3. Structure of Sympathetic Nervous System
◦ Another name for the sympathetic division is thoracolumbar division, which tells us where the
sympathetic preganglionic neurons originate.
◦ Their cell bodies are in the thoracic segments and some of the lumbar segments of the spinal
cord. Their axons extend to the sympathetic ganglia, most of which are located in two chains
just outside the spinal column.
◦ Within the ganglia are the synapses between preganglionic and postganglionic neurons; the
postganglionic axons then go to the visceral effectors.
◦ One preganglionic neuron often synapses with many postganglionic neurons to many effectors.
This anatomic arrangement has physiological importance: The sympathetic division brings
about widespread responses in many organs.
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4. Structure of Sympathetic Nervous System
◦ Pathway from Spinal Cord to
Sympathetic Trunk Ganglia
◦ Cell bodies of sympathetic
preganglionic neurons are
part of the lateral gray horns
of all thoracic segments and
of the first two lumbar
segments of the spinal cord
◦ After exiting through the
intervertebral foramina, the
myelinated preganglionic
sympathetic axons pass into
the anterior root of a spinal
nerve and enter a short
pathway called a white
ramus
◦ Pathways from Sympathetic
Trunk Ganglia to Visceral
Effectors
◦ Axons leave the sympathetic
trunk in four possible ways:
◦ (1) They can enter spinal
nerves;
◦ (2) they can form cephalic
periarterial nerves;
◦ (3) they can form sympathetic
nerves; and
◦ (4) they can form splanchnic
nerves.
Organisation of sympathetic
trunk ganglia
The cervical portion of
each sympathetic trunk is
located in the
neck and is subdivided into
• superior cervical
ganglion
• Middle cervical
ganglion
• Inferior cervical
ganglion
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5. Functions of sympathetic nervous system
The pupils of the eyes dilate.
Heart rate, force of heart contraction, and blood
pressure increase.
The airways dilate, allowing faster movement of air
into and out of the lungs.
The blood vessels that supply the kidneys and
gastrointestinal tract constrict, which decreases
blood flow through these tissues. The result is a
slowing of urine formation and digestive activities,
which are not essential during exercise.
Blood vessels that supply organs involved in
exercise or fighting off danger—skeletal muscles,
cardiac muscle, liver, and adipose tissue—dilate,
allowing greater blood flow through these tissues.
Liver cells perform glycogenolysis (breakdown of
glycogen to glucose), and adipose tissue cells
perform lipolysis (breakdown of triglycerides to
fatty acids and glycerol).
Release of glucose by the liver increases blood
glucose level.
Processes that are not essential for meeting the
stressful situation are inhibited. For example,
muscular movements of the gastrointestinal tract
and digestive secretions slow down or even stop.
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6. Structure of parasympathetic nervous system
◦ The other name for the parasympathetic division is the craniosacral division.
◦ The cell bodies of parasympathetic preganglionic neurons are in the brain stem and the sacral
segments of the spinal cord.
◦ Their axons are in cranial nerve pairs 3, 7, 9, and 10 and in some sacral nerves and extend to
the parasympathetic ganglia.
◦ These ganglia are very close to or actually in the visceral effector, and contain the
postganglionic cell bodies, with very short axons to the cells of the effector.
◦ One preganglionic neuron synapses with just a few postganglionic neurons to only one effector
and so very localized (one organ) responses are possible.
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7. Functions of parasympathetic nervous system
◦ The parasympathetic division dominates in relaxed (non-stress) situations to promote normal functioning of several organ systems.
◦ Digestion will be efficient, with increased secretions and peristalsis; defecation and urination may occur; and the heart will beat at a
normal resting rate.
◦ Notice that when an organ receives both sympathetic and parasympathetic impulses, the responses are opposites. Such an
arrangement makes maintaining an appropriate level of activity quite simple, as in changing the heart rate to meet the needs of a
situation.
◦ Notice also that some visceral effectors receive only sympathetic impulses. In such cases, the opposite response is brought about by
a decrease in sympathetic impulses. Secretion by the sweat glands is an example
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10. Spinal cord physiology
◦ About 100 million neurons and even more neuroglia compose the spinal cord, the part of the
central nervous system that extends from the brain. In length, the spinal cord extends from the
foramen magnum of the occipital bone to the disc between the first and second lumbar
vertebrae. The spinal cord and its associated spinal nerves contain neural circuits that control
some of your most rapid reactions to environmental changes.
◦ The spinal cord has two principal functions in maintaining homeostasis: nerve impulse
propagation and integration of information.
◦ The white matter tracts in the spinal cord are highways for nerve impulse propagation. Sensory
input travels along these tracts toward the brain, and motor output travels from the brain along
these tracts toward skeletal muscles and other effector tissues. T
◦ The gray matter of the spinal cord receives and integrates incoming and outgoing information
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11. Functions of the Spinal Cord and Spinal Nerves
1. The white matter of the spinal cord contains sensory and motor tracts, the “highways” for
conduction of sensory nerve impulses toward the brain and motor nerve impulses from the
brain toward effector tissues.
2. The spinal cord gray matter is a site for integration (summing) of excitatory postsynaptic
potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).
3. Spinal nerves and the nerves that branch from them connect the CNS to the sensory
receptors, muscles, and glands in all parts of the body.
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12. Spinal nerves
◦ Spinal nerves are associated with the spinal cord and, like all nerves of the peripheral nervous
system (PNS), are parallel bundles of axons and their associated neuroglial cells wrapped in
several layers of connective tissue. Spinal nerves connect the CNS to sensory receptors,
muscles, and glands in all parts of the body.
◦ The 31 pairs of spinal nerves are named and numbered according to the region and level of the
vertebral column from which they emerge.
◦ Not all spinal cord segments are aligned with their corresponding vertebrae. Spinal cord ends
near the level of the superior border of the second lumbar vertebra (L2), and that the roots of
the lumbar, sacral, and coccygeal nerves descend at an angle to reach their respective
foramina before emerging from the vertebral column.
◦ This arrangement constitutes the cauda equina.
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13. Spinal Nerves
◦ The first cervical pair of spinal nerves emerges from the spinal cord between the occipital bone
and the atlas (first cervical vertebra, or C1).
◦ Most of the remaining spinal nerves emerge from the spinal cord through the intervertebral
foramina between adjoining vertebrae.
◦ Spinal nerves C1–C7 exit the vertebral canal above their corresponding vertebrae. Spinal nerve
C8 exits the vertebral canal between vertebrae C7 and T1.
◦ Spinal nerves T1–L5 exit the vertebral canal below their corresponding vertebrae.
◦ From the spinal cord, the roots of the sacral spinal nerves (S1–S5) and the coccygeal spinal
nerves (Co1) enter the sacral canal, the part of the vertebral canal in the sacrum. Subsequently,
spinal nerves S1–S4 exit the sacral canal via the four pairs of anterior and posterior sacral
foramina, and spinal nerves S5 and Co1 exit the sacral canal via the sacral hiatus.
◦ A typical spinal nerve has two connections to the cord: a posterior root and an anterior root .
Because the posterior root contains sensory axons and the anterior root contains motor axons,
a spinal nerve is classified as a mixed nerve.
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19. References
◦ G J Tortora, B Derrickson; Tortora’s Principle of Anatomy and Physiology; Wiley; 15th edition; 2017: 446-472 & 526-546
◦ Valerie C. Scanlon, Tina Sanders; Essentials of Anatomy and physiology; F A Davis Company; 5th edition; 2007: 187-191
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20. This Photo by Unknown Author is licensed under CC BY-SA
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