Chapter 14The Autonomic Nervous System

Autonomic systemSystem of nerves mediating involuntary actionsRegulates body organ activityMaintains normal internal functionsDivided into sympathetic and parasympathetic divisionsAllows for varied nervous system responses in times of stress and rest

Figure 14 Section 1 1

A schematic of the somatic nervous system(SNS), which provides conscious and sub-Conscious control over skeletal muscles

BRAIN

Upper motorneurons in

primary motorcortex

Skeletalmuscle

Skeletalmuscle

Lowermotor

neurons

Spinal cord

Somaticmotornuclei ofspinal cord

Somatic motornuclei of brain

stem

Comparison of the Somatic and Autonomic Nervous Systems: Functional Organization

• Somatic nervous system (SNS)– Includes processes perceived or controlled consciously

– Somatic sensory portion• detects stimuli from special senses, skin, and proprioceptors

• sends information to CNS

– Somatic motor portion• transmits nerve signals from CNS to control skeletal muscles

Comparison of the Somatic and Autonomic Nervous Systems: Functional Organization

• Autonomic nervous system (ANS)– Includes processes regulated below conscious level

– Visceral sensory portion• detects stimuli from blood vessels and internal organs

– Autonomic motor portion (visceral motor)• transmits nerve signals to cardiac muscle, smooth muscle, and glands

– Functions to maintain homeostasis• constant internal environment

– Regulates:• heart rate and blood pressure

• respiratory rate, sweating, and digestion

– Keeps these variables within optimal ranges

Figure 15.1a

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Somatic Nervous System

Posterior root ganglion

(a)

Skeletal muscleSensory receptor

in skin

Anterior root

Somatic motor neuronsends nerve signals to skeletal muscle.

Somatic sensoryneuron detects stimuliand transmits nervesignals from skin, skeletal muscle, joints, and special senses (vision, hearing, etc.).

Figure 15.1b

Autonomic Nervous System

Autonomicganglion

Preganglionic autonomic motor neurontransmits nerve signals to a ganglionicmotor neuron.

Visceral sensoryneuron detectsstimuli within bloodvessels and smoothmuscle in the viscera.

(b)

Sensory receptorin viscera

Smooth musclein trachea

Ganglionic autonomicmotor neuron transmitsnerve signals to smoothmuscle, cardiac muscle,and glands.

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Divisions of the Autonomic Nervous Systems: Functional Differences

• Motor component of ANS– Subdivided into parasympathetic and sympathetic divisions

– Parasympathetic division• functions to maintain homeostasis at rest

• energy conservation and replenishing stage

• “rest-and-digest” division

– Sympathetic division• prepares the body for emergencies

• “fight-or-flight” division

• increased alertness and metabolic activity

• “three E’s”: emergency, exercise, or excitement

Figure 14.1 1

Autonomic Nervous System

Sympathetic Division Parasympathetic Division

In the sympathetic division, or thoracolumbar(thor-a-kō-LUM-bar) division, axons emerge from the thoracic and superior lumbar segments of the spinal cord and innervate ganglia relatively close to the spinal cord.

In the parasympathetic division, or cranio-sacral (krā-nē-ō-SĀ-krul) divions, axonsemerge from the brain stem and the sacralsegments of the spinal cord, and they innervateganglia very close to (or within) target organs.

The two main divisionsof the ANS: the sympatheticand parasympatheticdivisions

Cranial nerves(III, VII, IX, andX)

T1T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

L1

L2

S2

S3

S4

Sacral nerves(S2, S3, S4 only)

Lumbar nerves(L1, L2 only)

Thoracicnerves

Divisions of the Autonomic Nervous Systems: Anatomic Differences

• Parasympathetic– Preganglionic neuron in

brainstem or S2-S4 spinal cord

– Termed craniosacral division

– Ganglionic neuron innervating muscles or glands

– Preganglionic axons longer

– Postganglionic axons shorter

– Few preganglionic axons

– Ganglia close to or within effector

• Sympathetic– Preganglionic neuron in lateral

horns of T1-L2

– Termed thoracolumbar division

– Ganglionic neuron innervating muscles or glands

– Preganglionic axons shorter

– Postganglionic axons longer

– Many preganglionic axons

– Ganglia relatively close to spinal cord (in sympathetic trunk ganglia or prevertebral ganglia)

Figure 15.3

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Sympathetic Division

Autonomic Motor Nervous System

Parasympathetic Division

Origin:Preganglionic neurons located in lateral horns of T1– L2 segments of spinal cord (thoracolumbar)

Functions:• Activated in emergency situations• “Fight-or-flight” response• Also involved with homeostasis

T1–L2 segmentsof spinal cord

Sympathetic trunk

Origin:Preganglionic neurons located in brainstem nuclei and S2–S4 segments of spinal cord (craniosacral)

