1. PNS Efferent Division
Somatic & Autonomic

2. PNS Efferent Division
Somatic & Autonomic

3. 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

4. Comparison of Somatic and Autonomic Systems

5. Efferent Reflex Pathways

6. 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

7. Efferent Pathways: Motor & Autonomic
Figure 11-11: Summary of efferent pathways

8. Autonomic Nervous System Sympathetic & 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

9. 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)

10. 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

11. 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
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)

12. 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

13. 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

14. Autonomic Pathways

15. Synaptic Organization

16. 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)

17. 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.

18. 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.

19. 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.

20. Responses to Cholinergic Stimulation (continued)
Figure 9-1

21. 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

22. Responses to Adrenergic Stimulation

23. 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

24. Dual Antagonistic Innervation

25. 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

26. Sympathetic vs Parasympathetic

27. 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

28. Levels of Autonomic Control
Figure 16.12

29. 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)

30. Somatic Motor Controls Skeletal Muscles
Body movement
Appendages
Locomotion
Single neuron
CNS origin
Myelinated
Terminus
Branches
Neuromuscular junction

31. 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

32. 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

33. Neuromuscular Junction

34. 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

35. Figure 7.6: Events at a neuromuscular junction.
Fig. 7-6, p. 245

36. 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

37. 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

38. 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

39. Summary Efferent motor neurons control skeletal muscles
Single long myelinated neuron from CNS
Neuromuscular junction structure & mechanism