The role of the Sympathetic Nervous System in cardiovascular control

Question 1

What is present in the thoracic segments of the spinal cord?

Answer 1

Preganglionic sympathetic (visceral) motor neurons are present in the lateral (intermediolateral) horn of the grey matter

Question 2

Where do the visceral motor neurons send out axons to?

Answer 2

The visceral motor neurons send axons out of the ventral roots and through a ‘white communicating ramus’ nerve to end on postganglionic neurons in the (paravertebral) sympathetic ganglia, a series of ganglia anterior to the vertebrae.

Question 3

What connects the sympathetic ganglia together?

Answer 3

The sympathetic trunk

Question 4

Where do preganglionic axons travel before they synapse on the postganglionic neuron?

Answer 4

Preganglionic axons can travel in the sympathetic trunk to rostral or caudal ganglia before they synapse on the postganglionic neuron.

Question 5

What is the collective name for the sympathetic trunk and sympathetic ganglia?

Answer 5

Sympathetic chain

Question 6

Where does the sympathetic chain extend down to?

Answer 6

The sympathetic chain extends down to the second lumbar spinal root (L2)

Question 7

What can the axons of the postganglionic sympathetic nerves project out as?

Answer 7

The axons of the postganglionic sympathetic neurons project out either as separate nerves or as plexi around blood vessels to end on the smooth muscle of blood vessels.

Question 8

What do the thoracic sympathetic ganglia have the appearance of? What do they supply?

Answer 8

Beads on a string
They supply vasoconstrictor nerves to arterioles that supply oxygen to skeletal muscles in the chest, upper limbs and neck.

Question 9

Where are the sympathetic chains located?

Answer 9

One sympathetic chain is located on each side of the vertebral column: Each is made up of 25 pairs of ganglia joined by nerve trunks

Question 10

What does it mean if the sympathetic nervous system exhibits both divergence and convergence?

Answer 10

Divergence = pre-ganglionic fibres branch and connect to several postganglionic neurons at different levels of the chain
Convergence = a postganglionic neuron can receive synapses from many pre-ganglionic fibres
These two processes allow the sympathetic nervous system to have a co-ordinated action at many sites simultaneously

Question 11

What is located at the rostral end of the sympathetic chain?

Answer 11

At the rostral end of the sympathetic chain there are three cervical ganglia adjacent to cervical vertebrae. These cervical sympathetic ganglia are sometimes not or only thinly connected to the local cervical ventral roots but the preganglionic fibres rise up from C8, T1 and sometimes T2 spinal nerves.

Question 12

What is normally the largest of the cervical ganglia? What do these ganglia supply?

Answer 12

The largest of the cervical ganglia is normally the superior cervical ganglion.
These ganglia supply the sympathetic innervation of the head; this includes innervation of the iris of the eye, skin of the face and the salivary glands.

Question 13

What do the sympathetic postganglionic fibres for the head exist as? Where do they reach and what are their actions?

Answer 13

• The sympathetic postganglionic fibres for the head do not exist as separate nerves but form nerve plexuses around the carotid arteries and enter the skull together with the carotids. The fibres reach the eye, skin & salivary glands as plexuses around the smaller arteries supplying these tissues
  Sympathetic actions in the head are limited mainly to pupil dilation, changes in skin blood flow and reduction (thickening of saliva)

Question 14

Why does dilation of the pupil happen? (Clue: it does not help you see more clearly!)

Answer 14

• It appears that, like blushing, it is a non-verbal signal of sympathetic arousal.
  Sympathetic activity increases when appraising someone as a possible sexual partner.
  If the pupils of two individuals both dilate when they look at each other, this can be a sign of mutual sexual attraction, which may be perceived consciously or subconsciously.
  In fact, it appears that if someone’s pupils dilate when they look at you, your own pupils also dilate as a response.
  Positive feedback occurs and you both get dilated pupils as you “gaze into each other’s eyes”
  However, increased pupil size is an ambiguous signal, really just indicating arousal, as it also occurs (in you) when someone is threatening to attack you
  In this case it may trigger a ‘fight or flight’ response

Question 15

What type of axons do pre- and postganglionic neurons have?

