The autonomic nervous system has two divisions that are distinguished by where the nerves exit the central nervous system. Nerves exiting the brain and sacral spinal cord (blue in the above model) are collectively know as the "carnio-sacral outflow" or the parasympathetic division (PSNS). Nerves exiting other sections of the spinal cord (orange in the model)-- with the exception of the cervical region -- are known as the "thoraco-lumbar outflow" or the sympathetic division (SNS).
The ANS is the involuntary portion of the nervous system. Its nerves begin within the central nervous system and connect with target organs via two-neuron pathways. Nerves in the ANS are totally efferent withl impulses traveling from the central nervous system ending in target organs throughout the body. The cells within innervated organs have membrane receptors to which neurotransmitters attach. These cells are smooth muscle, cardiac pacemaker cells, cardiac muscle cells or glandular cells. Smooth muscle cells respond by either increasing or decreasing their muscle tone, the cardiac pacemaker increases or decreases the heart rate, the cardiac muscle contracts more or less strongly with each beat and glands increase or decrease their secretions.
The major point to always keep in mind is that many -- but not all -- organs are responding to neurotransmitters of both divisions simultaneously.
Scroll down for a description of the model's design.
As you read through each section below, scroll back up to the top of the page
and locate the part of the model being described.
The circle with a long hollow tail running the length of the model represents the brain and spinal cord.
Nerves of the autonomic nervous system contain numerous, parallel, two-neuron chains. A neuron consists of a cell body ( colored circle) and its axon represented by a line extending from it. The axon of the first neuron is solid while that of the second neuron is dashed.
Nerves consists of hundreds of individual neurons and when wrapped in connective tissue have an obvious bulge where the cell bodies of the second neurons are located. Such a bulge is called a "ganglion". It is for this reason that the first neurons in a nerve are described as "preganglionic" and the second neurons as "postganglionic." In the model ganglia can be thought of as cell bodies of the postganglionic neurons. These are the light blue and yellow circles outside the central nervous system. Similar circles drawn inside the brain and spinal cord (dark blue and orange) represent the cell bodies of preganglionic neurons. In the brain these regions are called "nuclei" and in the spinal cord they are "lateral grey horns".
The thorax houses the heart drawn to the right of the upper spinal cord. The circle inside the heart is the pacemaker. The tubular structure inside the heart represents small coronary arterioles.
The branching structure to the right of the heart represents the bronchial tree. The donut-shaped structures around the branches are the bronchial muscles that regulate the diameter of the airways. The tubular structure above the lungs represents the bronchial arteries.
The abdominopelvic region contains the gastrointestinal tract; the model shows the stomach, pyloric sphincter, small and large intestines and internal anal sphincter. These organs are illustrated at the lower right portion of the model. The tubular structure to the left of the small intestine represents splanchnic arteries that supply these organs. The arrows leaving it represent epinephrine (E) and norepinephrine (NE) diffusing to these organs.
The urinary bladder and its internal urethral sphincter are shown below the splanchnic artery and to the left of the large intestine.
An adrenal gland is shown just beneath the heart. Its central portion is the medulla which releases the hormones epinephrine (E) and norepinephrine (NE) into the capillaries (shown to the right) that pass through the gland.
Arteries are shown throughout the model as curved tubes encircled by a donut. The donut represents vascular smooth muscle that encircles resistance arteries. When these muscles contract the diameter of the vessel narrows and reduces blood flow; when they are relaxed the pressure of the blood causes the vessel diameter to enlarge and flow to increase.
A facial profile containing a blood vessel that supplies the linings (mucosae) of the nose (nasal) and mouth (oral) is shown in the upper-left corner. At the top-center of the model are two illustrations of the iris of the eye showing the pupil constricted and dilated. Just below and to the right of the dilated pupil is a representation of the functional unit of salivary glands.
Illustrations at the lower left of the model depict structures found in the skin. The oval structure to the left is an apocrine sweat gland entering the hair follicle. These are most abundant in the armpits and groin area. The structure running from the follicle to the lower surface of the skin is the arrector pili muscle. The smaller oval structure to the right of the follicle is an eccrine sweat gland. This is the 'normal sweat' that evaporates to cool the body. Below these structures is a dermal artery.
The spindle-shaped structure at center left represents the skeletal muscles. The tubular structure within it represents arteries.
