D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MOTOR SYSTEMS III
Lecture 20: Autonomic Nervous System
1.
Organization:
• autonomic nervous system: part of the motor division of the peripheral
nervous system (ppt. 1) that controls functions of internal organs (viscera): heart,
digestive tract, vascular system, urinary tract, reproductive organs; also iris & pupil
of eye, sweat glands and arrector pili muscles
• autonomic reflexes: visceral reflexes account for most autonomic activation
- afferents are subconscious sensory signals from various internal organs
- efferents are subconscious motor or secretomotor signals to the same organs to
control their functions
- efferent signals involve two neurons: preganglionic & postganglionic;
third order or higher neurons may be involved in GI innervation (enteric nervous
system)
2.
Divisions: efferent pathway involves two divisions: sympathetic and
parasympathetic; a third component includes the enteric nerve plexus of the GI tract
• sympathetic – thoracolumbar distribution (fig. 60 – 1 & ppt. 2)
- sympathetic trunks connect chain of paravertebral ganglia; associated with
spinal nerves via rami communicantes (fig. 60 – 2 & ppts. 3 & 4); white ramus carries
preganglionic fibers to synapse in ganglion of same segment or neighboring segments;
postganglionic fibers (C fibers) return to spinal nerve via gray ramus for distribution to
effector organs
Bio 659 lec 20
- p. 2 -
- alternative pathways involve synapses in prevertebral ganglia (e.g., celiac &
mesenteric ganglia) or synapses in adrenal medulla (secretes mainly epinephrine &
some norepinephrine); preganglionic fibers reach these ganglia via splanchnic nerves
- preganglionic fibers (cholinergic) originate from intermediolateral horn of cord
gray matter or from brainstem
- postganglionic fibers (adrenergic) are in paraveretebral or prevertebral ganglia
or in adrenal medulla; some postganglionic fibers (to sweat glands) are cholinergic
• parasympathetic – craniosacral distribution (fig. 60 – 3 & ppt. 5)
- cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal) and X
(vagus) and sacral nerves (mainly 2 through 4) distribute preganglionic fibers; vagus
carries 75% of parasympathetic distribution (all thoracic & abdominal distribution
except distal half of colon)
- majority of preganglionic (cholinergic) fibers (those in vagus n. and sacral
nn.) synapse with postganglionic (also cholinergic) neurons (near or in wall of target
organs)
- preganglionic fibers in some cranial nerves synapse with postganglionic neurons
in peripheral ganglia
- ciliary ganglion receives cranial nerve III fibers, submandibular &
sphenopalatine ganglia receive cranial nerve VII fibers and otic ganglion receives
cranial nerve IX fibers
• neurotransmitters and receptors (ppts. 6 - 8): acetylcholine is secreted by
cholinergic fibers; two types of receptors: nicotinic (postganglionic neurons) &
muscarinic (effector organ cells)
- norepinephrine is secreted by adrenergic fibers; several types of receptors
fall into two main categories: alpha (favor norepinephrine) and beta (favor
epinephrine)
Bio 659 lec 20
3.
- p. 3 -
Functions (table 60 – 2):
• sympathetic/parasympathetic tone: basal signal activity can be increased
(excitation) or decreased (inhibition) facilitating more precise control
- resting output from adrenal medulla generates much of the sympathetic tone
- sympathetic & parasympathetic systems are often antagonistic to each other;
however, in most instances, one or the other may dominate control of a particular body
function
• sympathetic (fight or flight, alarm reactions):
- sympathetic NS works in collaboration with adrenal medulla, which it
excites, to prepare the body systems for response to emergency: increased respiratory rate,
increased heart rate, stroke volume & cardiac output, increased glucose output by
liver, vasoconstriction of non essential vasculature (in favor of blood supply to
muscles & heart) (ppt. 9)
- respiratory and cardiovascular control centers (pons & medulla) regulate
SNS signals to respective organs (fig. 60 – 5 & ppt. 10)
• parasympathetic (rest & digest, serene states): parasympathetic NS is major
regulator of gastrointestinal function (salivary secretion, secretion of enzymes, GI
motility)
• other effects: pupillary constriction (PNS) & dilation (SNS); erection of penis
(PNS), ejaculation of semen (SNS)
• brain centers: some autonomic signals originate in various brain regions –
brainstem (cranial nerve nuclei), hypothalamus & other parts of limbic cortex (ppt.
11)