The sympathetic vasoconstrictor system: its control by the central nervous system

2021-05-27 02:44 PM

The vasomotor centre in the brain transmits parasympathetic signals via the X cord to the heart and sympathetic signals through the spinal cord and peripheral sympathetic fibres to virtually all the body's arteries, arterioles, and veins.

The function of local tissue control mechanisms plays a major role in regulating blood flow to tissues and organs of the body. Redistribution of blood flow to areas of the body, an increase or decrease in the pumping action of the heart, and a rapid correction of systemic arterial blood pressure.

The nervous system controls circulation mainly through the autonomic nervous system.

Sympathetic nerves carry many vasoconstrictors fibres and only a small number of vasodilator fibres. Vasoconstrictor fibres are distributed to all segments of the circulatory system but are more abundant in some tissues than in others. The sympathetic vasoconstrictor effect is particularly potent in the kidneys, intestines, spleen and skin but less effective in skeletal muscle and brain.

The vasomotor centre in the brain controls the vasoconstriction system

Distributed bilaterally mainly in the reticular substance of the medulla and lower than the third ventricle of the pons is called the vasomotor centre. This centre transmits parasympathetic signals via the X cord to the heart and sympathetic signals through the spinal cord and peripheral sympathetic fibres to virtually all the body's arteries, arterioles, and veins.

Figure. Anatomy of the sympathetic nervous system that controls circulation. Red dashed line, X channel carries parasympathetic signals to the heart.

Figure. Areas of the brain that play an important role in regulating the circulation. Dotted lines represent inhibition.

Although all the organs of the vasomotor centre are still unclear, conducting experiments can identify a few important areas of the centre:

1. The vasoconstrictor zone is distributed bilaterally in the anterior superior portion of the superior medulla. Nerve fibres originating in this region distribute their fibres to all levels of the spinal cord, where they stimulate the preganglionic vasoconstriction of the sympathetic system.

2. The vasodilatation zone is distributed bilaterally in the anterior and lateral part of the lower half of the ... neurons from this region go up to the vasoconstrictor zone and inhibit the activity of this region thereby causing vasodilation.

3. A sensory region is distributed bilaterally in a solitary nucleus accumbent in the medulla oblongata and inferior to the pons. The neurons of this area receive sensory nerve signals from the circulatory system via the X and IX nerves, and the output from the sensory area then helps control the activity of the vasoconstrictor and vasodilating regions of the central nervous system. vasomotor centre, thus creating a reflex that controls many circulatory functions. For example, reflex barotrauma to control arterial blood pressure.

Continuous partial constriction of normal blood vessels caused by sympathetic vasoconstrictor tone

Under normal conditions, the vasoconstrictor zone of the vasomotor centre continuously transmits signals to sympathetic vasoconstrictor fibres throughout the body at a rate of about 1.5 to 2 impulses per second. This continuous signal is called the sympathetic vasoconstrictor tone. This signal normally maintains the state of constriction of blood vessels, known as vasomotor tone.

Figure. Effect of general spinal anaesthesia on arterial pressure, showing a marked decrease in pressure due to loss of “vasomotor tone”.

The figure explains the importance of vasoconstrictor tone. Diagram of experimental monitoring of animals under general anaesthesia. Anaesthesia causes a blockade of sympathetic signalling from the spinal cord to the periphery. The result is a drop in blood pressure of 100 to 50 mmHg, demonstrating a loss of vasoconstrictor tone throughout the body. A few minutes later, a small amount of the hormone norepinephrine is injected into the bloodstream (norepinephrine is the predominant vasoconstrictor hormone at the end of the sympathetic vasoconstrictor). This injected hormone is transported into the bloodstream to the blood vessels, the vessels constrict, and the arterial blood pressure rises to a greater-than-normal threshold for 1 to 3 minutes, until norepinephrine is destroyed.