Inspection of the entire model shows "blue nerves" exiting the central nervous system from the brain and sacral spinal cord -- thus the name cranio-sacral outflow. It is also called the parasympathetic division because its activity is auxillary to the sympathetic division. The anatomy of this division is much simpler than the sympathetic division that is represented by "orange nerves" in the model.
Throughout the model the preganglionic cell bodies (dark blue circles) give rise to axons (solid blue lines) that extend toward their targets and end as neurosecretory terminals (blue bars) that secrete acetylcholine. Most targets are postganglionic neurons; one exception is the adrenal medulla. The postganglionic cell bodies (light blue circles) lie close to, or within, the target organs. Their axons (dashed blue line) are relatively short and also end in terminals (blue bars) that secrete acetylcholine.
Ganglia are groups of postganglionic (light blue) cell bodies. This is where the preganglionic axons secrete acetylcholine onto these cell bodies. The acetylcholine binds to and activates receptors on the cell bodies. As a result, the postganglionic neurons send an impulse down their axons causing their terminals to also release acetylcholine.
Because the axons of PSNS preganglionic neurons (solid lines) are long, the location of postganglionic cell bodies are near or within the target organs. In these ganglia they synapses with postganglionic cell bodies (light blue circles) that have short axons (dashed lines) spreading throughout the organ.
When the ganglia are within a target organ they are referred to as intramural ganglia as shown in the stomach. Intramural means 'within the wall'. When the ganglia are on or near the outer surface of a target organ they are referred to as terminal as shown near the heart.
For this tutorial the term 'visceral' means all organs except those in the dermis and skeletal muscles; we'll consider those to be 'somatic'. It is not the organ itself but rather specific cell types within the organ that are innervated. These cell types are:
Inspection of the entire model shows that these nerves contain axons forming two-neuron chains. The cell bodies of the first or preganglionic neurons (orange circles) are located within the thoracolumbar region of the spinal cord in the lateral grey horns. Their axons ( orange lines) leave the cord to synapse within ganglia with the cell bodies of the second or postganglionic neurons (yellow circles); axons of these neurons are shown as dashed orange lines.
For this tutorial the term 'visceral' means all organs except those in the dermis and skeletal muscles; we'll consider those to be 'somatic'. It is not the organ itself but rather specific cell types within the organ that are innervated. These cell types are:
As in the parasympathetic division the cell bodies of the postganglionic neurons are clustered together to form ganglia. Similarly it is acetylcholine that is secreted onto these cell bodies by the terminals of the preganglionic neurons. The postganglionic neurons respond by secreting norepinephrine.
Two chains of ganglia lie along each side, i.e. paravertebral, of the spinal cord --only one is shown in the entire model ... except in the cervical (neck) region. Each ganglion is a cluster of postganglionic cell bodies ( yellow circles). The nerve that links the ganglia into a chain consists of hundreds of preganglionic axons (orange lines) running up and down between the ganglia. The entire structure is called the ganglionic chain.
The first insert shows a preganglionic axon exiting the spinal cord. In this illustration it does not synapse in the nearest ganglion but instead turns upward and passes through several other ganglia before finally synapsing in the top one. A postganglionic axon (dashed orange line) exits this ganglion to travel smooth muscle encircling a blood vessel. We know that this blood vessel is within the body cavity (visceral) because the postganglionic axon goes directly from the ganglion to the target. The significance of this observation will become clear in the next section.
Somatic is defined as relating to the wall of the body cavity, particularly as distinguished from the head, limbs or viscera. Inspection of the lower left side of the entire model shows the somatic region of the body where arteries within skeletal muscles and structures in the skin (arrector pili muscle of hair follicles, eccrine sweat glands and dermal arteries) are located. The insert to the left shows a preganglionic axon (orange) leaving the spinal cord to synapse with a postganglionic cell body (yellow) in the ganglionic chain before passing into a skeletal muscle to target the smooth muscle of a blood vessel. The postganglionic axon (dashed orange line) appears to curve back to contact the spinal cord before heading to its target is in the somatic region of the body.
