Biol 2401 A & P LECTURE NOTES Chapter 16 Dr

BIOL 2401 A & P I LECTURE NOTES ANS Dr. Weis

AUTONOMIC NERVOUS SYSTEM :

is a portion of the peripheral nervous system motor efferent division to involuntary muscle
that controls the visceral functions of the body, (GVE). Its actions and reactions take place primarily without voluntary control.

It functions to maintain homeostasis by dual innervation creating counterbalance and regulation of concentrations of fluids and electrolytes.

The ANS is activated by many centers located in the spinal cord, brain stem, and hypothalmus.
The motor unit of ANS is a two neuron chain. The cell body of the first neuron is called the PREGANGLIONIC neuron and resides in the brain or spinal cord.
The axon is called the preganglionic axon and will synapse with the second motor neuron --> the postganglionic neuron at a ganglion (neuron cell body outside the CNS)

The POSTGANGLIONIC axon will then go to the effector organ. Conduction through this two neuron chain is slower due to light or absent myelination.

Impulse are transmitted to the body through two major subdivisions of the ANS :

Parasympathetic

Sympathetic

Division differences are distinguished by :

1. unique origin sites of preganglionic neurons

2. different axon lengths for preganglionic and postganglionic neurons

3. Ganglia location

General Organization of the Sympathetic Nervous System :

--> paravertebral chains to the side of the spinal cord

--> sympathetic nerves orginate in the lateral gray horns spinal cord between segments T1 - L2,
and pass from here into the sympathetic chain. The origin points of the sympathetic nervous system are referred to as thoracodorsal

--> from the chain to tissues and organs

The sympathetic nerves are different from skeletal motor nerves

Skeletal motor nerves have single motor fiber from the spinal cord to the skeletal muscle

Each sympathetic nerve is comprised of 2 fibers :

preganglionic

postganglionic

Preganglionic fibers leave the spinal nerve through the ventral rami (a portion of the rami communicans) and pass through the white ramus into one side of the sympathetic chain that lie on either side of the vertebral column.

The sympathetic chain is made up of paravertebral ganglia in the cervical, thoracic, lumbar, sacral, and coccygeal regions.

The fibers can then do one of three things :

1. Can synapse with post ganglionic neurons in the ganglion that it enters

2. pass up or down in the chain and synapse in one of the other ganglion of the chain

3. can pass for a variable distance through the chain

(bypassing paravertebral ganglia) and then terminate in an outlying sympathetic ganglion called a prevertebral ganglia such as :

a. celiac

b. superior mesenteric

c. inferior mesenteric

d. hypogastric

The postganglionic fibers can then originate from the sympathetic ganglion or one of the outlying ganglion.

Axons of those postganglionic fibers that originate in the paravertebral ganglia will enter the ventral ramus
of the spinal nerve by way of the gray rami communicans to travel to their destination in the various organs.

Sympathetic nerve fibers from the cord segments T1 - L2 are APPROXIMATELY distributed as follows :

1. T1...........head

2. T2...........neck

3. T3-T6........thorax (heart, lungs, upper limbs)

4. T7-T11.......abdomen (gi, renal, reproductive)

5. T12 - L2.....legs

THE ADRENAL MEDULLA

The adrenal medulla has a special association with the sympathetic nervous system.

Preganglionic sympathetic nerve fibers pass WITHOUT synapsing through the sympathetic chains and
finally end directly on specialized cells that secrete epinephrine & norepenipherine directly into the circulatory system.

These specialized cells are analagous (embryologically) to postganglionic neurons.

Stimulation of sympathetic nerves causes the release of epinephrine (80%) and norepinephrine (20%) into the blood stream.
These hormones have more prolonged effects because they are removed from the blood more slowly than a neurotransmitter.

