Wk8 Anatomy Of The ANS

Question 1

What is the ANS responsible for?

Answer 1

Fight/flight sympathetic 
THE Fs
- fight
- flight
- fright

Rest/digest parasympathetic
NON-Fs
- rest
- digest
- energy storage 

The autonomic nervous system is in a state of equilibrium during normal activity

Question 2

Overview of ANS

Answer 2

Somatic motor
▪ Spinal cord
▪ Single neuron
pathway
▪ Voluntary: Skeletal muscle

ANS motor
▪ Spinal cord
▪ 2 neuron pathway
▪ Involuntary:
• Cardiac muscle
• Smooth muscle 
• Glands

Question 3

ANS general principles

Answer 3

Pre-ganglionic fibre releases ACh
Synapse - ganglion in PNS
Post-ganglionic fibre - ACh or noradrenaline (target organ or tissue)

Question 4

Sympathetic nerves - key facts

Answer 4

• Thoracolumbar outflow (T1 – L2/L3 spinal level)
• ACh secreted by pre-ganglionic fibres
• NA secreted by post-ganglionic adrenergic fibres

Cell body in lateral horn of T1-L2 spinal cord
Ganglion in sympathetic chain or near target organ

• Head & neck Thorax • Abdomen
• Pelvis
• Blood vessels

Question 5

Sympathetic nerves exceptions

Answer 5

No ganglion to suprarenal gland

Direct innervation from sympathetic preganglionic neurons to adrenal medulla (chromaffin cells) for production of adrenaline

• Skin - sweat glands
• Skin – arrector pili

ACh secreted by postganglionic ‘sudomotor’ neurons (innervating sweat glands and arrector pili muscle of skin)

Question 6

Parasympathetic nerves summary

Answer 6

1. Cell body in brain stem (CNIII or CNX)—> ganglion near target organ —> head and neck, thorax, upper abdomen
2. Cell body in sacral spinal cord (S2-S4) —> ganglion near target organ —> lower abdomen and pelvis

Craniosacral outflow with acetylcholine (ACh) as primary neurotransmitter for both pre-ganglionic and post-ganglionic neurons

Question 7

The spinal cord

Answer 7

▪ Dorsal horn with dorsal root.
(somatic afferents synapse with CNS cell bodies)
▪ Ventral horn with ventral root (somatic efferent cell bodies and axons)
▪ Lateral horn (T1-L2 spinal cord segments) (pre-ganglionic sympathetic cell bodies)

Remember: Spinal cord to peripheral nervous system:
Horn → Root → Mixed spinal nerve → Dorsal/ventral ramus…

Question 8

What is the lateral horn?

Answer 8

pre- ganglionic sympathetic neuronal cell bodies

Question 9

Summary of the ANS

Answer 9

▪ The ANS is tonically activated and maintains homeostasis.
• Involuntary control of cardiac, smooth muscle and glands.
▪ Sympathetic outflow is thoracolumbar (T1-L2).
• Increase sympathetic activity in response to stress (fight/flight).
• Pre-ganglionic from the lateral horn of spinal cord = ACh
• Post ganglionic = NA (ACh for sweat glands and arrector pili).
▪ Parasympathetic outflow is craniosacral (cranial nerves and S2-S4).
• Increase parasympathetic activity in response to consumption of food.
• Pre- and post-ganglionic release ACh

Question 10

Actions of sympathetic ANS

Answer 10

Dilate pupil
Increase HR 
Constrict blood vessels
Dilate bronchial smooth muscle
Inhibit digestion
Inhibit salivation
Secretion of adrenaline 
Orgasm shoot

Question 11

Actions of parasympathetic ANS

Answer 11

Constrict pupil
Decrease HR
Constrict bronchial smooth muscle
Increase peristalsis
Stimulate salivation
Erection - point

Question 12

Sympathetic ganglia

Answer 12

On either side of the neck

Superior cervical ganglia (C2/C3 level) Middle (C6 level) cervical ganglia innervate
thyroid gland, trachea, oesophagus, heart
Inferior cervical ganglia (C7/T1 level (AKA stellate ganglia) innervate upper limb

Question 13

Abdominal autonomic ganglia

Answer 13

▪ Via thoracic splanchnic nerves Greater/lesser/least
▪ Pre-aortic (pre-vertebral) ganglia 
Coeliac
Superior mesenteric
Inferior mesenteric
▪ ANS fibres follow the blood vessels.

▪ ANS fibres follow the blood vessels.
▪ Visceral efferent = ANS motor
▪ Visceral afferent = visceral sensory

Question 14

Parasympathetic ganglia

Answer 14

▪ Parasympathetic pre- ganglionic fibres synapse on post ganglionic fibres at:
• Named ganglia in the head.
• OR in the wall of their target organ.

