CVPR 03-28-14 09-10am Autonomic Nervous System Physiology - Felsen Flashcards
Role of the autonomic NS
To maintain homeostasis in the internal environment amid changing external conditions, via widespread & complementary actions in organs systems in response to sensory stimuli
How the ANS works
It regulates the physiological systems over which we have only minimal conscious control, such as the cardiovascular, GI, and respiratory systems….. Changes in autonomic control of these systems elicit the responses we associate with stressful stimuli and other emotions.
ANS vs. Somatic motor system
ANS – involuntary, diffuse projections, slow action, innervates smooth/cardiac muscle & gland cells, disynaptic (pre/postganglionic neurons)…..SMS – voluntary, specific projections, rapid action, innervates skeletal muscle, monosynaptic
Autonomic NS – how it works
Unlike the somatic (motor) NS, the Autonomic NS is characterized by a 2-neuron link between CNS & peripheral target organ….Preganglionic neurons are located in the CNS (brainstem or spinal cord) and connect to Postganglionic neurons located in autonomic ganglia outside the CNS —> Postganglionic neurons project to the target organ where they innervate smooth muscle, cardiac muscle, or glands
Myelination w/in the 2-Neuron System of the ANS
Transmission over PREganglionic fibers is relatively fast, b/c they are composed of myelinated fibers….. POST ganglionic fibers are unmyelinated.
Autonomic nervous system - Division
The ANS consists of the Sympathetic NS & the Parasympathetic NS, as well as the Enteric semi-autonomous control of digestion
Ganglion vs. Nucleus
Ganglion = cluster of cell bodies in peripheral NS; Nucleus = cluster of cell bodies in CNS
Sympathetic vs. Parasympathetic – origin of preganglionic neurons
SYMP: thoracic & lumbar spinal cord….. PARA: brainstem & sacral spinal cord
Sympathetic vs. Parasympathetic –location of ganglia
SYMP: near spinal cord…..PARA: near target organs
Sympathetic vs. Parasympathetic – ratio of pre- to post-ganglionic neurons
SYMP: 1:10 pre to post…. PARA: 1:3
Sympathetic vs. Parasympathetic –Neurotransmitters of preganglionic neurons
BOTH: Acetylcholine (ACh)
Sympathetic vs. Parasympathetic – Postanglionic
SYMP: Norepinephrine (NE) and epinephrine…. PARA: ACh
Role of the Sympathetic nervous system
Initiates responses to emergency or stressful situations - “fight and flight” responses
Organization of the Sympathetic NS – Location of Preganglionic neurons
Preganglionic neurons are located in the intermediolateral column of the thoracic & upper lumbar segments of the spinal cord, as well as in splanchnic nerves arising from the lower 7 thoracic spinal level
Pathway of Preganglionic Sympathetic neurons located in the thoracic/upper lumbar spinal cord
Preganglionic Sympathetic axons exit the spinal cord in the ventral roots of corresponding spinal nerves –> Pass through white ramus communicans (myelinated) —> Enter sympathetic trunk and branch to innervate postganglionic neurons in multiple levels of the spinal trunk
Pathway of Preganglionic Sympathetic neurons located in splanchnic nerves
Preganglionic parasympathetic axons in splanchnic nerves pass through the spinal trunk to innervate postganglionic sympathetic neurons located in the celiac & mesenteric ganglia
Neurotransmitter relased by PREganglionic vs POSTganglionic neurons of the Sympathetic NS
Acetylcholine is the neurotransmitter released by preganglionic sympathetic neurons; Postganglionic sympathetic neurons release norepinephrine as their neurotransmitter.
