Transcript Chapter 8 Histology and Physiology of Muscles Skeletal Muscle Fibers

Chapter 8
Histology and
Physiology of
Muscles
Skeletal Muscle Fibers
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Functions of the Muscular System
1.
2.
3.
4.
5.
6.
Body movement (Skeletal)
Maintenance of posture (Skeletal)
Respiration (Skeletal)
Production of body heat (Skeletal)
Communication (Skeletal)
Constriction of organs and vessels
(Smooth)
7. Heartbeat (Cardiac)
Functional Characteristics of Muscle
1. Contractility:
ability to shorten forcibly
2. Excitability:
ability to receive and respond to stimuli
3. Extensibility:
ability to be stretched or extended
4. Elasticity:
ability to recoil and resume original resting
length
3 Types of Muscle Tissue
• skeletal, smooth, cardiac
• These types differ in
– Structure
– Location
– Function
– Means of activation
• Each muscle is a discrete organ
composed of muscle tissue, blood vessels,
nerve fibers, and connective tissue
Types of Muscle Tissue
• Skeletal muscles
– responsible for most body movements
– Maintain posture, stabilize joints, and generate heat
• Smooth muscle
– found in walls of hollow organs and tubes; moves
substances through them
– Helps maintain blood pressure
– Squeezes or propels substances (i.e., food, feces) through
organs
• Cardiac muscle
– found in heart; pumps blood throughout body
Tab. 8.1
Skeletal Muscle Structure
• Cells elongated, often called skeletal muscle
fibers
• Each cell contains several nuclei located around
periphery of fiber near plasma membrane
• Fibers appear striated due to actin and myosin
myofilaments
• A single fiber can extend from one end of a
muscle to the other!!!
• Contracts rapidly but tires easily
• controlled voluntarily (i.e., by conscious control)
Skeletal Muscle Structure
• Fascia: general term for connective tissue sheets
• 3 muscular fascia separate and compartmentalize
individual muscles or groups of muscles
– Epimysium: overcoat of dense collagenous
connective tissue that surrounds entire muscle
– Perimysium: fibrous connective tissue that surrounds
groups of muscle fibers called fascicles (bundles)
– Endomysium: fine sheath of connective tissue
composed of reticular fibers surrounding each muscle
fiber
Skeletal Muscle Structure
• connective tissue
provides a pathway for
blood vessels and
nerves to reach muscle
fibers
Fig. 8.1
Skeletal Muscle Structure
• Muscle connective
tissue blends with
other connective
tissue based
structures, such as
tendons
– connect muscle to
bone
Fig. 8.2
Skeletal Muscle Structure
• Muscle Fibers
– Sarcolemma: muscle cell plasma membrane
– Sarcoplasm: cytoplasm of a muscle cell
– Myo, mys, and sarco: prefixes used to refer
to muscle
• Muscle contraction depends on 2 kinds of
myofilaments: actin and myosin
• Myofibrils - densely packed, rod-like contractile
elements
• make up most of muscle volume
Fig. 8.2
Skeletal Muscle Structure:
ACTIN
• (thin) myofilaments
• 2 helical polymer strands of F actin (composed of G
actin), tropomyosin, and troponin
• G actin contains active sites to which myosin heads
attach during contraction
• Tropomyosin and troponin are regulatory subunits
bound to actin
Fig. 8.2
Skeletal Muscle Structure:
MYOSIN
• (thick) myofilaments
• consist of myosin molecules
• Each myosin molecule has
– A head with an ATPase, which breaks down ATP
– A hinge region, which enables head to move
– A rod
• A cross-bridge is formed when a myosin head binds
to the active site on G actin
Fig. 8.2
Skeletal Muscle Structure
• Sarcomeres
–
–
–
–
smallest contractile unit of a muscle
bound by Z disks that hold actin myofilaments
6 actin myofilaments surround a myosin myofilament
Myofibrils appear striated because of A bands and I
bands
Fig. 8.2
Skeletal Muscle Structure
• Thick filaments: extend the entire length of an A band
• Thin filaments: extend across the I band and partway
into A band
• Z-disc: coin-shaped sheet of proteins (connectins) that
anchors thin filaments and connects myofibrils to one
another
• Thin filaments do not overlap thick filaments in lighter H
zone
• M lines appear darker due to presence of protein desmin
• arrangement of myofibrils within a fiber is so organized a
perfectly aligned repeating series of dark A bands and
light I bands is evident
Fig.
8.3bc
Sliding Filament Model Summary
• Actin and myosin myofilaments do not change in length
during contraction
• Thin filaments slide past thick ones so actin and myosin
filaments overlap to a greater degree
– Upon stimulation, myosin heads bind to actin and sliding begins
– Each myosin head binds and detaches several times during
contraction (acting like a ratchet to generate tension and propel
the thin filaments to the center of sarcomere)
• In relaxed state, thin and thick filaments overlap only
slightly
• As this event occurs throughout sarcomeres, muscle
shortens
• I band and H zones become narrower during contraction,
and A band remains constant in length
Fig. 8.4
Sliding Filament Model
• Actin and myosin myofilaments in a relaxed
muscle (below) and a contracted muscle are the
same length.
• Myofilaments do not change length during
muscle contraction!
Fig. 8.4
Sliding Filament Model
• During contraction, actin myofilaments at each
end of the sarcomere slide past the myosin
myofilaments toward each other. As a result,
the Z disks are brought closer together, and the
sarcomere shortens
Fig. 8.4
Sliding Filament Model
• As the actin myofilaments slide over the myosin
myofilaments, H zones (yellow) and I bands
(blue) narrow.
