Lecture 11
The Autonomic Nervous System
General Properties of the Autonomic Nervous System
 General Actions
o Autonomic means “self-governed” – the ANS usually controls actions
without our conscious intent or awareness
o The ANS can be defined as a motor nervous system that controls glands,
cardiac muscle, and smooth muscle
o It can be called the visceral motor system for more direct comparisons to
the somatic nervous system, which controls skeletal muscles
o Visceral effectors do not depend on the ANS to function, but the ANS
adjusts their activity to the body’s changing needs (by speeding them up or
slowing them down, for example)
 Visceral Reflexes
o The ANS is responsible for unconscious, automatic, stereotyped responses
to stimulation that involve visceral receptors and effectors
o Autonomic activity involves a reflex arc that includes receptors, afferent
neurons leading to the CNS, interneurons in the CNS, efferent neurons
carrying motor signals away from the CNS, and effectors
 Example:
 high blood pressure activates baroreceptors in the carotid
and aortic arteries
 They transmit signals via the glossopharyngeal nerves to
the medulla oblongata
 The medulla integrates this information and sends efferent
signals back to the heart via the vagus nerve
 The vagus nerves slow down the heart and reduces blood
pressure
 Divisions of the Autonomic Nervous System
o The sympathetic division adapts the body for physical activity (the fight or
flight response)
 Increases alertness, heart rate, blood pressure, pulmonary airflow,
blood glucose, blood flow to cardiac and skeletal muscle
 Decreases blood flow to the skin and digestive tract
o The parasympathetic division has a calming effect on the body (the resting
and digesting state)
o The two operate simultaneously to produce a background rate of activity,
and the balance between sympathetic and parasympathetic activity is
shifted, depending on the needs of the body
 Neural Pathways
o The ANS has components of both the central and peripheral nervous
system
o In autonomic pathways, the signal must travel across two neurons to get to
the target organ
 The signal must cross a synapse where the two neurons meet in an
autonomic ganglion
 The first neuron, called the preganglionic neuron, has a soma in the
brainstem or spinal cord and an axon that terminates in a ganglion
 The second neuron, called the postganglionic neuron, extends from
the ganglion to the target cells.
Anatomy of the Autonomic Nervous System
 The Sympathetic Division
o Called the thoracolumbar division because it arises from the thoracic and
lumbar regions of the spinal cord
o The preganglionic neurons lead to the sympathetic chain of ganglia along
each side of the vertebral column
 Preganglionic neurons are myelinated
 The sympathetic chains extend above and below the thoracic and
lumbar regions
o After entering the sympathetic chain, preganglionic fibers may follow any
of three courses
 They may end in the ganglion and synapse immediately with a
postganglionic neuron
 They may travel up or down the chain and synapse in ganglia at
other levels
 They may pass through the chain without synapsing and continue
as splanchnic nerves
 These splanchnic nerves lead to the collateral ganglia
where they synapse with postganglionic neurons
 The Adrenal Glands
o The adrenal glands rest above the kidneys
o Each adrenal gland is actually two glands with different functions
 The adrenal cortex (outer gland) secretes steroid hormones
 The adrenal medulla (inner gland) is a modified sympathetic
ganglion
 It consists of modified postganglionic neurons
 It is innervated by preganglionic neurons
 When stimulated, the adrenal medulla secretes epinephrine
and norepinephrine into the bloodstream
 The Parasympathetic Division
o Called the craniosacral division because it arises from the brain and sacral
regions of the spinal cord
o The preganglionic neurons end in terminal ganglia in or near the target
organ
o Parasympathetic fibers leave the brainstem by way of four cranial nerves
 Oculomotor nerve – postganglionic fibers run to the papillary
constrictor muscles of the eye

Facial nerve – postganglionic fibers run to the salivary glands in
the floor of the mouth
 Glossopharyngeal nerve – postganglionic fibers run to the parotid
salivary glands below and anterior to the ears
 Vagus nerve – postganglionic fibers run to the heart, the lungs, the
esophagus, the stomach, liver, and proximal half of the colon
o Additional parasympathetic fibers arise from levels S2 to S4 of the spinal
cord
 Postganglionic nerve fibers run to the distal half of the large
intestine, the rectum, urinary bladder, and reproductive organs

