The Autonomic Nervous System

Chapter 15

Introduction The Autonomic Nervous System (ANS) is

the system of motor neurons that innervate the smooth muscle, cardiac muscle, and glands of the body

By controlling these effectors, the ANS regulates such visceral functions as … – Heart rate– Blood pressure– Digestion– Urination

Introduction The ANS is the general visceral motor

division of the peripheral nervous system and is distinct from the general somatic motor and brachial motor divisions which innervate skeletal muscles

Introduction The general visceral sensory system

continuously monitors the activities of the visceral organs so that the autonomic motor neurons can make adjustments as necessary to ensure optimal performance of the visceral organs

Introduction The stability of our internal environment

depends largely on the autonomic nervous system

Autonomic nervous system(ANS) receives signals from visceral organs

The ANS makes adjustments as necessary to ensure optical support for body systems

Comparison of ANS & PNS Recall that the somatic motor system

innervates skeletal muscles Each somatic motor neuron runs from the

central nervous system all the way to the muscle being innervated, and that each motor unit consists of a single neuron plus the skeletal muscle cells it innervates

Typical somatic motor axons are thick, heavily myelinated fibers that conduct nerve impulses rapidly

Comparison of ANS & PNS

Comparison of ANS & PNS In the somatic system

– Cell bodies are within the central nervous system

– Axons extend to the muscles they serve– Somatic motor fibers are thick, heavily

myelinated Type A fibers that conduct impulses very rapidly

Comparison of ANS & PNS In the autonomic nervous system

– The motor unit is a two neuron chain– The cell body of the first neuron, the

preganglionic neuron, resides in the brain or spinal cord

– Its axon, the preganglionic axon, synapses with the second motor neuron, the post- ganglionic neuron, in an autonomic ganglion outside the central nervous system

– The postganglionic axon then extends to the effector organ

Comparison of ANS & PNS

Compare the one motor neuron of the somatic motor division with the two neuron chain of the autonomic nervous system

Efferent Pathways and Ganglia

Axons of most preganglionic neurons run from the CNS to synapse in a peripheral autonomic ganglion with a postganglionic neuron

Efferent Pathways and Ganglia

Axons of postganglionic neurons run from the ganglion to the effectors (cardiac and smooth muscle fibers and glands)

Efferent Pathways and Ganglia Preganglionic axons are lightly myelinated

thin fibers Postganglionic axons are even thinner and

are unmyelinated Conduction though the autonomic chain is

slower than through the somatic motor Many pre and postganglionic fibers are

incorporated into spinal or cranial nerves for most of their course

Efferent Pathways and Ganglia Remember that autonomic ganglion are

motor ganglia, containing the cell bodies of motor neurons

They are sites of synapse and information transmission from pre to postganglionic neurons

Also note that the somatic motor division lacks ganglia entirely

Neurotransmitter Effects All somatic motor neurons release

acetylcholine at their synapses with their effectors, skeletal muscle fibers

The effect is always excitatory, and if stimulation reaches threshold, the skeletal muscle fibers contacts

Neurotransmitter Effects Neurotransmitters released onto visceral

effector organs by postganglionic autonomic fibers include– Norepinephrine secreted by most sympathetic

fibers– Acetylcholine released by parasympathetic

fibers Depending on the receptors present on the

target organ, its response to these neuro- transmitters may be either excitation or inhibition

Overlap of Somatic & Autonomic Higher brain centers regulate and

coordinate both somatic and visceral motor activities

Nearby spinal nerves and many cranial nerves contain both somatic and autonomic fibers

Most of the body’s adaptations to changing internal and external conditions involve both skeletal activity and enhanced response of visceral organs

Divisions of ANS There are two division of the ANS

– Parasympathetic– Sympathetic

Generally the two divisions have chains of two motor neurons that innervate same visceral organs but cause essentially opposite effects

If one division stimulates certain smooth muscle to contract or a gland to secrete, the other division inhibits that action

Through this process of duel innervation the two systems counterbalance each other

Divisions of ANS The sympathetic part mobilizes the body

during extreme situations (such as fear, exercise or rage)

The parasympathetic division allows us to unwind as it performs maintenance activities and conserves body energy

