2. Objectives
• Describe the location of the cell bodies of
preganglionic sympathetic and parasympathetic
neurons.
• Describe the location of postganglionic sympathetic
and parasympathetic neurons.
• Name the neurotransmitters that are released by
preganglionic autonomic neurons, postganglionic
sympathetic neurons, postganglionic parasympathetic
neurons, and adrenal medullary cells.
• Outline the functions of the autonomic nervous
system.
• List the ways that drugs act to increase or decrease the
activity of the components of the autonomic nervous
system.
3. Introduction
• Autonomic nervous system (ANS):
–Innervates organs whose functions are not
usually under voluntary control.
–Effectors include cardiac and smooth
muscles and glands.
• Effectors are part of visceral organs and
blood vessels.
4. Comparison of Somatic and
Autonomic Nervous Systems
• SNS provides motor fibers
to skeletal muscle fibers.
• In SNS, a single motor
neuron forms the efferent
[pathway from the CNS to
effectors.
• Acetylcholine, the
neurotransmitter of somatic
neurons is stimulatory to
skeletal muscle fibers.
• ANS provides motor fibers
to smooth and cardiac
muscles and glands.
• In ANS efferent pathway
consists of a two neuron
chain, the preganglionic
neuron in the CNS and
postganglionic in a ganglion.
• Neurotransmitters released
by ANS motor neurons-
acetylcholine and
norepinephrine -may cause
excitation or inhibition.
5. • Involuntary effectors are somewhat independent
of their innervation.
– Smooth muscles maintain resting tone in
absence of nerve stimulation.
• Denervation hypersensitivity:
–Damage to autonomic nerve makes its
target tissue more sensitive than normal
to stimulating agents.
For example Cardiac and many smooth muscles
can contract rhythmically in absence of nerve
stimulation.
Visceral Effector Organs
7. • Sensory input transmitted to brain centers that
integrate information can modify activity of
preganglionic autonomic neurons.
• Medulla:
– Most directly controls activity of autonomic
system.
– Location of centers for control of cardiovascular,
pulmonary, urinary, reproductive and digestive
systems.
• Hypothalamus:
– Regulates medulla.
• Cerebral cortex and limbic system:
– Responsible for visceral responses that are
characteristic of emotional states.
Control of the ANS by Higher Brain Centers
9. • Myelinated preganglionic fibers exit spinal cord
in ventral roots from T1 to L2 levels.
• Most sympathetic nerve fibers separate from
somatic motor fibers and synapse with
postganglionic neurons within paravertebral
ganglia.
–Ganglia within each row are interconnected,
forming a chain of ganglia that parallels spinal
cord to synapse with postganglionic neurons.
Sympathetic Division
10.
11. SEPARATION OF FUNCTION
• Sympathetic – Fight, flight, Fear.
• Parasympathetic – rest, digestion, calm
• Both involuntary
12. • Fight or flight response.
• Release of norepinephrine (NT) from
postganglionic fibers and epinephrine (NT)
from adrenal medulla.
• Mass activation prepares for intense
activity.
Sympathetic Effects
13. • Increased arterial pressure
• Increased blood flow to active muscles
concurrent with decreased blood flow to organs
such as the gastrointestinal tract and the kidneys
that are not needed for rapid motor activity
• Increased rates of cellular metabolism
throughout the body
• Increased blood glucose concentration
• Increased glycolysis in the liver and in muscle
• Increased muscle strength
• Increased mental activity
• Increased rate of blood coagulation
14. At other times, activation occurs in isolated
portions of the sympathetic nervous system.
Important examples are the following:
(1)During the process of heat regulation, the
sympathetics control sweating and blood flow in
the skin without affecting other organs
innervated by the sympathetics.
(2)Many "local reflexes" involving sensory afferent
fibers travel centrally in the peripheral nerves to
the sympathetic ganglia and spinal cord and
cause highly localized reflex responses. For
instance, heating a skin area causes local
vasodilation and enhanced local sweating,
whereas cooling causes opposite effects.
15.
16.
17.
18.
19.
20.
21. • Adrenal medulla secretes epinephrine (Epi) and
norepinephrine (NE) when stimulated by the
sympathetic nervous system.
• Modified sympathetic ganglion:
– Its cells are derived form the same embryonic
tissue that forms postganglionic sympathetic
neurons.
