SMOOTH  MUSCLE      

Learning objectives

1.  Describe  the  structure  and  func2on  of  smooth  muscle    2.  Explain  how  myofilaments  are  regulated.  3.  Name  three  ways  in  which  contrac2on  is  ini2ated.  4.  Explain  spontaneous  electrical  ac2vity  (pacemaker).  5.  Contrast  single  unit  and  mul2unit  smooth  muscles.  

•  The  sliding  filament  mechanism,  in  which  myosin                filaments  bind  to  and  move  ac2n  filaments,  is  the  

           basis  for  shortening  of  s2mulated  muscle.    

•  Myosin-‐ac2n  interac2ons  are  regulated  by  calcium.    

•  Changes  in  membrane  poten2al  are  linked  to  changes            in  internal  Ca++  leading  to  contrac2on  (E-‐C  Coupling).  

SHARED  PRINCIPLES  

Thin,  cigar  shaped  cells  with  central  nucleus.    No  T  tubules  &  no  troponin-‐tropomyosin.    ContracHon  is  regulated  by  thick  (myosin)  filament.      Thin  filaments  are  anchored  to  the  plasma  membrane  by  dense  bodies  (analogous  to  z  lines).      When  contracted,  cells  have  a  “postage    stamp”    shape.        

SMOOTH  MUSCLE  FIBER    

Removal  of  Ca++  leads  to  relaxaHon  by  dephosphorylaHng  myosin  light  chain.    

Ca++  REGULATES  MYOSIN      

TONIC:    amount  of  tension  generated  is          propor6onal  to  the  s6mulus  and        is  sustained  over  6me.      

 PHASIC:    single  contracHon  followed  by  

   relaxaHon  =  twitch.      

TYPES  OF  CONTRACTION  Slow  myosin  ATPase  =  slow  contracHon  

•  Mechanically  Gated  Channels  •  Ligand  Gated  Channels  or  Receptors  

   -‐  Autonomic  nervous  system  (NorEPI)      -‐  Hormones  (expl.  oxytocin)        -‐  Paracrine  agents  (expl.  K+,  H+)  

•  Voltage  Gated  Channels      -‐  Spontaneous  pacemaker  poten6als  

REGULATION  OF  CONTRACTION  

PACEMAKER  ACTIVITY  

Unstable  res2ng  membrane  poten2al  opens  Ca++  voltage  channels    resul2ng  in  rhythmic  pa]erns  of  ac2on  poten2als  and  contrac2ons.    Res2ng  membrane  poten2al  is  unstable  due  to  efflux  of  K+  via  K+  leak  channels.      

Mem

bran

e  po

tenH

al  (m

V)  

Time  (msec)  

threshold  

SINGLE  UNIT  &    MULTI  UNIT  FIBERS  In single-unit muscles: a few cells are innervated. Gap junctions spread the membrane potentials between neighboring cells = synchronous activity. (Expl: GI tract, uterus and small blood vessels)

In multi-unit muscle: each fiber is innervated. There are few or no gap junctions. These cells are not activated by stretch receptors. (Expl:  associated  with  hair  follicles)    

KEY  CONCEPTS  •  Smooth  muscle  is  an  involuntary,  non-‐striated  muscle  associated  with  blood  vessels  

and  visceral  organs.    •  Smooth  muscle  contains  overlapping  protein  myofilaments,  ac2n  and  myosin.  The  

rela2ve  sliding  of  which  produces  shortening  and  generates  force.  This  process  involves  cross  bridge  forma2on  between  ac2n  and  myosin  which  is  driven  by  ATP.    

•  Coupling  between  the  membrane  ac2on  poten2al  and  contrac2on  is  mediated  by  calcium  ions.  Ca++  regulates  myosin  to  enable  cross  bridge  forma2on  and  contrac2on.  

•  Smooth  muscle  is  regulated  by  the  autonomic  nervous  system.  Some  smooth  muscle  is  regulated  by  stretch  and/or  by  paracrine  factors.  

•  In  pacemaker  cells,  ac2on  poten2als  are  ini2ated  by  an  influx  of  extracellular    Ca++.        •  Some  smooth  muscle  exhibits  fused  tetanus  and  tonic  contrac2on.