• Innervates smooth muscle, cardiac muscle and glands

• Helps to maintain homeostasis

• Also called the involuntary or visceral motor system

THE AUTONOMIC NERVOUS SYSTEM

• Regulation– By brainstem, spinal cord, hypothalamus

and parts of the cerebrum

THE AUTONOMIC NERVOUS SYSTEM

• The motor units of the ANS consist of two neurons:– Preganglionic Neuron

• Cell body is in the CNS

• Synapses with another neuron before reaching the

effector

• The synapse occurs in an autonomic ganglion outside

the CNS

• (The somatic motor unit consists of one neuron with its

cell body in the CNS and its axon extending to the

effector)

– Postganglionic Neuron• Extends from the autonomic ganglion in the PNS to the

effector

AUTONOMIC GANGLIA AND CIRCUITS

AUTONOMIC GANGLIA AND CIRCIUTS

CNS PNS Effector

THE AUTONOMIC NERVOUS SYSTEM

• Fight or flight system activated in emergency situations

• Effects:– Increases heart rate

– Dilates bronchial tubes and pupils

– Constricts blood vessels

– Stimulates secretion of epinephrine and norepinephrine from the adrenal gland

– Stimulates sweat glands

– Inhibits digestion

– Aids in ejaculation in males

DIVISIONS OF THE ANS: The Sympathetic System (see table 14.4)

THE SYMPATHETIC SYSTEM

• Neurons– Emerge through the ventral roots

of spinal nerves T1 through L2 (thoracolumabar outflow)

THE SYMPATHETIC SYSTEM

• Preganglionic Sympathetic Neurons– Preganglionic bodies are

in the lateral horns of the spinal cord at the T1 through L2 levels

– Preganglionic sympathetic axons are short

– Preganglionic fibers pass through the ventral root into the white rami communicantes

THE SYMPATHETIC SYSTEM

• Preganglionic Sympathetic Neurons– Preganglionic

sympathetic neurons synapse with a postganglionic neuron in the paravertebral chain ganglia

• Paravertebral chain ganglia run lateral to the spinal cord on both sides

• There are 22 to 23 pair of paravertebral ganglia on both sides of the vertebral column

THE SYMPATHETIC SYSTEM

• Preganglionic Sympathetic Neurons– Preganglionic fibers

may ascend or descend within the chain to synapse within a ganglion at a different level

– Some sympathetic preganglionic fibers pass through the chain ganglion without synapsing

• These are called sympathetic splanchnic nerves

THE SYMPATHETIC SYSTEM

• Preganglionic Sympathetic Neurons– Sympathetic

splanchnic nerves synapse in ganglia anterior to the vertebral column (near the aorta)

– These ganglia are called prevertebral or collateral ganglia

– Sympathetic splanchnics innervate smooth muscles of the abdominal and pelvic viscera and their blood vessels

THE SYMPATHETIC SYSTEM

• Postganglionic Sympathetic Neurons– Post ganglionic

sympathetic axons are long

– Post ganglionic sympathetic neurons exit the paravertebral ganglia via the gray rami communicantes (unmyelinated) and re-enter the spinal nerve

– From here they continue on to the effector (viscera, blood vessels, sweat glands)

• All sympathetic preganglionic neurons release acetylcholine as a neurotransmitter (cholinergic)

• Sympathetic postganglionic neurons release norepinephrine (adrenergic) with the exception of neurons to blood vessels and sweat glands (cholinergic)

THE SYMPATHETIC SYSTEM

• Adrenal Medulla– Preganglionic sympathetic neurons innervate the

adrenal medulla– Cause release of epinephrine and norepinephrine into

the bloodstream– Has the same effect as the sympathetic system only

lasts 5 to 10 times longer

THE SYMPATHETIC SYSTEM

• Norepinephrine & Epinephrine– Norepinephrine and epinephrine both have

similar effects on the body – Epinephrine has a greater effect on cardiac

stimulation, raising B.P. and increasing metabolic rate

– Both are secreted by the adrenal medulla in response to sympathetic stimulation

– Same effect as stimulating organs via sympathetic nerves only lasts 5 to 10 times longer

THE SYMPATHETIC SYSTEM

• Effects:

– Constricts the pupils and bronchi

– Restores gland and digestive system

activity

– Slows heartrate

THE PARASYMPATHETIC SYSTEM

THE PARASYMPATHETIC SYSTEM

• Neurons:– Emerge with the

cranial nerves (III, VII, IX and X)

– Some emerge with the sacral spinal nerves

– Craniosacral outflow

C.N. III

C.N. VII

C.N. IX

C.N. X

Neuron cell bodies for the fibers traveling with cranial nerves are in the brainstem

Cell bodies for the fibers traveling with the sacral spinal nerves are in the lateral gray horns of spinal levels S2-S4

THE PARASYMPATHETIC SYSTEM

• Preganglionic Parasympathetic Neurons– Preganglionic neurons are

long– Preganglionic neurons travel

from the CNS almost all the way to the effector before synapsing with a postganglionic neuron

THE PARASYMPATHETIC SYSTEM

• Postganglionic Parasympathetic Neurons– Postganglionic

parasympathetic neurons are short

– Postganglionic neurons synapse with preganglionics on or near the effector organ in terminal ganglia (collectively called intramural ganglia)

– Postganglionic neurons travel from the terminal ganglia to the effector cells

THE PARASYMPATHETIC SYSTEM

• Cranial Outflow

– With C.N. III, VII, IX and X– Preganglionic fibers

travel with each cranial nerve

– Postganglionic fibers for C.N. III, VII and IX travel with C.N. X for distribution to the face

C.N. III

C.N. VII

C.N. IX

C.N. X

Vagus nerve accounts for 90% of all preganglionic parasympathetic fibers in the body

Parasympathetic fibers from the Vagus nerve supply almost every thoracic and abdominal organ

THE PARASYMPATHETIC SYSTEM

• Sacral Outflow– Axons run from the spinal

cord with the ventral rami of S2-S4

– Fibers branch into pelvic splanchnic nerves

– Most fibers go on to synapse in intramural ganglia near the effector organ

– Pelvic splanchnics innervate the distal large intestine, urinary bladder, ureters and reproductive organs

• Cholinergic Receptors– Activated by acetylcholine (Ach)– Two types:

• Muscarinic– Found on all effector cells stimulated by postganglionic

cholinergic fibers (all parasympathetic target organs and some sympathetic)

• Nicotinic– Found on motor end plates of skeletal muscle– Found on all postganglionic neurons (sympathetic and

parasympathetic)– Found on the hormone producing cells of the adrenal

medulla

ANS RECEPTORS

ANS RECEPTORS

• Adrenergic Receptors

– Activated by epinephrine and norepinephrine– Two types:

• Alpha ()– Epinephrine has a greater effect on these than

norepinephrine– Found on all sympathetic target organs except the heart– Usually stimulatory when NE or Epinephrine binds to them

• Beta ()– Found in the heart, adipose tissue and most sympathetic

organs– Usually inhibitory when either hormone binds to them

(except in the heart)– Cause dilation of blood vessels

ANS RECEPTORS

ANS RECEPTORS

VISCERAL REFLEXES

• Visceral Reflex Arcs