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