0-1 Chapter 15 - Autonomic Nervous System Flashcards
Autonomic Nervous System
- portion of the nervous system that operates in comparative secrecy
- it manages a multitude of unconscious processes responsible for the body’s homeostasis
autonomic nervous system (ANS)
a motor nervous system that controls glands, cardiac muscle, and smooth muscle
carries out actions involuntarily–without our conscious intent or awareness
visceral motor system
autonomic nervous system (ANS)
primary organs of the ANS
•viscera of thoracic and abdominal cavities •some structures of the body wall –cutaneous blood vessels –sweat glands –piloerector muscles
denervation hypersensitivity
exaggerated response of cardiac and smooth muscle if autonomic nerves are severed
visceral reflexes
unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors and somewhat slower responses
visceral reflex arc
–receptors–nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli
–afferent neurons –leading to the CNS
–interneurons–in the CNS
–efferent neurons –carry motor signals away from the CNS
–effectors–that make adjustments
Visceral Reflex to High BP
high blood pressure detected by arterial stretch receptors (1), afferent neuron (2) carries signal to CNS, efferent (3) signals travel to the heart (4), heart slows reducing blood pressure
Divisions of ANS
•two divisions innervate same target organs
–may have cooperative or contrasting effects
sympathetic division
parasympathetic division
sympathetic division
prepares body for physical activity –exercise, trauma, arousal, competition, anger, or fear
•increases heart rate, BP, airflow, blood glucose levels, etc
•reduces blood flow to the skin and digestive tract
parasympathetic division
calms many body functions reducing energy expenditure and assists in bodily maintenance
•digestion and waste elimination
•“resting and digesting” state
autonomic tone
normal background rate of activity that represents the balance of the two systems according to the body’s changing needs
parasympathetic tone
- maintains smooth muscle tone in intestines
* holds resting heart rate down to about 70 –80 beats per minute
sympathetic tone
keeps most blood vessels partially constricted and maintains blood pressure
balance between sympathetic and parasympathetic
sympathetic division excites the heart but inhibits digestive and urinary function, while parasympathetic has the opposite effect
Neural Pathways
ANS has components in both the central and peripheral nervous systems
–control nucleus in thehypothalamus and other brainstem regions
–motor neurons in the spinal cord and peripheral ganglia
–nerve fibers that travel through the cranial and spinal nerves
somatic motor pathway
–a motor neuron from the brainstem or spinal cord issues a myelinated axon that reaches all the way to the skeletal muscle
autonomic pathway
–signal must travel across two neurons to get to the target organ
–must cross a synapse where these two neurons meet in an autonomic ganglion
presynaptic neuron
the first neuron has a soma in the brainstem or spinal cord
postganglionic neuron
synapses with a postganglionic neuron whose axon extends the rest of the way to the target cell
ANS –two neurons from CNS to effectors
- presynaptic neuron whose cell body is in CNS
* postsynaptic neuron cell body in peripheral ganglion
Sympathetic Nervous System
also called the thoracolumbar division because it arises from the thoracic and lumbar regions of the spinal cord
relatively short preganglionic and long postganglionic fibers
preganglionic neurosomas
in lateral horns and nearby regions of the gray matter of spinal cord
–fibers exit spinal cord by way of spinal nerves T1 to L2
–lead to nearby sympathetic chain of ganglia (paravertebral ganglia)
sympathetic chain of ganglia
- series of longitudinal ganglia adjacent to both sides of the vertebral column from cervical to coccygeal levels
- usually 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal ganglion
- sympathetic nerve fibers are distributed to every level of the body
preganglionic fibers are
small myelinated fibers that travel form 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
postganglionic fibers extend
postganglionic fibers extend the rest of the way to the target organ
after entering the sympathetic chain, the preganglionic fibers may follow any of three courses
some end in ganglia
some travel up or down the chain
some pass through the chain without synapsing
nerve fibers leave the sympathetic chain by
spinal, sympathetic, and splanchnic nerves
spinal nerve route
- some postganglionic fibers exit a ganglion by way of the gray ramus
- returns to the spinal nerve and travels the rest of the way to the target organ
- most sweat glands, piloerector muscles, and blood vessels of the skin and skeletal muscles
sympathetic nerve route
•other nerves leave by way of sympathetic nerves that extend to the heart, lungs, esophagus and thoracic blood vessels
•these nerves form carotid plexus around each carotid artery of the neck
•issue fibers from there to the effectors in the head
–sweat, salivary, nasal glands, piloerector muscles, blood vessels, dilators of iris
•some fibers of superior and middle cervical ganglia form cardiac nerves to the heart
splanchnic nerve route
splanchnic nerve route
•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
abdominal aortic plexus
–wraps around abdominal aorta
–three major collateral ganglia in this plexus
•celiac, superior mesenteric, and inferior mesenteric
•postganglionic fibers accompany these arteries and their branches to their target organs
solar plexus
collective name for the celiac and superior mesenteric ganglia
•nerves radiate from ganglia like rays of the sun
neuronal divergence predominates
