Kontraksi dan Eksitasi Otot Polos

Musculosceletal System for Nursing

Yulius TirandaPSIK STIKes Muhammadiyah Palembang

Karakteristik khusus OtotOtot mempunyai 4 fungsi utama yaitu, kontraktilitas, eksitabilitas, ekstensibilitas dan elastisitas.1. Contractility (kontraktilitas)  adalah kemampuan otot untuk memendek dengan kekuatan tertentu. Ketika otot berkontraksi, hal tersebut menyebabkan pergerakan struktur internal otot (filamen otot) dan akan menngakibatkan tekanan pada organ dan pembuluh darah.2. Excitability (eksitabilitas) adalah kemampuan otot untuk merespon stimulus, dimana umumnya otot, khususnya otot rangka berkontraksi sebagai akibat stimulasi oleh saraf. Otot polos dan jantung dapat berkontraksi tanpa stimulus luar, tetapi keduanya juga berkontraksi akibat stimulus saraf dan hormon.3. Extensibility (ekstensibilitas) adalah dapat meregang pada panjang tertentu dengan derajat tertentu.4. Elasticity (elastisitas) adalah kemampuan otot untuk kembali ke kondisi semula setelah melakukan proses meregang.

SMOOTH MUSCLESmooth muscle tissue is nonstriated and involuntary and is classified into two types: visceral (single unit) smooth muscle (Figure 10.18a) and multiunit smooth muscle (Figure 10.18b).

▫Visceral (single unit) smooth muscle is found in the walls of hollow viscera and small blood vessels; the fibers are arranged in a network and function as a “single unit.”

▫Multiunit smooth muscle is found in large blood vessels, large airways, arrector pili muscles, and the iris of the eye. The fibers operate singly rather than as a unit.

Two Types of Smooth Muscle• Visceral (single-unit)

▫ in the walls of hollow viscera & small BV

▫ autorhythmic▫ gap junctions cause fibers

to contract in unison• Multiunit

▫ individual fibers with own motor neuron ending

▫ found in large arteries, large airways, arrector pili muscles,iris & ciliary body

Single-Unit Muscle

Multi-Unit Muscle

Multi vs. Single-Unit Muscle

Smooth Muscle•Fusiform cells•One nucleus per

cell•Nonstriated•Involuntary•Slow, wave-like

contractions

Smooth Muscle

• Cells are not striated• Fibers smaller than those in

skeletal muscle• Spindle-shaped; single, central

nucleus• More actin than myosin• No sarcomeres

▫ Not arranged as symmetrically as in skeletal muscle, thus NO striations.

• Caveolae: indentations in sarcolemma;▫ May act like T tubules

• Dense bodies instead of Z disks ▫ Have noncontractile intermediate

filaments

Smooth Muscle• Grouped into sheets in walls of hollow organs

Longitudinal layer – muscle fibers run parallel to organ’s long axis

Circular layer – muscle fibers run around circumference of the organ

Both layers participate in peristalsis

Smooth Muscle Cell

Microscopic Anatomy of the Smooth Muscle•Sarcoplasm of smooth muscle fibers

contains both thick and thin filaments which are not organized into sarcomeres.

•Smooth muscle fibers contain intermediate filaments which are attached to dense bodies. (Figure 10.19)

Microscopic Anatomy of Smooth Muscle

• Small, involuntary muscle cell -- tapering at ends

• Single, oval, centrally located nucleus• Lack T tubules & have little SR for Ca+2 storage

Microscopic Anatomy of Smooth Muscle

•Thick & thin myofilaments not orderly arranged so lacks sarcomeres

•Sliding of thick & thin filaments generates tension

•Transferred to intermediate filaments & dense bodies attached to sarcolemma

•Muscle fiber contracts and twists into a helix as it shortens -- relaxes by untwisting

Physiology of Smooth Muscle•Contraction starts slowly & lasts longer

▫no transverse tubules & very little SR▫Ca+2 must flows in from outside

•In smooth muscle, the regulator protein that binds calcium ions in the cytosol is calmodulin (in place of the role of troponin in striated muscle); ▫calmodulin activates the enzyme myosin

light chain kinase, which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate.

Smooth Muscle Tone•The prolonged presence of calcium ions

in the cytosol of smooth muscle fibers provides for smooth muscle tone, a state of continued partial contraction.

•Smooth muscle fibers can stretch considerably without developing tension; this phenomenon is termed the stress-relaxation response.

•Useful for maintaining blood pressure or a steady pressure on the contents of GI tract

DEVELOPMENT OF MUSCLE•With few exceptions, muscles develop

from mesoderm (Figure 6.13a)•Skeletal muscles of the head and

extremities develop from general mesoderm; the remainder of the skeletal muscles develop from mesoderm of somites (Figure 10.20a).

Fusion of Myoblasts into Muscle Fibers

• Mature muscle cells developed from 100 myoblasts that fuse together in the fetus. (multinucleated)

• Mature muscle cells are not known to divide.• Muscle growth is a result of cellular enlargement

(hypertrophy) & not cell division (hyperplasia)• Satellite cells retain the ability to regenerate new cells.

