LECTURE 11 ANS
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Autonomic nervous system
- the portion of the nervous system that is motor to smooth muscle, cardiac muscle, and glands (involuntary effectors)
- functional entity rather than an anatomic one, includes parts of CNS and PNS
two divisions of ANS
- divisions by some anatomic, pharmacologic, and functional criteria
- sympathetic and parasympathetic
sympathetic part ("division") of the ANS
- that portion of the autonomic nervous system that produces "fight or flight" actions
- sometimes called thoracolumbar division cause it's preganglionic neurons are located in those parts of the spinal cord
- these include such things as increased heart rate, pupillary diation, decreased salivation, and piloerection (goose bumps)
parasympathetic part ("division") of the ANS
- the portion of the autonomic nervous system that produces reactions generally antagonistic to those considered to be "fight or flight"
- often called the "homeostatic" division of ANS, (rest and digest)
- also known as "craniosacral" division of ANS cause it's preganglionic neurons are located in the brain and in the sacral parts of the spinal cord
What is unique about the way ANS impulses travel
- ANS uses a two neuron chain, peripherally
- First neuron goes partway; second goes the rest of they way. They synapse in-between
- first if pre, second is post ganglianic VE neuron
- the first neurons in the two neuron chain
- have cell bodies in the brain or spinal cord and their axons course peripherally within the cranial and spinal nerves
- synapse with second neurons called postganglionic neurons
are VE neurons
- the second neurons in the two neuron chain
- located at various autonomic ganglia within the PNS
- axons continue on to their effector organs by coursing within the named peripheral nerves
- are VE neurons
- Must be stimulated to a certain level by excitatory preganglionic impulses before they will fire (transmit impulse)
- if they receive a sign. # of inhibitory impulses during a given time period, will not reach threshold potential and will not fire
reason for having a two neuron chain in the ANS..
- appears two-fold
- first, each pre-ganglionic neuron can synapse on several postganglionic neurons. Allows "muliplication" where relatively small impulse output can cause large peripheral response. Net effect is that ANS operates on subconscious lever
- Second, the 2 neuron chain allows some PNS interaction btwn pre-ganglionic excitatory impulses and inhibitory impulses.
the amount of inhibitory impulses required to have a postganglionic neuron "fire"
smooth muscle tissue
- found in the walls of most organs of visceral body systems, within walls of blood vessels, of base of hair follicles
- also found within the iris (forms the pupillary sphincter m.'s which determine size of pupils, regulating amount of light that strikes the retina) and ciliary body (controls thickness of lens) of the eye
- some have inherent rhythmicty which ANS controls
cardiac muscle tissue
- found within heart and also extends for short distance into pulmonary trunk and aorta
- has inherent rhythmicity, but ANS can speed or slow frequency of contraction
- stimulated to secrete or can be inhibited form secreting by ANS
- both exocrine glands and endocrine glands receive antonomic innervation
when adrenal medulla is stimulated by sympathetic division...
it releases catecholamines which travel in bloodstream and stimulate sympathetic effectors all over the body
specific anatomic differences btwn sympathetic and parasympathetic parts of ANS - criteria
based primarily on the locations of cell bodies of the preganglionic and postganglionic neurons
sympathetic parts based on anatomic criteria
- preganglionic neurons are located in T1 to L2 segments of spinal cord (within lateral gray column).
- postganglionic neurons located in sympathetic trunk ganglia as well as in named ganglia primarily in neck, thorax, and ab.
- in general, sympathetic effectors are found throughout the body
parasympathetic parts based on anatomic criteria
- preganglionic neurons located in brain (w/in motor nerve nuclei of CN 3,7,9,10 and in sacral region of spinal cord (in area corresponding to lateral gray column)
- postganglionic neurons located primarily in maned ganglia in head (ass. w/ CN 3,7,9), in the sacral regin, and in terminal ganglia within walls of thoracic and abdominal organs
- In general, parasympathetic effectors are limited to head and to the viscera of the thorax, abdomen, and pelvis
pharmacologic differences btwn two divisions of ANS based on
- the specific neurotransmitters used at different locations
- BOTH divisions use acetylcholine at the ganglia (btwn pre- and postganglionic neurons)
- sympathetic division, postganglionic (neuron-to-effector-organ)neurons release the neurotransmitter norepinephrineparasympathetic division uses (again) acetylcholine as the postganglionic neurotransmitter
functional differences btwn two divisions of ANS relate to
- the "fight/flight" vs. homeostatic roles
- most changes in body induced by stimulation of sympathetic part can be deduced by considering what happens in an anger/fear situation
- parasympathetic division responses can be logically deduced by considering a resolution form anger/fear
must be noted that most ANS effectors receive...
