Muscles
Ch.12 Our patient for the day... Name: Eddy Age: Newborn
Whole-body muscle contractions
No relaxation
Severe difficulty breathing due to inadequate relaxation of breathing muscles
Diagnosed with tetanus Stepping on a rusty nail with a certain type of bacteria can be baaaaaad
Let’s talk about why. Our patient for the day... Name: Eddy Age: Newborn
Whole-body muscle contractions
No relaxation
Severe difficulty breathing due to inadequate relaxation of breathing muscles
Diagnosed with tetanus Signaling a muscle to start contraction
ACh Nicotinic receptor
Target: skeletal muscle CNS Figure 8.19a ESSENTIALS – Synaptic Communication
Neurotransmitter Release
An action potential depolarizes the axon terminal.
Synaptic vesicle Action potential with neurotransmitter The depolarization opens arrives at molecules voltage- gated Ca2+ channels, axon terminal and Ca2+ enters the cell.
Calcium entry triggers exocytosis of synaptic vesicle contents. Docking protein 2+ Ca Neurotransmitter diffuses Synaptic across synapse and binds to cleft post-synaptic receptors.
Neurotransmitter binding initiates a response in the postsynaptic cell. Voltage-gated Receptor 2+ Postsynaptic cell Ca channel Cell response
If this Somatic Motor Pathways Trigger Action Potentials Within Skeletal Muscle
Synaptic vesicle (ACh)
Ca2+ Ca2+
ACh Acetyl + choline Voltage-gated Ca2+ channels
AChE
Nicotinic receptors bind Skeletal muscle ACh, opening Na+ channels, fiber triggering action potentials and contraction in skeletal muscle. Why do APs in the muscle cause them to contract? To learn about something’s function, we can start by taking a look at its structure.
Let’s take a look under the microscope Anatomy review Skeletal muscle composed of many of muscle fibers/cells
Tendon Skeletal muscle (connects bone to muscle) Nerve and blood vessels Connective tissue
Muscle fascicle: bundle of fibers Connective tissue
Nucleus
Muscle fiber Skeletal muscle composed of many of muscle fibers/cells
Tendon Skeletal muscle (connects bone to Whole muscle muscle) Nerve and blood vessels Muscle fascicle Connective tissue
Muscle fiber/cell Muscle fascicle: bundle of fibers Connective tissue
Nucleus
Muscle fiber Figure 12.1
THE THREE TYPES OF MUSCLES HAVE DIFFERING STRUCTURES/APPEARANCES
Skeletal muscle fibers are large, multinucleate cells that appear striated (striped) under the microscope. Control the Nucleus movement of our skeleton
Muscle fiber (cell)
Striations *multinucleated=many nuclei in same cell
Cardiac muscle fibers are also Nucleus striated but they are smaller, branched, and uninucleate. Cells are joined in series by junctions Muscle fiber called intercalated disks.
Intercalated disk
Striations
Smooth muscle fibers are small, uninucleate, and lack striations.
