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BIOB32H3 Lecture Notes - Aorta, Paravertebral Ganglia, Cholinergic

Biological Sciences
Course Code
Kenneth Welch

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Autonomic Nervous System (ANS)
Autonomic Nervous System (ANS)
The ANS consists of motor neurons that:
Innervate smooth and cardiac muscle and glands
Make adjustments to ensure optimal support for body activities
Operate via subconscious control
Have viscera as most of their effectors
ANS Versus Somatic Nervous System (SNS)
The ANS differs from the SNS in the following three areas
Efferent pathways
Target organ responses
The effectors of the SNS are skeletal muscles
The effectors of the ANS are cardiac muscle, smooth muscle, and glands
Efferent Pathways
Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector
Axons of the ANS are a two-neuron chain
The preganglionic (first) neuron with a lightly myelinated axon
The gangionic (second) neuron that extends to an effector organ
Neurotransmitter Effects
All somatic motor neurons release ACh, which has an excitatory effect
In the ANS:
Preganglionic fibers release ACh
Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or
ANS effect on the target organ is dependent upon the neurotransmitter released and the
receptor type of the effector
Divisions of the ANS
The two divisions of the ANS are the sympathetic and parasympathetic
The sympathetic mobilizes the body during extreme situations
The parasympathetic performs maintenance activities and conserves body energy
The two divisions counterbalance each other’s activity
Role of the Parasympathetic Division
Concerned with keeping body energy use low
Involves the D activities digestion, defecation, and diuresis
Its activity is illustrated in a person who relaxes after a meal
Blood pressure, heart rate, and respiratory rates are low
Gastrointestinal tract activity is high
The skin is warm and the pupils are constricted
Role of the Sympathetic Division
The sympathetic division is the “fight-or-flight” system
Involves E activities exercise, excitement, emergency, and embarrassment
Promotes adjustments during exercise blood flow to organs is reduced, flow to muscles is
Its activity is illustrated by a person who is threatened
Heart rate increases, and breathing is rapid and deep
The skin is cold and sweaty, and the pupils dilate
Sympathetic Outflow
Is from nerves T1 through L2
Sympathetic neurons produce the lateral horns of the spinal cord
Preganglionic fibers pass through the white rami communicantes and synapse in the
paravertebral ganglia
Fibers from T5-L2 form splanchnic nerves and synapse in collateral ganglia
Postganglionic fibers innervate the numerous organs of the body
Sympathetic Trunks and Pathways
Preganglionic fibers pass through white rami communicantes and enter paravertebral
The paravertebral ganglia form part of the sympathetic chain
Typically there are 23 ganglia 3 cervical,
11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal
A pregangiolonic fiber follows one of three pathways upon entering the paravertebral
Synapse with the ganglionic neuron within the same ganglion
Ascend or descend the
sympathetic chain to
synapse in another
chain ganglion
Pass through the chain
ganglion and emerge
without synapsing
Pathways with Synapses in a Chain Ganglion
Postganglionic axons enter the ventral rami via the gray rami communicantes
These fibers innervate sweat glands and arrector pili muscles
Rami communicantes are associated only with the sympathetic division
Pathways to the Head
Preganglionic fibers emerge from T1-T4 and synapse in the superior cervical ganglion
These fibers:
Serve the skin and blood vessels of the head
Stimulate dilator muscles of the iris
Inhibit nasal and salivary glands
Pathways to the Thorax
Preganglionic fibers emerge from T1-T6 and synapse in the cervical chain ganglia
Postganglionic fibers emerge from the middle and inferior cervical ganglia and enter nerves
These fibers innervate the heart via the cardiac plexus, as well as innervating, the thyroid
and the skin
Other T1-T6 preganglionic fibers synapse in the nearest chain ganglia
Postganglionic fibers directly serve the heart, aorta, lungs, and esophagus
Pathways with Synapses in a Collateral Ganglion
These fibers (T5-L2) leave the sympathetic chain without synapsing
They form thoracic, lumbar, and sacral splanchnic nerves
Their ganglia include the celiac, the superior and inferior mesenterics, and the hypogastric
Pathways to the Abdomen
Sympathetic nerves innervating the abdomen have preganglionic fibers from T5-L2
They travel through the thoracic splanchnic nerves and synapse at the celiac and superior
mesenteric ganglia
Postganglionic fibers serve the stomach, intestines, liver, spleen, and kidneys
Pathways to the Pelvis
Preganglionic fibers originate from T10-L2
Most travel via the lumbar and sacral splanchnic nerves to the inferior mesenteric and
hypogastric ganglia
Postganglionic fibers serve the distal half of the large intestine, the urinary bladder, and the
reproductive organs
Pathways with Synapses in the Adrenal Medulla
Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulla
Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood
Visceral Reflexes
Visceral reflexes have the same elements as somatic reflexes
They are always polysynaptic pathways
Afferent fibers are found in spinal and autonomic nerves
Referred Pain
Pain arising from the viscera but is perceived as somatic in origin
This may be due to the fact that visceral pain afferents travel along the same pathways as
somatic pain fibers
Neurotransmitters and Receptors
Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the
ACh is released by all preganglionic axons and all parasympathetic postganglionic axons
Cholinergic fibers ACh-releasing fibers
Adrenergic fibers sympathetic postganglionic axons that release NE
Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type
Cholinergic Receptors
The two types of receptors that bind ACh are nicotinic and muscarinic
These are named after drugs that bind them and mimic ACh effects
Nicotinic Receptors
Nicotinic receptors are found on:
Motor end plates (somatic targets)
All ganglionic neurons of both sympathetic and parasympathetic divisions
The hormone-producing cells of the adrenal medulla
The effect of ACh binding to nicotinic receptors is always stimulatory
Muscarinic Receptors
Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic
The effect of ACh binding:
Can be either inhibitory or excitatory
Depends on the receptor type of the target organ