1. Epinephrine :: increases cardiac output and metabolic rate

2. Norepi :: constricts blood vessels to cause peripheral resistance.

The importance of the adrenal medulla ::

stimulates structures not innervated directly by the sympathetic fibers

provides back up to the sympathetic nervous system as a means of second stimulation (safety factor)

TO SUMMARIZE THE SYMPATHETIC NERVOUS SYSTEM ::

origin --> thoracodorsal

fibers --> short preganglionic axons long postganglionic axons

ganglia --> paravertebral chain outlying prevertebral groups

PARASYMPATHETIC NERVOUS SYSTEM

Anatomy :

parasympathetic fibers leave the CNS through several cranial nerves and sacral nerves (S1 - S4, primarily S2 & S3)

About 75% of the parasympathetic nerve fibers are in the VAGUS nerve (CN X).
The vagus nerve supplies the heart, lungs, esophagus, stomach, small intestine, proximal colon, liver, gall bladder, pancreas, and upper portion of the ureters.

Parasympathetic Nerves :

CN III : nuclei in midbrain, ciliary ganglia w/in eye

ciliary muscles of the eye :: affect papillary sphincter muscles to cause pupillary

constriction and affect the lens allowing it to bulge allowing for close vision.

CN III is known as the OCULOMOTOR n.

CN VII : nuclei in pons, ganglia in maxillary, submandibular or sublingual regions

lacrimal, nasal, submaxillary gland

CN VII is known as the FACIAL n.

CN IX : nuclei in medulla, otic ganglia near foramen ovale of the skull

parotid gland

CN IX is known as the GLOSSOPHARYNGEAL n.

To summarize : preganglionic fibers of these nerves lie within the cranial nerves.

Postganglionic neurons will tie in with CN V (trigeminal) to supply the head structures mentioned.

CN X : nuclei in medulla

terminal ganglia near organ innervated supply the neck and most of the organs in the thoracic and abdominal cavities

CN X is known as the VAGUS n.

Some branches of the vagus nerve will form various plexuses :

1. cardiac plexus --> heart to slow rate

2. Pulmonary plexus --> lungs, bronchi

3. Esophageal plexus --> esophagus

4. Aortic plexus --> along aorta

Sacral parasympathetic nerve fibers

originate in the lateral gray matter of spinal cord segments S2-S4

axons run in the ventral roots of the spinal nerve to the ventral rami, then branch to form the pelvic

splanchnic nerves that innervate :

descending colon

rectum

bladder and lower ureters

external genitalia

Parasympathetic system has preganglionic and postganglionic fibers.

The preganglionic fibers pass UNINTERUPTED to the origin that is to be excited by parasympathetic impulses.

In the wall of the organ are located the postganglionic neurons, which are very short fibers ranging from 1mm-cm.

TO SUMMARIZE : PARASYMPATHETIC NERVOUS SYSTEM :

origin --> cranial-sacral

fibers --> long preganglionic axons

short postganglionic axons

ganglia --> terminal ganglia (intramural) @ visceral organ

NEUROTRANSMITTERS of the Sympathetic and Parasympathetic System

Sympathetic and Parasympathetic secrete 1 of 2 neurotransmitters :

1. acetylcholine

2. norepinephrine

Those that secrete acetylcholine are called CHOLENERGIC

Those that secrete norepinephrine are called ADRENERGIC

ALL preganglionic neurons are CHOLENERGIC in BOTH the parasympathetic and sympathetic system and therefore will secrete acteylcholine.

Postganglionic parasympathetic neurons are cholenergic verses the postganglionic sympathetic, MOST of which are ADRENERGIC.

Recall that acetylcholine is synthesized in the terminal endings of cholenergic nerve fibers......

Acetyl CoA + choline -----> ACETYLCHOLINE

which in turn is broken down by the enzyme acetylcholinesterase.

RECEPTORS for ANS neurotransmitters :

Acetylcholine can activate two different types of receptors :

1. muscarinic

2. nicotinic

muscarinic is for effector cells of heart and smooth muscle

nicotinic is for postganglionic neuron @ both sympathetic & parasympathetic

neuromuscular junction with skeletal muscle

adrenal medulla.

ACH binding to nicotinic receptors is always stimulatory.

ACH binding to muscarinic receptors can be either stimulatory or inhibitory :

Muscarinic receptors of heart cause inhibitory

Muscarinic receptors of smooth muscle excitatory

This receptor difference is important because certain drugs can block or stimulate one or both of these receptors.

