ANS Flashcards
The regulation of mammalian bodily functions such as temperature, metabolism, heart rate, blood pressure etc., are achieved through a combination of endocrine organs that produce slow, long-term changes and through the autonomic nervous system that provides rapid but short term responses. The ANS is aka _____
the visceral nervous system
The autonomic nervous system or ANS is typically thought of as an ‘effector’ or motor system without sensory input. Why is that wrong?
This is incorrect since the ANS receives substantial sensory information at levels of the spinal cord, brainstem, and even the cerebral hemispheres. This sensory input permits rapid, single and multi-synaptic reflex responses.
This overly simplified diagram presents only the motor or efferent component of the ANS and demonstrates the anatomic differences between the somatic motor system and the autonomic efferent system. What are they?
The lower or somatic motor neuron of the somatic motor system sends its axon to directly innervate its target organ, the skeletal muscle.
In contrast, the autonomic efferent system is comprised of two efferent neurons, a preganglionic neuron and its fiber and a postganglionic neuron and its fiber that innervate smooth muscle, cardiac muscle, or glands. The innervation of smooth muscle permits regulation of functions such as blood pressure and peristalsis, the innervation of glands regulates secretions from salivary lacrimal and other glands, and the innervation of cardiac muscle controls heart rate and the force of heart muscle contraction.
The ANS can be subdivided into what 3 divisions:
1) the sympathetic or thoracolumbar system,
2) the parasympathetic or craniosacral system, and
3) the enteric nervous system.
The neurotransmitter of the preganglionic neurons for both the sympathetic and parasympathetic systems is what?
acetylcholine.
Note the adrenal gland receives a direct, preganglionic innervation with acetylcholine.
The neurotransmitter for the postganglionic neurons in the parasympathetic system is also acetylcholine but in the sympathetic system, it is _____.
norepinephrine
Note an exception to the latter rule is the postganglionic innervation of the sweat glands and erector pili muscles. They use acetylcholine as the neurotransmitter.
The enteric nervous system uses a variety of neurotransmitters and neuropeptides for synaptic transmission
This slide nicely summarizes the anatomical pathways and neurotransmitters of the ANS. The parasympathetic system is presented on the left and the sympathetic system is shown on the right
Note that the primary ANS neurons of origin that lie in the hypothalamus are not depicted in the slide. The secondary neurons of the sympathetic system that lie in the intermediolateral gray matter of the T1-L3 spinal cord are shown as are the cranial nerve nuclei and sacral neurons of the parasympathetic system. Note that the preganglionic neurons of both the sympathetic and parasympathetic system use acetylcholine, that is the blue axons. The postganglionic noradrenergic neurons of the sympathetic system are shown in red.
Note once again that the sympathetic neurotransmitter to the sweat glands, erector pili, and adrenal glands is acetylcholine.
Thus, under extreme conditions, for example, you are hiking in Yellowstone Park and startle a large bear behind a bush. It charges in your direction.
Under such conditions the sympathetic nervous system will trigger the body to make maximum use of its resources to enhance survival by dilating the pupils to permit full vision, constricting the blood vessels of the skin and gut thereby rerouting blood supply to the muscles to escape, dilating the bronchi to maximize oxygenation, increasing the heart rate and its force of contraction to increase blood supply and energy to the muscles. Simultaneously, sympathetic activity stimulates increased glucagon release by the pancreas, glycogen conversion to glucose by the liver for increased muscle metabolism, and release of epinephrine by the adrenal glands.
In contrast to the “fight or flight” physiology of the sympathetic system, the function of the parasympathetic system may be characterized as a “rest and digest” state for the organism. Activation of the parasympathetic system constricts the pupils, stimulates saliva production, constricts the bronchi, slows the rate and force of heart muscle contraction, stimulates digestion, and dilates gastric blood vessels. Hopefully its not the bear that reaches this state of affairs after its encounter with you.
This slide presents the anatomical organization of the preganglionic spinal outflow for the sympathetic system. Note the origin of the secondary neuron where?
in the intermediolateral (lateral horn) zone of the spinal cord gray matter
Note the pathway taken by the secondary neuron, shown in blue, as it exits the spinal cord via the ventral root and travels through a short segment of the peripheral nerve before entering the white communicating ramus to gain access to the paravertebral sympathetic chain ganglia. Within the paravertebral ganglia chain, the secondary neuron may take any of three courses. It may synapse immediately with a tertiary noradrenergic neuron, which in turn sends its axon back to the peripheral nerve via the gray communicating ramus. The secondary axon may travel rostral or caudal in the chain before synapsing on a tertiary noradrenergic neuron, or it may exit the chain to synapse in a more peripherally located prevertebral ganglion.