–each preganglionic cell branches and synapses on 10 to 20 postganglionic cells
–one preganglionic neuron can excite multiple postganglionic fibers leading to different target organs
–have relatively widespread effects
effectors in body wall are innervated by
sympathetic fibers in spinal nerves
effectors in head and thoracic cavity are innervated by
fibers in sympathetic nerves
effectors in abdominal cavity are innervated by
sympathetic fibers in splanchnic nerves
Adrenal Glands
- paired adrenal (suprarenal) glands on superior poles of the kidneys
- each is two glands with different functions
adrenal cortex
(outer layer)
•secretes steroid hormones
adrenal medulla
(inner core)
•essentially a sympathetic ganglion
•consists of modified postganglionic neurons without dendrites or axons
–stimulated by preganglionic sympathetic neurons that terminate on these cells
•secretes a mixture of hormones into bloodstream
–catecholamines -85% epinephrine(adrenaline) and 15% norepinephrine (noradrenaline)
–also function as neurotransmitters
sympathoadrenal system
is the closely related functioning adrenal medulla and sympathetic nervous system
parasympathetic division is also called the
craniosacral division
–arises from the brain and sacral regions of the spinal cord
–fibers travel in certain cranial and sacral nerves
terminal ganglia
in or near target organs
–long preganglionic, short postganglionic fibers
neuronal divergence
less than sympathetic division
–one preganglionic fiber reaches the target organ and then stimulates fewer than 5 postganglionic cells
Parasympathetic Cranial Nerves
- Oculomotor nerve (III)
- Facial nerve (VII)
- Glossopharyngeal nerve (IX)
- Vagus nerve (X)
Efferent Pathways
remaining parasympathetic fibers arise from levels S2to S4of the spinal cord
•form pelvic splanchnic nerves that lead to the inferior hypogastric plexus
•most form pelvic nerves to their terminal ganglion on the target organs
enteric nervous system
the nervous system of the digestive tract
–does not arise from the brainstem or spinal cord
–does innervate smooth muscle and glands
regulates motility
of esophagus, stomach, and intestines and secretion of digestive enzymes and acid
•normal digestive function also requires regulation by sympathetic and parasympathetic systems
megacolon
massive dilation of bowel accompanied by abdominal distension and chronic constipation
Hirschsprung disease
hereditary defect causing absence of enteric nervous system
how can different autonomic neurons have different effects? constricting some vessels but dilating others
2 fundamental reasons:
–sympathetic and parasympathetic fibers secrete different neurotransmitters
–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
•there are 2 classes of receptors for each of these neurotransmitters
Acetylcholine (ACh)
ACh is secreted by all preganglionic neurons in both divisions and the postganglionic parasympathetic neurons
–called cholinergic fibers
–any receptor that binds it is called cholinergic receptor
2 types of cholinergic receptors
muscarinic receptors
nicotinic receptors
muscarinic receptors
- all cardiac muscle, smooth muscle, and gland cells have muscarinic receptors
- excitatory or inhibitory due to subclasses of muscarinic receptors
nicotinic receptors
on all ANS postganglionic neurons, in the adrenal medulla, and at neuromuscular junctions of skeletal muscle
•excitatory when ACh binding occurs
Norepinephrine (NE)
NE is secreted by nearly all sympathetic postganglionic neurons
–called adrenergic fibers
–receptors for it called adrenergic receptors
alpha-adrenergic receptors
–usually excitatory
–2 subclasses use different second messengers (α1& α2)
beta-adrenergic receptors
–usually inhibitory
–2 subclasses with different effects, but both act through cAMP as a second messenger (β1& β2)
autonomic effects on glandular secretion are often
an indirect result of their effect on blood vessels
–vasodilation –increased blood flow –increased secretion
–vasoconstriction–decreased blood flow –decreased secretion
sympathetic effects tend to last longer than
parasympathetic effects
–ACh released by parasympathetics is broken down quickly at synapse
–NE by sympathetics is reabsorbed by nerve, diffuses to adjacent tissues, and much passes into bloodstream
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
–exerted because each division innervates different cells
cooperative effects
two divisions act on different effectors to produce a unified overall effect
Without Dual Innervation
some effectors receive only sympathetic fibers
vasomotor tone
a baseline firing frequency of sympathetics
ANS regulated by several levels of CNS
cerebral cortex
hypothalamus
midbrain, pons, and medulla oblongata contain
spinal cord reflexes
cerebral cortex
has an influence –anger, fear, anxiety
•powerful emotions 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 contain
•nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes
spinal cord reflexes
- defecation and micturition reflexes are 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
neuropharmacology
study of effects of drugs on the nervous system
sympathomimetics
sympathomimetics enhance sympathetic activity
–stimulate receptors or increase norepinephrine release
•cold medicines that dilate the bronchioles or constrict nasal blood vessels
sympatholytics
sympatholytics suppress sympathetic activity
–block receptors or inhibit norepinephrine release
•beta blockers reduce high BP interfering with effects of epinephrine/norepinephrine on heart and blood vessels
parasympathomimetics
parasympathomimetics enhance activity
parasympatholytics
parasympatholytics suppress activity
caffeine competes with
adenosine(the presence of which causes sleepiness) by binding to its receptors
