chapter 15 the autonomic nervous system

Autonomic nervous system (ANS)

a motor nervous system that controls glands, cardiac muscle, and smooth muscle

-also called visceral nervous system

-carries out actions involuntarily: without our conscious intent or awareness

visceral reflexes

unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors and somewhat slower responses

receptors (visceral reflex arc)

nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli

afferent neurons (visceral reflex arc)

leading to the CNS

interneurons (visceral reflex arc)

in the CNS

efferent neurons (visceral reflex arc)

carry motor signals away from the CNS

effectors (visceral reflex arc)

make adjustments

-do not depend on the ANS to function; only to adjust their activity to the body’s changing needs

ex: adjusting heart rate

visceral reflexes high blood pressure

high blood pressure is detected by arterial stretch receptors (1), afferent neurons (2) carry signals to CNS, efferent (3) signals travel to the heart, then (4) heart slows reducing blood pressure

-homeostatic negative feedback loop

sympathetic division

prepares body for physical activity: exercises, trauma, arousal, competition, anger, or fear

-increases heart rate, BP, airflow, blood glucose levels

-reduces blood flow to the skin, digestive tract, and urinary track

-fight or flight stage

parasympathetic division (ANS)

calms many body functions reducing energy expenditure and assist in bodily maintenance

-digestion and waste elimination

-reduces heart rate, BP

-resting and digesting state

-reduces energy expenditure

autonomic tone

normal background rate of activity that represents the balance of the two systems according to the bodys changing needs

somatic motor pathways (autonomic output pathways)

a motor neuron from the brainstem or spinal cord issues a myelinated axon that reaches all the way to the skeletal muscle

autonmonic pathway (autonomic output pathways)

1. must cross a synapse where these two neurons meet in an autonomic ganglion

2. preganglionic neuron: the first neuron has a soma in the brainstem or spinal cord

3. synapses with a postganglionic neuron whose axon extend the rest of the way to the target cell

sympathetic division (anatomy of ANS)

relatively short preganglionic and long postganglionic fibers

preganglionic neurosomas (anatomy of ANS)

lateral horns of spinal cord/column

-fibers exit spinal cord by way of spinal nerves T1 to L2

-lead to nearby sympathetic chain of ganglia

• series of longitudinal ganglia adjacent to both sides of the vertebral column from cervical to coccygeal levels

each paravertebral ganglion is connected to a spinal nerve by two branches: communicating rami

preganglionic fibers

small myelinated fibers that travel from spinal nerve to the ganglion by way of the white communicating ramus (myelinated)

postganglionic fibers

leave the ganglion by way of the gray communicating ramus (unmyelinated)

-forms a bridge back to the spinal nerve

extend the rest of the way to the target organ

nerve fibers route

leave the sympathetic chain by spinal, sympathetic, and splanchnic nerves

spinal nerve route (U-turn)

some postganglionic fibers exit a ganglion by way of the gray ramus

return to the spinal nerve and travel the rest of the wat to the target organ

most sweat glands, piloerector muscles, and blood vessels of the skin and skeletal muscle

sympathetic nerve route (elevator)

some travel up or down the chain and synapse in ganglia at other levels

-these fibers link to the paravertebral ganglia into a chain

extend to the heart, lungs, esophagus, and thoracic blood vessels

form carotid plexus around each carotid artery of the neck

issue fibers from there to the effects in the head

-sweat, salivary, nasal glands; piloerector muscles; blood vessels; dilators of iris

splanchnic nerve route (stop sign)

some fibers that arise from spinal nerves T5 to T12 pass through the sympathetic ganglia without synapsing

-continue on as the splanchnic nerves

-lead to second set of ganglia: collateral (prevertebral) ganglia and synapse there

collateral ganglia contribute to a network called the abdominal aortic plexus

-wraps around abdominal aorta

-three major collateral ganglia in the plexus

• celiac, superior mesenteric, and inferior mesenteric

• postganglionic fibers accompany these arteries and their branches to their target organs

the adrenal glands

paired adrenal (supraradenal glands) on superior poles of the kidneys

two glands with two different functions

consist of modified postganglionic neurons without dentaries or axons

stimulated by preganglionic sympathetic neurons that terminate on these cells

secretes a mixture of hormones into bloodstream - 85% epinephrine (adrenaline) and 15% norepinephrine (noradrenaline)

