Autonomic Nervous System Lecture Notes


General description:


1. Nervous system has great function of controlling and regulating body activities, in additions; to another system called (hormonal system) they both keep homeostasis of the body in different situations.

2. Nervous system, as a function, divided to voluntary system (where we can give orders from our consciousness) and involuntary system (all activities that we are not ware of them. And as anatomy, Nervous system divided to central (Brain & spinal cord) and peripheral (all nerves that emerge and enter from brain & spinal cord to different body sites.

* (Remember that Nerves = bundle of axons collected together)

3. Autonomic system means involuntary system, as to say, it works automatically without our knowledge.

4. Autonomic system divided to sympathetic and parasympathetic and almost always each one act oppositing the other (if sympathetic increase some thing, parasympathetic decrease it).

5. Both division supply following structures [heart muscle, smooth muscle, glands (most exocrine glands and some endocrine glands)] these structures or organs are called effector organs

6. Sympathetic & parasympathetic are part of peripheral Nervous system.

7. Principle: when we talk about a system, in nervous systems, you have to know that the system should have neuron bodies and axons (as to say nerves), and any system may have one or more type of neuron bodies regarding the site they lie in (i.e. not necessary the neurons lie in one place).

8.Sympathetic & parasympathetic system are simple, they have only two types of neurons (regarding the site)

First neurons lie in central Nervous system (CNS) and they extend axons that synapse with second neurons, the site of synapse, is called ganglion [pleural: ganglia], the site of synapse (ganglion) is out side (CNS) and may be near or far from CNS.

9. Sympathetic system over work in emergency situations or extreme situations and prepare the body for this, so it is called (fight or flight) system, While parasympathetic over work in rest & quiet situations and control activities that are not needed for emergency (like digestion or urination) so it is called (Rest & digest) system.

Anatomy of sympathetic & parasympathetic:

* As we said that each division of autonomic have two neurons; first: in CNS and second: outside CNS.


A- Sympathetic system:

First neurons: lie in CNS, axons emerge from thoracic and lumber regions of (spinal cord) and give axons to outside spinal cord. (This origin Called thoracolumber region).

Second neurons: they lie in region parallel to spinal cord (on left and right side of spinal cord) and they form a chain. on both sides so they are called (sympathetic chain) or (sympathetic ganglia).

* Axons of first neurons go through sympathetic ganglia (chain) to make synapse with second neurons, and most of them do this, but, some fibers (axons) go through sympathetic chain without synapsing and go away toward the (effector organ), but; in the half way, they synapse with (second neurons) to form (collateral ganglion), these ganglia lie in mid way between spinal cord and effector organ, like superior and inferior mesenteric ganglion,

* Notice that sympathetic chain is near spinal cord. 1 / 7

* After first axons synapse with second neuron, the second axons go to effector organ (heart or smooth muscle or glands).


Note: now we will use specific terms:

Axons of first neuron are called: sympathetic preganglionic nerves and axons of second neuron: sympathetic postganglionic nerves and the ganglia are called sympathetic ganglia.

B- Para sympathetic system:

First neuron: lie in CNS and axons emerge with cranial nerves (III, VII, IX, X) and from sacral region of spinal cord (S2, S3, S4).


1. Cranial nerves are nerves that emerge from cranium (Brain) they are 12 nerves.

2. Because parasympathetic originates from cranial & sacral regions, they are called craniosaeral origin.

Second neuron: first axons go toward effector organs until they reach the second neurons very near or inside the effector organ, where they synapse to form ganglion, and from this second neuron the axons extend very shortly to innervate effector organ.

Note: first neuron axons called parasympathetic preganglionic nerves; second neuron axons called parasympathetic postganglionic nerves, the ganglia are called parasympathetic ganglia.

* Notice that the ganglia are far away from CNS (reverse to sympathetic ganglia).

Note: now you can conclude why we say that; preganglionic nerves of sympathetic are short and postganglionic are long, and in parasympathetic the reverse, preganglionic is long and post ganglion are short, isn’t it?

General Function of Autonomic (Sympathetic & Parasympathetic).

* In normal conditions or rest state both divisions work together, may be with dominance of one on other, but, both of them has what we call a (Tone) of work, so we have (sympathetic Tone) and (parasympathetic Tone).

* When the demands of the body need some division to dominate in some situation (let’s say sympathetic division), the response will be increased sympathetic tone and decreased parasympathetic tone (in the same time).

