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Efferent Division: Autonomic and Somatic Motor Control

Chapter 11

Efferent Division of the Peripheral Nervous System

• Somatic motor neurons– Control skeletal muscles– Mostly voluntary

• Autonomic neurons– Control smooth and cardiac muscle, many glands,

lymphoid and some adipose tissues– Mostly involuntary– Sympathetic branch is fight-or-flight– Parasympathetic branch is rest-and-digest– Works with endocrine and behavioral state systems to

maintain homeostasis

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Figure 11.1

THE AUTONOMIC DIVISION

Homeostasis is a dynamic balancebetween the autonomic branches.

Rest-and-digest:Parasympatheticactivity dominates.

Fight-or-flight:Sympathetic activitydominates.

Parasympathetic Sympathetic

Figure 11.2

The hypothalamus, pons, and medulla initiate autonomic,endocrine, and behavioral responses.

Sensoryinput

Hypothalamicsensors

Somatic and visceralsensory neurons

Pons,medulla,

hypo-thalamus

Limbicsystem,cerebralcortex

Autonomicresponse

Endocrineresponse

Behavioralresponse

Figure 11.3

AUTONOMIC CONTROL CENTERS

Hypothalamus

Pons

Medulla

Temperature control

Water balance

Eatingbehavior

Respiratory center

Blood pressurecontrol

Secondaryrespiratory center

Urinary bladdercontrol

Antagonistic Control of the Autonomic Division

• Most internal organs are under antagonistic control– One autonomic branch is excitatory, and the other branch is

inhibitory– Example:

– Effector organ: heart– Sympathetic response increases heart rate– Parasympathetic response slows rate– exceptions sweat glands and smooth muscle in blood vessels

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Figure 11.5-3 (3 of 4)

*Hormonal epinephrine only **All parasympathetic responses are mediated by muscarinic receptors.

SYMPATHETIC RESPONSE

ADRENERGICRECEPTOR

PARASYMPATHETIC RESPONSE **

EFFECTORORGAN

Pupil of eye Dilates Constrictsα

Salivaryglands Mucus, enzymes Watery secretionα and β2

Heart Slows rateIncreases rateand force ofcontraction

β1

α β2

Arteriolesand veins

ConstrictsDilates

β2*

α, β2

α

α

Lungs Bronchiolesdilate

Bronchiolesconstrict

Digestivetract

Decreasesmotility andsecretion

Increasesmotility andsecretion

Exocrinepancreas

Decreasesenzymesecretion

Increasesenzymesecretion

Endocrinepancreas

Inhibits insulinsecretion

Stimulates insulin secretion

Autonomic neuronal pathways

Autonomic pathways consist of two neuronsthat synapse in an autonomic ganglion.

Preganglionicneuron

Postganglionicneuron

Autonomicganglion

Targettissue

CNS

– All autonomic pathways (symp. and parasymp.) have 2 neuron series

– 1st neuron=preganglionic neuron (CNS and projects)– 2nd neuron= resides in the autonomic ganglion (outside

CNS)= postganglionic neuron (projects to target)

Regions: Sympathetic vs Parasympathetic

– Main regional differences

1. point of origin in CNS

– Sympathetic originate in lumbar or thoracic region

– Parasympathetic originate in brain steam, sacral region

2. location of autonomic ganglia

– Parasympathetic located close to target tissue, so are the postganglionic neurons

– Sympathetic close to spinal cor

Figure 11.5

SYMPATHETIC PARASYMPATHETIC

Hypothalamus,Reticular formation

Pons

Medulla

Spinalcord

Sympathetic chain

FIGURE QUESTIONS

1. Name the regions of the CNS where the two branches originate.2. Describe where the ganglia for the two branches are located (relative to the spinal cord).

3. What is an advantage of having ganglia in the sympathetic chain linked to each other?

Sympathetic

Parasympathetic

KEY

Co1

L1

S1

T1

C12345678

23456789

1011

12

2

345

2345 Pelvic

nervesUterus

Testes

Penis

Urinary bladder

Kidney

Adrenal medulla

Pancreas

Intestines

Stomach

Liver

Ganglion

Engorgementand secretions

Stimulatescontraction

Increasesrenin secretion

Inhibitsdigestion

Release enzymesand insulin

Increases motilityand secretion

Increasesmotility and

secretionDecreases enzymes

and insulin

Induces erection

Release of urine

Induces ejaculation

Relaxes bladder

secretes catecholamines

Inhibits digestion

Inhibits digestion

Increases bilesecretion

Relaxesairways

Increases heart rateand contractility

Mucus and enzymessecreted

Pupil dilatesPupil constricts

Watery secretion

Slows heart rate

Constricts airways

Lungs

Heart

Salivary glands

EyeGanglion Hypothalamus,

Reticular formation

Pons

Medulla

Spinalcord

Vagusnerve

Vagus Nerve

• Contains about 75% of all parasympathetic fibers• Sensory information from internal organs to brain• Output from brain to organs

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Neurotransmitters: Sympathetic vs Parasympathetic

– Sympathetic and parasympathetic can be distinguished by receptors and neurotransmitters

– Preganglionic neurons: Both Sympathetic and parasympathetic: release ACh onto nicotinic cholinergic receptors on postganglionic neurons

– Postganglionic sympathetic neurons: secret norepinephrine (NE) onto adrenergic receptors on target cells

– Postganglionic parasympathetic neurons: secret ACh onto muscarinic cholinergic receptors on target cells.