Divisions of ANS Both the sympathetic

and parasympathetic divisions issue from the brain and spinal cord

Two neuron pathways are shown for both divisions

Solid lines indicate pre-ganglionic axons while broken lines indicate post-ganglionic axons

Sympathetic Division The sympathetic division is responsible for

the “fight, flight, or fright” response Its activity is evident during vigorous

exercise, excitement, or emergencies Physiological changes like a pounding

heart, fast and deep breathing, dilated eye pupils, and cold, sweaty skin are signs of the mobilization of the sympathetic division, which help us survive danger

Sympathetic Division Sympathetic responses prepare our

bodies to cope with physiological stressors

While sympathetic response may increases the capacities of some systems they may in fact inhibit “non-essential” functions such as digestion and urinary tract motility

Sympathetic Division The sympathetic system also innervates

blood vessels, sending signals to the smooth muscles in their walls

Even though sympathetic input causes the smooth muscle in some vessels (in skeletal muscle) to relax so that the vessel dilates, the bulk of sympathetic input signals smooth muscle in blood vessels to contract, producing vasoconstriction

Sympathetic Division Vasoconstriction results in the narrowing

of vessel diameter which forces the heart to work harder to pump blood around the vascular circuit

As a result sympathetic activity results in blood pressure to rise during excitement and stress

Role of Sympathetic Division During exercise the sympathetic division

also promotes physiological adjustments– Visceral blood supply is diminished– Blood is shunted to working musculature– Bronchioles of the lungs dilate to increase

ventilation– Liver releases more sugar into blood stream

to support metabolism

Role of Sympathetic Division Its activity is evident when we are excited or

find ourselves in emergency or threatening situations (frightened)

Pounding heart; rapid, deep breathing; cold, sweaty skin; and dilated eyes are signs

Also changes in brain wave patterns Its function is to provide the optimal

conditions for an appropriate response to some threat (run / see / think)

Parasympathetic Division The parasympathetic division is most

effective in non-stressful situations This division is chiefly concerned with

keeping body energy use as low as possible, even as it directs body processes such as digestion and elimination

Resting and digesting division

Autonomic Homeostasis Autonomic homeostasis is the dynamic

counteraction between the two divisions such that they balance each other during times when we are neither highly excited nor completed at rest

Divisions of ANS In addition to the functional differences

between the parasympathetic and sympathetic divisions , there are also anatomical and biochemical differences

Divisions of ANS The two divisions issue

from different regions of the CNS

The sympathetic can also be called the thoracolumbar division because its fibers emerge from the thoracic and lumbar parts of the spinal cord

Divisions of ANS The parasympathetic

division can also be termed the craniospinal division because its fibers emerge from the brain and spinal cord (sacral)

Comparison of ANS & PNS

A second difference between the two divisions is that sympathetic pathways have short pre-ganglionic fibers and long post-ganglionic fibers

Comparison of ANS & PNS

Parasympathetic pathways in contrast have long pre-ganglionic fibers and short post-ganglionic fibers

Divisions of ANS Therefore, all

sympathetic ganglia lie near the spinal cord and vertebral column, and all parasympathetic ganglia lie far from the CNS, in or near the organs innervated

Divisions of ANS The third

anatomical difference between the two divisions is that sympathetic axons branch profusely, while parasympathetic fibers do not

Divisions of ANS Extensive branching allows each

sympathetic neuron to influence a number of different visceral organs, enabling many organs to mobilize simultaneously during the “fight, flight or fright” response

Parasympathetic effects, by contrast are more localized and discrete

Divisions of ANS The main

biochemical difference between the two divisions involves the neurotransmitter release by the postganglionic axons

Divisions of ANS In the sympathetic division, most

postganglionic axons release norepinephine (also called noradrenaline) these fibers are termed adrenergic

The postganglionic neurotransmitter in the parasympathetic division is acetycholine (Ach) these fibers are termed cholinergic

The preganglionic axon terminals of both divisions always release acetylcholine

Divisions of ANS The main anatomical and physiological

differences between the parasympathetic and sympathetic divisions are summarized in Table 15.1