* Both sympathetic and parasympathetic innervate an organ, except for some organs examples: systemic blood vessels (not including blood vessels of sex organs), sweat glands, brain) all of them have only sympathetic innervations.

Note: sex organs are called (genitalia); their blood vessels are innervated by both sympathetic & parasympathetic.

* What is the wisdom of dual innervations of an organ by sympathetic & parasympathetic?!

Answer: for precise control of activities of this organ.


* Don’t forget each synapse or meeting between nerve & organ, need a neurotransmitter….

1. Axons of first neurons of both sympathetic & parasympathetic (i.e. preganglionic fibers) release (ACH) neurotransmitter, so, they are called cholinergic fibers.

2. Postganglionic fibers of parasympathetic, release ACH (cholinergic fibers).

3. Postganglionic fibers of sympathetic, release norepinephrine (noradrenalin) so the fibers are called adrenergic fibers. 2 / 7

Important note: above the kidney there is a gland called adrenal gland (Ad = above, renal = kidney), this gland consist of cortex and medulla, the medulla receive (preganglionic sympathetic fibers), as if it is (sympathetic ganglia).

* When adrenal medulla receive stimulation from preganglionic sympathetic fibers, in turn, the medulla will secrete norepinephrine & epinephrine directly to blood, and these substances will go to all body.

* So, the adrenal medulla enhances the action of sympathetic ..

Receptors of Neurotransmitters:

1. For ACH: there are nicotinic receptors (stimulated also by nicotine that present in tobacco) and Muscarinic receptors stimulated also by substance called muscarine derived from mushroom .

* Nicotinic receptors found in autonomic ganglia (sympathetic & parasympathetic) and in neuromuscular junction.

* Muscarinic receptors found in all effector organs of parasympathetic nerves (heart, smooth muscles, glands).

2. For epinephrine (adrenalin): there are Alpha receptors (α) and Beta receptors (β) [β1 and β2].

* They are distributed in effector organs of sympathetic nerves in some pattern (so that some organs has alpha, others has β1 and other β2 and others has mixture of receptors).

3. For Norepinephrine (Noradrenalin): only Alpha-receptors and β1 receptors.

* β1 receptors mainly present in heart


1. Alpha and β1 receptors are mainly stimulatory or excitatory for organ.

2. β2 is mainly inhibitory.

3. Notice that excitatory for muscles (heart or smooth muscles), means increase contraction, inhibitory means relaxation of muscle. And excitatory for gland, means increase secretion.

Summary of receptors

1- For postganglionic Parasympathetic, there are Muscarinic receptors.

2- For postganglionic sympathetic there are Alpha, β1, β 2 receptors.

3- Alpha, β 1 are stimulatory, β 2 inhibitory for organ.

4- Don’t forget; for preganglionic fibers, there are only nicotinic receptors)

(We mean by preganglionic, for both sympathetic & parasympathetic.)


1– Don’t forget that skeletal muscles don’t have autonomic innervations to their muscle fibers

2– Muscarinic, nicotine receptors are called cholinergic receptors because they are receptors for ACH, (and Alpha, β 1, β 2) are called adrenergic receptors, because they are for (Adrenaline & Noradrenalin)

3– Alpha receptors (recently) divided to alpha 1, alpha2 (the important here is alpha1)

Synapse between postganglionic fiber & effector organ:

* When the postganglionic fiber (what ever it is, sympathetic or parasympathetic) reach the effector organs, it does not form well organized structure like that in neuromuscular junction, but instead , The terminals of axon will have multiple bulges on the each axon terminal that are called Varicosities These varicosities are filled with vesicles, that contain the neurotransmitter, and neurotransmitter is released in response to arrival of Action potential 3 / 7

Fate of Neurotransmitters: –

1- ACH: will be destroyed by Ach esterase to acetate and choline some are return and other may go with blood stream .

2- Adrenaline & Noradrenalin are destroyed by enzymes called [Mono amino oxidase (MAO)] or by [Catechol -o- methyl transferase (COMT)]


1. Adrenaline & nor adrenaline are part of catecholamines.

* 2. Nor epinephrine is neurotransmitter and hormone (because it is secreted from adrenal medulla gland)

While adrenaline is just a hormone and not present as neurotransmitter.

Summary of ACH releasing sites:

Q. In where, ACH is released? Answer:

1- From all preganglionic fibers

2- From all postganglionic fibers of parasympathetic.

