Anatomy and Physiology Chapter 15 - The ANS
What are the three courses a preganglionic fiber may follow after entering the sympathetic ganglion chain?
1. Some end in the ganglion that they enter and synapse immediately with a postganglionic neuron 2. Some travel up or down the chain and synapse in ganglia at other levels. It is these fibers that link the paravertebral ganglia into the chain. They are the only route by which ganglia at the cervical, sacral, and coccygeal levels receive input. 3. Some pass through the chain without synapsing and continues as splanchnic nerves.
What are two reasons that can explain how different autonomic neurons have such contrasting effects?
1. Sympathetic and parasympathetic fibers secrete different neurotransmitters 2. Target cells respond in different ways even to the same neurotransmitter depending on what type of receptors they have for it
Parasympathetic fibers leave brainstem in following cranial nerve: Vagus nerve (X); what three networks does this nerve form and what are each of those networks responsible for?
90% of all parasympathetic preganglionic fibers carried by this nerve; the three networks are the cardiac plexus (which supplies fibers to the heart), pulmonary plexus (supplies fibers to bronchi and blood vessels in lungs), and esophageal plexus (supplies fibers that regulate swallowing). These plexuses give off VAGAL TRUNKS, which contribute to the extensive abdominal aortic plexus where sympathetic fibers synapse. The parasympathetic fibers however pass through this plexus without synapsing. They go lower.
Acetylcholine (ACh); what is it secreted by, and what are two categories of receptors?
ACh is secreted by the PREganglionic fibers in BOTH divisions, and the POSTganglionic fibers of the PARAsympathetic division. Two categories of receptors are muscarinic receptors and nicotinic receptors.
basic sympathetic wiring
Arises from thoracic and lumbar regions of spinal cord; short preganglionic and long postganglionic; axons exit via spinal nerves T1 to L2 and leads to sympathetic chain of ganglia. Lots of divergence.
Enteric Nervous System: what does it do, and what does it have to do with sympathetic/parasympathetic autonomic nervous system?
Digestive tract has own nervous system; innervates smooth muscle of tract. Has its own reflex arc. The enteric nervous system regulates the motility of the esophagus, stomach, and intestines and the secretion of digestive enzymes and acids. To function normally, also requires regulation by the sympathetic and parasympathetic systems.
True or False: Visceral effectors depend on the autonomic NS to function. Example: The heart cannot function when autonomic nerves are severed.
False: visceral effectors do not depend on the autonomic NS to function, but only to adjust their activity to the body's changing needs. The heart can go on beating without autonomic nerves, but it cannot adjust its rate based on what the body needs.
True or False: dual innervation is required for the ANS to produce opposite effects on an organ
False; dual innervation is not always necessary for the ANS to produce opposite effects on an organ. For example, sympathetic effect on blood pressure does not need parasympathetic effect to rise or lower BP. The firing frequency of sympathetic stimulation can control BP alone; i.e., the increase in frequency of firing rate makes blood vessels constrict more and therefore raises BP, but the reduction in firing frequency allows blood vessels to relax and lower BP.
How do signals travel in autonomic pathways?
In autonomic pathways, the signal must travel across two nerve fibers to get to the target organ, and it must cross a synapse where these two neurons meet in an autonomic ganglion.
Varicosities
In contrast to somatic motor neurons, postganglionic fibers of the ANS do not end by synapsing with a specific target cell, but rather with a chain of varicosities that diffusely release neurotransmitter into the tissue and may stimulate many cells simultaneously. Common varicosities: Ach and NE
Muscarine receptors; what is it and what organs have this kind of receptors?
It is a cholinergic receptor (ACh receptor); all cardiac muscle, smooth muscle, and gland cells that receive cholinergic innervation have muscarinic receptors. There are different subclasses of muscarinic receptors with different effects; thus, ACh excites intestinal smooth muscle by binding to one type of muscarinic receptor and inhibits cardiac muscle by binding to a different type. Works through a variety of second-messenger systems.
