Lecture 7: Skeletal Muscle Structure and Functions

function of mucles

movement, maintain posture, stabilize joints, generate heat

functional characteristics of muscles`

excitability, elasticity, contractility, extensibility

types of muscles

skeletal, smooth, cardiac

smooth muscle characteristics

• Location: wall of hollow organs, vessels, respiratory passageways

• Cell characteristics: tapered at each end, branching networks, nonstriated

• Control: Involuntary control

• Action: Produce peristalsis- contracts and relaxes slowly, may sustain contraction

cardiac muscle characteristics

• Location: wall of heart

• Cell characteristics: branching networks, special membranes (intercalated disks) between cells, single nucleus; lightly striated

• Control: Involuntary control

• Action: pumps blood out of heart; self-excitatory but influenced by nervous system and hormones

skeletal muscle characteristics

• Location: attached to bones

• Cell characteristics: long and cylindrical; multinucleated; heavily striated

• Control: Voluntary control

• Action: Produces movement at joints; stimulated by nervous system; contracts and relaxes rapidly

hierarchal organization of muscle

• bones

• muscle

  • surrounded by epimysium

• fascicles

  • surrounded by perimysium

• muscle fibers (cells)

  • covered by endomysium

• myofibrils

  • surrounded by sarcoplasmic reticulum

• sarcomere

• thick and thin filaments (myosin and actin)

the muscle fiber is surrounded by _____ and contains _______

endomysium, myofibrils

myofibrils are surrounded by _______, and contains ________

sarcoplasmic reticulum, sarcomeres (Z line to Z line)

sarcomeres contain

thick (myosin) and thin (actin) filaments

sarcomere

• basic unit of contraction

• has repetitive units of overlapping filaments w/ myofibrils

• Z-disc/line forms boundary b/w adjacent sarcomeres

myofilaments: M line

cytoskeletal proteins that stabilizes myosin

myofilaments: H zone

myosin only (no actin overlap)

myofilaments: A band

length of myosin (w/ some actin overlap); thick filaments

myofilaments: I band

actin ONLY (no myosin overlap); thin filaments

Nebulin

protein that aligns actin

actinin

protein that anchors actin to z-disk/line

titin

protein that provide elasticity and stabilize myosin

which nervous system branch is involved in skeletal muscle

somatic nervous system (of the peripheral nervous system)

skeletal muscle and nerves are organized into ________

motor units (neuron + all muscle fiber it innervates)

small motor units

has high precision BUT low force control

• a single motor neuron triggers fewer than 10 muscle fibers

  • example: in the eye and fingers

large motor units

has high force BUT low precision actions

• a single motor neuron triggers 1000-2000 in quadriceps, biceps, gastrocnemius

motor end plate

is the postsynaptic membrane of the muscle fiber

ACh combines with ________ _________ or is metabolized by ____________________

nicotinic receptors; acetylcholinesterase (AChE)

what are the effects of botulinum toxin (botox)?

blocks the release of ACh; interferes with synapsins that move vesicles containing ACh = prevents muscle contraction (paralysis)

what causes sliding of myofilaments?

sliding of myofilaments is triggered by myosin-actin cross bridge formations

what causes cross bridges?

when myosin head that extend out toward actin

myosin head contains

1. actin binding site

2. ATP binding site (location of myosin ATPase)

troponin

promote muscle contraction; frees binding site of actin filaments

tropomyosin

inhibit contraction; blocks binding sites

mechanism of skeletal muscle contraction (from motor end plate to cross bridge formation)

• AP to motor end plate (NMJ)

  • releases ACh into synaptic cleft

• ACh binds to receptors on sarcolemma (muscle cell membrane)

  • opens Na+ channels, sodium enters muscle fiber leading to depolarization and generation of new AP

• AP propagated and travels along sarcolemma and down through T-tubules

  • causing Ca2+ to be released to cytosol from the SR

• Ca2+ binds to troponin, changing its shape

• Tropomyosin moves away from myosin binding site on actin filament

• Myosin head attach to actin, forming cross bridge

• Binding of myosin trigger release of ADP and Pi causing myosin head to pivot and pull actin filament toward center of sarcomere = power stroke

  • sliding of actin over myosin shortens sarcomere = muscle contraction

in muscle contraction, which bands shorten and which DOES NOT shorten?

I and H bands: shorten

A bands: do not shorten

Role of ATP in muscle function

ATP hydrolyzed by ATPase and yields ADP, Pi and stored energy necessary for muscle contraction

1. myosin ATPase splits ATP to ADP and Pi

2. ADP and Pi remain attached to myosin

3. myosin attaches to actin

4. Pi is released, triggering power stroke

5. ADP then detaches to myosin and another ATP binds, causing cross-bridges to break

6. cross bridges detach and is ready to bind again

Role of Ca2+ in muscle function

1. Ca2+ binds to troponin

2. tropomyosin-troponin complex conformational change

3. exposures of myosin binding site in actin filament

4. cross bridges of myosin head with actin

role of Ca2+ in muscle contraction; receptors

DHP receptors detect the change in membrane potential as the action potential travels down the T-tubules; their activation opens ryanodine receptors in the SR, facilitating Ca²⁺ release

calcium is released from SR terminal cisternae thru calcium channels (ryanodine receptors - responsible for the release of calcium from the SR into the cytosol thru depolarization of T-tubules)

big picture: excitation-contraction coupling

1. NT release

2. depolarization (Na+ influx) of motor end plate

3. voltage activation of t-tubules (DHP receptor) that results in

4. activation of SR Ca2+ channels (ryanodine receptor)

what happens in muscle relaxation?

1. AP stop

2. Acetylcholinesterase (AChE) degrades and recycles ACh

3. DHP receptors returns to original conformation

4. Ca2+ release channels (Ry receptors) close

5. Ca2+ pumped back into SR through Ca2+-ATPase pumps (SERCA)

types of muscle fibers

1. slow twitch (type I)- red (red d/t myoglobin protein that carry oxygen)

2. intermediate (type IIa)

3. fast twitch (type IIb/x) - white (bigger stronger contractions)

• classified on basis of contraction speed

ATP generation of each of the muscle fibers

Type I - oxidative phosphorylation (36 molecules of ATP)

Type IIa - glycolysis (2 molecules of ATP)

**Type IIb/x - creatine phosphate (1 molecule of ATP)

Type I fiber characteristics

• aka slow oxidative (SO)

• red fibers

• aerobic metabolism

• fatigue-resistant

• well vascularized (high blood supply)

• numerous mitochondria (enzymes for aerobic metabolism)

• high myoglobin concentration

• small diameter

• ex) long distance race (marathon) “endurance”

myosin ATPase activity is slow

rate of Ca2+ uptake by SR is slow to intermediate

Type II fiber characteristics

fast twitch (type IIb or IIx fibers)

• aka fast glycolytic (FG)

• white fibers

• anaerobic metabolism (few mitochondria)

• large stores of glycogen (glycolysis)

• poorly vascularized

• highly fatigueable

• large diameter (POWER)

intermediate (type IIa fibers)

• aka fast oxidative glycolytic (FOG)

• aerobic metabolism

• moderate resistance to fatigue

myosin ATPase activity is fast

rate of Ca2+ uptake by SR is high