Q1
LONG ANSWER
10M
★ High-yield
Describe the organisation of the autonomic nervous system. Compare the sympathetic and parasympathetic divisions and their actions on the various organs.
Definition
The autonomic nervous system is the part of the nervous system that controls the involuntary functions of viscera, smooth muscle, cardiac muscle and glands, so as to maintain homeostasis.
- It is a two-neurone efferent system — preganglionic and postganglionic — unlike the single motor neurone of the somatic system
Comparison of the Two Divisions
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Outflow | Thoracolumbar (T1–L2/L3) | Craniosacral — CN III, VII, IX, X and S2, S3, S4 |
| Preganglionic fibre | Short | Long |
| Postganglionic fibre | Long | Short |
| Ganglia | Paravertebral chain and prevertebral, near the cord | In or near the effector organ |
| Pre : post ratio | 1 : 20 or more | 1 : 1 to 1 : 3 |
| Nature of response | Diffuse, mass discharge | Discrete, localised |
| Postganglionic transmitter | Noradrenaline | Acetylcholine |
| Function | "Fight, flight, fright" — catabolic, prepares for emergency | "Rest and digest" — anabolic, conserves energy |
| Survival without it | Possible, but no stress tolerance | Not possible in the long term |
- The vagus alone carries about 75% of all parasympathetic fibres
- The wide divergence of sympathetic fibres explains why sympathetic responses are generalised, while parasympathetic effects can be organ-specific
Cranial Parasympathetic Outflow
| Nerve | Nucleus | Ganglion | Supplies |
|---|---|---|---|
| III (oculomotor) | Edinger–Westphal | Ciliary | Sphincter pupillae, ciliary muscle |
| VII (facial) | Superior salivatory | Submandibular, pterygopalatine | Submandibular and sublingual glands, lacrimal gland |
| IX (glossopharyngeal) | Inferior salivatory | Otic | Parotid gland |
| X (vagus) | Dorsal nucleus, nucleus ambiguus | In the organ wall | Heart, lungs, and gut up to two-thirds of the transverse colon |
- Sacral outflow (S2–S4, the pelvic splanchnic nerves) supplies the distal colon, rectum, bladder and genitalia
Actions on Individual Organs
| Organ | Sympathetic | Parasympathetic |
|---|---|---|
| Eye — pupil | Dilates (α1) | Constricts (M3) |
| Eye — ciliary muscle | Relaxes (distance vision) | Contracts (accommodation) |
| Heart | ↑ Rate, force, conduction (β1) | ↓ Rate and conduction (M2) |
| Bronchi | Dilate (β2) | Constrict + secretion |
| Blood vessels | Constrict (α1); dilate in skeletal muscle (β2) | No supply to most vessels |
| GI tract | ↓ Motility, sphincters contract | ↑ Motility and secretion, sphincters relax |
| Bladder | Detrusor relaxes, sphincter contracts (storage) | Detrusor contracts (micturition) |
| Sweat glands | Secretion — but by ACh on muscarinic receptors | No supply |
| Liver | Glycogenolysis | Glycogen synthesis |
| Genitalia | Ejaculation | Erection |
CLINICAL PEARL
Remember: erection is Point (Parasympathetic), ejaculation is Shoot (Sympathetic).
Exceptions Worth Knowing
- Sweat glands — sympathetic supply, but cholinergic and muscarinic. Hence atropine causes dry skin, and sympathectomy causes anhidrosis
- Adrenal medulla — supplied by preganglionic sympathetic fibres directly; it is a modified ganglion
- Most blood vessels have only sympathetic supply — vasodilatation is produced by reducing sympathetic tone
- Skeletal muscle vessels also carry sympathetic cholinergic vasodilator fibres, active in anticipation of exercise
Autonomic Reflexes
| Reflex | Afferent | Efferent | Effect |
|---|---|---|---|
| Baroreceptor | IX, X | Sympathetic and vagus | BP regulation |
| Chemoreceptor | IX, X | Sympathetic | ↑ Ventilation and BP |
| Bainbridge | Vagus (atrial stretch) | Sympathetic | ↑ Heart rate on ↑ venous return |
| Oculocardiac | Trigeminal (V) | Vagus | Pressure on the eyeball → bradycardia |
| Micturition | Pelvic nerve | Pelvic (parasympathetic) | Bladder emptying |
| Defaecation | Pelvic nerve | Pelvic | Rectal emptying |
- Mass reflex — in chronic spinal transection above T6, a noxious stimulus below the lesion triggers widespread sympathetic discharge with severe hypertension, sweating and flushing — autonomic dysreflexia, a medical emergency
Applied Aspects
- Autonomic tone — both divisions discharge continuously, so a single system can produce both increase and decrease
- Diabetic autonomic neuropathy → postural hypotension, resting tachycardia, gastroparesis, impotence, loss of hypoglycaemia awareness
- Denervation hypersensitivity — a denervated structure becomes supersensitive to the circulating transmitter, from receptor up-regulation
- Sympathectomy is used for hyperhidrosis and some vascular disorders
- Vasovagal syncope — sudden withdrawal of sympathetic tone with vagal overactivity → bradycardia, hypotension and fainting
All questions in Autonomic Nervous System & Integrative Physiology
- Q1Describe the organisation of the autonomic nervous system. Compare the sympathetic and parasympathetic divisions and their actions on the various organs. ★
- Q2Enumerate the autonomic neurotransmitters and their receptors. Describe the actions of each receptor type and the drugs acting on them. ★
- Q3Describe the regulation of body temperature. Explain the mechanisms of heat production and heat loss, and the pathogenesis of fever. ★
- Q4Describe the cardiovascular, respiratory and metabolic changes that occur during exercise. Add a note on the effects of training. ★
- Q5Classify memory and describe the mechanism of its formation. Describe the areas concerned with speech and the types of aphasia. ★
- SN1Describe Horner syndrome with its features and causes. ★
- SN2Define fever. Describe its mechanism, phases and effects. ★
- SN3Enumerate the components of the limbic system and describe its functions. ★
- SN4Write a short note on circadian rhythm. Add a note on melatonin.
- SN5Describe heat stroke and differentiate it from heat exhaustion. Add a note on acclimatisation to heat. ★
- SN6Write a short note on denervation hypersensitivity.
- SN7Enumerate the functional areas of the cerebral cortex with the effects of their lesions. ★