adrenal cortex (outer layer)

secretes steroid hormones

adrenal medulla (inner core)

essentially a sympathetic ganglion

the parasympathetic division

arises from the brain and sacral regions of the spinal cord

-fibers travel in certain cranial and sacral nerves

-origin of long preganglionic neurons

-midbrain, pons, and medulla

-sacral spinal cord segments s2 to s4

pathways of long preganglionic fibers

fibers in cranial nerves III, VII, IX, X

fibers arising from sacral spinal cord

terminal ganglia in or near target organs

long preganglionic, short postganglionic fibers

neuronal divergence less than sympathetic division

one preganglionic fiber reaches the target organs and then stimulates fewer that five postganglionic cells

parasympathetic cranial nerve

oculomotor nerve(III): narrows pupil and focuses lens

facial nerve (VII): tear, nasal, and salivary glands

glossopharyngeal nerve (IX): parotid salivary gland

vagus nerve (X): viscera as far as proximal half of colon (resting and digesting)

distal half of colon, rectum, urinary bladder, and reproductive organs

the enteric nervous system

the nervous system of the digestive track

does not arise from the brainstem or spinal cord

does innervate smooth muscle glands

no components in the CNS

has its own reflex arcs '

regulates motility of esophagus, stomach, and instestine and secretion of digestive enzymes and acid

normal digestive function also requires regulation by sympathetic and parasympathetic systems

neurotransmitters and their receptors

effects determined by types of neurotransmitters released and types of receptors found on target cells

two fundamental reasons:

• sympathetic and parasympathetic fibers secrete different neurotransmitter’s

• target cells respond to the same neurotransmitter differently depending upon the type of receptor they have for it

  • all autonomic fibers secrete either acetylcholine or norepinephrine

autonomic effects on glandular secretion

often an indirect result of their effect on blood vessels

vasodilation

increased blood flow; increased secretion (small —> big)

vasoconstriction

decreased blood flow; decreased secretion (big —> small)

dual innervation

most viscera receive nerve fibers from both parasympathetic and sympathetic divisions

antagonistic effect

oppose each other

exerted through dual innervation of same effector cells

• heart rate decreases (parasympathetic)

• heart rate increase (sympathetic)

exerted bc each division innervates different cells

• pupillary dilator muscle (sympathetic) dilates pupil

• constrictor pupillae (parasympathetic) constricts pupil

1 does 1 thing and the other 1 does something else

cooperative effects

two divisions act on different effectors to produce a unified overall effect

when two divisions acts on different effectors to produce a unified effect

• parasympathetic increase salivary serous cell secretion

• sympathetic increase salivary mucous call secretion

works together to produce an effect

Ex: someone spitting Water=para, mucous= symp

control without dual innervation

some effectors receive only sympathetic fibers

• adrenal medulla, arrector pili muscles, sweat glands, and many blood vessels

• ex: regulation of blood pressure and routes of blood flow

sympathetic vasomotor tone

a baseline firing frequency of sympathetic

• keeps vessels in state of partial constriction

• increase in firing frequency -vasodilation (fast=constricting)

• decrease in firing frequency -vasoconstriction (slow = expanding)

sympathetic division acting alone

can exert opposite effect on the target organ through control of blood vessels

• during stress

  • blood vessels to muscles and heart dilate

  • blood vessels to skin constrict

• sympathetic division prioritized blood vessels to skeletal muscles and heart in times of emergency

• blood vessels to skin vasoconstrict to minimize blooding in injury occurs during stress or exercise

cerebral cortex

has an influence: anger, fear, anxiety

• powerful emptions influence the ANS because of the connections between our limbic system and the hypothalamus

hypothalamus

major visceral motor control center

• nuclei for primitive functions — hunger, thirst, sex

midbrain, pons, and medulla oblongata

nuclei for cardiac and vasomotor control; salivation, swallowing, sweating, bladder control, and pupillary changes

spinal cord reflexes

defecation and micturition reflex’s and integrated in spinal cord

we control these functions because of our control over skeletal muscle sphincters; if the spinal cord is damaged, the smooth muscle of bowel and bladder is controlled by autonomic reflexes built into the spinal cord.