Developmental Anatomy of the Muscular System

•Develops from mesoderm•Somite formation

▫blocks of mesoderm give rise to vertebrae and skeletal muscles of the back

•Muscles of head & limbs develop from general mesoderm

Regeneration of Muscle•Skeletal muscle fibers cannot divide after 1st year

▫growth is enlargement of existing cells▫repair

satellite cells & bone marrow produce some new cells

if not enough numbers---fibrosis occurs most often

•Cardiac muscle fibers cannot divide or regenerate▫all healing is done by fibrosis (scar formation)

•Smooth muscle fibers (regeneration is possible)▫cells can grow in size (hypertrophy)▫some cells (uterus) can divide (hyperplasia)▫new fibers can form from stem cells in BV walls

Aging and Muscle Tissue•Skeletal muscle starts to be replaced by fat

beginning at 30 ▫“use it or lose it”

•Slowing of reflexes & decrease in maximal strength•Change in fiber type to slow oxidative fibers may be

due to lack of use or may be result of aging

Excitation-Contraction Coupling:(below)

Regulation of Contraction by Calcium Ions • For skeletal muscle, calcium ions initiate contraction • Smooth muscle has no troponine but does contain the thin

filament protein tropomyosin and other notable proteins - caldesmon and calponin.

• Combination of calcium ions with calmodulin in smooth muscle

• Calmodulin: a special protein that reacts with four calcium ions.

• Three steps for activation and contraction by calmoduline 1. Calcium ions bind with calmodulin 2. Calmodulin-calcium conbination then joins with and

activates myosin kinase, an enzyme 3. One of the light chains of each myosin head becomes

phosphrelated by myosin kinase. The head binds with the actin filament (less clacum, reverse phosphorylation due to myosin phosphatase, then cessation of contractoin)

•No neuromuscular junctions of the highly structured type in smooth muscle, unlike skeletal muscles

•Instead the autonomic nerve fibers innervate smooth muscle

Smooth Muscle Contraction: Mechanism

Mekanisme Kontraksi pada Otot Polos 1. Pada saat sebuah hormon berikatan pada reseptor di membran

maka akan mengaktifkan sebuah molekul G protein akibat terjadinya mekanisme depolarisasi membran plasma.

2. Akibat depolarisasi membran plasma akan membuka kanal Ca2+ di permukaan membran plasma dan memicu proses difusi Ca2+ melalui kanal Ca2+ yang kemudian akan berkombinasi dengan calmodulin.

3. Calmodulin dengan Ca2+ yang telah membentuk ikatan kemudian melekat pada miosin kinase dan mengaktivasi protein kinase ini.

4. Aktivasi miosin kinase menempelkan phosphat dari ATP pada kepala miosin untuk mengaktifkan proses kontraktil.

5. Kemudian terjadilah sebuah siklus cross-bridge formation, pergerakan, dan pelepasan ikatan protein kontraktil yang terlibat.

6. Relaksasi pada otot polos terjadi ketika miosin phosphatase memindahkan phosphat dari miosin.

Smooth Muscle Relaxation: Mechanism

Properties of Single-Unit Smooth Muscle▫Gap junctions▫Pacemaker cells

with spontaneous depolarizations

▫Innervation to few cells

▫Tone = level of contraction without stimulation

▫Increases/decreases in tension

▫Graded Contractions No recruitment Vary intracellular

calcium▫Stretch Reflex

Relaxation in response to sudden or prolonged stretch

Release and uptake of calcium by the sarcoplasmic reticulum during contraction and relaxation of a skeletal muscle fiber.

Comparisons Among Skeletal, Smooth, and Cardiac Muscle

Comparison of smooth muscle contraction with skeletal muscle contraction •Most skeletal muscles contact and relax rapidly •Smooth muscle contraction is prolonged,

lasting hours or even days. •Some of the differences 1. Slow cycling of the myosin cross-bridges Attachment to actin and release from actin is

much slower. As little as 1/10 to 1/300 the frequency

Yet the fraction of time that the cross-bridges remain attached to the actin is greater in smooth muscl

2. Energy required to sustain smooth muscle contraction Only 1/10 to 1/300 as much energy is required to

sustain the same tension of contraction in smooth muscle

Explains in tension in intestines, urinary bladder, gallbladder (tonic muscle contaction of these. Energy saving)

3. Slowness of onset of contraction and relaxation of smooth muscle Total contraction time of 1 to 3 seconds 30 times as long as a single contraction of a skeletal

muscle Slow attachement and detachement Initiation of contraction due to calcium is much

slower to

4. Force of smooth muscle contraction Max. force of contraction of smooth muscle is

greater than that of skeletal muscle, as great as 4 to 6 kg/cm2 in comparison to 3 to 4 kg for skeletal muscle

This great force results from the prolonged attachment of the myosin cross-bridges to actin

5. Latch mechanism After full contraction, actin-myosin continue to

be attached (latched) continuously generating tension without using ATP. (a very low affinity for ATP)

This mechanism is called the latch mechanism and saves the smooth muscle cell a great deal of ATP

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