- BOTH sympathetic and parasympathetic innervation and the effect of stimulation is usually opposite for the 2 divisions
- A few effectors receive sympathetic only: including sweat glands, smooth m. in vessel walls, pupillary dilater m., arrector pili m at base of hair follicles, and adrenal medula
- Only effectore which receive exclusively parasympathetic innervation are ciliary m. of eye (controls thickness of lens) and the pupillary sphincter m. (limits the amount of light entering the posterior and vitreous chambers of the eyeball
- the small fasciculus of nerve fibers that connects the anterior branch of a spinal nerve to the ipsilateral sympathetic trunk of vertebral nerve.
- A typical ramus includes gray (unmyelinated) and white (myelinated) rami
the axons of all postganglionic neurons synapse on...
involuntary effectors (smooth muscle cells, cardiac muscle cells, glandular epithelium)
motor ganglia is same thing as...
- autonomic ganglia
- Note these are localized groups of postganglionic VE neurons found in the PNS
sympathetic vs. parasympathetic distribution in body
- sympathetic: everywhere, deep and superficial
- parasympathetic: more limited, involve deep stuff in head, neck, and trunk. Nothing in limbs or superficial
tissues in the body that DO something
specific locations of preganglionc neurons
- spinal cord ONLY T1-L2 segments
- in transverse section, in side swelling in gray matter (lateral horns in gray matter)
autonomic nervous system does NOT have...
structure of sympathetic part of ANS
- preganglionic neurons leave spinal cord through anterior rootlets of spinal nerves T1-L2
- course into the anterior branches of those spinal nerves and enter the white (myelinated) rami communicates and course to the ipsilateral sympathetic trunk
- may travel up and down trunk before synapse
the left and right sympathetic trunks (which extend along each side of the v. column) consists of a series of...
sympathetic trunk ganglia, which are joined to each other by interganglionic rami and joined to the anterior branches of the spinal nerves by rami communications
structure of parasympathetic part of the ANS
- para. preganglionic neurons leave CNA through cranial nerves 3,7,9.10 and through sacral segments of spinal cord
- synapse at autonomic ganglia in head, thorax, abdomen, and pelvis (rarely at sympathetic trunk ganglia)
- distributed to heart as well as smooth muscle and glands of the head, neck, thorax and abdomen
the parasympathetic fibers associated w/ oculomotor nerve are distibuted to...
- the pupilary sphincter m. of the iris and the ciliary m. of eye
- The pre/post synapes occur at the ciliary ganglion which is located w/in the orbit adjacent to the eyeball
In sympathetic pathways destined for the head or other peripheral effector organs, preganglionic neurons synapse on postganglionic neurons within ...
the sympathetic trunk ganglia
Axons from postganglionic neurons...
course back up the reami communicantes and are distributed by the branches of the spinal nerves to the various peripheral effector organs
white ramus vs. gray ramus
Since the preganglionic axons are typically myelinated and the postganglionic's are usually not, each ramus communicans is divided into a white ramus (preganglionic axons) and a gray ramus (postganglionic axons)
Cranial nerves which carry parasympathic preganglionic fibers
Oculomotor nerve, facial nerve, vagus nerve
parasympathetic fibers associated with the facial nerve..
- distributed to smooth m. and glands in nasal mucosa, pharynx, and palate, to the sublingual and submandibular glands and to the lacrimal gland in the orbit
- Pre/post synapes occure in the pterygopalatine ganglion and the submandibular ganglion
- result in salivation, lacrimation, and secretion of exocrine glands in nasal and oral mucosae
parasympathetic fibers associated with glossopharyngeal nerve...
