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Neuron

الكلية كلية طب الاسنان     القسم  العلوم الاساسية     المرحلة 2
أستاذ المادة حيدر حميد عباس الحيدري       09/09/2020 13:28:37
AUTONOMIC NERVOUS SYSTEM

The autonomic nervous system (ANS) is the portion of the nervous system that operates largely unconsciously to control entirely or partially the visceral functions of the body throughout the day. However, some visceral sensations do give rise to conscious recognition, such as hunger, nausea, and fullness of urinary bladder and rectum. Some visceral mechanisms, which have been considered inaccessible to conscious control can be brought under partial voluntary influence (blood pressure and heart rate can be influenced directly by persons skilled in the arts of yoga, meditation, and relaxation).
The ANS innervates smooth muscles, cardiac muscle, and glands of the body. ANS helps control arterial pressure, gastrointestinal tract (GIT) motility and secretion, urinary bladder emptying, sweating, body temperature, and many other activities.
The ANS often operates by centers in the spinal cord (such as emptying of urinary bladder, are integrated in the spinal cord), brainstem (such as centers for control of blood pressure, heart rate, and respiration), hypothalamus (such as temperature regulation center and centers to control hunger and thirst), and also portions of the cerebral cortex. Higher centers can transmit impulses to lower centers and influence autonomic control. For instance stimulation in appropriate areas of the hypothalamus can activate the medullary cardiovascular control centers strongly enough to increase the arterial pressure to more than double normal. Portions of the cerebral cortex can influence the autonomic activities by connections with lower centers in the hypothalamus and brainstem.

The ANS often operates by means of visceral reflexes i.e. sensory impulses initiated in visceral receptors are relayed via visceral afferent pathways to the central nervous system (CNS) and are integrated within it at different levels which in turn transmit reflex responses via efferent autonomic pathways to the visceral effectors to control their activities. Visceral reflexes can also be initiated by impulses passing through somatic afferent fibers.

ANS differs from the somatic motor system in the following ways:
[I] Anatomically: In somatic motor system, each motor pathway from spinal cord to skeletal muscle is composed of a single myelinated neuron. In the autonomic nervous system, the pathway from spinal cord to the stimulated organ is made up of two neurons instead of one:
A: Preganglionic neurons, Which are centrally located myelinated neurons (type B fibers) that
synapse on cell bodies of postganglionic neurons.
B: Postganglionic neurons That are located in all cases outside the CNS. The axons of
postganglionic neurons (mostly unmyelinated C fibers) end on visceral effectors.
[2] Functionally: Somatic motor neurons have only an excitatory effect on skeletal muscle, there are no peripheral inhibitory actions exerted on skeletal muscle. In the ANS the postganglionic fibers cause excitation or inhibition of the innervated organ.

The efferent autonomic signals are transmitted to the body through two major divisions, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS) (this is an anatomic and functional division).
Anatomic Organization of Sympathetic Nervous System
The preganglionic neurons have their cell bodies in the intermediolateral gray column of spinal cord segments (TI - L2) and their axons pass through the anterior nerve roots into the corresponding spinal nerves. Then soon leave the spinal nerves and pass into the paravertebral sympathetic ganglion chain (a chain of ganglia that extend along either side of vertebral column from base of skull to the coccyx). Then the course of the fibers can be one of the following ways:
[1]- They can pass up or down the chain to end in paravertebral ganglia at higher or lower levels than the point of entrance.
[2]- They can synapse with postganglionic neurons in the ganglia that they enter.
[3]- They can pass through the sympathetic chain without synapsing, then through one of the sympathetic nerves radiating outward from the chain and finally terminating in one of the prevertebral (collateral) ganglia such as the celiac ganglion.
[4]- They can pass through the sympathetic chain and prevertebral ganglia without synapsing and innervate adrenal medulla chromaffin cells, which secrete catecholamines.

The postganglionic neurons originate either in one of the sympathetic chain ganglia or in a prevertebral ganglion then travel to the various organs and smooth muscles in blood vessels’ wall, sweat glands and piloerector muscles connected with hair follicles in skin.

