Autonomic Nervous System (Physiology)

Question 1

Describe the organisation of neurones associated with sympathetic and parasympathetic nerves (i.e. what the terms pre- and postganglionic neurones mean)

Answer 1

• Sympathetic and parasympathetic nerves reach the tissues they innervate via two neurones.
• The junction between these two neurones is called a ganglion.
• The preganglionic neurones carry the signal towards the ganglion and the postganglionic neurones carry the signal away from the ganglion.

Question 2

Explain why autonomic nerves generally have opposing effects on the tissues they innervate

Answer 2

• The neurotransmitter released by both types of preganglionic neurones is acetylcholine.
• The sympathetic postganglionic neurones release noradrenaline whereas the parasympathetic postganglionic neurones release acetylcholine. This makes it clear why these nerves generally have opposing effects on their target tissues.
• Sympathetic nerves are referred to as noradrenergic and and parasympathetic nerves as cholinergic.
• Noradrenaline activates adrenergic receptors, these may be of type alpha or beta.
• Acetylcholine can act via nicotinic and muscarinic receptor types.
• The sympathetic nervous system often responds by mass discharge known as the fight or flight response. The parasympathetic nervous system usually causes specific localised responses.

Question 3

List the various tissues that are under autonomic control and describe how their activity is modified by both types of nerve

Answer 3

The eye

• Sympathetic: contraction of radial muscle (dilate pupil), no effect on circular muscle, relaxed ciliary muscles (thin lens).
• Parasympathetic: no effect on radial muscle, contraction of circular muscle (constrict pupil), contracted ciliary muscles (fat lens).

Integumentary system
- Sympathetic: sweating, hair raising.

Cardiovascular system

• Sympathetic: increase in heart rate and force of contraction, increased cardiac output, blood vessels of skin constrict, blood vessels of most viscera constrict, blood vessels of skeletal muscle dilate, mean blood pressure increases.
• Parasympathetic: decrease in heart rate and force of contraction, decreased cardiac output, blood vessels of skin dilate, blood vessels of most viscera dilate, no effect on blood vessels of skeletal muscle, mean blood pressure decreases.

Respiratory system

• Sympathetic: bronchi and bronchioles dilate.
• Parasympathetic: bronchi and bronchioles constrict.

Digestive system

• Sympathetic: decreases GI motility and secretion e.g. thick saliva.
• Parasympathetic: increases GI motility and secretion e.g. thin saliva.

Urinary system

• Sympathetic: no effect on detrusor muscle, contracts the internal sphincter, prevents urination.
• Parasympathetic: contraction of detrusor muscle, relaxation internal sphincter, causes urination.

Question 4

Explain the concept of autonomic tone

Answer 4

Each type of nerve innervating a particular organ or tissue exhibits a background level of activity called tone. The action of the tissue depends on which type of nerve exhibits greater activity.

Question 5

Explain the link between the sympathetic nervous system and the adrenal gland

Answer 5

• For the sympathetic nerves of the adrenal medulla there are no ganglions or postganglionic neurones.
• This is because the cells of the adrenal medulla are modified neuronal cells releasing noradrenaline and adrenaline. Therefore cells of the adrenal medulla are essentially postganglionic sympathetic neurones.
• During a flight or fight response the hypothalamus stimulates the release of noradrenaline and adrenaline from the adrenal gland and they enter the blood stream.
• Since they are released at the same time as tissues are stimulated by sympathetic nerves, the tissues of the body are stimulated both directly by nerves and indirectly by circulating hormones during the fight or flight response.