PPT Notes Chapter 14 Flashcards
Autonomic Nervous System (ANS)
The ANS consists of motor neurons that:
Innervate smooth and cardiac muscle and glands
Make adjustments to ensure optimal support for body activities
Operate via subconscious control
Other names
Involuntary nervous system [due to its subconscious control]
General visceral motor system [indicating the location of most of its effectors]
Somatic and Autonomic Nervous Systems
The two systems differ in:
Effectors
Efferent pathways (and their neurotransmitters)
Target organ responses to neurotransmitters
Innervation of Effectors
Somatic nervous system Skeletal muscles ANS Cardiac muscle Smooth muscle Glands
Efferent Pathways
Somatic nervous system; one motor neuron The cell bodies of the neurons are in the spinal cord and their axons extend to the skeletal muscle they innervate. ANS pathway is a two-neuron chain Pre- (before) Post- (after) Let’s take a look:
Neurotransmitter Effects
Somatic nervous system
All somatic motor neurons release acetylcholine (ACh)
Effects are always stimulatory
ANS
Preganglionic fibers release ACh
Postganglionic fibers release norepinephrine or ACh at effectors
Effect is either stimulatory or inhibitory, depending on type of receptors
Two Divisions of the ANS
Sympathetic division
Parasympathetic division
Dual innervation
Almost all visceral organs are served by both divisions, but they cause opposite effects
Functional Role of the Parasympathetic Division: “rest and digest”
Promotes maintenance activities and conserves body energy
Its activity is illustrated in a person who relaxes, reading, after a meal
Blood pressure, heart rate, and respiratory rates are low
Gastrointestinal tract activity is high
Pupils are constricted and lenses are accommodated for close vision
Promotes maintenance activities and conserves body energy
Its activity is illustrated in a person who relaxes, reading, after a meal
Blood pressure, heart rate, and respiratory rates are low
Gastrointestinal tract activity is high
Pupils are constricted and lenses are accommodated for close vision
Functional Role of the Sympathetic Division:“fight or flight”
Mobilizes the body during activity; is the “fight-or-flight” system
Promotes adjustments during exercise, or when threatened
Blood flow is shunted to skeletal muscles and heart
Bronchioles dilate
Liver releases glucose
Sympathetic (Thoracolumbar) Division
Preganglionic neurons are in spinal cord segments T1 – L2
Sympathetic neurons produce the lateral horns of the spinal cord
Preganglionic fibers pass through the white rami communicantes and enter sympathetic trunk (paravertebral, or chain) ganglia
Sympathetic Trunks and Pathways
There are 23 paravertebral ganglia in the sympathetic trunk (chain)
3 cervical
11 thoracic
4 lumbar
4 sacral
1 coccygeal
Upon entering a sympathetic trunk ganglion a preganglionic fiber may do one of the following:
Synapse with a ganglionic neuron within the same ganglion
Ascend or descend the sympathetic trunk to synapse in another trunk ganglion
Pass through the trunk ganglion and emerge without synapsing
Visceral Reflexes
Visceral reflex arcs have the same components as somatic reflexes
Main difference: visceral reflex arc has two neurons in the motor pathway
Referred Pain
Visceral pain afferents travel along the same pathway as somatic pain fibers, contributing to the phenomenon of referred pain.
Pain stimuli arising in the viscera are perceived as somatic in origin
Neurotransmitters
Cholinergic fibers release the neurotransmitter ACh
All ANS preganglionic axons
All parasympathetic postganglionic axons
Adrenergic fibers release the neurotransmitter NE
Most sympathetic postganglionic axons
Exceptions: sympathetic postganglionic fibers secrete ACh at sweat glands and some blood vessels in skeletal muscles
Receptors for Neurotransmitters
Cholinergic receptors for ACh
Adrenergic receptors for NE
Cholinergic Receptors
Two types of receptors bind ACh Nicotinic Muscarinic Named after drugs that bind to them and mimic ACh effects [nicotine & muscarine]
Nicotinic Receptors
Found on
Motor end plates of skeletal muscle cells (Chapter 9)
All ganglionic neurons (sympathetic and parasympathetic)
Hormone-producing cells of the adrenal medulla
Effect of ACh at nicotinic receptors is always stimulatory
Muscarinic Receptors
Found on
All effector cells stimulated by postganglionic cholinergic fibers
The effect of ACh at muscarinic receptors
Can be either inhibitory or excitatory
Depends on the receptor type of the target organ
Adrenergic Receptors
Two types Alpha () (subtypes 1, 2) Beta () (subtypes 1, 2 , 3) Effects of NE depend on which subclass of receptor predominates on the target organ
Interactions of the Autonomic Divisions
Most visceral organs have dual innervation
Dynamic antagonism allows for precise control of visceral activity
Sympathetic division increases heart and respiratory rates, and inhibits digestion and elimination
Parasympathetic division decreases heart and respiratory rates, and allows for digestion and the discarding of wastes
Sympathetic Tone
Sympathetic division controls blood pressure, even at rest
Sympathetic tone (vasomotor tone)
Keeps the blood vessels in a continual state of partial constriction
Sympathetic fibers fire more rapidly to constrict blood vessels and cause blood pressure to rise
Sympathetic fibers fire less rapidly to prompt vessels to dilate to decrease blood pressure
Alpha-blocker drugs interfere with vasomotor fibers and are used to treat hypertension
Parasympathetic Tone
Parasympathetic division normally dominates the heart and smooth muscle of digestive and urinary tract organs
Slows the heart
Dictates normal activity levels of the digestive and urinary tracts
The sympathetic division can override these effects during times of stress
Drugs that block parasympathetic responses increase heart rate and block fecal and urinary retention
Cooperative Effects
Best seen in control of the external genitalia
Parasympathetic fibers cause vasodilation; are responsible for erection of the penis or clitoris
Sympathetic fibers cause ejaculation of semen in males and reflex contraction of a female’s vagina
Unique Roles of the Sympathetic Division
The adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels receive only sympathetic fibers The sympathetic division controls Thermoregulatory responses to heat Release of renin from the kidneys [rennin is a protein involved with raising blood pressure] Metabolic effects Increases metabolic rates of cells Raises blood glucose levels Mobilizes fats for use as fuels
Localized Versus Diffuse Effects
Parasympathetic division: short-lived, highly localized control over effectors
Sympathetic division: long-lasting, bodywide effects
Effects of Sympathetic Activation
Sympathetic activation is long lasting because NE
Is inactivated more slowly than ACh
NE and epinephrine are released into the blood and remain there until destroyed by the liver
Control of ANS Functioning
Hypothalamus—main integrative center of ANS activity
The brainstem appears to exert the most direct influence over autonomic functions.
Other controls come from the cerebral cortex, the reticular formation of the brain stem, and the spinal cord
Hypothalamic Control
Control may be direct or indirect (through the reticular system)
Centers of the hypothalamus control
Heart activity and blood pressure
Body temperature, water balance, and endocrine activity
Emotional stages (rage, pleasure) and biological drives (hunger, thirst, sex)
Reactions to fear and the “fight-or-flight” system
