Transcript Muscular System: Histology and Physiology

Chapter 9
Muscular System:
Histology and Physiology
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Functions of the Muscular System
• Body movement (skeletal muscles attached to bones)
• Maintenance of posture
• Respiration (skeletal muscles of thorax are responsible
for the movement necessary for respiration)
• Production of body heat (when skeletal muscles
contact, heat is given off as a by-product)
• Communication (speaking, writing)
• Constriction of organs and vessels (contraction
of smooth muscle)
• Heart beat (contraction of cardiac muscle)
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General Functional
Characteristics of Muscle
• Contractility: ability of a muscle to shorten
with force
• Excitability: capacity of muscle to respond
to a stimulus (by nerve or hormone)
• Extensibility: muscle can be stretched to its
normal resting length and beyond to a
limited degree
• Elasticity: ability of muscle to recoil to
original resting length after stretched
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Types of Muscle Tissue
• Skeletal
– Responsible for locomotion, facial expressions, posture, respiratory
movements, other types of body movement
– Voluntary
• Smooth
– Walls of hollow organs, blood vessels, eye, glands, skin
– Some functions: propel urine, mix food in digestive tract,
dilating/constricting pupils, regulating blood flow
– In some locations, autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
• Cardiac
– Heart: major source of movement of blood
– Autorhythmic
– Controlled involuntarily by endocrine and autonomic nervous systems
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Skeletal Muscle Structure
• Composed of muscle cells
(fibers), connective tissue, blood
vessels, nerves
• Fibers are long, cylindrical,
multinucleated
• Tend to be smaller diameter in
small muscles and larger in large
muscles. 1 mm- 4 cm in length
• Develop from myoblasts (they are
converted to muscle fibers as contractile
proteins accumulate within their cytoplasm);
numbers remain constant (# of
muscle fibers remain constant after birth---so, enlargement of muscles is an increase in
size rather than #)
• Striated appearance due to light
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and dark banding
Connective Tissue Coverings of Muscle
• Layers
– External lamina. Delicate, reticular
fibers. Surrounds sarcolemma (P.M.)
– Endomysium. Loose C.T. with
reticular fibers.
– Perimysium. Denser C.T. surrounding
a group of muscle fibers. Each group
called a fasciculus
– Epimysium. C.T. that surrounds a
whole muscle (many fascicles)
• Fascia: connective tissue sheet
– Forms layer under the skin
– Holds muscles together and separates
them into functional groups.
– Allows free movements of muscles.
– Carries nerves (motor neurons, sensory
neurons), blood vessels, and
lymphatics.
– Continuous with connective tissue of
tendons and periosteum.
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Nerves and Blood Vessel
Supply
• Motor neurons: stimulate
muscle fibers to contract.
Nerve cells with cell
bodies in brain or spinal
cord; axons extend to
skeletal muscle fibers
through nerves
• Axons branch so that each
muscle fiber is innervated
• Capillary beds surround
muscle fibers
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Muscle Fibers
• Nuclei just inside sarcolemma
• Cell packed with myofibrils within cytoplasm
(sarcoplasm = cytoplasm without myofibrils)
– Threadlike (extends from one end of muscle fiber to the other)
– Composed of protein threads called myofilaments:
thin (actin 8nm) and thick (myosin 12nm)
– Sarcomeres: actin & myosin myofilaments form
highly ordered units called sarcomeres. They are
joined end to end to form the myofibrils.
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Parts of a Muscle
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Actin and Myosin Myofilaments
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Actin (Thin) Myofilaments
• Two strands of fibrous (F) actin form a
double helix extending the length of the
myofilament; attached at either end at
sarcomere.
– Composed of G actin monomers
each of which has an active site
– Actin site can bind myosin during
muscle contraction.
• Tropomyosin: an elongated protein
winds along the groove of the F actin
double helix.
•Troponin is composed of three subunits: one that binds to actin, a second that
binds to tropomyosin, and a third that binds to calcium ions. Spaced between
the ends of the tropomyosin molecules in the groove between the F actin
strands.
•The tropomyosin/troponin complex regulates the interaction between active
sites on G actin and myosin.
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Myosin (Thick) Myofilament
•
•
•
Many elongated myosin
molecules shaped like golf
clubs.