Organization of the Sympathetic NS –Location of Postganglionic neurons
Located in the ganglia of the sympathetic trunk (paravertebral chain ganglia) and in the celiac & mesenteric ganglia (those involved in GI tract regulation
Pathway of Sympathetic Postganglionic neurons located in the paravertebral chain ganglia
From the ganglia of the sympathetic trunk (paravertebral chain ganglia). The sympathetic postganglionic axons exit the spinal trunk through the gray rami communicans (unmyelinated) —> travel in spinal nerves to reach sweat glands, peripheral blood vessels, hair follicles, etc
Pathway of Sympathetic Postganglionic neurons located in the celiac & mesenteric ganglia
Axons from the postganglionic sympathetic neurons in the celiac & mesenteric ganglia form the perivascular plexuses innervating the abdominal viscera —> regulate GI tract
Adrenal medulla
Specialized component of the sympathetic nervous system; functions as neuroendocrine gland; its postganglionic cells secrete epi (80-90%) & norepi (10-20%), both of which bind to adrenergic receptors, into the blood stream….Thus, rather than acting as neurotransmitters, these agents become hormones to regulate activity of distant target tissues (sympathomimetic but more widespread & slower than normal sympathetic neurons)
Innervations of specialized postganglionic cells of the Adrenal medulla
Innervated by preganglionic sympathetic neurons located in thoracic levels 6-9 of the spinal cord.
Sympathetic NS – what it controls
Vasodilation & vasoconstriction are controlled primarily by the SNS….. Increased sympathetic output causes vasoconstriction (as well as increased HR & force of contraction), while decreased sympathetic output causes vasodilation….At rest, sympathetic output maintains the vasculature in an intermediate state between constriction & dilation
Role of the Parasympathetic nervous system
Involved in conservation & replenishing of resources; Classically associated w/ “rest and digest” responses; The actions of the PNS generally oppose those of the sympathetic NS.
Organization of the Parasympathetic NS –Location of Preganglionic neurons
Preganglionic neurons of the PNS are located in brainstem nuclei & in the sacral spinal cord ; characterized by long axons
Pathway of Parasympathetic Preganlionic neurons located in the brainstem
The parasympathetic preganglionic fibers located in the brainstem exit in cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus) —> innervate postganglionic neurons located in ganglia near or in the effector organ.
Pathway of Parasympathetic Preganglionic neurons located sacrally
The parasympathetic preganglionic axons exit the sacral portion of the parasympathetic system and travel in the splanchnic nerves to innervate ganglia associated with the colon, rectum, urinary bladder, and genital organs —> functions to control “emptying” (e.g. urination, defecation, and erection)
Organization of the Parasympathetic NS –Location of Postganglionic neurons
Located in/near effector organ, and thus characterized by short axons.
Neurotransmitters relased by PREganglionic vs POSTganglionic neurons of the Parasympathetic NS
Both pre- & post-ganglionic parasympathetic neurons use acetylcholine as a neurotransmitter
Increased parasympathetic output results in….
Decreased HF & force of contraction; While parasympathetic output may result in some small amount of vasodilation, most vasodilation is controlled by decreased sympathetic output.
Neurotransmitters of ANS preganglionic neurons
Usually, preganglionic neurons of sympathetic & parasympathetic system release NT acetylcholine (ACh) —> ACh acts on nicotinic cholinergic receptors on postganglionic neurons —> depolarization & firing of postganglionic neuron —> APs arrive at nerve terminal of postganglionic neuron then cause release of synaptic vesicles containing norepinephrine or ACh via exocytosis…… Co-localized agents (e.g. ATP, peptides) also participate.
Neurotransmitters of Sympathetic vs. Parasympathetic Postganglionic neurons
Sympathetic Postanglionic = Norepinephrine (NE); Parasympathetic Postganglionic = ACh
Receptor subtypes for ACh: Two types
Two types of cholinergic receptors to which ACh binds & elicits a cholinergic response.: 1. nicotinic (stimulated by nicotine) AND 2. muscarinic (stimulated by muscarine) receptors……. Cellular mechanisms by which ACh produces its effects are very different for nicotinic & muscarinic receptors.
Location of ACh receptors - Nicotinic vs. Muscarinic
Nicotinic receptors are mainly present in the cell body of postganglionic neurons of the autonomic ganglia; Muscarinic receptors are present on effector cells of cardiac muscle, smooth muscle, & glands.
