Download Animal Physiology, Chapter 19

Resistance exercise and microgravity are conditions that produce changes in skeletal muscles
Functions of musculoskeletal system
• Provide the basic form and shape of
organism
• Mechanical function of support
• Means of protection for vulnerable
organs
• Allows body movement
• Provides a set of levers
• Heat production
Homeostatic functions of skeletal system
• Maintain a constant state- invloves
behavioral responses to environmental
changes
• Erythrocytes and other formed elements
of the blood produced in bone marrow
• Stores minerals – calcium and
phosphorus
Characteristics of muscle fibers
•
•
•
•
Irritability
Contractibility
Extensibility
Elasticity
Figure 19.1 The power a muscle is capable of generating reflects its functional capabilities
Figure 19.2 Quadriceps muscles of the anterior thigh
Figure 19.3 A needle biopsy is used to obtain samples of muscle tissue
Structural; basis of contraction
•
•
•
•
Fascia
Connective tissue components
Nerve and blood supply
Components of a skeletal muscle fiber
Figure 19.8 Major muscles of the lower leg
Structure of Skeletal Muscle
(continued)
(coninued)
Mechanisms of Contraction
• Each myofibril contains myofilaments.
– Thick filaments:
• A bands contain thick filaments (primarily
composed of myosin).
– Thin filaments:
• I bands contain thin filaments (primarily
composed of actin).
– Center of each I band is Z disc.
Sliding Filament Theory of Contraction
(continued)
Excitation-Contraction Coupling
• Ca2+ attaches
to troponin.
• Tropomyosintroponin
complex
configuration
change occurs.
• Cross bridges
attach to actin.
(continued)
Mechanisms of Contraction
(continued)
• Sarcomere:
– Z disc to Z disc.
– M lines:
• Produced by
protein filaments
in a sarcomere.
– Anchor myosin
during
contraction.
• Titin:
– Elastic protein that
runs through the
myosin from M line
to Z disc.
• Contributes to
elastic recoil of
muscle.
Mechanisms of contraction- sliding filament theory
• A band – thick filaments
• I band – thin filaments
• Myosin cross-bridges extend out from the thick
filaments to the thin filaments
• Activity of c-b
• Sequence of events in stimulation and contraction of
muscle
Sliding Filament Theory of Contraction
(continued)
• Muscle contracts:
– Occurs because of sliding of thin filaments over and between
thick filaments towards center.
• Shortening the distance from Z disc to Z disc.
• A bands:
– Contain actin.
• Move closer together.
– Do not shorten.
• I bands:
– Distance between A bands of successive sarcomeres.
– Decrease in length.
• H bands shorten.
– Contain only myosin.
– Shorten during contraction.
Contraction
(continued)
Role of Ca2+ in Muscle Contraction
• Muscle Relaxation:
– [Ca2+] in sarcoplasm low when tropomyosin
blocks attachment.
• Prevents muscle contraction.
• Ca2+ is pumped back into the SR in the terminal
cisternae.
– Muscle relaxes.
Muscle in vivo
•
•
•
•
Twitch
Summation
Tetanus
Fatigue
Muscle Response to Varying Stimuli
• More rapidly delivered stimuli result in
incomplete tetanus
• If stimuli are given quickly enough,
complete tetanus results
Figure 9.15
Muscle Twitch Comparisons
Figure 9.14b
Treppe: The Staircase Effect
Figure 9.18
Twitch, Summation, and Tetanus
(continued)
Motor Unit
When somatic neuron is activated, all the muscle fibers it
innervates contract with all or none contractions.
• Innervation ratio:
– Ratio of motor neuron: muscle fibers.
• Fine neural control over the strength occurs
when many small motor units are involved.
• Recruitment:
– Larger and larger motor units are activated to
produce greater strength.
Motor Unit
(continued)
• Each somatic neuron
together with all the
muscle fibers it
innervates.
• Each muscle fiber
receives a single
axon terminal from a
somatic neuron.
• Each axon can have
collateral branches to
innervate an equal #
of fibers.
Neuromuscular Junction
Figure 9.7 (a-c)
Synaptic Cleft: Information Transfer
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
5
Degraded
neurotransmitter
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel closed
Ion channel (open)
Figure 11.18
Excitation-Contraction Coupling
• Na+ diffusion
produces end-plate
potential
(depolarization).
• + ions are attracted
to negative plasma
membrane.
• If depolarization
sufficient, threshold
occurs, producing
APs.
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
ACh
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2 Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
ADP
Pi
6
Ca2+
Ca2+
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
Ca2+
Ca2+
3 Calcium ions bind to troponin;
Ca2+
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
5 Removal of Ca2+ by active transport
into the SR after the action
potential ends.