Nicotinic receptors; where do they occur, how do they work, and are they excitatory or inhibitory?
It is a cholinergic receptor (ACh receptor); is always excitatory. These receptors are all all ANS ganglia synapses, on cells of adrenal medulla, and also at neuromuscular junctions on skeletal muscle. Work by opening ligand-gated ion channels.
Enteric nervous system: what are its two main components, and what is its influence on the digestive system?
Main components are two plexuses: myenteric (between muscle layers), and submucosal. Its influence on the digestive system is motility, secretion, and blood flow.
Dual Innervation
Most of the viscera receive nerve fibers from BOTH the sympathetic and parasympathetic divisions; in such cases, the two divisions may have either antagonistic or cooperative effects on the same organ.
The sympathetic nerve route: (preganglionic fibers after entering sympathetic chain)
Other POSTganglionic fibers leave by way of sympathetic nerves directly after chain; Extends to heart, lungs, esophagus, and thoracic blood vessels. These nerves form carotid plexus.
What is a possible explanation for the selectivity of parasympathetic wiring?
Parasympathetic wiring is based off of long preganglionic fibers and short postganglionic fibers; this being said, a postganglionic fiber reaches its target organ before even slight divergence can occur (because they are so short).
The superior branch of the facial nerve's (VII) parasympathetic fiber ends at the __________________________ ganglion, and its postganglionic fibers innervate:
Pterygopalatine; tear glands and glands of nasal cavity
The spinal nerve route: (preganglionic fibers after entering sympathetic chain)
Some POSTganglionic fibers exit a ganglion by way of the gray ramus, return to the spinal nerve or its subdivisions, and travel the rest of the way to the target organ.
The splanchnic nerve route
Some of the fibers that arise from spinal nerves T5 to T12 pas through the sympathetic ganglia without synapsing; beyond the ganglia they continue as splanchnic nerves, which lead to a second set of ganglia called collateral (prevertebral) ganglia. It is here that preganglionic fibers synapse with the postganglionics.
Celiac ganglion postganglionic target organs:
Stomach, spleen, liver, pancreas, small intestine, and kidneys
The inferior branch of the facial nerve's (VII) parasympathetic fiber ends at the __________________________ ganglion, and its postganglionic fibers innervate:
Submandibular; salivary glands in floor of the mouth
What levels of the CNS influences the ANS?
The ANS is influenced by several levels of the CNS: the cerebral cortex, the hypothalamus, the midbrain, pons, and medullar oblongata, and also the spinal cord itself have effects on the ANS.
Describe how the autonomic nervous system is influenced by the central nervous system's hypothalamus:
The major control center of the visceral motor system is the hypothalamus. This vital region contains many nuclei for primitive functions including hunger, thirst, thermoregulation, emotions, and sexuality.
Describe how the autonomic nervous system is influenced by the central nervous system's cerebral cortex:
The mind definitely influences the control over ANS: anger rises blood pressure, fear makes the heart race, thoughts of food make the stomach growl, etc. The limbic system, an ancient part of the cerebral cortex, is involved in many emotional responses and provides a pathway connecting sensory and mental experiences with the ANS.
Describe how the autonomic nervous system is influenced by the central nervous system's spinal cord:
The spinal cord integrates such autonomic reflexes as micturition (urination), defecation, erection, and ejaculation.
What is the solar plexus?
The term solar plexus is used by some authorities as a collective name for the celiac and superior mesenteric ganglia, and by others as a synonym for the celiac ganglion only. The term comes from the nerves radiating from the ganglion like the rays of the sun
Dual Innervation: Cooperative Effects
These effects are seen when the two divisions act on different effectors to produce a unified overall effect. For example, in salivation the parasympathetic division stimulates serous cells of the salivary glands to secrete a watery, enzyme-rich secretion while the sympathetic division stimulates the secretion of mucous. These are both necessary components of saliva.
