Muscle Physiology Slide Notes Flashcards
The limiting factor in muscle contraction
Calcium
Utilize more extracellular Ca2+ than intracellular
Cardiac and smooth muscle
Skeletal muscle function begins in the
Motor cortex (in cerebral cortex)
Connect brain to regions of the spinal cord
-where motor signals leave the ventral root
Rubrospinal and reticulospinal tracts
ACh receptors that are specific to skeletal muscle
Cholinergic-nicotinic receptors
Allows for calcium to be released from the SR
-a voltage gated Ca2+ channel
DHPR-RyR complex
Decreasing the amount of sarcoplasmic Ca2+ results in
Muscle relaxation
Transports calcium out of the cell and sodium into the cell
-a secondary mechanism to move calcium out of the cell
NCX
The biggest mechanism for sequestering Ca2+ back into the SR
SERCA
In skeletal muscle, crossbridge cycling is driven by
Intracellular Ca2+
Changes in Ca2+ are directly proportional to changes in contractility. These different levels of contraction are called the
Inotropic state
Allows for influx of Ca2+ into the myocardium
Type L channel
Targets bone and the kidneys and increases bone resorption, which increases plasma concentration of Ca2+
Parathyroid hormone (PTH)
Causes motor neuron and skeletal muscle hyperexcitability an Na+ channel stabiity
-can lead to muscle spasm
Hypocalcemia
Raises the threshold for voltage-gated Na+ channels which leads to muscle and neuron hypoexcitability
Hypercalcemia
The opposition to displacing load
Afterlod
Force generated to counter afterload
Preload
The preload is greater than the afterload and work is performed in
Isotonic Contraction
In cardiac muscle, the preload is the ability for
Ventricles to contract
In cardiac muscle, the afterload is
Aortic BP and pulminary arterial pressure
Blood only leaves the ventricles if
Preload > afterload
The maximal force a muscle can generate
Muscle tetanus
Phenomenon due to muscle elasticity
-stretch in muscle
Passive tension
Reaches a maxima shortly after contraction and then declines
-shows us that muscle generates most of its power early in contraction
Active tension
If muscles are not starting from L0, but rather from a shorter length, the power the muscle can generate will be
Lower than if it were at L0
Greatest at intermediate loads where F and V are moderate
Power
Describe the orderly recruitment principle
We want to move a load, so type 1 fibers are recruited and we are in a stage of isometric contraction, then some type 2 fibers are recruited and we get an isometric to isotonic conversion. Then a large amount of type 2 fibers are recruited and we reach maximal force of muscle contraction. But then after 50msec or so, we recruit type 1 fibers because they are the only ones with the oxidative capacity to sustain prolonged contraction
Musculotendon assessment mechanisms tell us if it is ok to keep contracting or if we need to stop. What are the two systems we have in place for this assessment?
- ) Golgi tendon organs
2. ) Muscle spindles
Stress sensors that will decrease muscle contraction via type 1b afferent fibers if necessary
Golgi tendon organs (GTOs)
Monitor change in muscle length and rate of change in length
Muscle spindles
What are the components of the intrafusal fibers of muscle spindles?
- ) Bag fibers
2. ) Chain fibers
What are bag fibers innervated with?
-Tells us about change in muscle length and provides us with proprioception
Type 1a afferents
Tells us about static length
Type II afferents (in chain fibers)
An increase in muscle fiber diameter
-predomiant
Hypertrophy
An increase in number of muscle fibers
-less common
Hyperplasia
Activated in response to stimuli induced by workload
- stimulate myoblasts
Satellite cells
The main source of muscle regeneration and growth
Satellite cells
Released due to anaerobic and aerobic work and function in autocrine and paracrine hypertrophy myogenesis
Myokines
Myokines promote
Osteogenesis, anti-inflammatory interactions, and insulin secretion
Have direct anabolic and anti-catabolic effects by
Increasing: Satellite cell proliferation and GH and IGF-1 levels
Decreasing: Glucocorticoid activity
Anabolic androgen steroids (i.e. testosterone and dihydrotestosterone)
Induces increase in nitrogen balance, proteogenesis, and increased hepatic IGF-1
Growth Hormone (GH)
Ubiquitously expressed in skeletal muscle and increases proteogenesis and regeneration of muscle
IGF-1
Are less effective without eachother, i.e. they are synergists
GH and IGF-1
What are three catabolic factors?
- ) Excess Ca2+
- ) Glucocorticoids
- ) Myostatin
Released as part of the immune/anti-inflammatory response and function to increase proteolysis and type II fiber atrophy
Glucocorticoids
Stress induced hormones that decrease activity of IGF-1
Glucocorticoids
Comes from the adrenal cortex and causes release of myostatin
Cortisol
Expressed within satellite cells where it blocks cell cycle progression and inhibits proliferation of satellite cells
Myostatin
Inhibition of myostatin results in
Unregulated muscle growth
Muscle fatigue will prevent cross-bridge cycling. What are two factors that will lead to muscle fatigue?
- ) Decreased pH (affects type II fibers)
2. ) K+ efflux
Which type of muscle fiber is most affected by aging?
Type II fibers
Has slow wave and spike potentials and is innervated by autonomic nerve fibers
Visceral smooth muscle
In visceral smooth muscle, what is the effect of the following stimulation:
- ) Sympathetic
- ) Parasympathetic
- ) relaxation
2. ) contraction
In vascular smooth muscle, epinephrine and norepinephrine activate α1 adrenoreceptors which lead to
Inhibition of SERCA, resulting in increased sacoplasmic Ca2+ and thus vasoconstriction
