Single Unit and Multiunit Smooth Muscles

Smooth muscles are often grouped into two functional categories: single-unit and multiunit (fig. 12.35). Single-unit smooth muscles have numerous gap junctions (electrical synapses) between adjacent cells that weld them together electrically; they thus behave as a single unit, much like cardiac muscle. Most smooth muscles—including those in the digestive tract and uterus—are single-unit.

Only some cells of single-unit smooth muscles receive autonomic innervation, but the ACh released by the axon can diffuse to other smooth muscle cells. Binding of ACh to its muscarinic receptors causes depolarization by closing K+ channels, as described in chapter 9 (see fig. 9.11). Such stimulation, however, only modifies the automatic behavior of single-unit smooth muscles. Single-unit smooth muscles display pacemaker activity, in which certain cells stimulate others in the mass. This is similar to the situation in cardiac muscle. Single-unit smooth muscles also display intrinsic, or myogenic, electrical activity and contraction in response to stretch. For example, the stretch induced by an increase in the volume of a ureter or a section of the digestive tract can stimulate myogenic contraction. Such contraction does not require stimulation by autonomic nerves.

Contraction of multiunit smooth muscles, by contrast, requires nerve stimulation. Multiunit smooth muscles have few, if

Table 12.9 Comparison of Skeletal, Cardiac, and Smooth Muscle

Skeletal Muscle

Cardiac Muscle

Smooth Muscle

Striated; actin and myosin arranged in sarcomeres

Striated; actin and myosin arranged in sarcomeres

Not striated; more actin than myosin; actin inserts into dense bodies and cell membrane

Well-developed sarcoplasmic reticulum and transverse tubules

Moderately developed sarcoplasmic reticulum and transverse tubules

Poorly developed sarcoplasmic reticulum; no transverse tubules

Contains troponin in the thin filaments

Contains troponin in the thin filaments

Contains calmodulin, a protein that, when bound to Ca2+, activates the enzyme myosin light-chain kinase

Ca2+ released into cytoplasm from sarcoplasmic reticulum

Ca2+ enters cytoplasm from sarcoplasmic reticulum and extracellular fluid

Ca2+ enters cytoplasm from extracellular fluid, sarcoplasmic reticulum, and perhaps mitochondria

Cannot contract without nerve stimulation; denervation results in muscle atrophy

Can contract without nerve stimulation; action potentials originate in pacemaker cells of heart

Maintains tone in absence of nerve stimulation; visceral smooth muscle produces pacemaker potentials; denervation results in hypersensitivity to stimulation

Muscle fibers stimulated independently; no gap junctions

Gap junctions present as intercalated discs

Gap junctions generally present

Fox: Human Physiology, Eighth Edition

12. Muscle: Mechanisms of Contraction and Neural Control

Text

© The McGraw-H Companies, 2003

Chapter Twelve

Multi Unit Smooth Muscle

Autonomic neuron

Synapses en passant

Smooth muscle cell

Gap junctions

Autonomic neuron

Synapses en passant

Figure 12.35 Single-unit and multiunit smooth muscle. In single-unit smooth muscle, the individual smooth muscle cells are electrically joined by gap junctions, so that depolarizations can spread from one cell to the next. In multiunit smooth muscle, each smooth muscle cell must be stimulated by an axon. The axons of autonomic neurons have varicosities, which release neurotransmitters, and which form synapses en passant with the smooth muscle cells.

Autonomic neuron

Synapses en passant

Smooth muscle cell

Gap junctions

Autonomic neuron

Synapses en passant

Figure 12.35 Single-unit and multiunit smooth muscle. In single-unit smooth muscle, the individual smooth muscle cells are electrically joined by gap junctions, so that depolarizations can spread from one cell to the next. In multiunit smooth muscle, each smooth muscle cell must be stimulated by an axon. The axons of autonomic neurons have varicosities, which release neurotransmitters, and which form synapses en passant with the smooth muscle cells.

any, gap junctions. The cells must thus be stimulated individually by nerve fibers. Examples of multiunit smooth muscles are the arrector pili muscles in the skin and the ciliary muscles attached to the lens of the eye.

