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Chapter 16
Anatomy & Physiology
Fifth Edition
Seeley/Stephens/Tate
(c) The McGraw-Hill Companies, Inc.
The Autonomic System ---simplified
• The peripheral nervous system consist of:
– Afferent neurons: from the periphery to CNS,
Reflex loop (sensory - ascending)
– Efferent neurons: from the CNS to periphery,
Somatic motors – skeletal muscles
ANS – smooth muscles, cardiac muscle and glands
(motor – descending)
• Innervation of these two systems differ:
– Afferent neurons connect directly to CNS through
synapses.
• Efferent neurons of somatic division connect directly from the
CNS to the skeletal muscles.
• Efferent neurons of ANS, which have come out from the spinal
cord goes through autonomic ganglions.
– Preganglionic neuron (myelinated)--- autonomic ganglion --postganglionic neuron (unmyelinated)
• There are two types of ANS; sympathetic division and
parasympathetic division.
– The sympathetic division: preganglionic fibers from the T and
L regions of the spinal cord make ganglia at the synaptic chain
ganglia. They usually stimulate tissue metabolism, increases
alertness, get ready for emergencies.
– The parasympathetic division: they originate from the
brainstem and the S region of the spinal cord. Conserve
energies and sedentary. Also digestion.
• The neuro transmitters of ANS
• Recall, the neurotransmitters for the afferent neurons
and somatic efferent neurons is Ach.
• In ANS both Ach and norepinephrine (NE), are used.
– Cholinergic neurons use Ach..
– Adrenergic neurons use NE.
– In the sympathetic division:
• Preganglionic neurons are cholinergic
• Most of the post ganglionic neurons are adrenergic
• While the postganglionic sweat glands, thermoregulaters
are cholinergic.
•
In the parasympathetic division
– Both preganglionic and postganglionic neurons are
cholingeric (nicotinic or muscarinic), they generally excitatory
(sm. muscles), but can be inhibitory (heart).
– There are other neurotransmitters of ANS, such as, fatty
acids like prostaglandins and peptides such as, gastrin,
somatostatin, dopamine, etc…
•
Receptors of ANS
– There are two types:
1. Cholinergic receptors: ( two subtypes)
•
Nicotinic receptors for nicotine (alkaloid from tobacco).
–
•
Found in postganglionic neurons in the autonomic neurons and skeletal
muscles.
Muscrininc receptors for muscarine ( from mushrooms)
–
Found in the effector cell responding to Ach.
• Ach on the nicotinic receptor opens Na+ channels (
excitatory)
• Ach on the muscarinic receptors modulate G-proteins
and the response could be either excitatory or
inhibitory. Examples,
– Ach + muscarine receptor of cardiac muscle is
inhibitory.
– Ach + muscarine receptor of smooth muscle is
excitatory.
2.
Adrenergic receptors
– NE is released from the adrenergic postganglionic neurons of the
sympathetic division ---- to adrenergic receptors of the effector
cells.
– Two types of adrenergic receptors could be found in the same
cell, alpha receptors and beta receptors.
– The action of NE on the adrenergic receptors;
• modulates G-proteins.
• The response could be either excitatory or inhibitory depending
on the tissues. For example:
–
–
–
NE + beta receptor of cardiac muscles is excitatory.
NE + beta receptor of the stomach smooth muscle is inhibitory.
Both epinephrine and NE can bind to alpha and beta receptors.
– Epinephrine binds to alpha or beta receptors equally.
–
NE binds to alpha more strongly than beta.
• Dual innervations of organs with sympathetic and
parasympathetic divisions with opposing or
cooperative effects.
• The sympathetic division has a more general effect
than the parasympathetic division, because
epinephrine and norepinephrine are released from the
adrenal medulla and circulates through the circulatory
system.
• Review clinical focus.
The End.