Abstract
The autonomic nervous system (ANS) is the portion of the nervous system that innervates smooth muscle and glands, thus influencing the function of internal organs that regulate heart rate (HR), arterial blood pressure, digestion, micturition, defecation, sweating and sexual function.
What is the autonomic nervous system?
The autonomic nervous system (ANS) is the portion of the nervous system that innervates smooth muscle and glands, thus influencing the function of internal organs that regulate heart rate (HR), arterial blood pressure, digestion, micturition, defecation, sweating and sexual function.
Describe the two divisions of the ANS
The ANS is subdivided into two separate nervous systems – sympathetic and parasympathetic – which usually have antagonistic effects. Most viscera are innervated by both divisions of the ANS. Some authorities separately identify the enteric nervous system as a distinct ANS subdivision. The ANS comes under the control of the hypothalamus (see Chapter 80).
The function of the sympathetic nervous system is often summarised by fight, flight or fright. Its role is concerned with preparing the body for stressful situations: increasing cardiac output (by increasing HR, stroke volume and myocardial contractility), vasoconstriction, venoconstriction, mobilising glucose stores and pupillary dilatation.
The function of the parasympathetic nervous system is often summarised by rest and digest. It carries out the basic functions required for life, including decreasing HR, salivation, stimulating peristalsis in the gut, urination and pupillary constriction.
Name some of the effects of sympathetic and parasympathetic nervous system activity on the viscera
The effects of sympathetic and parasympathetic activity on the viscera are perhaps best described in a table (see Table 59.1).
Table 59.1 Comparison of sympathetic and parasympathetic effects on viscera.
| Organ | Sympathetic nervous stimulation | Parasympathetic nervous stimulation |
|---|---|---|
| Heart | Increased HR, increased contractility | Reduced HR3 |
| Lung | Bronchodilatation | Bronchoconstriction, increased mucus production |
| Pupils | Pupillary dilatation | Pupillary constriction |
| Salivary glands | Inhibition of salivation | Stimulation of salivation |
| Arterioles | Vasoconstriction | No effect4 |
| Sweat glands | Activates sweating | No effect |
| Adrenal gland | Release of adrenaline and noradrenaline | No effect |
| Gastrointestinal tract | Inhibits peristalsis | Stimulates peristalsis |
| Bladder | Relaxes bladder | Contracts bladder |
| Penis | Ejaculation | Erection |
Describe the anatomy of the ANS in more detail
Irrespective of the ANS subdivision, action potentials reach the viscera through two sets of neurons: pre-ganglionic and post-ganglionic. The sympathetic and parasympathetic nervous systems differ in the length of these neurons, the location of their synapses (ganglia) and the neurotransmitter utilised (Figure 59.1).
Sympathetic nervous system:
– Sympathetic pre-ganglionic neurons originate from the lateral horn of the spinal cord between T1 and L2/3, the so-called thoracolumbar outflow of the ANS.
– Pre-ganglionic nerves emerge from the spinal cord with the spinal nerves in white rami communicantes,1 but separate shortly afterwards to form either paravertebral or prevertebral ganglia. The paravertebral ganglia form the sympathetic chain; important named prevertebral ganglia are the coeliac, the superior mesenteric and the inferior mesenteric (Figure 59.1).
– The sympathetic chain is often divided into four parts:
▪ Cervical, which supplies the head and neck.
▪ Thoracic: the upper thoracic sympathetic chain (T1–5) supplies the heart, lungs and aorta, whilst the lower thoracic sympathetic chain (T6–12) supplies the foregut and midgut viscera.
▪ Lumbar (or abdominal), which supplies the hindgut viscera.
▪ Sacral (or pelvic), which supplies the pelvic viscera.
– In the ganglia, pre-ganglionic neurons synapse with post-ganglionic neurons. The neurotransmitter at this synapse is acetylcholine (ACh), which acts upon nicotinic postsynaptic membrane receptors.
– Post-ganglionic neurons leave the ganglia through grey rami communicantes2 and travel within peripheral nerves to synapse with their target organ. The neurotransmitter at this synapse is noradrenaline, which acts upon post-ganglionic adrenergic receptors.
