Autonomic
Nervous System
Dr Farah Nabil Abbas
MBChB, MSc, PhD
Chair Dept. of Physiology
The Autonomic Nervous System
The Autonomic Nervous System (ANS) is that portion of the
nervous system that controls the visceral functions of the body e.g.
arterial pressure, GI motility, secretion, urinary bladder emptying,
sweating, & body temperature.
One of the most striking characteristics of the autonomic nervous
system is the rapidity and intensity with which it can change visceral
functions. For instance, within 3 to 5 seconds it can increase the heart
rate to twice normal, and within 10 to 15 seconds the arterial pressure
can be doubled; or, at the other extreme, the arterial pressure can be
decreased low enough within 10 to 15 seconds to cause fainting.
Sweating can begin within seconds, and the urinary bladder may
empty involuntarily, also within seconds
CNS
CNS
preganglionic
ganglia
sensory
Somatic Reflex
motor
sensory
Autonomic Reflex
General Organization of the Autonomic Nervous
System
The autonomic nervous system is activated mainly by centers
located in the spinal cord, brain stem, and hypothalamus. Also,
portions of the cerebral cortex, especially of the limbic cortex, can
transmit signals to the lower centers and in this way influence
autonomic control.
The autonomic nervous system also often operates by means of
visceral reflexes. That is, subconscious sensory signals from a visceral
organ can enter the autonomic ganglia, the brain stem, or the
hypothalamus and then return subconscious reflex responses directly
back to the visceral organ to control its activities.
ANS has 2 subdivisions:
Sympathetic: fight or flight.
2. Parasympathetic: vegetative functions.
1.
Physiological Anatomy:
Sympathetic Nervous System
CNS – Spinal cord – T1-L2
Intermediolateral horn  (preganglionic)  Anterior horn 
white ramus  (Ganglia) in the sympathetic chain
(paravertebral)  postganglionic  gray ramus  spinal
nerve
Fate of Preganglionic Fibers:
1.Synapse at the ganglia.
2.Goes up or down, then synapse in a ganglia.
3.Synapse at an outlying sympathetic ganglia.
Parasympathetic Nervous System
1. CNS – Cranial outflow
X vagus – 75% of parasympathetic outflow  thoracic
III oculomotor: sphincter of pupils ciliary muscle
VII facial: lacrimal, nasal, sub maxillary glands.
IX glossopharyngeal: parotid gland.
2.Sacral outflow –
nervi egents – lower GI - bladder
Preganglionic: long, ganglia, outlying in the wall of the organ
Postganglionic: short, 1mm
Neurotransmitters of the ANS:
Cholinergic fibers secrete acetyl choline. Adrenergic fibers
secrete norepinephrine (noradrenalin).
Non-adrenergic non-cholinergic (NANC)
1.
All preganglionic neurons (sympathetic &
parasympathetic) cholinergic
2.
Postganglionic parasympathetic cholinergic
3.
Postganglionic sympathetic adrenergic except:
postganglionic sympathetic to sweat glands and few blood
vessels cholinergic
Mechanism of secretion of Ach & NE:
Action potential  depolarization  increased permeability to Ca+2  secretion of
neurotransmitter
Synthesis of Ach & Its Destruction:
Synthesis:
Acetyl-CoA + choline
Axoplasm to vesicle
Cholineacetyl transferase
Destruction:
Ach Acetylcholine esterase Choline + acetate
Present in terminal nerve ending
receptor organ (tissue acetylcholine esterase)
serum acetylcholine
destruction in seconds
Ach
Synthesis of NE & Its Removal:
Synthesis:
Tyrosine
hydroxylation
DOPA
decarboxylation
Dopamine
Epinephrine methylation Norepinephrine hydroxylation transport into
vesicles
Removal:
1. Re-uptake into nerve ending 50-80%
2. Diffusion into body fluids & blood.
3. Destruction by monoamine oxidase catechol-O-methyl transferase
14
Receptors:
On cell membrane of effecter cells – proteins
1. binding  change of molecular structure of R  alteration of
+
+2
permeability of cell membrane to Na , Ca , Cl  influx.
K+  efflux  alteration of membrane potential  action potential of
smooth muscles  electronic effects on glands  secretion
2. R-transmitter complex activates enzymes adenyl cyclase cyclic
AMP
Types of Acetylcholine Rs:
1. Muscarinic R
2. Nicotinic R
In effector cells of:
1. Muscarinic Rs  postganglionic parasympathetic  to sweat glands
and some blood vessels.
2. Nicotinic Rs  In synapse between the pre & post- ganglionic fibers
of both sympathetic and parasympathetic ganglia  in skeletal
muscle fibers.
Adrenergic Receptors (Rs):
α , β adrenergic receptors (Rs)
NE
stimulates
Epinephrine
α Receptors (mainly), β receptors (very slightly)
stimulates
α , β receptors usually
- certain α functions are excitatory , others inhibitory
- certain β functions are excitatory , inhibitory
e.g. on blood vessels 
α  vasoconstrictor
β  vasodilator
on the heart β  cardio acceleration
-Sympathetic versus parasympathetic effects:
-Some time reciprocal
-However most organs are dominantly controlled by one of them.
Autonomic Effects on Various Organs of the Body
Eye:
pupil
Ciliary muscle
Glands: Nasal
Lacrimal, salivary
Gastric, pancreatic
Sweat glands
Heart: muscle
coronaries
Systemic Blood
vessels: Abdominal
Skin
Muscle
Lungs: bronchi
Gut: Lumen
Sphincter
Penis
Basal metabolism
Sympathetic stimulation
Dilated
Slight relaxation
Vasoconstriction
Slight secretion
Parasympathetic stimulation
Constricted
Contracted
Stimulation of thin, copious
secretion with enzymes
Copious sweating
Increase rate
Increase force of contraction
Dilated (β), constricted (α)
Decrease rate
Decrease force of contraction
Dilated
Constricted
Constricted
Dilated (β), constricted (α)
Dilated
Decrease peristalsis
Increase tone
Ejaculation
Increased
Constricted
Increase peristalsis
Decrease tone
Erection
-
Sympathetic & Parasympathetic Tones:
The system is continually active
Basal rate of activity  tone
The tone can be increased or decreased
e.g. sympathetic tone to blood vessels constricted to ½ max diameter
increased sympathetic tone  more vasoconstriction
inhibitory tone  vasodilatation
e.g. parasympathetic tone to GIT
increased tone  increased motility & secretion
decreased tone  decreased motility & secretion
Basal adrenal medulla secretion  tone
Denervation of BV  decreased tone  maximal dilatation
After days or weeks  intrinsic tone  normal diameter
Denervation supersensitivity
Autonomic Reflexes:
-CV reflexes
Baroreceptor reflex of BP
Increased blood pressure  stimulates Rs  CNS  decreased
sympathetic tone  decreased blood pressure
-GI reflexes
Distension of the rectum  stretch Rs  reflex defecation
Higher centers of autonomic functions
Reticular substances in medulla, pons, midbrain, CV, respiratory, GI
control centers, Hypothalamus, cerebral cortex
Functions of the adrenal
Sympathetic stimulation – large quantities of epinephrine (80%) & norepinephrine
(20%)
1. Epinephrine & norepinephrine have similar effects, but with the following
differences:
2. Epinephrine has greater effects on the heart
3. Epinephrine has a weak constriction on blood vessels of muscles
NEincrease total peripheral resistance
Increase blood pressure
4. Epinephrine causes a greater increase in BMR (basal metabolic rate)
The effects of adrenal medulla stimulation are more prolonged and widely
distributed in the body.