AUTONOMIC AUTONOMIC NERVOUS SYSTEMNERVOUS SYSTEM

The autonomic system controls theThe autonomic system controls thevisceral functions of the body: arterial visceral functions of the body: arterial

pressure, gastrointestinal motility and pressure, gastrointestinal motility and secretion, urinary bladder emptying, secretion, urinary bladder emptying, sweating, body temperature, and many sweating, body temperature, and many other activities.other activities.

There are two major subdivisions in There are two major subdivisions in the autonomic nervous system:the autonomic nervous system:

a) sympathetic nervous system, anda) sympathetic nervous system, and b) parasympathetic nervous system.b) parasympathetic nervous system.

General scheme of the reflex arc of General scheme of the reflex arc of the autonomic nervous systemthe autonomic nervous system

receptor – afferent nervereceptor – afferent nerve

fiber – nerve center –fiber – nerve center –

preganglionic efferentpreganglionic efferent

nerve fiber – peripheralnerve fiber – peripheral

ganglion – ganglion – postganglionicpostganglionic

efferent nerve fiber –efferent nerve fiber –

visceral organ.visceral organ.

EffectorsEffectors

Somatic nervous systemSomatic nervous system– Skeletal musclesSkeletal muscles

ANSANS– Cardiac muscleCardiac muscle– Smooth muscleSmooth muscle– GlandsGlands

Efferent PathwaysEfferent Pathways Somatic nervous systemSomatic nervous system

– A, thick, heavily myelinated somatic motor A, thick, heavily myelinated somatic motor fiber makes up each pathway from the CNS fiber makes up each pathway from the CNS to the muscleto the muscle

ANS pathway is a two-neuron chainANS pathway is a two-neuron chain

1.1.Preganglionic neuron (in CNS) has a thin, Preganglionic neuron (in CNS) has a thin, lightly myelinated preganglionic axonlightly myelinated preganglionic axon

2.2.Ganglionic neuron in autonomic ganglion has Ganglionic neuron in autonomic ganglion has an unmyelinated postganglionic axon that an unmyelinated postganglionic axon that extends to the effector organextends to the effector organ

Neurotransmitter Neurotransmitter EffectsEffects

Somatic nervous systemSomatic nervous system

– All somatic motor neurons release All somatic motor neurons release acetylcholine (ACh)acetylcholine (ACh)

– Effects are always stimulatoryEffects are always stimulatory ANSANS

– Preganglionic fibers release AChPreganglionic fibers release ACh– Postganglionic fibers release Postganglionic fibers release

norepinephrine or ACh at effectorsnorepinephrine or ACh at effectors– Effect is either stimulatory or inhibitory, Effect is either stimulatory or inhibitory,

depending on type of receptorsdepending on type of receptors

Divisions of the ANSDivisions of the ANS

1.1. Sympathetic divisionSympathetic division

2.2. Parasympathetic divisionParasympathetic division Dual innervationsDual innervations

–Almost all visceral organs are Almost all visceral organs are served by both divisions, but served by both divisions, but they cause opposite effectsthey cause opposite effects

SYMPATHETIC NERVOUS SYMPATHETIC NERVOUS

SYSTEMSYSTEM Preganglionic neurons (sympathetic nervePreganglionic neurons (sympathetic nervecenters) lie in the spinal cord from T-1 to L-2centers) lie in the spinal cord from T-1 to L-2segments (in the intermediolateral horn).segments (in the intermediolateral horn).

Postganglionic neurons lie either in the twoPostganglionic neurons lie either in the twoparavertebral sympathetic chains of ganglia paravertebral sympathetic chains of ganglia

orortwo prevertebral ganglia (the celiac andtwo prevertebral ganglia (the celiac andgipogastric). The postganglionic nerve fibersgipogastric). The postganglionic nerve fiberstravel to their destinations in the varioustravel to their destinations in the variousorgans.organs.

Approximate distribution of the Approximate distribution of the sympathetic innervation:sympathetic innervation:

From T-1 to the headFrom T-1 to the head From T-2 into the neckFrom T-2 into the neck From T-3, 4, 5, 6 into the thoraxFrom T-3, 4, 5, 6 into the thorax From T-7, 8, 9, 10, 11 into the From T-7, 8, 9, 10, 11 into the

abdomenabdomen From T-12, L-1, 2 into the legs.From T-12, L-1, 2 into the legs.