▪ 4parasympathetic ganglia (on either side of head):
Ciliary
Otic 
Pterygopalatine 
Submandibular
COPS

▪ Ganglia are in wall of target organ (intramural).
▪ X Vagus
• Foregut and midgut organs
▪ Pelvic splanchnic nerves 
• S2–S4
• Wall of hindgut or pelvic organs

Question 15

Enteric nervous system

Answer 15

Myenteric plexus
▪ Peristalsis
Submucosal plexus
▪ Glandular secretion

Question 16

Hirschprung’s disease

Answer 16

Congenital absence of myenteric and submucosal plexus neurons (ganglion cells) in parts of intestine

Question 17

Autonomic nerve supply to the gut

Answer 17

Parasympathetic
▪ Vagus nerve to distal third of transverse colon (splenic flexure)
▪ Pelvic splanchnic nerves (from S2 – S4) to hindgut and pelvic organs

Parasympathetic
▪ Vagus nerve
▪ Pelvic splanchnic nerves (S2 – S4)
Sympathetic (thoracic + lumbar splanchnic nerves)
▪ Celiac ganglion (foregut)
▪ Superior mesenteric ganglion (midgut)
▪ Inferior mesenteric ganglion (hindgut)
Form autonomic nerve plexi around arteries.
▪ Autonomic nerves follow blood vessels to reach target organ

Question 18

Sympathetic nervous stem

Answer 18

1) Pre-ganglionic neurones depart the lateral horn (T1 –L2)
2) Pass through the ventral root
3) Enters the sympathetic chain via the white ramus communicans (myelinated)
4) Synapse at sympathetic ganglion
5) Post-ganglionic neurons depart via the grey ramus communicans (unmyelinated)
6) The dorsal ramus will supply the back, the ventral ramus will supply the anterior body and limbs

Question 19

Routes in the sympathetic chain

Answer 19

Superior Cervical Ganglion

1. Pupil Dilation
2. Inhibit Salivation

ABDOMEN (pre-vertebral/pre-aortic ganglia)

1. Coeliac Ganglion
2. Superior Mesenteric Ganglion
3. Inferior Mesenteric Ganglion

Question 20

Horner’s syndrome - left side

Answer 20

Constricted pupil (miosis)
Partial ptosis
Anhydrosis (reduced sweating on same side)

Some potential causes are:
• Trauma (e.g. blunt or surgical trauma in the root of the neck)
• Tumour (e.g. lung, thyroid)
• Multiple sclerosis and spinal cord lesions
• Cervical rib (impinging on sympathetic chain)

Question 21

Pupillary constriction

Answer 21

Miosis
Sphincter pupillae muscle contracts: pupil size decreases
Parasympathetic

Mydriasis
Dilator pupillae muscle contracts: pupil size increases
Sympathetic

Question 22

Stellate block

Answer 22

▪ Local anaesthetic at stellate ganglion
• Sympathetic paralysis in head, neck and upper limb on same side.
▪ Successful stellate block is demonstrated by
1. a warm, dry hand;
2. Horner syndrome, a constricted pupil (pupillary constrictor is unopposed by
dilator pupillae;
3. ptosis (drooping) of the upper eyelid because of paralysis of smooth muscle fibres within the levator muscle of the upper eyelid

Question 23

Summary

Answer 23

▪ The sympathetic chain allows pre-ganglionic neurons to synapse, ascend or descend or to pass through to other sympathetic ganglia.
▪ The route taken by the pre-ganglionic fibre is:
• Lateral horn – Ventral root, spinal nerve, white ramus communicans –
sympathetic chain/through to pre-aortic ganglia
The route taken by the post-ganglionic fibre is:
• Sympathetic chain – grey ramus communicans – spinal nerve – dorsal/ventral ramus to body.
▪ Horner’s syndrome is a paralysis of the sympathetic neurons in the head, neck and upper limb. Characterised by pupillary constriction, warm skin/anhidrosis, partial ptosis
▪ The adrenal gland is directly innervated by pre-ganglionic fibres. Chromaffin cells secrete adrenaline into the blood.

Question 24

What can sympathetic pregangionic neurones do?

Answer 24

▪ Synapse with a postganglionic neuron in a sympathetic ganglion at the same spinal level
▪ OR ascend the sympathetic chain to synapse with a postganglionic neuron in a higher sympathetic ganglion
▪ OR descend the sympathetic chain to synapse with a
postganglionic neuron in a lower sympathetic ganglion
▪ OR pass through the sympathetic ganglion to reach a splanchnic nerve and travel to the thoracic/abdominal organs

Question 25

Key themes

Answer 25

▪ Resolve the paradox that, despite an outflow restricted to 14 or 15 ventral roots,
all 31 spinal nerve trunks acquire sympathetic fibres.
▪ Appreciate that the sympathetic ganglia along the abdominal aorta are activated
by preganglionic fibres, as is the adrenal medulla.
▪ Pay special attention to the autonomic innervation of the eye, discussed both
here and in Chapter 23 .
▪ Appreciate that the four parasympathetic ganglia in the head are functionally
similar to intramural ganglia elsewhere.
▪ Be aware that the pelvic ganglia are mixed autonomic ganglia.
▪ Realise that the preganglionic neurons of both divisions are cholinergic and that
the target receptors in all of the autonomic ganglia are nicotinic.
▪ Appreciate that visceral afferents utilise autonomic pathways to gain access to
the nervous system. They are especially important in the context of thoracic and abdominal pain.