BIOL 2305
Peripheral Nervous System – Efferent Division
Somatic & Autonomic
Efferent division provides communication link between CNS and the activities of muscle and glands –
the “effector organs”
Two Branches of the Peripheral Efferent Division:
Autonomic (ANS) – involuntary cardiac, smooth muscle, exocrine glands, and some endocrine glands
Somatic – voluntary movement and secretions
Autonomic Pathways consist of a 2 neuron chain:
The first motor neuron, called the preganglionic neuron, has its cell body in the CNS and its
myelinated axon called the preganglionic fiber extends to autonomic ganglion.
The second motor neuron called the postganglionic neuron has its soma in that autonomic ganglion
and its unmyelinated axon called the postganglionic fiber extends directly to the effector cell or organ
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Efferent Pathways
Somatic:
Single, heavily myelinated axon of the somatic motor neuron extends from the CNS to the
effector (lacks ganglia)
ANS (Parasympathetic & Sympathetic):
Pathways in the ANS are a two-neuron chain
1. The preganglionic (first) neuron has a lightly myelinated axon
2. The postganglionic (second) unmyelinated neuron extends to an effector organ via
the postganglionic axon
Comparison of Somatic and Autonomic Systems
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Autonomic Nervous System
The ANS operates without conscious control; it is primarily regulated by the hypothalamus and the
medulla oblongata with input from the limbic system and other regions of the cerebrum.
The afferent component of the ANS consists of general visceral sensory neurons:
Interoreceptors such as chemoreceptors (CO2 levels) and mechanoreceptors (degree of stretch
of organs and vessels)
Afferent signals are not consciously recognized unless intense enough to cause pain or nausea
from damaged viscera, fullness of bladder, angina pectoris (inadequate blood flow to heart)
The efferent component consists of the autonomic motor neurons that excite or inhibit visceral activities
of effector tissues: cardiac muscle, smooth muscle, glands
Efferent responses include automatic activities beyond conscious control - dilation/constriction
of pupils, accommodation of lens, dilation of blood vessels, heartbeat, GI/GU movement,
glandular secretions.
Efferent Reflex Pathways
Efferent motor neuron synapses at NMJ
with skeletal muscle
Preganglionic neuron synapses at
sympathetic chain ganglion
Efferent motor neuron releases NT (ACh)
at NMJ
Postganglionic neuron releases NT
(NE/ACh) at varicosities onto
viscera/glands
Voluntary
Involuntary
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Synapses in Autonomic Nerves
Varicosities
Form strings of synapses (called boutons)
NT synthesized in varicosities
AP from axon triggers voltage-gated Ca2+
channels
Influx of Ca2+ stimulates exocytosis of NT
NT released into general ISF in synaptic cleft
NT diffuses slowly through interstitial fluid to
receptors
No motor end plate (meaning
receptors on the muscle are not
clustered)
Impact:
Large area
Slow acting
Long duration
NT removed by reuptake, degradation, or
diffusion
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Efferent Pathways: Motor & Autonomic
Autonomic Nervous System: Sympathetic & Parasympathetic
Regulation of the internal environment generally “autonomous” (outside of our conscious control)
ANS innervates smooth muscle, cardiac muscle, and glands
“Visceromotor” – movement of viscera; effector targets are visceral organs and blood vessels
Involve 2 neurons that synapse in a peripheral ganglion:
Preganglionic neuron is myelinated
Postganglionic neuron is unmyelinated
Autonomic nerves release NT’s that may be stimulatory (NE) or inhibitory (ACh)
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)
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Two Divisions of the ANS (Sympathetic and Parasympathetic)
Both have preganglionic neurons that originate in CNS.
Both have postganglionic neurons that originate outside of the CNS in ganglia.
Differences: Sympathetic vs Parasympathetic
Parasympathetic Nervous System (PNS)
Fibers originate from cranial and sacral areas of
CNS
CN 3, 7, 9, 10
Sacral 2, 3, 4
Preganglionic fibers are longer
Very short postganglionic fibers
Preganglionic fibers release acetylcholine (ACh)
Postganglionic fibers release acetylcholine
(ACh)
Sympathetic Nervous System (SNS)
SNS
Fibers originate in thoracic and lumbar regions
of spinal cord
T1 – L2
Most preganglionic fibers are short
Long postganglionic fibers
Preganglionic fibers release acetylcholine (ACh)
Most postganglionic fibers release
norepinephrine (noradrenaline, NE)
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Functional Differences
Sympathetic - “fight or flight”
Catabolic (expend energy)
Release of norepinephrine from postganglionic fibers and epinephrine from adrenal
medulla
Mass activation prepares for intense activity
↑Heart rate
↑ Bronchodilation
↑ Blood [glucose]
Parasympathetic - “feed & breed”, “rest & digest”
Maintain homeostasis
Normally not activated as a whole, stimulation of separate parasympathetic nerves.
