Lecture 12 - Visceral Motor System Flashcards
What is the efferent division of the visceral NS?
Autonomic NS
Does the autonomic NS have both CNS and PNS components?
YUP
Main function of autonomic NS?
Regulate the activity of vital organs
If consciousness of a patient is greatly affected, can the autonomic NS still be functioning?
YUP
What are the 3 branches of the autonomic NS?
- Sympathetic NS
- Parasympathetic NS
- Enteric NS
Sympathetic stimulation on heart?
- Increased heart rate
2. Increase in force of contraction
Sympathetic stimulation on blood vessels?
Constriction
Sympathetic stimulation on lungs?
Bronchodilation
Sympathetic stimulation on GIT?
- Decreased motility
- Sphincter contraction
- Decreased secretions
Parasympathetic stimulation on heart?
- Decreased heart
2. Decreased force of contraction (weak effect)
Parasympathetic stimulation on blood vessels?
No effect except in genitalia (dilation)
Parasympathetic stimulation on lungs?
Bronchoconstriction
Parasympathetic stimulation on GIT?
- Increased motility
- Sphincter relaxation
- Increased secretions
Describe the autonomic NS pathway.
Preganglionic (CNS) neuron bodies in brainstem and spinal cord -> Preganglionic fibers (CNS) => Postganglionic (PNS) primary motor neurons (bodies and fibers) -> viscera
Are the preganglionic neuron bodies in the CNS sympathetic or parasympathetic?
- Brainstem: parasympathetic
2. Spinal cord: both
Where are the preganglionic neuron bodies found in the spinal cord?
Ventral horn
What are the 3 autonomic ganglia of the PNS?
- Paravertebral (chain) ganglia
- Prevertebral ganglia
- Intramural ganglia
What are the 2 main differences between the somatic efferent pathway and the autonomic pathway?
- LMNs synapse directly onto skeletal muscle allowing for precise innervation vs autonomic pathways involve 2 neurons and is diffuse with divergent targeting and indirect control
- LMNs are heavily myelinated therefore have faster conduction vs ANS preganglionic neurons are slightly myelinated and postganglionic ones are not at all, therefore have slower conduction
Describe the sympathetic outflow pathway.
Preganglionic neuron in spinal cord => paravertebral or prevertebral ganglion => postganglionic axon => viscera
What is the difference between paravertebral and prevertebral ganglia?
Paravertebral ganglia are closer to the spinal cord vs prevertebral ganglia are closer to the end organs
Describe the parasympathetic outflow pathway.
Preganglionic neuron => intramural ganglion => postganglionic axon => viscera
Does the autonomic system have myotomes?
NOPE
Where do the sympathetic preganglionic neurons synapse along the spinal cord? What do we call this part?
T1 to L2 spinal segments = Intermediolateral column
Where do the parasympathetic preganglionic neurons synapse along the spinal cord?
- Brainstem cranial nerve nuclei (III, VII, IX, X)
2. S2 to S4 spinal segments (sacral spinal cord)
What part of the spinal cord horns are sympathetic preganglionic neurons found?
Lateral horns
What part of the spinal cord horns are parasympathetic preganglionic neurons found?
Intermediate zone
Where are the intramural ganglia of the parasympathetic NS located?
Generally near target tissues (often on blood vessel)
What is the NT released by both pre and post ganglionic parasympathetic neurons?
ACh
Do most organs have dual sympathetic/parasympathetic innervation?
YUP with a few exceptions
Parasympathetic NS: how does the length of the preganglionic axons compare to the postganglionic axon?
Longer preganglionic axon
What is the primary control center of the autonomic NS?
Hypo
Sympathetic NS: how does the length of the preganglionic axons compare to the postganglionic axon?
Longer postganglionic axons
What is the NT released by preganglionic sympathetic neurons?
ACh
What is the NT released by postganglionic sympathetic neurons?
NE, except for sweat glands (ACh)
What is the vagus nerve made of? What does it innervate?
Preganglionic fibers from the dorsal motor nucleus in the medulla synapsing on postganglionic neurons in intramural ganglia near many viscera in thorax and abdomen
What part of the autonomic system is the vagus nerve part of?
Parasympathetic
Where is the dorsal motor nucleus of the vagus nerve found?
Cranial nerve nucleus for the vagus nerve in the medulla
What is the vagovagal reflex?
