Central Nervous System
Sympathetic Nervous System
Fig 1 below shows sympathetic nerves emerging from spinal cord segments T1 to L2/3 (Note: there are a total of 12 thoracic segments and 5 lumbar segments). A chain of ganglia, the paravertebral chain, runs parallel to the vertebral column on either side.
In Fig 1, ganglia are represented as circular structures.
In general, there is one ganglion for each spinal root, not just the thoracic and lumbar segments but the cervical and sacral regions as well. In some cases, the ganglia are fused. Thus, for example, the sympathetic supply to the head and neck arises from the superior cervical ganglion which is formed by fusion of the ganglia equivalent to cervical segments 1 to 4. However, the preganglionic supply to the cervical ganglia arises from spinal cord segments T1 and T2 and travels in the paravertebral chain to reach the cervical ganglia.
Some organs, such as the heart, have both a sympathetic and a parasympathetic nerve supply.
In the peripheral circulation, the autonomic nerve supply to blood vessels is almost entirely sympathetic. Therefore, the generalisation that sympathetic and parasympathetic nervous systems have equivalent and opposite effects on tisses is not tenable.
Motor Functions of the SNS
Characteristics of SNS mediated stress response include:
- An increase in heart rate
- Vasoconstriction in tissues including the kidneys, gastrointestinal tract, skin and non-exercising skeletal muscle
- Decrease in insulin secretion
- Increase in blood glucose concentration as a result of glycogenolysis, particularly in the liver
- Dilation of the airways in the lung
- Reduction in gastrointestinal smooth muscle motility and a reduction in digestive juice secretion
- Increase in sweating
- Dilation of the pupils of the eyes
- Increase in catecholamine secretion from the adrenal medulla (suprarenal medulla)
- Contraction of the spleen
- Reduction in blood clotting time
- Piloerection – hair follicles erect
Sensory Function of SNS
It is common practice to think primarily of the motor functions of the autonomic NS. A clear example is in the regulation of heart rate.
Both parasympathetic and sympathetic nerves also have important sensory (afferent) roles carrying information regarding visceral function back to the brain.
Examples of the sensory roles for the ANS include the arterial baroreceptors. The locations of the carotid sinus and the aortic arch baroreceptors are shown in the figure above. A baroreceptor consists of modified autonomic nerve endings which are sensitive to stretch.
The largest concentration of autonomic afferent fibres is found in the vagus nerve. This nerve trunk carries information from stretch receptors in many hollow structures such as blood vessels, heart, gastrointestinal tract and lungs. In addition, blood gas information deriving from chemoreceptors and blood [glucose] data is carried in the vagus. All this information enters the brain at the level of the medulla.Sensory (afferent) nerves in the vagus trunk outnumber the motor (efferent) nerves by 9:1. Many visceral nociceptive (pain) receptors are connected to the sympathetic nervous system.