Chapter 15: ... Flashcards
Autonomic Nervous System (ANS)
Motor nervous system which innervates involuntary effectors
* Cardiac and smooth muscle, glands
Primary target organs:
* Thoracic viscera
* Abdominopelvic viscera
* Some body wall structures
- Actions occur without voluntary input or conscious awareness
Effectors do not require ANS innervation for normal function
* ANS adjust organ function to meet changing needs of body
Notes on Reflexes
Any quick, involuntary, stereotyped reaction to stimulation
* Muscle OR gland
Characteristics
* Require stimulation
* Involuntary
* Quick
* Stereotyped
Two main categories
* Somatic reflexes involve skeletal muscle
* Visceral reflexes involve cardiac and smooth muscle or glands
* aka autonomic reflexes
Somatic Vs Visceral Reflexes
Somatic Reflexes
* Somatic receptors
* In skin, muscles, tendons
* Somatic effectors
* Skeletal muscle
* Faster
Visceral Reflexes
* Visceral receptors
* In organs
* Visceral effectors
* Cardiac and smooth muscle,
glands
* Slower
Components of a Reflex Arc
Receptor
* Detects stimulus
Sensory (afferent) neuron
* Carries information to integrating center
Integrating center
* Within gray matter of CNS
* Involves interneurons
Motor (efferent) neuron
* Carries commands to effector
Effector
* Carries out response
Divisions of the ANS
Two divisions
* Sympathetic
* Parasympathetic
- Differ in structure and function
Typically organs innervated by both divisions
* May have opposing or cooperative effects
- Some organs may be innervated by only one division
Sympathetic Vs Parasympathetic
Sympathetic Division
* Adapts body for physical activity
* “Fight or flight”
* More blood flow to cardiac and
skeletal muscle
* Response to stress, exercise,
danger, fear, anger, trauma, etc.
* Increased energy expenditure
Parasympathetic Division
* Calming effect on body
* “Rest and digest”
* More blood flow to skin and
digestive tract
* Reduced energy expenditure and
normal bodily maintenance
* Digestion and waste elimination
Autonomic Tone
- Both divisions are usually active simultaneously
Autonomic tone is the background rate of ANS activity
* Balance between sympathetic and parasympathetic tone
* Shifts according to body’s needs
Neither division is exclusively excitatory or inhibitory
* Sympathetic division excites heart, inhibits digestive organs
* Parasympathetic division inhibits heart, excites digestive organs
Somatic Vs Autonomic Motor Division
Somatic
* One lower motor neuron
* No ganglia
* Acetylcholine (ACh)
* Axons end in neuromuscular
junctions
* Skeletal muscle
* Faster conduction
* Usually voluntary
* Always excitatory
Autonomic
* Two lower motor neurons
* Ganglia
* Either ACh or NE
* Axons end in varicosities
* Cardiac and smooth muscle,
glands
* Slower conduction
* Usually involuntary
* Can be excitatory or
inhibitory
Pre- and Postganglionic Neurons
Preganglionic neuron lies before autonomic ganglia
* Neurosoma in brain or spinal cord
* Myelinated
* Axon projects to autonomic ganglion
* Uses ACh exclusively
Postganglionic neuron lies after autonomic ganglia
* Neurosoma lies in autonomic ganglion
* Unmyelinated
* Axon projects to effector
* Uses either ACh or NE
* End in varicosities
ANS Division Structural Differences
Location of preganglionic neuron neurosoma
* SNS: spinal cord (T1-L2) = thoracolumbar
* PNS: brainstem and spinal cord (S2–S4) = craniosacral
Location of ganglia
* SNS: close to vertebral column
* PNS: close to or within target organs
Neurotransmitter utilized
* Both preganglionic: ACh
* SNS postganglionic: mostly norepinephrine (NE)
* PNS postganglionic: ACh
Length of pre- and postganglionic axons
* SNS: pre = shorter, post = longer
* PNS: pre = longer, post = shorter
Neural divergence
* SNS: extensive (many branches)
* PNS: minimal (fewer branches)
Effects
* SNS: more widespread, general
* PNS: more local, specific
Sympathetic Neurons
Preganglionic
* Neurosomas within thoracic and lumbar spinal cord
* Lateral horns and nearby gray matter
* Use ACh only
* Axons exit via spinal nerves (T1 – L2)
Postganglionic
* Neurosomas within autonomic ganglia
* Primarily use NE
* Axons innervate body wall structures and viscera
Sympathetic Ganglia
Sympathetic chain (paravertebral) ganglia
* Chains lateral to spinal cord
* Runs from cervical to coccygeal level
* Postganglionic axons innervate head, neck, thoracic cavity,
abdominopelvic cavity
Collateral (prevertebral) ganglia
* Anterior to vertebral column
* Named for nearby blood vessels
* Three in total
* Postganglionic axons innervate abdominopelvic structures
* Travel along with blood vessels to effectors
SNS Effector Innervation
- Postganglionic fibers leave sympathetic chain to innervate effector\
Via spinal nerves
* Sympathetic axons travel within spinal nerves
* Overall: structures within muscles and body wall
* Sweat glands, piloerector muscles, blood vessels of skin and skeletal muscles
Via sympathetic nerves
* Heart, lungs, esophagus, thoracic blood vessels
* Form carotid plexus, fibers innervate effectors of head
* Overall: head and thoracic cavity
Via splanchnic nerves
* Preganglionic axons don’t synapse within sympathetic ganglia
* Form splanchnic nerves which synapse at collateral ganglia
* Overall: structures within abdominopelvic cavity
The Adrenal (Suprarenal) Glands
- Positioned on top of kidneys
Divided into two regions
* Adrenal cortex: endocrine gland, secretes steroids
* Adrenal medulla: modified sympathetic ganglion
Medulla produces catecholamine hormones
* Epinephrine, norepinephrine, dopamine
