Autonomic Nervous System pdf - د. غسان

The Autonomic Nervous System (ANS):

After studying these lectures, you should be able to. .

1. Compare the structures and pathways of the autonomic system with

those involved in the control of skeletal muscle.

2. Explain how autonomic innervation of involuntary effectors differs

from the innervation of skeletal muscle.

3. Describe the structure and general functions of the sympathetic

division of the autonomic system.

4. Describe the structure and general functions of the parasympathetic

division of the autonomic system.

5. List the neurotransmitters of the preganglionic and postganglionic

neurons of the sympathetic and parasympathetic systems.

6. Describe the structural and functional relationships between the

sympathetic system and the adrenal medulla.

7. Distinguish between the different types of adrenergic receptors and

explain the physiological and clinical significance of these receptors.

8. Explain how the autonomic system is controlled by the brain.

The ANS coordinates cardiovascular, respiratory, digestive, urinary and

reproductive functions.

This system helps to control arterial pressure, gastrointestinal motility,

gastrointestinal secretions, urinary bladder, sweating, body temperature, and
many other activities. Some of theses activity regulated partially and some

others entirely regulated by ANS.

The most striking characteristic of ANS is the rapidity and intensity with

which it can change visceral functions. For example, it can increase heart rate

twice normal within 3 to 5 seconds, and within 10 to 15 seconds the arterial

pressure can be double; or at other extreme, the arterial pressure can

decrease within 5 seconds to fainting!

Basic anatomy of ANS:

• Preganglionic neuron

– Cell body in brain or spinal cord.
– Axon is myelinated type B fiber that extends to autonomic ganglion.

• Postganglionic neuron

– Cell body lies outside the CNS in an autonomic ganglion
–

Axon is unmyelinated type C fiber that terminates in a visceral

effector.

The ANS is composed of 2 anatomically and functionally distinct

divisions, the sympathetic system and the parasympathetic system. Both

systems are tonically active. In other words, they provide some degree of

nervous input to a given tissue at all times. Therefore, the frequency of

discharge of neurons in both systems can either increase or decrease. As a

result, tissue activity may be either enhanced or inhibited. This characteristic

of the ANS improves its ability to more precisely regulate a tissue's function.

Without tonic activity, nervous input to a tissue could only increase.

Many tissues are innervated by both systems. Because the sympathetic

system and the parasympathetic system typically have opposing effects on a

given tissue, increasing the activity of one system while simultaneously

decreasing the activity of the other results in very rapid and precise control of

a tissue's function.

Sympathetic Division

The sympathetic division is also called the thoracolumbar division of

the autonomic system because its preganglionic fibers exit the spinal cord

from the first thoracic (T1) to the second lumbar (L2) levels. Most

sympathetic nerve fibers, however, separate from the somatic motor fibers

and synapse with postganglionic neurons within a double row of sympathetic
ganglia, called paravertebral ganglia, located on either side of the spinal

cord. Ganglia within each row are interconnected, forming a sympathetic

chain of ganglia that parallels the spinal cord on each lateral side.

The myelinated preganglionic sympathetic axons exit the spinal cord in

the ventral roots of spinal nerves, but they soon diverge from the spinal

nerves within short pathways. The axons within each ramus enter the

sympathetic chain of ganglia, where they can travel to ganglia at different

levels and synapse with postganglionic sympathetic neurons.

These ganglion chains, which run parallel immediately along either side

of the spinal cord, each consist of 22 ganglia. The preganglionic neuron may

exit the spinal cord and synapse with a postganglionic neuron in a ganglion at

the same spinal cord level from which it arises. The preganglionic neuron may

also travel more upward or downward in the ganglion chain to synapse with

postganglionic neurons in ganglia at other levels. In fact, a single
preganglionic neuron may synapse with several postganglionic neurons in

many different ganglia.

The long postganglionic neurons originating in the ganglion chain then

travel outward and terminate on the effector tissues. This divergence of the

preganglionic neuron results in coordinated sympathetic stimulation to

tissues throughout the body. The concurrent stimulation of many organs and

tissues in the body is referred to as a mass sympathetic discharge.

Other preganglionic neurons exit the spinal cord and pass through the

ganglion chain without synapsing with a postganglionic neuron. Instead, the

axons of these neurons travel more peripherally and synapse with

postganglionic neurons in one of the sympathetic collateral ganglia.

These include the celiac, superior mesenteric and inferior mesenteric

ganglia. Postganglionic fibers that arise from the collateral ganglia innervate

organs of the digestive, urinary, and reproductive systems.

