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AUTONOMIC NERVOUS SYSTEM
Somatic efferent and ANS
Somatic Efferent
Control …is over skeletal muscles.
External environment
This division of the PNS responds to some change in the external
environment.
single somatic efferent neuron
Extends from the CNS to the effector (skeletal muscle)
Acetlycholine
… is the NT released at the synapse (motor endplate.)
Autonomic nervous system
(visceral efferent)
Regulates visceral activities
Organs of the thoracic and abdominal regions; sweat and sebaceous
glands, etc.
No conscious control
…over visceral activities.
Stable internal environment
The goal of the ANS is to maintain a stable internal environment.
Autonomic nervous system
Regulated by centers in
Brain stem and cord
2 efferent neurons
One from CNS to ganglion
preganglionic
One to visceral effector
postganglionic
ganglion
CNS
preganglionic
postganglionic
Acetlycholine (ACh) or norepinephrine (Ne)
Are the NTs released at the synapses.
Structure of the ANS
Visceral efferent neurons organized into
Nerves (neurons)
Pre- or postganglionic nerves.
Ganglia
Where the pre- or postganglionic neurons form a synapse; this is where
the postganglionic soma is located.
Plexuses
Tangles of visceral efferent nerves.
Entirely motor
Carry motor information to visceral effectors.
1) Efferent neurons
preganglionic
Soma in brain or cord
Myelinated axons
Synapses with postganglionic neuron
Within a ganglion
CNS
1) Efferent neurons
Postganglionic
autonomic ganglia
Somas are located within autonomic ganglia or target organs
Unmyelinated axons
are found with postganglionic neurons
Synapses with effector Usually smooth muscle, heart or gland
CNS
1) Efferent neurons
Sympathetic division of the ANS has its preganglionic somas in the...
thoracolumbar regions of the cord
helps body respond to stress
This division of the ANS is called the “fight or flight’ division.
If the body is under some threat or stress, the sympathetic
stimulation goes up.
This causes an increase in heart and respiration rate, sweating, blood
flow to the CNS and muscles.
This causes a decrease blood flow to some visceral organs, such as
the intestines and the kidneys.
CNS
1) Efferent neurons
Parasympatheic division of the ANS has its preganglionic somas in the...
craniosacral regions of the cord
active under normal conditions
This division of the ANS is called the “rest and repose” division.
The parasympathetic division is stimulated under normal or resting
conditions, when you want to cover from stress; build up reserves
and maintain the body.
This causes a decrease in heart and respiration rate, sweating, blood
flow to the CNS and muscles.
This causes an increase blood flow to some visceral organs, such as
the intestines and the kidneys.
CNS
Dual innervation
The heart, each visceral organ and gland is innervated by both the
sympathetic and parasympathetic divisions of the ANS.
Although each effector is under the control of both divisions, they
are not simultaneously stimulated by both.
Which division predominates at any given time is determined by the
demands on the body at that time.
2) Autonomic ganglia
a) sympatheic trunk or chain ganglia (paravertebral or lateral ganglia)
Located close and lateral to the spinal cord.
Stimulate primarily...
Sympathetic division
Preganglionic fibers are short
2) Autonomic ganglia
b) prevertebral ganglia (collateral ganglia)
Located anterior to the spinal cord.
Stimulate primarily...
Sympathetic division
Preganglionic fibers are short
b) prevertebral
ganglia (collateral
ganglia)
Some examples of
prevertebral ganglia
Largest
Celiac
Postganglionic fibers from this
ganglion innervate organs of
the abdomen; liver, gallbladder,
stomach, pancreas, spleen.
Superior mesenteric ganglia
Innervate the small intestine and part of the large.
inferior mesenteric ganglia
Innervate most of the large intestine, kidney, bladder and
reproductive organs.
2) Autonomic ganglia
c) terminal ganglia
Located within target visceral organs.
Stimulate primarily...
Parasympathetic division
Preganglionic fibers are very long
3) Plexuses
Interlaced networks
Similar to the plexuses
of the somatic
efferent PNS.
Tangles of visceral
efferent nerve fibers.
Solar plexus or Celiac
plexus
Postgangionic fibers from
the celiac ganglion pass
through here.
Physiology of the ANS
Autonomic fibers release NT at synapse
Cholinergic
…fibers release Ach at the synapse.
The preganglionic fibers of the sympathetic division are cholinergic.
Ach is released to bind to the postsynaptic (postganglionic) neuron.
Adrenergic
…fibers release norepinephrine (NE) at the synapse.
The postganglionic fibers of the sympathetic division are adrenergic.
Responses
NE tends to stimulate the effectors.
Catechol-O-methyl transferase This enzyme, found in the synaptic gap, breaks down NE.
It has a similar function as acetylcholinesterase, but works at a
slower rate.
As a result some NE tends to diffuse out of the synapse into the
surrounding tissues, where it may exert an effect.
So the effects of sympathetic stimulation are more wide spread and
last longer the parasympathetic stimulation.
Responses
Parasympathetic:
Both pre- and postganglionic fibers are cholinergic.
As Ach is rapidly broken down by acetylcholinesterase, it does not
have a chance to diffuse from the synapse.
Parasympathetic effects tend to be localized and short in duration.
Visceral Autonomic Reflexes
…similar to somatic reflexes.
Target organs are...
smooth muscle Such as digestive system, vessels, reproductive.etc.
cardiac muscle
Heart
gland secretion Salivary, sweat, etc.
Visceral Autonomic Reflex Arc
receptor
afferent neuron
association neuron
visceral efferent preganglionic
neuron visceral efferent postganglionic
neuron
visceral effector Visceral sensations
conscious sensations
Visceral sensations reach the conscious
levels of the mind if the a voluntary action is
required to deal with the sensation; as in
urination or defecation.
Control by higher centers
Hypothalamus
For example; if hypothalamus determines
that the the water content of the blood is
too low, it will send signals to the higher
brain centers to provide a sensation of
thirst.
Cerebral cortex
The cerebrum will respond to the thirst
sensation by stimulating drinking behaviors;
a voluntary action.