The next insert shows the spinal cord and an attached nerve. Below the nerve is one of the ganglia from a chain. There are two short nerves connecting the ganglion to the main nerve. The preganglionic axon (solid orange line) leaves the cord, bypasses the first small nerve and enters the second. This small branch is called the white ramus ... white because the axon is myelinated. The postganglionic axon (dashed orange line) reenters the main nerve via the other small nerve called the grey ramus ... because its axon is not myelinated. It re-enters the main nerve to travel to the somatic region of the body.
The exception to the 'two neuron rule' is the innervation of the adrenal medulla shown in this insert There are no postganglionic neurons involved...there are only preganglionic neurons (orange). Their axons exit the cord, pass through the nearest sympathetic ganglion (yellow circle) of the chain, penetrate the cortex (i.e., outer most part) of the gland to terminate within the medulla (i.e., inner most part) of the gland.
The next insert shows three large, yellow circles; these represent prevertebral/collateral ganglia which are at the midline of the body near the aorta. They are called "prevertebral" because they lie "in front of the vertebrae." They are also called "collateral" which means "running side by side" because preganglionic neurons from both the right and left side of the spinal cord come together in these midline ganglia. The top ganglion is called the celiac, the middle is called the superior mesenteric, and the lower is called the inferior mesenteric. Their names are derived from the arteries they are near.
The insert to the left depicts the basic pattern that the two-neuron pathway uses to reach targets in the abdominal and pelvic regions. Preganglionic axons (orange line) pass through nearby paravertebral/chain ganglia (small yellow circles) after exiting the spinal cord. The nerves containing these axons are referred to as 'splanchnic' meaning 'related to visceral organs; usually those below the diaphragm'. From top to bottom they are named the greater, lesser and least splanchnic nerves.
These preganglionic axons are longer than typical ones and they continue toward the midline of the body where collateral ganglia large (yellow circles) are located. Postganglionic axons (dashed orange lines) exit the ganglia and proceed to organs of the gastrointestinal tract and urinary bladder.
Throughout these tutorials the receptors that increase the activity of the cell are color-coded green and the those that decrease the activity of the cell are color-coded red. A more meaningful way to understand this is to view the relationship between activation of a receptor and the response of the cell as either direct (green receptor) or inverse (red receptor).
It is important to remember that both divisions of the ANS respond simultaneously although one may override the other depending on the circumstances. It's like driving a car with one foot on the accelerator and the other on the break at the same time!
Cholinergic means "having to do with acetylcholine". The neurotransmitter acetylcholine is released from the terminals of both pre- and postganglionic neurons of the PSNS but from only the preganglionic neurons of the SNS. There are two categories of cholinergic receptors that respond to the presence of acetylcholine-- nicotinic and muscarinic.
Nicotine injected into laboratory animals causes some organs to respond as if acetylcholine had been injected. Thus, the receptors to which both acetylcholine and nicotine can attach are called "nicotinic." There are several subtypes of nicotinic receptors but only the N1 variety is associated with the autonomic nervous system. p>
N1 receptors are located on all postganglionic cell bodies and on cells of the adrenal medulla.
Further down is a typical long parasympathetic preganglionic (dark blue) axon. It synapses with parasympathetic postganglionic cell bodies within an intramural ganglion. These responds by releasing their own transmitters.
Muscarine, derived from the mushroom Amanita muscaria, injected into laboratory animals causes some organs to respond as if acetylcholine had been injected. Thus, the receptors to which both acetylcholine and muscarine can attach are called "muscarinic."
Of the five known subtypes, M4 and M5 are restricted to the brain. The subtypes associated with the ANS are M1, M2 and M3. Binding of acetylcholine to M1 and M3 receptors causes an increase the activity of the cell. Binding of acetylcholine to M2 receptors causes a decrease the activity of the cell. The main model shows M1 receptors only on gastric glands lining the stomach, M2 receptors only in the heart and M3 receptors located at numerous sites.
Acetylcholine activates M1 receptors on gastric glands increasing the secretion of acidic 'gastric juice' into the lumen of the stomach. The relationship between acetylcholine and the gland is direct.
The relationship between acetylcholine and the response of these targets is inverse; binding of acetylcholine to M2 receptors causes both the heart rate and the contractile force to decrease.
M3 receptors are found in many locations in the main model. Acetylcholine activation of M3 receptors results in increased responses in these organs. Responses are:
Inspect the entire model to locate the organs that have M3 receptors.