Finally, the preganglionic neuron may travel to the adrenal medulla and

synapse directly with this glandular tissue. The cells of the adrenal medulla

have the same embryonic origin as neural tissue and, in fact, function as

modified postganglionic neurons. Instead of the release of neurotransmitter

directly at the synapse with an effector tissue, the secretory products of the

adrenal medulla are picked up by the blood and travel throughout the body to

all of the effector tissues of the sympathetic system.

An important feature of this system, which is quite distinct from the

parasympathetic system, is that the postganglionic neurons of the
sympathetic system travel within each of the 31 pairs of spinal nerves. This

allows for the distribution of sympathetic nerve fibers to the effectors of the

skin including blood vessels and sweat glands. In fact, most innervated blood

vessels in the entire body, primarily arterioles and veins, receive only

sympathetic nerve fibers. Therefore, vascular smooth muscle tone and

sweating are regulated by the sympathetic system only. In addition, the

sympathetic system innervates structures of the head (eye, salivary glands,

and mucus membranes of the nasal cavity), thoracic viscera (heart, lungs) and

viscera of the abdominal and pelvic cavities (eg, stomach, intestines, pancreas,

spleen, adrenal medulla, and urinary bladder).

Parasympathetic Division

The preganglionic neurons of the parasympathetic system arise from

several nuclei of the brainstem and from the sacral region of the spinal cord

(segments S2-S4). The axons of the preganglionic neurons are quite long

compared to those of the sympathetic system and synapse with

postganglionic neurons within terminal ganglia which are close to or

embedded within the effector tissues. The axons of the postganglionic

neurons, which are very short, then provide input to the cells of that effector
tissue.

The preganglionic neurons that arise from the brainstem exit the CNS

through the cranial nerves. The occulomotor nerve (III) innervates the eyes;

the facial nerve (VII) innervates the lacrimal gland, the salivary glands and

the mucus membranes of the nasal cavity; the glossopharyngeal nerve (IX)

innervates the parotid (salivary) gland; and the vagus nerve (X) innervates

the viscera of the thorax and the abdomen (e.g., heart, lungs, stomach,

pancreas, small intestine, upper half of the large intestine, and liver). The

physiological significance of this nerve in terms of the influence of the

parasympathetic system is clearly illustrated by its widespread distribution

and the fact that 75% of all parasympathetic fibers are in the vagus nerve. The

preganglionic neurons that arise from the sacral region of the spinal cord exit
the CNS and join together to form the pelvic nerves. These nerves innervate

the viscera of the pelvic cavity (eg, lower half of the large intestine and organs

of the renal and reproductive systems).

Because the terminal ganglia are located within the innervated tissue,

there is typically little divergence in the parasympathetic system compared to

the sympathetic system. In many organs, there is a 1:1 ratio of preganglionic

fibers to postganglionic fibers. Therefore, the effects of the parasympathetic

system tend to be more discrete and localized, with only specific tissues being

stimulated at any given moment, compared to the sympathetic system where

a more diffuse discharge.

Neurotransmitters of the Autonomic Nervous

System:

The 2 most common neurotransmitters released by neurons of the ANS

are acetylcholine and norepinephrine. Neurotransmitters are synthesized in

the axon varicosities and stored in vesicles for subsequent release.

Nerve fibers that release acetylcholine are referred to as cholinergic

fibers. These include all preganglionic fibers of the ANS, both sympathetic and

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parasympathetic systems; all postganglionic fibers of the parasympathetic
system; and sympathetic postganglionic fibers innervating sweat glands.

Nerve fibers that release norepinephrine are referred to as adrenergic fibers.

Most sympathetic postganglionic fibers release norepinephrine.

Distinguishing Features of Neurotransmitters of the Autonomic

Nervous System were summarized in this table.

As previously mentioned, the cells of the adrenal medulla are

considered modified sympathetic postganglionic neurons. Instead of a

neurotransmitter, these cells release hormones into the blood. Approximately

20% of the hormonal output of the adrenal medulla is norepinephrine. The

remaining 80% is epinephrine. Unlike true postganglionic neurons in the

sympathetic system, the adrenal medulla contains an enzyme that methylates
norepinephrine to form epinephrine. The synthesis of epinephrine, also

known as adrenaline, is enhanced under conditions of stress. These 2

hormones released by the adrenal medulla are collectively referred to as the

catecholamines.

