PNS Efferent Division

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PNS Efferent Division

• Somatic Autonomic

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PNS Efferent Division

• Somatic Autonomic

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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

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Comparison of Somatic and Autonomic Systems
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Efferent Reflex Pathways
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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
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Efferent Pathways Motor Autonomic
Figure 11-11 Summary of efferent pathways
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Autonomic Nervous SystemSympathetic
Parasympathetic

• Regulation of the internal environment
  generally outside 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

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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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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
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Anatomical Differences between the Sympathetic
and Parasympathetic Divisions

• SNS
• 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)

• 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

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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

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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

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Autonomic Pathways
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Synaptic Organization
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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 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)

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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.

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Adrenergic and Cholinergic NTs (continued)

• 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.

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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.

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Responses to Cholinergic Stimulation (continued)
Figure 9-1
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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

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Responses to Adrenergic Stimulation
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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

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Dual Antagonistic Innervation
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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

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Sympathetic vs Parasympathetic
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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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Levels of Autonomic Control
Figure 16.12
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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 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)

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Somatic Motor Controls Skeletal Muscles

• Body movement
• Appendages
• Locomotion
• Single neuron
• CNS origin
• Myelinated
• Terminus
• Branches
• Neuromuscular junction

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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

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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

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Neuromuscular Junction
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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

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Fig. 7-6, p. 245
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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

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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

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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

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Summary

• Efferent motor neurons control skeletal muscles
• Single long myelinated neuron from CNS
• Neuromuscular junction structure mechanism