Autonomic nervous system

Question 1

Autonomic nervous system (ANS)

Answer 1

The involuntary component of the nervous system, which can be sub-divided into the sympathetic, parasympathetic and enteric nervous systems. Responsible for maintaining homeostasis, vision, and reproduction.

Question 2

ANS efferent pathways

Answer 2

ANS motor neurones innervate involuntary effectors, such as smooth muscle, cardiac muscle, or glands. They constitute general visceral efferent (GVE) nerves, which typically involve 2 neurones; a pre-ganglionic fibre and a postganglionic fibre.

Question 3

Pre-ganglionic fibres

Answer 3

Autonomic efferent neurones with their cell bodies in the CNS, which secrete ACh at the synapse within a ganglion.

Question 4

Post-ganglionic fibres

Answer 4

Autonomic efferent neurones with their cell bodies in ganglia within the PNS, which secrete ACh or noradrenaline (NA) onto a target organ/tissue.

Question 5

Sympathetic nerves

Answer 5

Constitute a thoracolumbar outflow. Pre-ganglionic fibres are short, have their cell bodies in the lateral horn of spinal cord levels T1-L2/L3 and secrete ACh. Post-ganglionic fibres are longer, have their cell bodies in sympathetic chain ganglia/pre-aortic ganglia, and secrete NA.

Question 6

Exceptions to sympathetic innervation

Answer 6

The sympathetic innervation of the adrenal gland is direct, with only one neurone travelling to the neuroendocrine chromaffin cells of the adrenal medulla, stimulating the secretion of adrenaline.

The sweat glands and arretor pili muscles in the skin are innervated by sympathetic post-ganglionic sudomotor neurones, which secrete ACh rather than NA.

Question 7

Parasympathetic nerves

Answer 7

Constitute a cranial sacral outflow . Pre-ganglionic fibres are longer, have their cell bodies in the brain stem or spinal cord levels S2-S4 and secrete ACh. Post-ganglionic fibres are short, have their cell bodies in ganglia near the target organ and secrete ACh.

Question 8

Actions of the sympathetic nervous system

Answer 8

• pupil dilation
• increase heart rate
• vasoconstriction
• bronchodilation
• relax smooth muscle (eg. GI tract)
• reduce secretion (tears, saliva, gastric acid)
• secrete adrenaline
• ejaculation/orgasm

Question 9

Actions of the parasympathetic nervous system

Answer 9

• pupil constriction
• decrease heart rate
• bronchoconstriction
• contract smooth muscle (eg. peristalsis)
• stimulate salivation
• increase secretion
• erection

Question 10

Sympathetic chain ganglia

Answer 10

Sympathetic ganglia of the head and neck:

• superior cervical ganglia (C2/C3)
• middle cervical ganglia (C6); innervates the thyroid, trachea, oesophagus, heart
• inferior cervical ganglia (C7/T1, aka stellate ganglia); innervates the upper limb

Question 11

Pre-aortic ganglia

Answer 11

Sympathetic ganglia of the abdomen and pelvis:

• coelic ganglia
• superior mesensteric ganglia
• inferior mesenteric ganglia

The greater, lesser and least splanchnic nerves provide fibres to these ganglia. Post-ganglionic autonomic nerve fibres follow the blood vessels to reach their target organs.

Question 12

Parasympathetic ganglia

Answer 12

4 named head ganglia:

• ciliary ganglion–> eye (CN III)
• pterygopalatine ganglion–>lacrimal gland, mucous membranes (CN VII)
• submandibular ganglion–>submandibular and sublingual glands (CN VII)
• otic ganglion–> parotid gland (CN IX)

CN X and pelvic splanchnic nerves from S2-S4 do not have named ganglia. Intramural ganglia are found in the wall of their target organ.

Question 13

Autonomic nerve supply to the gut

Answer 13

Parasympathetic- CN X innervates the distal 1/3 of the colon, while the pelvic splanchnic nerves innervate the hindgut and pelvic organs.

Sympathetic- thoracic and lumbar splanchnic nerves innervate the foregut (from coeliac ganglion), midgut (from superior mesenteric ganglion) and hindgut (from inferior mesenteric ganglion).

Question 14

Sympathetic chain pathway

Answer 14

Pre-ganglionic fibres originate in the lateral horn of spinal cord levels T1-L2/L3. They exit via the ventral root, then join the mixed spinal nerve. They enter the sympathetic ganglion via the white ramus communicans, where they synapse with post ganglionic fibres. These exit via the grey ramus communicans to target organs/tissues.

