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تحميل الدليل التدريبي

أسئلة شائعة




 Nervous System




Gad El - Mawla A. Gad.

Professor of Physiology







The physiological functions are regulated according to the body needs and in response to the changes in the internal or external environment. This occurs by two main regulatory mechanisms:

A) Chemical regulation; by hormones, vitamins, enzymes, ions, gases and chemical transmitters. Chemical regulation is characterized by having a slow onset and a prolonged duration of action.

B) Nervous regulation; by the nervous system. It is characterized by a rapid onset and a short duration of action.  

The two regulatory mechanisms are complementary and can, therefore influence each other e.g. certain chemical substances can, either stimulate or inhibit the nervous system and by turn influence the nervous regulation. On the other hand, stimulation of the nervous system leads to release of chemical substances e.g. hormones which influence the chemical regulation.

Nervous system:

The structural unit of the nervous system is the neuron.

The functional unit of the nervous system is the reflex arc. In its simplest form it is composed of two neurons; an afferent and efferent neurons and a center uniting them. The afferent neuron carries impulses, generated by the receptors, to the central nervous system. The efferent neuron carries impulses from the central nervous system (CNS) to the effector organ.

Divisions of the nervous system:

(I) Central nervous system (CNS):

It is the part of the nervous system which lies inside the bony cavities (skull and vertebral column). It is divided into:

A) Brain which includes:

1)  Cerebrum:

a)     Cerebral cortex (2 cerebral hemispheres).

b)    Sub-cortical centers (thalamus, hypothalamus and basal ganglia).

2)  Cerebellum:

3)  Brian stem (mid brain, pons and medulla oblongata).

B) Spinal Cord which is divided into the following: Cervical region (8 segments ) , Thoracic region (12 segments),  Lumbar region (5 segments), Sacral region (5 segments ), Coccygeal region (1 segment)

 (II) Peripheral nervous system:  In includes the different nerves connecting the central nervous system with the various organs of the body. The peripheral nervous system can be classified by many ways:

A) Anatomical classification:

1)     Cranial nerves: (Connected with the brain), There are 12 pairs of cranial nerves:



Sensory function

Motor function


Olfactory nerve

Smell sensation



Optic nerve

Visual sensation.



Oculomotor nerve


To muscles of the eye except 2 muscles.


Trochlear nerve


To one muscle of the eye.


Trigeminal nerve

From the face

To muscles of mastication


Abducent nerve


To one muscle of the eye.


Facial Nerve

Taste from tongue

To muscles of face. Secretory to salivary and lacrimal glands.


Auditory nerve

Auditory sensation and equilibrium



Glossopharyngeal nerve

Taste from tongue and pharynx.

Secretory to salivary glands.


Vagus nerve

From thoracic and abdominal viscera

To thoracic and abdominal viscera.


Accessory nerve


To muscles of shoulders & back of neck.


Hypoglossal nerve


To muscles of tongue.

2)     Spinal nerves: (connected with the spinal cord)

There are 31 pairs of  nerves; one pair of nerves arises from each segment of the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5sacral and one coccygeal).

B) Physiological classification:

The peripheral nervous system, can be classified according to its function into:

1) Somatic nervous system, supplies the body wall and extremities by:

a) Somatic sensory fibers, carry sensory informations to the central nervous system from skin, skeletal muscles, and bones. Their mother cells are present in the dorsal root ganglia of the spinal nerves or in similar ganglia of the cranial nerves. Somatic sensory (afferent) fibers carry the cutaneous sensations (pain, temperature and touch), deep sensations (sense of pressure, position, and movements)and the afferent impulses of the somatic reflexes (e.g. withdrawal reflex ).

b) Somatic motor fibers, carry motor orders from the central nervous system to the skeletal (voluntary ) muscles.

N.B: All spinal nerves contain both sensory and motor fibers i.e. mixed nerves. Some cranial nerves are purely sensory ( Olfactory, Optic and Auditory nerves), some are purely motor( Oculomotor, Trochlear and Abducent nerves), and the remaining are mixed nerves.