Adrenaline is secreted by the adrenal medulla. Adrenaline was the original name for what we now call epinephrine. Stimulation of the adrenal medulla by the sympathetic nervous system causes the secretion of both epinephrine (E) (80%) and norepinephrine (NE)(20%) into the bloodstream.
There are two categories of receptors: alpha and beta. There are also subtypes of each. Useful generalizations concerning these are:
Alpha 1 receptors are more responsive to NE than to E. This is interesting as alpha 1 receptors are abundantly found on vascular smooth muscle (donuts around blood vessels) as seen in the model. The relationship between the activated receptor and the cell's response is direct -- the activity (contraction of circular smooth muscle) is increased and blood flow is reduced. Alpha 1 receptors are located on:
Increase in tone of vascular smooth muscles reduces the ability of blood pressure to expand vessel diameter thus reducing blood flow to downstream organs. Increased tone in urinary and gastrointestinal sphincters reduces the passage of contents past them. Increased tone of the dilator muscle of the iris enlarges the pupil. Increased tone of arrector pili muscles pulls on the hair follicle causing hair to 'stand on end'.
Alpha 2 receptors (not shown in the model) are located adjacent to secretory terminals of some neurons mostly within the CNS. Current research is involved with determining if alpha 2 receptors are located elsewhere.
The relationship between beta 1 activation by E and/or NE is direct -- the cell's activity is increased. Beta 1 receptors are located on:
The cardiac pacemaker responds by increasing the heart rate. Simultaneously the myocardium contracts more forcefully. The physiology of the response of salivary duct cells to beta 1 activation is unclear; it appears that beta 1 activation of certain duct cells reabsorbs some water in the slowly passing saliva making it more viscous. The secretory portion of both types of sweat gland is stimulated only by E and NE from the blood, not via nerves.
Note that beta 1 activation in the heart is both neural and hormonal. In the salivary glands the activation is solely neural while in both type sweat glands it is solely due to E and NE in the blood.
Beta 2 receptors are located on smooth muscle. The relationship between E (hormonal) / NE (neurological) activation and the response of the cell is inverse -- activity (muscle tone) of the cell is decreased. Beta 2 receptors are located on smooth muscle in:
The model emphasizes the main locations of beta 2 receptors throughout the GI tract -- stomach, small intestine, colon, rectum. Their activation results in decreased muscle tone and motility. In the urinary bladder this decreased muscle tone enables greater filling.
The model shows relaxation of the circular smooth muscle in the bronchial tree is due to epinephrine -- note the lack of innervation at this location. Beta 2 receptors are also on small coronary arterioles thus increasing hormonally induced blood flow within the musculature of the heart. These receptors are the primary receptor in skeletal muscles arteries resulting in enhanced blood flow especially when epinephrine is present.
Inspection of the main model to determine which arteries have alpha 1 sites and which have beta 2 sites. Remember that epinephrine is the best activator of B2 sites.
Also notice there are beta 3 (B3)receptors on the smooth muscle cells of the urinary bladder that decrease their tone in response to receptor activation allowing increased filling.
The main model illustrates several organs in the head: the vessels of the oral and nasal mucosae, the iris of the eye and salivary glands.
These arteries supply the lining of the mouth and nasal cavities. Arterial diameter is due to blood pressure pushing against the wall. If the tone of the encircling smooth muscle is increased the diameter can't expand as much thus restricting blood flow. If this tone is decreased then blood pressure can expand the diameter and increase blood flow.
Sympathetic nerves arising from the superior cervical ganglion innervate the encircling smooth muscle of these arteries. Norepinephrine activated alpha 1 receptors cause the cells to increase their tone and decrease blood flow. Epinephrine is especially potent in further increasing this tone resulting in additional vasoconstriction. The result is 'dry mouth' and 'ease of nasal air flow' that is part of the 'fight-or-flight' response characteristic of a high level of sympathetic activity. Notice there is no parasympathetic innervation at this site.
Click on 'Model' for larger view.
The iris contains two layers of smooth muscle cells. Pupillary dilation is caused by SNS activity releasing norepinephrine on the alpha 1 receptors within the dilator muscle. Pupillary constriction is caused by PSNS activity releasing acetylcholine on the muscarinic 3 receptors within the constrictor muscle.
