Download Sympathetic Preganglionic Neurons

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Anatomy & Physiology II
Chapter 15 (Autonomic Nervous System)
ANS is a part of the Peripheral Nervous System, along with the Somatic nervous System.
It is controlled by the brain stem & hypothalamus.
It gets its sensory input from the autonomic sensory neurons.
-These neurons get their information from receptors in internal organs (aorta: chemo- & baroreceptors)
Autonomic motor neurons respond by exciting or inhibiting effectors (cardiac, smooth muscle, glands)
ANS controls, for example, increased heart rate, sweating & pupillary dilatation due to fear.
One can learn to consciously control ANS by using yoga, meditation or biofeedback methods.
Autonomic motor pathways consist of two motor neurons:
1. Preganglionic neuron – situated in the CNS, its’ myelinated axon travels via a cranial or spinal
nerve to an autonomic ganglion (outside CNS), where it synapses with a postganglionic neuron.
2. Postganglionic neuron – situated in the PNS, its’ unmyelinated axon travels from the autonomic
ganglion to an effector (cardiac, smooth muscle, glands)
-The effector is either excited to increase secretion in a gland or cause muscle contraction OR
- The effector is inhibited to decrease secretion in a gland or cause muscle relaxation.
[Somatic Nervous System always excites to cause skeletal muscle contraction]
Autonomic motor neurons release neurotransmitters Acetylcholine or Norepinephrine.
[Somatic motor neurons releases only Acetylcholine]
The motor part of ANS consists of two divisions:
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Sympathetic (Thoracolumbar division)
Parasympathetic. (Craniosacral division)
Most organs receive impulses from both sympathetic & parasympathetic divisions (Dual Innervation)
-Each division opposes the action of the other (one is excitatory, while the other is inhibitory)
STRUCTURE OF AUTOMONIC NERVOUS SYSTEM
I. PREGANGLIONIC NEURONS:
Sympathetic Preganglionic Neurons – They arise from the T1 to L2 segments of spinal cord and
extend to the sympathetic ganglia near the vertebra. They have short axons.
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Parasympathetic Preganglionic Neurons – They arise from the brain stem (nuclei of III, VII, IX & X
cranial nerves) and sacral segments (S2, S3, S4) of the spinal cord. They have long axons.
-Their axons travel via the anterior (motor) root of the cranial & spinal nerves to the internal organs.
Vagus (X) nerve carries about 80% of parasympathetic outflow.
-It supplies the terminal ganglia in the walls of heart, lungs and most abdominal organs.
Parasympathetic axons from the sacral segments supply ganglia in the ascending colon & pelvic organs.
II. AUTONOMIC GANGLIA:
They are divided into:
1. Sympathetic ganglia:
A) Sympathetic trunk ganglia – They are situated laterally on both sides of the vertebral column.
-Their postganglionic axons supply sympathetic nerve impulses to the organs above diaphragm*.
In the neck, they are divided into:
i) Superior Cervical Ganglion – supplies sympathetic nerves to iris, face, nasopharynx & heart.
ii) Middle Cervical Ganglion – supplies sympathetic nerves to heart.
iii) Inferior Cervical ganglion - supplies sympathetic nerves to heart.
In the thorax, postganglionic fibers from sympathetic trunk ganglia supply heart, lungs & bronchi.
B) Prevertebral ganglia - They are situated anterior to the vertebral column near abdominal aorta.
-Their postganglionic axons supply sympathetic nerve impulses to the organs below diaphragm*.
They include:
i) Celiac Ganglion – supplies sympathetic nerves to liver, gall bladder, kidneys, adrenals, ducts etc.
ii) Superior Mesenteric Ganglion - supplies sympathetic nerves to intestines & mesentery
iii) Inferior Mesenteric Ganglion - supplies sympathetic nerves to colon & pelvic organs.
C) Adrenal Medulla – Even though it is not a ganglion, its cells are similar to postganglionic neurons.
- Its cells release hormones epinephrine (80%) & norepinephrine (20%).
- These hormones intensify sympathetic responses (increase BP, heart rate, respiration rate, sweating)
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2. Parasympathetic ganglia –
A) Terminal ganglia - They are located close to or in the wall of the organ they innervate.
