Download THE ORGANIZATION OF SKELETAL MUSCLES

MECHANISM OF SKELETAL MUSCLE CONTRACTION
DIFFERENTIATE BASIS OF SMOOTH MUSCLE
CONTRACTION
LEARNING OBJECTIVES
• Types of muscle tissue
• Functional characteristics
• Structural organization of skeletal muscle
• Ultrastructure of Sarcomere
• Mechanism of muscle contraction
• Excitation/contraction sequence
• Sliding filament theory
• Relaxation
CLASSIFICATION OF THE MUSCLE
 According to the structure: Striated Muscle, Smooth
Muscle
 According to the nerve innervation: Voluntary Muscle,
Involuntary Muscle
 According to the Function: Skeletal Muscle, Cardiac
Contraction, Smooth Muscle
Skeletal Muscle
Cardiac Muscle
Smooth Muscle
SKELETAL MUSCLE INNERVATION
ILLUSTRATION OF THE
NEUROMUSCULAR JUNCTION (NMJ)
NERVE TERMINAL CA2+ CHANNELS
 Structurally similar to Na+ channels
 Functionally similar to Na+ channels except
 activation occurs at more positive potentials
 activation and inactivation much slower than Na+
channels
NEUROMUSCULAR TRANSMISSION
Axon
Axon Terminal
Skeletal Muscle
Nerve action
potential invades + axon terminal + -
-
-
Neuromuscular
Transmission:
+
Step by Step
-
+
+
+
Look - +
here + -
-+
+
-++
-
Ca2+ induces fusion of
vesicles with nerve
terminal membrane.
ACh ACh
Ca2+
ACh
Ca2+
Na+
Na+
Na+
K+
Na+
Na+
K+
Na+
K+
ACh
Na+
K+
Na+
Na+
K+
Outside
Muscle membrane
Na+
Na+
K+
K+
Na+
K+
Inside
K+
Na+
K+
K+
K+
K+
Na+
END PLATE POTENTIAL (EPP)
Presynaptic
terminal
VNa
Muscle Membrane
Voltage (mV)
The movement of Na+ and K+
depolarizes muscle membrane
potential (EPP)
0
EPP
Threshold
-90 mV
VK
Presynaptic
AP
Time (msec)
Outside
Muscle membrane
Inside
ACh Receptor Channels
Voltage-gated
Na Channels
Inward Rectifier
K Channels
STRUCTURAL REALITY
SKELETAL MUSCLE
 Human body contains over 400 skeletal muscles
 40-50% of total body weight
 Functions of skeletal muscle
 Force production for locomotion and breathing
 Force production for postural support
 Heat production during cold stress
 Fascicles: bundles, CT(connective tissue) covering on
each one
 Muscle fibers: muscle cells
STRUCTURAL ORGANIZATION OF SKELETAL
MUSCLE
•
•
•
•
•
Organ
Fascicles
Cell
Myofibrils
Myofilaments
THE ORGANIZATION OF SKELETAL MUSCLES
Figure 10.1
THE ORGANIZATION OF SKELETAL MUSCLES
• Tissues:
– Blood vessels
– Nerves – branches to each fiber
– Connective Tissue (Fig 9.2, Table 9.1)
• Endomysium –wraps each fiber
• Perimysium –wraps fibers into fascicles
• Epimysium –wraps fascicles into a muscle
• All are continuous with each other and the
tendons.
STRUCTURE OF SKELETAL MUSCLE:
MICROSTRUCTURE
 Sarcolemma
 Transverse (T) tubule
 Longitudinal tubule (Sarcoplasmic reticulum, SR)
 Myofibrils
 Actin(thin filament)
 Troponin
 Tropomyosin
 Myosin(thick filament)
THE ORGANIZATION OF SKELETAL MUSCLES
• Cell membrane = sarcolemma
• Cell interior gel = sarcoplasm with myoglobin
• Organelles
– Mitochondria, multiple nuclei, etc. squeezed
between myofibrils.
– Myofibrils aligned in such a way as to produce
alternating light (I) and dark (A) bands or
striations.
4. Sarcomere
• arrangement of myofibrils
a. Z disk – attaches actin
b. I band – actin myofilament
c. A band – both actin and myosin
H zone – only myosin
5. T Tubules
• invagination of sarcolemma
6. Sarcoplasmic Reticulum
• high conc. of calcium
WITHIN THE SARCOPLASM
Triad
 Transverse tubules
 Sarcoplasmic reticulum -Storage sites for calcium
 Terminal cisternae - Storage sites for calcium
MICROSTRUCTURE OF
SKELETAL MUSCLE (MYOFIBRIL)
SARCOMERE
 Sarcomere : bundle of alternating thick and
thin filaments
 Sarcomeres join end to end to form myofibrils
 Thousands per fiber, depending on length of
muscle
 Alternating thick and thin filaments create
appearance of striations
THE ORGANIZATION OF SKELETAL MUSCLES
• Myofibrils – hundreds to thousands per cell contain the
contractile proteins = myofilaments
– Actin – thin filaments
– Myosin – thick filaments
THE ORGANIZATION OF SKELETAL
MUSCLES
• Bands
– A bands = actin &
myosin overlap
– I bands = actin only
– H zone in A band –
myosin only
– Z disc – attachment of
actin and myosin;
distance between Z
discs = sarcomere
THE ORGANIZATION OF SKELETAL
MUSCLES
• Closer look at a
sarcomere
Myosin head is hinged
Bends and straightens during contraction
THICK FILAMENTS (MYOSIN)
 Bundle of myosin proteins shaped like double-headed golf
clubs
 Myosin heads have two binding sites
 Actin binding site forms cross bridge
 Nucleotide binding site binds ATP (Myosin ATPase)
 Hydrolysis of ATP provides energy to generate power
stroke
THIN FILAMENTS (ACTIN)
Backbone: two strands of polymerized globular
actin – fibrous actin
 Each actin has myosin binding site
Troponin
Binds Ca2+; regulates muscle contraction
Tropomyosin
Lies in groove of actin helix
Blocks myosin binding
 sites in absence of Ca2+
 Thick filament: Myosin (head and tail)
 Thin filament: Actin, Tropomyosin, Troponin
(calcium binding site)
THIN FILAMENTS (ACTIN)
Backbone: two strands of polymerized globular
actin – fibrous actin
 Each actin has myosin binding site
Troponin
Binds Ca2+; regulates muscle contraction
Tropomyosin
Lies in groove of actin helix
Blocks myosin binding
 sites in absence of Ca2+
SARCOMERE
• I bands
• A bands
• H zone
• Z lines
• M line
THE ORGANIZATION OF SKELETAL MUSCLES
• Ultrastructure of Sarcomere,
– Myosin – 2 globular heads whose tails are
intertwined. Heads are the “business” end, i.e. form
cross bridges with actin.
– Actin – globular proteins arranged like 2 strands of
beads twisted together in a helix.
– Tropomyosin – protein filaments give strength and
cover active sites on actin.
– Troponin – controls position of tropomyosin.
THE ORGANIZATION OF SKELETAL MUSCLES
• Sarcoplasmic reticulum – smooth ER, regulates
intracellular calcium; forms paired terminal cisternae at
A-I junctions.
• T-tubules – invaginations of sarcolemma that reach each
A and I band junction, traveling between paired terminal
cisternae = triad.
– Communicate with external environment, carry
electrical impulses into muscle mass.
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