2) Autonomic (visceral ) nervous system, supplies the internal organs (the viscera) by:

a) Autonomic sensory fibers; carry sensory information from the internal organs (heart, lungs, stomach, intestine, kidney, urinary bladder, rectum etc) to the central nervous system. Autonomic sensory (afferent) fibers carry the visceral sensations (pain, sense of desire of micturition and defecation etc), and the afferent impulses of autonomic reflexes (e.g. micturition reflex).

b) Autonomic motor fibers, carry motor orders from the central nervous system to the smooth (involuntary) muscles, cardiac muscle, and glands.

Differences between somatic and autonomic nervous Systems:


Somatic nervous system
Autonomic nervous system
With the body wall and extremities i.e. skin and skeletal system (muscles, joints and bones)
With the smooth muscles, cardiac muscle and glands
Sensory (afferent)


1.      Carry cutaneous sensation and deep sensations
2.      Carry afferent impulses of somatic reflexes which control activity of skeletal muscles (e.g. withdrawal ref.)
1. Carry    visceral sensation.

2. Carry afferent impulses of autonomic reflexes which control the activity of the viscera (e.g. autonomic reflex) 

Motor (efferent)


1.Innervate skeletal (voluntary) muscles


1.Innervate smooth (involuntary) muscles, cardiac muscles, and glands.


2. Voluntary.
2. Involuntary.
3. Arise form the anterior horn cells (AHC) in the spinal cord and the somatic motor nuclei in the brain stem.
3. Arise from the lateral horn cells (LHC) in the spinal cord and the visceral motor nuclei in the brain stem.
4. Only one neuron arises from the AHC to the skeletal muscles i.e. motor fibers do not synapse outside CNS.
4. Two neurons (a) preganglionic neuron arises from LHC and synapses in autonomic ganglia and (b) postganglionic neuron passes from the ganglia to the viscera 
5. Thick myelinated fibers (type A)
5. Preganglionic fibers are thin myelinated (type B) and post- ganglionic fibers are unmyelinated (type C)


6. Stimulation of somatic motor fibers always leads to excitation (contraction) of skeletal muscles.
6. Stimulation of  autonomic motor fibers may lead to either excitation or inhibition of effector organs
7. Cutting of somatic motor nerves leads to paralysis
7. Cutting of autonomic motor nerves does not lead to paralysis
8.Chemical transmitter is acetylcholine
8.Chemical transmitter at preganglionic nerve endings is acetylcholine and at postganglionic nerve endings are acetylcholine and noradrelaline.




Somatic reflex                           Autonomic reflex          
Autonomic nervous system

The autonomic (visceral or involuntary) nervous system is that part of the nervous system which controls the activity of the viscera, there are two divisions of the autonomic nervous system; the sympathetic (thoraco - lumbar) and the parasympathetic (cranio - sacral) divisions. The parasympathetic postganglionic fibers liberate acetylcholine, so they are called cholinergic fibers. Most sympathetic postganglionic fibers liberate noradrenaline, so they are called adrenergic. However, sympathetic postganglionic fibers to the sweat glands and blood vessels of skeletal muscles liberate acetylcholine i.e. cholinergic.

Sympathetic nervous system:

It is the part of the autonomic nervous system which prepares the body for vigorous activity (fight or flight) in response to emergency situations (fear, intense muscular exercise, stress, haemorrhage, cold, etc)

Origin: The sympathetic nervous system originates from the lateral horn of the spinal gray matter from the first thoracic to the second (sometimes the third) lumbar segments, hence the name thoraco - lumber outflow.

Course and distribution: The sympathetic preganlionic fibers leave the spinal cord with the ventral roots of the spinal nerves. The preganglionic fibers are thin myelinated fibers (type B) and appear whitish in colour. They are called white rami communicants. The preganglionic fibers synapse (relay) in the sympathetic ganglia with a large number of postganglionic neurons. The sympathetic ganglion acts as a distributing center. The ratio of preganglionic to postganglionic fibers is 1:20 or more, so the sympathetic actions are widespread. The postganglionic fibers (non myelinated) proceed to supply the visceral structures.

The sympathetic ganglia are:

1- Lateral (paravertebral) ganglia:

These ganglia lie on both sides of the vertebral column forming the sympathetic chains. They extend from the base of the skull to the front of the coccyx where they meet at the coccygeal ganglion. The number of ganglia in the sympathetic chain is about 22-24 on each side and are named according to the region. In cervical region, there are three ganglia (superior, middle, and inferior cervical ganglia). They are relatively large and represent the fusion of two or more smaller ganglia. The superior cervical ganglion is the largest of the three ganglia. The middle cervical ganglion lies near the inferior ganglion and it may be absent. The inferior cervical ganglion is usually fused with the first thoracic ganglion and occasionally with the second as well, to form the stellate ganglion.

