Organization of Motor
Systems
The motor systems encompass 2
divisions of the PNS
• Somatic (‘voluntary’) – controls skeletal
muscle
• Autonomic (‘involuntary’) – controls
visceral effectors:
–
–
–
–
–
Smooth muscle surrounding blood vessels
Cardiac muscle
GI tract smooth muscle
Exocrine glands (salivary, sweat, gastric secretion, etc)
metabolic effects in variety of tissues (SNS)
Before we start, just a little about receptors
• Receptors divided into two classes:
– Ionotropic – receptor is an ion channel that opens a conductive
pathway when bound with transmitter – effect may be
depolarization, hyperpolarization or stabilization.
– Metabotropic – transmitter-receptor binding initiates a chemical
2nd message within the target cell – this typically activates ion
channels and transduces an electrical effect – but it doesn’t
always have to do so to be effective.
• Drugs that act on receptors may be
– agonists (acting like or facilitating the action of the native
transmitter)
– antagonists (opposing the action of the native transmitter)
Are there excitatory and inhibitory transmitters and
receptors?
• Although we sometimes speak as if this were so,
• In most cases, the outcome of applying a
particular transmitter to a tissue depends entirely
on what happens in the target tissue after the
receptor has been activated, and not on any
intrinsic property of the transmitter or receptor
Comparison of Autonomic and
Somatic Motor Systems
• The next slide compares the basic
organization, transmitter identities, and
receptor identities in the two branches of
the autonomic system with the somatic
system.
Comparison of ANS
and SNS
Some Important things to know about the motor
systems slide
• In all systems, at the 1st synapse outside the CNS, the
transmitter is always Ach and the receptors nicotinic.
• Nicotinic receptors are always excitatory; muscarinic and
adrenergic receptors may be excitatory or inhibitory
depending on the effect of the 2nd messengers coupled
to the receptor in each tissue.
• Single innervation of effectors in the somatic system vs
(typically) dual innervation in the autonomic system.
• Sympathetic ganglia are generally remote from the target
organ whereas parasympathetic ones are generally in or
on the target.
The nature of effectors in the
autonomic system
• Target tissues may include smooth muscle,
ducted glands, and, in the case of the
sympathetic branch, the metabolic activity of a
number of tissues, including the liver, adipose
tissue, skeletal muscle, and the brain itself.
• Generally, autonomic inputs do not have a off/on
effect on their targets. Instead, they modulate
tissue activities that are already ongoing.
Smooth muscle as an effector of the
autonomic system
• Characteristics of smooth muscle:
– Cells small
– Surrounds hollow internal organs and blood
vessels
– Contractile machinery diffuse (no sarcomeres)
– Electrical activation is dependent on external
Ca++
Control features of smooth muscle
• Two basic types:
• 1. Single unit
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–
–
–
Cells coupled by gap junctions
Spontaneous activity due to pacemakers
ANS does not need to make a synapse on each cell
ANS modulates spontaneous activity
• 2. Multi-unit
– Few gap junctions; little or no spontaneous activity
– ANS must make synapses on each cell
– Contractile activity reflects the balance of S versus PS inputs
There are two routes to activate smooth muscle
Electromechanical:
Chemical message > depolarization > Ca++ entry
> contractile activity
Pharmacomechanical:
Chemical message > intracellular 2nd message >
Ca++ release from internal stores > contractile
activity
Anatomical summary of the ANS
Basic
Anatomical
Layout of the
Autonomic
Nervous System
The Sympathetic Branch
• Cell bodies of preganglionic neurons are in thoracic and
upper lumbar cord
• Preganglionic axons pass out through T1-L2 ventral
roots as B and C fibers – small to very small.
• They may synapse in paravertebral ganglion of the same
segment, or turn to go rostrally or caudally to other
segments, or continue through a splanchnic nerve to a
prevertebral ganglion among the viscera
– Celiac ganglion – organs of upper abdomen
– Superior mesenteric ganglion – intestines
– Inferior mesenteric ganglion – distal colon, bladder, genitalia
Sympathetic branch, continued.
