NERVOUS TISSUE - Tissue possessing the properties of irritability and conductivity. Irritability is a sensitivity to changes in the environment (stimuli) resulting in a response of some kind. Conductivity is the transmission of a wave of electrochemical excitation. These two qualities reach their highest expression in nervous tissue.

THE NEURON - a nerve cell consisting of its cell body and all of its processes.

1. The neuron asssociates with other neurons by means of synapses. The synapse is a very tiny space between the tip of a telodendria of one nerve cell and the dendrite of another.

2. The neuron is the structural and functional unit of the nervous system.

I. The Cell Body (Perikaryon) - is the portion of the nerve cell that surrounds the nucleus.

A. Size varies from 4 to 135um.

B. Shape depends on the number of processes from the cell body:

1.   Unipolar neuron - a globular cell with a single bifurcated process, e.g., cells of the dorsal root ganglion.

2.  Bipolar neuron - spindle-shaped cell with a process at each end, e.g., cells of the acoustic ganglia, some retinal neurons and olfactory neurons.

3.  Multipolar neurons - cells having more than two to several processes. This type of neuron represents the most common one found in the nervous system.

C. The population of neurons in the body is huge. The cerebral cortex of the human brain, alone, contains about 14 billion cells.

D. The nucleus is singular and located centrally in the cell body. The nucleus contains:

1.  One prominent nucleolus

2.  A Barr body in the nuclei of neurons of the female. This structure is a condensed or deactivated X chromosome.

3.  Most of the chromatin is in a dispersed state (euchromatin) giving the nucleus a reticular appearance under light microscopy (LM).

E. The perikaryon contains a number of organelles:

1.  There is a large amount of rough endoplasmic reticulum (RER), free ribosomes and polyribosomes indicative of a high level of protein synthesis.

2.  Under LM irregular clumps of intensely basophilic material called Nissl bodies can be found widely distributed throughout the cytoplasm. Under electron microscopy (EM), these Nissl bodies are found to be composed of RER with associated ribosomes and polyribosomes. After injury to the cell, these bodies disperse their contents throughout the cytoplasm. This produces a darkening of the cell. This effect is referred to as the Nissl Reaction or chromatolysis.

3.  Golgi complexes and mitochondria are numerous.

4.  Neurofibrils are present in both the cell body and in cell processes. These are composed of microtubules and microfilaments as seen under EM. When neurons are stained with heavy metals, e.g., silver stain, the neurofibrils are seen as thick (2 to 3 um) interwoven strands. The neurofibrils are thought to be responsible for the intracellular transport of materials throughout the cell.

          F. Inclusions in the perikaryon include:

1. fat droplets

2. glycogen in embryonic cells

3. pigment granules, e.g., melanin, lipofuchsin tend to accumulate with age.

4. iron containing granules accumulate with age

5. catecholamine granules are found in neurons of the autonomic nervous system

6. vesicles containing neurosecretory products can be identified by the Gamori staining technique.They are called Herring bodies. These vesicles are only found in the terminal ends of neurons.

II. Cell Processes - nerve fibers
     The cytoplasm of the nerve cell is drawn out into long, thread-like processes to provide the conduction pathways for nerve impulses.

A. Dendrons or dendrites conduct impulses towards the cell body:

1. Multipolar neurons have many dendrons.

2. Uni or Bipolar neurons have one process which functions as a dendron.

3. The dendron arises as a broad stem that branches freely into a bush-like   process.

4. Branches of dendrons appear spiny due to lateral twigs called gemmules which are swollen or knobby at the end.

5. Dendrons derminate an axons or cell bodies of other neurons.

6. Dendrons are naked, i.e., have no myelin sheath.

7. Although they are short extensions of the cell body, they may include Nissl bodies and neurofibrils.

B. Axon conducts impulses away from the cell body.

1. Only one axon per cell.

2. Axon is longer and more slender than the dendron.

3. The axon begins from a swelling on the cell body called the axon hillock. This

region is often devoid of Nissl bodies.

4. The diameter of the axon remains uniform.

5. The axon terminates in a set of branches called the telodendria.

6. The axon may have slender, lateral branches called collaterals.

7. An accessory sheath may cover the axon. This sheath can be formed by Schwann cells (PNS) or by the oligodendria (CNS).

8. Nerve fibers tend to run in bundles in the central nervous system (tracts) and in the peripheral nervous system (nerves).