- distributed to parotid gland
- pre/post synapse occur in otic ganglion and result in salivation
parasympathic fibers associated with the vagus nerve
- distributed to the hear and the smooth muscle and glands in the lower respiratory organs, AND most digestive organs
- most pre/post synapses occur within organs innervated
- These autonomic ganglia, too small to see w/o magnif & not indiv. named, called termianl (intramural) gangliathese impulses decrease heart rate, cause bronchoconstriction, stimulate secretion in various glands of lower resp. system and the allimentary canal
autonomic (motor) ganglia
- localized groups of postganglionic VE neurons found in PNS
- scattered in specific locations in head, neck and trunk
- three groups: sympathetic trunk ganglia, terminal ganglia, and named ganglia
- should be distinguished from sensory ganglia (see next card)
- groups of cell bodies of sensory (SA and VA) neurons
- all associated with spinal and cranial nerves, are close to brain and spinal cord
sympathetic trunk ganglia
- located along right and left sympathetic trunks.
- since sympathetic trunks lie parallel and close to vertebral column, often called paravertebral ganglia (="near the vertebrae)
- associated with sympathetic pathways, especially those distributed to peripherally located effectors
- autonomic ganglia found in the walls of organs innervated
- often microscopic
- involved in BOTH sympathetic and parasympathetic pathways
- also called intramural ganglia
- those autonomic ganglia that are large enough to see, structurally independent of major organs, are not located along sympathetic trunks
- involved in BOTH sympathetic and parasympathetic pathways
All autonomic ganglia consist of groups of...
- postganglionic neuron cell bodies along the "two neuron chain" of the ANS
- these VE motor neurons whose axons continue on the synapse directly with smooth m.c., cardiac m.c., or glandular epithelium
what role does medulla oblongata have in ANS
- has nuclei that control motor output to effectors in cardiovascular, respiratory, digestive, urinary, and reproductive systems
- the hypothalamus also has some control
- agents which stimulate the sympathetic division of the ANS (or block the parasympathetic division)
- envoke some of the fight/flight types of responses
- EX: can be instilled onto eye to dilate the pupil for exam of retina; asthmatic individuals use for bronchiodilator
- EX: epinephrine, ephedrine, etc..
those which cause physiologic changes that mimic stimulation of the parasympathetic part of the ANS
- a precipitous drop in blood pressure mediated by temporary inhibition of the sympathetic pathways (due to emotional disturbance) which stimulate the normal tone of the smooth muscle in vessel walls.
- one of several types of circulatory shock and can be precipitated by psychological (emotional) states
why does the ANS not constitute a third part of the nervous system (along with PNS and CNS)
- All the components of the ANS are part of either the CNS or the PNS.
- ANS is a functional subdivision rather than a third anatomic one
Why (how) does the ANS function at a subconscious level?
The total CNS output required is very small due to the "multiplication" effect whereby each preganglionic neuron synapses with numerous postganglionic neurons. The total CNS output can be so small it's not even noticed - thus subconscious
Explain what is meant by prevertebral, paravertebral, and terminal ganglia?
- These apply to various groups of autonomic ganglia.
- Those within the sympathetic trunks are also called paravertebral ganglia since they are positioned along each side of the vertebra column.
- Prevertebral ganglia are named ganglia anterior to ("pre") the vertebral column.
- Terminal (intramural) ganglia are those located in or close to the walls of the organs innervated
What are the differences btwn sensory ganglia and motor (autonomic) ganglia?
- sensory ganglia contain cell bodies of VA or SA neurons, are located near the origins of cranial or spinal nerves, and synapses DO NOT OCCUR WITHIN THEM.
- Motor ganglia contain the cell bodies of postganglionic VE neurons, are located in numerous places, and synapses btwn pre- and postganglionic neurons occur within them
Most autonomic effector organs are innervated by both divisions of the ANS. How are those effectors which are innervated by only one division ?
Effector organs having only sympathetic or parasympathetic inputs are controlled by the number of impulses they receive
What are the pharmacologic differences btwn the two divisions of the ANS
- Oversimplified, both divisions use the same neurotransmitter at the ganglia (acetylcholine), but they use different ones at the effector organs.
