central Nervous System [Brain & Spinal Cord]
PNS = Peripheral Nervous System [Cranial Nerves & Spinal Nerves

Neuron Terminology

Initial Segment - First part of an axon
Axon hillock - junction of cell body & axon
Trigger Zone - Located between axon hillock and initial segment. Nerve impulse originates here.
Axon Collateral - Branch of an axon
Axoplasm - Cytoplasm of the axon
Axolemma - Plasma membrane of an axon

Structural Classification of Neurons

Multipolar Neuron - has many processes extending from the cell body, Location: Brain & Spinal Cord
Bipolar Neuron - has two processes extending from the cell body, Location: Retina of the eye, inner ear, olfactory area of the brain
Unipolar Neuron - has one process extending from cell body, Location: ganglia of spinal nerves & ganglia of cranial nerves {PNS}

Examples of CNS Neurons

Purkinje Cell - located in cerebellum of brain
Pyramidal Cell - located in cerebral cortex of brain

Neuroglia = Nerve Glue

Neuroglia is found around both CNS and PNS neurons.
Neuroglia contains neuroglial cells.
For each neuron there are 5-50 neuroglial cells. Neurons are unable to multiply, but neuroglial cells can multiply.
Cancer of neuroglial cells of the brain is called Gliomas.

CNS Neuroglial Cells
There are four kinds of neuroglial cells in the CNS:

1. Astrocytes - Star-shaped cells.
Functions: a. Potassium ion (K-) balance b. metabolism of neurotransmitters such as Ach c. form Blood Brain Barrier (BBB).

2. Oligodendrocytes - Maximum of 15 processes. Main Function: Forms myelin sheath around CNS axons.

3. Microglial Cells - Main Function: Phagocytize microbes and cellular debris.

4. Ependymal Cells - Lines ventricles of brain and central canal of spinal cord.
Main Function: Synthesis and circulation of CSF (Cerebrospinal Fluid)

PNS Neuroglial Cells
There are two kinds of neuroglial cells in the PNS:

1. Schwann Cell (Neurolemmocyte) - A schwann cell myelinates a small distance (1mm length) of a PNS axon. A schwann cell spirals around the PNS axon 100 times - the myelin sheath is the 100 twists of the plasma membrane of the Schwann cell around the PNS axon.
Main Function: Forms myelin sheath around PNS axons.

The outermost layer of the plasma membrane of the Schwann cell containing its entire cytoplasm and the nucleus is called Neurolemma. Because of the presence of a neurolemma, the Schwann cell is called Neurolemmocyte.

2. Satellite Cell - These are flat cells found around the cell bodies of PNS neurons. Functions: a. Provide structural support to the PNS cell bodies b. help exchange of materials between the PNS cell bodies and interstitial fluid.

Nucleus vs. Ganglion

Nucleus - Cluster of neuronal cell bodies in CNS
Ganglion - Cluster of neuronal cell bodies in PNS

White Matter vs. Gray Matter

White Matter - Aggregations (clusters) of myelinated axons. - Myelin sheath is made of proteins & lipids which make the myelinated axons look white in appearance. Hence the name white matter! - Nodes of Ranvier - Unmyelinated portions of myelinated axons. Nodes of Ranvier are present in both PNS and CNS neurons.
Gray Matter - Aggregations of unmyelinated axons. Unmyelinated axons look gray in the absence of the myelin sheath because of the absence of proteins & lipids.

	Created by Bianca Tobar about 7 years ago

Chapter 12, 14. 16 and 17 from A&P Tortuga and related topics covered in Dr. K's green book