Anatomy of ANS The sympathetic and parasympathetic

divisions are distinguished by– Unique sites of origin– Different lengths of their fibers– Location of their ganglia

Anatomy of ANS Unique origin sites

– Parasympathetic fibers emerge from the brain and from the spinal cord at the sacral level

– Sympathetic fibers originate from the thoracic and lumbar regions of the spinal cord

Anatomy of ANS Different Lengths of their Fibers

– Parasympathetic division has long preganglionic and short postganglionic fibers

– Sympathetic is the opposite with short preganglionic and long postganglionic fibers

Anatomy of ANS Length of their Ganglia

– Most parasympathetic ganglia are located in the visceral effector organs

– Sympathetic ganglia lie close to the spinal cord

Parasympathetic Division The parasympathetic emerge from

opposite ends of the central nervous system

The preganglionic axons extend from the CNS nearly all the way to the structures to be innervated

Parasympathetic Division

The preganglionic neurons synapse with the ganglionic neurons located in terminal ganglia

Very short post ganglionic axons issue from the terminal ganglia and synapse with effector cells in their immediate area

Parasympathetic Division

Several cranial nerves contain outflow of the parasympathetic

Preganglionic fibers run in the oculomotor, facial, glossopharyngeal, and vagus nerve

Cranial Outflow Oculomotor nerve III

– The parasympathetic fibers of the oculomotor nerves innervate smooth muscles of the eye

• Constrictor muscles of iris cause pupil to constrict

• Ciliary muscle within the orbits of the eye controls lense shape for visual focusing

• Allow the eye to focus on close objects in the visual field

Cranial Outflow Facial Nerves VII

– The parasympathetic fibers of the facial nerves stimulate the secretory activity of many large glands of the head

• The pathway activates the nasal glands and the lacrimal glands of the eyes

• The preganglionic fibers then run to synapse with ganglionic neurons in the pterygopalatine ganglia stimulating the submandibular and sublingual salivary glands

Cranial Outflow Glossopharyngeal

(IX)– The

parasympathetic nerves originate in the medulla and activate the parotid salivary gland

Cranial Outflow Vagus nerves (X)

– The major portion of the parasympathetic cranial outflow is via the vagus nerves

– The two vagus nerves account for an estimated 90% of all preganglionic parasympathetic fibers in the body

– They provide fibers to the neck and contribute to nerve plexuses that serve virtually every organ in the thoracic and abdominal cavity

Cranial Outflow The vagus nerve

fibers arise from the dorsal motor of the medulla and terminate by synapsing in terminal ganglia that are usually located in the walls of the target organ

Cranial Outflow Most of the terminal ganglia are not

individually named; instead they are collectively called intramural ganglia, literally ganglia “within the walls”

As the vagus nerves passes into the thorax, they send branches to autonomic plexuses– Cardiac plexuses– Pulmonary plexuses– Esophageal plexuses

Cranial Outflow When the vagus

nerves reach the esophagus, their fibers intermingle to form the anterior and posterior vagal trunks

Each trunk carries fibers from both vargus nerves

Cranial Outflow The vagal trunks

ride the esophagus down to enter the abdominal cavity

They send fibers to form the aortic plexuses (formed by the celiac, superior mesenteric and hypogastric)

Cranial Outflow Abdominal organs

which receive vagal innervation include the liver, gallbladder, stomach, small intestine, kidneys, pancreas, and the proximal half of the large intestine

The rest of the cavity are innervated by the sacral outflow

Sacral Outflow The sacral outflow

arises from neurons located in the lateral horn of the spinal cord at S2 - S4

The axons of these neurons run in the ventral roots of the spinal nerves to the ventral rami

Sacral Outflow From the ventral

rami the neurons branch to form the pelvic splanchnic nerves

Most neurons synapse in the intramural ganglia located in the walls of the distal large intestine, urinary bladder and reproductive organs

Sympathetic Division The sympathetic division innervates

more organs It supplies not only the visceral organs in

the internal body cavities, but also the visceral structures in the superficial part of the body– Sweat glands– Arrector pili– Arteries and veins

Sympathetic Division All preganglionic

fibers in the sympathetic division arise from cell bodies of preganglionic neurons located in spinal cord segments from T1 through L2

