Muscle Physiology 2 Flashcards
Induces an upregulation in 1) mitochondrial number and function, 2) ATP and phosphocreatine content, 3) enhanced levels of intramuscular glycogen stores, and 4) an increase in intramuscular triglyceride levels
Exercise
Muscle fibers express type β2 adrenoceptors. Acute adrenergic stimulation causes an
Increase in forceful contraction of Type 2b fibers
Decrease in force develops in Type 1 fibers
Chronic stimulation of the skeletal muscle adrenoreceptor system via a β2 agonist (ex: albuterol) exerts and
Anabolic effect
Controls various aspects of muscle metabolism by controlling SERCA
-upregulates rate of contraction and relaxation but does NOT increase contractile force
Thyroid hormone
The effects of thyroid hormone are most profound in
Type 1 muscle
Although the rate of contraction-relaxation is augmented by thyroid hormone, the overall efficiency of energy turnover is
Decreased
Age associated loss in muscle mass
Sarcopenia
The process of sarcopenia actually begins sometime around age 30 years, after which muscle loss can approach
3-8% per decade
Decrease proteogenesis and the rate of amino acid transport and inhibit IGF-1 expression
Glucocorticoids (ex: prednisone and dexamethasone)
A transcription factor required for satellite cell differentiation into myofibers
-inhibited by glucocorticoids
Myogenin
Functions as an endocrine organ
Muscle
Aerobic and anaerobic muscle work is correlated with the secretion of growth factors and immune mediators from muscle fibers, collectively known as
Myokines
Can modulate muscle hypertrophy and myogenesis
Myokines
Consists of 1) a motor neuron and relatively few muscle fibers and/or 2) numerous motor neurons innervating single and/or small groups of muscle fibers.
One motor unit in muscles mediating fine control (i.e. hands)
These small diameter fibers are extremely well vascularized (hence the bright red appearance), and can effectively utilize oxidative metabolism for energy
Type 1 muscle fibers
Type 1 muscle fibers contain an abundance of
Myoglobin and mitochondria
Function as a hybrid between type 1 and type 2b fibers
Type 2a fibers
Are relatively large in diameter, this giving them a greater potential for force of contraction
Fast fibers
Fast fibers have a relatively low complement of mitochondira, but an extensive
SR
What are the two types of efferent neurons that innervate skeletal muscle fibers?
α and ƴ motor neurons
Mediate essentially all of the voluntary contractile responses of skeletal muscle
α neurons
Smaller, slower neurons that innervate the intrafusal muscle fibers within the muscle spindles
ƴ neurons
Contain sensory receptors that are designed to detect changes in muscle length and the rate of change of length
Muscle spindles
Intrafusal fibers run in parallel to the force generating extrafusal fibers. The intrafusal fiber ends are innervated by
Type II afferent fibers and ƴ motor neurons
The ends (chain) of intrafusal fibers can be stimulated to contract in response to
ƴ motor neuron activity
In contrast, the central portion of intrafusal fibers (bag) can not contract but is sensitive to
Stretch
Have receptors that encircle the center of the intrafusal fibers
-Impulses are continually being transmitted via
these sensory neurons to the spinal cord
Type 1a afferent neurons
Stretch and/or an increase in the rate of stretch can up-regulate the number and frequency of APs transmitted by
Type 1a afferent neurons
During muscle contraction, the bag region of
muscle spindles initially go slack, and type II and Ia activity is suppressed. If sustained contraction is required, descending (motor) tracts stimulate
ƴ motor neuron activity
ƴ motor neurons then stimulate contraction of the
Intrafusal muscle ends
Contraction of the fiber ends causes stretch within the
center (bag) portion of intrafusal fibers, and this stretch reactivates
Type 1a afferent activity
What happens when the limit of contraction/re-stretch of intrafusal fibers is reached?
Isotonic muscle contraction ceases
Continuously provides the CNS with information about muscle length and rate of change in length in order to allow and sustain the full range of skeletal muscle contraction
The muscle spindle
Located in musculotendon junctions where they serve as an early warning system to the CNS that muscle tension has suddenly increased
-serve as a protective mechanism against muscle tear
Golgi tendon organs (GTOs)
If load (due to passive stretch or active contraction) causes an increase in tension in the muscle, the activity of GTOs is up-regulated, and there is an increase in the frequency of
Type 1b action potentials
Activation of type 1b muscle fibers stimulates
Decrease in muscle tension
Stimulate inhibitory interneurons and antagonistic muscles
Type 1b afferents
Defined as a reversible decrease in contractile force in
response to an increase in stimulation frequency or duration
Muscle fatigue
Force generation is especially restrained due to a breakdown in pCr, attributing to an insufficient rate of ADP phosphorylation and the accumulation of
Inorganic phosphate
What are the differences between smooth and skeletal muscle cells?
