Muscular System

Locomotion

( movement of the parts or the whole body).

Types of muscles

• According to the histological structure

1- Striated muscles, contain striations.

2- Unstriated muscles, no striations (smooth).

• According to the position :

1- Skeletal muscles, present around the skeleton.

2- Cardiac muscles, present in the heart.

3- Visceral muscles, present in the viscera and urinary bladder.

• According to the ability of controlling the contraction:

1- Voluntary muscles, contract under our control.

2- Unvoluntary muscles, contract without control of us.

Skeletal muscle

• according to protein in myosin: two types:a) Slow unit

* red - dark- respond slowly- long twitch duration (up to 100 ms) - low mitochondrial ATP ase activity ,

* Produce ATP slowly

* In large limp muscle

* Ex. marathon runners (more red fibers)

b) Fast unit* Pale(white) - short twitch duration ( 7 ms) - * produce

ATP quickly – more powerful contractions – fine ,skilled movement

* Ex. Weighlifters

Skeletal (voluntary; striated) muscle

• Attach to skeleton• Function in movement (bone)• Contract voluntary

C.N.S. neuromuscular junction• Connect to the bone through the beginning

(origin) other end (insertion) • If the origin formed of 2 parts(Biceps) or 3

(Triceps)• Ends with tendons

Structure

Each muscle composed of many myofibrils(muscle fibers).

Cylindrical shape, multinucleated.

Each myofibril composed of many myofibrils .

Each myofibril composed of many repeated sarcomeres.

Myofibrile appear as alternated dark and light bands.

Light band is called Isotropic band (I),

Dark band is called Anisotropic band (A).

Each sarcomere composed of many myofilaments.

( thin ) actin , troponin , tropomysin

( thick ) myosin .

Molecular structure:Myosin:

• Thick helical protein filament.

• Consists of heads and tails.

• By proteiolytic enzyme(trypsin) broken into:

** light meromyosin (LMM) : H-zone

No ATPase activity and can’t combine with actin.

No heads at the H-zone.

** Heavy meromyosin (HMM) contain ATP ase activity , can combine with actin.

• Release the heads from the tails if treated with pepsin enzyme.

• The head of the myosin molecule forms cross bridge with the adjacent actin filament.

Actin:•Thin filaments actin with small amount of tropomyosin and

troponin.

• In ionic environment of the cell, actin exist in the fibrous form (F-actin) containing 2 chains of actin monomer coiled around each other.

• In the absence of salts, actin becomes globular (G-actin).

•The change from F- actin to G- actin is reversible and is called ( G F ) transformation;

• Troponin and tropomyosin

(Regulatory agent in muscle contraction)

At rest ,tropomyosin block the binding of actin to myosin.

• Troponin is formed of 3 subunits:

Troponin (I) inhibit the interaction of myosin to actin.

Troponin (C) which contain the binding sites of Ca++ that initiate the contraction of the myofilaments.

Troponin (T) binds the other troponin component to tropomyosin.

Classification according to the movement:

• 1- a) Flexor; bends one part upon another.

• b) Extensor; straightens or extends part.

• 2- a) Adductor; draws a part towards the axis

• b) Abductor; draws a part away of the axis

• 3- a) Depressor; lowers a part.

• b) Elevator; elevate or raise a part.

• 4- a) Constrictor; constricts or close an organ.

• b) Dilator; cause dilation of the same organ.

• 5- Rotator Muscle; rotates on part on another

CARDIAC MUSCLES:

They contract involuntary, and rythimitically. They obey all or non law.

They are auto stimulated (pace maker).

Striated muscle

Pump blood

Adapted for resistant to fatigue

**A lot mitochondria (aerobic respiration)

**Numerous myoglobins (O2 storing pigment)

**Good blood supply

Intercalated discs (support contration) double membrane separating

adjacent cells.

Cell junctions fuse together the plasma membranes of cardiac muscle cells, allow the cells to contract as a unit.

Smooth muscle

Unstriated

Walls of hollow organs

( gastrointestinal tube & respiratory system )

Two types

multi-unit: without interconnection,iris

single-unit :(walls of viscera)

Spindle shape –single nucleus

Arranged in sheets

Few mitochondria(depend on anaerobic glycolysis)

Contain myosin & actin (not arranged into sarcomeres)

Stimulated by

Autonomic nervous system ( involuntary)

Hormons ( vasoconstriction & vasodilators) adrenalin

EFFECT OF STIMULUS:

a- In skeletal Muscle:

• Threshold Stimulus:

no responds occur.

• Minimal Stimulus:

responds by minimal observed line.

• Submaximal Stimulus:

increasing the stimulus, the number of responding fibers increase

• Maximum Stimulus:

all muscle fibers contract.

• Supra-maximum Stimulus:

increasing the stimulus, no more effect.

B- In cardiac Muscle:

• Sub-minimal stimulus;

no responds occur.

• Threshold value;

all muscle fibers respond,increasing

stimulus no more effect will resulted.

Steps of Contraction: • Release of transmitter at motor end

plate.

• Generation of action potential.

• Inward spread of depolarization along T-tubules.

• Release of Ca++ ions.

• Binding of Ca++ ions to troponin C.

• Uncovering myosin binding sites on actin.

• Formation of cross-Linkage between actin and myosin

• Sliding of actin over myosin, producing shortening.

Steps Of Relaxation:

•Ca++ ions pumped back into sarcoplasmic reticulum.

•Release of Ca++ ions from troponin.

•Cessation of interaction between actin and myosin.

Changes accompanying muscle contraction:

Thermal Change:

Heat of contraction and relaxation.

PH Changes:

Accompanied the chemical changes, acid by

splitting ATP or base by breakdown of CP.

Electrical change:

Direct or indirect depolarization through the

connected nerve.

Chemical change:

(Aerobic or anaerobic) oxidative respiration,

CP is an immediate source of ATP ( phosphrelate

ADP).

Mechanical changes:

Sliding of actin filament over the myosin filaments,

( cross bridge).