The activation of target tissue receptors causes the effects associated with the sympathetic system. The two exceptions mentioned above are the postganglionic neurons of sweat glands and the chromaffin cells of the adrenal medulla. The postganglionic neurons of sweat glands release acetylcholine for the activation of muscarinic receptors. The chromaffin cells of the adrenal medulla are analogous to post-ganglionic neurons—the adrenal medulla develops in tandem with the sympathetic nervous system and acts as a modified sympathetic ganglion.
Within this endocrine gland, the pre-ganglionic neurons create synapses with chromaffin cells and stimulate the chromaffin cells to release norepinephrine and epinephrine directly into the blood. In all cases, the axon enters the paravertebral ganglion at the level of its originating spinal nerve. After this, it can then either create a synapse in this ganglion, ascend to a more superior ganglion, or descend to a more inferior paravertebral ganglion and make a synapse there, or it can descend to a prevertebral ganglion and create a synapse there with the postsynaptic cell.
The postsynaptic cell then goes on to innervate the targeted end effector i. Because paravertebral and prevertebral ganglia are relatively close to the spinal cord, presynaptic neurons are generally much shorter than their postsynaptic counterparts, which must extend throughout the body to reach their destinations.
In the cranium, preganglionic fibers cranial nerves III, VII, and IX usually arise from specific nuclei in the central nervous system CNS and create a synapse at one of four parasympathetic ganglia: ciliary, pterygopalatine, otic, or submandibular. From these four ganglia the postsynaptic fibers complete their journey to target tissues via cranial nerve V the trigeminal ganglion with its ophthalmic, maxillary, and mandibular branches.
The vagus nerve does not participate in these cranial ganglia, as most of its fibers are destined for a broad array of ganglia on or near the thoracic viscera esophagus, trachea, heart, lungs and the abdominal viscera stomach, pancreas, liver, kidneys. The pelvic splanchnic efferent preganglionic nerve cell bodies reside in the lateral gray horn of the spinal cord at the S2—S4 spinal levels.
Their axons continue away from the CNS to synapse at an autonomic ganglion close to the organ of innervation. This differs from the sympathetic nervous system, where synapses between pre- and post-ganglionic efferent nerves in general occur at ganglia that are farther away from the target organ.
The parasympathetic nervous system uses acetylcholine ACh as its chief neurotransmitter, although peptides such as cholecystokinin may act on the PSNS as a neurotransmitter. The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors.
Most transmissions occur in two stages: When stimulated, the preganglionic nerve releases ACh at the ganglion, which acts on the nicotinic receptors of the postganglionic neurons. The postganglionic nerve then releases ACh to stimulate the muscarinic receptors of the target organ. Autonomic plexuses are formed from sympathetic and parasympathetic fibers that innervate and regulate the overall activity of visceral organs.
Autonomic plexuses are formed from sympathetic postganglionic axons, parasympathetic preganglionic axons, and some visceral sensory axons. The nerves in the each plexus are close to each other, as in the plexuses of the somatic nervous system, but typically do not interact or synpase together. Sympathetic trunk : This section of the sympathetic trunk shows both the celiac and the hypogastric plexus. Instead, they provide a complex innervation pattern to the target organs, since most organs are innervated by both divisions of the autonomic nervous system.
The autonomic plexuses include the cardiac plexus, the pulmonary plexus, the esophageal plexus, and abdominal aortic plexus, and the superior and inferior hypogastric plexuses.
The cardiac plexus is a plexus of nerves situated at the base of the heart that innervates the heart. The superficial part of the cardiac plexus lies beneath the arch of the aorta, in front of the right pulmonary artery. It is formed by the superior cardiac branch of the left sympathetic trunk and the lower superior cervical cardiac branch of the left vagus nerve.
A small ganglion, the cardiac ganglion of Wrisberg, is occasionally found connected with these nerves at their point of junction. The pulmonary plexus is an autonomic plexus formed from pulmonary branches of vagus nerve and the sympathetic trunk.
It supplies the bronchial tree and the visceral pleura. The esophageal plexus is formed by nerve fibers from two sources: the branches of the vagus nerve and the visceral branches of the sympathetic trunk. The esophageal plexus and the cardiac plexus contain the same types of fibers and are both considered thoracic autonomic plexus es.
