Muscle Physiology Flashcards
skeletal muscle cells
Voluntary
attached to bones or skin
very long, cylindrical
multinucleated
not self stimulated
no rhythmic contractions
fatigues easily
cardiac muscle cells
involuntary
branching
only in heart
self stimulated
striated
does not fatigue
cardiac muscles are… (characteristics)
striated
elongated
branched cells (linked via intercalated disks)
desmosomes (cardiac muscle cells)
specialized cell junctions along intercalated disks between cells “spot welds”
withstand high mechanical stress from myocardial beating
gap junctions (cardiac muscle cells)
provide electrical conduction that enables intercalated cells to beat as a single conductive unit (syncytium)
what enters cytoplasm from both the sarcoplasmic reticulum stores and extracellular sources
Calcium
- L-type calcium channel enables Ca2+-induced calcium release
autorhythmicity
intrinsic pacemaker cells (cardiac muscle cells)
What type of respiration do cardiac muscle cells primarily use?
aerobic respiration
able to moderately resist fatigue (large mitochondria)
high myoglobin content for O2 storage
cardiac myocytes don’t _____ but are …
don’t fatigue easily BUT they are very sensitive to ischemia/hypoxia
smooth muscle cell’s shape and nucleus?
spindle-shaped and mononucleated
T/F: smooth muscle cells form sheets on the walls of hollow organs and some blood vessels
TRUE
smooth muscle cells: voluntary or involuntary control?
involuntary control
slow, rhythmic (wave-like) contractions
T/F: smooth muscles cells are striated
FALSE:
smooth muscle cells are NOT striated (not organized into sarcomeres)
smooth muscle cell organization
thin (actin) fibers are attached to the cell wall and to dense bodies in the cytoplasm between myosin bundles
smooth muscle cell activation causes what?
When activated, thin actin fibers slide over the myosin bundles causing shortening of the cell walls
smooth muscle cells contracted vs contracted state
crumples up when contracted
motor unit
the functional unit of skeletal muscle
a motor unit is composed of…
composed of an alpha motor neuron and all the muscle fibers (myofibrils) it innervates
alpha motor neuron axon characteristics
synapse with up to thousands of muscle fibers BUT each muscle fiber is connected to only ONE alpha motor neuron
motor unit recruitment
the process by which more and more motor units are put into action
the greater the number/size of the motor units recruited, the more powerful the contraction
sarcomere
a structural unit of myofibril in striated muscle
gives Striated Muscle its “striped” appearance
sarcomere is composed of…
myosin (thick)
actin (thin)
both protein filaments
arrangement of sarcomere
sarcomere: what does the z-disk do?
anchor for actin filaments
sarcomere: what is the I-band?
space between A-bands containing only actin and z-disk
sarcomere: what is the H-zone?
forms the center (lightest part) of the A-band
contains myosin filaments only
shortens during contraction
sarcomere: M-line
middle/midline of the H-zone and the A-band
accessory proteins anchor myosin to M-line
sarcomere: A-band
darker, thicker band containing both myosin and actin
length does not change during contraction
sarcomere: titins
elastic filaments that anchor myosin to z-discs (springs)
what sarcomere components shorten during muscle contraction?
I-band
H-zone
A-band does NOT shorten
sliding filament theory
During contraction, the heads of each thick (myosin) filament form a cross-bridge with adjacent thin (actin) filaments.
During excitation-contraction coupling, the actin filaments slide toward the M-line (center) shortening the sarcomere
sliding filament theory is a ______- dependent process
ATP-dependent process
T/F: during muscle contraction, the length of the actin and myosin filaments shorten
FALSE: the length of the filaments do not shorten
the I-band and H-band shorten *actin and myosin are pulled together
excitation-contraction coupling
Physiological conversion of electrical stimuli (action potential (APs )) to mechanical responses (contraction) at the neuromuscular junction
6 steps of excitation-contraction coupling
cross-bridge formation
during relaxation is cytoplasmic Ca2+ low or high?
low Ca2+ in relaxation
during muscle relaxation (low Ca2+), what blocks the myosin binding site on actin?
tropomyosin block binding site on actin (prevents contraction)
during muscle contraction (high Ca2+), what is activated?
troponin is activated by Ca2+
troponin pulls tropomyosin off the actin’s myosin binding site
myosin head is able to bind to actin and contract muscle
what does high Ca2+ concentration do?