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Responses

  • Iiro
    Why is nerve stimulation not required in cardiac muscles?
    6 years ago
  • KELLY
    Why do multiunit smooth muscles have few gap junctions?
    6 years ago
  • ted
    How is multiunit smooth muscle like skeletal muscle?
    6 years ago
  • Georgette
    What is multi unit smooth muscle?
    6 years ago
  • fiona
    What are multiunit smooth muscle cells?
    6 years ago
  • mustafa
    Do cardiac muscles behave as a single unit?
    6 years ago
  • charles
    Are smooth muscles in the gi tract single unit muscle cells?
    6 years ago
  • Hyiab
    Do smooth muscles have pacemaker cells?
    6 years ago
  • Ralph
    What is single unit smooth cell ?
    6 years ago
  • Maeve Walker
    Does multiunit smooth muscle tissue have gap junctions?
    6 years ago
  • sam graham
    What is single unit innervations?
    6 years ago
  • ebony
    What is the function of a multi unit smooth muscle?
    6 years ago
  • kristian
    Which muscle cell is not stimulates by the Autonomic nervous system?
    6 years ago
  • liya
    Where is singleunit smooth muscle and multiunit smooth muscle found?
    6 years ago
  • paladin
    Where are multiunit smooth muscles found?
    6 years ago
  • monica
    How many gap junctions are between the fibers of multiunit smooth muscle?
    6 years ago
  • viljo
    Are gap junctions and pacemaker cells multi unit?
    6 years ago
  • iolo puddifoot
    Are skeletal muscle fibers joined by gap junctions?
    6 years ago
  • sara
    Does a multiunit smooth muscle display rhythmicity?
    5 years ago
  • Leigh
    Are multiunit smooth muscle capable of depolarization?
    5 years ago
  • jill
    Are multiunit smooth muscle cells are stimulated in pairs?
    5 years ago
  • tim
    What means multiunit vs singleunit smooth muscle cell?
    5 years ago
  • matthew
    Does smooth muscle display ryhmnicity?
    3 years ago
  • petra
    Is there en passant nerve ending for multiunit smooth muscle?
    3 years ago
  • primula
    Why do multi unit smooth muscles not display action potentials?
    2 years ago
  • Berylla Brockhouse
    Do multi unit smooth muscles exhibit myogenicity?
    1 year ago
  • edward
    What is difference between single unit and multi unit cels?
    1 year ago
  • procopio giordano
    Is single unit smooth muscle bidirectional electrical synapse?
    1 year ago
  • MARTHA
    Do cardiac muscle fibers function as a single uniy?
    1 year ago
  • ANJA EHRLICHMANN
    Why no action potential in multiunit smooth muscle?
    1 year ago
  • virginia
    What are the mechanism action of single unit vs multi unit?
    1 year ago
  • VALENTINO
    How do a single unit muscle differ from multiunit muscle innervation and contraction?
    1 year ago
  • piia-noora
    What is the single unit of heart.?
    12 months ago
  • angelo
    Why is good to have digestive smooth muscle single unit and internal eye muscles multi unit?
    10 months ago
  • dwight creech
    Why is good to have the digestive smooth muscle single unit and the internal eye muscles multiunit?
    10 months ago
  • sandy
    Is multi unit smooth muscle contain myogenic nature?
    10 months ago
  • Mathilda
    Is there a difference between visceral and multi unit smooth muscles?
    9 months ago
  • fethawi
    What neurotransmitter goes to multi unit smooth muscle?
    8 months ago
  • Marcel
    Do multiunit smooth muscle display action potential when stimulated?
    8 months ago
  • yohannes
    Where can find multiunit smooth muscle?
    7 months ago
  • erling brownlock
    What stimulates unitary smooth muscles?
    7 months ago
  • sebastian
    What example of multi unit smooth muscle found in the body?
    7 months ago
  • robert
    Where is visceral or single unit smooth muscle located?
    7 months ago
  • Dina
    What are the types of smooth muscle?
    7 months ago
  • andrea
    Are multiunit smooth muscles foind ike?
    6 months ago
  • cynthia
    What is single unit smmoth muscle in physiology?
    5 months ago
  • Lisa
    Does smooth muscle have pacemaker cells?
    4 months ago
  • Marco
    Is there smooth muscle in the heart?
    3 months ago
  • Shelby Murphy
    Why iris of contains multi unite smooth muscle?
    3 months ago
  • Ilmari
    Why is multi unit smooth muscle present in ciliary muscles?
    2 months ago
  • sesto
    How do singleunit and multiunit smooth muscle differ in innervation and contractile behavior?
    6 days ago

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