– Dual innervation: input from both sympathetic and parasympathetic

Figure 11.6 Sympathetic and parasympathetic neurotransmitters and receptors

Parasympathetic pathwaysuse acetylcholine.

Sympathetic pathwaysuse acetylcholine andnorepinephrine.

FIGURE QUESTIONS

CNS CNS

ACh

Nicotinicreceptor

Autonomicganglion

Norepinephrine

Target tissue

AChMuscarinicreceptor

Adrenergicreceptor

1. Identify all: - cholinergic neurons - adrenergic neurons - preganglionic neurons - postganglionic neurons2. Which pathway will have longer preganglionic neurons? (Hint: See Fig. 11.5.)

T T

Control of smooth muscle, cardiac muscle, and glands

– Neuroeffector junction (target=effector) : the synapse between a postganglionic autonomic neuron and its target cell

– Autonomic postganglionic axons end with a series of swollen areas at their distal ends called varicosity

– neurotransmitter is released into the interstitial fluid and diffuses to wherever the receptors are located results in less directed communication than skeletal muscle

– neurotransmitters are synthesized in varicosities– monoamine oxidase (MAO) the main enzyme

responsible for degradation of catecholamines (Ach)

Figure 11.7a (1 of 10)

Autonomic varicosities release neurotransmitter over the surface of target cells.

Mitochondrion

VaricosityVesicle containingneurotransmitter

Axon ofpostganglionic

autonomicneuron

VaricositiesSmooth muscle cells

Figure 11.7b (2 of 10)

NE is metabolized bymonoamine oxidase (MAO).

NE can be taken back intosynaptic vesicles for re-release.

NE is removed from the synapse.

Receptor activation ceases whenNE diffuses away from the synapse.

NE binds to adrenergicreceptor on target.

Ca2+ entry triggersexocytosis of synaptic vesicles.

Depolarization opensvoltage-gated Ca2+ channels.

Action potential arrives atthe varicosity.

Norepinephrine (NE) release and removal at a sympathetic neuroeffector junction

Axon varicosity

MAOTyrosine

Axon

Exocytosis

NEAction

potential

Voltage-gatedCa2+ channel

Ca2+

NEDiffuses away

Bloodvessel

Adrenergicreceptor

ResponseTarget cell

Active transport

G

Postganglionic Autonomic Neurotransmitters– Sympathetic Pathways: secrete NE that binds to

receptors on target cells– 2 types of Adrenergic receptors:

– Alpha receptors– Beta receptors: 3 types

– B1, B2, B3

Table 11.2 Properties of Adrenergic Receptors

Parasympathetic Branch

• Acetylcholine• Muscarinic receptors

– G protein–coupled– Second messenger pathways– At least five subtypes

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Adrenal Medulla

• Neuroendocrine tissue• Associated with the sympathetic branch• Primary neurohormone is epinephrine

– Secreted into the blood

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Figure 11.8

THE ADRENAL MEDULLA SECRETES EPINEPHRINE INTO THE BLOOD.

Adrenal gland

Kidney

Spinal cordPreganglionicsympathetic

neuron

ACh

Adrenal medulla

The chromaffincell is a modifiedpostganglionicsympatheticneuron.

Epinephrine is aneurohormone thatenters the blood.

To targettissues

Blood vessel

Adrenal cortex is atrue endocrine gland.

Adrenal medulla is amodified sympatheticganglion.

Table 11.3 Agonists and Antagonists of Neurotransmitter Receptors

Table 11.4 Comparison of Sympathetic and Parasympathetic Branches

– Both sympathetic and parasympathetic pathways have 2 neurons (preganglionic and postganglionic). Exception: adrenal medulla where postganglionic cells is a chromaffin cells that secretes NE.

– ALL preganglionic autonomic neurons secrete ACh onto nicotinic receptors

– Postganglionic neurons:

– MOST sympathetic neurons secret NE onto adrenergic receptors

– MOST parasympathetic neurons secrete ACh onto muscarinic receipts

– Pathway origination:

– Sympathetic pathways originate in the thoracic and lumbar regions of the spinal cord

– Parasympathetic pathways leave the CNS at the brain stem or the sacral region.