• Sympathoadrenal system:
– Stimulated by mass activation of the sympathetic
nervous system.
– Innervated by preganglionic sympathetic fibers.
Adrenal Glands
22. Parasympathetic Division
• Preganglionic fibers originate in midbrain, medulla,
pons; and in the 2-4 sacral levels of the spinal
column.(craniosacral)
• Preganglionic fibers synapse in terminal ganglia
located next to or within organs innervated.
• Most parasympathetic fibers do not travel within
spinal nerves.
– Do not innervate blood vessels, sweat glands, and
arrector pili muscles.
23. • Normally not activated as a whole.
– Stimulation of separate parasympathetic nerves.
• Release ACh as NT.
• Relaxing effects:
– Decreases HR.
– Dilates visceral blood vessels.
– Increases digestive activity.
Parasympathetic Effects
24. Comparisons
• SYMPATHETIC
• Short preganglionic fibers
• Neurotransmitter:
norepinephrine
• Turn OFF most gut activities.
• Constrict blood vessels to
splanchnopleure.
• PARASYMPATHETIC
• Long preganglionic fibers
• Neurotransmitter:
acetylcholine
• Turn ON most gut activities.
• Dilate blood vessels to
splanchnopleure.
25.
26.
27. The neurons that are cholinergic
1. all preganglionic neurons,
2. all parasympathetic postganglionic neurons
3. sympathetic postganglionic neurons that innervate
sweat glands and
4. sympathetic postganglionic neurons that end on
blood vessels in some skeletal muscles and produce
vasodilation when stimulated (sympathetic
vasodilator nerves).
Adrenergic
The remaining sympathetic postganglionic neurons
are noradrenergic (ie, release norepinephrine). The
adrenal medulla is essentially a sympathetic ganglion
in which the postganglionic cells have lost their axons
and secrete norepinephrine and epinephrine directly
into the bloodstream. The cholinergic preganglionic
neurons to these cells have consequently become the
secretomotor nerve supply of this gland.
28. Receptors on the Effector Organs
• (1) causing a change in cell membrane
permeability to one or more ions or
• (2) activating or inactivating an enzyme
attached to the other end of the receptor
protein, where it protrudes into the interior of
the cell.
29. Two Principal Types of Acetylcholine
Receptors
• Muscarinic and Nicotinic Receptors
• Acetylcholine activates both of them.
• Muscarinic receptors are found on all effector cells that
are stimulated by the postganglionic cholinergic
neurons of either the parasympathetic nervous system
or the sympathetic system.
• Nicotinic receptors are found in the autonomic ganglia
at the synapses between the preganglionic and
postganglionic neurons of both the sympathetic and
parasympathetic systems. (Nicotinic receptors are also
present at many nonautonomic nerve endings-for
instance, at the neuromuscular junctions in skeletal
muscle
30. Adrenergic Receptors-Alpha and Beta
Receptors
• beta receptors in turn are divided into beta1, beta2 and
beta3
• alpha receptors are divided into alpha1 and alpha2
receptors.
• Norepinephrine excites mainly alpha receptors but
excites the beta receptors to a lesser extent as well.
• epinephrine excites both types of receptors
approximately equally.
• Therefore, the relative effects of norepinephrine and
epinephrine on different effector organs are
determined by the types of receptors in the organs. If
they are all beta receptors, epinephrine will be the
more effective excitant.
31. Most visceral organs receive dual innervation (innervation by
both sympathetic and parasympathetic fibers).
Antagonistic effects:
◦ Sympathetic and parasympathetic fibers innervate the same
cells.
Actions counteract each other.
Heart rate.
Complementary:
◦ Sympathetic and parasympathetic stimulation produces
similar effects.
Salivary gland secretion.
Cooperative:
◦ Sympathetic and parasympathetic stimulation produce
different effects that work together to produce desired effect.
Micturition.
Organs With Dual Innervation
32. • Regulation achieved by increasing or decreasing
firing rate.
• Adrenal medulla, arrector pili muscle, sweat
glands, and most blood vessels receive only
sympathetic innervation.