• A bands, equal to length of myosin
myofilaments, do not narrow because length of
myosin myofilaments does not change
Fig. 8.4
Sliding Filament Model
• In a fully contracted muscle, ends of
actin myofilaments overlap at center of the
sarcomere and H zone disappears
Fig. 8.4
Physiology of Skeletal Muscle Fibers
• Membrane Potentials!!!
– nervous system stimulates muscles to contract
through electric signals called action potentials
– Plasma membranes are polarized, which means there
is a charge difference (resting membrane potential)
across plasma membrane
– inside of plasma membrane is negative as compared
to the outside in a resting cell
– action potential: reversal of resting membrane
potential so that inside of plasma membrane
becomes positive
Physiology
• Ion Channels
– Assist with
production of
action
potentials
• Ligand-gated
channels
• Voltage-gated
channels
Fig. 8.5
Fig. 8.6
Physiology of Skeletal Muscle Fibers
• Action Potentials
– Depolarization results from an increase in the
permeability of the plasma membrane to Na+
– If depolarization reaches threshold, an action
potential is produced
– The depolarization phase of the action potential
results from the opening of many Na+ channels
Fig. 8.6
Physiology of Skeletal Muscle Fibers
• Action Potentials
– The repolarization phase of the action
potential occurs when the Na+ channels close
and K+ channels open briefly
Fig. 8.6
Fig. 8.6
Physiology of Skeletal Muscle Fibers
• Action Potentials
– Occur in an all-or-none fashion
• stimulus below threshold produces no action
potential
• stimulus at threshold or stronger will produce an
action potential
– Propagate (travel) across plasma membranes
Physiology of Skeletal Muscle Fibers
• Nerve Stimulus of Skeletal Muscle
– Skeletal muscles are stimulated by motor
neurons of the somatic nervous system
– Axons of these neurons travel in nerves to
muscle cells
– Axons of motor neurons branch profusely as
they enter muscles
– Each axonal branch forms a neuromuscular
junction with a single muscle fiber
Physiology of Skeletal Muscle Fibers
• neuromuscular junction is formed from:
– Axonal endings
• small membranous sacs (synaptic vesicles)
• Contain neurotransmitter acetylcholine (ACh)
– Motor end plate of a muscle
• Specific part of the sarcolemma
• Contains ACh receptors
• Though exceedingly close, axonal ends and
muscle fibers are always separated by a space
called the synaptic cleft
Fig. 8.7
Neuromuscular Junction Physiology
1. action potential (orange
arrow) arrives at
presynaptic terminal and
causes voltage-gated Ca2+
channels in the
presynaptic membrane to
open
2. Calcium ions enter the
presynaptic terminal and
initiate the release of the
neurotransmitter
acetylcholine (ACh) from
synaptic vesicles
3. ACh is released into the
synaptic cleft by exocytosis
Fig. 8.8
Neuromuscular Junction Physiology
4. ACh diffuses across the
synaptic cleft and binds to
ligand-gated Na+ channels on
the postsynaptic membrane
5. Ligand-gated Na+ channels
open and Na+ enters the
postsynaptic cell, causing the
postsynaptic membrane to
depolarize. If depolarization
passes threshold, an action
potential is generated along
the postsynaptic membrane
6. ACh is removed from the
ligand-gated Na+ channels,
which then close
Fig. 8.8
Neuromuscular Junction Physiology
7. enzyme acetylcholinesterase, which
is attached to postsynaptic
membrane, removes acetylcholine
from synaptic cleft by breaking it
down into acetic acid and choline
8. Choline is symported with Na+ into
presynaptic terminal, where it can be
recycled to make ACh. Acetic acid
diffuses away from synaptic cleft
9. ACh is reformed within presynaptic
terminal using acetic acid generated
from metabolism and choline
recycled from synaptic cleft. ACh is
then taken up by synaptic vesicles
Fig. 8.8
Fig. 8.8
Excitation-Contraction Coupling
• In order to contract, a skeletal muscle
must:
– Be stimulated by a nerve ending
– Propagate an electrical current, or action
potential, along its sarcolemma
– Have a rise in intracellular Ca2+ levels, the
final trigger for contraction
• Linking the electrical signal to the
contraction is excitation-contraction
coupling
Excitation-Contraction Coupling
• Invaginations of sarcolemma form T tubules, which wrap
around sarcomeres and penetrate into cell’s interior at
each A band –I band junction
• Sarcoplasmic reticulum (SR) is
an elaborate, smooth
endoplasmic reticulum that
mostly runs longitudinal and
surrounds each myofibril
– Paired terminal cisternae form
perpendicular cross channels
– Functions in regulation of
intracellular calcium levels
• A triad is a T tubule and two
terminal cisternae
Fig. 8.9
Excitation-Contraction Coupling
1. An action potential produced at
presynaptic terminal in the
neuromuscular junction is propagated
along sarcolemma of the skeletal
muscle. depolarization also spreads
along membrane of T tubules
2. depolarization of T tubule causes
gated Ca2+ channels in SR to open,
resulting in an increase in
permeability of SR to Ca2+, especially
in terminal cisternae. Calcium ions
then diffuse from SR into sarcoplasm
3. Calcium ions released from SR bind
to troponin molecules. troponin
molecules bound to G actin
molecules are released, causing
tropomyosin to move, exposing
active sites on G actin
4. Once active sites on G actin
molecules are exposed, heads of
myosin myofilaments bind to them to
form cross-bridges
Fig. 8.11
Fig. 8.11
Fig. 8.11
Fig. 8.11
Fig. 8.11
Fig. 8.11