10-motor system (muscle)

8/3/2019 10-Motor System (Muscle)

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Muscle (Motor)

Lecturer: Dr. R. Ahangari

University of Central Florida, OrlandoHuman Physiology by Linda S.Constanzo



Functions of muscle tissue:

1- Movement

Skeletal muscle: attached to skeleton, moves body by moving the bones

Maintenance of posture: enables the body to remain sitting or standing

Smooth muscle: squeezes fluids and other substances through hollow organs2- Joint stabilization

3- Heat generation: contractions produce heat, keeps normal body temperature

Muscle tissue



Types of muscles:

Three types

1- Skeletal muscle tissue: packaged into skeletal muscles

Makes up 40% of body weight. Cells are striated

2- Cardiac muscle tissue: occurs only in the walls of the heart

3- Smooth muscle tissue: in the walls of hollow organs. Cells lack striations



Skeletal muscle:Has a nerve and a blood supply.

neuromuscular junction: Where nerve contacts the muscle

Origin on bone is at less movable attachment

Insertion is on more movable attachmentOrigin and insertions are by tendon or aponeurosis



Motor unitMotor unit recruitment is the

progressive activation of a muscle

by successive recruitment of contractile units (motor units) to

accomplish increasing gradations

of contractile strength.

A motor unit consists of one motor

neuron and all of the muscle fibersit contracts.



Characteristics of the skeletal muscle:

The skeletal muscle cells are called: fibers

Fibers are long (10-100µm in diameter and several cm long) and cylindrical

Cells are multinucleate, nuclei are peripherally located

Plasma membrane is called a sarcolemma

Cytoplasm is called sarcoplasm

Contain Myofibrils: myofibrils are long rods within cytoplasm, responsible

for striations of the skeletal muscle

A myofibril is a long row of repeating segments called sarcomeres.



Sarcomere:

Is the basic unit of contraction of skeletal muscle from one Z line to the next.

Z disc (Z line): boundaries of each sarcomereThin (actin) filaments: extend from Z disc toward the center of the sarcomere

Thick (myosin) filaments: located in the center of the sarcomere

Overlap inner ends of the thin filaments contain ATPase enzymes

A bands: full length of the thick filament, includes inner end of thin filaments

H zone: center part of A band where no thin filaments occur M line: in center of H zone, contains tiny rods that hold thick filaments together

I band: region with only thin filaments, lies within two adjacent sarcomeres



Muscle contraction:

when a nerve cell stimulates a muscle fiber, it sets up an impulse in the

Sarcolemma that signals the Sarcoplasmic reticulum to release Calcium ions.

Released Ca++ diffuses through cytoplasm and triggers the sliding filamentmechanism.

Impulses further conducted by t tubules (deep invaginations of the sarcolemma)



Mechanism of contraction:

Sliding filament theory:

Myosin heads attach to actin

in the thin filaments

Then pivot to pull thin filaments

inward toward the center of the

sarcomere

Contraction mechanism is activatedby binding of Ca++ to the thin

filaments and powered by ATP.



Thin Filament: Actin *

*



Thin Filament: Tropomyosin *

*

*



Thin Filament: Troponin *

A complex of 3 subunits: TnI, TnC, TnT

*

Troponin has 3 subunits that together form a complex:

Tn C (troponin C) binds to calcium, initiates muscle contraction

Tn T (troponin T), binds troponin complex to tropomyosin

Tn I (troponin inhibitor), inhibits actin binding to myosin heads in resting muscle



The Sliding f ilament hypothesis is a proposal to explain how a

skeletal muscle can contr act.

Muscle Contraction:

Innervation of muscle by neurotransmitter ACETYLCHOLINE

Calcium released from sarcoplasmic reticulum (SR) into the cytoplasm

Calcium binds to Troponin ± C

This releases Troponin ± I from the actin binding site; the actin binding site is exposed

Actin can now bind to myosin

When heads of myosin molecules bind to actin filaments, myosin ATPase is activated

ATP (energy form) is released and induces flexion of the myosin heads

Flexion of myosin heads slides actin filaments towards the A band

Myosin heads detach, reattach to next neighboring actin filament

T

he thin filaments slide in between the thick filaments (muscle contraction).Mechanism continues until calcium is taken back into the SR



Changes in striation during the contraction of skeletal muscle:Electron micrographs

1- sarcomere that is relaxed

2- sarcomere that is partly contracted

3-sarcomere that is fully contracted



Muscle sensors1. Types of muscle sensors

A. Muscle spindles (groups Ia and II afferents) are arranged in parallel with

extrafusal fibers.

