The Autonomic Nervous System
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cranial nerves | Main Anatomy
Last updated 30 March 2006
- The autonomic nervous system (ANS) is the part of the
nervous system, both afferent and efferent, that
innervates various body systems that are not under
- These include the constriction and dilation of blood
vessels, the activity of the viscera, the secretion of
glands (fibres supplying glands are called secretomotor
fibres), and assist the endocrine system to maintain a
constant internal environment (homeostasis).
- The afferent neurons
have their peripheral receptors in the wall of viscera
and blood vessels, and their cell bodies in the dorsal
root ganglia or cranial nerve ganglia. Their central
processes end in the dorsal grey column of the spinal
cord or the brainstem.
- The efferent neurons
supply the smooth muscles in the wall of hollow viscera
and blood vessels. They can be either excitatory or
Sympathetic and Parasympathetic
- Functionally, the ANS is divided into sympathetic and
parasympathetic systems. Read "Table 4. Functions of
the Autonomic Nervous System (ANS), p. 29" from
Moore's "Clinically Oriented Anatomy 3rd Edition"
for a larger list.
- The sympathetic system
prepares the body for "fright, fight or
flight". It increases the heart rate and ventricular
contraction, dilates the blood vessels in skeletal
muscles, constricts blood vessels in the skin and guts,
increases blood sugar level, stimulates sweating, dilates
the pupils, inhibits activities of the guts and gastric
- The parasympathetic
system is more active at rest, having in general
anabolic effects. For example, it slows down the heart
rate, constricts the pupils, increases gastric secretion
and intestinal motility.
General Layout of the ANS
The simplified classical view of the design of the ANS is as
The efferent pathway is made up of 2 neurons:
- a preganglionic neuron, (myelinated), located in the
spinal cord or brainstem, synapsing with;
- postganglionic neuron(s) (unmyelinated) in an autonomic
- In general the sympathetic ganglia are further from the
target organs, and the parasympathetic ganglia are
located near or in the wall of target organs.
- The sympathetic outflow from the CNS is thoracolumbar, emerging from
the spinal cord segments T1-L3.
- The sympathetic ganglia form a string of beads called the
sympathetic trunk, lying
on either side of the vertebral bodies.
- The two trunks extend from C1 to the level of the coccyx
where they unite at the midline at the ganglion
- The preganglionic fibres leave the spinal cord with the
ventral root of the corresponding spinal nerve, and pass
to the corresponding sympathetic ganglion via the white ramus communicans (white
because the fibres are myelinated).
In that ganglion, they may:
- Synapse with postganglionic fibres which leave the
sympathetic trunk in the grey rums
communicans, rejoins the same ventral ramus,
then are distributed to the target organs (smooth muscles
of blood vessels, sweat glands...)
- Travel up or down the trunk to synapse with
postganglionic fibres in other ganglia (e.g. cervical,
lumbar, sacral, which do not receive direct rami
communcantes from the cervical, lower lumbar or sacral
- Pass through the ganglion without relay to synapse in prevertebral ganglion (e.g.
coeliac ganglion, known to lay people as "solar
Sympathetic Supply to the Head and
The preganglionic fibres, which supply the head and neck,
arise from the spinal segments T1-T2.
They enter the sympathetic trunk and travel upward to synapse in
one of the three ganglia in the neck:
- The superior cervical ganglion
(located anterior to vertebrae C1, C2).
- The middle cervical ganglion
(small, often absent, lying anterior to C6).
- The inferior cervical ganglion
which is usually fused with the first thoracic ganglion
to form the cervicothoracic
(or stellate) ganglion (located anterior to
C7 and the neck of the first rib).
Each of these ganglia gives rise to:
- Cardiac branches.
- Branches to blood vessels, sweat glands, hair follicles
in the neck and head. The superior ganglion sends
branches to spinal nerves C1-C4, the middle ganglion to
spinal nerves C5-C6, and the cervicothoracic ganglion to
spinal nerves C7, 8-T1.