Question 15

Horner’s syndrome

Answer 15

The loss of sympathetic function from the neurones of the inferior cervical (stellate) ganglion, resulting in constricted pupils (miosis), partial ptosis (eyelid drooping) and anhydrosis (reduced sweating). May be caused by trauma, tumour, multiple sclerosis, spinal cord lesions, or a cervical rib impinging on the sympathetic chain.

Question 16

Stellate block

Answer 16

The application of a local anaesthetic to the stellate ganglion, resulting in sympathetic paralysis in the head, neck and upper limb on the same side (Horner’s syndrome).

Question 17

Parasympathetic ganglia and plexuses

Answer 17

• CN III (oculomotor); ciliary ganglion
• CN VII (facial); pterygopalatine and submandibular ganglia
• CN IX (glossopharyngeal); otic ganglion
• CN X (vagus); cardiac/pulmonary plexus (thoracic visceral plexus)
• Pelvic splanchnic nerves; prevertebral plexus

Question 18

Autonomic sensory components

Answer 18

Receptors in the viscera detect internal stimuli (not consciously perceived) eg. baroreceptors detect BP.
Receptors in the somatosensory organs detect external stimuli.

Question 19

Autonomic motor components

Answer 19

Sympathetic, parasympathetic and enteric efferent neurones innervate involuntary effectors, such as smooth muscle, cardiac muscle and glands.

Question 20

Autonomic innervation

Answer 20

Pre-ganglionic neurones may branch to synapse with many post-ganglionic neurones. These can release neurotransmitter at varicosities branching across a large area of target tissue, to activate many effectors. This results in a diffuse and highly coordinated response.

Question 21

Divisions of the ANS

Answer 21

• sympathetic (alert and active); emergency, defense response, fight/flight/fright
• parasympathetic (rest and digest); energy conservation, digestion, emptying
• enteric; gastrointestinal function

Question 22

Dual innervation

Answer 22

Most organs are innervated by both the sympathetic and parasympathetic nervous systems, which act antagonistically. For example, in cardiac muscle, the SNS increases heart rate, while the PNS decreases it. In the GI tract, the SNS decreases motility, while the PNS increases it.

Question 23

Pupil constriction

Answer 23

The pupil is controlled by two muscles, which are dually innervated, bringing about a combined action. The sympathetic nervous system stimulates contraction of the radial dilator pupillae, resulting in dilation of the pupil. The parasympathetic nervous system stimulates contraction of the circular sphincter pupillae, resulting in constriction of the pupil.

Question 24

Single innervation

Answer 24

Some structures are only innervated by one division of the ANS. For example, the sweat glands, most blood vessels and smooth muscle are innervated only by the sympathetic nervous system.

Question 25

Vasomotor tone

Answer 25

In most blood vessels, the sympathetic nervous system stimulates vasoconstriction. The blood vessels of the head and reproductive organs are dually innervated, so the parasympathetic nervous system also stimulates vasodilation.

Question 26

Exception to vasomotor tone

Answer 26

Unlike the rest of the blood vessels, those supplying the heart dilate with sympathetic activation. They express more beta adrenergenic receptors, which stimulate vasodilation in response to adrenaline. Most blood vessels express more alpha adrenergenic receptors, which stimulate vasoconstriction in response to NA.

Question 27

Production of saliva

Answer 27

The blood vessels in the head and neck supply the salivary glands with the components of saliva, including water and ions. Vasoconstriction by the sympathetic nervous system reduces the supply, resulting in a smaller volume of thick and sticky saliva. Vasodilation by the parasympathetic nervous system increases the supply, resulting in a larger volume of watery saliva.

Question 28

Reproductive organs

Answer 28

In reproductive tissues and organs, both divisions of the ANS work together. For example, in the male sex organs, the parasympathetic nervous system is responsible for erection (point), while the sympathetic nervous system is responsible for ejaculation (shoot).

Question 29

Interaction of autonomic and somatic nervous systems

Answer 29

A specific sensory stimulus may bring about both somatic and autonomic responses, for example:

• focusing the eyes on an object; pupil/lens adjustments (ANS), eye movements (SNS)
• response to cold; vasoconstriction in the skin (ANS), shivering (SNS)