Transcript Autonomic Nervous System

Lecture 9:
Autonomic Nervous System
•Controls the function of internal
organs.
•The autonomic nervous system
provides internal homeostasis.
•Autonomic reflexes control blood
pressure, heart rate, respiration, water
balance, body temperature and other
homeostatic functions.
•Divided into two major subdivisions:
the sympathetic and parasympathetic
divisions. The two divisions cannot be
readily distinguished except according
to the type of situation in which they
are most active.
Resulting Homeostasis Following a Balance
Between Sympathetic and Parasympathetic
Control
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The fight-or-flight response is the quick protective reaction
of the body
Two Neurons Form Synapses in Ganglia
•Ganglia can modulate information passing through them with the
assistance of intrinsic neurons.
•A single preganglionic neuron synapses with as many as 8-9
postganglionic neurons. Allows control of multiple pathways.
Neuroeffector Junction
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•Varicosities release neurotransmitters. Lay close to targets.
•Diffused transmitter interacts with receptors, wherever they are.
Less directed communication, but more tissue affected.
Sympathetic and Parasympathetic Pathways
Sympathetic Pathway
•Originate from the thoracic and sacral regions of the spinal
cord.
•Ganglia are found in the sympathetic chain.
•Postganglionic neurons have long axons.
Parasympathetic Pathway
•Preganglionic cell bodies are found in the brain stem or the sacral
region of the spinal cord.
•Parasympathetic ganglia are found near their target organ.
•Preganglionic axons are long.
Vagus Nerve
•Carry sensory neurons and
parasympathetic fibres to most
internal organs.
•Vagotomy- performed in the late
19th and early 20th centuries to treat
stomach ulcers by removing the
parasympathetic innervation which
causes acid secretion. Nowadays, its
treated with drugs.
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•Otto Loewi: Discovered chemical
synaptic transmission by showing that
the slowing of the heart beat upon
vagal nerve stimulation resulted from
the release of ACh from nerve
endings. The details of his important
experiment came to him in his dream.
He won the Nobel Prize in 1936.
Adrenal Medulla Secretes Catecholamines
•The adrenal medulla is a modified sympathetic ganglion.
Postganglionic neurons lack axons.
•NE and E are secteted in response to alarm signals from the CNS into
the bloodstream for distribution throughout the body.
ACh and NE are the Primary Neurotransmitters
•Neurons which secrete ACh are termed cholinergic and those which
secrete NE are termed adrenergic.
•Adrenergic neurons are mainly sympathetic.
•All preganglionic neurons are cholinergic. Parasymapthetic neurons
release ACh at neuroeffector synapses.
•A few sympathetic neurons release ACh instead of NE and they are
termed sympathetic cholinergic neurons.
•Non-adrenergic, non-cholinergic neurons
•They secrete neither ACh or NE. They can also secrete transmitters
such as VIP, substance P, NO, ATP, along with ACh and NE.
•They are assigned to either autonomic division according to their
anatomical organization.
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•Geoffrey Burnstock formulated the concept of purinergic
signaling. ATP is a purine nucleotide. It is a
neurotransmitter in the ANS; functions by activating
‘purinoceptors’.
Neurotransmitter Release
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•Neurotransmitters are synthesized and packaged into vesicles within
the varicosities.
•Presynaptic receptors can modulate release. Facilitate or inhibit it.
•Substances are co-released along with transmitters.
Termination of Neurotransmitter Release
•Neurotransmitters can be metabolized in the extracellular fluid by
enzymes (eg. ACh by AChE).
•Transported back into the axon terminal (eg.NE).
•The concentration of neurotransmitter in the synaptic cleft and
the number of receptors on target cells determine the magnitude of
the synaptic response.
Cholinergic Neurotransmitter Receptors
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•nAChR open cation channels which pass Na+ and K+. Net Na+ ion
influx which depolarizes postsynaptic cell.
•mAChR are G-protein coupled channels linked to 2nd messengers or
gated to K+ channels. The tissue response varies with receptor
subtype.
Adrenergic Receptors
•Adrenergic receptor activates responses which lasts for long periods
resulting from 2nd messenger activation. It modifies protein
synthesis or existing proteins.
•Three subtypes: a, b1 and b2.
• a receptors- Cause muscle contraction. In the digestive tract, they
cause relaxation.
• b1 receptors- enhance cardiac contraction and speed up electrical
conduction in cardiac muscle.
• b2 receptors- activation by epinephrine triggers smooth muscle
relaxation.
•Beta-blockers are used clinically to treat high blood pressure; one
of the most common disorders in the US.
Homeostatic Regulation by the Autonomic Divisions
•The two autonomic divisions demonstrate all properties of
homeostasis:
•1) Maintenance of the internal environment.
•2) “Up-down” regulation by varying the tonic control.
•3) Antagonistic control.
•4) Variable tissue responses that result from different membrane
receptors. Eg. blood vessels which contain receptors which
mediate vasoconstriction, whereas another class of receptors cause
vasodilation.
•Eg. Sympathetic stimulation increases the heart rate whereas
parasympathetic stimulation decreases it. Thus, the heart rate could
be altered by changing the relative contribution of the two
divisions. Example displays (2), (3) and (4).
Summary of Autonomic Pathways
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Autonomic Control is Regulated by the Brain
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•CNS control of autonomic divisions is closely linked to centres
in the medulla, pons and hypothalamus.
Hypothalamus Integrates Many Autonomic
Reflexes
•Controls homeostasis and behavioural drives.
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•Receives input from the limbic system.
•Best known hypothalamic response is the fight-or-flight
response. It increases sympathetic output: pounding heart,
sweaty palms and increased blood pressure in response to fear.
•Catecholamine release from the adrenal medulla spreads this
response far and wide through the bloodstream.
References
1. Tortora, G.J. & Grabowski, S.R (2003). Principles of
Anatomy & Physiology.New Jersey: John Wiley &
Sons. Ch.17, pp.565-581.
2. Silverthorn, D.U (1998). Human Physiology: An
Integrated Approach. New Jersey: Prentice Hall.
Ch.11, pp.307-318.