- Vascular branches which pass to the vertebral, common,
internal and external carotid arteries. Some of these
branches "hitch-hike" along the arteries and
their branches to reach their targets in the head and
neck. The superior ganglion sends branches along the
internal and external carotid arteries to reach
structures in the orbit, face, nasal and oral cavities
and pharynx. The middle ganglion sends branches along the
inferior thyroid artery to reach the larynx, trachea and
upper oesophagus. The inferior ganglion sends branches to
the subclavian and vertebral arteries.
The Parasympathetic System
The parasympathetic system outflow is craniosacral,
- The oculomotor, facial, glossopharyngeal, and vagus
- The spinal cord segments S2, 3, 4. These fibres travel in
the branches of sacral nerves S2-S4 (nervi erigentes) to
the pelvic viscera.
- The parasympathetic system supplies the heart, glands,
and smooth muscles of the viscera not
the sweat glands, blood vessels or erector pilorum
Parasympathetic Supply to the Head
- Click here for a
diagram of the parasympathetic ganglia along the branches
of the facial (CN VII) and glossopharyngeal (CN IX)
There are four ganglia in the head: ciliary, pteryopalataline,
otic, and submandibular.
- The parasympathetic fibres enter the orbit with the
inferior division of the nerve and synapse in the ciliary ganglion, which is
located just lateral to the optic nerve.
- The postganglionic fibres with the short ciliary nerves
enter the eye to supply the sphincter pupillae (that
constricts the pupil) and ciliary muscles.
- Parasympathetic fibres also travel along the superior
branch of the oculomotor nerve to supply the smooth
muscle component of levator palpebrae superioris.
2. Facial Nerve
- The parasympathetic fibres supply the lacrimal gland, the
mucous glands of the nose and palate, the submandibular
and sublingual salivary glands.
- The fibres which supply the lacrimal, nasal and palatine
glands leave the facial nerve (at its genu inside the
petrous bone as the greater petrosal nerve), passes
through the pterygoid canal (at the root of the pterygoid
process as the nerve of the pterygoid canal), and
terminates in the pterygopalatine
ganglion (located in the pterygopalatine
- The postganglionic fibres travel in the branches of the
ganglion or the maxillary nerve to reach the nasal and
- The fibres supplying the lacrimal gland travel with the
zygomatic nerve (a branch of the maxillary nerve, CN V2)
to enter the orbit, then pass into the lacrimal branch of
the ophthalmic nerve to reach the gland.
- Other preganglionic fibres leave the facial nerve in its chorda tympani branch. This
nerve passes over the internal surface of the tympanic
membrane before emerging from the skull through a small
fissure, the petrotympanic fissure, in the temporal bone.
It joints the lingual nerve (a branch of the mandibular
nerve) to travel to, and synapse in, the submandibular ganglion, which
is suspended from the lingual nerve.
- The postganglionic fibres either pass into the
submandibular gland or rejoin the lingual nerve to reach
the sublingual gland.
- The parasympathetic fibres supply the parotid gland.
- The preganglionic fibres travel with the tympanic branch
into the tympanic plexus in the middle ear. The fibres
emerging from this plexus as the lesser
petrosal nerve, which synapses in the otic ganglion (located medial
to the mandibular nerve, just below the foramen ovale).
- The postganglionic fibres then hitchhike the
auriculotemporal nerve (a branch of the mandibular nerve,
CN V2) to reach the parotid gland.
- Other preganglionic fibres of CN IX are secretomotor to
the oropharynx. These fibres pass the pharynx via the
pharyngeal plexus. They synapse on the postganglionic
cells, which are distributed throughout the wall of the
4. Vagus Nerve
- The parasympathetic fibres supply the glands of the
laryngopharynx, the larynx, the cardiac muscles, and the
smooth muscles and glands of the thoracic and abdominal
- Preganglionic fibres travel with the branches of the
vagus nerve to reach their targets in the neck, thorax,
and abdomen. Then they synapse on postganglionic cells,
which are located in the wall of the target organs,
rather than in specific ganglion.