Download MTC42: control of smooth muscle 11/10/07

MTC42: CONTROL OF SMOOTH MUSCLE
11/10/07
Feedback loops control and regulate the action of the ANS, for example:
For vasoconstriction, the brain stem cardiovascular centre senses current blood pressure and produces a
sympathetic output to peripheral blood vessels to increase or decrease constriction and alter heart rate which alter
blood pressure. The new blood pressure is then sensed again and so on.
LEARNING OUTCOMES
Describe in outline the organisation of the autonomic nervous system
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The autonomic nervous system controls a wide range of metabolic, cardiopulmonary and other
visceral requirements of our body which continues whether we are asleep or awake
In most cases we are unaware of autonomic nervous activity within our bodies
The ANS has three divisions:
o Sympathetic – arising from the spinal cord (thoraco-lumbar)
o Parasympathetic – arising from the brain stem (cranio-sacral)
o Enteric – surrounding the gastrointestinal tract
Cell bodies of ANS preganglionic neurons are found in the brain stem and spinal cord (within the CNS)
and send axons (now part of the PNS) out to make synaptic contact with peripheral nerves
Preganglionic neurons are generally mylinated and meet postganglionic neurons (at the ganglion)
whose axons reach out to target organs and muscles
The sympathetic division has its ganglia located distant from the target organ; parasympathetic ganglia
are found close to the target
Preganglionic cells use acetyl choline (ACh) as a transmitter in both divisions
Most postganglionic cells in the sympathetic division use noradrenaline (norepinefrin) as a transmitter
whereas in the parasympathetic division ACh is used again
Parasympathetic postganglionic receptors can be either nicotinic (fast) using ligand-gated sodium
channels or muscarinic (slow) using potassium leak channels
The sympathetic division is associated with the fight or flight reaction whereas the parasympathetic
division deals with rest and digest
Provide examples of the effects of the autonomic nervous system on smooth muscle
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Varicosities in nerves transmit chemical signals to smooth muscle cells amongst the interstitial fluid
In the heart, sympathetic effects can increase rate and force of vascular contraction which increases
conduction velocity to constrict coronary vessels (alpha); parasympathetic effects have the opposite
effect and dilate coronary vessels
In the lungs, sympathetic effects relax bronchial muscle (beta 2) whereas parasympathetic effects
contract it
In the eye, sympathetic effects dilate the pupil and relax ciliary muscle whereas parasympathetic
effects have the opposite effects as well as stimulating lacrimation
α-receptors on smooth muscle raise intracellular calcium
β-receptors on smooth muscle decrease intracellular calcium