Ca2+
4 Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Figure 9.10
Muscle Metabolism: Energy for Contraction
Figure 9.20
Metabolism of Skeletal Muscles
• Metabolism
of skeletal muscle
• Lactate threshold:
– % of max. 02 uptake at which there is a significant
rise in blood [lactate].
• Healthy individual, significant blood [lactate] appears at 50–
70% V02 max.
• During light exercise:
– Most energy is derived from aerobic respiration of fatty acids.
• During moderate exercise:
– Energy is derived equally from fatty acids and glucose.
• During heavy exercise:
– Glucose supplies 2/3 of the energy for muscles.
• Liver increases glycogenolysis.
• During exercise, the GLUT-4 carrier protein is moved to
the muscle cell’s plasma membrane.
Metabolism of Skeletal Muscles
(continued)
• Oxygen debt:
– Oxygen that was withdrawn from hemoglobin and
myoglobin during exercise.
– Extra 02 required for metabolism tissue warmed
during exercise.
– 02 needed for metabolism of lactic acid produced
during anaerobic respiration.
• When person stops exercising, rate of oxygen
uptake does not immediately return to preexercise levels.
– Returns slowly.
Metabolism of Skeletal Muscles
(continued)
• Phosphocreatine (creatine phosphate):
– Rapid source of renewal of ATP.
– ADP combines with creatine phosphate.
• [Phosphocreatine] is 3 times [ATP].
– Ready source of high-energy phosphate.
Muscle Fatigue
• Any exercise induced reduction in the ability to maintain
muscle to generate force or power.
– Sustained muscle contraction fatigue is due to an accumulation
of ECF K+.
• Repolarization phase of AP.
• During moderate exercise fatigue occurs when slowtwitch fibers deplete their glycogen reserve.
• Fast twitch fibers are recruited, converting glucose to
lactic acid.
– Interferes with Ca2+ transport.
• Central fatigue:
– Muscle fatigue caused by changes in CNS rather than fatigue of
muscles themselves.
Muscle Tone
• Muscle tone:
– Is the constant, slightly contracted state of all
muscles, which does not produce active
movements
– Keeps the muscles firm, healthy, and ready to
respond to stimulus
• Spinal reflexes account for muscle tone by:
– Activating one motor unit and then another
– Responding to activation of stretch receptors
in muscles and tendons
Slow- and Fast-Twitch Fibers
• Slow- and Fast-Twitch Fibers
– Skeletal muscle fibers can be divided on
basis of contraction speed:
• Slow-twitch (type I fibers).
• Fast-twitch (type II fibers).
• Differences due to different myosin
ATPase isoenzymes that are slow or
fast.
Slow- and Fast-Twitch Fibers
(continued)
• Slow-twitch (type I fibers):
– Red fibers.
– High oxidative capacity for aerobic respiration.
– Resistant to fatigue.
– Have rich capillary supply.
– Numerous mitochondria and aerobic
enzymes.
– High [myoglobin].
• Soleus muscle in the leg.
Slow- and Fast-Twitch Fibers
(continued)
• Fast-twitch (type IIX fibers):
–
–
–
–
–
White fibers.
Adapted to respire anaerobically.
Have large stores of glycogen.
Have few capillaries.
Have few mitochondria.
• Extraocular muscles that position the eye.
• Intermediate (type II A) fibers:
– Great aerobic ability.
– Resistant to fatigue.
• People vary genetically in proportion of fastand slow-twitch fibers in their muscles.
Characteristics of Muscle Fiber Types
Figure 19.10 Record speeds achieved by athletes decrease with age
Figure 19.11 Remodeling of motor units with aging
Figure 19.12 Dystrophin connects F-actin of the cytoskeleton to the sarcolemma
Figure 19.13 Costameres
Figure 19.4 VEGF responses to a single bout of endurance exercise
Figure 19.5 Endurance training increases the number of mitochondria
Figure 19.6 Changes in fast fiber types during training and detraining
Figure 19.7 Stretch or stretch combined with electrical stimulation increased protein synthesis
Effects of endurance training
• Effects of endurance training
– Improve ability to obtain ATP from oxidative
phosphorylation
– Increase size and # of mitochondria
– Less lactic acid produced per given amount of
exercise
– Increase myoglobin count
– Increase intramuscular triglyceride content
– Increase lipoprotein lipase
– Increase proportion of energy derived from fat
– Lower rate of glycogen depletion during exercise
– Improve efficiency in extracting O2 from blood