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Integrative Physiology Ch. 11 - Efferent Division

The efferent division of the PNS can be subdivided into: (1) Somatic motor neurons which control skeletal muscle, and (2) autonomic neurons which control smooth muscle, cardiac muscle, many glands, and some adipose tissue
The autonomic division is further subdivided into three branches: The sympathetic, parasympathetic and enteric (covered in digestive system chapter) nervous systems
Fight-or-flight response An action in which the brain triggers massive simultaneous sympathetic discharge. The heart speeds up; blood vessels to the muscles of the arms, legs, and heart dilate; the liver produces glucose for muscles
How is digestion affected during the fight or flight response? Digestion becomes a low priority so blood is diverted from the gastrointestinal tract to skeletal muscles
The massive sympathetic discharge that occurs in fight-or-flight situations is mediated through the _____ Hypothalamus
How is homeostasis reached in the context of sympathetic vs. parasympathetic activity Homeostasis is a dynamic balance between the autonomic branches. Autonomic control of body function “see-saws” back and forth between the sympathetic and parasympathetic branches as they cooperate to fine-tune various processes
The autonomic nervous system works closely with the _____ system and _____ system to maintain homeostasis in the body Endocrine; behavioral state
What is the pathway that describes how sensory input (which will eventually result in autonomic response) leads to their control centers? Sensory input from hypothalamic receptors (e.g. osmolarity, temperature) and input from somatic/visceral receptors are sent to both the (1) pons, medulla, hypothalamus and (2) limbic system, cerebral cortex
How does the sensory input received by the control centers lead to responses in the three systems: autonomic, endocrine, and behavioral state? The pons/medulla/hypothalamus create autonomic, endocrine, and behavioral responses. The limbic system and cerebral cortex lead to behavioral responses. Also: the two control centers send messages to EACH OTHER
In the context of autonomic homeostasis, most internal organs are under _____ control Antagonistic control
Antagonistic control One autonomic branch is excitatory and the other branch is inhibitory. E.g., sympathetic innervations increases heart rate while parasympathetic innervations decreases it
Exceptions to the dual antagonistic innervation Sweat glands and smooth muscle in blood vessels. These tissues are innervated by the sympathetic branch only and rely strictly on tonic (up-down) control
Although the two autonomic branches are usually antagonistic in their control of a given target tissue, they… …sometimes work cooperatively on different tissues to achieve a common goal. E.g., blood flow for penile erection is under the parasympathetic branch, and muscle control for sperm ejaculation is directed by the sympathetic branch
In autonomic pathways does the released chemical signal always determine the response? No, in some pathways the receptor determines the response. E.g., adrenergic receptors come in multiple types: some result in vasoconstriction while others result in vasodilation – while they’re both activated by catecholamines
All autonomic pathways (sympathetic and parasympathetic) consist of two neurons in series (and where do they originate?): The preganglionic neuron (originating in the CNS) and projects to an autonomic ganglion outside the CNS where it synapses with the second neuron in the pathway, the postganglionic neuron (originating outside the CNS)
How is the postganglionic neuron oriented? Its cell body is within an autonomic ganglion and projects its axon to the target tissue
How does divergence occur in autonomic pathways On average, one preganglionic neuron entering a ganglion synapses with eight or nine postganglionic neurons. Each postganglionic neuron may then innervate a different target. Thus, one preganglionic neuron can affect many cells
Are the autonomic ganglia simply a way station for the transfer of signals from preganglionic neurons to postganglionic neurons? No, they also contain neurons completely within them which act as mini-integrating centers, receiving sensory input from the periphery of the body and modulating outgoing autonomic signals to target tissues
An example of an autonomic division of the body totally integrated by autonomic ganglia Enteric nervous system
How do the two autonomic branches (sympathetic and parasympathetic) differ anatomically? (1) pathways’ point of origin in the CNS, and (2) the location of the autonomic ganglia
How do the two autonomic branches (sympathetic and parasympathetic) differ anatomically – sympathetic Originate in the thoracic and lumbar regions of the spinal cord. Found in two chains that run along either side of the bony vertebral column, with additional ganglia along the descending aorta. Short preganglionic neurons, long postganglionic
Where are the ganglia of the sympathetic system typically located? Near the spinal cord
How do the two autonomic branches (sympathetic and parasympathetic) differ anatomically – parasympathetic Many originate in the brain stem. Their axons leave the brain in several cranial nerves. Others originate in the sacral region and control pelvic organs. Since their ganglia are usually located near target organs: long preganglionic/short postganglionic
Parasympathetic innervation goes primarily to the Head, neck, and internal organs
The major parasympathetic tract The vagus nerve (cranial nerve X), which contains about 75% of all parasympathetic fibers
What kind of information does the vagus nerve carry? Both sensory information from internal organs to the brain and parasympathetic output from the brain to organs
Vagotomy A surgical procedure in which the vagus nerve is cut, was an experimental technique used in the 19th and early 20th centuries to study the effects of the autonomic nervous system on various organs.