Nucleus
Muscle fiber The basic functional unit of a myofibril is the sarcomere A band Sarcomere Z disk Z disk
ANATOMY SUMMARY Myofibril
M line I band H zone
Titin
Z disk Z disk M line Myosin crossbridges
M line Thick filaments Thin filaments A band: overlap of thick and thin filaments
I band: only thin filamentsTitin
Troponin Nebulin Myosin heads
Hinge Myosin tail region
Tropomyosin G-actin molecule
Myosin molecule Actin chain The basic functional unit of a myofibril is the sarcomere A band Sarcomere Z disk Z disk
ANATOMY SUMMARY Myofibril
M line I band H zone
Titin
Z disk Z disk M line Myosin crossbridges
M line Thick filaments Thin filaments
Titin
Troponin Nebulin Myosin heads
Hinge Myosin tail region
Tropomyosin G-actin molecule
Myosin molecule Actin chain Muscle cells and tissue use unique names for some common things End anatomy review Muscle fibers composed of subunits called myofibrils
Mitochondria (energy source)
Sarcoplasmic reticulum
Nucleus Thick Thin filament filament
T-tubules Now let’s look at one individual muscle fiber/cell and its part Myofibril Sarcolemma
Sarcolemma=Cell membrane Sarcoplasmic Retic.=Modfied Endoplasmic Reticulum T-tubules=invaginations of sarcolemma Muscle fibers composed of subunits called myofibrils
Mitochondria (energy source) Sarcoplasmic reticulum (Ca2+ storage)
Nucleus
T-tubules
Sarcolemma (cell membrane)
Sarcolemma=Cell membrane Sarcoplasmic Retic.=Modified Endoplasmic Reticulum T-tubules=invaginations of sarcolemma Figure 12.10a ESSENTIALS – Excitation-Contraction Coupling and Relaxation Slide 2 https://www.youtube.com/watch?v=8Hu5W_tFXLs
Axon terminal of somatic motor neuron KEY DHP = dihydropyridine L-type calcium channel
RyR = ryanodine receptor-channel Muscle fiber ACh nt ial Let’s look in a bit more detail about how e
Somatic motor neuron releases calcium+ gets released from the sarcoplasmic Na ACh a neuromuscular junction.
Action pot pot Action reticulum.... Motor end plate + RyR Net entry of Na through ACh receptor-channel initiates a T-tubule muscle action potential. Ca2+ Sarcoplasmic reticulum Z disk DHP Troponin Actin Tropomyosin M line Myosin head
Myosin thick filament
© 2013 Pearson Education, Inc. Figure 12.4 T-TUBULES HELP SPREAD ACTION POTENTIAL IN FIBER T-tubules are extensions of the cell membrane (sarcolemma) that associate with the ends (terminal cisternae) of the sarcoplasmic reticulum.
T-tubule brings action potentials into interior Sarcoplasmic of muscle fiber. reticulum stores Ca2+. Sarcolemma AP in muscle fiber
Triad Thick Thin Terminal filament filament cisterna Figure 12.10b ESSENTIALS – Excitation-Contraction Coupling and Relaxation Slide 5 https://www.youtube.com/watch?v=IOkn1ldFO60
DHP KEY DHP = dihydropyridine L-type calcium channel
RyR = ryanodine receptor-channel RyR
Action potential in t-tubule Let’s look in a bit more detailalters conformationabout ofhow DHP calcium gets released from thereceptor. sarcoplasmic DHP doesn’t open! DHP receptor opens RyR Ca2+ RyR reticulum.... release channels in sarco- DHP plasmic reticulum, and Ca2+ enters cytoplasm.
Ca2+ released Ca2+ binds to troponin, allowing actin-myosin binding.
Myosin thick filament Myosin heads execute power stroke.
Distance actin moves Actin filament slides toward center of sarcomere.
© 2013 Pearson Education, Inc. Figure 12.8b (2 of 7) Slide 5 Figure 12.10b ESSENTIALS – Excitation-Contraction Coupling and Relaxation https://www.youtube.com/watch?v=sIH8uOg8ddw Slide 5
2+ Cytosolic Ca2+ Ca levels increase in cytosol. DHP Tropomyosin KEY shifts, exposing DHP = dihydropyridine L-type binding site on Ca 2+ binds calcium channelto actin. troponinRyR = ryanodine (TN). receptor-channel RyR
Action potential in t-tubule Let’s look in a bit more detailTroponin-Caalters conformationabout2+ ofhow DHP TN complexreceptor. DHP pulls doesn’t open! calcium gets released from thetropomyosin sarcoplasmic Actin 2+ awayDHP receptor from opensactin RyR’s Ca RyR reticulum....moves release channels in sarco- myosin-binding site.2+ DHP ADP plasmic reticulum, and Ca enters cytoplasm. Power stroke Ca2+ released P Ca2+ binds to troponin, i Myosinallowing actin-myosin binds strongly binding. to actin and completes power stroke. Myosin thick filament Myosin heads execute power stroke.