Receptors for Autonomic Neurotransmitters

As discussed in the previous section, all of the effects of the ANS in

tissues and organs throughout the body, including smooth muscle contraction

or relaxation, alteration of myocardial activity, and increased or decreased

glandular secretion, are carried out by only 3 substances, acetylcholine,

norepinephrine, and epinephrine. Furthermore, each of these substances may

stimulate activity in some tissues and inhibit activity in others.

The cholinergic nerve fibers:

Cholinergic Neurons

• Cholinergic neurons release the neurotransmitter

In the ANS, the cholinergic neurons include:

1) All sympathetic and parasympathetic preganglionic neurons

2) Sympathetic postganglionic neurons that innervate most sweat

glands

3) All parasympathetic postganglionic neurons

Acetylcholine is stored in synaptic vesicles and released by exocytosis.

It diffuses across the synaptic cleft and binds with specific cholinergic

receptors, integral proteins in the postsynaptic plasma membrane.

●

Excitation or inhibition depending upon receptor subtype and organ

involved

●

The two types of cholinergic receptors are nicotinic and muscarinic

receptors.

●

Activation of nicotinic receptors causes excitation of the postsynaptic

cell.

=norepinephrine & epinephrine

●

Nicotinic receptors are found on the cell bodies of all postganglionic

neurons, both sympathetic and parasympathetic, in the ganglia of the ANS

(and at neuromuscular junction). Acetylcholine released from the

preganglionic neurons binds to these nicotinic receptors and causes a rapid

increase in the cellular permeability to Na

ions and Ca

ions. The resulting

influx of these 2 cations causes depolarization and excitation of the

postganglionic neurons the ANS pathways.

●

Nicotine mimics the action of acetylcholine by binding to these

receptors.

●

Muscarinic receptors are found on the cell membranes of the effector

tissues and are linked to G proteins and second messenger systems which
carry out the intracellular effects.

●

Activation of muscarinic receptors can cause either excitation or

inhibition depending on the cell that bears the receptors. For example,

muscarinic receptor stimulation in the myocardium is inhibitory and

decreases heart rate while stimulation of these receptors in the lungs is

excitatory, causing contraction of airway smooth muscle and

bronchoconstriction.

●

Muscarinic receptors are found on plasma membranes of all

parasympathetic effectors. Examples: smooth muscle, cardiac muscle and

glands.

The adrenergic nerve fibers:

The ANS, adrenergic neurons release norepinephrine (noradrenalin).
●

Most sympathetic postganglionic neurons are adrenergic.

NE is synthesized and stored in synaptic vesicles and released by

exocytosis.

●

Molecules of NE diffuse across the synaptic cleft and bind to specific

adrenergic receptors on the postsynaptic membrane, causing either excitation

or inhibition of the effector cell.

●

The main types of adrenergic receptors are alpha and beta receptors.

These receptors are found on visceral effectors innervated by most

sympathetic postganglionic axons.

These receptors are further classified into subtypes.

– Alpha1 and Beta1 receptors produce excitation

– Alpha2 and Beta2 receptors cause inhibition

●

Effects triggered by adrenergic neurons typically are longer lasting

than those triggered by cholinergic neurons

●

Cells of most effectors contain either alpha or beta receptors.

Norepinephrine stimulates alpha receptors more strongly than beta receptors

●

All of these receptors are linked to G proteins and second messenger

systems which carry out the intracellular effects.

Alpha receptors are the more abundant of the adrenergic receptors. Of

the 2 subtypes, α

receptors are more widely distributed on the effector

tissues. Alpha one receptor stimulation leads to an increase in intracellular

calcium. As a result, these receptors tend to be excitatory. For example,

stimulation of α

receptors causes contraction of vascular smooth muscle

resulting in vasoconstriction and increased glandular secretion by way of

exocytosis.

Termination of Neurotransmitter Activity

For any substance to serve effectively as a neurotransmitter, it must be

rapidly inactivated or removed from the synapse or, in this case, the

neuroeffector junction. This is necessary in order to allow new signals to get

through and influence effector tissue function.

●

The primary mechanism used by cholinergic synapses is enzymatic

degradation. Acetylcholinesterase hydrolyzes acetylcholine to its component

choline and acetate. It is one of the fastest acting enzymes in the body and

acetylcholine removal occurs in less than 1 msec.

●

The most important mechanism for the removal of norepinephrine

from the neuroeffector junction is the reuptake of this neurotransmitter into

the sympathetic nerve that released it. Norepinephrine may then be

metabolized intraneuronally by monoamine oxidase (MAO).

