|
Muscle and Movement - Chapter 19
I.
The musculoskeletal system:
A. Provides the basic form and shape of the
organism
B. Mechanical function of support
C. Means of protection for vulnerable organ
within body
D. Allows body movement; provides a set of
levers E. Heat production
II. The skeletal system - bone and cartilage.
A. Homeostatic function - 3 ways:
1. Maintain a constant state - involves
behavioral responses to environmental changes- produces locomotion
2. Erythrocytes and other formed elements of
the blood produced in the red marrow
3. Stores minerals - calcium and phosphorus,
sodium, magnesium, and manganese
B. Movement - muscles responsible for
movement of the skeleton are attached to he bones by stringy connective
tissue- tendons
1. Movement produced by a contracting muscle
involves a stationary base- origin, and a moving base- insertion
2.
All muscles produce their effects by contraction. Opposite movements at a joint are due to the existence of antagonistic muscle pairs,
which oppose one another
in their contraction e.g. biceps and triceps
3.
Characteristics of muscle fibers
a. Irritability - property of receiving and
responding to stimuli
b. Contractibility - ability to shorten and
thicken
c. Extensibility - ability to be stretched or
extended
d. Elasticity - ability to return to original
shape after contraction or extension
4. Kinds -
a. Skeletal muscle tissue - attached to
bones, striated, voluntary
b. Smooth muscle tissue - located in viscera.
nonstriated and involuntary c. Cardiac muscle tissue - forms the walls of heart, striated and involuntary
III. Structural basis of contraction-- 490
A. Fascia
1. Sheet or broad band of fibrous connective
tissue underneath the skin or
around muscle
2. 3 types - superficial, deep and subserous
B. Connective tissue components
1. The entire muscle is covered by the
epimysium.
Fasciculi
are covered by perimysium. Fibers are
covered by endomysium.
2. Tendons and aponeuroses (broad flat
tendon) attach muscle to bone
C. Nerve and blood supply
Nerves
convey impulses, and blood provides nutrients and oxygen for contraction
D. Components of a skeletal muscle fiber
1. Multinucleated, striated
2.
Parallel bundles - myofibrils - actin and myosin, troponin
and tropomyosin -
repeated units - sarcomeres
a. Sarcolemma - cell membrane; sarcoplasm -
cytoplasm
sarcoplasmic reticulum - membranous
channels, T tubules
b. Each fiber contains myofilaments;
myofilaments are compartmentalized
intosarcomeres
c. Dark bands - A bands - anisotropic bands
d. Light bands - I bands - isotropic bands
e. I bands bisected by dark Z lines,
sarcomeres bounded by Z lines
IV. Mechanisms of contraction
A. A bands - thick filaments - myosin
1.
Edges of A band - thin filaments overlapped with thick
2. Central regions of A bands- only thick
filament-H bands
3. I bands - only thin filaments - protein =
actin
4. Thin filaments composed of: globular actin
subunits,
B. Myosin cross-bridges extend out from the
thick filaments to the thin filaments
1.
At rest - cross-bridges - not attached to actin
a.
Cross-bridge heads - function as ATPase enzymes
b.
ATP --- ADP + P, activating the cross-bridges
2.
Contraction - A nerve impulse travels over the sarcolemma and enters
the transverse (T) tubules.
3. Action potential in the T tubules
stimulate the release of calcium from the sarcoplasmic reticulum
triggering the contractile process
4. Ca++ binds with troponin, conformation of tropomyosin exposes the binding sites on the actin to the c-b
5.
When the activated cross-bridges attach to actin, power
stroke occurs and
in the process ADP + P are released
6. At the end of the power stroke -
cross-bridges bond to a new ATP.
This allows the detachment from
actin and repeats cycle.
(Rigor mortis - inability of
cross-bridges to detach from actin due to a lack of ATP)
C. The activity of the c-b causes the thin
filaments to slide towards the centers of the sarcomeres
1. The filaments slide; not shortened during
muscle contraction
2. A bands stay the same length, H and I
bands shortens
3. Chemical energy is translated in to
mechanical energy
- rocking or rotation of myosin head produces force, and
this force is
transmitted to the thick filament
ATP - not needed to produce c.b. bridge
force, but is required to detach the myosin head from actin filament.
-
c.b.
attachment requires intracellular free calcium -at conc. above 10 -7 M.