Nicotinic receptor – what it is & what ACh does to it
A ligand-gated, non-selective cation channel….. When ACh binds it, the ion channel opens & allows rapid movement of Na+ & K+ across the membrane—> depolarization & excitation
Muscarinic receptor – what it is & what ACh does to it
Located in cell membrane, linked to a GTP-binding (G) protein….. When ACh binds the extracellular site of the muscarinic receptor protein, a conformational change occurs w/in it —> “activation” of G protein coupled to the receptor —> G protein then either stimulates or inhibits other intracellular effectors such as enzymes or ion channels —> physiological response.
Five subtypes of muscarinic receptors
M1-M5; Different muscarinic receptors are coupled to different effectors within a cell….. Thus, simple ACh can produce various physiological responses in different cell types, depending on which muscarinic receptor subtype is activated
Muscarinic receptors & hyperpolarization
In Atrial cells, ACh released from vagus nerve binds to muscarinic receptor —> activates G protein, which opens K channels —> hyperpolarization & slowing down of HR
Muscarinic receptors & depolarization
In some cells, activation of muscarinic receptors causes a different type of K+ channel (than is opened in Atrial cells by ACh binding to muscarinic receptors) to close —> sustained depolarization of the neuron.
Receptors of ACh vs. NE
ACh: nicotinic vs. muscarinic….. NE: alpha- & beta-adrenergic
Receptor subtypes for Norepinephrine vs. Epinephrine
Like w/ cholinergic receptors, there are multiple receptor subtypes for NE (NT for adrenergic system)….. Adrenergic receptors consist of alpha (α) and beta (β) receptors, subdivided into α1, α2, β1, and β2….NE only activates α1, α2, and β1 receptors….. Epinephrine (released from adrenal medulla into bloodstream) activates all 4 subtypes.
Activity of the autonomic nervous system is regulated by…
…information coming from visceral afferents and from higher CNS centers.
Inputs to preganglionic neurons…
Visceral sensory input <– hypothalamus (in forebrain)
Autonomic reflexes – where they are organized
Autonomic reflexes are organized in the spinal cord & brainstem
Organization of the Baroreflex (an autonomic reflex)
Baroreflex is organized in the medulla w/afferent info transmitted in glossopharyngeal nerve (cranial nerve IX) from the carotid sinus to the dorsal vagal complex of the medulla, which in turn contains neurons projecting to regions of the medulla that regulate sympathetic & parasympathetic activity.
Location of Organization of the respiratory reflexes (autonomic reflexes)
Organized in the pons
Location of organization of the pupillary reflexes (autonomic reflexes)
Organized in the midbrain; allow visual system to rxt to different light levels (dilations/constriction); unconscious, involuntary
Cardiovascular regulation by nervous system
Sympathetic/parasympathetic control of HR & BP; Humoral response (via bloodsream); Baroreceptor reflex
Norepinephrine & Sympathetic regulation of the CV system
Sympathetic stimulation –> increase NE —> increased BP (increase HR & contractile force; vasoconstriction)
Acetylcholine & Parasympathetic regulation of the CV system
Parasympathetic stimulation —> increase ACh —> decreased BP (decreased BP & contractile force)
Baroreceptor reflex
Low BP —> relative increase in sympathetic output (send info to medulla)….. High BP —> relative increase in parasympathetic output
Hypothalamus & the ANS
“Head ganglion” of the autonomic nervous system, b/c it integrates info from several brain regions in order to convey the needs of the organism to the preganglionic autonomic centers in the brainstem & spinal cord….. It also coordinates humoral (i.e., hormonal) response via pituitary & somatic motor response….. It also coordinates the visceromotor response via the ANS, in order to maintain homeostasis. [EX: Hypothalamus stimulates the release of vasopressin from the posterior pituitary into the bloodstream, resulting in vasoconstriction.]
Humoral regulation via Hypothalamus
Hypothalamus controls release of hormones via pituitary (Posterior pituitary (part of the brain) & Anterior pituitary (a gland))
Vasopressin (ADH)
Low BP detected by subfornical organ —> release of vasopressin from posterior pituitary —> causes vasoconstriction & increases water retention via kidneys —> increased BP