●

The circulating catecholamines, epinephrine and norepinephrine, are

inactivated by catechol-O-methyltransferase (COMT) in the liver.

● Certain postganglionic autonomic axons produce their effects

through mechanisms that do not involve either norepinephrine or

acetylcholine. This can be demonstrated experimentally by the inability of

drugs that block adrenergic and cholinergic effects from inhibiting the

actions of those autonomic axons.

These axons, consequently, have been termed “nonadrenergic,

noncholinergic fibers.” Proposed neurotransmitters for these axons include

ATP, a polypeptide called vasoactive intestinal peptide (VIP), and nitric oxide

(NO).

The nonadrenergic, noncholinergic parasympathetic axons that

innervate the blood vessels of the penis cause the smooth muscles of these

vessels to relax, thereby producing vasodilation and a consequent erection of

the penis.

In a similar manner, nitric oxide appears to function as the autonomic

neurotransmitter that causes vasodilation of cerebral arteries.

Studies suggest that nitric oxide is not stored in synaptic vesicles, as are

other neurotransmitters, but instead is produced immediately when Ca2+

enters the axon terminal in response to action potentials. This Ca2+

indirectly activates nitric oxide synthetase, the enzyme that forms nitric

oxide from the amino acid L-arginine. Nitric oxide then diffuses across the

synaptic cleft and promotes relaxation of the postsynaptic smooth muscle

cells.

Nitric oxide can produce relaxation of smooth muscles in many

organs, including the stomach, small intestine, large intestine, and urinary
bladder.

Indeed, nitric oxide is a member of a class of local tissue regulatory

molecules called paracrine regulators. Regulation can therefore be a complex

process involving the interacting effects of different neurotransmitters,

hormones, and paracrine regulators.

Functions of the Autonomic Nervous System

The 2 divisions of the ANS are dominant under different conditions. As

stated previously, the sympathetic system is activated during emergency

“fight-or-flight” reactions and during exercise. The parasympathetic system is

predominant during quiet conditions (“rest and digest”). As such, the

physiological effects caused by each system are quite predictable. In other
words, all of the changes in organ and tissue function induced by the

sympathetic system work together to support strenuous physical activity and

the changes induced by the parasympathetic system are appropriate for when

the body is resting.

The “fight-or-flight” reaction elicited by the sympathetic system is

essentially a whole body response.

Changes in organ and tissue function throughout the body are

coordinated so that there is an increase in the delivery of well-oxygenated,

nutrient-rich blood to the working skeletal muscles. Both heart rate and

myocardial contractility are increased so that the heart pumps more blood per

minute. Sympathetic stimulation of vascular smooth muscle causes

widespread vasoconstriction, particularly in the organs of the gastrointestinal

system and in the kidneys. This vasoconstriction serves to “redirect” or

redistribute the blood away from these metabolically inactive tissues and

toward the contracting muscles. Bronchodilation in the lungs facilitates the

movement of air in and out of the lungs so that the uptake of oxygen from the

atmosphere and the elimination of carbon dioxide from the body are

maximized. An enhanced rate of glycogenolysis (breakdown of glycogen into

its component glucose molecules) and gluconeogenesis (formation of new

glucose from noncarbohydrate sources) in the liver increases the

concentration of glucose molecules in the blood. This is necessary for the

brain as glucose is the only nutrient molecule that it can utilize to form

metabolic energy. An enhanced rate of lipolysis in adipose tissue increases the

concentration of fatty acid molecules in the blood. Skeletal muscles then

utilize these fatty acids to form metabolic energy for contraction. Generalized

sweating elicited by the sympathetic system enables the individual to
thermoregulate during these conditions of increased physical activity and

heat production. Finally, the eye is adjusted such that the pupil dilates letting

more light in toward the retina (mydriasis) and the lens adapts for distance

vision.

The parasympathetic system decreases heart rate which helps to

conserve energy under resting conditions.

Salivary secretion is enhanced to facilitate the swallowing of food.

Gastric motility and secretion are stimulated to begin the processing of

ingested food. Intestinal motility and secretion are also stimulated to continue

the processing and to facilitate the absorption of these nutrients. Both

exocrine and endocrine secretion from the pancreas is promoted. Enzymes

released from the exocrine glands of the pancreas contribute to the chemical

breakdown of the food in the intestine and insulin released from the

pancreatic islets promotes the storage of nutrient molecules within the tissues

once they are absorbed into the body. Another bodily maintenance type of

function caused by the parasympathetic system is contraction of the urinary

bladder which results in urination. Finally, the eye is adjusted such that the

pupil contracts (miosis) and the lens adapts for near vision.

