Term
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Definition
•innervation of skeletal muscles (movement)
•CNS control (corticospinal or pyramidal tracts)
•somatic innervations consist of a single neuron (final common motor neuron) arising in spinal cord and extending via the ventral root to the skeletal muscles
•releases acetylcholine (ACh)
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Term
Autonomic nervous system
Functional Considerations |
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Definition
•Functional considerations:
–mediates control of vegetative or involuntary functions
–innervation of cardiac muscle, vascular and nonvascular smooth muscle and exocrine glands
–functions in these systems often occur without conscious control
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Term
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Definition
•Anatomical considerations:
–In contrast to somatic efferents, autonomic innervations consist of 2 sequential neurons
–These sequential neurons are the preganglionic and postganglionic neurons which synapse at autonomic ganglia
–The autonomic nervous system consists of two divisions:
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•sympathetic
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•parasympathetic
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Term
Sympathetic nervous system |
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Definition
•preganglionic neurons exit the spinal cord at the thoraco-lumbar level to synapse with postganglionic nerves at para-vertebral ganglia (22 pairs on each side of spinal cord) or prevertebral ganglia (celiac, mesenteric) in the abdomen.
•The adrenal medulla is considered to be a modified sympathetic ganglion; the medulla is embryonically and anatomically homologous to the sympathetic ganglia
•Sympathetic innervations usually consist of one short preganglionic fiber synapsing with several (one or more) long postganglionic fibers in the sympathetic ganglia
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Term
Ramifications of Sympathetic Nervous System Compared with Parasympathetic |
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Definition
-•there is a greater ramification of sympathetic fibers compared to the parasympathetic system (the ratio of pre to postganglionic fibers @ 1:20)
•diffuse action; “fight or flight” responses (i.e., stress)
•not essential for life
•this system is normally active with the degree of activity varying from moment to moment and organ to organ
•this system constantly adjusts to a changing environment, especially during rage or fright
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Term
Typical sympathetic responses include:
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Definition
•increase in heart rate
•shift in blood flow to muscles
•increase in blood glucose levels
•dilation of the pupils
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Term
Parasympathetic nervous system |
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Definition
•preganglionic neurons originate in the cranial nerves of the brain stem and the sacral portion of the spinal cord
•these neurons synapse with post-ganglionic neurons in ganglia very close or in the organs innervated
•Parasympathetic innervations typically consist of one long preganglionic fiber synapsing with one short post-ganglionic fiber in the parasympathetic ganglia.
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Term
Parasympathetic nervous system compared to sympathetic |
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Definition
•this system is more circumscribed than the sympathetic system, although a 1:1 ratio of pre to postganglionic fibers is not always the case
•discrete action; conservation and restoration of energy, localized control of discrete functions
•essential for life
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Term
Typical parasympathetic responses include:
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Definition
•slowing the heart rate
•lowering blood pressure
•protecting the retina from light
•emptying the bladder
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Term
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Definition
•The sympathetic and parasympathetic systems usually do not function independently; i.e., they are physiological antagonists.
•Often when one system inhibits a process, the other system will augment the level of activity so that the total response depends on the influence of both systems, although this is not always the case.
•The integration of these system regulates functions below the level of consciousness.
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Term
Neurochemical classification of Peripheral Nervous System |
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Definition
Acetylcholine (ACh or Cholinergic) synapses include:
–All preganglionic fibers outside CNS (sympathetic & parasympathetic)
–All parasympathetic postganglionic nerve endings (ACh is the transmitter)
•Exception: sympathetic postganglionic nerve endings of sweat glands
–somatic motor neurons innervating skeletal muscle
•Noradrenergic (NE) synapses:
–all postganglionic sympathetic fibers (except those to sweat glands)
–adrenal medulla (norepinephrine & epinephrine)
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Term
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Definition
Chemistry
•acetylcholine is synthesized from acetyl co-enzyme A and choline using the enzyme choline acetyl transferase
•the major means of inactivation of acetylcholine is degradation in the synapse using the enzyme acetylcholine esterase
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Term
•Acetylcholine (Cholinergic) Receptors
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Definition
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•Muscarinic receptors
–postganglionic parasympathetic fibers innervating heart, smooth muscle and exocrine glands
–exception: postganglionic sympathetic fibers innervating sweat glands
–blocked by antimuscarinic agents (e.g., atropine)
•Nicotinic receptors
–classically a biphasic response is observed with stimulation at low doses and inhibition at high doses
–sympathetic and parasympathetic autonomic ganglia and the adrenal medulla
–effects blocked with ganglionic blockers (e.g., trimethaphan, hexamethonium)
–neuromuscular junction of skeletal muscle
–effects blocked with neuromuscular blockers (e.g., curare)
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Term
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Definition
Chemistry
•norepinephrine is ultimately synthesized from tyrosine using the enzyme tyrosine hydroxylase which converts tyrosine to DOPA
•aromatic L-amino acid decarboxylase converts DOPA to dopamine
•dopamine b-hydroxylase converts dopamine to norepinephrine
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Term
Norepinephrine inactivation |
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Definition
•Phenylethanolamine N-methyl-transferase converts norepinephrine to epinephrine
•the major means of inactivation of norepinephrine is reuptake back into the presynaptic neuron from which it was released
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Term
Norepinephrine Receptors (alpha) |
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Definition
•a1 (alpha 1)
•vascular smooth muscle, genitourinary smooth muscle, liver (contraction)
•intestinal smooth muscle (hyperpolarization and relaxation)
•heart (increased contractile force, arrhythmias)
•a2 (alpha 2)
•pancreatic islets (b cells, decreased insulin secretion)
•platelets (aggregation)
•vascular smooth muscle (contraction)
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Term
Norepinephrine Receptors (beta) |
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Definition
•b1 (beta 1)
•heart (increased force and rate of contraction, AV nodal conduction velocity)
•juxtaglomerular cells (increased renin secretion)
•b2 (beta 2)
•smooth muscle [vascular, bronchial, gastrointestinal, genitourinary] (relaxation)
•skeletal muscle (glycogenolysis; uptake of K+)
•liver (glycogenolysis; gluconeogenesis)
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Term
Potential ways to affect autonomic neurotransmission |
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Definition
•Synthesis
–availability of precursors for the NT
–availability of synthesis enzymes
•Storage (vesicles)
–protects the NT from degradation
–provides for the quantal release of the NT
•Release (Ca2+ dependent exocytosis)
–agents could interfere with or enhance the release of the NT
•Receptor activation
–Agonist - high affinity and high intrinsic activity
–Antagonist - high affinity but NO intrinsic activity
•Termination of NT effect
–Acetylcholine - metabolism in synaptic cleft via acetylcholine esterase
–Norepinephrine - reuptake into presynaptic neuron
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