Midterm 2- skeletal muscle conraction Flashcards
what are the two systems of the peripheral nervous system?
somatic
- posture and movement
- includes muscle you can actively control (voluntary)
autonomic
- sympathetic and parasympathetic
- largely subconscious regulation
what are the somatic motor control regions?
Morton cortex
corticospinl tract
ventral horn of spinal cord
what are the events leading up to contraction?
ACH release from somatic motor neuron at NMJ
excitation contraction (E-C Coupling)
contraction-relaxation cycle (sliding filament theory)
what is the neuromuscular junction (MNJ and what are its three components?
the MNJ is the synapse of the motor neuron on a muscle fiber
three components
- motor neuron’s presynaptic axon terminal filled w synaptic vesicles and mitochondria
- synaptic cleft
- post synaptic membrane on muscle fiber
consists of
- axon terminals
- motor end plants on skeletal muscle membrane
- Schwann cells (which form a thin layer covering the top of the axon terminal; held with insulation and increasing conduction)
walk through the steps that occur at the MNJ when a stimulus takes place?
- A stimulus comes down the axon, and causes Ca 2+ to move down its electrochemical gradient (bc Ca is more concentrated on out); stimulus is an action potential and it causes Ca gates to open, allowing ca to flow
- Ca entry causes synaptic vesicles to just with pre synaptic membrane and release ACh into the synaptic cleft
- the motor end plate is the muscle cell membrane that is across the axon terminal; the motor end plate contains nicotinic receptors
- two ACh bind to ACh receptor on the muscle cell
- ACH receptors open nonspecific monocovalent cation channels; allow Na and K to pass
- there is a net influx of Na, causing the muscle fiber to depolarize
- ACH is broken down by acetylcholinesterase
explain the importance of the cation influx causing depolarization
once two ACH bind to a nicotinic receptor, nonspecific cation channels open
there’s an influx of sodium, causing depolarization (which is a graded potential)
IF THIS GRADED POTENTIAL IS STRONG ENOUGH, AN ACTION POTENTIAL WILL RESULT
these Ads can run along t-tubules, which stimulate calcium release from SR, which in turn causes myofibrils to contract
order these from largest to smallest
skeletal muscle
myofibril
fascicle
fiber (cell)
muscle > fascicle > fiber (cell) > myofibril
skeletal muscle is made up of muscle fascicles
muscle fascicles are bundles of muscle fibers (cells)
muscle fiber (cells) contain multiple myofibrils
where are t-tubules located, and what do they do?
the fiber surface has crevices, which are t-tubules
action potentials run along t-tubules to stimulate calcium release from SR
calcium causes myofibrils to contract
what are myofibrils made of?
myofibrils are organized myofilaments (myosin, actin, titin)
what are the generic “steps” of the sliding filament theory?
rigor state
ATP binds
ATP hydrolysis
calcium/cocked
Phosphate release (power stroke)
end stroke
explain the rigor state (SFT)
at first, myosin is tightly bound to the actin filament at a 45 degree angle
ATP is not bound yet
explain the ATP-binding stage (SFT)
when ATP binds to myosin, the myosin head dissociates from actin filament
explain ATP hydrolysis (SFT)
myosin ATPase hydrolyzes the bound ATP in ADP and P
this hydrolysis causes the myosin head to swing forward to become 90 degrees relative to the actin filament
explain calcium/cocked (SFT)
ADP and P are still bound to myosin protein
when the myosin head swings to 90 degrees, it binds weekly to actin filaments (about 1-3 actin units further than previous binding location)
in this stage, the myosin head is “cocked” and ready to perform a powerstroke
explain phosphate release/power stroke (SFT)
the phosphate group is released
this causes the power stroke, where the myosin head rotates back to a 45 degree angle
this pushes the actin filament, which is how the muscle is able to shorten (since this occurs multiple times in a short amount of time)
explain end stroke (SFT)
at the end of power stroke, myosin releases ADP
the myosin head is now in a tightly bound rigor state
myosin cross bridge is at a 45 degree angle relative to the filaments
myosin is now reset and ready to perform another power stroke once more ATP arrives
explain Botulinum Toxin impact
botox
blocks Ca
causes paralysis
presynaptic effect
explain curare’s impact
block nicotinic receptor at NMJ (it is a nicotinic receptor antagonist)
causes muscle relaxation; flack paralysis
fibers CANNOT CONTRACT
post synaptic effect
what are some force generation variables?
Contraction force of the muscle fiber itself
- how long is the sarcomere
- how thick aerie fibers
- how much intracellular calcium is released?
Number of muscle fibers
- how any fibers are in the motor unit
what is a twitch?
one cycle of excitation-contraction coupling
what is tetanus and how is it achieved?
tetanus is max possible tension
many twitches sum to produce tetanus
what is the difference between fused and unfused tetanus?
unfused:
- APs arrive at the motor end plate at a high frequency, causing the muscle fiber to contract repeatedly to max tension
- the rte of stimulation is high, but it is not at its max value
- muscle fibers able to relax somewhat between contractions
fused:
- APs arrive at motor endplate at a very high frequency, causing muscle fiber to contract to max tension
- the rate of muscle fiber stimulation is at its max value
- muscle fiber cannot relax between contractions and remains at max tension
how does twitch summation occur?
how does tetanus occur?
it occurs when a muscle fiber is restimulated before it has completely relaxed; the second twitch it added onto the first twitch, resulting in summation
tetanus occurs when a muscle fiber is stimulated so rapidly that it does not have an opportunity to relax at all between stimuli
explain the size principle and what it hopes to achieve
the size principle ensures that forces generated by the muscle fibers are the right amount for the task
the size principle maximizes energy efficiency and avoids fatigue
it activates fatigue-resistant muscle fibers before fatiguable muscle fibers (recruits small motor units first)
allows for fine motor control mediated by small motor units
explain the difference between small motor units and large motor units
small motor units
- contract slowly
- lower force generation
- fine motor control (think fingers)
- recruited by few action potentials
- high membrane resistance means APs propagate and cause muscle fiber contraction
large motor units
- contract quickly
- higher force generated
- coarse control (think quads)
- recruited by many action potentials
- low membrane resistance means APs do not travel (WHY IS THIS); needs large stimulus to cause fiber contraction
all of the fibers within a motor unit are…
All of the fibers within a motor unit are the SAME TYPE
explain properties of glycolysis
fast
inefficient (does not yield as much ATP)
does not rely on mitochondria
anaerobic
uses up energy faster than it can be produced
“white meat”
low levels of myoglobin, fewer mitochondria, not very vascular
large diameter fibers (large contractions)
explain properties of oxidative phosphorylation
slow
efficient (generates lots of ATP)
needs mitochondria
aerobic
keeps energy levels high enough to persist
red eat
high levels of myoglobin, lots of mitochondria, heavily vascularize
small diameter fibers (less contraction)
target of somatic pathway:
skeletal muscles
target of autonomic pathway:
cardiac muscle, smooth muscle, glands