Adrenal Medulla

A mass sympathetic discharge, which typically occurs during the “fight-

or-flight” response and during exercise, involves the simultaneous stimulation

of organs and tissues throughout the body. Included among these tissues are

the adrenal medullae which release epinephrine (80%) and norepinephrine

(20%) into the blood. In large part, the indirect effects of these

catecholamines are similar to and, therefore, reinforce those of direct

sympathetic stimulation. However, there are some important differences in
the effects of the circulating catecholamines and those of norepinephrine

released from sympathetic nerves.

Figure 2: The direct release of NE to the blood.

●

The duration of activity of the catecholamines is significantly longer

than that of neuronally released norepinephrine. Therefore, the effects on the

tissues are more prolonged. This difference has to do with the mechanism of
inactivation of these substances. Norepinephrine is immediately removed

from the neuroeffector synapse by way of reuptake into the postganglionic

neuron. This rapid removal limits the duration of the effect of this

neurotransmitter. In contrast, there are no enzymes in the blood to degrade

the catecholamines. Instead, the catecholamines are inactivated by COMT in

the liver. As one might expect, the hepatic clearance of these hormones from

the blood would require several passes through the circulation. Therefore, the

catecholamines are available to cause their effects for a comparatively longer

period of time (up to 1-2 minutes as opposed to milliseconds).

●

Because they travel in the blood, organs and tissues throughout the

body are exposed to the catecholamines. Therefore, they are capable of
stimulating tissues that are not directly innervated by sympathetic nerve

fibers: airways smooth muscle, hepatocytes, and adipose tissue, in particular.

As a result, the catecholamines have a much wider breadth of activity

compared to norepinephrine released from sympathetic nerves. It also causes

a great increase in the basal metabolic rate (BMR).

●

The third important feature that distinguishes the catecholamines

from neuronally released norepinephrine involves epinephrine's affinity for

receptors. Norepinephrine has a very limited affinity for these receptors.

Therefore, circulating epinephrine causes effects that differ from those of

direct sympathetic innervations including a greater stimulatory effect on the

heart and relaxation of smooth muscle (vascular, bronchial, gastrointestinal,
and genitourinary).

Epinephrine and norepinephrine have equal affinity for β

receptors, the

predominant adrenergic receptor on the heart. However, the human heart

also contains a small percentage of β

receptors which, like β

receptors are

excitatory. Therefore, epinephrine is capable of stimulating a greater number
of receptors and of causing a greater stimulatory effect on the myocardium.

Beta two adrenergic receptors are also found on smooth muscle in

several organ systems. These receptors tend to be inhibitory and cause

relaxation of the smooth muscle. Vascular smooth muscle in skeletal muscle

contains both α

and β

receptors. Norepinephrine, which stimulates only the

excitatory α

receptors, causes strong vasoconstriction. However,

epinephrine, which stimulates both types of receptors, causes only weak

vasoconstriction. The vasodilatation resulting from β

receptor stimulation

opposes and, therefore, weakens the vasoconstriction resulting from α

receptor stimulation. Given that skeletal muscle may account for 40% of an

adult's body weight, the potential difference in vasoconstriction, blood
pressure, and the distribution of blood flow could be quite significant.

Another noteworthy example of the relaxation of smooth muscle by way

of β

receptor stimulation involves the airways. Bronchodilation, or the

opening of the airways, facilitates airflow in the lungs. Any direct sympathetic

innervation to the lungs is irrelevant in this respect, as only circulating

epinephrine is capable of stimulating these receptors on airway smooth

muscle.

Synthesis of NE:

Tyrosine

═

(hydroxylation)

═►

DOPA

═

(decarboxylation)

══►

Dopamine transport in vesicles ═ (hydroxylation) ═►nor epinephrine ═

(methylation) ═► Epinephrine

When NE or E come in contact with receptors on cell membrane of the

effector cells → receptor transmitter complex →activated enzyme adenyl

cyclase → cyclic AMP → Phosphorylation of Voltage-dependent Ca

channels

→ increase intracellular Ca

Sympathetic and parasympathetic tones:

The ANS is continuously active. The basal rate of activity is called

"Tone". The tone can be increased or decreased.

