LECTURE 16: AUTONOMIC AND NEUROENDOCRINE SYSTEMS
REQUIRED READING: Kandel text, Chapter 49
Autonomic nervous system mediates visceral reflex responses that
are involuntary and largely unconscious
Autonomic nervous system consists of:
1)
Motor neurons which act on smooth muscle, cardiac muscle,
and exocrine glands
2)
“Preganglionic” CNS neurons whose axons synapse on these
motor neurons
3)
Visceral sensory neurons
4)
Branches and subsets of external-sensing neurons (including
somatosensory , olfactory, and retinal)
Autonomic postganglionic neurons release neurotransmitters which
act through metabotropic receptors on target cells
Autonomic responses are coordinated with one another and with
behavioral responses and emotions through the hypothalamus
in the CNS
nicotinic
receptors
always
cholinergic
cholinergic
or
adrenergic
muscarinic
or
adrenergic
receptors
EXAMPLES OF AUTONOMIC FUNCTION
Regulation of heartbeat rate
Regulation of vascular constriction/dilation
Pupil and lens ocular reflexes
Exocrine gland secretion
Glucose mobilization
Sweating and hair follicle erection
Bladder filling and emptying
Sexual responses
Alimentary and bronchial reflexes
Gut peristalsis
THREE DIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM
Sympathetic nervous system
Sympathetic system controls visceral responses that prepare the body for rapid, intense activity,
often refered to as FIGHT-OR-FLIGHT REACTION.
Responses include accelerated heartbeat, central artery constriction, peripheral vascular dilation,
liver glycogen metabolism, & rapid breathing.
Other sympathetic responses also work in balance with countering parasympathetic responses
to maintain body homeostasis (counteraction to body stress).
Parasympathetic nervous system
Parasympathetic responses sometimes refered to as the REST-AND-DIGEST STATE.
Almost all visceral targets receive both sympathetic & parasympathetic neuronal inputs.
Enteric nervous system
Enteric neurons form plexuses that surround and extend along the length of the gut, including
stomach, small and large intestines.
Enteric system activate coordinated contraction of smooth muscles to cause peristaltic
constriction of the gut.
Most of enteric nervous system functions independently of higher CNS control.
ANATOMY OF SYMPATHETIC & PARASYMPATHETIC NERVOUS SYSTEM
Most SYMPATHETIC
postganglionic neurons are
adrenergic
(release E or NE)
Most PARASYMPATHETIC
postganglionic neurons are
cholingeric
Site of spinal cord lesion injury can be rapidly assessed by surveying damaged and surviving autonomic reflex responses
ANATOMY OF SYMPATHETIC & PARASYMPATHETIC NERVOUS SYSTEM
Generalized “FIGHT” response
mediated by sympathetic
activation of the adrenal gland,
triggering epinephrine
secretion into circulation
ANATOMY OF ENTERIC NERVOUS SYSTEM
SENSORIMOTOR CONNECTIONS IN ENTERIC NERVOUS SYSTEM
ARE PREDOMINANTLY LOCAL
A local circuitry drives peristalsis
in the intestines
Pressure-sensing neuron senses gut distension
PERISTALSIS
Acts through interneurons to activate enteric
motor neurons with axons projecting rostrally
causing squeezing of circular muscle behind the distension
Simultaneous inhibition of other motor neurons
with axons projecting caudally relaxes downstream
circular muscle
FOOD
DISTENSION
PRESSURE SENSING NEURON
CIRCULAR MUSCLE
MOTOR NEURONS
POST-GANGLIONIC NEUROTRANSMISSION LACKS TYPICAL
PRE- AND POST-SYNAPTIC SPECIALIZATIONS
Post-ganglionic neuron’s axon terminal lacks clear-vesicle docking machinery.
Multiple axonal swellings (varicosities) are sites of neurotransmitter vesicle accumulation.
Post-synaptic target (smooth muscle, gland, etc.) lacks post-synaptic density.
Target cell neurotransmitter receptors are broadly distributed on surface.
Released neurotransmitter acts diffusely over distances up to 1 mm.
Highly branched axons with multiple varicosities enable post-ganglionic neuron
to act upon many cells in the target structure.