In the thoracic, lumbar, and sacral regions, the ganglia are segmentally arranged. There are 10-12 thoracic, 4 lumbar, and 4-5 sacral ganglia. In the sacral region, the two sympathetic chains gradually approach each other and fuse at the unpaired coccygeal ganglion.

2- Collateral ( prevertebral ) ganglia:

These ganglia lie between the sympathetic chain and the viscera. They are closely related to the aorta and its branches and named according to these branches e.g. celiac, superior mesenteric, inferior mesenteric and aortico-renal ganglia. From these ganglia, postganglionic fibers arise and pass with the blood vessels to supply the viscera.

3- Terminal sympathetic ganglia:

A few, relatively small sympathetic ganglia lie closer to the visceral organs and especially close to the rectum, urinary bladder and reproductive organs in the pelvis. These ganglia send out short postganglionic fibers to the viscera.

Functions of the sympathetic nervous system:

1) Sympathetic to head and neck:

Origin: From the lateral horn cells (LHC) of the first and second thoracic segments.

Relay: In the superior cervical ganglion.


i) In the eye:

-         Motor to the dilator pupillae muscle leading to dilatation of the pupil.

-         Motor to the smooth muscles of the eye lids leading to elevation of the upper and lowering of the lower eye lids i.e. widening of the palpebral fissure.

-         Motor to the smooth muscles of the eye ball (Muller’s muscle) present behind the eye leading to forward protrusion of the eye i.e. exophthalmos.

-         Vasoconstriction of the blood vessels of the lacrimal glands.

-         Relaxation of the ciliary muscle helping the eye to see far objects.

ii) In the salivary glands:

Acini: Salivary secretion which is little in volume, viscid, and rich in organic substances.

Smooth muscles around both the acini and the ducts: Contraction which leads to squeezing of the duct content outside.

Blood vessels: Vasoconstriction.

iii) In the skin:

-         Sweat secretion.

-         Contraction of pilo-erector muscles causing erection of hairs.

-         Vasoconstriction and vasodilatation to the skin blood vessels, but the vasoconstrictive action is more powerful.

iv) In the brain:

-         No effect or slight vasoconstriction of the cerebral blood vessels.

2) Sympathetic to thorax:

Origin: From the L.H.C. of upper four or five thoracic segments.

Relay: In the superior, middle, inferior cervical ganglia and the upper four thoracic ganglia.


i) In the heart:

-         Stimulation of all properties of the cardiac muscle leading to an increase in the heart rate, force of cardiac contraction, conduction velocity and excitability.

-         Dilatation of the coronary blood vessels leading to an increase in the blood supply to the cardiac muscle.

ii) In the lungs:

-         Relaxation of the muscles of the bronchi and bronchioles leading to widening of the air passages.

-         Decreased mucous secretion in the air passages.

-          Slight vasoconstriction of the pulmonary blood vessels.

3) Sympathetic to abdomen:

Origin: From the L.H.C. of thoracic segments 6 - 12.

Relay: Preganglionic fibers form the splanchnic nerves which relay in collateral ganglia (celiac, superior mesenteric and aortico - renal) and terminal ganglia.


i) In the gastrointestinal tract:

-         Relaxation of the smooth muscles of the stomach, small intestine and proximal part of the large intestine but motor to the sphincters.

ii) In the liver:

-         Glycogenolysis i.e. conversion of the liver glycogen into blood glucose. This effect is produced by adrenaline secreted from the supra renal gland.

-         Relaxation of the wall of the gall bladder and motor to its sphincter.

iii) In the spleen:

-         Contraction of the smooth muscles present in the splenic capsule and trabeculae leading to the addition of blood rich in red and white blood cells into the general circulation.

iv) In the blood vessels:

-         Vasoconstriction to the blood vessels of the stomach, intestine, liver, pancreas and kidneys.

v) In the supra renal medulla:

-         Secretion of two hormones (adrenaline 80% and noradrenaline 20%) into the general circulation. These hormones have almost the same effects throughout the body as direct sympathetic stimulation except:

1-    Their effects are more prolonged than direct sympathetic stimulation (about 10 times) because these hormones are slowly removed from the blood.

