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PNS Efferent Division
Somatic & Autonomic
PNS Efferent Division
Somatic & Autonomic
Efferent Pathways
• Heavily myelinated axons of the somatic motor neurons
extend from the CNS to the effector (lacks ganglia)
• Pathways in the ANS are a two-neuron chain –The
preganglionic (first) neuron has a lightly myelinated
axon. The ganglionic (second) unmyelinated neuron
extends to an effector organ via the postganglionic axon
Comparison of Somatic and
Autonomic Systems
Efferent Reflex Pathways
Synapses in Autonomic Nerves
• Varicosities
• NT released to ECF
• No cleft
• Impact
• Large area
• Slow acting
• Long duration
Figure 11-8: Varicosities of autonomic neurons
Efferent Pathways: Motor & Autonomic
Figure 11-11: Summary of efferent pathways
Autonomic Nervous System
Sympathetic & Parasympathetic
• Regulation of the “internal” environment generally outside
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of our conscious control: “autonomous”
Innervates organs that are not usually under voluntary
control - glands, smooth/cardiac muscle
Efferent (motor) systems “visceromotor” - effectors are
part of visceral organs and blood vessels
Involve 2 neurons that synapse in a peripheral ganglion
Presynaptic neuron is myelinated and postsynaptic
neuron is unmyelinated
Autonomic nerves release NT that may be stimulatory or
inhibitory
ANS
• Autonomic nerve pathway
• Extends from CNS to an innervated organ
• Two-neuron chain
• Preganglionic fiber (synapses with cell body of second
neuron)
• Postganglionic fiber (innervates effector organ)
Divisions of the ANS
•
•
Both have
preganglionic neurons
that originate in CNS.
Both have
postganglionic
neurons that originate
outside of the CNS in
ganglia.
Figure 9-6
Anatomical Differences between the
Sympathetic and Parasympathetic Divisions
PNS
•
Fibers originate from
cranial and sacral
areas of CNS
•
Preganglionic fibers
are longer
•
Very short
postganglionic fibers
•
Preganglionic fibers
release acetylcholine
(Ach)
•
Postganglionic fibers
release acetylcholine
SNS
•
•
•
•
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Fibers originate in
thoracic and lumbar
regions of spinal cord
Most preganglionic
fibers are short
Long postganglionic
fibers
Preganglionic fibers
release acetylcholine
(Ach)
Most postganglionic
fibers release
noradrenaline
(norepinephrine)
Neurochemistry of the ANS
• All preganglionic fibers release acetylcholine (=cholinergic)
• Postganglionic PARASYMPATHETIC fibers release
acetylcholine(=cholinergic)
• Postganglionic SYMPATHETIC fibers release
norepinephrine(=adrenergic)
• Exceptions:
• Adrenal medullary chromaffin cells secrete epinephrine
• Sympathetic nerves innervating sweat glands secrete acetylcholine
• Sympathetic nerves innervating blood vessels in skeletal muscle
secrete acetylcholine
• Sympathetic nerves innervating renal blood vessels secrete
dopamine
Functional Differences
• Sympathetic - “fight or flight”
• Catabolic (expend energy)
• Release of norepinephrine (NT) from postganglionic fibers
and epinephrine (NT) from adrenal medulla.
• Mass activation prepares for intense activity.
• Heart rate (HR) increases.
• Bronchioles dilate.
• Blood [glucose] increases.
• Parasympathetic - “feed & breed”, “rest & digest”
• Maintain homeostasis
• Normally not activated as a whole, stimulation of separate
parasympathetic nerves.
• Release ACh as NT.
• Relaxing effects:
• Decreases HR.
• Dilates visceral blood vessels.
• Increases digestive activity.
• Dual innervation of many organs — having a brake
and an accelerator provides more control
Autonomic Pathways
Synaptic Organization
Adrenal Glands
• Adrenal medulla secretes epinephrine (Epi) and
norepinephrine (NE) when stimulated by the sympathetic
nervous system.
•
Modified sympathetic
ganglion, derived from same
embryonic tissue that forms
postganglionic sympathetic
neurons.
•
Sympathoadrenal system:
•
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mass activation of the
sympathetic nervous system.
Innervated by preganglionic
sympathetic fibers.
Stimulation of preganglionic
fiber prompts secretion of
hormones into blood
•
About 20% of hormone
release is norepinephrine
• About 80% of hormone
released is epinephrine
(adrenaline)
Adrenergic and Cholinergic NTs
•
•
ACh is NT for all
preganglionic fibers of both
sympathetic and
parasympathetic nervous
systems.
Transmission at these
synapses is termed
cholinergic:
•
•
ACh is NT released by
most postganglionic
parasympathetic fibers at
synapse with effector.
Axons of postganglionic
neurons have numerous
varicosities along the axon
that contain NT.
Adrenergic and Cholinergic NTs
•
•
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NT released by most
postganglionic sympathetic
nerve fibers is NE.
Epi, released by the adrenal
medulla is synthesized from
the same precursor as NE.
Transmission at these
synapses is called
adrenergic
Collectively called
catecholamines.
(continued)
Responses to Cholinergic Stimulation
• All somatic motor neurons, all preganglionic and most
postganglionic parasympathetic neurons are
cholinergic.
• Release ACh as NT.
• Somatic motor neurons and all preganglionic autonomic
neurons are excitatory.
