Neuro Histology

NEUROHISTOLOGY AND NEUROCHEMISTRY

Neurohistology

I. Nervous Tissue: Overview -develops from ectoderm (neural tube and neural crest). -consists of neurons and glial cells.

II. Neurons -constitute the genetic, anatomic, trophic, and functional units of the nervous system (known as the neuron doctrine). -have lost the capacity to undergo cell division. -have the capacity to receive impulses from receptor organs or other neurons. -have the capacity to transmit impulses to other neurons or effector organs. -consist of the cell body and its processes, dendrites, and a single axon.

A. Classification of neurons (Figure)

-is according to number of processes (unipolar, bipolar, or multipolar), axonal length, function, and neurotransmitter.

1. Processesa. Unipolar or pseudounipolar neurons -are sensory neurons located in the dorsal root and cranial nerve ganglia and in the mesencephalic nucleus of the trigeminal nerve (CN V) of the brainstem.

b. Bipolar neurons -are located in the vestibular and cochlear ganglia of the vestibulocochlear nerve (CN VIII), the retina, and the olfactory epithelium (CN I). c. Multipolar neurons -possess one axon and more than one dendrite. -are the largest population of nerve cells in the nervous system. -include motor neurons, interneurons, pyramidal cells of the cerebral cortex, and Purkinje' cells of the cerebellar cortex. 2. Axonal length a. Goigi type I neurons -have long axons (e.g., giant pyramidal cells of Betz of the motor cortex). b. Goigi type II neurons -have short axons (e.g., interneurons). 3. Function a. Motor neurons -conduct impulses to muscles, glands, and blood vessels (e.g., ventral horn cells). b. Sensory neurons -receive stimuli from the external and internal environment (e.g., dorsal root ganglion cells). c. Interneurons -are intercalated or internuncial neurons that interconnect motor or sensory neurons. 4. Neurotransmitter (cholinergic neurons) -elaborates acetylcholine as a neurotransmitter (e.g., ventral horn motor neurons).

B. Nerve cell body -is also called the soma or perikaryon. -contains the organelles found in other cells, including a large nucleus and a prominent nucleolus. -has receptor molecules on its pbsmalemmal surface that confer sensitivity to various neurotransmitters. -incorporates or gives rise to the following structures:

1. Nissl substance -is characteristic of nerve cells and consists of rosettes of polysomes and rough endoplasmic reticulum. -plays a role in protein synthesis. -is abundant throughout cytoplasm and dendrites but is not found in the axon hillock or in the axon.

2. Lysosomes -are membrane-bound dense bodies that contain hydrolytic enzymes and are involved in the process of intracellular digestion. -A genetic defect in the synthesis oflysosomal enzymes results in a storage disease [e.g., Tay-Sachs disease (GM2 gangliosidosis)]. 3. Filamentous protein structures -form an internal supportive network, the cytoskeleton, consisting of: a. Microtubules (25 nm in diameter) -are found in the cell body, dendrites, and axons. -playa role in the development and maintenance of cell shape. -playa role in the intracellular transport of peptide vesicles and organelles.

b. Neurofilaments (10 nm in diameter) -consist of spiral protein threads that playa role in developing and regenerating nerve fibers. -degenerate in Alzheimer·disease to form neurofibrillary tangles. -contain neurofilament protein, which is exclusive to neurons and their precursors. c. Microfilaments (5 nm in diameter) -are composed of actin. -are found in the tips of growing axons. -facilitate movement of plasma membrane and growth of nerve cell processes.

4. Inclusion bodies -include pigment granules: a. Lipofuscin (lipochrome) granules -are common pigmented inclusions of cytoplasm that accumulate' with aging. -are considered to be residual bodies derived from lysosomes. b. Neuromelanin (melanin) -is a blackish pigment in the neurons of the substantia nigra and locus ceruleus. -disappears from the substantia nigra and the locus ceruleus inParkinson disease c. Lewy bodies -are eosinophilic intracytoplasmic inclusion bodies found in the sub. I stantia nigra in patients with Parkinson disease.

d. Hirano bodies -are intracytoplasmic inclusion bodies found in the hippocampus in patients with Alzheimer disease. e. Negri and lyssa bodies -are intracytoplasmic inclusion bodies found in people with rabies.

5. Dendrites -are processes that extend from the cell body. -contain cytoplasm similar in composition to that of the cell body; however, no Golgi apparatus is present. -conduct in a decremental fashion but may be capable of generating action potentials. -receive synaptic input and transmit it toward the cell body.

6. Axons -arise from either the cell body or a dendrite. -originate from the axon hillock, which lacks Nissl substance. -give rise to collateral branches. -may be myelinated or unmyelinated. -generate, propagate, and transmit action potentials. -end distally in terminal boutons in synapses with neurons, muscle cells, and glands.

7. Nerve fibers The nerve fibers are classified by different methods. The basis of classification differs in each method. Following are the various methods of classification of the nerve fibers.

1. DEPENDING UPON THE STRUCTURE

Depending upon the structure, the nerve fibers are classified into two types namely: 1. The myelinated nerve fibers- nerve fibers are covered by myelin sheath. 2. The nonmyelinated nerve fibers-The nerve fibers of this type do not have myelin sheath.

2. DEPENDING UPON THE DISTRIBUTION • Somatic nerve fibers supply the skeletal muscles of the body. • Autonomic nerve fibers supply the various internal organs of the body.

3. DEPENDING UPON THE SOURCE OF ORIGIN • Nerve fibers arising from the brain are called cranial nerves. • Nerve fibers arising from the spinal cord are called spinal nerves.

4.DEPENDING UPON THE FUNCTION • The motor nerve fibers carry motor impulses from central nervous system to different parts of the body. These nerve fibers are also called the efferent nerve fibers. • The sensory nerve fibers carry sensory impulses form different parts of the body to the central nervous system. These nerve fibers are also known as afferent nerve fibers.

5.DEPENDING UPON THE CHEMICAL NEUROTRANSMITTER 1. Adrenergic Nerve Fibers Adrenergic nerve fibers secrete noradrenaline. 2. Cholinergic Nerve Fibers- secrete acetylcholine 6.DEPENDING UPON THE DIAMETER AND CONDUCTION

A alpha fibers are also called type Ia fibers. A beta nerve fibers are called type II fibers. A delta fibers are known as type III fibers. C fibers are also known as Type IV fibers. • Except C type of fibers, all the nerve fibers are myelinated • The velocity of impulse through nerve fiber is directly proportional to the thickness if the fibers

8. Myelin sheath -is produced in the peripheral nervous system (PNS) by Schwann cells. -is produced in the central nervous system (CNS) by oligodendrocytes. -is interrupted by the nodes of Ranvier. -consists of a spirally wrapped plasma membrane.

9. Synapses -are the sites of functional contact of a nerve cell with another nerve cell, an effector cell, or a sensory receptor cell.