Functions:• “Rest-and-digest” response• Brings body to homeostasis

CN III (oculomotor)

CN VII (facial)

CN IX (glossopharyngeal)

CN X (vagus)

S2–S4 segmentsof spinal cord

Pelvic splanchnicnerves

Figure 15.4

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(b)(a)

Parasympathetic Division

Preganglionicneuron Long preganglionic axon

Ganglionicneuron

Autonomic ganglion(close to or within effector organ wall)

Sympathetic Division

Short, branchingpreganglionic axon

Preganglionicneuron

Autonomic ganglion(close to the vertebral column)

Ganglionic neuron

Long postganglionic axonShort

postganglionicaxon

The parasympathetic division has short axons with relatively few branches and ganglia located close to or within the wall of the organ.

The sympathetic division has long axons with many branches and ganglia located in the sympathetic trunk or prevertebral ganglia

Parasympathetic Division: Cranial Components

• Cranial components– Oculomotor(lens & iris), facial(lacrimal glands), and glossopharyngeal

nerves(parotid)• convey parasympathetic information to the head

– Vagus nerve• parasympathetic information for thoracic and most abdominal organs

• increased mucus production and decreased airway diameter

• decrease in heart rate and force

• causes increased smooth muscle motility and secretory activity

Parasympathetic Division: Cranial Components

The oculomotor nerve innervates the ciliary and constrictor muscles of the eye.The facial nerve innervates the lacrimal, submandibular, and sublingual glands.The glossopharnygeal nerve innervates the parotid salivary gland.The vagus nerve innervates the thoracic organs, most of the abdominal organs, and the gonads.

Which four cranial nerves have a parasympathetic component? What organs are innervated by each?

Parasympathetic Division: Pelvic Splanchnic Nerves

• Target organs– Distal portion of large intestine, rectum

– Bladder, distal ureter

– Most reproductive organs, and others

– Causes:• smooth muscle motility

• secretory activity in digestive tract

• contraction in bladder wall

• erection of clitoris and penis

See Table 15.3: Parasympathetic Division Outflow

Figure 15.5

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Preganglionic

Postganglionic

Lacrimal gland

Parotid salivary gland

Heart

Submandibular salivary gland

Lung

Esophagus

Liver

Gallbladder

Stomach

Spleen

Kidney

Ureter

Pancreas

Small intestine

Descending colon

Rectum

Bladder

Penis

Vagina

Uterus

Pelvic splanchnic nerves

CN X

CN IX

CN VII Submandibular ganglion

Otic ganglion

Cardiac plexus

Hypogastric plexus

Abdominal aortic plexus

Esophageal plexus

Pulmonary plexus

Ciliary ganglion

Pterygopalatine ganglion

CN III

Trachea

Sublingual salivary gland

Spinal cord

Testis Ovary

S2

S3

S4

Pons

Figure 15.7

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Superior venacava

Descendingthoracic aorta

Azygos vein

Diaphragm

Greater thoracicsplanchnic nerve

Sympathetic trunkganglia

Intercostal nerve

Sympathetic trunk

Gray ramusWhite ramus

© The McGraw-Hill Companies, Inc./Photo and Dissection by Christine Eckel

Sympathetic Division: Clinical View: Horner Syndrome– Injury of cervical sympathetic trunk or T1 trunk ganglion

– Symptoms on same side of head

– Impairment of sympathetic signaling

– Ptosis• drooping of superior eyelid• due to paralysis of superior tarsal muscle

– Miosis• constricted pupil• due to paralysis of pupil dilator muscle

– Anhydrosis• lack of sweating• sweat glands not receiving sympathetic innervation

– Facial flushing• due to vasodilation due to lack of sympathetic innervation

Sympathetic Division: Sympathetic Pathways

– adrenal medulla pathway

• internal region of adrenal gland, adrenal medulla

• directly innervated by preganglionic sympathetic axons

• preganglionic neuron

– extends through sympathetic trunk and prevertebral ganglia

• upon stimulation, epinephrine and norepinephrine produced in medulla

– circulate within blood

– help prolong fight-or-flight response

– prolong effects of sympathetic stimulation

Figure 15.8d

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Splanchnic nerve

Prevertebral ganglion(no synapse occurs)

Adrenalmedulla

Gray ramus

White ramus

(d) Adrenal medulla pathway

Overview of ANS Neurotransmitters

• Neurotransmitters– Acetylcholine (ACh) and norepinephrine (NE)

– Bind to specific receptors on postsynaptic cell

– Cause stimulation or inhibition, depending on receptor

• Acetylcholine– Synthesized and released by cholinergic neurons

• all sympathetic and parasympathetic preganglionic neurons• all parasympathetic ganglionic neurons• neurons innervating sweat glands and blood vessels of skeletal muscle