– Ganglia location:

– MOST Sympathetic ganglia are located close to the spinal cord.

– MOST parasympathetic ganglia are located close to the target tissu

Sympathetic vs Parasympathetic

Figure 11.9-2 ESSENTIALS – Efferent Divisions of the Nervous System

SOMATIC MOTOR PATHWAY

CNSACh Nicotinic receptor

Target:skeletal muscle

Figure 11.9-3 ESSENTIALS – Efferent Divisions of the Nervous System

AUTONOMIC PATHWAYS

Parasympathetic Pathway

Sympathetic Pathway

Adrenal SympatheticPathway

Autonomic targets:CNS

CNS

CNS

ACh ACh

Nicotinic receptor

Nicotinic receptor

Ganglion Muscarinicreceptor

α receptor

β2 receptor

β1 receptor

E

E

Adrenal medulla

Adrenal cortexBlood vessel

KEY

FIGURE QUESTIONS

• Smooth and cardiac muscles

• Some endocrine and exocrine glands• Some adipose tissue

Using the figure, compare: (a) number of neurons in somatic motor and autonomic pathways (b) receptors on target cells of somatic motor, sympathetic, and parasympathetic pathways (c) neurotransmitters used on target cells of somatic motor, sympathetic, and parasympathetic pathways (d) receptor subtypes for epinephrine with subtypes for norepinephrine

ACh = acetylcholine

E = epinephrineNE = norepinephrine

ACh

NE

Figure 11.9-4 ESSENTIALS – Efferent Divisions of the Nervous System

Comparison of Somatic Motor and Autonomic Divisions

SOMATIC MOTOR AUTONOMIC

Number of neurons in efferent path 1 2

ACh/muscarinic or NE/α- or β-adrenergicACh/nicotinicNeurotransmitter/receptor atneuron-target synapse

Target tissue Skeletal muscle Smooth and cardiac muscle; some endocrineand exocrine glands; some adipose tissue

Neurotransmitter released from Axon terminals Varicosities and axon terminals

Effects on target tissue Excitatory only: muscle contracts Excitatory or inhibitory

Peripheral components foundoutside the CNS

Axons only Preganglionic axons, ganglia, postganglionicneurons

Summary of function Posture and movement Visceral function, including movement in internalorgans and secretion; control of metabolism

Somatic Motor Division

• Single neuron – CNS origin (either brain or ventral horn of spinal cord)

and projects to target tissue (always skeletal muscle)– Always excitatory– Myelinated

• Terminus – Branches close to target– Single neuron to control many muscle fibers at once– axon terminals lie on the surface of skeletal muscles– Neuromuscular junction: synapse of a somatic motor

neuron on a muscle fiber

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Figure 11.10a ESSENTIALS – Somatic Motor Neurons and the Neuromuscular Junction

The neuromuscular junctionconsists of axon terminals, motor end plates on the muscle membrane, and Schwann cell sheaths.

Somatic motor neuronbranches at itsdistal end.

Skeletalmuscle fiber

Motor end plate

The Neuromuscular Junction

Postsynaptic membrane is modified into amotor end plate.

Synaptic cleft

Presynapticmembrane

Synaptic vesicle(ACh)

Nicotinic AChreceptors

The neuromuscular junctionNMJ has 3 components:

1. motor neuron’s presynaptic axon terminal is filled with synaptic vesicles and mitochondria

2. the synaptic cleft

3. postsynaptic membrane of the skeletal muscle fiber

Motor End Plate: Neuromuscular Junction

Schwann cell sheath

Axon terminal

MitochondrionMotor end plate

The motor end plate is aregion of muscle membrane thatcontains high concentrations ofACh receptors.

Figure 11.10d ESSENTIALS – Somatic Motor Neurons and the Neuromuscular Junction

An action potential arrives at the axon terminal, causingvoltage-gated Ca2+ channels to open. Calcium entry causessynaptic vesicles to fuse with the presynaptic membraneand release ACh into the synaptic cleft.

Synapticvesicle

(ACh)

ACh

AChE

Ca2+ Ca2+

Acetyl + cholineVoltage-gatedCa2+ channels

Skeletal musclefiber

Nicotinicreceptor

Acetylcholine (ACh) is metabolizedby acetylcholinesterase (AChE).

Postsynaptic membrane is modified into amotor end plate.

Synaptic cleft

Presynapticmembrane

Figure 11.10e ESSENTIALS – Somatic Motor Neurons and the Neuromuscular Junction

Na+

Na+K+

K+

ACh

Closed channel Open channel

The nicotinic cholinergic receptor binds two AChmolecules, opening a nonspecific monovalentcation channel. The open channel allows Na+

and K+ to pass. Net Na+ influx depolarizes themuscle fiber.