W3 skeletal muscle

Question 1

• Skeletal muscle

Answer 1

o Elongated multi-nucleated cells grouped into bundles surrounded by connective tissue sheaths and extend from their proximal site of origin to their distal attachment

Question 2

• Epimysium (E)

Answer 2

o Dense connective tissue that surrounds the entire muscle

o Also called deep facia

Question 3

• Perimysium (P)

Answer 3

o Connective tissue that surrounds groups of fibers to form fascicle
o Conveys large blood vessels and nerves into muscles

Question 4

• Endomysium (En)

Answer 4

o Delicate connective tissue that surrounds individual muscle fibers

Question 5

• Type I muscle fibers

Answer 5

o	Show up light on histology
o	Slow/red muscle fibers
o	More efficient over long periods of time 
o	Postural maintenance and endurance
o	ATP via oxidative phosphorylation 
o	High myoglobin levels 
o	Low speed of contractions
o	Large motor units (many muscle fibers innervated by single neuron)

Question 6

• Type II muscle fibers

Answer 6

o	Shows up dark on histology
o	Slow/white 
o	Short bursts of speed 
o	Fatigue quickly
o	ATP via anaerobic glycolysis 
o	High levels of glycogen 
o	Speed of contraction is high
o	Smaller motor units (small number of muscle fibers innervated by single neuron)

Question 7

• Sarcolemma

Answer 7

o Plasma membrane of muscle cells
o Contains numerous ion transport proteins and receptors
o Surrounded by basal lamina
o Invaginates finger-like projections to form transverse tubule system

Question 8

• T tubules

Answer 8

o Formed by sarcolemma

o Make contact with sarcoplasmic reticulum and transmit depolarization signal deep in the muscle

Question 9

• Sarcoplasmic reticulum (SR)

Answer 9

o Internal membrane network with no connection to extracellular space
o Makes junctional complexes with T tubules to form triads
o SR is site of intracellular Ca++ storage and release
o Important in regulation of contraction

Question 10

• Myofibrils

Answer 10

o Composed of thin filaments (actin) and thick filaments (myosin)
o Occupies 80% of cytoplasm
o Surrounded by mitochondria

Question 11

• Desmin

Answer 11

o Intermediate filament
o Anchored to inside of sarcolemma and extends from one myofibril to another
o Help organize cytoplasm and facilitate coordinated contraction

Question 12

• Sarcomeres

Answer 12

o	Z lines
o	I band
o	A band
o	H band
o	M line

Question 13

• Z lines

Answer 13

o Define boundaries of sarcomere

o Contains actin binding protein: alpha actinin

Question 14

• I band

Answer 14

o Composed of actin thin filaments

o Width decreases during contraction

Question 15

• A band

Answer 15

o Composed of overlapping thick and thin filaments

o Width remains constant during contraction

Question 16

• H band

Answer 16

o Central region of A band which contains only thick filaments
o Width decreases during contraction

Question 17

• M line

Answer 17

o Middle of H band

o Overlapping tails of thick filaments

Question 18

• Thin filaments

Answer 18

o G actin monomers
o Form double stranded and twisted F actin filament
o G actin monomers have binding sites for myosin, regularly spaced
o Forms a complex with tropomyosin and troponin complex (I, T, C)
o Tropomyosin runs in groove of F actin filaments and bins to troponin T (TnT)
o TnI inhibits binding of myosin
o TnC binds Ca++
o Nebulin molecule extends from Z line along the thin filament and stabilizes the thin filament

Question 19

• Thick filaments

Answer 19

o Composed of myosin (two identical HCs and two LCs)
o Each HC has a globular head that binds to actin and ATPase domain
o HC also has alpha helical coiled tail to myosin can self-assemble into a bipolar aggregate or filament
o Globular head and tail are separated by moveable hinge region
o ATP hydrolysis causes movement at hinge joint and affects interaction with actin
o LC plays a role in regulating activity of myosin head and titin