The Enteric Nervous System
o The digestive tract has a nervous system of its own
o The enteric nervous system regulates the motility of the esophagus,
stomach, and intestines and the secretion of digestive enzymes and acid
Autonomic Effects
 Neurotransmitters and Receptors
o Some nerve fibers of the ANS are cholinergic
 They secrete acetylcholine (Ach)
 Examples:
 Preganglionic fibers of both divisions
 Postganglionic fibers of the parasympathetic division
 A few sympathetic postganglionic fibers (those of some
sweat glands and blood vessels)
o Some nerve fibers of the ANS are adrenergic
 They secrete norepinephrine (NE)
 Examples:
 Most sympathetic postganlionic fibers
 Dual Innervation
o Most of the viscera receive nerve fibers from both the sympathetic and
parasysmpathetic division
 Example: the sympathetic division dilates the pupil, and the
parasympathetic division constricts the pupil
o Dual innervation is not always necessary – some have sympathetic
stimulation only
 Sympathetic fibers to a blood vessel have a baseline sympathetic
tone that keeps the vessel in a state of partial constriction
 An increase in stimulation causes vasoconstriction
 A drop in stimulation causes vasodilation
 Central Control of Autonomic Function
o The midbrain, pons, and medulla oblongata house the nuclei of cranial
nerves that mediate several autonomic responses
 Pupillary constriction
 Lacrimal and salivary gland secretion
 Blood pressure regulation
 Control of thoracic and abdominal viscera
o These regions of the brain are influenced by the hypothalamus
o The hypothalamus is influenced by the cerebral cortex and limbic system
 Side Note: What is the Limbic System?
 Ring (limbus) of structures on the medial side of the
cerebral hemisphere incirclilng the corpus callosum and the
thalamus
 Experiments have shown that it hase significant roles in
emotion and memory
 Plays roles in feelings of anger, fear, hunger, pleasure – so
an emotion like fear can have an effect on the body by
indirectly affecting the autonomic nervous system
o The spinal cord contains nuclei of autonomic control
 Defecation and micturition are reflexes if the brain is unable to
consciously inhibit them
Effects of the Sympathetic and Parasympathetic Nervous Systems
Target
Eye



Dilator of Pupil
Constrictor of Pupil
Ciliary Muscle and Lens
Sympathetic
Effect
Parasympathetic
Effect
Pupillary dilation
No effect
Relaxation for
No effect
Pupillary constriction
Contraction for
 Lacrimal Gland
Integumentary System
 Sweat Glands
 Piloerector Muscles
Adrenal Medulla
Circulatory System
 Heart Rate and Force
 Deep Coronary Arteries
 Blood Vessels of Viscera
 Blood Vessels of Skeletal Muscle
 Blood vessels of skin
Respiratory System
 Bronchi and Bronchioles
Urinary System
 Kidneys
 Bladder Wall
 Internal Urinary Sphincter
Digestive System
 Salivary Glands
 Gastrointestinal Motility
 Gastrointestinal Secretion
 Liver
 Pancreatic Enzyme Secretion
Reproductive System
 Penile or Clitoral Erection
 Glandular Secretion
 Orgasms, Smooth Muscle Roles
 Uterus
far vision
none
near vision
Secretion
Secretion
Hair erection
Hormone secretion
No effect
No effect
No effect
Increased
Vasodilation
Vasoconstriction
Vasodilation
Vasoconstriction
Decreased
Vasoconstriction
Vasodilation
No effect
Vasodilation
Bronchodilation
Bronchoconstriction
Reduced urine output No effect
No effect
Contraction
Contraction
Relaxation
mucus secretions
Decreased
Decreased
Glycogen breakdown
Decreased
saliva secretions
Increased
Increased
Glycogen synthesis
Increased
No effect
No effect
Stimulation
Relaxation
Labor Contractions
Stimulation
Stimulation
No effect
No effect