Neuro 7 & 8 – The Autonomic Nervous System
Anil Chopra
- Describe the sympathetic and parasympathetic pathways and their central/spinal connections.
Autonomic nervous system split into sympathetic and parasympathetic nervous systems.
Sympathetic Nervous System
Principally involved with “fight or flight”
Regulates a number of bodily functions.
Has short preganglionic fibres, and long postganglionic fibres.
Have ganglia in the sympathetic trunks wither side of the spinal cord.
Preganglionic fibres emerge from thoracic and lumbar parts of the spinal cord.
It also innervates the adrenal medulla directly and the adrenal medulla releases its products directly into the bloodstream.
Parasympathetic Nervous System
Principally involved with “rest and digest”.
Has long preganglionic fibres and short postganglionic fibres.
Ganglia usually in effector organ.
Fibres emerge from cranial and sacral parts of the CNS.
2, 5 & 6. Identify the neurotransmitter substances released at different levels within the autonomic nervous system and describe the steps involved in their biosynthesis and metabolism. Classify the cholinoreceptors found within the autonomic nervous system and identify the principal loci of the nicotinic receptors and the muscarinic cholinoreceptors. Identify the principal loci of the adrenoreceptors. Classify them as α and β sub-classes and note that they are G-protein coupled.
Chemical Transmission
Parasympathetic
- both preganglionic fibres and postganglionic fibres use Acetylcholine Ach.
Sympathetic
-preganglionic fibres use acetylcholine Ach and postganglionic fibres use noradrenaline NA.
-exception is the sweat gland which uses acetylcholine in the postganglionic fibre.
-Adrenal medulla is also special in that synaptic pre-ganglionic fibres innervate the chromaffine cells of the adrenal medulla to secrete catecholamines to such as adrenaline and noradrenaline.
- Describe the influence of the sympathetic and parasympathetic nervous systems on the principal systems/organs of the body and understand the dual innervation and autonomic tone.
Cardiovascular System
Cardiac output = mean arterial blood pressure ÷ total peripheral resistance
Total Peripheral Resistance
TPR changes because of changes in sympathetic tone to the blood vessels. Increasing sympathetic tone results in vasoconstriction which increases TPR. Decreasing sympathetic tone results in vasodilation which decreases TPR.
Cardiac Output
Sympathetic activity increases the heart rate and force of contraction. Parasympathetic activity decreases heart rate and force of contraction.
As the sympathetic and parasympathetic both can affect the SAN but with opposite effects, it is an example of dual innervation.
Both changes in the cardiac output and peripheral resistance will change the mean arterial blood pressure.
Vasodilation can also be due to:
-blood vessels which have cholinergenic fibres or adrenergenic β receptors (skeletal muscle)
-vasodilators such as CO2 [H+], nitric oxide, histamine.
-Decrease of sympathetic tone.
-Parasympathetic stimulation of the penis.
Postural Change
When an upright posture is suddenly gained:
Gastrointestinal Tract
Sympathetic activity decreases motility and tone and causes sphincters to contract. General secretion is decreased.
Parasympathetic activity increases motility and tone and causes sphincters to relax. General secretion is increased.
Also has dual innervation.
Bladder
Mainly under sympathetic activity. When pressure builds up in the bladder, the pelvic nerve stimulates the detrussor to contract.
This causes a decrease in sympathetic activity which results in the internal sphincter relaxing.
The external sphincter only relaxes under voluntary control.
Lungs
Sympathetic activity causes bronchi to dilate and so oxygen delivery to the lungs is increased.
Penis
Sympathetic stimulation causes vasoconstriction and ejaculation.
Parasympathetic stimulation causes vasodilation and erection.
(Point and shoot)
Eye
Sympathetic activity results in relaxation of the ciliary muscle do the lens gets longer and thinner.
Also results in contraction of the radial muscle so the pupil is wider (dilated).
Parasympathetic activity results in the contraction of ciliary muscle so the lens gets shorter and thicker for near vision.
Also results in papillary light reflux:
8. Describe the main abnormalities in autonomic failure differentiating between localised and generalised disorders.
Postural Hypotension
Drop in systolic blood pressure 20mmHg
Diastolic blood pressure 10mmHg
when standing (60° head up tilt)
Symptoms include:
-dizziness
-visual disturbances
-loss of consciousness
-cognitive defects
-muscle ache (coat hanger ache)
-oliguria (decreased urine production)
-weakness, fatigue
-fainting (syncope)
-joint hypermobility
This is because of a reduced sympathetic response to the lowering of blood pressure when standing up suddenly.