Question 20

• Titin

Answer 20

o thick filament that extends from Z line to center of sarcomere and controls assemble of thick filaments and provides elasticity to sarcomere

Question 21

• skeletal muscle in contraction

Answer 21

o thin filaments slide toward center of sarcomere
o length of thick and thin filaments does not change
o length of sarcomere decreases because thick and thin slide past one another causing Z lines to move closer together
o width of H and I band decrease
o sliding and tension facilitated by conformational changes in myofibrillar proteins through ATP hydrolysis and release of Ca++

Question 22

• steps in a single cycle

Answer 22

o ATP binds to myosin head causing dissociation of myosin from actin thin filament
o ATP is hydrolyzed by myosin head ATPase causing conformation change in myosin and movement at the hinge so angle between head and tail increases. This lines up the head with a new myosin binding site on the thin filament
o Myosin head attaches to thin filament
o Binding of myosin causes the release of Pi and a conformational change in the myosin and movement at the hinge such that the angel between the head and tail decreases. This causes a sliding of thin filament toward the center of the sarcomere.
o ADP released from myosin head and new cycle begins

Question 23

• Calcium cycle during contraction

Answer 23

o Depolarization spreads from sarcolemma to the T tubules which transport the signal to interior of muscle cell
o Depolarization of T tubule is sensed by L Type Ca++ channels in the membrane of the T tubule that induces opening of ryanodine sensitive Ca++ channels in the membrane of SR
o Ca++ stored in SR is release into cytoplasm of the muscle fiber increasing resting level of Ca++ from 10-7 mM to 10-5 mM
o Ca++ binds to TnC causing conformational change in the whole troponin complex
o Change in troponin alters conformation of tropomyosin and leads to uncovering of the myosin-binding sites on actin allowing thick filament to bind to actin
o Ca++ dependent ATPase mediates return of Ca++ to SR during relaxation

Question 24

• Neuron and muscle interactions

Answer 24

o Each fiber is innervated by axon of motor neuron via motor end plate (neuromuscular junction)
o Contraction begins with Ach release from synaptic vesicles
o Ach binds to receptors on sarcolemma and causes depolarization
o Action potential spreads to the T tubules which transmit signal to interior muscle cell and cause Ca++ release
o Acetylcholinesterase is released from basal lamina of synaptic cleft and eliminates Ach

Question 25

• Motor unit

Answer 25

o Motor axon and all the muscle fibers
o Can be small and large
o Small motor units have fewer muscle cells innervated per axon compared to large motor units
o Fine motor control muscles have small motor units
o Muscles for powerful movements have large motor units
o All motor units have single type of muscle fiber (can change if innervating motor neuron changes from one type to another)

Question 26

• Motor unit in damage

Answer 26

o Can change from one type to another
o Denervation causes atrophy and become flattened and angulated
o Reinnervation restores fiber size and shape, but could be innervated by a different type
o In ongoing axonal or neuronal damage and dropout: residual motor axons may innervate increasingly large numbers of myofibers leading to enlargement of motor units

Question 27

• Fiber type grouping

Answer 27

o Motor units being composed of one muscle fiber type

Question 28

• Grouped atrophy

Answer 28

o If axon is damaged, all fibers in a motor unit will atrophy

Question 29

• Sensory components of skeletal muscle

Answer 29

o Proprioceptors (sensory organs) and tendons provide feedback on contractile state of muscle, tendon tension and position of joint
o Muscle spindles have intrafusal muscle fibers and afferent nerve endings
o Stretching a muscle stimulates nerve endings and is sensed by spinal cord
o Appropriate motor neuron is stimulated and the stretched muscle contracts
o Simultaneously, sensory synapses on interneuron that inhibits motor neuron to antagonistic muscle allowing it to relax

Question 30

• Golgi tendon organ

Answer 30

o Bare sensory nerve ending in tendons that inhibits muscle contraction when it senses excess tension placed on the tendon by a muscle
o At the same time causes contraction of the antagonistic muscle group