Regulation of Gastrointestinal Function Nerves and Smooth Muscle Flashcards
Smooth muscle is unstriated, does it have sarcomeres? Z lines? Dense bodies?
No sarcomeres
No Z lines
Yes on the Dense bodies, whatever they are
Does tropomyosin block smooth muscle binding sites?
No
What activates myosin in smooth muscle?
Ca2+-dependent phosphorylation of
regulatory light chains
In smooth muscle innervation there are multiple contacts between neuron and smooth muscle cell-No true NMJ. What is the term for the axonal swelling at each contact point? What is contained there-in?
Varicosities
neurotransmitters
What are the two main types of smooth muscle?
- Multi-unit smooth
- single-unit smooth
What are the two subtypes of single-unit smooth muscle?
- Visceral smooth
- Unitary smooth
What allows multi-unit cells finer motor control?
Electrical isolation
What allows unitary muscle cells coordinated contraction?
Gap junctions
In multi-unit smooth muscle, how is synaptic input distributed?
Each smooth muscle cell recieves its own synaptic input.
How is innervation distributed to the unitary smooth muscle?
Only a few of the smooth muscles recieve direct synaptic input. Stimulus is propagated through gap junctions.
Characterize in one word the stimulation of multi-unit s. muscle cells.
Neurogenic
Characterize in one word the stimulation of single-unit smooth muscle cells.
Myogenic
Single-unit smooth muscle cells are myogenic and may exhibit pacemaker activity. How is the oscillation of membrane potential described?
Slow wave
Where does one find single-unit S.M. cells?
(5)
- Gastrointestinal tract
- -Uterus
- -Ureter
- -Bladder
- -Some blood vessels
Where does one find multi-unit smooth muscle cells?
- Eye: ciliary body, iris
- -Skin: piloerector m.m.
- -Some blood vessels
- -Vas deferens
What are the two types of potentials we utilize in the GI tract?
- Graded Potentials
- Slow-wave potentials
Graded potentials are hyperpolarizing or depolarizing, what kinds of summation (2) and stimulation (3) do they involve?
Summation
- Spatial
- Temporal
Stimulation
- mechanical
- humoral
- neural
Slow-wave potentials involve G.I. pacemaker cells which initiate spontaneous electrical activity. What are these called? Where are they found?
- Interstitial cells of Cajal
- Between longitudinal and circular muscles of the muscularis externa layer.
Repetitive oscillation in Vm = ?
Generally thought to result from?
slow-wave potential
cyclic opening of Ca2+ channels, followed by K+ channels
Smooth muscle action potentials are generated in response to? How do they compare to the sk. muscle?
Neural and/or hormonal modulation of membrane potential to threshold.
Slower and longer than sk. muscle
Describe the depolarization and repolarization of smooth muscle.
Depolarization: opening of slow Ca2+ channels (slow inactivation); Na+
Repolarization: delayed opening voltage-gated or Ca2+-activated K+ channels
Identify steps 1-7
What is of great importance in this, per Dr. Choudry?
- Voltage gated calcium channels open
- action potential spike
- Calcium influx, rise in internal Calcium concentration
- open Calcium dependent K+ cannels
- slow hyperpolarization
- voltage-gated Calcium channels close, and internal [Ca2+] decreases
- Ca2+ dependent K+ channels close
Calcium influx and potassium efflux
What type of contraction do phasic smooth muscles exhibit? Where are they found?
short-lived
GI tract and bladder
What type of contraction do tonic smooth muscle exhibit?
prolonged contraction
Identify which of the three steps shown is voltage dependent or independent.
- Voltage gated
- voltage independent
- voltage independent
How can hormones & neurotransmitters can initiate increased [Ca2+]i?
Via G-protein coupled receptors (voltage independent)
What are the three sources of Ca2+ in smooth muscle?
- voltage gated calcium channels
- sarcoplasmic reticulum
- store-operated calcium channels (SOCs)
What are the three ways calcium is sequestered in smooth muscle?
- SERCA of the SR
- Ca2+-ATPase pump (sarcolemma)
- 3Na+-1Ca2+ antiporter (sarcomlemma)
What is the role of Ca2+ in the Excitation-Contraction Coupling of Smooth Muscle?
The activation of myosin
How does Calcium activate myosin?
- 4 Ca2+ binds calmodulin
- Ca2+- calmodulin complex binds & activates myosin light-chain kinase (MLCK)
- Activated MLCK phosphorylates myosin regulatory light chain
- Conformational change of myosin head & increased ATPase activity allows actin interaction & power stroke
How is contraction force regulated in smooth muscle?
relative balance between phosphorylated and dephosphorylated MLCs
- [Ca2+]i
- Sensitivity to Ca2+ of MLCK
_______________________:
Maintenance of tension in tonic smooth muscle contractions for prolonged periods of time
-Relatively low ATP consumption
(< 300x that of skeletal m. for same force generation)
- Force maintained at lower level of MLCK phosphorylation
- Myosin remains attached to actin for longer duration
Latch State
What is the significance of the latch state?
Allows the ability to maintain smooth muscle tone without fatigue.
What are the extrinsic innervations of the G.I? Intrinsic?
Extrinsic: Parasympathetic and sympathetic
Intrinsic: submucosal and myenteric plexuses
What are the four general GI regulatory mechanisms?
- Autonomous smooth muscle
- extrinsic nerves
- Intrinsic nerve plexuses
- gastrointestinal hormones
What are some examples of GI regulatory hormones?
- Gastrin, Secretin, CCK, GIP
- Paracrines: Somatostatin, Histamine
Increased parasympathetic activity promotes?
digestive and absorptive processes; increases motility and secretions
What impact does increased ACh have on…
- salivation
- defecation
- lacrimation
- GIT motility
- urination
- muscle contraction
Increases all
What impact does decreased ACh have on…
- salivation
- defecation
- lacrimation
- GIT motility
- urination
- muscle contraction
Decrease all
What are the prevertebral ganglia? (3)
- Celiac
- Superior Mesenteric
- Inferior Mesenteric
Increased sympathetic activity promotes?
decreased motility and secretions; increased sphincter constriction
What is the enteric nervous system?
What does it do? (3)
Division of the ANS
- Coordinates and relays Parasympathetic and sympathetic inputs to the GI tract.
- Coordinates local reflexes within the GI tract
- Controls most GI functions (motility and secretions)
Can the ENS act independently of extrinsic innervation?
Yes
Where is Auerbach’s plexus found?
(A.K.A. the myenteric plexus)
-Between the outer longitudinal and inner circular muscle layers of the muscularis externa throughout the GI tract
Where is Meissner’s plexus found?
A.K.A. the submucosal plexus
In the submucosa of the small and large intestines
Identify the indicated structures
What does the myenteric plexus primarily control?
Motility of the GI smooth muscle
What does the submucosal plexus primarily control?
Secretion and blood flow
What do mechanoreceptors monitor in the GI lumen? Chemoreceptors?
- distention
- pH, nutrients