The abdominal aortic plexus is formed by branches derived, on either side, from the celiac plexus and ganglia, and receives filaments from some of the lumbar ganglia. It is situated on the sides and front of the aorta, between the origins of the superior and inferior mesenteric arteries. From this plexus arise parts of the spermatic, the inferior mesenteric, and the hypogastric plexuses; it also distributes filaments to the inferior vena cava. The superior hypogastric plexus in older texts, hypogastric plexus or presacral nerve is a plexus of nerves situated on the vertebral bodies below the bifurcation of the abdominal aorta.
The inferior hypogastric plexus pelvic plexus in some texts is a plexus of nerves that supplies the viscera of the pelvic cavity. The inferior hypogastric plexus is a paired structure, with each situated on the side of the rectum in the male, and at the sides of the rectum and vagina in the female. Parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system that lie near or within the organs they innervate.
Nerves that supply parasympathetic fibers to the parasympathetic ganglia of the head include the oculomotor nerve ciliary ganglion ; the facial nerve pterygopalatine ganglion, submandibular ganglion ; the glossopharyngeal nerve otic ganglion ; the vagus nerve no named ganglion ; and the pelvic splanchnic nerves no named ganglion.
Parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system, blue fibers. These paired ganglia supply all parasympathetic innervation to the head and neck: ciliary ganglion spincter pupillae, ciliary muscle , pterygopalatine ganglion lacrimal gland, glands of nasal cavity , submandibular ganglion submandibular and sublingual glands , and otic ganglion parotid gland. Nerve innervation of the autonomic nervous system : The parasympathetic nervous system, shown in blue, is a division of the autonomic nervous system.
Each has three roots entering the ganglion motor, sympathetic, and sensory roots and a variable number of exiting branches. The nerves that supply parasympathetic fibers to the parasympathetic ganglia of the head include the oculomotor nerve ciliary ganglion , the facial nerve pterygopalatine ganglion, submandibular ganglion , the glossopharyngeal nerve otic ganglion , the vagus nerve, and the pelvic splanchnic nerves.
Because of its location, the parasympathetic system is commonly referred to as having craniosacral outflow, in contrast to the sympathetic nervous system, which is said to have thoracolumbar outflow. Sympathetic ganglia are the ganglia of the sympathetic nervous system that initiate fight-or-flight, stress-mediated responses.
The sympathetic ganglia are the ganglia of the sympathetic nervous system the red lines in the diagram below. One of the strategies to increase ACh neurotransmission is the administration of choline in the diet. However, this has not been effective, probably because the administration of choline does not increase the availability of choline in the CNS. The majority of the ACh in nerve endings is contained in clear as viewed in the electron microscope um vesicles.
A small amount is also free in the cytosol. Vesicle-bound ACh is not accessible to degradation by acetylcholinesterase see below. The uptake of ACh into storage vesicle occurs through an energy-dependent pump that acidifies the vesicle.
No useful pharmacological agents are available to modify cholinergic function through interaction with the storage of ACh. Interestingly, the gene for VAChT is contained on the first intron of the choline acetyltransferase gene. This proximity implies the two important cholinergic proteins are probably regulated coordinately.
You will recall that the miniature endplate potentials and the quantal release in response to action potentials at the neuromuscular junction are due to the release of packets of ACh from individual storage vesicles Chapter 5. Many toxins are known that interfere with these processes and are effective in preventing ACh secretion. The examples in Figure There are two broad classes of cholinergic receptors: nicotinic and muscarinic. This classification is based on two chemical agents that mimic the effects of ACh at the receptor site nicotine and muscarine.
ACh binds to the two a subunits. The bottom half shows the molecular structure of each a subunit of the nicotinic receptor based on cDNA derived amino acid sequence. A funnel-shaped internal ion channel is surrounded by the five subunits. Muscarinic receptors, classified as G protein coupled receptors GPCR , are located at parasympathetic autonomically innervated visceral organs, on the sweat glands and piloerector muscles and both post-synaptically and pre-synaptically in the CNS see Table I.
The muscarinic receptor is composed of a single polypeptide. Because each of these regions of the protein is markedly hydrophobic, they span the cell membrane seven times as depicted in Figure The fifth internal loop and the carboxyl-terminal tail of the polypeptide receptor are believed to be the site of the interaction of the muscarinic receptor with G proteins see right.