Note:Note: some some sympatheticsympathetic

preganglionic nerve fibers pass allpreganglionic nerve fibers pass allthe way (without synapsing) fromthe way (without synapsing) fromthe spinal cord into the adrenalthe spinal cord into the adrenalmedullae. They end directly onmedullae. They end directly onmodified neuronal cells that modified neuronal cells that

secretesecreteepinerphine epinerphine and and norepinerphinenorepinerphineinto the blood stream.into the blood stream.

Role of the Sympathetic Role of the Sympathetic DivisionDivision

Mobilizes the body during activity; is Mobilizes the body during activity; is the “fight-or-flight” systemthe “fight-or-flight” system

Promotes adjustments during Promotes adjustments during exercise, or when threatenedexercise, or when threatened– Blood flow is shunted to skeletal Blood flow is shunted to skeletal

muscles and heartmuscles and heart– Bronchioles dilate Bronchioles dilate – Liver releases glucoseLiver releases glucose

PARASYMPATHETIC NERVOUS PARASYMPATHETIC NERVOUS SYSTEMSYSTEM

Preganglionic neurons liePreganglionic neurons liea)a) into the brain stem into the brain stem

(nuclei of the cranial (nuclei of the cranial nerves III, VII, IX, and X) nerves III, VII, IX, and X) and and

b)b) into the sacral segments into the sacral segments of the spinal cord.of the spinal cord.

Postganglionic neurons Postganglionic neurons lie inlie in

the wall of the innervated the wall of the innervated organsorgans

(except a few (except a few parasympatheticparasympathetic

nerves). Postganglionic nerve nerves). Postganglionic nerve fibers are short (from a few fibers are short (from a few

mm to cm).mm to cm).

Role of the Parasympathetic Role of the Parasympathetic DivisionDivision

Promotes maintenance activities and Promotes maintenance activities and conserves body energyconserves body energy

Its activity is illustrated in a person Its activity is illustrated in a person who relaxes, reading, after a mealwho relaxes, reading, after a meal– Blood pressure, heart rate, and Blood pressure, heart rate, and

respiratory rates are lowrespiratory rates are low– Gastrointestinal tract activity is highGastrointestinal tract activity is high– Pupils are constricted and lenses are Pupils are constricted and lenses are

accommodated for close visionaccommodated for close vision

Distribution of parasympathetic Distribution of parasympathetic innervation:innervation:

III cranial nerve – ciliary ganglion – ciliary muscle of III cranial nerve – ciliary ganglion – ciliary muscle of eye; papillary sphincter;eye; papillary sphincter;

VII cranial nerve – VII cranial nerve – a) sphenopalatine ganglion – lacrimal glands; nasal a) sphenopalatine ganglion – lacrimal glands; nasal

glands;glands;b) submandibular ganglion – submandibular glands;b) submandibular ganglion – submandibular glands;

IX cranial nerve – optic ganglion – parotid gland;IX cranial nerve – optic ganglion – parotid gland;

X cranial nerve – into the thorax and abdomen;X cranial nerve – into the thorax and abdomen;

Sacral segments of the spinal cord – into the pelvis Sacral segments of the spinal cord – into the pelvis (rectum, bladder).(rectum, bladder).

NEUROTRANSMITTERS:NEUROTRANSMITTERS:

1.1. All preganglionic neurons are cholinergic in both All preganglionic neurons are cholinergic in both sympathetic and parasympathetic nervous sympathetic and parasympathetic nervous systems;systems;

2.2. All postganglionic neurons of the parasympathetic All postganglionic neurons of the parasympathetic nervous system are cholinergic;nervous system are cholinergic;

3.3. Most of the postganglionic neurons of the Most of the postganglionic neurons of the sympathetic nervous system are adrenergic;sympathetic nervous system are adrenergic;

NOTE:NOTE: postganglionic sympathetic nerve fibers postganglionic sympathetic nerve fibersto the sweat glands, the piloerector muscles and ato the sweat glands, the piloerector muscles and afew blood vessels are cholinergic as an exception.few blood vessels are cholinergic as an exception.