Answer 15

Pre- have small myelinated (type Ab) axons

Post- have unmyelinated (type C) axons

Question 16

How many neurons are there in the sympathetic efferent system between spinal cord and smooth muscle? How many in the somatic?

Answer 16

Two

One in the somatic

Question 17

Via what does the sympathetic innervation of the heart occur? What about the lungs? And the oesophagus?

Answer 17

Via postganglionic branches from the upper thoracic sympathetic chain which form a plexus (nerve mesh) around the aorta and atria; the cardiac plexus.
Innervation of the lungs is via a similar plexus around the large pulmonary arteries; this is the pulmonary plexus.
There is also a plexus around the oesophagus, the oesophageal plexus.

Question 18

What happens in the final regions of the postganglionic nerves?

Answer 18

Postganglionic nerves do not have a single synaptic terminal but instead form a series of ‘varicosities’ (swellings) along their final regions.
These varicosities contain noradrenaline in synaptic vesicles which is released from the varicosities as the action potential passes over them.
This diffuses across a gap to (alpha) adrenoreceptors present on the vascular smooth muscle under the varicosities.
The adrenoreceptors bind the noradrenaline and cause the smooth muscle to contract and thus constrict the blood vessel.

Question 19

What happens to the sympathetic chain in the abdomen? What do they supply innervation to? What are their names and where do they arise?

Answer 19

The main sympathetic chain continues below the diaphragm but in addition there are ganglia which have migrated away from the spinal cord and form plexuses consisting of one large or several closely adjacent small ganglia and a mesh of pre and post-ganglionic neurons.
These are called prevertebral plexuses and ganglia.
They supply sympathetic innervation to the gut
The long preganglionic axons to the coeliac (celiac) and mesenteric plexi are called the greater and lesser splanchnic nerves, arising in the thorax at T5-T12

Question 20

What other ganglia are located in the abdomen? How are they grouped?

Answer 20

In the abdomen the sympathetic system contains a set of extra ganglia and plexuses in addition to those in the sympathetic chain.
They are grouped into the coeliac (celiac) ganglion & plexus, superior mesenteric ganglion and plexus, inferior mesenteric ganglion and plexus, and hypogastric plexus

Question 21

How are the abdominal prevertebral ganglia situated relative to each other?

Answer 21

The abdominal prevertebral sympathetic ganglia are close together & adjacent to the abdominal aorta.
Sometimes the ganglia from the two sides of the body fuse to form a single ganglion such as for the case of the superior mesenteric ganglion

Question 22

What is responsible for sympathetic supply to stomach, liver and pancreas? What about to the rest of the gut?

Answer 22

Celiac ganglion

Superior and inferior mesenteric ganglia

Question 23

Where do some fibres in the splanchic nerve go to?

Answer 23

Pass through celiac ganglion and travel to the adrenal gland located on top of the kidney

Question 24

What does acetylcholine released by the preganglionic fibres stimulate?

Answer 24

Stimulates chromaffin cells in the adrenal medulla to release adrenaline (epinephrine is USA name) into the capillaries which carry it into the venous blood.

Question 25

If a preganglionic neurone doesn’t stop at the ganglia, where might it go?

Answer 25

Travel to the adrenal gland (really a post-ganglionic neurone that didn’t grow an axon)
The adrenal gland secretes adrenaline into the capillaries of the gland
Blood carries this to all parts of the body

Question 26

What is the one exception to the rule that the sympathetic post-ganglionic fibres release noradrenaline?

