The ANS and CVS

Question 1

Describe the critical anatomic features of the ANS, such as ganglia, rami and pre/ post ganglions.

Answer 1

The autonomic nervous system is an efferent pathway. It has a preganglionic nerve, connected to the CNS, and a postganglionic nerve, connected to an effector tissue via a neuro effector junction.

Question 2

Describe key sympathetic and parasympathetic branches of the ANS, including where preganglionic fibres leave the CNS, the location of ganglia and the relative length of the fibres

Answer 2

• The parasympathetic system has fibres predominantly coming from the cranial nerves or from the sacral region, forming the cranio-sacral outflow. They tend to have long preganglionic neurones and short post ganglionic neurones. It innervates far fewer structures than the sympathetic system
• The sympathetic system has fibres coming predominantly from the thoracic and lumbar regions, known as the thoraco-lumbar outflow. Preganglionic fibres tend to be short and postganglionic fibres tend to be long. There is an associated paravertebral chain of ganglia running along the vertebrae here.

Question 3

Name the chemical transmitters at the pre and post synapses, for both sympathetic and parasympathetic fibres. State also the type of receptor upon which it normally acts.

Answer 3

Preganglionic fibres of both the sympathetic and parasympathetic branches release the neurotransmitter acetylcholine. This binds to ligand gated nicotinic receptors on the postganglionic nerve cell.

Most postganglionic parasympathetic nerve terminals release acetylcholine, whilst most of the postganglionic sympathetic nerve terminals release noradrenaline. Noradrenaline acts upon adrenoreceptors, whilst acetylcholine acts upon muscarinic receptors.

Question 4

Name the usual chemical transmitters released from post-ganglionic neurones of the sympathetic system and the types of receptor upon which it normally acts.

Answer 4

Acetylcholine – muscarinic receptors. Noradrenaline – adrenoreceptors. Both are GPCRs.

Question 5

State the distribution of different types of adrenoreceptors around the body

Answer 5

Broadly speaking, alpha adrenoreceptors are found within the smooth muscle of the vascular system, whilst beta adrenoreceptors are found within the heart, smooth muscle of the airways of the lung, adipose tissue and some blood vessels – particularly in skeletal muscle.

Question 6

State the action of the sympathetic nervous system on blood vessels in different organs

Answer 6

The sympathetic nervous system innervates the smooth muscle in the walls of arteries, arterioles and veins. Sympathetic activity causes constriction of arterioles (vasoconstriction) by the action of alpha 1 adrenoceptors. There is constant activity in the sympathetic nervous system – known as sympathetic vasomotor tone

Question 7

Explain the action of the sympathetic and parasympathetic system upon heart rate and force of ventricular contraction.

Answer 7

• Parasympathetic innervates few and specific vascular tissue, such as erectile tissue where it facilitates engorgement, and gut tissue, where it stimulates activity and thus the production of vasodilator metabolites
• In the heart, parasympathetic activity slows down the heart rate (acetylcholine - M2 muscarinic receptors) whilst sympathetic activity speeds up the heart rate (noradrenaline - beta 1 adrenoceptors) There is basal activity of both systems, however if you were to block the parasympathetic drive then heart rate would reach 100bpm, the intrinsic pacemaker cell rhythm.
• The sympathetic system also innervates ventricular myocytes where they increase the force of contraction of heart muscle. Adrenaline from the adrenal medulla can also increase heart rate. All in all, the ANS provides the CVS with the means to control: total peripheral resistance, distribution of blood flow and cardiac output.