The detailed physiological processes involved in saliva production are a 'hot topic' of research as of this writing. Salivary glands have flask-shaped acini of glandular cells that secrete amylase and/or mucus, along with fluid the consistency of blood plasma, into the hollow centers. The walls of the ducts have 'duct cells' that modify the composition of the passing saliva. Both divisions of the ANS are involved in the behavior of these glands.
Saliva is constantly produced because both divisions are always working simultaneously. The volume and consistency of the saliva depends on which division predominates at the given moment.
Acetylcholine activation of muscarinic 3 receptors on the glandular acini increases the secretion of saliva resulting in a high volume of watery saliva. This is characteristic of the 'rest-and-digest' response when parasympathetic activity dominates.
Norepinephrine activation of B1 receptors in certain duct cells results in low volume, viscous saliva. There is a good blood supply to these glands so that epinephrine decreases the volume even further under excessive sympathetic stimulation. The resulting 'dry mouth' is characteristic of the 'fight-or-flight' response at such times.
The myocardium, consisting of cardiac muscle, is the contractile part of the heart; the pacemaker (circle inside heart) consists of autorhythmic cells that establishes the heart rate. The coronary arterioles shown within the heart muscle are not the large coronary arteries that lie on the surface of the heart. These smaller arterioles are embedded within the myocardium. This organ is neutrally regulated by both divisions of the ANS as well as hormonally.
The pacemaker can function without any innervation because it has an intrinsic mechanism that establishes a baseline heart rate. Acetylcholine released from parasympathetic postganglionic neurons (dashed blue line) binds to muscarinic 2 (M2)receptors resulting in a decreased rate. These neurons (dashed blue lines) arise from a terminal ganglion (light blue) near the surface of the heart. The preganglionic neurons (blue line) from the brain travel to this site within the vagus nerve.
The heart's atria (upper chambers), consisting of myocardial cells, also have muscarinic 2 (M2) receptors. When acetylcholine binds to these cells their force of contraction is decreased. There appears to be little parasympathetic innervation, if any, of the heart's ventricles (lower chambers) and this is currently under investigation.
Norepinephrine released from sympathetic postganglionic neurons (dashed orange lines) binds to beta 1 receptors in the pacemaker resulting in an increased heart rate. These neurons arise from several cervical ganglia (yellow circles) in the ganglionic chain. The preganglionic neurons (orange line) from the spinal cord travel through the ganglionic chain from the thoracic region of the cord.
Both the atria and ventricles have beta 1 (B1) receptors. Norepinephrine released from sympathetic postganglionic neurons (dashed orange lines) binds to beta 1 receptors result in increased contractile force. This innervation is like that of sympathetic fibers to the pacemaker.
The presence of epinephrine and norepinephrine diffusing from the blood (arrows) enhances the increased heart rate and contractile force as described above. Additionally, there are beta 2 receptors on smooth muscle encircling coronary arterioles. Epinephrine binding with these receptors decreases the muscle tone of the vascular wall. As a result the diameter enlarges allowing for increased blood flow to the myocardium.
The parasympathetic division (blue neurons) is the dominant neural control mechanism of this system -- there is no direct sympathetic neural involvement. Local controls play a major role in the regulation of this system. (Refer to Respiratory System in the main menu of this site).
The PSNS preganglionic neurons that reach the respiratory tree travel within the vagus nerve. They synapse with terminal ganglia (light blue circle) within the lungs. The postganglionic neurons from these ganglia release acetylcholine on M3 receptors on smooth muscle encircling the smaller cartilage-free bronchioles. PSNS activation causes an increase in the tone of these muscles causing bronchoconstriction and breathing difficulty. There is no SNS innervation of these smooth muscles!
However, there are also beta 2 receptors on these smooth muscle fibers. They are activated when epinephrine and norepinephrine diffuse to them from the bloodstream. Recall that beta 2 receptors are significantly more responsive to epinephrine than norepinephrine. Thus, as SNS activity increases more epinephrine from the adrenal medulla will be released into the bloodstream. In this situation, the smooth muscle fibers will relax and allow the expansion of the chest wall and lowering of the diaphragm to expand the bronchioles and facilitate air flow.
