Download The Autonomic Nervous System

The Autonomic Nervous System
i.
Introduction:
a. Main site of action is on: Smooth muscle
Cardiac muscle &
Glands
b. This is a specific motor output portion of PNS
c. Structurally ANS include:
Autonomic sensory neuron
Integrating center in CNS.
Autonomic motor neuron.
d. Functionally: operates without conscious control.
Its activity is controlled by centers in
CNS.(hypothalamus, brain stem &
limbic system).
e. Focus is mainly on comparison, organization &
Action of its two major branches: Sympathetic
& Parasympathetic
2. Comparison of somatic & autonomic nervous
system.
a. Somatic nervous system:
--Include both sensory & motor neurons.
--Sensory neurons convey input from receptors for
special senses (vision, hearing) and from receptors
for somatic senses.( pain, temperature, tactile &
proprioception ).
--Sensations are perceived consciously.
--Motor neurons in somatic NS innervate skeletal
muscles and produce voluntary movements.
--Effects of motor neuron are always excitation.
b. Autonomic nervous system:
--Input comes from autonomic sensory receptors.
(chemoreceptors, baroreceptors)
--Sensory signals are not consciously perceived.
-- only visceral pain and anginal pain are perceived.
--Motor neurons innervate smooth muscle, cardiac
muscle and glands
--Effects can be both excitatory and inhibitory
c. Motor output in ANS:
 ANS has two divisions: Sympathetic and
Parasympathetic
 most organs have direct innervation & have
opposing actions. Example: sympathetic
stimulation increases heart rate & parasympathetic
stimulation decreases heart rate.
3. Anatomy of autonomic motor pathway:
a. Components: i. Preganglionic neuron.
--Cell bodies in CNS.
--Axons are myelinated B fiber and end in
autonomic ganglia.
II. Post-ganglionic neuron
--Cell bodies are in the ganglia.
--Axons are of small diameter and unmyelinated
c fiber and end in the effector organs.
--Lies entirely outside the CNS.
b. Preganglionic neuron
--In the sympathetic division, cell bodies are in the
lateral horns of the 12 thoracic & 1,2 lumber segments
of the spinal cord. (Thoraco-lumbar outflows.)
--In parasympathetic division, cell bodies of the
preganglionic neurons are located in the nuclei of the
four cranial nerves in the brain stem & in the lateral
gray horn of the 2nd. through 4th. sacral segments.(
cranio-sacral outflows).
c. Postganglionic neuron:
 Sympathetic preganglionic neuron may end in the
Postganglionic neuron in either of 3 ways:
--Synapse directly at the ganglia.
--may ascend or descend & then synapse with
Postganglionic neuron.
--or may continue without synapsing at the ganglia to
end in a Postganglionic neuron at prevertebral ganglia.
 One sympathetic preganglionic neuron may synapse
with 20 or more Postganglionic neurons. This
explains why sympathetic system response are
generalized and affects entire body simultaneously.
 In parasympathetic preganglionic neuron:
--after entering the ganglia, presynaptic neuron
usually synapse with only 4-5 Postganglionic
neurons, all of which supply a single visceral
effector.
 Thus parasympathetic response are localised.
4. Structure of the sympathetic division:
 Cell bodies are in the lateral horns of all thoracic
& first two lumber segments.
 Preganglionic neuron leaves spinal cord along with
the anterior root of spinal nerve.
 After exiting, the myelinated preganglionic axons
enter the white rami communicates to enter
sympathetic trunk ganglia on the same side.
 The Postganglionic neuron from the ganglia leave
through the gray rami communicates
(unmyelinated) & joins the spinal nerve to go to
their effector organs.
 Sympathetic preganglionic fibers also extend to
the adrenal medulla—So stimulation of
sympathetic preganglionic neuron, adrenal
medulla releases epinephrine or nor-epinephrine
rather than acetylcholine.
5. Structure of parasympathetic division:
--Cell bodies of the preganglionic neurons are in the
nuclei of brain stem & in the lateral horn of the 2nd.
through 4th. Sacral segments of the spinal cord.
--Cranial parasympathetic outflow: include
preganglionic neurons from brain stem to four cranial
nerves.
--Sacral parasympathetic outflow: consists of
preganglionic axons in anterior root of the second
through fourth sacral nerves.
--No rami communicates present here.
--Postganglionic axons are very short.