• Norepinephrine– Synthesized and released by adrenergic neurons

• most other sympathetic ganglionic neurons• form network of swellings at target organ

– termed varicosities– contain stored NE– NE released along length of axon

Figure 15.10

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Preganglionic axon

Autonomic ganglion

Ganglionic neuron cell body

Postganglionic axon

Varicosities Smooth muscle cells

Mitochondrion

Synaptic vesiclecontaining NE

Varicosity

Cholinergic Receptors• Nicotinic receptors cholinergic receptor

– Sensitive to the drug nicotine

– Found on all ganglionic neurons and adrenal medulla

– When bound: always produces a stimulatory response• open ion channels greater movement of Na+ into cell than K+ out of cell

• Muscarinic receptors cholinergic receptor

– Responsive to muscarine, a mushroom toxin– Found in: all target membranes in parasympathetic division selected sympathetic

cells

• e.g., sweat glands in skin, blood vessels in skeletal muscle

• decreases heartbeat rate

• Biogenic amines (monoamines)– Bind adrenergic receptors

– Catecholamines, subcategory• have catechol ring structure in molecule

• include dopamine, norepinephrine, and epinephrine

Figure 15.9

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Sympathetic PathwaysParasympathetic Pathway

Preganglionic axonreleases ACh.

Ganglionic neuron cell body and dendrites always contain receptors for ACh.

Postganglionic axon releases ACh or NE.

Target cells contain either ACh receptors (bind ACh) or NE receptors (bind NE). Target cell Target cell

(e.g., sweat glands and blood vessels in skeletal muscle)

Target cell(e.g., most other body structures)

Adrenergicreceptors

NE

Nicotinicreceptors

AChAChACh

Nicotinicreceptors

Nicotinicreceptors

Muscarinicreceptors

Muscarinicreceptors

AChACh

Interactions Between the Parasympathetic and Sympathetic Divisions: Dual Innervation

Antagonistic Effects– Parasympathetic and sympathetic effects usually antagonistic

– E.g., control of heart rate• parasympathetic stimulation slowing heart rate

• sympathetic stimulation increasing heart rate

– E.g., control of muscular activity in GI tract• parasympathetic stimulation accelerating rate of contraction and motility

• sympathetic stimulation decreasing motility

– E.g., control of pupil diameter in the eye• parasympathetic stimulation of circular muscle layer of iris

– causes pupil constriction

• sympathetic stimulation of radial muscle layer of iris– causes pupil dilation

Interactions Between the Parasympathetic and Sympathetic Divisions: Dual Innervation

Cooperative Effects– When both parasympathetic and sympathetic produce single result

– E.g., male sexual function• penis erect due to parasympathetic innervation

• ejaculation due to sympathetic innervation

Parasympathetic and Sympathetic Interactions: Systems Controlled Only by Sympathetic Division

• Opposing effects without dual innervation– E.g., blood vessels innervated by sympathetic axons only

• cause increased smooth muscle contraction and blood pressure

• vasodilation achieved by decreasing stimulation below autonomic tone

– E.g., sweat glands in the trunk and arrector pili muscles in the skin• cause sweating and “goosebumps”

– E.g., neurosecretory cells of adrenal medulla• release epinephrine and norepinephrine, prolonging fight-or-flight effects

Figure 14.6 1

Parasympathetic and Sympathetic Interactions: Systems Controlled Only by Sympathetic Division

Clinical View: Raynaud Syndrome

– Sudden constriction of small arteries of digits

– Results in loss of normal hue of distal skin

– Accompanied by pain

– Triggered by cold or emotional stress

– Due to exaggerated local sympathetic response

– More common in women

Control and Integration of Autonomic System Function: Autonomic Reflexes

Clinical View: Autonomic Dysreflexia– Causes blood pressure to rise profoundly

– Stimulates a sympathetic reflex • causes systemic vasoconstriction

• marked increase in blood pressure

– Caused by hyperactivity of ANS after a spinal cord injury

– Initial response to injury is spinal shock, with loss of autonomic reflexes

– Abnormal response to lack of innervation, denervation hypersensitivity• e.g., involuntary relaxation of internal urethral sphincter

• due to spinal cord reflex

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Cerebrum

Hypothalamus

Spinal cord

Brainstem

Conscious activities in thecerebrum affect hypothalamuscontrol of the ANS

Integration and command centerfor autonomic functions; involved in emotions

Contains major ANS reflexcenters

Contains ANS reflex centers fordefecation and urination

Involvement of CNSANS is a regulated nervous system, not independentInfluenced by four CNS regions:

cerebrum, hypothalamus, brainstem, and spinal cord

Figure 14.6 1

Figure 14.6 1

Sympathetic

• Adapts body for physical activity

• Fight or flight• Subtle effects

Parasympathetic

• Calming effect on functions

• Reduced energy expenditure and normal bodily maintenance