Neurotransmitters and receptors used for sympathetic and parasympathetic pathways: preganglionic neurons Both sympathetic and parasympathetic preganglionic neurons release acetylcholine (ACh) onto nicotinic cholinergic receptors on the postganglionic cell
Neurotransmitters and receptors used for sympathetic and parasympathetic pathways: postganglionic neurons Postganglionic sympathetic neurons: secrete norepinephrine (NE) onto adrenergic receptors on the target cell. Postganglionic parasympathetic neurons: secrete acetylcholine onto muscarinic cholinergic receptors on the target cell
Sympathetic cholinergic neurons The exception to the rule that sympathetic postganglionic neurons secrete only norepinephrine. Some—such as those terminating on sweat glands—secrete ACh rather than norepinephrine: sympathetic cholinergic neurons
Nonadrenergic, noncholinergic neurons A small number of autonomic neurons that secrete neither norepinephrine nor acetylcholine. Instead, their neurotransmitters include substance P, somatostatin, vasoactive intestinal peptide (VIP), adenosine, NO, and ATP
Which branch (sympathetic vs. parasympathetic) are the nonadrenergic, noncholinergic neurons assigned to? They assigned to either branch, according to where their preganglionic fibers leave the nerve cord
Neuroeffector junction The synapse between a postganglionic autonomic neuron and its target cell
How do the structures of autonomic synapses differ from the normal “model” synapse? Autonomic postganglionic axons end with a series of varicosities at their distal ends. Instead of the neurotransmitter being secreted directly onto target cell receptors, they’re released into the IF for a less-directed response
IF Interstitial fluid
Varicosities A series of swollen areas on the distal ends of the autonomic postganglionic axons, like beads spaced out along a string, which contain vesicles filled with neurotransmitter
How is the release of autonomic neurotransmitters modulated? A variety of sources, including: (1) varicosities contain receptors for hormones and for paracrines such as histamine; (2) some preganglionic neurons co-secrete neuropeptides which act as neuromodulators on postganglionic neurons
In the autonomic division, neurotransmitter synthesis takes place where and by what? Review: what are the primary autonomic neurotransmitters? In the axon varicosities by cytoplasmic enzymes. Primary autonomic neurotransmitters: acetylcholine and norepinephrine
The major factor in the control that an autonomic neuron exerts on its target The concentration of neurotransmitter in the synapse (IF): the more neurotransmitter means a longer or stronger response
Autonomic neurotransmitter activation of its receptor terminates when: The neurotransmitter either (1) diffuses away, (2) is metabolized by enzymes in the ECF, or (3) is actively transported into cells around the synapse (the uptake of neurotransmitter by varicosities allows reuse of the chemicals)
The acetylcholinesterase analog for the breakdown of norepinephrine. Also: where does it occur? Monoamine oxidase (MAO). After NE is taken up by the varicosity it enters the mitochondria where it is broken down by MAO (or it is repackaged into vesicles and reused)
Postganglionic autonomic neurotransmitters: receptor types Sympathetic: Alpha- and beta-adrenergic; parasympathetic: nicotinic and muscarinic cholinergic
Postganglionic autonomic neurotransmitters: synthesized from Sympathetic: norepinephrine is synthesized from tyrosine; parasympathetic: acetylcholine is synthesized from Acetyl CoA + choline
Postganglionic autonomic neurotransmitters: inactivation enzyme Sympathetic: MAO in mitochondria of the varicosity; parasympathetic: acetylcholinesterase (AChE) in synaptic cleft
Postganglionic autonomic neurotransmitters: varicosity membrane transporters for Sympathetic: norepinephrine; parasympathetic: choline
Alpha receptors The most common sympathetic receptor. It responds strongly to norepinephrine and only weakly to epinephrine
Three main subtypes of beta receptors Beta-1-receptors: respond equally strongly to norepinephrine and epinephrine; beta-2-receptors: more sensitive to epinephrine than to norepinephrine; beta-3-receptors: more sensitive to norepinephrine than to epinephrine
Unique characteristic about beta-2-receptors They’re not innervated (no sympathetic neurons terminate near them), which limits their exposure to the neurotransmitter norepinephrine
Where are beta-1, beta-2, and beta-3-receptors primarily found? Beta-1: heart/kidney; beta-2: certain blood vessels and smooth muscle of some organs; beta-3: primarily on adipose tissue
All adrenergic receptors are of what class of receptors? Significance? GPCR (rather than ion channels). This means the target cell response is slower to start and lasts longer
If the adrenergic receptors are all of the same class, how do they differ in function? Their second-messenger pathways are different
Second messenger pathways activated upon catecholamine binding to beta receptors Increases cyclic AMP and triggers the phosphorylation of intracellular proteins
Second messenger pathways activated upon catecholamine binding to alpha-1-receptors Phospholipase C is activated, creating inositol triphosphate (IP3) and diacylglycerol (DAG). DAG initiates a cascade that phosphorylates proteins. IP3 opens Ca^2+ channels. Ultimately muscle contraction OR exocytosis occurs
Second messenger pathways activated upon catecholamine binding to alpha-2-receptors Decreases intracellular cyclic AMP and causes smooth muscle relaxation (gastrointestinal tract) or decreased secretion (pancreas)
The adrenal medulla Modified sympathetic ganglion. Specialized neuroendocrine tissue associated with the sympathetic nervous system. It primarily secretes epinephrine (rather than norepinephrine).
