HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 12 PERIPHERAL NERVOUS SYSTEM

Connected to the brain by cranial nerves or to the cord by roots combining to form nerves are sensory, relay and effector structures, which send raw sensory data to the central nervous system and receive from it and carry out its instructions.

A PERIPHERAL NERVE

l Nerves fibres present may be:
l centripetal sensory fibres,
2 centrifugal motor fibres to skeletal muscle,
3 centrifugal autonomic fibres to glands, and smooth muscles.

2 Connective tissue wrappings
l Epineurium around the whole nerve trunk with blood and lymphatic vessels (vasa nervorum), collagen and fibroblasts, and fat cells.
2 Perineurium around each fasciculus of nerve fibres: the site of the blood-nerve barrier. Perineurial cells are tightly attached.
3 Endoneurium around each individual myelinated nerve fibre, but separated from its Schwann cells by a basal lamina.

3 Cross-section of nerve in LM shows:
l Close-to-round shape with no lumen; CT coat and divisions.
2 Nuclei of Schwann cells, fibroblasts and a few capillaries.
3 Axons and some remnant of myelin (so-called neurokeratin) around them (with H & E staining); or
4 brownish-black rings (myelin with an unstained axon within each) (osmium tetroxide treatment).
5 The eosin of H & E shows the collagen of epi- and perineurium, which remain very pale yellow with osmium. Osmium tetroxide will, however, show intensely black the fat in the adipocytes, usually present in epineurium.

4 Single nerve fibre
Single fibres that have branched off from nerves to pass to and enter some kind of end-organ remain unseen unless special techniques are used, although the CT capsule and supporting cells of the end-organ are usually discernible with HE staining. The fibre-revealing techniques are EM, silver impregnation, or histochemical ones for cholinesterase, neuropeptides, and catecholamines (Chapter 30.D.2.).

B RECEPTORS AND SENSORY FIBRES

Sensory fibres (except for those of cranial nerves) are derived from the dorsal root/spinal ganglion cells lying just outside the spinal cord. The fibre is T-shaped with one branch entering the cord as an element of the dorsal root, and the other coming from a sensory receptor of one of the following kinds *.

l Skin and some mucous membranes (exteroceptors)
l *Meissner's corpuscles - common in dermal papillae of fingers, palms, nipple, etc.
2 *Krause's end-bulbs and Ruffini's end-organs - in external genitalia, dermis, tongue, joints, etc.
3 *Pacinian corpuscles - large, lamellated bodies, in external genitalia, also lie more deeply under the skin, in tendons, mesentery, joints, etc.
... Receptors/endings l to 3 are definitely encapsulated.
4 *Merkel`s discs - intra-epithelial in lower layers of the epidermis and oral epithelium.
5 *Free nerve endings - also intra-epithelial.
6 *Palisade/peritrichal endings around a hair follicle.

How far the morphology of a receptor can be related to a special sensitivity to a particular modality, e.g., pain, touch, cold, is disputed.

All receptor axon terminals lack myelin and contain vesicles, mitochondria, and filaments.

2 Muscle, joint and tendon (proprioceptors)
l *Golgi tendon organ - branching nerve fibres with thickenings between a tendon's collagen fibres. Joint and ligament receptors are similar, but some take more specialized forms, e.g., Pacinian.
2 *Muscle spindle

(a) A spindle-shaped C.T. capsule encloses several thin specialized intrafusal skeletal muscle fibres. Away from the muscle fibre's central region with clustered or linear nuclei (nuclear-bag and nuclear-chain fibres) are plate or trail motor endings served by mostly gamma (fusimotor) nerve fibres.
(b) Two types of sensory ending on the fibres - primary annulospiral and secondary flower-spray - increase the combinations of innervations.
(c) The larger extrafusal muscle fibres stimulate the attached spindle by their movements, while its sensitivity is varied by its own alpha and gamma motor control.
(d) The spindle informs on the complexities of muscle contraction, movement and stretch, and gives a `muscle sense'.

3 (Mechano-receptors of the vestibular apparatus (Chapter 14.C.) inform the CNS independently of the results of the muscles' actions in terms of changed position and movement of the head. Skin receptors likewise contribute to 'proprioception' in the lax sense.

3 Viscera (interoceptors)
l Carotid body - chemoreceptor for blood O₂ tension; has sinusoids with blood passing in close relation to glomus/Type I cells. Clusters of these cells, with their cored vesicles, are innervated by axons of the glossopharyngeal nerve. The intermixed sustentacular/Type II cells are glial, and have no known role in signal transduction. The aortic body is similar in structure and function, and connects with the vagus nerve.
2 Carotid sinus and aortic arch - pressoreceptors/baroceptors (for blood pressure) set within the vessel's wall.
3 *In lung, gut, bladder, and other viscera - measuring distension, motility and chemical irritation.

4 Brain (other intero-chemoreceptors)
l In hypothalamus: for blood osmolarity, glucose, hormones (and for temperature).
2 In medulla: for CO₂ tension of the blood.

5 Special senses (extero-chemoreceptors)
1 Olfactory mucosa (smell)

(a) Three cell kinds are in the pseudostratified, columnar neuro-epithelium:
1. (i) Bipolar sensory cell, with an axon going directly through the ethmoid bone to the olfactory bulb and, at the apical end, several, long, cilium-like processes (olfactory hairs) extending into the surface coat of mucus and lipid.
2. (ii) Sustentacular cell has pigment in its apical cytoplasm and is equipped with microvilli.
3. (iii) Basal cells to divide and replace sustentacular and sensory cells.
(b) All cell types rest on a BL penetrated by axons that become grouped as unmyelinated nerves in the lamina propria.
(c) Bowman's compound serous glands also contribute secretion to the surface. Unlike other serous cells, they have much smooth ER.