"Splanchnic" means having to do with the viscera (i.e., internal organs). Sympathetic preganglionic axons (orange lines) emerge from the cord and pass through ganglia of the sympathetic chain (line of yellow circles) without synapsing. They gather into three nerves called the greater, lesser & least splanchnic nerves. Though not obvious in the model the greater consists of bilateral outflow from T5-9, the lesser of outflow from T 10 & 11 and the least from T 12.
Parasympathetic preganglionic axons (blue line) emerge from the sacral region of the cord to form the pelvic splanchnic nerve. These neurons penetrate pelvic organs to synapse with intramural ganglia (light blue circle). From here, postganglionic axons (dashed blue line) synapse with their receptors within the organ.
These ganglia (large yellow circles) are named the celiac, superior mesenteric and inferior mesenteric. They sit just ventral to the aorta and the nerves entering and leaving them are intertwined forming a plexus. This entire region is called the 'solar plexus'. Within these ganglia preganglionic neurons release acetylcholine on nicotinic 1 (not shown) receptors on cell bodies of postganglionic neurons. These neurons respond by sending impulses down their axons to release norepinephrine on the organs they innervate. An example is the innervation of an abdominal vessel shown in the insert.
The adrenal medulla is unique because it is the only organ directly innervated by preganglionic axons (orange line), instead of postganglionic axons. These preganglionic axons (orange lines) form a branch of the greater splanchnic nerve as indicated in the model. Acetylcholine activation of nicotinic 1 (N1) receptors of chromaffin cells in the medulla increases their secretion of epinephrine (80%) and norepinephrine (20%) into capillaries. It is this release of epinephrine that is a hallmark of the "fight-or-flight response" resulting from excessive sympathetic stimulation.
This organ system contains a nervous system of its own called the enteric (i.e., intestinal) nervous system; some classify this as a third division of the autonomic nervous system.
Click on 'Model' for larger image.
The vagus nerve contains parasympathetic preganglionic neurons (blue lines) that penetrate the stomach wall to synapse within a plexus of postganglionic cell bodies (light blue circle). This plexus (myenteric/Auerbach's) lies between the outer longitudinal and inner circular smooth muscle layers (not shown) of the stomach wall. When stimulated by acetylcholine from the preganglionic neurons, the postganglionic neurons will secrete acetylcholine that activates muscarinic 3 (M3) receptors on smooth muscle of both muscle layers. The result is increased motility of the stomach.
Another plexus lies between the circular smooth muscle layer and the mucosa. The axons of these parasympathetic postganglionic neurons connect with mucosal gastric glands that bear muscarinic 1 (M1) receptors. Acetylcholine activated M1 receptors cause an increase in gastric juice secretion.
The celiac ganglion (top yellow circle) houses the cell bodies of sympathetic postganglionic neurons whose axons (dashed orange lines) lead to beta 2 receptors of both muscle layers of the stomach wall. Norepinephrine activated beta 2 receptors cause decreased motility of the stomach wall. Other neurons innervate the pyloric sphincter releasing norepinephrine onto the alpha 1 receptors located on these smooth muscle cells. When activated by NE they respond by increasing their tone making passage of material from the stomach to the intestine more difficult. Note that there are no sympathetic postganglionic neurons innervating the gastric glands.
Like the stomach, the proximal small intestine is innervated by branches of the vagus nerve (blue line). Synapses occur within the myenteric plexus(light blue circle). Acetylcholine released from postganglionic neurons (also called enteric neurons) binds with muscarinic 3 (M3) receptors on the intestinal smooth muscle to increase motility.
The remainder of the small intestine has little parasympathetic innervation. However, the colon and rectum do have parasympathetic innervation from the pelvic nerve (blue line). Again, the postganglionic neurons are the enteric neurons of the myenteric plexus. The smooth muscle receptors are muscarinic 3 (M3) and activation by acetylcholine increases motility in these sections.
Click on 'Model' for larger image.
The small intestine is innervated by postganglionic axons (dashed orange line) originating in the celiac ganglion. The colon (large intestine) is innervated by neurons from the superior mesenteric ganglion while in the rectum they originate in the inferior mesenteric ganglion. Norepinephrine is released from these to binds with beta 2 (B2) receptors on the smooth muscle layers to decrease their motility. Additionally, neurons from the inferior mesenteric also innervate the internal anal and urethral sphincters to release norepinephrine onto alpha 1 receptors causing increased tone in those tissues.
