Skeletal Muscle (physiology) Flashcards
Are Skeletal muscles voluntary or involuntary muscles?
-voluntary
Where do our thoughts of voluntarily movement come from?
- Primary motor cortex
- sends to
What is the neuromuscular junction?
- synapse between somatic motor neuron and skeletal muscle fiber
Where is the origin of the motor neuron (cell body) of the neuromuscular junction?
- anterior horn of the spinal cord
What happens at the axon terminal directly above the muscle fiber?
- relay message by
- releasing neurotransmitter (ACh) to bind on the sarcolemma
The sarcolemma contains what kind of tubules?
-transverse tubule (t-tubule)
What is the function of the motor end plate?
-increase surface area for receptors to accept the neurotransmitters (ACh) from the vesicles
What is the goal of the neurotransmitter (ACh) being released into the muscle fiber?
-excite the muscle fiber for contraction
What type of receptor is needed for the muscle fiber to be excited?
- nicotinic cholinergic receptor
- when ACh binds, Sodium comes in , and a little K+ leaves
Once the muscle is excited it leads to what process?
-excitation-contraction coupling
What is the muscle fiber threshold?
-50mv
What is an end plate potential?
- localized depolarization (EPP)
- due to entry of NA+ ion
What kind of electrical signal does End plate potential give off?
- EPSP (excitatory postsynaptic potential)
- is a graded potential
Where is the end plate potential created?
Motor end plate
Can graded potentials be summed?
Yes
Where is the action potential created on the muscle fiber? Where does it go after?
- sarcolemma
- T tubule
Once the action potential reaches the t-tubule, which receptor conformed (altered)?
- DHP receptor (dihydropyridine) #3
- voltage sensitive
- linked to the sarcoplasmic reticulum
The DHP receptor (dihydropyridine) opens which ion channel in the sarcoplasmic reticulum?
- RyR Ca2+ (ryanodine Ca2+ release channels) #4
- Ca2+ enters cytoplasm
Once Ca2+ is released in the cytoplasm, where do they bind to?
- Ca2+ binds to troponin (of the thin filament) #5
- allows actin-myosin binding
Explain how the filaments are arranged in a relaxed state (not contracted)
- myosin head cocked
- tropomyosin partially blocks binding site on actin
- myosin is weakly bound to actin
-no calcium ions are available in a relaxed state
Explain what happens to the filaments during the initiation of contraction
- Cystolic Ca2+ increases
- pulls on the tropomyosin, exposing binding site on actin
-Actin is able to bind to myosin head
What is the importance of calcium ions during the initiation of contraction
- pulls the tropomyosin out of the way to expose the binding site on actin, so myosin can bind
What is the sliding filament theory?
When a muscle is contracted, what happens to the sarcomere (length)?
- shortens with contraction
- H zone and I band both shorten while A band remains constant
When a muscle undergoes relaxation, what is the movement of Ca2+?
- sarcoplasmic Ca2+ ATPase pumps Ca2+ back into SR. (Primary active transport) #8
- Decrease in cystolic Ca2+ causes Ca2+ to unbind from troponin #9
- Tropomyosin recovers binding site. When myosin heads release, elastic elements pull filaments back to their relaxed position #10
What is a muscle twitch?
-a single contraction/relaxation cycle in a muscle fiber and produces tension (force)
What is the latent period during the muscle twitch?
-time from start of muscle action potential to start of muscle tension development
=time required for excitation-contraction coupling to occur
The tension of a muscle twitch depends on what?
-sarcomere length: the more # of cross bridges(myosin binding with actin)=increase in tension
What is the optimal sarcomere operating length?
-80%-120% of resting length
How is the optimal sarcomere length maintained?
-CNS maintains resting muscle length near optimal by maintaining a muscle tone
Describe the summation of muscle twitches
-occurs when successive stimuli arrive before the relaxation phase has been completed
When does incomplete (unfused) tetanus occur during a muscle twitch?
-when the muscle fiber relaxes slightly between stimuli and tension increases
When does a complete (fused) tenanus occur during a muscle twitch?
- when a muscle fiber doesn’t relax between stimuli and reaches max tension development
- relaxation phase is eliminated
What are the characteristics of a slow twitch muscle fiber(type I)?
- most used
- dark red (myoglobin)
- oxidative;aerobic
- slow
- fatigue resistant
What are the characteristics of fast twitch muscle fibers (type II)?
- least used (jumping, quick, fine movements)
- glycoltic (more anaerobic)
- pale color
- fast
- large in diameter
- short contractions
- easily fatigued
What is a motor unit?
- a motor neuron that innervates a group of muscle fibers
What is fatigue?
-when muscle can’t generate/sustain power output
What are the 2 types of fatigue
-central and peripheral fatigue
What does the new research say about the cause of muscle fatigue?
- failure of Exictation-contraction coupling
- lactate accumulation is no longer the cause of fatigue
What are the two types of contraction mechanics?
- isotonic contraction
- isometric contraction
What happens to the muscle during a isotonic contraction?
-muscle contracts, shorts and creates enough force to move a load
What happens to the muscle during an isometric contraction?
-the muscle contracts but does not shorten. The force created cannot move the load
Voluntary movements can be divided into what 3 phases?
1) Planning
2) initiation
3) execution
Where are the muscle spindles (proprioceptors) found ?
-buried among the extrafusal fibers of the muscle
What is the function of the muscle spindles (proprioceptor)?
- detects stretch of muscle fibers
- sends info to CNS
- maintains muscle tone
- spindles are tonically active and firing even when muscle is relaxed
- generates muscle spindle reflex
- can trigger a stretch reflex
What is the function of the golgi tendon organs (proprioceptor)?
-responds to muscle tension during isometric contraction
- golgi tendon reflex
- protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load
What are the 2 somatic motor reflexes?
- monosynaptic reflex
- polysynaptic reflex
What are the connections of the monosynaptic reflex?
-has a single synapse between the afferent and efferent neurons
What are the connections of the polysynaptic reflex?
-has two or more synapses. This somatic motor reflex has both synapses in the CNS
Describe the patellar tendon (knee jerk) reflex
-is a monosynaptic stretch reflex and reciprocal inhibition of the antagonistic muscle
Describe the crossed extensor reflex
- a flexion reflex in one limb causes extension in the opposite limb.
- The coordination of reflexes with postural adjustments is essential for maintaining balance