* Also act as neurotransmitters
Selected Effects of Increased SNS Stimulation
Increased cardiac output
* Heart rate
* Contraction strength
Systemic vasoconstriction, raised blood pressure
* Vasodilation of coronary arteries
Increased airflow
* Bronchodilation
- Increased metabolic rate and nutrient breakdown
- Increased sweat secretion
- Pupillary dilation
- Inhibition of digestive and urinary activity
Increased secretion from medulla
* Hormones prolong SNS effects
Parasympathetic Neurons
Preganglionic
* Neurosomas within brainstem and sacral spinal cord
* Use ACh only
* Axons travel via cranial and sacral nerves
Postganglionic
* Neurosomas within autonomic ganglia
* Use ACh only
* Axons do not typically innervate body wall structures
* Some exceptions
Parasympathetic Ganglia
Terminal ganglia
* Lie in or near target organ
Intramural ganglia
* Embedded within the wall of target organ
* Subtype of terminal ganglion
PNS Effector Innervation
- Parasympathetic axons travel via cranial or splanchnic nerves
Cranial nerves
* From neurosomas in brainstem
* CN III, VII, IX innervate structures of head
* CN X handles innervation to thoracic and abdominal cavities
Pelvic splanchnic nerves
* From neurosomas in the sacral spinal cord
* Give rise to pelvic nerves
* Innervate pelvic structures
Cranial Nerves and the PNS
Oculomotor nerve III
* Innervates muscles of eye
Facial nerve VII
* Innervates lacrimal glands, salivary glands, nasal glands
* Splits, associated with two ganglia
Glossopharyngeal nerve IX
* Innervates salivary glands
Vagus nerve X
* Majority (90%) of parasympathetic axons
* Thoracic and abdominal organs
* Forms three plexuses
Selected Effects of PNS Stimulation
Decreased cardiac output
* Decreased heart rate
Decreased airflow
* Bronchoconstriction
Decreased metabolic rate
* Increased nutrient digestion, synthesis, and storage
Pupillary constriction
* Stimulation of digestive and urinary activity
ANS and Neurotransmitters
- SNS and PNS typically have contrasting effects on target organs
How: postganglionic neurons release different NTs
* SNS: NE
* PNS: ACh
Same NT from the same division can have contrasting effects
* EX: vasoconstriction versus bronchodilation (both SNS NE)
How: different receptors for NTs exist
* Permits NTs to have varying effects based on which receptor is
present
Acetylcholine (ACh)
Released by
* All preganglionic neurons
* All PNS postganglionic neurons
* Some of SNS postganglionic neurons
* Sweat glands, blood vessels
- Neurons that release ACh named
cholinergic neurons (fibers) - Receptors that bind ACh named
cholinergic receptors
Cholinergic Receptors
Muscarinic
* Smooth and cardiac muscle, gland cells
* Subtypes with excitatory and inhibitory effects
* Requires use of second messenger systems
Nicotinic
* Locations
* All autonomic ganglia synapses
* Adrenal medulla
* Neuromuscular junctions
* Always excitatory!
* Open gated ion channels, produces EPSPs
Norepinephrine (NE)
- Secreted by majority of sympathetic
postganglionic neurons - Neurons that release NE named
adrenergic neurons (fibers) - Receptors that bind NE named
adrenergic receptors
Adrenergic Receptors
Alpha (α) receptors
* Primarily excitatory
* Two subclasses permit contrasting effects on effectors
* Uses second messenger systems
Beta (β) receptors
* Primarily inhibitory
* Two subclasses permit contrasting effects on effectors
* Use second messenger systems
Neurotransmitters and Effect Duration
- Sympathetic effects last longer than parasympathetic effects
Acetylcholine
* Degraded by acetylcholinesterase enzyme
Norepinephrine
* Reabsorbed by neuron
* Diffuses into nearby tissues
* Enters bloodstream
* Ultimately degraded by enzymes or the liver
Dual Innervation
- Most organs innervated by both SNS and PNS
Antagonistic effects
* Divisions have opposing effects
* Target same or different effector cells
* Ex: heart, pupil, digestive organs
Cooperative effects
* Divisions work together to produce unified effect
* Divisions act on different effector cells
* Salivation, male sexual activity
- Divisions may not innervate organs equally or have equal influence
Control Without Dual Innervation
- Organ receives innervation from either SNS or PNS
Changes occur due to alterations of autonomic tone
* Sympathetic tone
* Parasympathetic tone
Blood pressure regulation via vasomotor tone
* Increased SNS tone vasoconstriction, raised blood pressure
* Decreased SNS tone vasodilation, lowered blood pressure
Several organs receive only SNS innervation
* Many blood vessels, adrenal medulla, arrector muscles, sweat
glands
CNS Control Over ANS
The autonomic nervous system is a division of the nervous
system
* Not fully independent
Activity of the central nervous system can adjust ANS activity
* Allows adjustments based on current body conditions
CNS Structures Which Influence ANS
Cerebral cortex
* Sensory and emotional experiences to alter ANS activity
* Due to limbic system
* Features connections to hypothalamus
Hypothalamus
* Major control center for autonomic nervous system
* Houses nuclei which regulate specific functions
* Hunger, thirst, thermoregulation, emotion, sexuality, etc.
* Has connections to limbic system
* Has connections to brainstem
Brainstem
* House autonomic nuclei
* Cardiac center, vasomotor center, respiratory centers, salivation, etc.
* Reticular formation
Spinal cord
* Integrates some autonomic reflexes
* Micturition, defecation, erection, ejaculation
* Note: brain can consciously inhibit some of these reflexes