This slide presents the anatomic organization of the parasympathetic system from the brainstem and sacral spinal cord S2 –S4 segment.
Note the long projection of the preganglionic and the relatively short projection of the postganglionic nerves to their targets.
The ________ nucleus lies in the midbrain and sends fibers via cranial nerve III to the ciliary ganglion which controls pupillary constriction.
Edinger-Westphal
The superior salivatory nucleus in the _____________ ________ sends projections via cranial nerve VII to the pterygopalatine ganglion that controls the lacrimal gland and to the submandibular ganglion that controls secretion of the salivary glands.
pontine tegmentum (tegmentum means floor)
The _______ ________ nucleus lies just below the superior salivatory nucleus in the dorsal pons near the medulla and projects via cranial nerve IX to the otic ganglion that controls the parotid gland secretion.
inferior salivatory
A_t the middle medulla level_ the dorsal motor nucleus of the vagus and the nucleus ambiguus send projections where?
via cranial nerve X to ganglia within their targeted organs including the heart and gut. They decrease heart rate, constrict the bronchial tree, dilate intestinal blood vessels, and increase peristalsis.
This slide summarizes the essential components of the enteric nervous system.
The activity of the intestines is partially regulated by the central autonomic nervous system through parasympathetic and sympathetic innervation. However, the gut contains a separate set of local neurons that create two networks, the myenteric (Auerbach’s) plexus and the submucosal (Meissner’s) plexus. These two local neuronal networks function mostly independently of the central ANS.
Thus, many intestinal functions such as peristalsis and glandular secretions occur near normally in the absence of input from the ANS.
The figure depicts the parasympathetic and sympathetic innervation of the gut and also the location of the myenteric plexus between the longitudinal and circular muscle layers and the submucosal plexus beneath the circular muscle layer.
Note that the final common pathway of gut motility is through the enteric nervous system with _______ as the effector neurotransmitter.
acetylcholine
I include this slide to make you aware that the ANS functions through a complex integrated set of pathways and centers with sensory information reaching the ANS via the nucleus of the solitary tract with outflow of the ANS regulated by the amygdala and hypothalamus.
This slide presents the anatomical pathway of the autonomic innervation of the eye. Note the sympathetic neurons that innervate the dilator muscles of the iris and the tarsal muscle of the eyelid.
Note that the origin of the primary sympathetic neurons in the hypothalamus is not shown, the path through the brainstem and spinal cord (shown in green) to the T1 level, and the synapse in the intermediolateral gray area of the spinal cord.
Note the exit of the sympathetic pathway to the eye at T1-T2 and entry into the paravertebral ganglion with the second synapse in the superior cervical ganglion. Note the path of the tertiary (postganglionic) neuron within the carotid sheath and the entry of the sympathetic fibers into the calvarium with the carotid artery. The sympathetic fibers travel along the short and long ciliary nerves to reach the tarsal muscles and the dilators of the iris.
The figure below presents an individual with left sided ptosis (drooped eyelid) and meiosis (small pupil) or a Horner’s syndrome. What causes HS?
Note that the slide depicts only afferents from these receptors traveling via cranial nerve X (vagus) when in fact they travel in both IX and X.
Chemoreceptors, that is chemical receptors, located in the carotid body provide information on the concentration of oxygen and carbon dioxide in the blood to the solitary nucleus via cranial nerves IX and X.
Interneurons connect the solitary nucleus to the nucleus ambiguus and the dorsal motor nucleus of the vagus which send efferent parasympathetic fibers to the heart.
The solitary nucleus also connects to the midbrain sympathetic neurons that send efferent sympathetic fibers to the heart.
The parasympathetic system innervates the bladder detrusor muscle and causes it to contract to empty the bladder. The sympathetic system also innervates the bladder detrusor muscle but causes what?
it to relax, thereby allowing the bladder to fill up with urine. In males, the sympathetic system also innervates the internal urethral sphincter and causes contraction of the sphincter muscle, closure of the urethra and urine retention. The internal sphincter is absent in females.
How does the PMC empty the bladder?
To empty the bladder, the PMC stimulates parasympathetic contraction of the detrusor muscle, inhibits sympathetic relaxation of this muscle, inhibits sympathetically mediated contraction of the internal sphincter in males and inhibits the voluntary control of the external sphincter in both males and females.
This slide summarizes some of the typical signs and symptoms caused by dysfunction of the autonomic nervous system.