-
When action
potential cease , calcium is removed from the sarcoplasm and
store in the sarcoplamic reticulum
E. Sequence of
events in stimulation and contraction of muscle
1. Stimulation
a. Sarcolemma depolarized
b. T system depolarized
c. Ca ++ released from sarcoplasmic reticulum
d. Ca ++ diffuses to thin filament
2. Contraction
a. Ca ++ binds to troponin
b. Troponin-Ca ++ complex removes tropomyosin
blockage of actin sites
c. Heads of thick filament (containing
preexisting
myosin-ATP complex) form cross bridges
to actin strand
d. Cross bridges swivel as ATP is hydrolyzed
and ADP is released
3.
Relaxation
a. Ca ++ sequestered from thin filament by
sarcoplasmic reticulum
b. Calcium diffuses from thin filament toward
sarcoplasmic reticulum
c. Ca ++ released from troponin-Ca ++ complex
d. Troponin permits tropomyosin return to
blocking position
e. Myosin-actin cross bridges break
f. ATP-myosin complex reformed in heads of
thick filament
V. Muscle in vitro can exhibit twitch, summation, and tetanus 498
A. Twitch - rapid contraction and relaxation
B. A whole muscle can produce a twitch in
response to a single electrical pulse in vitro
1. The stronger the electric shock - stronger
the twitch
2.
Graded contraction - different no. of fibers contracting
C. Summation of fiber twitches - occur so
rapidly - muscle produces a smooth sustained contraction - tetanus VI. Contraction of muscle fibers in vivo - stimulated by motor neurons
A. Each somatic motor axon branches to
innervate a number of muscle fibers. B. The motor neuron and the muscle fibers - motor unit
1. When a muscle is composed of many motor
units - hand - fine control
2. Large muscle of the leg - relatively few
motor units
3. Sustained contractions are produced by the
asynchronous stimulation of different motor units.
4. Motor unit -
a. Transmits the stimulus to a skeletal
muscle for contraction
b.
Motor end plate - region of the sarcolemma specialized to
receive stimulus
c. Myoneural junction - area of contact
between a motor neuron and muscle fiber
d.
Muscle fibers of a motor unit contract to their fullest extend or not
at all
e. The weakest stimulus capable of causing
contraction is a minimal or threshold stimulus
VII. Isometric vs. isotonic contraction: isometric- muscle exerts
tension without shortening;
twitch,
summation and tetanus
Isotonic
- shortening occur-- force, work and power
Isometric - no shortening -- tension increases
VIII. Physiology-
A. When a nerve impulse reaches the motor end
plate,
the neuron releases acetylcholine - electrical changes occur
B. This change releases calcium ions-
activate the myosin, catalyzing the breakdown of ATP
C. The energy released - causes the sliding
of the myofilaments.
IX. Neural control of skeletal muscles-
stretch receptor-spindles - stimulate the production of nerve impulse in
sensory neurons when the muscle is stretched
A. Stretch reflex-- somatic neurons -
1.
Alpha motorneurons - innervate muscle fibers -
2. Gamma motorneurons - innervate muscle
spindles
B. Muscle spindles and Golgi tendon organ-
length and tension detectors in the muscle
1. Golgi tendon organ- detect the tension
that the muscle exerts on its tendon
E.g.
If a muscle is stretched extensively, it will relax- inhibitory
effects produced by Golgi tendon organ
2. Spindles - several intrafusal fibers
wrapped together and in parallel with the extrafusal fibers (muscle
fibers)
Number
of spindles increases with finer degree of control
3. Stretch the muscle - stretch the spindles
- this excite sensory endings -
a. Impulses in the sensory neurons -->
spinal cord -->in the dorsal root of spinal nerves
b. Sensory neuron synapse directly with an
alpha neuron within spinal cord monosynaptic reflex
c. Alpha motor neuron stimulate the
extrafusal muscle fibers to contract stretch reflex
4.
Gamma neurons - make the spindles sensitive to stretch, better able to
monitor the length of muscle fibers
C. Neurons in the brain that affect the lower
motor neuron - higher motor neurons
1. Fibers of neurons from motor cortex,
descend to the lower motor neuron
2. The left side of the brain controls the
musculature on the right side
3. Damage of the cerebellum produces
intention tremor.
4. The degeneration of fibers in the basal
ganglia - use dopamine as a transmitter produces Parkinson's disease.