- There, they sympathetic division uses norepinephrine and the parasympathetic division use acetylcholine
Would a drug that stimulated acetylcholine receptors cause sympathetic or parasympathetic responses?
- In theory, both divisions would be stimulated because both use ACh as a neurotransmitter in ganglia.
- In reality, there are different kinds of ACh receptors (muscarinic, nicotinic) and various pharmaceuticals may stimulate both or only one
Since all preganglionic sympathetic neurons are locate in the thoracic part of the spinal cord (T1-L2), how do sympathetic neurons get distributed to other parts of the body?
- Sympathetic nerve fibers extend the sympathetic trunks into the pelvis inferiorly and the vertebral nerves extend them superiorly into the head.
- Postganglionic sympathetic neurons are carried by all of the spinal nerves. Cephalic distribution follows the major arterial channels in the head
Do skeletal muscles receive any sympathetic neurons?
Yes, but not to the skeletal muscle cells, themselves. The sympathetic pathways are destined for the smooth muscle tissue within the blood vessel walls.
What is the difference in the two divisions of the ANS in regard to general areas of distribution?
- The sympathetic division is distributed to essentially all parts of the body.
- The parasympathetic pathways are limited to the head, neck, and trunk
What would be the general effect of a parasympathetic blocking agent?
Since the output of parasympathetic division is blocked, the sympathetic responses would be more obvious. This is due to the face that most involuntary effectors receive antagonistic input from the two divisions of the ANS
Explain why rami communicates in the thoracic region include both white and gray rami, but those in the cervical region are gray only?
- The white rami are bundles of axons of preganglionic sympathetic neurons. All of these exit the spinal cord from T1-L2.
- The gray rami are bundles of postganglionic axons leavign the sympathetic trunks (and vertebral nerves) to rejoin the spinal nerves for distribution with them
What is wrong with the statement "The only synapses in the peripheral nervous system are those occurring in the autonomic ganglia."?
Those synapses are the only ones btwn neurons in the PNS but there are also synapses btwn the motor and effector organs and btwn sensory receptors and sensory neurons
Is the sympathetic division of the ANS "excitatory" or "inhibitory"?
Some pathways are excitatory and others are inhibitory. Sympathetic fibers to salivary glands and smooth muscle in the intestinal wall, for example, are inhibitory
Which cranial nerves carry sympathetic preganglionic neurons?
- None of them (all sympathetic preganglionics originate in the spinal cord.)
- The oculomotor, facial, glossopharyngeal, and vagus nerves (CN3,7,9,10) carry parasympathetic preganglionic neurons
Are there any sensory pathways found in autonomic structures?
Yes, many sensory neurons follow autonomic pathways, but are not considered to be part of the ANS, per se
Drugs like ephedrine are called sympathominietics because they stimulate sympathetic effectors. What typical types of body responses would you expect after taking a drug of this type?
typical sympathetic responses like increased heart rate, pupillary dilatation, etc.
What are the basis for the saying ... "I was so mad that I couldn't see (straight)?
Anger causes a generalized sympathetic response including marked dilation of the pupils of the eyes. This may allow so much light to reach the retinae that the photoreceptors are over driven and vision is impaired
Most of the effects of the two divisions of the ANS are antagonistic to each other. Give an example where the two divisions work together cooperatively.
In male reproductive system the parasympathetic division controls erection of the penis and the sympathetic division is responsible for ejaculation
Some parasympathomimetic drugs, like muscarine, stimulate acetylcholine receptors at parasympathetic effector organs. Others, like nicotine, stimulate ganglionic transmission and also stimulate skeletal muscle cells. Compare the types of autonomic responses you would expect from these two drugs.
Nicotine should increase both sympathetic and parasympathetic output since it stimulates ganglionic transmission. Muscarine should result in only parasympathetic responses
Why are parasympathetic blocking agents considered to be sympathomimetic drugs?
most involuntary effectors receive both sympathetic and parasympathetic innervation and their effects are usually antagonistic to each other. Thus, a parasympathetic blocker would enhance sympathetic responses
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