The Autonomic Nervous System Chapter 15. Introduction n The Autonomic Nervous System (ANS) is the system of motor neurons that innervate the smooth muscle,
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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 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 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 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
Overview of the ANS The autonomic nervous system differs in…
– Its effectors– Its efferent pathways– Its target organs
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