The sympathetic division controls arteries in the abdominal cavity. The circular smooth muscle encircling the arteries supplying the gastrointestinal tract have alpha 1 receptors. They are innervated by sympathetic postganglionic neurons (dashed orange lines). There are many splanchnic arteries with innervations originating in all three collateral ganglia. Norepinephrine activated alpha 1 receptors causes the encircling smooth muscle to constrict reducing blood flow to the GI tract. Blood-borne epinephrine (E) has the same effect. There is no parasympathetic innervation.
The preganglionic neurons (blue line) of the pelvic splanchnic nerves innervate the bladder wall. They secrete acetylcholine onto the cell bodies of postganglionic neurons within intramural ganglia (light blue circle) in the wall. Postganglionic axons (dashed blue line) release acetylcholine to activate muscarinic 3 receptors on smooth muscle cells. These cells respond by increasing their tone so that the bladder is not as compliant to filling.
The bladder wall musculature also has beta 3 receptors that are innervated by postganglionic neurons from the hypogastric nerve arising from the ganglionic chain. Activated beta 3 receptors cause decreased muscle tone in the bladder that facilitates filling.
The smooth muscle of the neck of the bladder (internal urethral sphincter) has alpha 1 receptors. Activation by norepinephrine from postganglionic neurons arising from the inferior mesenteric ganglion causes an increase in tone helping to maintain the urine in the expanding bladder.
'Somatic' means the body wall as opposed to the internal viscera. Inspection of the model indicates that these organs are structures in the dermis (arrector pili muscle of hair follicles, eccrine sweat glands and dermal arteries) and arteries in skeletal muscles. The insert shows a very short preganglionic axon (orange line) leaving the spinal cord to synapse with a postganglionic cell body (yellow) in the ganglionic chain. The postganglionic axon (dashed orange line) appears to curve back to contact the spinal cord before heading to its target is in the somatic region of the body.
The insert below shows these neurons within an outline of the nerve itself. When the preganglionic axon (orange line) leaves the cord it travels a short distance then breaks out of the main nerve to enter a ganglion in the chain. This branch is called the white ramus. The axon of the postganglionic neuron (dashed orange line) leaves the ganglion via another short branch called the grey ramus. This axon re-enters the main nerve to travel to the somatic region of the body. This seemingly unusual arrangement is characteristic of sympathetic neurons heading to the somatic region of the body. Note the lack of parasympathetic neurons in this region.
The vast majority of somatic vessels (top right of model) are within skeletal muscles. The encircling smooth muscle has beta 2 receptors -- remember that B2 receptors give a strong response to epinephrine. Activation of beta 2 receptors causes a decrease in tone of these muscles and vasodilation that increases blood flow. Under high sympathetic activity epinephrine diffusing from the blood enhances this response.
These small smooth muscles originate on the lower surface of the dermis and insert on the side of hair follicles. Follicles are not part of the actual hair but are tubular depressions that house much of the hair. Norepinephrine activated alpha 1 receptors increases the tone of these cells. The response is that the follicle is pulled such that the enclosed hair sticks up straighter (hair standing on end) and some of the skin is pushed up into a bump (Goose bump).
The most abundant and widespread type of sweat gland is the eccrine gland . Their ducts lead directly to a pore at the surface of the skin. These produce a watery secretion primarily useful in thermoregulation -- evaporation from the skin surface decrease body heat. Their innervation is quite unusual because the postganglionic neurons secrete acetylcholine instead of the expected norepinephrine -- cholinergic sympathetic nerves. Some texts state some vessels also have sympathetic cholinergic innervation but this is not the case in humans. Activation of muscarinic 3 receptors on the secretory cells of these glands causes increased sweat production.
The secretory cells of eccrine sweat glands also have beta 1 receptors. However, there is no sympathetic adrenergic (i.e., norepinephrine) innervation and epinephrine is responsible for their activation; sweating is increased -- 'nervous' sweat'.
Apocrine sweat glands are predominantly located in the armpits and groin area. Their ducts do not lead to pores at the surface but rather into the side of hair follicles. From there the sweat is moved to the surface. The secretion is thick and contains many organic compounds that bacterial break down producing a foul odor. There is no innervation to these glands. Epinephrine stimulated beta 1 receptors induce production of this type sweat.