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The Nerve Tissue & The Nervous System

The Nerve Tissue & The Nervous System

Cerebral Cortex

The different cell types that constitute the cerebral cortex are distributed in layers with one or more cell types predominant in each layer, fibers exhibiting a radial arrangement are also present.

Six distinct layers are recognized:

1. The molecular layer: is the outermost layer; its peripheral portion is composed predominantly of horizontally directed neuronal processes. Both dendrites & axons, deep in the molecular layer lie the infrequent, stellate or spindle-shaped horizontal cells of Cajal.

Overlying & covering the molecular cell layer is the Pia Mater.

2. External granular layer: is composed of small pyramidal  which appear high in number & little amount distance between these cells.

3. Layer of medium sized pyramidal cells: the cells in this layer appear larger than the layer before it, & present distance between these cells.

4. Internal granular layer or outer granular layer: This layer is composed of small stellate which appear high in number.

5. Internal layer of large pyramidal cells: in this layer the cells appear prominent in all characteristic features.

6. The multiform layer: lacks the pyramidal cells, composed of fusiform cells, granule cell, stellate cell, & cell of martinatti. All of these cells vary in size.

The Peripheral Nervous System

The main components of the peripheral nervous system are the nerves, ganglia, & nerve endings.

·        Nerve fibers

Consists of axons enveloped by special sheath. Groups of nerve fibers constitute the tracts of the brain, spinal cord & peripheral nerves. They are two types of fibers: myelinated & unmyelinated nerve fibers.

·        Myelinated nerve fibers:

In myelinated fibers of the peripheral nervous system, the plasmalemma of the covering Schwann cell, the layers of membrane of the sheath cell unite & form myelin, a lipoprotein complex.

Each axon is surrounded by myelin formed by sequential series of Schwann cells. The myelin sheath shows gaps along its path called the nodes of Ranvier. The distance between two nodes is called an internode segment.

·        The structure of peripheral nervous system

In the peripheral nervous system, the nerve fibers are grouped in bundles to form the nerves. They have an external fibrous coat of dense connective tissue called epineurium, which also fills the space between the bundle is surrounded by the perineurium, the bundles of nerve fibers are called fasciles, within the perineurial sheath run the Schwann cell-sheathed axons & their connective tissue, the endoneurium, which consist of a thin layer of reticular fibers which are produced by Schwann cells, & found the other cells called fibroblasts are dark & elongated in the connective tissue of the fasciles.

·        Ganglia

Ganglia are ovoid structures containing neuronal cell bodies & glial cell supported by connective tissue, one of nerves enters & another exits from each ganglion. The direction of the nerve impulse determines whether the ganglion will be a sensory or autonomic ganglion.

·        Sensory ganglia

They receive afferent impulses that go to the central nervous system. Two types:

1. Cranial Ganglia: are associated with cranial nerves.

2. Spinal Ganglia: are associated with the dorsal root of spinal cord.

The neurons of these ganglia are pseudounipolar & arranged in peripheral of ganglia.

·        Autonomic ganglia

They appear as bulbous dilatations in autonomic nerves some are located within the organ specially in the wall of GIT. The ganglia have multipolar neurons, the latter are enveloped by a layer of satellite cells & distribution irregular in whole of ganglion.