2 Taste bud (taste)
(a) Barrel-shaped; lying within the stratified squamous epithelium of the tongue's circumvallate and fungiform papillae
(b) At the apex towards the opening of the taste pore, project processes of two fusiform cell kinds: (i) thin, neuro-epithelial receptor cells (dark, light, and intermediate), and (ii) paler sustentacular or supporting cells.
(c) Receptor cells have axons from the facial and glossopharyngeal nerves terminating synaptically upon them. (Taste buds in the pharynx, epiglottis and oesophagus are served by the vagus nerve.)
(d) Towards the bottom of the taste bud are basal cells, proliferating slowly to replace receptor cells.
(e) Von Ebner's glands in the lamina propria send a serous secretion into the trench around the vallate papilla, in whose walls the taste buds lie.

C MOTOR ENDINGS AND MOTOR FIBRES

l Skeletal-muscle motor fibres are derived from motor neurons/motoneurons within the CNS, either in the ventral-horn grey matter of the spinal cord or in motor nuclei of cranial nerves.
l Light microscopic view (after-gold-chloride impregnation). Nerve fibre branches to serve several skeletal muscle fibres, terminating on each as an irregular branching net lying on a small area of the muscle cell, its sole plate.

2 End-plate or neuromuscular/myoneural junction (EM morphology)

(a) The sarcolemma is depressed into a trough.
(b) The swollen ending of the axon lies in the trough.
(c) The sarcolemma lining the trough invaginates into many small junctional folds/secondary synaptic clefts. It has ACh receptors.
(d) The space between the sarcolemma and axolemma is filled by a basal lamina-like material.
(e) The axon terminal has mitochondria, and vesicles containing the chemical transmitter substance, acetylcholine, which, when released by electrical activity in the axon, indirectly causes the muscle fibre to contract.
(f) Schwann cells cover the axon and its terminal bag.

3 A motor unit comprises a motoneuron and all muscle fibres on which it has end-plates. .

2 Postganglionic autonomic nerve fibre terminals
l Control smooth muscle contraction and exocrine glandular secretion, or go to the heart muscle and adrenal medullary cells.
2 Axons lie against, or sometimes within, invaginations of the muscle fibres or glandular cells, making mostly en passant contacts,
3 but specialized sarcolemmal structures comparable with a motor end-plate's are not present.
4 The nerve fibres are, however, widely dispersed as a plexus between the smooth muscle fibres, and contain many vesicles concentrated periodically.
5 These vesicles may contain one of the two principal transmitter substances - acetylcholine (ACh) and norepinephrine (Ne)/noradrenaline, along with other chemicals, e.g., peptides. Some neurons and fibres are neither cholinergic nor adrenergic. A chemical mapping of the PNS (crucial to pharmacology) is under way, including the sensory pathways to autonomic ganglia.

3 Origin of autonomic nerve fibres
l Parasympathetic

Cranial nerves: III,VII,IX,X   |______  have parasympathic
Sacral pelvic nerves           |        preganglionic fibres (ACh)

These run near or into the organ to be controlled before synapsing with local parasympathetic ganglion neurons (e.g., of Auerbach`s plexus), whose own short post-ganglionic fibres (ACh) innervate the muscle or glandular tissue.

2 Sympathetic
Thoraco-lumbar outflow has sympathetic preganglionic fibres (ACh) synapsing with neurons of the sympathetic ganglion chain along the vertebral bodies or going farther to ganglia, e.g., coeliac, serving a visceral or cranial region. Sympathetic post-ganglionic fibres (usually Ne) thence pass to the muscle or gland to be controlled. (Ac - acetylcholine is the transmitter substance; the fibre is called cholinergic. Ne - norepinephrine is the transmitter substance; the fibre is called adrenergic.)
Beware. There can be more than one transmitter, for example, ATP can be a cotransmitter for both sympathetic and parasympathetic neurons, making these also purinergic.

3 Adrenal medulla
Receives direct, cholinergic, preganglionic, sympathetic fibres whose activity causes the release of norepinephrine (and epinephrine/adrenaline) into the blood, thus contributing to a widespread sympathetic tone.

D GANGLIA (relay structures)

l Spinal/dorsal root ganglion (no synapse involved)
l Has a collagenous connective tissue investment.
2 Many bundles of thick, myelinated, nerve fibres separate
3 groups of large, round-bodied nerve cells.
4 Each neuron has a thin CT capsule like an endoneurium.
5 Between capsule and neuron is a layer of small satellite cells of a glial nature.
6 Neuron has only one process (not a dendrite) branching into two near to the soma. The thinner axon runs centrally via a dorsal root into the spinal cord, the thicker runs peripherally to a nervous receptor.

2 Autonomic ganglion (compared with a spinal ganglion)
l Fewer myelinated fibres are present.
2 Neurons and fibres are interspersed.
3 Neurons are smaller and have dendrites, with preganglionic fibres synapsing upon them.
4 Many of the neurons' own axons (post-ganglionic fibres) are unmyelinated.

In a cross-sectional view, several unmyelinated fibres share one Schwann cell, lying in many deep invaginations of its membrane. In the gut, enteric glia take the place of Schwann cells.

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159