Organs Without Dual Innervation
33. Denervation Supersensitivity
In response to loss of innervation by
postganglionic sympathetic or
parasympathetic axons – there is increased
responsiveness of visceral target organ to
neurotransmitter or agonist that stimulates
adrenergic / muscarnic receptors.
Leads to exagerrated pressor responses to
adrenergic agonists
34. Sympathomimetic Drugs
Epinephrine and methoxamine are
sympathomimetic drugs.
They differ from one another in the degree to
which they stimulate different sympathetic
effector organs and in their duration of action.
Norepinephrine and epinephrine have actions as
short as 1 to 2 minutes, whereas the actions of
some other commonly used sympathomimetic
drugs last for 30 minutes to 2 hours.
Important drugs that stimulate specific
adrenergic receptors are phenylephrine (alpha
receptors), isoproterenol (beta receptors), and
albuterol (only beta2 receptors).
35. Drugs That Cause Release of
Norepinephrine from Nerve Endings
Ephedrine, tyramine, and amphetamine.
Their effect is to cause release of norepinephrine
from its storage vesicles in the sympathetic nerve
endings.
The released norepinephrine in turn causes the
sympathetic effects.
36. Drugs That Block Adrenergic Activity
• The synthesis and storage of norepinephrine in the
sympathetic nerve endings can be prevented. The best
known drug that causes this effect is reserpine.
• Release of norepinephrine from the sympathetic
endings can be blocked. This can be caused by
guanethidine.
• The sympathetic alpha receptors can be blocked. Two
drugs that cause this effect are phenoxybenzamine and
phentolamine.
• The sympathetic beta receptors can be blocked. A drug
that blocks beta1 and beta2 receptors is propranolol.
One that blocks mainly beta1 receptors is metoprolol.
• Sympathetic activity can be blocked by drugs that block
transmission of nerve impulses through the autonomic
ganglia an important drug for blockade of both
sympathetic and parasympathetic transmission
through the ganglia is hexamethonium.
37. Drugs That Act on Cholinergic Effector
Organs Parasympathomimetic Drugs
(Cholinergic Drugs)
• Two commonly used parasympathomimetic drugs
are pilocarpine and methacholine.
• They act directly on the muscarinic type of
cholinergic receptors.
• Neostigmine, pyridostigmine, and ambenonium.
These drugs inhibit acetylcholinesterase, thus
preventing rapid destruction of the acetylcholine
liberated at parasympathetic nerve endings. As a
consequence, the quantity of acetylcholine
increases with successive stimuli and the degree
of action also increases.
38. Drugs That Block Cholinergic Activity at
Effector Organs-
• Antimuscarinic Drugs Atropine and similar
drugs, such as homatropine and scopolamine,
block the action of acetylcholine on the
muscarinic type of cholinergic effector organs.
These drugs do not affect the nicotinic action
of acetylcholine on the postganglionic neurons
or on skeletal muscle.
39. Drugs That Stimulate Postganglionic
Neurons
• Accetylcholine
• Nicotine is another drug that can stimulate postganglionic
neurons in the same manner as acetylcholine because the
membranes of these neurons all contain the nicotinic type
of acetylcholine receptor. Therefore, drugs that cause
autonomic effects by stimulating postganglionic neurons
are called nicotinic drugs.
• Some other drugs, such as methacholine, have both
nicotinic and muscarinic actions, whereas pilocarpine has
only muscarinic actions. Nicotine excites both the
sympathetic and parasympathetic postganglionic neurons
at the same time, resulting in strong sympathetic
vasoconstriction in the abdominal organs and limbs but at
the same time resulting in parasympathetic effects such as
increased gastrointestinal activity and, sometimes, slowing
of the heart.
40. Ganglionic Blocking Drugs
• Many important drugs block impulse transmission
from the autonomic preganglionic neurons to the
postganglionic neurons, including tetraethyl
ammonium ion, hexamethonium ion, and pentolinium.
• These drugs block acetylcholine stimulation of the
postganglionic neurons in both the sympathetic and
the parasympathetic systems simultaneously.
• They are often used for blocking sympathetic activity
but seldom for blocking parasympathetic activity
because their effects of sympathetic blockade usually
far overshadow the effects of parasympathetic
blockade.
• The ganglionic blocking drugs especially can reduce the
arterial pressure in many patients with hypertension,
but these drugs are not useful clinically because their
effects are difficult to control.