Stomach becomes distended as it fills with food => vagus nerve afferent stimulates medulla => medulla stimulates vagus nerve efferent to stimulate parasympathetic responses like HCl and gastrin secretion in stomach and increased gut motility
Doe the vagus nerve only have an efferent component?
NOPE, afferent also
What is vasovagal syncope? What causes this? For who is it most common?
Vagus nerve hyperactivation associated fainting due to stimulation by GI illness, pain, or stress leading to low BP and heart rate which causes fainting
Most common in young people
What is a rare symptom of vasovagal syncope?
Localized seizure-like activity
What part of the autonomic system are the pelvic splanchnic nerves part of?
Parasympathetic
What are the pelvic splanchnic nerves made of?
Preganglionic fibers from the sacral spinal cord synapsing on postganglionic neurons in intramural ganglia near viscera in abdomen and pelvis
What 2 physiological processes do the pelvic splanchnic nerves control?
- Urination
2. Erection
What are the 5 types of nerves of the bladder? Function for each?
3 efferents:
- Sympathetic NS: contracts internal sphincter = KEEP PEE IN
- Parasympathetic NS (splanchnic nerves): contracts wall muscles and relaxes internal sphincter = PEE
- Somatic motor: controls external sphincter
2 afferents:
- Sympathetic NS: stimulation of efferent neurons due to mild stretch
- Parasympathetic NS (splanchnic nerves): stimulation of efferent neurons due to full stretch
Why do newborns and infants pee themselves?
They have not yet developed the ability to control the bladder’s external sphincter
What are the drugs Viagra, Cialis, and Levitra used to treat? How do they work? Side effects?
- Urine retention due to prostate enlargement
- Erectile dysfunction
Cause release of ACh from parasympathetic NS => NO release by endothelial cells => increase in cGMP in smooth muscles => decrease in [Ca++] => relaxation
Side effects: decrease in BP and heart rate leading to fainting
What organ releases norepi/epi as hormones instead of NTs? What is it composed of? What is it innervated by?
Adrenal medulla composed of modified postganglionic neurons = chromaffin cells innervated by preganglionic sympathetic neurons
Where is the adrenal medulla located in the body?
Right on top of each kidney
Are visceral reflexes mono or polysynaptic?
Always polysynaptic
What are the 2 visceral reflexes pathways? Describe each.
- Long reflex: afferent neuron => interneuron in gray matter of the brain or spinal cord => preganglionic neuron => autonomic postganglion (sympathetic or parasympathetic) => postganglionic neuron
- Short reflex: afferent neuron => interneuron in autonomic postganglion (sympathetic or parasympathetic) => postganglionic neuron
What is the main function of the baroreceptor reflex?
Regulation of heart rate and BP
Describe the pathway of the baroreceptor reflex.
Stretch receptors in walls of internal carotid arteries and aorta => visceral afferents of cranial nerves 9 and 10 (accessory and hypoglossal) => cardioregulatory center in medulla => parasympathetic (cranial nerve X) and sympathetic efferent fibers synapse on heart and SNS on adrenal medulla CV center
What autonomic NS is activated by a BP increase? How does it reduce it?
Parasympathetic NS => decreased heart rate => decreased BP
What autonomic NS is activated by a BP decrease? How does it increase it?
Sympathetic NS => increased heart rate => increased BP
What type of receptors are autonomic postganglionic ones?
Ionotropic ACh receptors
What type of receptors are visceral autonomic ones? What do they regulate?
GPCRs
Regulate intracellular signaling and glycogenolysis
Describe the parasympathetic NT pathway.
Preganglionic neuron releases ACh =>
postganglionic
nicotinic receptors leading to ACh release =>
target structure
muscarinic receptors => inhibition or excitation of target structure
How can parasympathetic innervation on a target structure lead to either inhibition or activation? Example?
Muscarinic receptors can have different effects depending on location
Example:
- Heart is inhibited via hyperpolarization leading to bradycardia
- Digestive tract smooth muscle is excited by depolarization leading to contractions
What is bradycardia?
Abnormally slow heart action
List the 5 types of parasympathetic muscarinic receptors on target tissues and their effect.
M1: contraction M2: relaxation M3: contraction M4: ? M5: ?
Describe the sympathetic NT pathway.