Answer 26

The sweat glands
Sweating is triggered by sympathetic outflow, however, the post-ganglionic nerve fibres use acetylcholine, not noradrenaline, as their transmitter
Blood vessels supplying the sweat glands are spontaneously constricted at rest
The acetylcholine triggers nitric oxide release from the vascular endothelium
This relaxes and dilates the vessels and thus increases their blood flow; Sodium concentration is high in the extravascular space, and this causes water to pass out of the blood and into the glands to be secreted as sweat

Question 27

What can the sympathetic nervous system be considered? What does it enable the body to do?

Answer 27

The sympathetic nervous system can be considered the ‘accelerator pedal’ of the human engine; it enables the body to increase its PHYSICAL (skeletal muscular) ACTIVITY.
At the start of exercise, the sympathetic nerve centres in the brainstem stimulate the respiratory centres (which are also in the brainstem) and stimulate an increased rate and depth of breathing.

Question 28

What is the function of adrenaline?

Answer 28

Adrenaline, released into the blood by sympathetic stimulation of the chromaffin cells in the adrenal medulla, relaxes the smooth muscle around lung airways, which allows them to dilate, thus reducing airflow resistance and improving air delivery to the alveoli.
Sympathetic nerves to the heart increase the heart rate and adrenaline in the cardiac arteries increases the force of contraction of the heart muscle.
These two actions together increase the cardiac output by three or four times so more oxygenated blood can be delivered to the active muscles.
Adrenaline also stimulates the liver and non-active muscles to release glucose into the blood stream to provide energy for the active muscles.
It also stimulates lipolysis which releases free fatty acids into the blood: these can be used as fuel by the active muscles.

Question 29

What are released when exercise begins? What do these do? What is the result of their release?

Answer 29

When exercise begins chemical vasodilators* are released from the exercising muscle(s) and enter the local blood vessels. These factors dilate the blood vessels in the active muscles and thus increase the local blood flow so that more oxygen can be delivered.
However this vasodilation in the active muscles reduces total peripheral resistance and would, if not compensated for, cause a drop in blood pressure.
*For many years there was controversy over the nature of these local vasodilators. However it is now generally accepted that adenosine and nitric oxide, released from the endothelium lining the blood vessels, are the key exercise-induced vasodilator factors.

Question 30

What happens to sympathetic outflow at the start of exercise? What is the result of this?

Answer 30

Sympathetic outflow
This produces a general vasoconstriction in the non-exercising muscles and the digestive tract.
The result is that the decrease in vascular resistance in the exercising muscles is compensated by an increase in vascular resistance in the nonactive muscles: The net result is that a large part of the cardiac output is redirected to the active muscles because of their vasodilation but total peripheral resistance (and thus blood pressure) is still maintained.

Question 31

What happens to blood flow to the heart during exercise?

Answer 31

During exercise the blood flow through the coronary arteries of the heart increases as the cardiac muscle works harder and faster to increases blood flow to the exercising muscles. (There are no sympathetic vasoconstrictor actions in the heart)

Question 32

Why is blood flow to the brain similar during both rest and exercise?

Answer 32

Postganglionic sympathetic fibres travel into the brain in the carotid arteries and then along the small arteries and arterioles.
Some of them enter the eye to produce pupil dilation, some innervate the salivary glands, and others terminate on vascular smooth muscle in the brain.
However, as brain blood flow is constant, the functional significance of these latter fibres is not clear.
There are no alpha adrenoreceptors on cerebral vessels.
Overall brain blood flow during exercise does not change significantly, but the distribution of the blood to different areas of the brain may alter (e.g. increased flow to the motor and premotor cortices, decreased flow to less active regions)

Question 33

What are the names for the receptors for adrenaline and noradrenaline? What are the two main types?

Answer 33

Receptors for adrenaline and noradrenaline are called adrenoreceptors: there are two main types: alpha and beta

Question 34

What are the key features of alpha receptors?

