Chapter 7The Peripheral Nervous System:

Efferent Division

Outline

• Autonomic nervous system

• Somatic nervous System

• Neuromuscular Junction

Outline

• Autonomic nervous system– Anatomy

• Pre and post fibers, sympathetic ganglia chain, collateral ganglia, terminal ganglia

– Neurotransmitters• Pre = Acetylcholine (ach)• Post = adrenaline (epinepherine) noradrenaline (norepinepherine)

– Sympathetic and parasympathetic branches• Dual innervation• Sympathetic dominance “fight or flight”• Parasympathetic dominance “rest and digest”

– Receptor types• Cholinergic, muscarinic, nicotinic• Adrenergic. 1 2 1 2

– CNS control

PNS: Efferent Division

• Communication link by which CNS controls activities of muscles and glands

• Two divisions of PNS

– Autonomic nervous system (ANS)• Involuntary branch of PNS

• Innervates cardiac muscle, smooth muscle, most exocrine glands, some endocrine glands, and adipose tissue

– Somatic nervous system• Subject to voluntary control

• Innervates skeletal muscle

ANS

• Autonomic nerve pathway

– Extends from CNS to an innervated organ

• Ganglion = neuronal cell bodies in the PNS

• Nuclei = neuronal cell bodies in the CNS

– Two-neuron chain• Preganglionic fiber (synapses with cell body of second

neuron)

• Postganglionic fiber (innervates effector organ)

Sympathetic Nervous System Parasympathetic Nervous System

Fibers originate in thoracic and lumbar regions of spinal cord

Fibers originate from cranial and sacral areas of CNS

Most preganglionic fibers are short Preganglionic fibers are longer

Long postganglionic fibers Very short postganglionic fibers

Preganglionic fibers release acetylcholine (Ach)

Preganglionic fibers release acetylcholine (Ach)

Most postganglionic fibers release noradrenaline (norepinephrine)

Postganglionic fibers release acetylcholine

• Most visceral organs innervated by both sympathetic and parasympathetic fibers

• In general produce opposite effects in a particular organ

• Dual innervation of organs by both branches of ANS allows precise control over organ’s activity

Sympathetic preganglionic fiber

Autonomiceffectors

Parasympathetic preganglionic fiberParasympathetic postganglionic fiber

Epinephrine

NorepinephrineAcetylcholine

Sympathetic postganglionic fiber

Muscarinicreceptor

ACh

Terminalganglion

Nicotinicreceptor

Sympatheticganglion chain

AdrenalMedulla

BloodE

Muscarinicreceptor

Collateralganglion

Nicotinicreceptor

Terminalganglion

AChACh

β1 receptor

β2 receptor

NE

E

E

α receptor

NE

E

ACh

NE

Adiposetissue

Mostendocrineglandsand someendocrineglands

Smoothmuscle

Cardiacmuscle

KEY

AChNE

EFig. 7-2, p. 241

Nicotinicreceptor

Nicotinicreceptor

ANS

• Sympathetic system dominates in emergency or stressful (“fight-or-flight”) situations

– Promotes responses that prepare body for strenuous physical activity

• Parasympathetic system dominates in quiet, relaxed (“rest-and-digest”) situations

– Promotes body-maintenance activities such as digestion

Eye

Nasalmucosa

Sympathetic

Spinal nerves

Sympathetictrunk

Splanchinonerves

Liver

Gallbladder

PancreasAdrenal gland

Kidney

Smallintestine

Colon

Rectum

Urinary bladderGenitalia

Lung

Heart

Spinalnerves

Cranialnerves

Salivaryglands

ParasympatheticParotidgland

Trachea

Lacrimal gland

Stomach

SpleenSympathetic preganglionic fiber

Parasympathetic postganglionic fiber

Parasympathetic preganglionic fiber

Sympathetic postganglionic fiber

Fig. 7-3, p. 237

Table 7-02a p244

ANS

• Exceptions to general rule of dual reciprocal innervation by the two branches of autonomic nervous system

– Most arterioles and veins receive only sympathetic nerve fibers (arteries and capillaries are not innervated)

– Most sweat glands are innervated only by sympathetic nerves

– Salivary glands are innervated by both ANS divisions but activity is not antagonistic – both stimulate salivary secretion

ANS

• Adrenal medulla is a modified part of sympathetic nervous system

– Modified sympathetic ganglion that does not give rise to postganglionic 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)

• Broadcast vs. localized

Table 7-1 p242

Sympathetic preganglionic fiber

Autonomiceffectors

Parasympathetic preganglionic fiberParasympathetic postganglionic fiber

Epinephrine

NorepinephrineAcetylcholine

Sympathetic postganglionic fiber

Muscarinicreceptor

ACh

Terminalganglion

Nicotinicreceptor

Sympatheticganglion chain

AdrenalMedulla

BloodE

Muscarinicreceptor

Collateralganglion

Nicotinicreceptor

Terminalganglion

AChACh

β1 receptor

β2 receptor

NE

E

E

α receptor

NE

E

ACh

NE

Adiposetissue

Mostendocrineglandsand someendocrineglands

Smoothmuscle

Cardiacmuscle

KEY

AChNE

EFig. 7-2, p. 241

Nicotinicreceptor

Nicotinicreceptor

Autonomic Neurotransmitter Receptors

• Tissues innervated by autonomic nervous system have one or more of several different receptor types for postganglionic chemical messengers– Alter tissue/cell response instead of chemical message– Alter the distribution (localized – varicosity, broadcast –

adrenal medulla)– Cholinergic receptors – bind to ACh

• Nicotinic receptors – found on postganglionic cell bodies of all autonomic ganglia

