Enteric Nervous System, Visceral Sensation & Clinical Concepts

What is the enteric nervous system (ENS)?

The ENS is a complex network of neurons within the wall of the gastrointestinal (GI) tract that can function independently of the CNS.

💡 Often called the “second brain.”

Why is the ENS called the “second brain”?

Because it:

• Has complex neural networks

• Integrates information locally

• Can function independently of the CNS

What are the two plexuses of the enteric nervous system?

• Myenteric plexus

• Submucosal plexus

Where is the myenteric plexus located?

Within the muscular wall of the GI tract.

What is the function of the myenteric plexus?

Controls GI muscle activity and motility (peristalsis).Controls GI muscle activity and motility (peristalsis).

Where is the submucosal plexus located?

Deep to the mucosa within the wall of the GI tract.

What is the function of the submucosal plexus?

Regulates:

• Secretions

• Blood flow

• Activity of the gut lining

Besides motor neurons, what other neurons are found in the ENS?

• Interneurons

• Intrinsic primary afferent neurons

What do intrinsic primary afferent neurons do?

Receive local sensory information and initiate local reflexes.

Which GI functions are controlled by local enteric reflexes?

• Peristalsis

• Secretions

• Vasomotion

• Immune activity

How do the sympathetic and parasympathetic systems affect the ENS?

They modulate ENS activity but do not control it completely.

What is the overall role of the sympathetic nervous system?

Prepares the body for stress (“fight or flight”) by mobilising energy.

What is the overall role of the parasympathetic nervous system?

Maintains homeostasis by promoting rest, digestion and energy conservation

What is the primary function of the sympathetic nervous system?

Regulation of blood vessels

How are most blood vessels affected by increased sympathetic activity?

Vasoconstriction

What happens if sympathetic activity decreases?

Vasodilation

Which blood vessels undergo sympathetic vasodilation?

• Coronary vessels

• Skeletal muscle vessels

• External genitalia

What is vasomotion?

Constriction or dilation of blood vessels under autonomic control.

What information do visceral afferent fibres usually carry?

Information about the body’s internal environment.

Why are we usually unaware of visceral afferent information?

Because it is processed unconsciously to regulate visceral reflexes.

What do visceral reflexes regulate?

Functions such as:

• Heart rate

• Blood pressure

• Respiration

• Vascular resistance

When visceral sensation reaches consciousness, how is it usually perceived?When visceral sensation reaches consciousness, how is it usually perceived?

As pain.

Why is visceral pain difficult to localise?

Because visceral afferent fibres have fewer pain receptors and less precise sensory pathways than somatic nerves.

What stimuli can produce visceral pain? 6

• Sudden distension

• Spasm

• Strong contraction

• Chemical irritation

• Mechanical stimulation

• Ischaemia

What is ischaemia?

Inadequate blood supply to a tissue.

Why is ischaemia a common cause of visceral pain?

Because reduced blood supply lowers the threshold for pain receptors.

Does normal visceral activity usually produce conscious sensation?

No.

Only abnormal stimulation usually produces pain.

Which autonomic fibres do most visceral reflex fibres accompany?

Parasympathetic fibres.Parasympathetic fibres.

Which autonomic fibres do most visceral pain fibres accompany?

Sympathetic fibres.

Most visceral pain from the heart and abdominal organs travels with which division of the ANS?

Sympathetic fibres.

Somatic pain vs visceral pain

Somatic	Visceral
Well localised	Poorly localised
Skin, muscles, joints	Internal organs

Visceral reflex fibres vs visceral pain fibres

Reflex	Pain
Mainly parasympathetic	Mainly sympathetic

Sympathetic vs parasympathetic overall roles

Sympathetic	Parasympathetic
Fight or flight	Rest and digest
Catabolic	Anabolic

What are the three divisions of the visceral motor system?

• Sympathetic

• Parasympathetic

• Enteric

Summarise the enteric nervous system.

• “Second brain”

• Two plexuses:

  • Myenteric

  • Submucosal

• Controls GI motility and secretion

• Functions independently

• Modulated by sympathetic and parasympathetic systems

• Where is the myenteric plexus?

• Where is the submucosal plexus?

• Which one controls muscle?

• Which one controls the mucosa?

Summarise the organisation of the nervous system.