RECEPTORS FOR RECEPTORS FOR NEUROTRANSMITTERSNEUROTRANSMITTERS

1.1. There are two principle types of acetylcholine There are two principle types of acetylcholine receptors in the postsynaptic membrane:receptors in the postsynaptic membrane:

nicotinic receptors – in the synapses between nicotinic receptors – in the synapses between preganglionic and postganglionic neurons of both the preganglionic and postganglionic neurons of both the sympathetic and parasympathetic nervous systems;sympathetic and parasympathetic nervous systems;

muscarinic receptors – between postganglionic muscarinic receptors – between postganglionic parasympathetic nerve firbers and innervated cells.parasympathetic nerve firbers and innervated cells.

NOTE:NOTE: specific drugs are used to stimulate or block one or specific drugs are used to stimulate or block one orthe other of the two types of receptorsthe other of the two types of receptors

2.2. There are two major types of adrenergic There are two major types of adrenergic receptors in the postsynaptic membrane of receptors in the postsynaptic membrane of innervated cells: innervated cells: alpha-receptorsalpha-receptors (alpha-1 and (alpha-1 and alpha-2) andalpha-2) and

beta-receptorsbeta-receptors (beta-1 and beta-2). (beta-1 and beta-2).

Norepinerphine and epinerphineNorepinerphine and epinerphine have have somewhat different effects in exciting the alpha- somewhat different effects in exciting the alpha- and beta- receptors:and beta- receptors:

= = norepinerphinenorepinerphine excites mainly alpha-receptors excites mainly alpha-receptors (and much less beta-receptors); (and much less beta-receptors);

= = epinerphineepinerphine excites both types of receptors excites both types of receptors equally. equally.

Effects are determined by the types of Effects are determined by the types of receptors in the organs.receptors in the organs.

AUTONOMIC EFFECTS ON VARIOUS ORGANS OF THE BODYAUTONOMIC EFFECTS ON VARIOUS ORGANS OF THE BODY

CONTROL OF BRAIN STEM CONTROL OF BRAIN STEM AUTONOMIC CENTERS BY THE AUTONOMIC CENTERS BY THE

HYPOTHALAMUSHYPOTHALAMUS Control of medullary Control of medullary

cardiovascular centers;cardiovascular centers;

Controls body Controls body temperature temperature

(heat control);(heat control);

Feeding control;Feeding control;

Control of salt and water Control of salt and water balance.balance.

The hypothalamus sends output signals in three The hypothalamus sends output signals in three direction:direction:

– downwards to the brain stem, mainly into the reticular downwards to the brain stem, mainly into the reticular areas and then into the autonomic nervous system;areas and then into the autonomic nervous system;

– upward toward many higher areas of the diencephalons upward toward many higher areas of the diencephalons and cerebellum, especially to the anterior thalamus and and cerebellum, especially to the anterior thalamus and limbic cortex;limbic cortex;

– into the infundibulum to control most of the secretary into the infundibulum to control most of the secretary functions of both the posterior and anterior pituitary glands.functions of both the posterior and anterior pituitary glands.

The hypothalamus controls most of the vegetative The hypothalamus controls most of the vegetative and endocrine functions of the body as well as many and endocrine functions of the body as well as many aspects of emotional behavior.aspects of emotional behavior.

Developmental Aspects of the Developmental Aspects of the ANSANS

During youth, ANS impairments During youth, ANS impairments are usually due to injuryare usually due to injury

In old age, ANS efficiency declines, In old age, ANS efficiency declines, partially due to structural changes partially due to structural changes at preganglionic axon terminalsat preganglionic axon terminals

Developmental Aspects of the Developmental Aspects of the ANSANS

Effects of age on ANSEffects of age on ANS– ConstipationConstipation– Dry eyesDry eyes– Frequent eye infectionsFrequent eye infections– Orthostatic hypotensionOrthostatic hypotension

Low blood pressure occurs because Low blood pressure occurs because aging pressure receptors respond aging pressure receptors respond less to changes in blood pressure less to changes in blood pressure with changes in body position and with changes in body position and because of slowed responses by because of slowed responses by sympathetic vasoconstrictor centers sympathetic vasoconstrictor centers

Role of the Autonomic nervous System in ExerciseRole of the Autonomic nervous System in Exercise