B. Golgi tendon organ (group Ib afferents) are arranged in series with extrafusal

muscle fibers. The detect muscle tension.

C. Pacinian corpuscles (group II afferents) are distributed throughout muscle. They

detect vibration.

D. Free nerve endings (group III and IV afferents) detect noxious stimuli.

Type of muscle fibersa. Extrafusal fibers

-make up the bulk of muscle.

-are innervated by alpha-motoneurons.

-provide the force for muscle contraction.

b. Intrafusal fibers

-are smaller than extrafusal muscle fibers.

-are innervated by gamma-motoneurons.

-are encapsulated in sheaths to form muscle spindles.

-run in parallel with extrafusal fibers. But not for entire length of the muscle.



How the muscle spindle works

-Muscle spindle reflexes oppose (correct for) in

creases in muscle length (stretch).

Sensory information about muscle length isreceived by group Ia (velocity) and group II (static)

afferent fibers.

When a muscle is stretched, the muscle spindle is

also stretched, stimulating group Ia and group II

afferent fibers.

Stimulation of groupIa afferents stimulates alpha-motoneurons in the spinal cord. This stimulation in

turn causes contraction and shortening of the

muscle. Thus, the original stretch is opposed

muscle length is maintained.

Function of gamma-motoneurons

-innervate intrafusal muscle fibers.-adjust the sensitivity of the muscle spindle so that it

will respond appropriately during muscle

contraction.

-alpha-motoneurons and gamma-motoneurons are

coactivated so that muscle spindles remain

sensitive to changes in muscle length duringcontraction



Smooth muscle:Cells are spindle-shaped, cells are non-striated and contain no sarcomere.

Separated by endomysium.

Contain one centrally located nucleus.

Grouped into sheets in walls of hollow organs.e.g.: Walls of circulatory vessels, Respiratory tubes, Digestive tubes

Urinary organs, Reproductive organs, Inside the eye etc.

Are in longitudinal layer or circular layer, both layers participate in contraction .

Contraction: Extracellular Ca++ diffusing into the smooth muscle cell is responsible for

sustained contractions. Opening of ca++ channels is graded by the amount of depolarization, the greater the depolarization, the more Ca++ will enter the cell and

the stronger will be the smooth muscle contraction. Ca++ combines with a protein ,

calmodulin, the calmodulin-Ca++ combines with and activates myosin light-chain

kinase, an enzyme that catalyzes the phosphorylation of myosin light chains, then it

binds to actin and thereby produce a contraction.

Relaxation of the smooth muscle follows the closing of the Ca++ channels.



Disorders of muscles:

Muscular dystrophy:

A group of inherited muscle degenerating

disease appearing in childhood.

The affected muscles enlarge with fat

and connective tissue but the muscle

fibers actually degenerate

Duchenne muscular dystrophy

Sex-linked recessive inherited disease,

males are most exclusively affected,

1/3500 boys, diagnosed between age 2-

10, muscle weakens, first pelvic

muscles affected, then muscles of shoulder and head, rarely live over 20

years.

Fibers lack a submembrane protein

called dystrophin.



Myotonic dystrophy

Myotonic dystrophy is an inherited disorder in which the muscles contract but havedecreasing power to relax. With this condition, the muscles also become weak and

waste away. Myotonic dystrophy can cause mental deficiency, hair loss and

cataracts.

Onset of this rare disorder commonly occurs during young adulthood. However, it

can occur at any age and is extremely variable in degree of severity.

The myotonic dystrophy gene, found on chromosome 19, codes for a protein kinase

that is found in skeletal muscle, where it likely plays a regulatory role.

10-motor system (muscle)

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