3- Some post ganglionic fibers of sympathetic (As in nerves that supply sweat glands)

4- at neuromuscular junction.

5- at several synapses in CNS

Blocking of receptors:

1- ACH receptors

a- Nicotinic receptors = blocked by curare

b- Muscarinic receptors = blocked by Atropine

Stimulation of receptors:

E.g. Nicotinic & Muscarinic: by carbachol, pilocarbine,

E.g. β2 stimulator: salbutamol (ventolin)

Note: The substances that make stimulation to receptor is called agonist and substance that make blocking are called antagonist

How receptors work:

* As we said Alpha, B1, are stimulatory, so they work by increasing intracellular Ca+2 , while B2 inhibitory , so they lead to decrease in intracellular Ca+2

* Second messenger (inside cell) for β 1, β 2 receptors is cyclic AMP or (cAMP) [second messenger is the substance that do the wanted action inside the cell after binding of neurotransmitter on the surface of the cell]

Functions of sympathetic & parasympathetic on each organ: –

1- Heart & rate and contractility:

* Sympathetic: increase heart rate & force of contraction of whole heart by β 1 action,

* Parasympathetic: decrease rate, and force of contraction of Atria only, No effect on coronaries

2- Blood vessels:

* Sympathetic: make vasoconstriction (contraction of smooth muscle of vessels) by alpha-receptor action (in most organs), or make vasodilatation by B2 receptor through adrenaline secreted from adrenal medulla

* Parasympathetic: has no effect on systemic blood vessel, except, for blood vessels of penis and clitoris, it makes vasodilatation which result in erection of these sex organs

3- Bronchioles of lung

* Sympathetic: bronchodilator by B2 action (relaxation of smooth muscles of bronchioles)

* Parasympathetic: bronchoconstriction (narrowing), and stimulate secretion of mucus in Bronchioles

4- Digestive tract:

* Sympathetic: Decrease movement (peristalsis), increase contraction the sphincters, so that prevent forward movement, weak inhibition of secretions

* Parasympathetic: increase movement (peristalsis), decrease contraction of sphincters and increase secretions of digestive system (digestive juices)

5- Urinary bladder

* Sympathetic: relaxation

* Parasympathetic: contraction (emptying of bladder)

6- Eye pupil:

* Sympathetic: dilation of pupil (to take more light enter to eye) and adjacement of eye for Far vision

* Parasympathetic: constriction of pupil (less light pass) and adjacement for near vision

7- Liver – glycogenolysis

* Sympathetic: increase glycogenolysis (to produce glucose, which is fuel ) * Parasympathetic: NONE

8- Lipolysis (to produce fatty acids which are fuel also)

* Sympathetic: increase lipolysis (to produce source of fuel i.e. energy) * Parasympathetic: none

9- Skin:

* Sympathetic: make vasoconstriction for the blood vessels of skin (so that the blood that is present in skin will divert to more important organs: brain, heart and kidney)

* Parasympathetic: none

10- Exocrine secretions: (pancreatic secretion)

* Sympathetic: decrease secretion * Parasympathetic: increase secretion

11- Salivary glands

* Sympathetic: stimulate small volume, watery saliva

* Parasympathetic: stimulate large volume, enzyme rich saliva

12- Adrenal medulla: 5 / 7

* Sympathetic: stimulate release of epinephrine & norepinephrin * Parasympathetic: none

13- Endocrine pancreases:

* Sympathetic inhibit insulin secretion, stimulate glucagons secretion

* Para sympathetic: stimulate insulin, inhibit glucagons

14- Sweat glands:

* Sympathetic stimulate sweating

* Parasympathetic: none

15- Genitalia (sex organs)

* Sympathetic: in male: ejaculation and orgasmic contraction, in female: orgasmic contraction

* Parasympathetic: erection of penis in male and clitoris in female

16- Brain activity:

* Sympathetic: increase alertness

*Parasympathetic: none.


1- Blood vessels has only Alpha-receptors and B2 receptors (except for vessels of heart has B2 only)

2- Size of these blood vessels is controlled by sympathetic Tone and through Alpha receptors only —- why? because sympathetic postganglionic fibers secrete or release norepinephrin only , and this substance don’t bind with β2 ( only with alpha & β1 ) , SO , when the tone increase — this mean vasoconstriction of blood vessels ( blood pressure will rise ) , and when tone decrease ( vasodilatation ) [ blood pressure will decrease ]

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