Myelinated motor nerves branch out within the perimysial connective tissue, where each nerve gives rise to several terminal twigs. At the site of innervation, the nerve loses its myelin sheath and forms a dilated termination (terminal bouton) that sits within a trough on the muscle cell surface. This structure is called the motor end-plate or myoneural junction. At this site the axon is covered by a thin cytoplasmic layer of Schwann cells. Within the axon terminal there are numerous mitochondria and synaptic vesicles, the latter containing the neurotransmitter acetylcholine. Between the axon and the muscle is a space, the synaptic cleft, in which lies an amorphous basal lamina matrix. At the junction, the sarcolemma is thrown into numerous deep junctional folds. In the sarcoplasm below the folds lie several nuclei and numerous mitochondria, ribosomes, and glycogen granules.
When an action potential invades the motor end-plate, acetylcholine is liberated from the axon terminal, diffuses through the cleft, and binds to acetylcholine receptors in the sarcolemma of the junctional folds. Binding of the transmitter makes the sarcolemma more permeable to sodium, which results in membrane depolarization (Fig.31).
Excess acetylcholine is hydrolyzed by the enzyme cholinesterase bound to the synaptic cleft basal lamina.
Fig.31.
Smooth muscle can either receive innervation via the autonomic nervous system, or hormones through the blood system can regulate them. Innervation involves nerve endings forming synapses with smooth muscle cells. They are usually in the form of swelling of axons which contain synaptic vesicles (norepinephrine or acetylcholine).
RECEPTORS
Information about the external world is conveyed to the CNS by sensory units called receptors. These structures transduce stimuli (heat, pressure, pain, etc) into signals that are capable of triggering action potentials in sensory nerves. The receptors constitute dendritic nerve endings of sensory neurons.
Morphologically, the receptors can be divided into two group, simple and compound.
Simple receptors are merely free branched or unbranched nerve endings. Free nerve endings consist of terminal branches of sensory nerve only. Another types of simple receptors may posses some sheath of glial cells.
Heat, cold and pain – these sensations are mainly mediated by free nerve endings that branch in the dermis, penetrate the basement membrane, and extend into the lower cell layers of the epidermis.
Compound receptors involve organization of associated non-neural tissues to complement the function of the neural receptors. The degree of organization may range mere encapsulation (a connective tissue capsule) to highly sophisticated arrangements such as in the eye and ear.
The sense of touch and pressure is detected by Meissner’s corpuscles, Ruffini’s endings, Merkel’s touch corpuscles, sensory endings around hairs, and perhaps by free nerve endings.
The best-studied mechanoreceptor is the Pacinian corpuscle ( or, Lamellated Corpuscle, or Corpuscle of Vater-Pacini). It is composed of a core with an inner and an outer region, and a capsule, which surrounds the core. The inner core invests the afferent nerve fiber, which loses its myelin sheath soon after entering the corpuscle. The core cells are modified Schwann cells, while the components of the capsule are 20-70 layers of fibroblasts alternating with thin collagen fibers, it resembles a sliced onion in histological sections. These corpuscles are found in the dermis and hypodermis and are also present in mesenteries and periosteum.
Meissner’s corpuscles ( or, Tactile Corpuscles) are composed of fibroblasts and thick collagen fibers surrounding an inner core of Schwann cells and nerve terminals. In histolologic sections, the horizontal arrangement of groups of Schwann cells forms layers that enfold the nerve terminals as they wind their way upward. They respond to tactile stimuli and are present mainly in the hairless skin of the palms, soles, nipples, and lips
Bulbous Corpuscles (of Krause) are spherical structures. They consist of capsule within which a nerve fiber terminates in a club-shaped manner.
Tactile Menisci (Merkel cell receptors) are small disc-like structures seen in relation to specialized epithelial cells (Merkel cells) present in the stratum spinosum of the epidermis. The discs are expanded ends of nerve fibers.
Ruffini’s endings are spindle-shaped encapsulated nerve endings found in joints and in the dermis. Within a fibrocellular sheath there are collagen fibers amongst which there are numerous unmyelinated endings of myelinated nerve fibers.
Дата добавления: 2015-12-07; просмотров: 114 | Нарушение авторских прав