2-    Adrenaline has a powerful metabolic effect. It increases the metabolic rate, blood glucose level (due to stimulation of glycogenolysis), free fatty acids and triglycrides (due to stimulation of lipolysis).

4) Sympathetic to pelvis:

Origin: From the L.H.C. of the first, second and sometimes the third lumbar segments.

Relay: In the collateral and terminal ganglia.


i) In the urinary bladder:

-         Relaxation of the smooth muscles of the wall of the uninary bladder and contraction of the internal uretheral sphincter thus leading to retention of urine (help filling of the bladder).

ii) In the rectum:

-         Relaxation of the smooth muscles of the wall of the rectum and contraction of the internal anal sphincter leading to retention of the feces (help storage of feces).

iii) In the sex organs:

-         Contraction of the smooth muscles of the vas deferens, seminal vesicle, ejaculatory duct and prostate leading to ejaculation of semen.

-          Vasoconstriction of the blood vessels of the pelvic viscera including that of the external genital organs leading to shrinkage of the penis and clitoris.

5) Sympathetic to upper limbs, lower limbs, thoracic and abdominal walls:


Upper limbs: From the L.H.C. of thoracic 5 to 9.

Lower limbs: From the L.H.C. of thoracic 10 to lumbar 2.

Thoracic and abdominal walls: From the LHC of all   thoracic and upper 2 lumbar.

Relay: In the sympathetic chain. The postganlionic fibers join the spinal nerves as gray rami to supply the involuntary structures in the skin and skeletal muscles.


1.     In the skin: Discussed before.

2.     In the skeletal muscles:

a)     Vasodilatation of the skeletal muscle blood vessels.

b)    Orbelli phenomenon: Sympathetic stimulation of the skeletal muscles causes better contraction, delayed fatigue, and early recovery of the muscle after fatigue. It is due to:

i) An increase in the blood flow to the muscles as a result of vasodilatation. This provides the muscles with more oxygen and glucose and removes waste products from them.

ii) An increase in the sensitivity of the motor end plate to the action of acetylcholine.

iii) Activation of phosphorylase enzyme which is needed for glycogen breakdown and release of energy in the muscle.

Parasympathetic nervous system:

It is that part of the autonomic nervous system which control the activity of the viscera during rest and sleep. It  deals with anabolic activities leading to restoration and conservation of body energy. In other words; in times of danger, the sympathetic system prepares the body for violent activity, when the danger is over, the parasympathetic system reverses these changes.

Origin: The parasympathetic division (Cranio - sacral outflow) is composed of two parts:

A) The cranial part arises from the visceral motor nuclei of the following cranial nerves:


III, Oculomotor nerve: arises from Edinger Westphal nucleus in the mid brain.

VII, Facial nerve: arises from the superior salivary (salivatory) nucleus in the pons.

IX, Glossopharyngeal nerve: arises from the inferior salivary (salivatory) nucleus in the medulla oblongata.

X, Vagus nerve: arises from the dorsal motor nucleus of the vagus in the medulla oblongata.

B) The sacral part: arises from the second, third and fourth sacral segments. The preganglionic fibers unite to form the pelvic nerve.

Course and distribution:

The preganglionic parasympathetic fibers are thin myelinated fibers (type B) like that of the sympathetic but they are relatively longer. They synapse in the parasympathetic ganglia with few postganglionic neurons. The axons of the postganglionic neurons are very short and unmyelinated (type C). The ratio of  the preganglionic to the postganglionic neurons in the parasympathetic ganglia is only 1:2. Therefore, the activity of the parasympathetic system is more localized.

The postganglionic fibers of the Oculomotor, Facial and Glossopharyngeal nerves supply the involuntary structures in the head, those of the Vagus nerve supply the thoracic and abdominal viscera, while those the pelvic nerve supply the pelvic viscera.

The parasympathetic ganglia are:

1)    The ciliary ganglion for relay of the Oculomotor nerve.

2)    The spheno-palatine and the submandibular ganglia for relay of the Facial nerve.

3)    The Otic ganglia for relay of the Glossopharyngeal nerve.

4)    The terminal (intrinsic) ganglia for relay of the Vagus and pelvic nerves. These ganglia lie on the surface or within the effector organs.

