Peripheral Nervous System (PNS)

Peripheral Nervous System (PNS)

The PNS comprises all nervous tissue outside the brain and spinal cord. It consists of groups of neurones (ganglion cells), called ganglia, feltworks of nerve fibres, called plexuses, and bundles of parallel nerve fibres that form the nerves and nerve roots. Nerve fibres, which originate from neurones within the CNS and pass out of the CNS in cranial and spinal nerves, are called efferent or motor fibers. Nerve fibres which originate from nerve cells outside the CNS but enter the CNS by way of the cranial or spinal nerves are called afferent or sensory nerve fibres.

The principal neurotransmitters in the PNS are acetylcholine and noradrenalin.

Peripheral Nerves

Afferent, sensory fibres enter the spinal cord via the dorsal roots, while efferent, motor fibres leave the spinal cord via the ventral roots. Dorsal and ventral roots merge to form the spinal nerves, which consequently contain both sensory and motor fibres. As the spinal nerves travel into the periphery they split into branches and the exact composition of the nerve in terms of motor and sensory fibres is, of course, determined by the structures the nerve will innervate.

One nerve fibre consists of an axon and its nerve sheath. Each axon in the peripheral nervous system is surrounded by a sheath of Schwann cells. An individual Schwann cell may surround the axon for several hundred micrometers, and it may, in the case of unmyelinated nerve fibers, surround up to 30 separate axons. The axons are housed within infoldings of the Schwann cell cytoplasm and cell membrane, the mesaxon.

In the case of myelinated nerve fibres, Schwann cells form a sheath around one axon and surround this axon with several double layers (up to hundreds) of cell membrane. The myelin sheath formed by the Schwann cell insulates the axon, improves its ability to conduct and, thus, provides the basis for the fast saltatory transmission of impulses. Each Schwann cell forms a myelin segment, in which the cell nucleus is located approximately in the middle of the segment. The node of Ranvier is the place along the course of the axon where two myelin segments abut.

Fibre types in peripheral nerves:

• Type A fibres (myelinated) are 4 - 20 µm in diameter and conduct impulses at high velocities (15 - 120 m per second). Examples: motor fibers, which innervate skeletal muscles, and sensory fibres.
• Type B fibres (myelinated) are 1 - 4 µm in diameter and conduct impulses with a velocity of 3 - 14 m per second. Example: preganglionic autonomic fibres.
• Type C fibres (unmyelinated) are 0.2 - 1 µm thick and conduct impulses at velocities ranging from 0.2 to 2 m per second. Examples: autonomic and sensory fibres.

Peripheral nerves contain a considerable amount of connective tissue. The entire nerve is surrounded by a thick layer of dense connective tissue, the epineurium. Nerve fibres are frequently grouped into distinct bundles, fascicles, within the nerve. The layer of connective tissue surrounding the individual bundles is called perineurium. The perineurium is formed by several layers of flattened cells, which maintain the appropriate microenvironment for the nerve fibres surrounded by them. The space between individual nerve fibres is filled by loose connective tissue, the endoneurium.

Fibrocytes, macrophages and mast cells are present in the endoneurium.
Nerves are richly supplied by intraneural blood vessels, which form numerous anastomoses. Arteries pass into the epineurium, form arteriolar networks beneath the perineurium and give off capillaries to the endoneurium.

Suitable Slides

sections of peripheral nerve - H&E, osmium or plucked preparations of peripheral nerve - osmium

Peripheral Nerve, cat - osmium
Which structures can be recognized in peripheral nerves depends on the stain that has been used in the preparation. Osmium gives a black color to lipids. In osmium stained preparations it is possible to observe the myelin sheath surrounding the axon. A good impression of the different sizes of the nerve fibres may be obtained. The axon is usually not well preserved. It may only form a little dark spot somewhere within the dark ring which represents the myelin sheath. Lipid droplets in fat cells, which can be found in the connective tissue around nerves, stand out as large (much larger than the nerve fibres), round, homogeneously stained areas.
Draw the nerve at low magnification (you may include some of the stained lipid droplets) and a small section of it at high magnification.

Peripheral Nerve, rat - H&E
In longitudinal H&E stained sections it is possible to identify the axon running in its myelin sheath, nodes of Ranvier and Schwann cell nuclei. Components of the connective tissue elements, which accompany the nerve, should be visible and identifiable in both longitudinal and transverse sections. H&E stained and transversely cut preparations give a good picture of the axon in the middle of a ring-like structure (sometimes fussy), which represents the remains of the myelin sheath. Due to their small size and the lack of a myelin sheath, type C fibres are very difficult to detect in either osmium or H&E stains.
Draw part of the longitudinally and transversely sectioned nerve at high magnification. Include Schwann cell nuclei, myelin sheath, axons and, if possible, nodes of Ranvier.

Title: Peripheral Nervous System (PNS)
by:om at 2013-02-15T03:34:00+07:00
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Peripheral Nervous System (PNS)