• Postganglionic axons are unmyelinated C
fibers that may pass through gray ramus
to spinal nerve running to body wall, or
into a sympathetic nerve to abdominal
organs
• Adrenal medulla – preganglionics are in
T10 and T11 – they go through the celiac
ganglion to adrenal medulla where they
synapse on chromaffin cells.
The Parasympathetic Branch
• Preganglionic cell bodies are in brainstem nuclei
– associated with particular cranial nerve roots, or
– in S2-S4.
• Ganglionic synapses are typically in or at least
close to the target organs.
• Cranial: ciliary ganglion – Cranial N. III;
sphenopalatine and submaxillary ganglia – C. N.
VII, Otic ganglion – C.N. IX
• Abdomen - splenic flexure of colon marks the
boundary between turf of the vagus N. (C.N. X)
and the sacral part of the P.S.B.
The cholinergic synapse
• Presynaptic: acetylcholine is synthesized
from glucose
• Postsynaptic: receptors may be nicotinic
(ionotropic-excitatory) or muscarinic (2nd
messenger); there are at least 4 subtypes
of muscarinic receptors
• After release, Ach is degraded by
acetylcholinesterase and choline is
reabsorbed by presynaptic terminal.
Cholinergic synapses as drug targets
• Vesicle release blocker: botulinum toxin
• Nicotinic receptor blockers: curare, cobra toxin,
hexamethonium
• Muscarinic receptor blockers: muscarine (toxin
from fly agaric mushroom Amantia muscarina),
atropine and scopalomine (Belladonna alkaloids)
• Cholinesterase inhibitors: physostigmine,
neostigmine, nerve gases, insecticides
• Choline reuptake inhibitor: hemicholinium
The Adrenergic Synapse
• Norepinephrine synthesized by synaptic
terminal:
– Tyrosine > DOPA>Dopamine>NE
Norepinephrine is recovered by the presynaptic
cell and may be recycled or degraded
(monoamine oxidase)
Adrenergic synapse,cont.
• 4 main classes of adrenergic receptors,
characterized by agonist specificity
• Alpha 1: sweat glands, blood vessels
• Alpha 2: GI tract, presynaptic terminals
• Beta 1: heart muscle
• Beta 2: Lung airway; blood vessels in
heart, skeletal muscle
The adrenergic synapse as a drug target
• Alpha agonists: phenylephrine, pseudephedrine,
amphetamines (decongestants, stimulants)
• Alpha blockers: phentolamine,
phenoxybenzamine, ergot alkaloids, yohimbine
• Beta agonist: isoproterenol
• Beta blockers: propranolol and its congeners
(antihypertensives)
• Monoamine oxidase inhibitors (antidepressants)
Higher Centers of Autonomic Function
• Location: Brainstem, hypothalamus and other
parts of the limbic system
• Physiological variables controlled by the limbic
system and A.N.S.:
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Body temperature
Blood pressure
Blood glucose
Digestive activities
Sex response
Emotions
An example – baroreceptor reflex and blood
pressure regulation
Medullary Respiratory Center
Limbic System
Medullary CV center (brainstem)
Vagus
(inhibits)
Baroreceptors
(carotid sinus, aortic
arch)
Sympathetics
(stimulate)
Vasculature
Heart
Reflexive response to a drop in blood
pressure versus the medullary setpoint
• Sensed by baroreceptors
• Integrated in MCV center
• Results of reflex activation:
– Increases outflow of action potentials in thoracic
sympathetics to heart
• Heart rate rises
• Stroke force increases
– decreases or shuts off vagal outflow to heart
• Parasympathetic tone removed – facilitates rate increase
– Increases sympathetic outflow to blood vessels
• Arterioles - Peripheral resistance rises
• Veins - Venous capacitance falls, so blood moves to arterial
side of circulation.