6. Neurotransmitters in ANS and in receptors:
--Based on the production and release of
neurotransmitters, ANS are classified as either;
 Cholinergic
 Adrenergic
a. Cholinergic neurons and receptors:
 these neurons release acetylcholine.
 They include:
i. all symp. & parasym. Preganglionic
neurons
ii. symp. Postganglionic neuron innervates
sweat gland.
iii. All parasym. Postganglionic neurons
--there are two types of cholinergic receptors, both of
which bind Ach. They are :
-Nicotinic—at motor end plate of all NMJ,
dendrites & cell bodies of autonomic neurons.
-Muscarinic—in plasma membrane of all smooth
muscles, cardiac muscle & glands.
--Activation of nicotinic receptors leads to
depolarization & excitation of postganglionic neuron
and skeletal muscles.
--Activation of muscarinic receptors by Ach: may lead
to depolarization & hyperpolarization depending on
the presence of these receptors.
--Effects of cholinergic fibers are brief as Ach. Is
quickly destroyed by acetylcholineesterase, AchE.
b. Adrenergic neuron & receptors:
--these neurons release nor-epinephrine (NE).
--They are mostly sympathetic postganglionic neuron.
--There are two main types of receptors: alpha( ) &
Beta( ) receptors.
--Action of Norepinephrine is at synapse is longer than
Ach.
7. Physiological effects of ANS:
 Most body organs are innervated by both divisions
of autonomic NS
 Sympathetic and parasymp. Systems act opposite
to each other.
 The balance between the two systems is regulated
by the hypothalamus.
 --Their actions are different because:
i. Postganglionic neurons release different
neurotransmitter.
ii. Effector organs also possess different
cholinergic and adrenergic receptors.
7a. Sympathetic responses: effects are long lasting
& widespread
--Dominates in physical & emotional stress.
--Supports vigorous physical activity & rapid
production of ATP.
--Fear, embarrassment & rage excitement are
sympathetic functions.
--(E- situation exercise, emergency, excitement &
embarrassment)
--Fight or Flight response: pupil dilate
--Heart rate rises
--BP increases.
--Airway dilates
--Blood vessels to kidney & GIT constrict.
--Blood vessels to sk. Muscles, liver & cardiac muscle
dilate and blood glucose level rises
b. Parasympathetic responses
--In contrast to sympathetic, parasympathetic system
enhances “rest and digest” activities.
--Support body functions that conserve and restore
body energy.
--It also reduces body functions that support physical
activity.
--“SLUDD” : S= salivation.
L= lacrimation.
U= Urination.
D= digestion.
D= defecation
--Parasympathetic stimulation decreases:
--Heart rate
--Diameter of airway passage.
--Diameter of pupil.
--Although fear excites sympathetic system, but
‘paradoxical fear’ (when backed into a corner with no
escape route) stimulate parasympathetic system
leading to urination and defecation.
8. Integration and control of autonomic functions:
a. Autonomic reflexes: These reflexes play a great role
in regulating controlled condition in the body.
Example:
i. Blood pressure is maintained by adjusting
heart rate, force of ventricular contraction &
blood vessel diameter.
ii. Respiration: is maintained by regulating
diameter of bronchial trees.
iii. Digestion: by adjusting mobility and muscle
tone of GIT.
iv. Urination & defecation: by adjusting the
spincters
9. Autonomic control by higher centers:
 Hypothalamus is the major control & integration
center of the ANS.
 Hypothalamus receives sensory inputs from
visceral organs such as olfaction, gestation, as well
as input related to change in body temperature,
osmolarity & levels of various substances in the
blood.
 In addition, it receives input(emotion) from limbic
system.
--
 Output from hypothalamus influences autonomic
centers in the brain stem, such as
Cardiovascular centers
Swallowing center
Salivation center &
Vomiting center.
 In spinal cord: such as defecation center &
Urination center.
The Autonomic Nervous System
1. Introduction
2. Comparison of somatic and autonomic NS
3. Autonomic motor neurons
Sympathetic
Parasympathetic
4. Anatomy of autonomic motor pathway
Preganglionic neuron
Postganglionic neuron
5. Structure of sympathetic division
6. Structure of parasympathetic division
7. Neurotransmitters in autonomic NS
Cholinergic neurons & receptors
Adrenergic neurons & receptors
8. Physiologic effects of ANS stimulation
Sympathetic response
Parasympathetic response
9. Integration & control of autonomic functions
10. Control of higher centers on autonomic functions