Release ACh as NT onto effector organs
Relaxing effects:
↓ Heart Rate
↑ Vasodilation (to visceral organs)
↑ digestive activity (motility/secretions)
Dual innervation of many organs — having a brake and an accelerator provides more control
Neurochemistry of the ANS
All preganglionic fibers release acetylcholine (cholinergic)
Postganglionic PARASYMPATHETIC fibers release acetylcholine (cholinergic)
Postganglionic SYMPATHETIC fibers release norepinephrine (adrenergic)
Some exceptions:
Adrenal medullary chromaffin cells secrete epinephrine (80%) and norepinephrine (20%) into
bloodstream
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
Autonomic Pathways
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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:
mass activation of the sympathetic nervous system.
Innervated by preganglionic sympatheti c fibers.
Secretion of hormones into blood
About 20% of hormone release is norepinephrine
About 80% of hormone released is epinephrine
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Adrenergic and Cholinergic NTs
Axons of postganglionic neurons have numerous varicosities along the axon that contain NT
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.
NT released by most postganglionic sympathetic nerve fibers is NE
Transmission at these synapses is called adrenergic
Note: Epi, released by the adrenal medulla, is synthesized from the same precursor as NE
Epi and NE collectively called catecholamines
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 nicotinic receptor binding sites
Opens a Na+/K+ channel
Always excitatory
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Responses to Cholinergic Stimulation
Nicotinic receptors (ligand-gated) open a Na+ & K+ channels; excitatory only
Muscarinic receptors involve G-proteins; excitatory or inhibitory
M1 receptor – common in exocrine glands and in the CNS; IP3 pathway
M2 receptor – receptors are located in the heart, cause a decrease in cAMP in the cell,
inhibition of voltage-gated Ca2+ channels, and efflux of K+
M3 receptor – smooth muscle in bronchioles. In blood vessels it causes increased synthesis of
nitric oxide by endothelial cells causing vasodilation, stimulates secretion in many glands; IP3
pathway
M4 receptor – found in the CNS; decrease cAMP in the cell and, thus, produce generally
inhibitory locomotor effects
M5 receptor – location is not well known
Cholinergic Stimulation (ACh)
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Responses to Adrenergic Stimulation
Excitatory or inhibitory effects, depending on receptor on target cell
All act through G-proteins
Alpha adrenergic responses:
α1 receptors - vasoconstriction in GI tract, skin, kidney – IP3, DAG, Ca2+
α2 receptors - sphincter constriction in GI tract, inhibit insulin release – reduced cAMP
Beta adrenergic responses:
β1 receptors - increases HR and force of contraction, increase renin secretion – cAMP
β2 receptors - bronchiole dilation, vasodilation skeketal muscle , decrease motility GI tract–
cAMP
β3 receptors - stimulates lipolysis
Responses to Adrenergic Stimulation
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Responses to Adrenergic Stimulation
Dual Innervation
Organs With Dual Innervation:
Innervated by both sympathetic and
parasympathetic fibers)
Antagonistic –
Sympathetic and parasympathetic fibers
innervate the same cells.
Actions counteract each other (e.g.:
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
Organs Without Dual Innervation:
Regulation achieved by increasing or
decreasing firing rate
Examples:
Adrenal medulla, arrector pili muscle, sweat glands, and most blood vessels
receive only sympathetic innervation
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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
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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:
Primal sympathetic response to anger or fear is brought on by hypothalamus through medulla
Medulla:
Most directly controls activity of ANS
Location of centers for control of cardiovascular, pulmonary, urinary, reproductive, and
digestive systems.
Spinal cord:
Some autonomic reflexes integrated at spinal cord (urination, erection)
Somatic Motor Controls Skeletal Muscles
Body movement
Appendages
Locomotion
Single neuron
CNS origin
Heavily myelinated
Terminus
Branches (motor unit)
Neuromuscular junction (motor end plate)
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 (NMJ) with
a muscle fiber
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Neuromuscular Junction (NMJ)
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 degraded by the
enzyme acetylcholinesterase
(AChE)
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Motor Unit: Neuromuscular Functional Unit
Motor 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
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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
Efferent motor neurons control skeletal muscles
Single long myelinated neuron from CNS
Neuromuscular junction structure & mechanism
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