Regulation of cardiac activity by the vasomotor centre

The vasomotor centre regulates the amount of vasoconstriction while controlling cardiac activity. The lateral region of the vasomotor centre transmits excitatory signals via sympathetic fibres to the heart when it needs increased rate and muscle contraction. Conversely, when it needs to reduce cardiac pumping, the medial region of the vasomotor centre sends signals to the region adjacent to the tonic motor nucleus of the X cord, when parasympathetic signals through the X cord to the heart decrease the heart rate. heart and contraction of the heart. Thus, the vasomotor centre not only increases but also decreases the activity of the heart. Normally, heart rate and myocardial contractility are increased when vasoconstriction is present and decreased when vasoconstriction is inhibited.

Control of vasomotor centre by a higher nerve centre

A large number of small neurons distributed throughout the reticular substance of the pons, midbrain, and medial brain can both stimulate and inhibit the vasomotor centre. In the reticular, normal neurons superior to and lateral to the reticular cause excitation, while the medial and anterior regions cause inhibition.

The hypothalamus plays an important role in the control of the vasoconstrictor system because it can cause strong inhibition or stimulation of the vasomotor centre. The posterior lateral region of the hypothalamus is primarily excitatory, while the anterior region may cause mild or inhibitory inhibition, depending on the specific region stimulated.

Many regions of the cerebral cortex can stimulate or inhibit the vasomotor centre. Motor cortex stimulation causes vasoconstrictor centre stimulation because the signal is transmitted to the hypothalamus and then to the vasomotor centre. In addition, stimulation of the anterior temporal lobe, orbital area of ​​the frontal cortex, anterior part of the cingulate gyrus, amygdala, septum, and hippocampus can stimulate or inhibit the vasomotor centre, depending on the specific region of the brain. stimulated area and intensity of stimulation. As such, there are many regions of the brain that affect the cardiovascular system.

Norepinephrine is a neurotransmitter of sympathetic vasoconstriction

The substance secreted at the end of vasoconstrictor fibres is mainly norepinephrine, which acts directly on the alpha receptors of the vascular smooth muscle to cause vasoconstriction.

Adrenal medulla and its relationship to the sympathetic vasoconstrictor system

Sympathetic signals are transmitted to the adrenal medulla at the same time as the blood vessels. This signal causes the adrenal medulla to secrete both epinephrine and norepinephrine into the bloodstream and be transported throughout the body, acting directly on all blood vessels causing vasodilation. In some tissue’s epinephrine causes vasoconstriction because it acts on beta receptors, causing dilation rather than vasoconstriction.

Sympathetic vasodilation is controlled by the central nervous system

Sympathetic fibres to skeletal muscle carry sympathetic vasodilators along with vasoconstrictor fibres. In some animals, such as the cat, vasodilator fibres release acetylcholine at the terminal crest, although in humans the vasodilator effect is thought to be due to epinephrine stimulation of beta receptors in muscle vessels.

The pathway that controls the vasodilatation of the central nervous system is shown by the dashed line in the figure. The main area of ​​the brain that controls this system is the anterior hypothalamus.

The role of the sympathetic vasodilator system

Sympathetic vasodilatation does not play a major role in the regulation of circulation in humans because a complete blockade of sympathetic nerves to stressed muscles affects the muscle's ability to regulate blood flow in many physiological states. Several experiments have shown that when exercise is initiated, the sympathetic system can cause initial vasodilation in skeletal muscle allowing an increase in blood flow even before the muscle needs increased nutrition. Evidence in humans indicates that the sympathetic vasodilating response in skeletal muscle may be immediate by circulating epinephrine, which stimulates beta receptors, or by NO release from vascular endothelial cells in response to the response. response to acetylcholine.

Emotional Syncope and Vasovagal Syncope

A notable vasodilating reaction occurs in over-excited people causing syncope. In this case, the muscular vasodilatation system is activated while the cardiac inhibitory centre of the X wire transmits a strong signal to the heart, causing a marked slowing of the heart rate. Arterial blood pressure drops rapidly, reducing blood flow to the brain causing loss of consciousness. All of these effects are called vasovagal syncope. Emotional syncope begins with a disturbance in the cerebral cortex. This pathway is in order from the vasodilating centre of the anterior hypothalamus to the X cord centre of the medulla, to the heart via the X cord, and through the spinal cord to the sympathetic vasodilating nerve of the muscle.