Answer 34

o Noradrenaline acts mainly on alpha receptors
o There are two types of a-receptor: α1 and α2
o Alpha 1 is the main receptor on vascular smooth muscle. Acts to increase contraction.
o Alpha 2 receptors are found on presynaptic sympathetic nerve terminals & varicosities they act to reduce noradrenaline release by negative feedback; they reduce calcium inflow into the terminal during an action potential and thus reduce vesicle fusion with the synaptic membrane

Question 35

What are alpha 1 antagonists (e.g. prazosin) used for?

Answer 35

To reduce blood pressure

Question 36

What are alpha 2 agonists (e.g. clonidine) used for?

Answer 36

Reduce blood pressure by inhibiting noradrenaline release

Question 37

What are the key features of beta receptors?

Answer 37

o There are three types of beta-receptor: beta 1, beta 2, & beta 3.
o Adrenaline acts mainly on beta-receptors
o Beta 1 receptors found mainly in heart (and kidneys). Act to increase force & rate of contraction of myocardium
o Beta 2 receptors are found mainly in bronchial smooth muscle (but also in smooth muscle of gastrointestinal tract, liver, uterus). Act to relax the muscle
o Beta receptor blocking drugs are used to reduce heart rate & force in stressed or hypertensive patients. The original beta blocker was propranolol, but it blocked both beta 1 and beta 2 receptors, so was unsuitable for asthmatics. Modern beta blockers e.g. Bisoprolol are selective for beta 1 receptors.
o Selective Beta 2 agonist drugs are given to relax bronchial smooth muscle in asthmatics. Can be short acting (SABAs) e.g. salbutamol, fenoterol, or long acting (LABAs) e.g. clenbuterol
o Beta 3 receptors increase glucose release from liver and non-active muscle, and trigger lipolysis and gluconeogenesis in adipose tissue.

Question 38

Summarise the role of adrenoreceptors in exercise.

Answer 38

Both alpha and beta receptors are stimulated by sympathetic activity during exercise.
Alpha 1 receptors constrict vessels in the non-active muscles and gut and so redistribute the blood to the active exercising muscles.
Beta 1 receptors increase the rate and force of cardiac contraction and thus increase cardiac output, while beta 2 receptors relax bronchial smooth muscle to decrease airway resistance and increase oxygen uptake.
The result is a smoothly co-ordinated mechanism that increases the oxygen supply to the exercising muscles.
- Beta 3 receptors increase glucose release from the liver and non-active muscle and increase glucose uptake into the active muscle.
Thus the exercising muscle gets both more blood and more metabolic fuel.
Beta 3 receptors also increase lipolysis from adipose tissue to provide extra substrates like free fatty acids for the working muscles.

Question 39

Summarise the role of the sympathetic nervous system in dealing with haemorrhage.

Answer 39

The sympathetic system is vital for maintaining blood pressure after haemorrhage.
When haemorrhage occurs sympathetic activity contracts venous smooth muscle via alpha receptors.
Veins because of their large size act as reservoirs for blood.
Constricting the veins increases venous return which will compensate for blood lost by haemorrhage.
Thus the venous return to the heart is kept constant and thus the blood pressure can be maintained.
If the blood loss is severe, then arterial vasoconstriction of vessels in non-essential organs like the gut and skin reduces their blood flow and helps maintain blood pressure and perfusion of vital organs like heart, brain and kidney.
Renin release from the kidney helps maintain blood pressure by formation of angiotensin (see later lectures).
Stimulation of antidiuretic hormone (see later lectures) release will reduce urine flow and thus reduce fluid loss.
Adrenaline stimulates release of megakaryocytes and platelets from bone marrow; this increases the clotting ability of the blood to reduce blood loss.
Adrenaline also increases the oxygen carrying capacity of the blood by increasing the release of erythrocytes from bone marrow.

Question 40

How does bleeding differ from the capillaries, veins and arteries?

Answer 40

Capillary: slow, even flow of bright red blood
Venous: steady, slow flow of dark red blood
Arterial: spurting, pulsating flow of bright red blood