4.0 Nervous System 2: Autonomic Nervous System

Question 1

Identify the different parts of the autonomic nervous system (ANS) and the reason for their classifications.

Answer 1

The autonomic nervous system ( ANS ) is part of the peripheral nervous system and has both sensory (afferent) and motor (efferent) components.

Its functions are largely below the level of consciousness such as regulation of cardiac function, digestion, respiration, salivation, sweating, pupil diameter, urination, and sexual arousal. The system regulating gut function is often referred to as the enteric nervous system and not everyone includes this as a part of the ANS.

The ANS is classically separated into two divisions,

sympathetic nervous system (SNS)
- arises from Thoraco-lumbar region of spinal cord
- Preganglionic arise in the central nervous system and synapse with a postganglionic neuron that supplies the target organ.
- Preganglionic: short
- Postganglionic: long

parasympathetic nervous system (PNS)
- arises from Cranial and sacral region of spinal cord
- Preganglionic arise in the central nervous system and synapse with a postganglionic neuron that supplies the target organ.
- Preganglionic: long
- Postganglionic: short

Question 2

Describe the sympathetic and parasympathetic division, and how they affect each organ.

Answer 2

sympathetic and parasympathetic responses and receptors
(see table in OFC anat folder - make copy of this )

Question 3

Recall the neurotransmitters, receptor class and receptor subtypes involved in the sympathetic and parasympathetic divisions of the nervous system.

Answer 3

SYMPATHETIC
As a general rule, the neurotransmitter released from the preganglionic neurons is acetylcholine (ACh) in BOTH the sympathetic and parasympathetic nervous system (see left) and binds to nicotinic ACh receptors on the post-ganglionic cell in the autonomic ganglia to activate the postganglionic neuron.

The post-ganglionic sympathetic neurotransmitter in the sympathetic nervous system is noradrenaline (NAd) (also known as norepinepherine).

An exception to the above is that postganglionic ACh is released at sweat glands, and the fibres are thus called sympathetic cholinergic fibres.

PARASYMPATHETIC
As in the sympathetic division, the parasympathetic pre-ganglionic neurons release ACh onto nicotinic ACh receptors (nAChR) at the autonomic ganglia.

The postganglionic neuron is thus activated, with the neurotransmitter released at the end terminal also being ACh.

Question 4

Describe the enteric division and the mechanisms by which it works.

Answer 4

The enteric nervous system
- is found in the wall of the gastrointestinal tract, from the oesophagus to the distal colon.
- includes afferent and efferent neurons, and interneurons.

These neurons are collected into two plexuses: the myenteric (Auerbach’s), found between the longitudinal and circular smooth muscle layers, and submucosal (Meissner’s), found in the submucosa. These plexuses receive sensory input from receptors in the wall of the gut that respond to mechanical, thermal, osmotic, and chemical stimuli. Local reflexes adjust gut secretion, such as gastric acid, and motility in response to these stimuli.

These plexuses also receive preganglionic parasympathetic fibers and postganglionic sympathetic fibers and are thus influenced by factors outside of the gastrointestinal tract. The parasympathetic and sympathetic postganglionic neurotransmitters are ACh and NAd, respectively.