Functions of the parasympathetic nervous system:

A) Cranial parasympathetic outflow:

III: Oculomotor nerve:

Origin: From the Edinger Westphal nucleus in the mid brain.

Relay and Functions: In the ciliary ganglion, from which postganglionic fibers run in the short ciliary nerves to causes:

-         Contraction of the constrictor pupillae muscle leading to narrowing of the pupil i.e. miosis.

-         Contraction of the ciliary muscle leading to an increase in convexity of the lens which helps accommodation of the eye to near vision.

VII: Facial nerve:

Origin: From the superior salivary nucleus in the pons.

Relay and functions: In the spheno-palatine ganglion, from which postganglionic fibers pass to the lacrimal glands causing secretion of tears and vasodilatation, and also to the mucous membranes of nose, soft palate and pharynx causing secretion of mucous and vasodilatation.

The chorda tympani branch of the Facial nerve relay in the submandibular ganglion from which the postganglionic fibers pass to the submandibular and sublingual salivary glands causing salivary secretion (large in volume, watery and poor in organic substances i.e. true secretion) and vasodilatation. The chorda tympani nerve causes also vasodilatation in the mucous membranes of the anterior two thirds of the tongue and floor of the mouth.

IX: Glossopharyngeal nerve:

Origin: From the inferior salivary nucleus in the medulla oblongata.

Relay and functions: In the Otic ganglion from which postganglionic fibers pass to the parotid salivary gland causing secretion (true secretion) and vasodilatation. These fibers also cause vasodilatation in the posterior third of the tongue.

X: Vagus nerve:

Origin: From the dorsal motor nucleus of the vagus present in the medulla oblongata.

Relay: In the terminal ganglia present in the thoracic and abdominal viscera.

Functions: About 75% of all parasympathetic nerve fibers are in the vagus nerves, passing to the entire thoracic and abdominal viscera.

1) In the thorax:

i) In the heart:

-         Inhibition of all properties of the cardiac muscle leading to decrease in the heart rate, force of atrial contraction, conduction velocity and excitability.

-         Vasoconstriction of the coronary blood vessels.

ii) In the lungs:

-         Motor to the smooth muscles of the bronchi and bronchioles.

-         Secretory to the mucous glands in the air passages.

-         Vasodilatation to the pulmonary blood vessels.

2) In the abdomen:

i) In the gastrointestinal tract:

-         Motor to the smooth muscles of the esophagus, stomach, small intestine and proximal part of the large intestine but inhibitory to their sphincters. In other words, it helps deglutition, gastric motility and evacuation, and stimulates peristaltic movements in the intestine.

-         Secretory to the gastric glands, producing gastric juice rich in HCl, and secretory to the duodenal (Brunner’s) glands, producing mucous secretion.

ii) In the liver:

-         Stimulates secretion of hepatic bile.

-         Motor to the wall of the gall bladder and inhibitory to its sphincter (sphincter of Oddi).

iii) In the pancreas:

-         Secretion of the pancreatic juice which is rich in enzymes.

-         Stimulation of insulin secretion from the beta cells of the islets of Langerhans.

B) Sacral parasympathetic outflow:

Origin: From the sacral segments 2,3 and 4. The preganglionic fibers unite to form the pelvic nerve.

Relay: In the terminal ganglia present in the wall of the pelvic viscera.


i) In the urinary bladder:

-         Motor to the wall of the urinary bladder and inhibitory to internal uretheral sphincter leading to micturition.

ii) In the distal part of the large intestine and rectum:

-         Motor to the wall and inhibitory to the internal anal sphincter leading to defecation.

iii) In the sex organs:

-         Vasodilatation of the blood vessels of the pelvic viscera including that of the sex organs leading to erection of penis and clitoris and congestion of the labia. So, the pelvic nerve is sometimes called the “nervous erigens”.

-         Secretory to the seminal vesicles and prostate.

Sympathetic and parasympathetic innervation of the viscera:

Many internal organs receive double nerve supply from the sympathetic and parasympathetic nervous systems. The actions produced by the two systems are usually antagonistic. For example, stimulation of sympathetic nerves causes increase in all cardiac properties, dilatation of the bronchioles, decrease in the gastrointestinal motility and contraction of the sphincters. Stimulation of parasympathetic nerves to these organs has the opposite effects i.e. decrease in all cardiac properties, constriction of the bronchioles, increase in the gastrointestinal motility and relaxation of the sphincters.