– nicotine• Muscarinic receptors – found on effector cell membranes

– Mushroom poison– Adrenergic receptors – bind to norepinephrine and

epinephrine• Alpha (α) receptors• Beta (β) receptors

Autonomic Neurotransmitter Receptors

– Cholinergic receptors – bind to ACh• Nicotinic receptors – found on postganglionic cell

bodies of all autonomic ganglia– Nicotine– Respond to Ach, opens Na and K channels, triggers AP

• Muscarinic receptors – found on effector cell membranes

– Mushroom poison– Smooth muscle, cardiac muscle, glands– 5 subtypes– G protein linked

Autonomic Neurotransmitter Receptors

– Adrenergic receptors – bind to norepinephrine and epinephrine

• G protein coupled

• Alpha (α) receptors - Ca second messenger system 1= stimulates cyclic amp, sympathetic tissues, vessel

constriction, contraction of smooth muscle, constrictor

2 blocks cyclic amp, decreases smooth muscle contraction in the digestive tract, inhibitory to the effector organ

• Beta (β) receptors - cyclic amp

Found in the heart, increases contractility and rate

Found in the arterioles and airways, dilator

Table 7-3 p246

Autonomic Agonists and Antagonists

• Agonists

– Bind to same receptor as neurotransmitter

– Elicit an effect that mimics that of neurotransmitter

• Antagonists

– Bind with receptor

– Block neurotransmitter’s response

Distinguishing Characteristics of Sympathetic and Parasympathetic Nervous Systems

Outline• Somatic nervous system

– Anatomy• Motor neurons• Continuous to effector• Voluntary• Final common pathway• ALS = Amyotropic lateral

sclerosis, death of motor neurons

• Neuromuscular junction– Anatomy

• Muscle fiber, terminal button• “motor end plate”

– Ach release• Presynaptic vesicular• Postsynaptic intracellular

– Ach activation of end plate potentials

• Activation of nicotinic receptors

• Inflow of sodium and potassium

• Ap propagated along the muscle fiber

– Ach breakdown• AChe

Somatic Nervous System

• Consists of axons of motor neurons of motor neurons that originate in spinal cord or brain stem and end on skeletal muscle

• Motor neuron releases neurotransmitter, Ach, which stimulates muscle contraction

• Motor neurons are final common pathway by which various regions of CNS exert control over skeletal muscle activity

– These areas of CNS include spinal cord, motor regions of cortex, basal nuclei, cerebellum, and brain stem

Outline

• Neuromuscular junction– Anatomy

• Muscle fiber, terminal button• “motor end plate”

– Ach release• Presynaptic vesicular• Postsynaptic intracellular

– Ach activation of end plate potentials• Activation of nicotinic receptors• Inflow of sodium and potassium• Ap propagated along the muscle fiber

– Ach breakdown• AChe

Axon terminals

Neuromuscularjunction

Spinal cord (section)

Muscle

Axonterminals

Axons of two efferentmotor neuron

Musclefibers

Terminalbutton

Terminal buttons

Neuro-muscularjunction

Muscle fibers

Fig. 7-4, p. 251

Muscle fibers innervated by red motor neuron

Muscle fibers innervated by red motor neuron

Contractile elements within muscle fiber

Acetylcholine-gatedreceptor-channel (fornonspecific cation traffic)

Action potentialpropagationin muscle fiber

Voltage-gatedCa2+ channel

Action potentialpropagationin motor neuron

Na+

Na+

Na+

Plasma membraneof muscle fiber

Voltage-gatedNa+ channel

Terminal button

Motor end plate

Vesicle ofacetylcholine

Ca2+

Myelin sheath

Axon terminal ofmotor neuron

K+Acetylcholinesterase

Fig. 7-5, p. 252

1

2

34

6

7

88

7

6

95

Comparison of Somatic and Autonomic Nervous System

Neuromuscular Junction

• Acetylcholinesterase – Inactivates ACh– Ends end-plate potential and the action potential

and resultant contraction• Neuromuscular junction is vulnerable to chemical

agents and diseases– Black widow spider venom causes explosive

release of ACh– Botulism toxin blocks release of ACh – Curare blocks action of ACh at receptor sites– Organophosphates prevent inactivation of ACh– Myasthenia gravis inactivates ACh receptor sites

Neuromuscular conditions and compounds

• ALS– Lou Gehrig’s Disease

• death of motor neurons– Black widow venom

• Explosive release of Ach• Paralysis of the diaphram

– Botulinum toxin• Inhibits Ach release

– Curare• Ach receptor blocker (chloinergic)

– Organophosphates• Ache inhibition

– Mysanthia Gravis• Autoimmune, body attacks Ach receptors