• Postganglionic axons, may be excitatory or inhibitory.
• Muscarinic receptors:
• ACh binds to receptor.
• Requires the mediation of G-proteins.
• Nicotinic receptors (ligand-gated):
• ACh binds to 2 nicotinic receptor binding sites.
• Opens a Na+/K+ channel.
• Always excitatory.
Responses to Cholinergic Stimulation
(continued)
Figure 9-1
Responses to Adrenergic Stimulation
• Has both excitatory and inhibitory effects
• All act through G-proteins
• Alpha adrenergic responses due to Ca2+
• A1 : excitatory constricts smooth muscles
• A2 : inhibitory decreases contraction of smooth muscle
• Beta adrenergic responses due to cAMP
• B1 : excitatory increases HR and force of contraction
• B2 : inhibitory relaxes bronchial smooth muscles
Responses to Adrenergic Stimulation
Organs With Dual Innervation
• Most visceral organs receive dual innervation (innervation
by both sympathetic and parasympathetic fibers).
•
Antagonistic effects:
• Sympathetic and parasympathetic fibers innervate the same cells.
• Actions counteract each other ex. Heart rate
•
Complementary - sympathetic and parasympathetic stimulation
produces similar effects ex. salivary gland secretion
•
Cooperative - Sympathetic and parasympathetic stimulation produce
different effects that work together to produce desired effect ex.
• Parasympathetic fibers penile erection
• Sympathetic fibers ejaculation
Dual Antagonistic Innervation
Organs Without Dual Innervation
• Regulation achieved by increasing or
decreasing firing rate
• Adrenal medulla, arrector pili muscle, sweat
glands, and most blood vessels receive
only sympathetic innervation
Sympathetic vs Parasympathetic
Levels of ANS Control
• The hypothalamus is
the main integration
center of ANS activity
• Subconscious
cerebral input via
limbic lobe
connections
influences
hypothalamic
function
• Other controls come
from the cerebral
cortex, the reticular
formation, and the
spinal cord
Levels of Autonomic Control
Figure 16.12
Regulation of the ANS by CNS
• Prefontal association cortex and limbic system -
Responsible for visceral responses that are
characteristic of emotional states and behavior
• Hypothalamus – sympathetic response to anger or
fear is brought on by hypothalamus through
medulla
• Medulla:
• Most directly controls activity of autonomic system
• Location of centers for control of cardiovascular
pulmonary, urinary, reproductive and digestive systems.
• Some autonomic reflexes integrated at spinal cord
(urination, erection)
Somatic Motor Controls Skeletal Muscles
• Body movement
• Appendages
• Locomotion
• Single neuron
• CNS origin
• Myelinated
• Terminus
• Branches
• Neuromuscular junction
Somatic Efferent
• Consists of the axons of motor neurons which
originate in the spinal cord and terminate on
skeletal muscle
• Acetylcholine released from a motor neuron
stimulates muscle contraction
• Motor neurons are the final common pathway by
which various regions of the CNS exert control
over skeletal muscle activity
• The areas of the CNS that influence skeletal
muscle activity by acting through the motor
neurons are the spinal cord, motor regions of the
cortex, basal nuclei, cerebellum, and brain stem
Nerve Stimulus
• Skeletal muscles are stimulated
by motor neurons of the
somatic nervous system
• Axons of these neurons travel
in nerves to muscle cells
• Axons of motor neurons branch
repeatedly as they enter
muscles
• Each axonal ending forms a
neuromuscular junction with a
muscle fiber
Neuromuscular Junction
Neuromuscular Junction
•When a nerve impulse reaches the
neuromuscular junction:
•Voltage-regulated calcium channels in the axon
membrane open and allow Ca2+ to enter the axon
•Ca2+ inside the axon terminal causes some of the
synaptic vesicles to fuse with the axon membrane
and release ACh into the synaptic cleft (exocytosis)
•ACh diffuses across the synaptic cleft and attaches
to ACh receptors on the sarcolemma
•Binding of ACh to receptors on the sarcolemma
initiates an action potential in the muscle
•ACh is quickly destroyed by acetylcholinesterase
Fig. 7-6, p. 245
Motor Unit: Neuromuscular Functional Unit
• A motor neuron and all
the muscle fibers it
supplies is called a Motor
Unit
• Each muscle has at least
one motor nerve that may
contain hundreds of motor
neuron axons.
• Axons branch into
terminals, each forming a
neuromuscular junction
with a single muscle fiber
Motor Unit
• The number of muscle fibers per motor unit
can vary from a few to several hundred
• Muscles that control fine movements
(fingers, eyes) have small motor units
• Large weight-bearing muscles (thighs, hips)
have large motor units
• Muscle fibers in a single motor unit are
spread throughout the muscle. As a result,
stimulation of a single motor unit causes
weak contraction of the entire muscle
Summary
• Autonomic branches:
sympathetic and parasympathetic
• Regulate glands, smooth & cardiac muscles
• Team with endocrine to regulate homeostasis
• Are regulated by hypothalamus, pons & medulla
• Have pathways with 2 neurons and a ganglion
• Use varicosities to release NTs
• Have diverse receptors: tonic & antagonistic
regulation
Summary
• Efferent motor neurons control skeletal
muscles
• Single long myelinated neuron from CNS
• Neuromuscular junction structure &
mechanism