The two systems may act synergically e.g.:

(1)     During salivary secretion, both sympathetic and parasympathetic fibers are stimulated. Sympathetic stimulation produces saliva which is little in volume viscid and rich in enzymes, at the same time parasympathetic stimulation produces saliva which is large in volume, watery and rich in electrolytes.

(2) During sexual intercourse, parasympathetic stimulation produces erection of penis and clitoris and secretion of seminal vesicles and prostate, while sympathetic stimulation produces ejaculation of semen and shrinkage of penis and clitoris.

Some organs are supplied only one division of the autonomic nervous system. The organs supplied by the sympathetic fibers only are: the dilator pupillae, muscle, the ventricles of the heart, most of the blood vessels, the spleen, the adrenal medulla and the skin. The organs supplied by the parasympathetic fibers only are the constrictor pupillae muscles, and glands of stomach and pancreas.

Sympathetic and parasympathetic tones:

The sympathetic and parasympathetic systems are continuously active and the basal rates of activity are known as the sympathetic or parasympathetic tones.

The value of this tone in that it allows a single nervous system to increase or decrease the activity of the stimulated organ. For example, the sympathetic tone normally keeps almost all the blood vessels constricted to approximately half their maximum diameter. By increasing the degree of sympathetic stimulation, the vessels can be constricted even more; but on the other hand, by inhibiting the normal tone, the vessels can be dilated.   

The sympathetic tone results from the continuous impulses through the sympathetic nerves and also from the basal secretion of adrenaline and noradrenaline from the adrenal medulla.

After cutting the sympathetic nerves, the sympathetic vasoconstrictor tone is lost resulting in immediate and almost maximal vasodilatation. However, after sometime the intrinsic tone in the smooth muscles of the blood vessels increases resulting in restoring of the vasoconstriction.

The parasympathetic tone is very important in the heart (vagus tone) to decrease the inherent high rhythm of the sino-artial node (SAN). The cause of the vagus tone is the continuous afferent impulses from the baro-receptors present in the aortic arch and carotid sinus. The parasympathetic tone is also important in the gastrointestinal tract to maintain its normal functions. Surgical removal of the parasympathetic supply to the gastrointestinal tract (vagotomy) may cause serious and prolonged gastric and intestinal atony.

Localized and generalized actions of sympathetic nervous system:

a) The sympathetic tone is the basal activity of the sympathetic nervous system in the resting condition. It keeps the blood vessels slightly constricted. It is important in maintaining the blood pressure at normal level.

b) Isolated discharge: In some cases sympathetic stimulation occurs in isolated portion of the system e.g. in hot weather, the sympathetic stimulation affects only the sweat glands and cutaneous blood vessels without affecting the other organs innervated by the sympathetic nervous system.

c) Mass discharge: The sympathetic nervous system usually discharges as a complete unit (mass discharge). This occurs in emergency (fear, muscular exercise, stress, hemorrhage, etc). This results in a widespread response throughout the body which prepare the person for “fight or flight”. The widespread response to sympathetic stimulation increases in many ways the capacity of the body to perform severe muscular activity e.g.:

1.     In the eye, there is a pupillary dilatation, exophthalmos and widening of the palpebral fissure which increases the field of vision.

2.     In the heart, there is an increase in the heart rate and myocardial contraction which increase the cardiac output.

3.     In the lungs there is a bronchiolar dilatation which favors oxygenation.

4.     Stimulation of the adrenal medulla to release adrenaline and noradrenaline into the circulation. The effects of both are mainly metabolic, to increase the supply of energy:

a.   In the liver, glycogenolysis is increased by adrenaline and the blood sugar level rises.

b.  In the muscles, glycolysis is increased and the lactic acid level rises.

c.   In the adipose tissues, the process of lipolysis is stimulated and the level of free fatty acids increases.

5.     Contraction of splenic capsule which adds blood rich in red and white blood cells into the general circulation.

6.     Dilatation of the blood vessels of the skeletal muscles and cardiac muscle and vasoconstriction to other areas. This help in shifting a great part of the cardiac output to the active areas.