Adrenal cortex The outer portion of the adrenal gland; it is a true endocrine gland of epidermal origin that secretes steroid hormones
The preganglionic -> postganglionic (note: they’re not officially “postganglionic neurons”; this is an exception to the pattern) pathway through the adrenal medulla Preganglionic sympathetic neurons project from the spinal cord to the adrenal medulla where they synapse specialized neurons called chromaffin cells which secrete large amounts of epinephrine directly into the blood
Chromaffin cells Modified postganglionic neurons that lack the axons which would normally project to target cells. Instead, they secrete epinephrine directly into the blood. Ergo: preganglionic neurons synapse chromaffin cells, not postganglionic neurons
Class of receptors that make up all muscarinic receptors GPCR
Recap: types of cholinergic receptors Nicotinic and muscarinic
Recap: types of adrenergic receptors Alpha and beta
Agonists and antagonists for muscarinic receptors Agonist: muscarine; antagonists: atropine, scopolamine
Agonists and antagonists for nicotinic receptors Agonist: nicotine; antagonists: curare, etc.
Agonists and antagonists for alpha (adrenergic) receptors Agonist: phenylephrine; antagonists: “alpha-blockers”
Agonists and antagonists for beta (adrenergic) receptors Agonist: isoproterenol; antagonists: “beta-blockers”: propranolol (beta-1 and beta-2), metoprolol (beta-1 only)
What is the effect of cocaine on neurotransmitters? It acts as an indirect agonist that blocks the reuptake of NE into adrenergic nerve terminals, thereby extending NE’s excitatory effect on the target
How do antidepressants affect neurotransmitters? Tricyclic antidepressants and SSRIs act on membrane transporters for neurotransmitters; MAO inhibitors act on metabolism
Urinary incontinence Loss of bladder control
How do somatic motor pathways differ anatomically and functionally from autonomic pathways? Somatic motor pathways have a single neuron that originates in the CNS and projects its axon to the target tissue, which is always a skeletal muscle. Unlike autonomic pathways (excitatory OR inhibitory), somatic = only excitatory
Somatic motor neurons: (1) neurotransmitter/receptor; (2) target tissues; (3) neurotransmitter released from; (4) effects on target; (5) peripheral components outside CNS; (6) summary of function (1) ACh/nicotinic; (2) skeletal muscle; (3) axon terminals; (4) excitatory only: muscle contracts; (5) axons only; (6) posture and movement
Where are the cell bodies of somatic motor neurons located Either in the ventral horn of the spinal cord or in the brain, with a long single axon projecting to the skeletal muscle target (myelinated obviously; can reach a over a meter in length, e.g., those that innervate the toes)
Branching of somatic motor neurons They branch close to their targets. Each branch divides into a cluster of enlarged axon terminals that lie on the surface of the skeletal muscle fiber. This branching allows a single motor neuron to control many muscle fibers at once
Neuromuscular junction The synapse of a somatic motor neuron on a muscle fiber. AKA NMJ.
Three components of the NMJ (1) The motor neuron’s presynaptic axon terminal filled with synaptic vesicles and mitochondria; (2) the synaptic cleft, and (3) the postsynaptic membrane of the skeletal muscle fiber
What role do Schwann cells play at the NMJ Schwann cells form a thin layer covering the top of axon terminals. They provide insulation to speed up action potential and they also create a variety of signal molecule which play a critical role in the formation and maintenance of NMJs
What is occurring on the postsynaptic side of the NMJ? The muscle cell membrane opposite the axon terminal is modified into a motor end plate. The synaptic cleft is filled with a fibrous matrix whose collagen fibers hold the axon and motor end plate in proper alignment.
Motor end plate A series of folds that look like shallow gutters. Along the upper edge of each gutter, nicotinic ACh receptor (nAChR) channels cluster in an active zone
Enzyme located in the synaptic cleft of the NMJ AChE – to degrade ACh into acetyl and choline
Are nAChR channels of skeletal muscles identical to those found on neurons? No; skeletal muscle has alpha, beta, gamma, and epsilon subunit isoforms while neuronal nAChR has only alpha and beta isoforms
Review: ACh binding to nAChR The nicotinic cholinergic receptor binds TWO ACh molecules, opening a nonspecific monovalent cation channel. The result: Na+ and K+ can now pass into the target
Created by: Intellex_



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