X. Skeletal muscle type and energy usage-
A. 3 types of muscle fibers-
1.
Slow-twitch red fibers - adapted for aerobic respiration - resistant
to fatigue
2. Fast-twitch white fibers - adapted for anaerobic respiration
3.
Intermediate - fast twitch, but adapted for aerobic respiration
B. Muscle fatigue - caused by
1.
Sustained maximal contraction - may be produced by the
accumulation of
extracellular K as a result of high levels of nerve activity
2. Rhythmic moderate exercise - result of
anaerobic respiration by fast twitch fibers
a. Production of lactic acid-- lowers the
intracellular pH -- inhibit glycolysis and decrease ATP b. Decreased ATP inhibits
excitation-contraction coupling- possibly due to a cellular loss of Ca
C. Physical training affects the characteristics
of the muscle fibers -
1.
Endurance training - increases the aerobic capacity of all muscle
fiber types;
reliance of anaerobic
respiration and their susceptibility to fatigue is decreased
2.
Strength training causes hypertrophy of the muscle fibers - an
increase in the size
and number of myofibrils.
CHARACTERISTICS OF
RED, INTERMEDIATE AND WHITE MUSCLE FIBERS
XI.
Cardiac and smooth muscle
A. Cardiac muscle - striated and contains
sarcomeres
1.
Cells are quadrangular and contain centrally placed nuclei
2. Fibers form a continuous, branching
network that contract as a unit
3.
Intercalated discs provide strength and aid impulse conduction
4. Action potentials in the heart originate
in myocardial cells; stimulation by neuronsnot necessary
5. Fibers contain many short individual
cells electrically coupled to one another via gap junction for through
conduction of the action potential
6. Ionic mechanisms - specialized for pacemaker activity in the atria and for
prolonged action potentials in the ventricle
B. Smooth muscle -
lack sarcomeres, not striated
1.
Contain myosin and actin, not arranged in sarcomeres
2.
Myosin - very long; can contract even when they are greatly stretched.
3. Phosphorylation of cross bridges is
required for their bonding to actin
4.
Groups of these cells are connected to each other by gap
junctions that permit
the
electronic spread of current from cell to cell
5. Transmitter substances from the autonomic
axons is released at a given varicosity .
(Do not form intimate junction) and diffuses over some distance -
affecting a number of smooth
muscle cells.
6. Autonomic and hormonal control -
involuntary contracts and relaxes slowly
than striated muscle
7. Calcium ions have a regulatory function-
when stimulated by
graded depolarization, calcium
ions enter smooth muscle, and instead of binding to troponin
(absent in smooth muscle), Ca
binds to calmodulin. Ca-calmodulin
activates a protein
kinase
that phosphorylates myosin heads (cross-bridges)
XI. Hydrostatic skeleton- 2 types-
A. Consists of a chamber with flexible walls completely filled with water or body fluid. the wall contains muscle that attached to each other via connective tissue. 1. Muscles contract, create a
tension in the wall that pressurizes the fluid
2. As water is incompressible, the chamber’s vol =
constant.
3. If one part
of the chamber is
B. No fluid-filled chamber- filled with muscles; intracellular
fluid of the muscles act
as
the hydraulic fluid-- muscular hydrostats.
E.g.
squid’s tentacles, elephant trunk muscles-- longitudinal and circumferential-
Long.
Muscles shorten and widen the tentacle, circ muscles elongate
and
thin the tentacle. Radial muscles
elongate and thin tentacles,
oblique
muscles twist the tentacles
XII. Swim bladders - buoyancy- originates as evagination of GI tract
A. Connection to esophagus -physotome fish -
fill by gulping air; permits gas to escape
B. Completely closed swim bladder -
physoclist fish -- gases in bladder originates from
Some comparisons of skeletal, cardiac,
and smooth muscles
Effects of endurance training -
1. Improved ability to obtain ATP from oxidative phosphorylation
2. Increased size and number of mitochondria
3. Less lactic acid produced per given amount of exercise
4. Increases myoglobin count
5. Increased intramuscular triglyceride content
6. Increased lipoprotein lipase - enzymes needed to utilize lipids
from blood
7. Increased proportion of energy derived from fat, less from
carbohydrates
8. Lower rate of glycogen depletion during exercise
9. Improved efficiency in extracting oxygen from blood
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||