Dual Innervation: Antagonistic Effects
These effects between the two divisions OPPOSE each other. For example, the sympathetic division speeds up the heart and the parasympathetic division slows it down. In some cases these effects are exerted through dual innervation of the same effector cells, as in the heart, where nerve fibers of both divisions terminate on the same muscle cells. In other cases, they innervate different effector cells with opposite effects on organ function. In the iris of the eye, the sympathetic fibers innervate pupillary dilator and parasympathetic fibers innervate the constrictor.
Describe how the autonomic nervous system is influenced by the central nervous system's midbrain, pons, and medulla oblongata:
These regions house numerous ANS nuclei like the centers for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, pupillary constriction/dilation, etc. Travel by way of parasympathetic nerves (oculomotor, facial, glossopharyngeal, and vagus nerves).
Parasympathetic fibers leave brainstem in following cranial nerve: Glossopharyngeal nerve (IX); what do its preganglionic fibers form after leaving it, what ganglia does its preganglionic fibers end at, and what does its postganglionic fibers innervate?
This nerve carries parasympathetic fibers concerned with salivation. The preganglionic fibers leave this nerve soon after its origin and form the TYMPANIC NERVE. A continuation of this nerve crosses the middle-ear cavity and ends at the OTIC GANGLION near the foramen ovale. The postganglionic fibers then follow the trigeminal nerve to the parotid salivary gland.
Parasympathetic fibers leave brainstem in following cranial nerve: Oculomotor nerve (III); where does its preganglionic fibers terminate and what does it control?
This nerve carries parasympathetic fibers that control the lens and pupil of the eye. The preganglionic fibers enter the orbit and terminate in the CILIARY GANGLION behind the eyeball. Postganglionic fibers enter the eyeball and innervate the ciliary muscle, which thickens the lens, and the pupillary constrictor, which narrows the pupil
Parasympathetic fibers leave brainstem in following cranial nerve: Facial nerve (VII); what part of brain does it arise from, which TWO ganglias does this nerves preganglionic nerves synapse, and what does it innervate?
This nerve carries parasympathetic fibers that regulate the tear glands, salivary glands, and nasal glands. Soon after this nerve emerges from the PONS, its parasympathetic fibers split away and form two smaller branches. The superior branch ends at the PTERYGOPALATINE GANGLION near the junction of the maxilla and palatine bone. Postganglionic fibers then continue to the tear glands and glands of the nasal cavity, palate, and other areas of the oral cavity. The inferior branch crosses the middle-ear cavity and ends at the SUBMANDIBULAR GANGLION near the angle of the mandible. Postganglionic fibers from here supply salivary glands in the floor of the mouth.
Norepinephrine (NE); what is it secreted by and what are the two principal categories of NE receptors?
This neurotransmitter is secreted by nearly all sympathetic postganglionic fibers. The two principal categories of NE receptors are alpha-adrenergic receptors and beta-adrenergic receptors.
True or False: Sympathetic effects last longer than parasympathetic effects. Explain.
True; after a parasympathetic fiber secretes Ach into a synapse, it is quickly broken down by AChE and its effects last only a few seconds. The NE released however by a sympathetic fiber, has various fates that sometimes allow NE and epinephrine to circulate throughout the body for several minutes before finally being degraded.
True or False: Each division of the ANS (sympathetic and parasympathetic) can have contrasting effects on different organs.
True; for example, the parasympathetic division contracts the wall of the urinary bladder but relaxes the internal urethral sphincter. Both actions are necessary for the expulsion of urine and employs acetylcholine for both purposes.
Alpha-adrenergic receptors
Usually excitatory with some inhibitory effects. Has two subclasses: alpha1 and alpha2. Receptors of the alpha1 type act through calcium ions as a second messenger, whereas alpha2 receptors inhibit the synthesis of cyclic AMP (cAMP).