The higher the Ca2+ concentration, the greater the # of tropomyosin molecules moved to expose myosin binding sites
cross-bridge cycle
cross bridge cyle can continue as long as…
- muscle is activated
- ATP is available
- the physical limit of shortening the sarcomere has not been reached
Main events of Skeletal muscle contraction
sarcomere length-tension relationship
The lower limit of contractile ability (75% of resting length)
Optimal operating length (80-120%)
—— muscle is slightly stretched with a slight overlap between myofibrils.
The maximum limit the sarcomere can stretch (170%).
3 energy sources for muscle contraction
creatine phosphate
glycogenolysis/anaerobic resp
aerobic resp
muscle contraction: creatine phosphate
direct phosphorylation
energy source: CP
oxygen use: none
produces 1 ATP per CP
muscle twitch
response of a muscle to a single action potential on its motor neuron
muscle twitch threshold
minimum voltage necessary to produce contraction
a single brief stimulus at that voltage produces a quick all or nothing cycle of contraction & relaxation (i.e., a twitch)
phases of muscle twitch
latent period
contraction phase
relaxation phase
refractory period
multiple motor unit summation
“Recruitment”
Increasing the strength of the stimulus at a constant frequency to recruit additional motor units and thereby increase the tension developed
tetanus
prolonged contraction without relaxation of muscles
NOT sustainable –> fatigue
unfused (incomplete) tetanus
“staircase effect”/treppe
wave summating at a frequency sufficient to produce periods of incomplete relaxation between contractions
wave summation
“temporal summation”
Increasing frequency of a stimulus held at a constant intensity
to shorten the muscle and move the object, the tension generated by the
muscle must ______ the load from the object
to shorten the muscle and move the object, the tension generated by the muscle must EXCEED the load from the object
tension vs load (muscle physiology)
tension: the force exerted on an object by a contracting muscle
load: the force exerted on the muscle by an object
isometric contraction
load exceeds the tension (muscle doesn’t shorten)
isotonic contraction
the tension (force) generated by the muscle is greater than the load (muscle shortens)
fractionation
All motor units in a muscle do not need to activate
However, the more motor units activated, or the larger the motor units recruited, the greater the tension (i.e., force) achievable by the muscle
Henneman’s “Size Principle”
Motor unit recruitment depends on the demand (i.e., load).
Motor units are recruited in the order of the size of the motor unit based on the force needed
Size Principle: under increasing load
motor units are recruited from smallest to largest
Type I (slow) motor units are recruited first for light exercise then Type IIa (fast) then Type IIb (slow)
Type I (slow)
Type IIa (fast)
Type IIb (slow)
slow vs fast twitch muscle fibers
slow (Type I)
Fast (Type II)
reflex arc
a neural pathway that controls a reflex (muscle phys)
somatic
skeletal muscle
autonomic
smooth and cardiac muscle
6 basic components of reflex arcs
sensory receptor
sensory (afferent) neuron
integration center (CNS)
interneuron
motor (efferent neuron)
effector (muscle)
example of monosynaptic reflex
Knee jerk (stretch reflex)
stimulus: muscle stretch
result: contraction of stretched muscle
example of a disynaptic reflex
golgi (deep) tendon reflex
stimulus: muscle contraction
result: relaxation of muscle
what type of muscle
rigor mortis
If cellular energy stores are depleted (as happens after death) the cross bridges cannot
detach due to lack of ATP and the cycle stops in the attached state. This produces
stiffness of the muscle known as “rigor.”