In the head they include:
i) Ciliary Ganglion – supplies ciliary body & circular muscle of iris in the eye (Origin from III nerve)
ii) Pterygoplalatine Ganglion – supplies palate & lacrimal glands (Origin from VII nerve)
iii) Submandibular Ganglion – supplies submandibular region (Origin from VII nerve)
iv) Otic Ganglion – Supplies the parotid gland (Origin from IX nerve nucleus)
In the thorax & abdomen, axons of vagus (X) nerve supply terminal ganglia situated in the walls of:
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Heart
Bronchi & lungs
Abdominal organs
III. POSTGANGLIONIC NEURONS:
1. Sympathetic Postganglionic Neurons:
Their axons are long as they have to travel a long distance from their ganglia situated near vertebra.
Sympathetic preganglionic neurons entering sympathetic trunk ganglia may:
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synapse in the sympathetic trunk ganglion
continue through sympathetic trunk ganglion and then synapse in the prevertebral ganglion
ascend or descend to higher or lower sympathetic trunk ganglion
Extend all the way to the adrenal medulla
A sympathetic preganglionic neuron may synapse with 20 or more postganglionic neurons.
-This leads to one preganglionic neuron simultaneously affecting many effectors (heart, lungs, bowels)
2. Parasympathetic Postganglionic Neurons:
Their axons are short as they arise from the terminal ganglia situated in the organ walls.
A parasympathetic preganglionic neuron may synapse with only 4-5 postganglionic neurons supplying
the same effector.
-This leads to parasympathetic response causing localized action in a single effector.
FUNCTIONS OF THE AUTONOMIC NERVOUS SYSTEM
ANS excites or inhibits the actions of effectors in the body (cardiac, smooth muscle, glands)
-This happens through the action of neurotransmitters acetyl choline & norepinephrine.
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ANS NEUROTRANSMITTERS –
Neurotransmitters are released where:
 neurons synapse (preganglionic to postganglionic) OR
 neuron & effector (cardiac muscle, smooth muscle or gland) synapse – neuroeffector junction
Cholinergic neurons release acetylcholine at their synapse. They include:
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All Parasympathetic neurons (preganglionic & postganglionic)
All sympathetic preganglionic neurons
Few Sympathetic postganglionic neurons (most are adrenergic neurons)
Cholinergic neurons maintain short-term, precise control over its effectors due to:
 rapid inactivation and
 localized action of acetylcholine.
Adrenergic neurons use the neurotransmitter norepinephrine.
Adrenergic neurons maintain long-term, generalized control on their effectors due to:
 slow inactivation and
 secretion of norepinephrine in the blood circulation
-Due to this, effects of sympathetic stimulation last longer than effects of parasympathetic stimulation.
ACTIVITIES OF ANS –
Hypothalamus maintains balance (tone) between opposing effects of symp- & parasympathetic activity.
The following structures have only sympathetic innervation:
 Kidneys
 Adrenal medulla
 Sweat glands
 Arrector pili muscles and
 Most blood vessels
-Opposing effect in these structures is produced by the increase or decrease of sympathetic tone.
Sympathetic Activities –
During physical or emotional stress, sympathetic tone increases.
This stress leads to usage of energy due to flight-or-fight response, characterized by:
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Increased rate & force of contraction of heart
Dilatation of pulmonary airways
Dilatation of pupils
Increased activity and circulation to heart, liver, skeletal muscles (to face the danger)
Decreased activity and circulation in GIT, kidneys etc. (to conserve energy for emergency)
Breakdown of glycogen & fat to produce ATP
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Parasympathetic Activities –
During rest & recovery, parasympathetic tone increases.
This helps body conserve energy to restore normalcy, and is characterized by:
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Increased digestion of food to store energy
Increased secretion of juices (salivation, digestive enzymes)
Decreased rate & force of contraction of the heart
Constriction of pulmonary airways
Constriction of pupils
Paradoxical fear - Stimulation of parasympathetic system when either flight or fight is useless (as
occurs when death is imminent).
Paradoxical fear leads to relaxation of sphincters (urination, defecation).
-X-