Interactions between the Sympathetic and

Parasympathetic Divisions:

Most visceral organs receive dual innervation—they are

innervated by both sympathetic and parasympathetic fibers. In this

condition, the effects of the two divisions of the autonomic system

may be antagonistic, complementary, or cooperative

Antagonistic Effects

The effects of sympathetic and parasympathetic innervation of

the pacemaker region of the heart is the best example of the

antagonism of these two systems. In this case, sympathetic and
parasympathetic fibers innervate the same cells. Adrenergic

stimulation from sympathetic fibers increases the heart rate,

whereas the release of acetylcholine from parasympathetic fibers

decreases the heart rate. A reverse of this antagonism is seen in the

digestive tract, where sympathetic nerves inhibit and

parasympathetic nerves stimulate intestinal movements and

secretions.

Complementary and Cooperative Effects

The effects of sympathetic and parasympathetic nerves are

generally antagonistic; in a few cases, however, they can be

complementary or cooperative. The effects are complementary
when sympathetic and parasympathetic stimulation produce similar

effects. The effects are cooperative, or synergistic, when sympathetic

and parasympathetic stimulation produce different effects that work

together to promote a single action.

The effects of sympathetic and parasympathetic stimulation on

salivary gland secretion are complementary. The secretion of watery

saliva is stimulated by parasympathetic nerves, which also stimulate
the secretion of other exocrine glands in the digestive tract.

Sympathetic nerves stimulate the constriction of blood vessels

throughout the digestive tract. The resultant decrease in blood flow

to the salivary glands causes the production of thicker, more viscous

saliva.

The effects of sympathetic and parasympathetic stimulation on

the reproductive and urinary systems are cooperative.

Erection of the penis, for example, is due to vasodilation

resulting from parasympathetic nerve stimulation; ejaculation is due

to stimulation through sympathetic nerves. The two divisions of the

autonomic system thus cooperate to enable sexual function in the

male.

Organs without Dual Innervation

Although most organs are innervated by both sympathetic and

parasympathetic nerves, some—including the adrenal medulla,

arrector pili muscles, sweat glands, and most blood vessels—receive

only sympathetic innervation. In these cases, regulation is achieved

by increases or decreases in the tone (firing rate) of the sympathetic

fibers. Constriction of cutaneous blood vessels, for example, is

produced by increased sympathetic activity that stimulates alpha-

adrenergic receptors, and vasodilation results from decreased

sympathetic nerve stimulation.

The sweat glands in the trunk secrete a watery fluid in

response to cholinergic sympathetic stimulation. Evaporation of this

dilute sweat helps to cool the body. At the end of exercise,

sympathetic stimulation is reduced, causing cutaneous blood vessels

to dilate. This increases blood flow to the skin, which helps to

eliminate metabolic heat.

Regulation of Autonomic Nervous System Activity

The efferent nervous activity of the ANS is largely regulated by

autonomic reflexes. In many of these reflexes, sensory information is

transmitted to homeostatic control centers, in particular, those located in the

hypothalamus and brainstem. Much of the sensory input from the thoracic

and abdominal viscera is transmitted to the brainstem by afferent fibers of

cranial nerve X, the vagus nerve. Other cranial nerves also contribute sensory

input to the hypothalamus and the brainstem. This input is integrated and a

response is carried out by the transmission of nerve signals that modify the
activity of preganglionic autonomic neurons. Many important variables in the

body are monitored and regulated in the hypothalamus and the brainstem

including heart rate, blood pressure, gastrointestinal peristalsis and glandular

secretion, body temperature, hunger, thirst, plasma volume, and plasma

osmolarity.

An example of this type of autonomic reflex is the baroreceptor reflex.

Baroreceptors located in some of the major systemic arteries are sensory

receptors that monitor blood pressure. If blood pressure decreases, the

number of sensory impulses transmitted from the baroreceptors to the

vasomotor center in the brainstem also decreases. As a result of this change in

baroreceptor stimulation and sensory input to the brainstem, ANS activity to
the heart and blood vessels is adjusted to increase heart rate and vascular

resistance so that blood pressure increases to its normal value.

These neural control centers in the hypothalamus and the brainstem

may also be influenced by higher brain areas. Specifically, the cerebral cortex

and the limbic system influence ANS activities associated with emotional

responses by way of hypothalamic-brainstem pathways. For example,

blushing during an embarrassing moment, a response most likely originating

in the frontal association cortex, involves vasodilation of blood vessels to the

face. Other emotional responses influenced by these higher brain areas

include fainting, breaking out in a cold sweat, and a racing heart rate.

Some autonomic reflexes may be processed at the level of the spinal

cord. These include the micturition reflex (urination) and the defecation

reflex. Although these reflexes are subject to influence from higher nervous

centers, they may occur without input from the brain.

End