Beta-adrenergic receptors
Usually inhibitory with some excitatory effects (NE relaxes and dilates the bronchioles). Also has two subclasses: beta1 and beta2. Mediate different effects, but both types act through cAMP as second messenger.
Adrenal medulla secretes
a mixture of hormones into the bloodstream; epinephrine and norepinephrine and dopamine. These hormones, the catecholamines, are neurotransmitters
All autonomic nerve fibers secrete either:
acetylcholine or norepinephrine
Sympathetic division
adapts the body in many ways for physical activity; it increases alertness, heart rate, blood pressure, pulmonary airflow, blood glucose concentration, and blood flow to cardiac and skeletal muscle by at the same time it reduces blood flow to the skin and digestive tract.
basic parasympathetic wiring
also called the craniosacral division because it arises from the brain and sacral region of the spinal cord. Has long pre-ganglia and short post-ganglia. Has much less convergence and divergence; more selective/narrow in its stimulation of target organs
high blood pressure activates visceral _______________; this stimulates stretch receptors called _________________ in the carotid arteries and aorta, and they transit signals via the glossopharyngeal nerves to the brainstem. The medulla integrates this with other information and transmits signals back to the heart by way of the vagus nerves. The vagus nerves slow down the heart and consequentially reduce blood pressure.
baroreflex; baroreceptors
The Autonomic Nervous System
can be defined as a motor nervous system that controls glands, cardiac muscle, and smooth muscle. It is also called the visceral motor system, to distinguish it from the somatic nervous system that controls the skeletal muscles.
The collateral ganglia contribute to a network called the abdominal aortic plexus; what are the three major collateral ganglia in this plexus?
celiac, superior mesenteric, and inferior mesenteric ganglia
In the thoracolumbar region, each paravertebral ganglion is connected to a spinal nerve by two branches called ________________ ___________.
communicating rami
Unmyelinated postganglionic fibers leave the ganglion by way of the:
gray communicating ramus, named for its lack of myelin and uller color. This ramus returns to the spinal nerve.
Parasympathetic division
has a calming effect on the body functions; it is association with reduced energy expenditure and normal bodily maintenance, including functions such as digestion and waste elimination.
Sympathoadrenal system
is a combination of the adrenal medulla and the sympathetic nervous system, they are so closely related in development and function
Adrenal medulla
is essentially a sympathetic ganglion. It consists of modified postganglionic neurons without dendrites or axons.
Preganglionic fiber
myelinated and leads from a soma in the brainstem or spinal cord to the autonomic ganglion
Autonomic tone
parasympathetic and sympathetic exhibit a background rate activity; the balance between them is called autonomic tone.
Visceral reflex arc includes:
receptors, afferent neurons leading to an integrating center in the CNS, interneurons in the CNS, efferent neurons carrying motor signals away from the CNS, and finally an effector that carries out the end response
Inferior mesenteric ganglion post ganglionic target organs:
rectum, urinary bladder, and reproductive organs
Celiac and superior mesenteric ganglia postganglionic target organs:
small intestine, colon, and kidneys
The route postganglionic fibers take to most sweat glands, piloerector muscles, and blood vessels of the skin and skeletal muscle:
spinal nerve route
In summary, effectors in the abdominopelvic cavity is innervated by:
splanchnic nerves
Adrenal cortex secretes
steroid hormones
The two divisions of the ANS:
sympathetic division and the parasympathetic division
In summary, effectors in the muscles and body wall are innervated mainly by:
sympathetic fibers in the spinal nerves
These nerves form a carotid plexus around each carotid artery:
sympathetic nerve route, sympathetic nerves
In summary, effectors in the head and thoracic cavity are innervated mainly by:
sympathetic nerves
Visceral Reflexes
unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors and slower responses.
Postganglionic fiber
unmyelinated fiber that leads from ganglion to the target cells
The preganglionic fibers are small myelinated fibers that travel from the spinal nerve to the ganglion by way of the:
white communicating ramus, which gets its name from color and name from the myelin.