The site of agonist binding is a circular pocket formed by the upper portions of the seven membrane-spanning regions. ACh has excitatory actions at the neuromuscular junction, at autonomic ganglion, at certain glandular tissues and in the CNS. It has inhibitory actions at certain smooth muscles and at cardiac muscle. The biochemical responses to stimulation of muscarinic receptor involve the receptor occupancy causing an altered conformation of an associated GTP-binding protein G protein.
In response to the altered conformation of the muscarinic receptor, the a subunit of the G protein releases bound guanosine diphosphate GDP and simultaneously binds guanosine triphosphate GTP. This hydrolysis terminates the action of the G protein. The rate of hydrolysis of the GTP thus dictates the length of time the G protein remains activated.
Nicotinic acetylcholine receptors: Two different subtypes of nicotinic acetylcholine receptors with alpha and beta subunits are shown. The acetylcholine binding sites are indicated by ACh. The sympathetic and parasympathetic autonomic nervous systems cooperatively modulate internal physiology to maintain homeostasis.
Describe the interactions between the sympathetic and parasympathetic divisions of the autonomic nervous system. Some processes that are modulated by the sympathetic and parasympathetic systems but that are not easily labeled as fight or rest include the maintenance of blood pressure when standing and the maintenance of regular heart rhythms.
Sympathetic and parasympathetic divisions typically function in opposition to each other. However, this opposition is better termed complementary in nature rather than antagonistic.
For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Consider sympathetic as fight or flight and parasympathetic as rest and digest or feed and breed. The subdivisions of the autonomic nervous system : In the autonomic nervous system, preganglionic neurons connect the CNS to the ganglion.
However, many instances of sympathetic and parasympathetic activity cannot be ascribed to fight or rest situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second-to-second modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles.
More generally, these two systems should be seen as permanently modulating vital functions, usually in an antagonistic fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below. The SNS promotes a fight-or-flight response, corresponds with arousal and energy generation, and performs the following functions:. The medulla oblongata, in the lower half of the brainstem, is the control center of the autonomic nervous system. The autonomic nervous system ANS is the part of the peripheral nervous system that controls involuntary functions that are critical for survival.
The ANS participates in the regulation of heart rate, digestion, respiratory rate, pupil dilation, and sexual arousal, among other bodily processes. Within the brain, the ANS is located in the medulla oblongata in the lower brainstem. The brain stem with pituitary and pineal glands : The medulla is a subregion of the brainstem and is a major control center for the autonomic nervous system. The hypothalamus acts to integrate autonomic functions and receives autonomic regulatory feedback from the limbic system to do so.
The ANS is classically divided into two subdivisions, the sympathetic division and the parasympathetic division. Preganglionic fibres of both the sympathetic and parasympathetic nervous systems secrete acetylcholine — thus nicotinic receptors see below predominate in the autonomic ganglia. Sympathetic postganglionic fibres are mostly adrenergic in nature — i.
The effect of postganglionic nerve stimulation will depend upon the receptors present at the effector site — usually alpha and beta adrenoreceptors. The effects are terminated by noradrenaline re-uptake in to the nerve terminals. A special case within the sympathetic nervous system is the nerve to the adrenal medulla. The adrenal medulla, which can be thought of as a modified autonomic ganglion, in turn secretes adrenaline in to the systemic circulation.
Parasympathetic postganglionic fibres release acetylcholine. Most effects are mediated via muscarinic receptors and actions are terminated as acetylcholine is hydrolysed by acetylcholinesterase within the synaptic cleft.
Neurotransmitters bind with specific receptors at target cells to produce their effects. Different receptor subtypes exist in each of the divisions of the ANS, and the intracellular response in the target cell and hence the target organ, is specific to the receptor type. Within the sympathetic nervous system, effects are generally mediated by adrenoreceptors. In the parasympathetic system effects are mediated generally by muscarinic acetylcholine receptors.
A further special case is that of sympathetic postganglionic fibres supplying sweat glands. These fibres secrete acetylcholine and exert their effects through muscarinic receptors.
Adrenoreceptors are subdivided into alpha, and beta receptors. Alpha receptors are G-protein linked receptors. Alpha-1 receptors act via the G-protein subgroup Gz and phospholipase C to increase cytosolic calcium levels. This leads to mainly excitatory effects — such as smooth muscle contraction. Alpha-1 receptors are widespread in the peripheral vascular tree and stimulation causes vasoconstriction, increased systemic vascular resistance and diversion of blood flow from the peripheries to the vital organs.
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