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Review of Somatic Nervous System
-Somatic sensory neurons conduct signals from special and general senses
- Axons of somatic motor neurons extend all the way from the CNS to the effectors (skeletal muscle)
- All somatic motor neurons release ACh, which brings on contraction - all are excitatory
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Autonomic sensory neurons
-conduct signals from interoceptors - sensory receptors located in blood vessels, visceral organs, muscles, and nervous system that monitor condition in the internal environment
- Two types of interoceptors
- - chemoreceptors - chemical balance - monitor blood CO2 level
- - mechanoreceptors - mechanical - detect the degree of stretch in the walls of organs or blood vessels
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Autonomic ganglion
- axons of myelinated autonomic motor neurons extend from the CNS to the autonomic ganglion. it synapses with a second autonomic motor neuron that extends from th autonomic ganglion to the effector via an unmylinated axon
- preganglionic autonomic motor neurons release ACh
- postganglionic autonomic motor neurons release either ACh or Norepinephrine
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Dual innervation
most organs receive impulses from both sympathetic and parasympathetic neurons - in general signals from one division of the ANS will stimulate while signals from the other division will decrease organs activity
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Two branches of ANS
1. Sympathetic division - fight-or-flight division - activities result in increased alertness and metabolic activities in order to prepare the body for an emergency situation
2. Parasympathetic division - rest-and-digest - activities conserve and restore body energy during times of rest or digesting a meal
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Autonomic motor pathways
- Autonomic motor pathways have two neurons
- 1) Preganglionic neurons - cell body within the CNS
- - Axon is myelinated, type B and part of a cranial or spinal nerve
- 2) Postganglionic neurons - cell body within autonomic ganglion
- - Axon is unmyelinated, type C and terminates in a visceral effector
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Preganglionic neurons
In sympathetic division - cell bodies lie within lateral horns of all thoracic segments and the first two lumbar segements - also called the thoracolumbar division
in parasympathetic division - cell bodies lie within nuclei of four cranial nerves in the brain stem and in lateral horns of the second through the fourth sacral segments - also called the craniosacral division
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two major groups of autonomic ganglia
- 1. Sympathetic ganglia
- - Sympathetic trunk ganglia : postganglionic axons innervate organs above the diaphram
- Prevertebral ganglia : postganglionic axons innervate organs below the diaphram
- 2. Parasympathetic division
- - Terminal ganglia : located close to or entirely within visceral organs
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Postganglionic neurons
Once sympathetic preganglionic neurons reach the sympathetic trunk ganglion they synapse with postganglionic neurons in one of three way
- 1. in the first ganglion it reaches
- 2. in a ganglion superior or inferior to the first one it reaches (travels up or down the sympathetic trunk ganglion)
- 3. In a prevertebral ganglion by bypassing the sympathetic trunk ganglion
- sympathetic preganglionic neurons may synapse with many postganglionic neurons (divergent circuit) - widespread effects
- Parasympathetic neurons synapse with fewer postganglionic neurons near or within the effector - localized effects
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Autonomic Plexuses
networks of both sympathetic and parasympathetic axons in the thorax, abdomen and pelvis
located in close proximity to bloodvessels
- 1. cardiac plexus - supplies the heart
- 2. pulmonary plexus - supplies the bronchial tree
- 3. celiac or solar plexus - surrounds the celiac trunk
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Pathways from spinal cord to sympathetic trunk ganglia
- 1. preganglionic axons exit the spinal cord as part of a spinal nerve through the anterior root
- 2. Preganglionic axons then travel through the white ramus communicans (farther from spinal cord - swings out further past the sympathetic trunk ganglion) TYPE B - Myelinated
- 3. postganglionic axons travel through the grey ramus communicans (close to spinal cord ) TYPE C - Unmyelinated
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Splanchnic nerves
- 1.preganglionic axons that pass through the sympathetic trunk ganglion without synapsing
- 2. they become part of splanchnic nerves
- 3. that terminate in the prevertebral ganglia
- 4. from there they synapse with postganglionic neurons that take the signal to effectors
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Classification of autonomic neurons
- classified as either
- 1. cholinergic
- 2. adrenergic
- depending on type of neurotransmitter released
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Cholinergic neurons and receptors
- - release ACh
- -include ALL sympathetic and parasympathetic preganglionic neurons
- - includes ALL parasympathetic postganglionic neurons
- -includes ONLY sympathetic postganglionic neurons that innervate sweat glands
Cholinergic receptors are integral membrane proteins in the postsynaptic plasma membrane
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Two types of cholinergic receptors (receive ACh)
classified by other substances they receive
1. Nicotinic receptors - only excitatory response (depolarization) - located in plasma membrane and cell bodies of all sympathetic and parasympathetic postganglionic neurons and the NMJ
2. muscarinic receptors - excitatory or inhibitory response (depolarization or hyperpolarization) - located in the plasma membrane of all effectors innervated by parasympathetic postganglionic neurons
Acetylcholinesterase - enzyme - breaks down ACh and removes it from synaptic gap so cholinergic neurons have brief outcomes
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Adrenergic neurons and receptors
- adrenergic neurons always release norepinephrine - can be excitatory or inhibitory depending on receptor
- - includes most sympathetic postganglionic receptors
adrenergic receptors - bind both norepinephrine and epinephrine - may bring on depolarization or hyperpolarization
- Types of adrenergic receptors:
- 1. Alpha 1 & 2
- 2. Beta 1, 2, & 3
- Alpha 1 & Beta 1 are excitatory
- Alpha 2 and Beta 2 are inhimitory
- Beta 3 is excitatory - thermogenesis - only in babies -found in BAT (Brown Adipose Tissue)
Adrenergic neurons have prolonged effects because COMT and TAO break down epinephrine and norepinephrien slower than Acetylcholinesterase breaks down ACh
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Agonist
substance that binds to and activates a receptor (minics effects of natural neurotransmitter)
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Antagonist
substance that binds to and blocks a receptor (prevents natural neurotransmitter from exerting its effect)
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Autonomic reflexes
-help regulate blood pressure, digestion, defication, and urination
- invole a typical components of a reflex arc
- 1. receptor
- 2. sensory neuron
- 3. integratin center
- 4. motor neurons
- 5. effector
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Autonomic control by higher centers
Hypothalimus is major control center for ANS
- hypothalamic nuclei have synapses with both sympathetic and parasympathetic division of the ANS
- receives sensory input regarding visceral functions, olfaction, taste, temperature, and levels of chemical in fluids
- hypothalamic output influences ANS centers in both the brain stem and the spinal cord
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glycogenolysis
breakdown of glycogen to glucose in the liver - occurs during the parasympathetic response
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