Molecule consists of two heavy
myosin molecules wound
together to form a rod portion
lying parallel to the myosin
myofilament and two heads
that extend laterally.
Myosin heads
1. Can bind to active sites on the actin molecules to form cross-bridges.
2. Attached to the rod portion by a hinge region that can bend and straighten
during contraction.
3. Have ATPase activity: activity that breaks down adenosine triphosphate
(ATP), releasing energy. Part of the energy is used to bend the hinge
region of the myosin molecule during contraction
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Sarcomeres: Z Disk to Z Disk
• Z disk: filamentous network of
protein. Serves as attachment
for actin myofilaments
• Striated appearance
– I bands: from Z disks to ends of
thick filaments
– A bands: length of thick
filaments
– H zone: region in A band where
actin and myosin do not overlap
– M line: middle of H zone;
delicate filaments holding myosin
in place
•In muscle fibers, A and I bands of parallel myofibrils are aligned.
•Titin filaments: elastic chains of amino acids; make muscles
extensible and elastic
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Sliding Filament Model
• Actin myofilaments sliding over
myosin to shorten sarcomeres
– Actin and myosin do not change
length
– Shortening sarcomeres
responsible for skeletal muscle
contraction
• During relaxation, sarcomeres
lengthen because of some external
force, like forces produced by other
muscles (contraction of antagonistic
muscles) or by gravity.
- agonist = muscle that accomplishes
a certain movement,
such as flexion.
- antagonist = muscle acting in
opposition to agonist.
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Sarcomere Shortening
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Physiology of Skeletal Muscle Fibers
• Nervous system controls muscle contractions through action potentials
• Resting membrane potentials
– Membrane voltage difference across membranes (polarized)
• Inside cell more negative due to accumulation of large protein molecules.
More K+ on inside than outside. K+ leaks out (through leak channels) but not
completely because negative molecules hold some back.
• Outside cell more positive and more Na+ on outside than inside.
• Na+ /K+ pump maintains this situation.
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– Must exist for action potential to occur
Ion Channels
• Types
– Ligand-gated. Ligands are molecules that bind to
receptors. Receptor: protein or glycoprotein with a
receptor site
• Example: neurotransmitters
• Gate is closed until neurotransmitter attaches to
receptor molecule. When Ach (acetylcholine) attaches
to receptor on muscle cell, Na gate opens. Na moves
into cell due to concentration gradient
– Voltage-gated
• Open and close in response to small voltage changes
across plasma membrane
• Each is specific for one type of ion
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Action Potentials
• Phases
– Depolarization: Inside of plasma membrane
becomes less negative. If change reaches
threshold, depolarization occurs
– Repolarization: return of resting membrane
potential. Note that during repolarization, the
membrane potential drops lower than its
original resting potential, then rebounds. This
is because Na plus K together are higher, but
then Na/K pump restores the resting potential
• All-or-none principle: like camera flash system
• Propagate: Spread from one location to another.
Action potential does not move along the
membrane: new action potential at each
successive location.
• Frequency: number of action potential produced
per unit of time
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Gated Ion Channels and
the Action Potential
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Action Potential Propagation
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Neuromuscular Junction
• Synapse: axon terminal
resting in an
invagination of the
sarcolemma
• Neuromuscular
junction (NMJ):
– Presynaptic terminal:
axon terminal with
synaptic vesicles
– Synaptic cleft: space
– Postsynaptic membrane
or motor end-plate
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Function of Neuromuscular Junction
• Synaptic vesicles
– Neurotransmitter: substance
released from a presynaptic
membrane that diffuses across
the synaptic cleft and
stimulates (or inhibits) the
production of an action
potential in the postsynaptic
membrane.
• Acetylcholine
– Acetylcholinesterase: A
degrading enzyme in synaptic
cleft. Prevents accumulation of
ACh
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Excitation-Contraction Coupling
• Mechanism by which an action
potential causes muscle fiber
contraction
• Involves
– Sarcolemma
– Transverse (T) tubules: invaginations
of sarcolemma
– Terminal cisternae
– Sarcoplasmic reticulum: smooth ER
– Triad: T tubule, two adjacent
terminal cisternae
– Ca2+
– Troponin
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Action Potentials and Muscle Contraction
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Cross-Bridge Movement
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