Special nature of sympathetic nerve endings in the adrenal medulla: Preganglionic nerve fibers pass all the way without synapsing from the spinal cord to the sympathetic chains, to splanchnic nerves to adrenal medullae, there they end directly on special cells that secrete epinephrine and norepinephrine and some dopamine directly’ into the blood stream. These cells are modified neuronal cells analogous to postganglionic neurons and even have rudimentary nerve fibers that secrete the hormones.
Visceral afferent fibers (usually myelinated), conveyed along the sympathetic division of the ANS, travel from the viscera through the sympathetic ganglia without synapsing. They enter the spinal nerve via the white rami communicantes and reach their cell bodies in the dorsal root ganglion of the corresponding spinal nerve. The central axons then enter the spinal cord and may form the afferent component of a local reflex arc. Others may pass up to higher autonomic centers in the brain.

Anatomic Organization of Parasympathetic Nervous System
PNS has a cranial outflow and a sacral one. In the cranial outflow the preganglionic fibers leave the CNS through cranial nerves Ill, VII, IX, and X. In the sacral outflow the preganglionic neurons originate in the intermediate gray column of spinal cord segments S2 and S3 and occasionally SI and S4. Their fibers leave the spinal cord through the anterior roots of the corresponding spinal nerves and then leave the spinal nerves and form the pelvic nerve on each side of spinal cord. The preganglionic fibers in both outflows end on short postganglionic neurons in ganglia located near or on the visceral structures.
Visceral afferent fibers (usually myelinated) conveyed along the parasympathetic division of the ANS, travel from the viscera to their cell bodies located either in the equivalent of dorsal root ganglia for the cranial nerves (VII, IX, and X) or in the dorsal root ganglia of the sacrospinal nerves. The central axons then enter the CNS and take part in the formation of local reflex arcs, or pass to higher centers of the autonomic nervous system.

Chemical Transmission at Autonomic Junctions
Transmission at the synaptic junctions between pre- and postganglionic neurons and between the postganglionic neurons and the autonomic effectors is chemically mediated. The principal transmitter agents involved are acetylcholine (Ach) and norepinephrine (noradrenaline), although dopamine is also secreted by interneurons in the sympathetic ganglia and gonadotropin-releasing hormone (GnRH) is secreted by some of the preganglionic neurons. In addition, there are cotransmitters in autonomic neurons: in some neurons VIP (vasoactive intestinal polypeptide) is released with Ach. and in some neurons ATP and neuropeptide Y with norepinephrine (NE).
Those neurons that secrete Ach are said to be cholinergic, and those that secrete NE are said to be noradrenergic (the term adrenergic is also applied to these neurons). On the basis of the chemical mediator released, the ANS is divided into cholinergic and noradrenergic divisions.
The neurons that are cholinergic are:
I- All preganglionic neurons in SNS and PNS.
2- All or almost all of the postganglionic neurons of PNS.
3- Postganglionic sympathetic nerve fibers to the sweat glands and to few blood vessels in skeletal muscles, which produce vasodilatation when stimulated. The remaining postganglionic sympathetic neurons are noradrenergic.

Secretion of Ach and NE by postganglionic Nerve Endings: A few of the postganglionic autonomic nerve endings, especially those of the parasympathetic nerves, are similar to but much smaller than those of the skeletal neuromuscular junction, however, some of the parasympathetic nerve fibers and almost all the sympathetic fibers merely touch the effector cells of the organs that they innervate as they pass by; in some instances, they terminate in connective tissue located adjacent to the cells that are to be stimulated. Where these filaments pass over or near the effector cells, they usually have bulbous enlargements called varicosities; it is in these varicosities that their transmitter vesicles of Ach or NE are found. Also in the varicosities are large number of mitochondria to supply the ATP required to energize Ach and NE synthesis.
When an action potential spreads over the terminal fibers, the depolarization process increases the permeability of the fiber membrane to Ca2+, allowing them to diffuse into the nerve terminals or nerve varicosities. There the Ca interacts with those secretory vesicles that are adjacent to the membrane, causing them to fuse with the membrane and to empty their contents to the


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