7.     Stimulation of the sweat secretion which increases the heat loss from the body.

8.     Other functions of minor importance are temporarily inhibited e.g. gastrointestinal motility.

Localized and generalized actions of Parasympathetic nervous system:

In contrast to the sympathetic system, most of  the control functions of the  parasympathetic system are very specific e.g.:

a) Parasympathetic cardiovascular reflexes usually act only on the heart to increase or decrease its rate of beating.

b) Parasympathetic reflexes frequently cause secretion mainly in the mouth or in other cases secretion in the stomach without affecting other organs supplied by the parasympathetic system.

Yet there is often association between closely related parasympathetic functions e.g.:

1-    Salivary secretion, gastric secretion and pancreatic secretion frequently occur at the same time.

2- Also, the rectal emptying reflex often initiates a bladder emptying reflex resulting in simultaneous emptying of both the bladder and the rectum.


          Interruption of the high sympathetic tone to the body or to an organ can be done surgically by removal of the sympathetic supply to that organ or medically by the use of sympathetic blocking drugs. For surgical sympathectomy to be effective, it must be done preganglionic to avoid denervation supersensitivity. It is used to treat the following conditions:

1-Vascular disorders in which there is an element of vasospasm. Removal of the sympathetic supply will relief spasm and cause vasodilatation. Therefore sympathectomy may be used in the treatment of Raynoud’s disease, essential hypertension, ischemic ulceration and gangrene.

2-Severe pain: Many painful conditions can be relieved by removal of the afferent sympathetic fibers e.g. sever pain of cancer bladder, angina pectoris and dysmenorrhea.

3-Hyperhydrosis which is a congenital disorder characterized by excessive sweating.

It is important to note that sympathectomy is restricted to the cases where the medical treatment (sympathetic blockers) fails to produce satisfactory response.


Vagal denervation (vagotomy) of the stomach eliminates direct vagal stimulation of the parietal cells and decreases parietal cells sensitivity to gastrin hormone. As a result, the basal and stimulated acid secretions are both reduced to about one-third of their preoperative levels. Vagotomy is used in the treatment of peptic ulcer. It has some complications e.g. gastric distension due to decreased gastric motility and abdominal distension due to decreased intestinal motility.

Control of autonomic functions:

Most of the autonomic functions are mediated through the autonomic reflexes as in the gastrointestinal, cardiovascular, genitourinary and respiratory functions. The autonomic reflexes begin by afferent fibers from the viscera which synapse at the lateral horn cells in the spinal cord or at the visceral part of the cranial nuclei in the brain stem. The efferent fibers emerge from the central nervous system as preganglionic fibers which relay in the autonomic ganglia before they reach the effector organs.


The autonomic reflexes are controlled by centers present at the following sites:

1-Spinal cord: For primitive autonomic reflexes.

2-Brain stem 

a- The medulla oblongata contains centers that control heart rate, arterial blood pressure, respiration, gastrointestinal motility and secretion, adrenaline secretion and vomiting.

b- The pons contains centers that control respiration and salivary secretion.

c-The mid brain contains centers that controls micturition and pupillary response to light and near vision.     

3-Hypothalamus: The hypothalamus plays an important role in maintaining the internal environment constant by affecting the autonomic nervous system and the hormonal secretion. The hypothalamus can affect the activity of almost all the brain stem autonomic centers. In general stimulation of the anterior nuclei of the hypothalamus increases the parasympathetic functions (decrease heart rate, decrease blood pressure. pupillary constriction, increase motility of the gastrointestinal tract and bladder contraction). Stimulation of the posterior nuclei increases the sympathetic functions (increase heart rate, increase blood pressure, pupillary dilatation and erection of hairs).

4-Cerebral cortex: Certain areas in the cerebral cortex (visceral or limbic cortex) can modify the autonomic functions through its connection with the hypothalamus and reticular formation This occurs as in the following conditions:

-       Certain cardiovascular and gastrointestinal functions are influenced by psychological factors. If these factors are strong enough they may cause autonomic induced diseases as peptic ulcer, constipation, and heart palpitation.

-        Voluntary control of micturition and defecation.

-       Increase blood flow in the skeletal muscles in anticipation of voluntary muscular and sexual activities.

-       Some conscious control can be produced by yoga players on a number of autonomic functions including heart rate, respiratory rate, gastrointestinal motility etc.

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