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Main Model

Anterior : Schwann cell

Peripheral Nervous System

The PNS includes all neuronal elements outside the brain and spinal cord. The peripheral nerves are the cranial and spinal nerves.

The PNS contains two supporting cell types:

1. Schwann cells, analogous to the oligodendrocytes of the CNS.

2. Satellite cells, Schwann cell - like surrounding the cell bodies of neurons in sensory and autonomic ganglia.

Individual nerve fibers of the PNS are ensheathed by Schwann cells. In myelinated fibers, individual Schwann cells wrap around the axon, forming a myelin sheath analogous to that of the oligodendrocytes of the CNS. In unmyelinated fibers, a single Schwann cell envelops several axons.

There are two important differences between Schwann cells and oligodendrocytes:

1. A single Schwann cell forms only one internodal segment of myelin, whereas a single oligodendrocyte may form 40 or 50 internodes.

2. Unmyelinated fibers in the PNS are embedded in Schwann cells, whereas those in the CNS are not ensheathed by oligodendrocytes but may have an investment of astrocytes.

Structure of a Peripheral Nerve

Connective tissue coverings divide the peripheral nerve into three segments, each with unique structural characteristics:

1. The epineurium.

2. The perineurium.

3. The endoneurium.

The epineurium is formed by type I collagen and fibroblasts and covers the entire nerve. It contains arteries, veins and lymphatic vessels.

Within the nerve, the perineurium segregates axons into fascicles. The perineurium consists of several concentric layers of neuroepithelial perineurial cells with two distinct characteristics:

1. A basal lamina, consisting of type IV collagen and laminin, surrounds the layers of perineurial cells.

2. Perineurial cells are joined to each other by tight junctions to form a protective diffusion barrier: the blood-nerve barrier, responsible for maintaining the physiologic microenvironment of the endoneurium.

The endoneurium surrounds individual axons and their associated Schwann cells and myelin sheaths. It consists of type III collagen fibrils, a few fibroblasts, macrophages, mast cells and endoneurial capillaries between individual the axons lor nerve fibers.

Multiple unmyelinated axons are individually encased within recesses of the cytoplasm of Schwann cells. Unmyelinated axons do not undergo the spiral concentric lamination and myelin formation. For future reference in neuropathology, keep in mind the Luxol fast blue staining method widely used for myelin staining.

Additional components of the blood-nerve barrier are the endothelial cells of the endoneurial capillaries. Endoneurial capillaries derive from the vasa nervorum and are lined by continuous endothelial cells joined by tight junctions.

Pathology: Schwannomas

Schwannomas are benign encapsulated tumors consisting of Schwann cells. Keep in mind that Schwann cells are present in all peripheral nerves. Therefore, schwannomas can be found in many sites (intracranial, intraspinal and extraspinal locations).

Schwannomas can develop at the surface or inside of a nerve fascicle and display spindle cells (called Antoni A pattern) or multipolar cells (called Antoni B pattern), the latter representing the result of a degenerative process. All schwannomas are immunoreactive for S-100 protein (a calmodulin-like cytosolic protein present in cells derived from the neural crest), type IV collagen and laminin. Schwannomas need to be distinguished from neurofibromas, that may contain Schwann cells.

Pathology: Segmental Demyelination and Axonal Degeneration

Diseases affecting Schwann cells lead to a loss of myelin, or segmental demyelination. Damage to the neuron and its axon leads to axonal degeneration (wallerian degeneration, first described by the English physiologist Augustus Volney Waller, 1816-1870).

Axonal degeneration may be followed by axonal regeneration. The motor unit is the functional unit of the neuromuscular system. Therefore, segmental demyelination and axonal degeneration affect the motor unit and cause muscle paralysis and atrophy. Physiotherapy for the paralyzed muscles is necessary to prevent muscle degeneration before regenerating motor axons can reach the motor unit.

Neurotrophins play a significant role in the survival of neurons uncoupled from a peripheral target.

Segmental demyelination occurs when the function of the Schwann cell is abnormal or there is damage to the myelin sheath, for example, a traumatic nerve injury. If the nerve fiber is completely severed, the chances of recovery decrease unless a nerve segment is grafted.

The presence of the endoneurium is essential for the proliferation of Schwann cells. Schwann cells guide an axonal sprout, derived from the proximal axonal stump, to reach the end organ (for example, a muscle).

Several sprouts can grow into the connective tissue and, together with proliferative Schwann cells, form a mass called an traumatic neuroma. Traumatic neuromas prevent regrowth of the axon after trauma and must be surgically removed to allow reinnervation of the peripheral end organ.

Axonal regeneration is a very slow process. It starts 2 weeks after injury and is completed, if successful, after several months. Schwann cells remyelinate the denuded portion of the axon, but the length of internodal myelin is shorter.

Axonal degeneration results from the primary destruction of the axon by metabolic or toxic damage and is followed by demyelination and degeneration of the neuronal cell body. This process is known as a "dying back" neuropathy.

Regeneration of nerve fibers in the CNS is not possible at present because of the following factors:

1. An endoneurium is not present.

2. Oligodendrocytes do not proliferate in contrast to Schwann cells, and a single oligodendrocyte serves a large number of axons.

3. Astrocytes deposit scar tissue (the astrocytic plaque).

Sensory (Spinal) Ganglia

A cluster of neurons forms a ganglion (plural ganglia). A ganglion can be sensory (dorsal root ganglia and trigeminal ganglion) or motor (visceromotor or autonomic ganglia). Axons derived from a ganglion are organized as nerves, rami (singular ramus), or roots.

Sensory ganglia of the posterior spinal nerve roots and the trunks of the trigeminal, facial, glossopharyngeal, and vagal cranial nerves have a similar organization.

A connective tissue capsule, representing the continuation of the epineurium and perineurium, surrounds each ganglion.

Neurons are pseudounipolar, with a single stem myelinated process leaving each cell body. The short process bifurcates into a peripheral centrifugal branch into one ramus of the spinal nerve and a centripetal branch into the spinal cord.

The neuronal cell body is surrounded by a layer of flattened satellite cells, similar to Schwann cells and continuous with them as they enclose the peripheral and central process of each neuron.

Following stimulation of the peripheral sensory receptor, nerve impulse reach the T-bifurcation junction bypassing the neuronal cell body, traveling from the peripheral axon to the centripetal axon.