Smooth muscle cells:
- ) are smaller
- ) have 1 central nucleus
- ) unstriated
- ) small SR
- ) high actin to myosin ratio (15:1)
Smooth muscles have many dense bodies, some of which are attached to the cell membrane. Dense bodies serve the same function as
Skeletal Z-discs
Connected by gap junctions which allow innervation of only a few muscle fibers to result in excitation of all muscle fibers in a region
Visceral smooth muscle
Visceral smooth muscle acts as a
Single unit
What contains more vascular smooth muscle, arteries or veins?
Arteries
The contraction and relaxation of vascular smooth muscle (i.e. vascular smooth muscle tone) is under the
control of the
Sympathetic nervous system
Contraction of smooth muscle occurs by a
-similar to skeletal muscle
Sliding filament mechanism
Because actin filaments are attached to dense bodies within the cell membrane, fiber shortening pulls the cell membrane inward causing the muscle cell to appear
Crinkled
With smooth muscle, as with skeletal muscle, the contractile system is activated by an increase in
Sarcoplasmic Ca2+
In smooth muscle, Ca2+ is obtained from extracellular sources following cell depolarization and activation of
Voltage-gated Ca2+ channels
Can each activate Ca2+ channels in the SR membrane through which SR stores of Ca2+ enter the sarcoplasm in smooth muscle cells
Ca2+-induced Ca2+ release and/or IP3-mediated signal transduction
In smooth muscles, when sarcoplasmic Ca2+ concentrations are sufficiently elevated, Ca2+ binds the calcium sensor known as
Calmodulin
The Ca2+/Calmodulin complex then activates
Myosin light chain kinase (MLCK)
Activation of MLCK causes phosphorylation of myosins, which gives them a very high affinity for
Actin
Dephosphorylates myosin in smooth muscle cells, resulting in the cessation of contration
MLCK phosphatase
Prolonged in comparison to skeletal muscle contraction
Smooth muscle contraction
In order to accomplish these relatively long duration contractions, smooth muscle utilizes
Oxidative metabolism
Slow, sustained contraction is caused by slow cycling of the
Myosin crossbridges
In smooth muscle cells, contraction requires very little
ATP
MLCK phosphatase is activated, crossbridge cycling ceases, and muscle relaxation occurs when
Ca2+/Calmodulin levels fall
In contrast to skeletal muscle, smooth muscle can shorten from 50% to 75% of its relaxed length
without altering the overlapping of
Myosin and actin filaments
As opposed to the linear contraction of skeletal muscle, smooth muscle can contract upon itself in
Multiple dimensions
In smooth muscle, stretch of the smooth muscle fibers results in Ca2+ inflix, which activates the
Contractile system (increases active tension)
The elastic properties of smooth muscle aids in the process of
- allows for an alleviation in pressure, which follows an increase in volume
- especially advantageous to hollow organs
Stress relaxation
What is Laplace’s Law?
P = T/r
where,
P = pressure T = tension r = radius
The concept of stress relaxation is especially important in the
-less important in vascular tissues
Bladder
ANS post ganglionic neurons have axons which have swollen areas called
-in close proximity to the smooth muscles they innervate
Varicosities
Function as diffuse neuromuscular junctions
Variscosities
Have vesicles containing the neurotransmitter, ACh (parasympathetic nervous system), or norepinephrine (sympathetic nervous system)
Variscosities
There are different types of action potentials that can be generated by smooth muscle. The main type of potential is called a
Slow wave potential
Not action potentials, but are rhythmic, subthreshold, pacemaker-type changes in membrane depolarization and repolarization
Slow wave potentials
The depolarization phase of a slow wave is induced by the opening of Ca2+ channels. These Ca2+ channels can be
Ligand- or voltage-gated
Slow waves set what is known as the
Basal electrical rythm of the gut
Smooth muscle potential which is similar to skeletal muscle APs.
-Depolarization is mediated by activation of Ca2+ and/or Na+ chanels
Spike potential
Both outward IK+ and ICa2+ can be involved in repolarization of
Spike potentials
100-1000 msec slower than spike potentials due to delayed repolarization.
-Like spike potentials, depolarization is mediated by inward ICa2+ and/or INa+
Plateau potentials
In a plateau potential, depolarization is followed by a transient repolarization phase, which is dependent on
Outward IK+
An example of a tissue that utilizes both spike potentials and plateau potentials
Myometrium
Ion flux is different in
Smooth muscle than in skeletal
In smooth muscle, the RMP is
-50 to -60 mV
In smooth muscle, the threshold potential is
-30 to -35 mV
Stimulation of smooth muscle increases Na+ influx, this in turn initiates
Membrane depolarization
The depolarization phase of a spike potential is due
predominantly to
Ca2+ influx
Participates not only in the depolarization phase of the AP in smooth muscle, but also in the initiation of smooth muscle contraction
Ca2+
