Transcript The Autonomic Nervous System

Lecture
The Autonomic Nervous System
The Autonomic Reflex:
It is a system that designate both Visceral and Somatic neurons
that serve as afferent pathways, making the Autonomic Reflex.
Review:
Components of the Autonomic Reflex
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Sensory inflow of neurons from the viscera (afferent
pathway)
Higher autonomic centres in the cerebral cortex,
thalamus, hypothalamus and the medulla of the
brain
Lower autonomic centres in the brainstem and spinal
cord.
Motor outflow of 2 neurons chain made of
preganglionic and postganglionic parts ending in
effectors organs (Efferent pathways).
Autonomic Centres
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Those centres are in the brain and spinal
cord receive sensory inflow from the viscera.
The sensory neurons convey information
about events in the viscera (visceral
afferent), which are less known with respect
to their exact pathway.
Both visceral and somatic afferents serve as
afferent pathways for autonomic reflexes.
The effects of sympathetic stimulation:
Stress state
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The sympathetic is activated during conditions of
stress and emergencies: e.g. fear, anxiety,
hypoglycemia etc.
These conditions need to be prepared for “fight or
flight”.
The effect of sympathetic is to promote mechanisms
which increase energy metabolism and the efficiency
of body supply systems ( e.g. circulatory and
respiratory systems).
The two organs representing the sympathetic effects
are:
The eye:
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The effects of the sympathetic on the eyeball
and vision is seen in the eye structures:
► Iris
dilation of the pupil.
► Ciliary muscle relaxation (far vision).
►Lacremal glands none
The heart
The effects of the sympathetic on the heart muscle
are:
► S.A. node:
Increased Heart Rate (HR)
► Myocardium: Increased strength of
contraction.
►Coronary vessels:
dilation.
►Blood vessels to skeletal muscles: dilation.
►Skin:
Vasoconstriction and sweating
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Effects of parasympathetic:
Relaxing state
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The physiological effects of stimulating
parasympathetic branch are related to activities of
relaxing actions on the vegetative functions of the
body (e.g. feeding, resting and sexual stimulation):
The responses of the digestive system to food intake
are due to the parasympathetic, among these:Increased secretions of the salivary glands, the
stomach, the pancreas, intestines and increased
motility and relaxation of sphincters to allow
digestion and flow of digesta.
The eye
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The effects of the parasympathetic on the
eyeball and vision is seen in the eye
structures:
Iris
constriction.
Ciliary muscle
contraction.
Lacrimal glands
Lacrimation.
The heart
The effects of the parasympathetic on the
heart muscle are:
► S.A. node
decreased HR.
►Myocardium
no effect?
►Coronary vessels
dilation
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Depending on innervation type:
Sympathetic
Parasympathetic
Vegetative functions:
Organ
symp.
Salivary
↓
Pancreas
↓
Stomach
↓
Intestine
↓
Sphincters constriction
Ureters
relaxation
Bladder
contraction
Internal sphincter: Ejaculation
Male organs:
secretion of
adrenaline &
Adrenal medulla: noradrenaline
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Parasymp.
↑
↑
↑
↑
relaxation
constriction
relaxation
Erection
none
-
Secretion &
Motility
Chemical transmitters
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The effect of sympathetic and parasympathetic
stimulation are produced by release of chemical
substances or transmitters which serve two
functions:
(1) transmit the nerve impulses
(2) combine with receptors in target cells so as to
produce the desired physiological effect.
In all ganglia, the transmitter substance is Ach,
therefore all the preganglionic fibres release
acetylcholine at the synapses between the pre-and
postganglionic neurons.
The adrenal medulla
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It is considered as ganglia-homologous with a
sympathetic system. The fibres reaching the medulla
come from preganglionic neurons and therefore
release acetylcholine which synapse on interneurons
that secrete dopamine as an inhibitory transmitter.
The function of these dopamine interneurons is to
lessen (damp) or inhibit transmission at the ganglia
and therefore to limit preganglionic stimulation.
Acetylcholine & Noradrenaline
They are the main transmitters released by the postganglionic
fibres.
 All parasympathetic postganglionic fibres release acetylcholine
(Ach).
 Acetylcholine receptors are either nicotinic or muscarinic
BESIDES
 Some postganglionic cholinergic neurons also release
Vasoactive Intestinal Peptide (VIP).
 Most of the sympathetic postganglionic fibres release
noradrenaline e.g. sweat glands & blood vessels in skeletal
muscles are exceptions and release Ach.
 In addition:
NANC
There are some which are Non-adrenergic, Non-cholinergic
(NANC) found in the intrinsic nerves of the gut, regulate GIT
motility and release of gut hormones:
 These are:
Dopamine
Encephalin
GABA
Gastrin -Releasing-Peptide (GRP)
Nitric Oxide (NO)
Vasoactive Intestinal Peptide (VIP)
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Adrenergic receptors
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They are either Alpha 1&2-stimulating or Beta 1&2inhibitory.
Affect vagal tone to the heart rate.
Certain drugs have been found to block or inhibit
some of the effects of alpha or beta receptor
stimulation.
On the basis of this; such drugs has been classified
as alpha-blockers & beta blockers. The organs
affected are classified accordingly, e.g.
Phenoxybenzamine (Beta receptor blocker), while
Dihydroergotamine is B1 and B2 blocker.
Higher control of the ANS
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As ANS is a reflexes which their centres in the spinal cord or
cranial nerve nuclei in the brainstem, there are several areas of
the brain which are concerned with autonomic regulation, e.g.
hypothalamus regulate sympathetic functions of the blood
pressure and heart rate.
The limbic system (responsible for instinctive behavior and
emotions) as it is situated closely to the hypothalamus
(responsible of vegetative or visceral functions) and are related
to each other. The nuclei of the hypothalamus receive
information from various parts of the CNS and gives outputs to
the cerebrum , the brainstem and to the anterior thalamus and
the limbic cortex in the cerebrum.