It is also called the thoracolumbar

Sympathetic Division After leaving the cord

via the ventral root, the preganglionic sympathetic fibers pass through a white ramus communicans to enter the adjoining chain (paravertebral) ganglion forming part of the sympathetic trunk or chain

Sympathetic Division The sympathetic

trunks flank each side of the vertebral column and appear as strands of white beads

Sympathetic Division Although the

sympathetic trunks extend from the neck to the pelvis, sympathetic fibers arise only from the thoracic and lumbar spinal cord segments

Sympathetic Division The ganglia vary in size, position, and

number, but there are typically 23 ganglia in each sympathetic chain…– 3 cervical– 11 thoracic– 4 lumbar– 4 sacral– 1 coccygeal

Sympathetic Division Once a preganglionic

axon reaches a paravertebral ganglion one of three things can happen to it

Sympathetic Division Once a preganglionic

axon reaches a paravertebral ganglion one of three things can happen to it…1. It can synapse with a

ganglionic neuron within the same chain ganglion

Sympathetic Division Once a preganglionic

axon reaches a paravertebral ganglion one of three things can happen to it…2. It can ascend or descend

the sympathetic chain to synapse in another chain ganglion

Sympathetic Division Once a preganglionic

axon reaches a paravertebral ganglion one of three things can happen to it…3. It can pass through the

chain ganglion and emerge from the sympathetic chain without synapsing

Sympathetic Division Preganglionic fibers

which emerge from the sympathetic chain without synapsing help to form the splanchnic nerves which synapse with prevertebral or collateral ganglia

Sympathetic Division The prevertebral

ganglia are located anterior to the vertebral column

Sympathetic Division Unlike the paravertebral ganglia the

prevertebral ganglia . . .– Are neither paired nor segmentally arranged– They occur only in the abdomen and pelvis

Sympathetic Division Note: Regardless of where the synapse occurs,

all sympathetic ganglia lie close to the spinal cord

The postganglionic fibers which run from the ganglion to the organs are typically much longer than the preganglionic fibers

Visceral Reflexes The visceral sensory neurons are the first

link in the autonomic reflexes These neurons send information

concerning chemical changes, stretch, and irritation of the viscera

Visceral Reflexes Visceral reflex arcs have essentially the

same components as somatic reflex arcs– Receptor– Sensory neuron– Integration center– Motor neuron– Effector

Visceral Reflexes

Visceral reflex arcs differ in that they have a two-neuron chain

Visceral Reflexes Nearly all sympathetic and parasympathetic

fibers are accompanied by afferent fibers conducting sensory impulses from glands or muscular structures

Thus, peripheral processes of visceral sensory neurons are found in cranial nerves, VII, IX, and X, the splanchnic nerves, and the sympathetic trunk, as well as the spinal nerves

Visceral Reflexes Like sensory neurons serving somatic

structures (skeletal muscles and skin) The cell bodies of visceral sensory

neurons are located in the sensory ganglia of associated cranial nerves or in the dorsal root ganglia of the spinal cord

Visceral Reflexes Visceral sensory reflexes are also found

within sympathetic ganglia where synapses with preganglionic neurons occur

Complete three-neuron reflex arcs (sensory, motor, and intrinsic neurons) exist within the walls of the gastro-intestinal tract– Enteric nervous system– Controls gastrointestinal activity

Visceral Reflexes The fact that

visceral pain travels along the same pathways as somatic pain fibers helps to explain the phenomenon of referred pain in which pain stimuli arising in the viscera is perceived as somatic in origin

Visceral Reflexes A heart attach may

produce a sensation of pain that radiates to the superior thoracic wall and along the medial aspect of the left arm

Visceral Reflexes Since the same

spinal segments (T1-T5) innervate both the heart and the regions to which pain signals from heart tissue are referred, the brain interprets most such inputs as coming from the somatic pathway

Visceral Reflexes Additional

cutaneous areas to which visceral pain is referred

Overview of the ANS The autonomic nervous system differs in…

– Its effectors– Its efferent pathways– Its target organs

Effectors of ANS The somatic nervous system stimulates

skeletal muscles The ANS innervates cardiac and smooth

muscles and glands