Preganglionic neuron releases ACh => postganglionic nicotinic receptors leading to NE release => target strcuture adrenergic receptors (β1, α1) => β1: tachycardia OR α1: blood vessel constriction
What is tachychardia?
Abnormally fast heart action
What % of epi is released by the adrenal medulla?
70%
What are beta blockers used for? What type?
Beta 1 blockers to treat hypertension (sometimes non-selective beta blockers are used, but not as safe, especially for asthmatics)
Where are beta 1 adrenergic receptors located?
Heart
Where are beta 2 adrenergic receptors located? What do they cause?
Lungs, uterus, gut: relaxation
Do alpha adrenergic receptors have more affinity for norepi or epi?
Norepi
Do beta adrenergic receptors have more affinity for norepi or epi?
Epi
What is an example of a non-selective beta blocker?
Propanolol
What is an example of a beta 1 blocker?
Metoprolol
Is smooth muscle striated?
NOPE
Why are smooth muscles unstriated?
Because their myosin (thick) and actin (thin) filaments are not organized the same way as in skeletal muscle
What controls smooth muscles?
- Autonomic NS
2. Hormones
Is cardiac muscle a smooth muscle?
NOPE
4 functions of smooth muscles? Give an example for each.
- Motion (pilomotors = hair raising muscles)
- Expelling content of hollow organs (G.I. tract)
- Changing cross-section dimensions of tubular organs
(airways, blood vessels) - Changing dimensions of passive organs (ciliary
muscles/lens)
What are smooth muscles?
All internal contractile organs except the heart
What are the 2 types of smooth muscle? 2 names for each
Which one is more prevalent?
- Multiunit = neurogenic
2. Unitary = myogenic (majority of smooth muscles)
Where are multiunit smooth muscles found?
In areas that require fine motor movement: iris, hair follicles (pili erector), ciliary muscle, large blood vessels, airways
Describe the cell membranes of multiunit smooth muscles. What does this allow?
They are electrically isolated to allow for finer muscle control.
What does it mean for multiunit smooth muscle cells to be neurogenic?
They have few muscle fibers/cells per neuron
What do unitary smooth muscle cells form?
Syncytium = functional group of interconnected cells
What is the activity of unitary smooth muscle cells like?
Contraction INITIATED by the myocyte itself, not innervation or hormones
Which smooth muscles have a higher innervation ratio: unitary or multiunit?
Multiunit
Where are unitary smooth muscles found? 6 locations
- GI tract
- Uterus
- Urethra
- Arterioles
- Capillaries
- Sphincters
What permits coordinated contraction of myogenic smooth muscles?
Gap junctions
Can unitary smooth muscles be innervated by the autonomic nervous system as well?
Yup
How do smooth muscle synapses differ from those at the NMJ? 2 ways
- They include varicosities that release NTs into a WIDE synaptic cleft, so the junction is more diffuse making the signaling less discrete and slower than at the NMJ
- The possynaptic receptors are GPCRs, allowing for graded potentials
Describe the 3 different action potentials that go through smooth muscle cells.
What is each caused by?
For what type of smooth muscle does the 3rd one happen?
- Single spike: electrical stimulation, hormone, or stretch
- Plateau: due calcium influx through channels that inactivate poorly
- Slow wave: much slower, below threshold and due to intracellular calcium: unitary smooth muscle
Describe the 7 steps for muscle contraction in smooth muscles.
- Calcium influx into muscle cell (or released from sarcoplasmic reticulum)
- Calmodulin binds calcium for Ca2+-CAM
- Ca2+-CAM binds and activates myosin light chain kinase (MLCK)
- Myosin hydrolyzes ATP recocking the myosin head for it to be perpendicular to actin = cross bridge release
- MLCK phosphorylates the myosin head activating it
- Myosin (bound to ADP + Pi) binds to actin
- Myosin releases ADP + Pi = contraction (crawling on actin filaments)
- ATP binds to myosin= actin release
- Myosin light chain phosphatase (MLCP) dephosphorylates myosin, inactivating it
- Calcium is resorbed by SR/sarcolemma by Ca-ATPase
How does skeletal muscle contraction differ from smooth muscle contraction?
- Intracellular calcium is key
- Ca2+ causes a fast physical change with troponin on actin
- Contraction is more rapid
Why is smooth muscle cell contraction slower than in skeletal muscle?
Because it’s a 2 step enzymatic process with calmodulin activating MLCK