DIFFUSE TRANSMISSION FROM GANGLIONIC AXONS FACILITATED
DISCOVERY OF THE FIRST CHEMICAL NEUROTRANSMITTER
Parasympathetic vagus nerve activity slows heartbeat rate,
while sympathetic accelerator nerve activity speeds heartbeat rate
TWO BEATING FROG HEARTS DISSECTED AND MAINTAINED IN SMALL VOLUME SOLUTION;
HEART #1 DISSECTED WITH INNERVATING NERVES ATTACHED
HEART #2 DISSECTED WITHOUT NERVES
Stimulation of vagal nerve slowed beating of heart #1
After stimulation, transfer of heart #1’s bathing solution to heart #2 slowed its beating
Stimulation of accelerator nerve speeds beating of heart #1
After stimulation, transfer of heart #1’s solution to heart #2 sped its beating
THEREFORE, NERVE-INDUCED CARDIAC RESPONSES ARE THROUGH
SECRETED CHEMICAL NEUROTRANSMITTERS
(Vagal transmitter later shown to be ACh, accelerator transmitter is NE)
MECHANISMS OF AUTONOMIC MODULATION OF CARDIAC FUNCTION
Parasympathetic release of acetylcholine reduces cardiac output in two ways
1)
2)
Muscarinic generation of Gbg directly activates a potassium channel (GIRK) in pacemaker
cardiocytes, which slows their depolarization and rate of heartbeat.
Muscarinic generation of Gai in heart muscle lowers cAMP and PKA levels, causing
reduced opening of L-type calcium channels, thereby reducing force of heart contraction.
Sympathetic release of norepinephrine increases cardiac output in two ways
1)
2)
b1-adrenergic generation of Gas in
pacemaker cardiocytes elevates cAMP and PKA levels,
which reduces the threshold voltage for action potential initiation, thereby increasing
rate of heartbeat.
b1-adrenergic elevation of cAMP and PKA in heart muscle increases opening of L-type
calcium channels, thereby increasing force of heart contraction.
SENSORY INPUTS TO AUTONOMIC FUNCTION
Our bodies sense deleterious changes and undertake automatic responses
to maintain homeostasis.
Sensory inputs eliciting autonomic responses include:
1)
External sensations which trigger corrective reflexes
Examples: a) Ocular reflexes -- pupil dilation or constriction in response to light,
lens stretching to adjust focus
http://library.med.utah.edu/kw/hyperbrain/anim/reflex.html
b) Painful laceration -- vasoconstriction to limit blood loss
sympathetic activation of coordinated fight/flight responses
2)
Visceral sensations induce homeostatic responses
Examples:
a) Opposing sympathetic/parasympathetic control of heartbeat and blood pressure -If sympathetic activity drives heartbeat and artery constriction too much,
pressure-sensitive sensory afferents in the aorta trigger the baroreceptor reflex,
which includes parasympathetic vagal input to heart and induction of
arterial dilation
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter21/animation__baroreceptor_reflex_control_of_blood_pressure.html
Reciprocally, pressure-sensitive sensory afferents in the cardiac right atriam sense distention
triggering the right atrial reflex, by which sympathetic accerelerator nerve firing speeds rate
b)
Irritants to oronasal cavities act through parasympathetic ganglia
to trigger nasal and lacrimal glandular secretions
PREGANGLIONIC FIBERS RELEASE SMALL MOLECULE AND PEPTIDE
NEUROTRANSMITTERS TO ELICIT COMPLEX GANGLIONIC NEURON RESPONSES
Single or low-frequency preganglionic firing releases Ach which activates nicotinic
receptors triggering fast EPSP in postganglionic neuron.
High-frequency stimulation releases more Ach and LHRH peptide. The complex
postganglionic response consists of fast EPSP, slow IPSP mediated by
muscarinic receptor activation of GIRKs, and delayed EPSP resulting
from LHRH binding to peptidergic receptors.
SENSORY PATHWAYS OF SYMPATHETIC AND PARASYMPATHETIC SYSTEMS
PASS LOOP THROUGH BRAIN STEM, BUT ALSO PROJECT TO CONSCIOUS CORTICAL AREAS
ASCENDING VISCERAL
SENSORY PATHWAYS
DESCENDING AUTONOMIC
RESPONSE PATHWAYS
HYPOTHALAMUS COORDINATES PHYSIOLOGY AND BEHAVIOR
IN RESPONSE TO VISCERAL SENSORY INPUTS
EXAMPLE: BLOOD OSMOLARITY HOMEOSTASIS
VISCERAL SENSORY INPUTS
Blood pressure
Blood osmolarity
HYPOTHALAMUS COORDINATED OUTPUTS
Autonomic -- action on smooth muscles in central and peripheral vasculature
Behavioral -- conscious thirst which drives search for fluid intake
Endocrine -- secretion of vasopressin into blood, which promotes water
resorption by kidneys
HYPOTHALAMUS CONTROLS HORMONE RELEASE FROM PITUITARY GLAND
BOTH DIRECTLY AND INDIRECTLY