In the adrenergic receptors, those that respond to norepinephrine or epinephrine, also have two receptor types :

1. alpha receptors, ( a1 & a2 ) usually stimulatory

2. beta receptors, ( b 1 , b 2, & b 3) usually inhibitory

(beta 1 receptors in the heart are stimulatory)

Epinephrine excites both alpha and beta receptors

Norepinephrine primarily excites alpha receptors.

Therefore, the effects of norepinephrine and epinephrine depend on the receptor type of receptors in the organs.

Beta 1 Heart stimulatory incr. rate/strength

Kidney stimulatory renin hormone release

Beta 2 Lungs inhibitory relaxes smooth muscle

B.V. to sk. m. inhibitory relaxes, vasodilates

pancreas stimulates insulin release

Beta 3 adipose stimulates lipolysis of fat cells

Alpha 1 B.V. other organs stimulates constricts

pupils inhibits relaxes, dilates

Alpha 2 adrenergic axons inhibits decrease NE release

platelets stimulates promotes blood clotting

From the effects on different visceral functions of the body, it can be seen that the parasympathetic
and sympathetic stimulation causes excitatory effects in some organs, but inhibitory effects in others.

The two systems may work reciprically to each other as when the sympathetic stimulation excites a particular organ, the parasympathetic system often inhibits it.

SUMMARY OF ANS FUNCTIONS

PARASYMPATHETIC SYSTEM

eye ---> constrict pupil

focus on things that are near

glands --> salivation, gastrointestinal, lacrimal

gi sys --> increase degree of activity

promote peristalsis

relax sphincters

heart ---> decrease activity (pumping)

lungs ---> mild bronchiolar constrict

blood vessels ---> some dilation, mainly to digestive organs

some constriction to skeletal muscles

SYMPATHETIC FUNCTIONS

eye ---> dilate pupils

glands --> sweat glands (apocrine)

gi ---> inhibits peristalsis (slows food)

heart ---> increase activity (rate and force)

effective pump, increase metabolism

blood vessel ---> constriction, therefore, heart + blood vessel = increase blood press

lungs ---> dilate bronchi

metabolic effects on glucose......increase release

increase metabolic rate & increase mental alertness

However, most organs are dominantly controlled by one or the other of the two systems to maintain tone.

Since most visceral organs are innervated by both sympathetic and parasympathetic fibers,

they therefore receive DUAL INNERVATION.

Overall, integration of the ANS is controlled by the hypothalamus

medial, anterior region for parasympathetic

lateral, posterior region for sympathetic

Various centers exert their effects by relays through the brainstem's RAS (reticular activating system).

The sympathetic and parasympathetic system are continually active, and the basal rates of activity are known as TONE.

So we can have sympathetic tone & parasympathetic tone.

One system can dominate and its TONE is important in that it allows that division the sole ability to increase or decrease the activity of a stimulated organ.

Examples ::

Sympathetic tone for blood vessels (B.V.)

Parasympathetic tone for the heart, smooth muscle of GI and smooth muscle of urinary system

If the nerve is cut, the innervated organ loses its sympathetic or parasympathetic tone.
Over time, compensation occurs and the organ will develop its own intrinsic tone to return it to function close to its normal basal level.

Large portions of the sympathetic nervous system often become stimulated simultaneously called MASS DISCHARGE.

This increases the body to perform vigorous muscle activity.
These effects permit the person to perform far more strenuous physical activity than would otherwise be possible.
This sympathetic stress reaction is frequently referred to as the "fight or flight" reaction.

ANS ::

is the primary visceral motor control system

does have sensory (visceral sensory neurons) from glands, smooth muscle, heart

that are associated with CN or dorsal root of Sp N.

Since visceral changes (pain) afferents travel along some of the

Same pathways as somatic pain, this creates a phenomenon called referred pain.

That is, visceral pain is perceived as somatic pain.

Cutaneous areas have been mapped (see text and lab book)

Regulation of ANS :

spinal cord, brainstem, hypothalamus, cerebral cortex

A. Spinal Cord .. preganglionic ANS motor neurons

involved in visceral reflex arcs

(same as somatic reflex arc, but the motor neuron is a 2 neuron chain)

B. Brain stem .. reticular formation (RAS)

medulla --> cardiac & respiratory centers

pons --> higher respiratory centers

midbrain --> pupillary response

C. Hypothalamus .. overall integration of ANS