SN7
SHORT NOTE
5M
Compare the sympathetic and parasympathetic nervous systems. Add a note on Horner syndrome.
Definition
The autonomic nervous system is the part of the nervous system that controls the viscera, glands and smooth muscle, functioning largely without conscious control.
Comparison of the Two Divisions
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Outflow | Thoracolumbar — T1 to L2 | Craniosacral — III, VII, IX, X and S2, S3, S4 |
| Ganglia | Near the cord — paravertebral chain and prevertebral | Near or IN the organ |
| Preganglionic fibre | Short | Long |
| Postganglionic fibre | Long | Short |
| Ratio of pre- to postganglionic | 1 : 20 or more — diffuse effect | 1 : 1 or 1 : 3 — discrete effect |
| Preganglionic transmitter | Acetylcholine | Acetylcholine |
| Postganglionic transmitter | Noradrenaline (except sweat glands, which are cholinergic) | Acetylcholine |
| Overall role | "Fight, fright and flight" | "Rest and digest" |
The Sympathetic Chain
- Extends from the base of the skull to the coccyx, where the two chains join at the ganglion impar
- Cervical — three ganglia: superior, middle and inferior (often fused with the first thoracic as the stellate ganglion)
- Thoracic — 11 or 12; lumbar — 4; sacral — 4 or 5
- White rami communicantes (myelinated, preganglionic) exist only from T1 to L2; grey rami (unmyelinated, postganglionic) join every spinal nerve
- Splanchnic nerves are preganglionic fibres that pass through the chain without relay, to the prevertebral ganglia
Chief Actions
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Eye | Dilator pupillae — mydriasis | Sphincter pupillae — miosis; accommodation |
| Heart | Rate and force | Rate |
| Bronchi | Dilate | Constrict |
| Gut | Motility; sphincters contract | Motility; sphincters relax |
| Bladder | Detrusor relaxes; internal sphincter contracts | Detrusor contracts; sphincter relaxes |
| Blood vessels | Vasoconstriction (skin, gut); vasodilatation in muscle | Little effect |
| Sweat glands | Secretion — cholinergic sympathetic | No supply |
Applied Aspects
- HORNER syndrome — interruption of the sympathetic supply to the head: partial ptosis, miosis, anhidrosis and apparent enophthalmos. Causes include a Pancoast tumour of the lung apex, carotid dissection, a cervical rib, and brainstem stroke
- Referred visceral pain follows the sympathetic afferents to the segments of the cord — cardiac pain to T1–T5, appendix to T10, ureter to T11–L2
- Hirschsprung disease — absence of the parasympathetic ganglion cells of the myenteric plexus; the aganglionic segment is contracted and the bowel above it dilates
- Autonomic neuropathy in diabetes causes postural hypotension, gastroparesis, impotence and painless myocardial infarction
- Cervical sympathectomy was used for hyperhidrosis and Raynaud disease; the stellate ganglion is spared where possible, or Horner syndrome results
All questions in Neuroanatomy
- Q1Describe the external features and internal structure of the spinal cord. Describe its ascending and descending tracts and add a note on its lesions. ★
- Q2Describe the functional areas of the cerebral cortex and the blood supply of the cerebrum. Add a note on aphasia. ★
- Q3Describe the cerebellum under divisions, peduncles, connections and functions. Add a note on the signs of cerebellar disease. ★
- Q4Describe the parts of the brainstem and the arrangement of the cranial nerve nuclei. Add a note on the brainstem syndromes. ★
- Q5Describe the meninges and the ventricular system. Explain the formation, circulation and absorption of cerebrospinal fluid and add a note on hydrocephalus. ★
- SN1Describe the internal capsule with its parts, contents and blood supply. Add a note on capsular stroke. ★
- SN2Describe the circle of Willis with its formation and significance. Add a note on berry aneurysm. ★
- SN3Describe the basal ganglia with their connections and functions. Add a note on their disorders. ★
- SN4Describe the thalamus with its nuclei and connections. Add a note on the thalamic syndrome. ★
- SN5Describe the visual pathway and the effects of lesions at different levels. Add a note on the light reflex. ★
- SN6Describe the hypothalamus with its nuclei, connections and functions. ★
- SN7Compare the sympathetic and parasympathetic nervous systems. Add a note on Horner syndrome.