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GI Smooth M. (general properties of smooth muscle (innervation (no true…

- - - - located b/w inner circular & outer longitudinal layers of muscularis externa
      - intramuscular ICCs (IM-ICCs):
        
        w/in circular layers of smooth m. bundles
        
        important in mediating neurotransmission d/t synaptic connections w/ varicose terminals of enteric motor neurons
        
        electrically coupled to smooth m. via gap jxns
      - generates spontaneous, repetitive oscillations of resting mp (plateau potentials; not APs)
      - "GI pacemaker cells" in region of myenteric plexus (MY-ICCs)
    - - cyclic opening of Ca-channels (depolar.) >> opening of K-channels (hyperpolar.)
      - depolarization
        
        membrane potential closer to threshold >> inc. probability of AP
        
        (+) L-type Ca-channels >> Ca entry >> contraction
        
        electrotonic conduction via gap jxns
      - AP produced on top of slow wave (spike potential) >> can initiate contraction
      - naturally organize contractile pattern into series of phasic contraction
    - - gastric (body): 3-5/min
        
        coordinated contractions allow for churning
        
        pacemaker area (in proximal corpus) >> perstaltic waves >> distal antrum
      - duo: 12-20/min
      - terminal ileum: 8-9/min
      - not influenced by neural or hormonal input
      - colon: 6-8/min
  - - - Voltage-gated Ca-channels opens >> AP spike
      - Ca-influx >> inc. intra-[Ca]
      - opens Ca-dependent K-channels >> K efflux >> slow hyperpolarization
      - Voltage-gated Ca-channels close >> dec. intra-[Ca]
      - Ca-dependent K-channels close >> depolarization
    - - higher slow wave potential rises >> inc. frequency of spike potentials
      - range = 1-10 spikes/second
      - last 10-20 msec (10-40x longer than APs in large nerve fibers)
      - Ca-Na channels
        
        unlike nerve fibers w/ Na-channels
        
        slower to open & close >> inc duration of AP
    - - depolarization:
        
        opening of slow Ca-channels
        
        Na
      - repolarization:
        
        delayed opening of voltage-gated or Ca-activated K-channels
      - minimal overshoot
  - - - less (-) >> more excitable
      - stimuli:
        
        stretching
        
        ACh (parasympathetic nn.)
        
        specific GI hormones (gastrin)
    - - more (-) >> less excitable
      - stimuli:
        
        NE (or epi)
        
        stimulation of sympathetic nn. that secrete NE
- - - - densely innervated
      - recruitment
      - allows finer motor control d/t separate motor units
      - electrical isolation: each cell receives own synaptic input
      - controlled by neural input or hormones (autonomic efferents)
      - little or no gap jxns
      - has pacemaker ability but rarely used
    - - eye: ciliary body & iris
      - skin: pilo-erector mm.
      - some large blood vessels
        *vascular smooth m. is described as having properties of both multi- & single-unit
      - vas deferens
    - - initiates contraction
      - contributes to gradation
  - - - GI tract
      - bladder & ureters
      - small blood vessels
      - uterus
    - - coordinated contraction: contract as single unit
      - pacemaker potentials & slow-wave potentials
      - gap junctions >> electrical syncytium
      - one nerve for many cells; few receive direct synaptic input
    - - modifies contraction
      - excite or inhibit
      - contributes to gradation
- - - - proposed MOA:
        
        dephosphor. "latch" cross bridges eventually release
        
        desphosphor. of myosin while bound to actin >> slows detachment
        
        must be rephosphor. by MLCK in order to bind again
      - **maintains smooth m. tone w/o fatigue
      - prolonged contraction at low ATP cost
      - lower level of MLCK phosphor.
      - slower cross bridge detachment kinetics
    - - vascular smooth m.
        (important for maintaining BP)
      - sphincters (LES, ICS, IAS)
      - bronchioles of lungs
    - - continuous repetitive spike potentials
      - hormones or orther facts that cause continuous partial depolarization
      - continuous Ca entry via mechanisms not assoc. w/ mp
  - - - SI
      - urinary bladder
      - esophagus
      - gastric antrum
- - - - depolarization >> opens L-type Ca-channels
    - - SR Ca-stores depleted >> opens SOCs
  - - - Pharmacomechanical coupling
        
        via G-protein coupled receptors (cascade)
        
        hormones & NTs >> inc. intra-[Ca]
      - via:
        
        Ca-induced Ca release (CICR) via ryanodine receptor (RYR)
        
        majority: IP3-activated Ca-channels on SR
- - - - d/t many gap jxns >> low-resistance movement of ions
      - also, muscle bundles fuse at many points
      - action potential elicited w/in bundle transmitted thruout
      - distance traveled related to excitability of muscle