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¥Preganglionic neuron
§cell body in brain or spinal cord
§axon is myelinated type B fiber that extends
to autonomic ganglion
¥Postganglionic neuron
§cell body lies outside the CNS in an
autonomic ganglion
§axon is unmyelinated type C fiber that
terminates in a visceral effector
¥The output (efferent) part of the ANS is divided into two
principal parts:
¥
§The Sympathetic division
§
§The Parasympathetic division
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¥Organs that receive impulses from both sympathetic and
parasympathetic fibers are said to have dual innervation.
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Cholinergic
neurons include:
- All sympathetic and parasympathetic preganglionic neurons.
- Sympathetic postganglionic neurons that
innervate sweat glands.
- All parasympathetic postganglionic
neurons.
Nicotinic
receptors:
- are present in dendrites & cell
bodies of sympathetic and parasympathetic postganglionic neurons.
- present in NMJ as well as in chromaffin
cells of adrenal gland.
Muscarinic receptors:
- are present in plasma membranes of all
effectors innervated by parasympathetic postganglionic neurons.
- are present in sympathetic
postganglionic neuron that innervate sweat glands.
Activation
of nicotinic receptors by Ach causes depolarization and thus excitation.
-
Activation of muscarinic receptors by Ach causes depolarization (excitation) and
sometimes hyperpolarization
(inhibition).
¥Adrenergic neurons release norepinephrine
(NE)
§from postganglionic sympathetic neurons only.
¥
¥Excites or inhibits organs
depending on
receptors
¥
¥The main types of adrenergic receptors are
alpha
and beta
receptors:
§α1 and β1 receptors
produce excitation
§α2 and β2 receptors cause
inhibition
§β3 receptors
(brown fat) increase thermogenesis
¥
¥Effects triggered by adrenergic neurons typically
are longer
lasting than those triggered by
cholinergic neurons.
¥Most body organs receive dual innervation
§innervation by both
sympathetic & parasympathetic
¥
¥Hypothalamus regulates balance (tone) between sympathetic and parasympathetic activity
levels
¥
¥Some organs have only sympathetic innervation
§sweat glands,
adrenal medulla & many blood vessels
**
Long lasting due to lingering of NE in
synaptic gap and release of norepinephrine by the
adrenal gland
¥Dominance by the sympathetic system is caused by physical or
emotional stress - “E situations”.
§
§emergency,
embarrassment, excitement, exercise
¥
¥Alarm reaction = flight or fight response
§
§dilation of
pupils
§increase of
heart rate, force of contraction & BP
§decrease in
blood flow to nonessential organs
§increase in
blood flow to skeletal & cardiac muscle
§airways dilate
& respiratory rate increases
§blood glucose
level increase
¥Enhance “rest-and-digest”
activities
¥
¥Mechanisms that help
conserve and
restore body energy during times of rest
¥
¥Normally dominate over
sympathetic
impulses
¥
¥SLUDD type responses =
Salivation, Lacrimation, Urination, Digestion
& Defecation
in addition to 3
“decreases”- ↓HR, ↓diameter
of airways and ↓diameter of pupil
¥A visceral autonomic reflex adjusts the activity of a
visceral effector, often unconsciously.
§changes in blood
pressure, digestive functions etc
§filling &
emptying of bladder or defecation
¥
¥Autonomic reflexes occur over autonomic reflex arcs.
Components of that reflex arc:
§sensory receptor
§sensory neuron
§integrating
center
§pre &
postganglionic motor neurons
§visceral
effectors
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¥Not
aware of autonomic responses because control center is in lower regions of the
brain.
¥
¥Hypothalamus
is major control center
¥
§input:
emotions and visceral sensory information
smell,
taste, temperature, osmolarity of blood, etc.
§
§output:
to nuclei in brainstem and spinal cord
§
§posterior
& lateral
portions control sympathetic NS: increase heart rate, inhibition GI tract, increase
temperature.
§
§anterior
& medial
portions control parasympathetic NS: decrease heart rate, lower blood
pressure, increased GI tract secretion and mobility.
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