Distance actin moves ActinActin filament filament slides toward moves.center of sarcomere. See video for how myosin uses ATP to complete power stroke © 2013 Pearson Education, Inc. © 2013 Pearson Education, Inc. After death, an animal can experience rigor mortis, the state of rigidity due to sustained muscle contraction....even after death.
Rigor mortis is due to lack of production of ATP. WTH?!
NO ATP leads to sustained muscle contraction?!?!
Let’s watch the video again... https://www.youtube.com/watch?v=sIH8uOg8ddw A normal muscle cell at rest (no sarcoplasmic Ca2+, ATP hydrolyzed into ADP + P by myosin)
Ca2+ released Ca2+ not released
A normal excited, muscle cell Binding sites for myosin unexposed, (sarcoplasmic retic. Ca2+ released) No contraction A normal excited, muscle cell (sarcoplasmic retic. Ca2+ released) Myosin binds to actin
ATP present? Myosin detaches and hydrolyzes ATP. Myosin resets
If Ca2+ still around too, generate another power stroke
Myosin releases ADP and P, generates power stroke/force A normal excited, muscle cell (sarcoplasmic retic. Ca2+ released) Myosin binds to actin Rigor mortis: hours after death, SR membrane breaks down.
Sarcoplasm Ca2+ Force generated Available ATP Myosin gets stuck in this Myosin releases ADP and P, generates conformation power stroke/force A normal excited, muscle cell (sarcoplasmic retic. Ca2+ released) Myosin binds to actin How do we get the muscle to relax normally?
Myosin releases ADP and P, generates power stroke/force FigureFigure 12.10b12.10c ESSENTIALSESSENTIALS –– Excitation-ContractionExcitation-Contraction Coupling Coupling and and Relaxation Relaxation SlideSlide 5 3
https://www.youtube.com/watch?v=IOkn1ldFO60KEY DHP = dihydropyridine L-type calcium channel KEY RyR = ryanodine receptor-channel DHP = dihydropyridine L-type calcium channel Action potential in t-tubule Let’s look in a bit more detailaltersRyR conformation =about ryanodine receptor-channel ofhow DHP receptor. calcium gets released from theSarcoplasmic sarcoplasmic Ca2+-ATPase DHPpumps receptor Ca2+ backopens into RyR SR. Ca 2+ reticulum.... release channels in sarco- plasmic reticulum, and Ca2+ entersDecrease cytoplasm. in free cytosolic ATP [Ca2+] causes Ca2+ to unbind Ca2+ releasesCa2+ released Ca2+ from troponin. Ca2+ binds to troponin, allowing actin-myosin binding. Tropomyosin re-covers binding site. When myosin heads release, elastic elements pull MyosinMyosin thickthick filamentfilament Myosinfilaments heads back execute to their power relaxed stroke.position.
Distance actinactin movesmoves Actin filament slides toward center of sarcomere. AP stops, RyR closes, Ca2+ return > Ca2+ release: Muscle relaxes © 2013 Pearson Education, Inc. © 2013 Pearson Education, Inc. You have decided to move your arm. So, you need to send an efferent signal to your arm from your CNS and make your arm muscle contract. Which ion is most important for triggering muscle contraction during this whole process?
A.) K+ B.) Na+ C.) Ca2+ D.) Acetylcholine A muscle cell has been depolarized and already is having its action potential. Which ion is most important for triggering muscle contraction after this point?
A.) K+ B.) Na+ C.) Ca2+ D.) Norepinephrine Some unique and interesting properties of muscle fibers 1. Delay between excitation and contraction Figure 12.11
There’s a delay between excitation of the muscle and contraction
Action potentials in the axon terminal (top graph) and in the muscle fiber (middle graph) are Motor Neuron Action Potential followed by a muscle twitch (bottom graph).
+30 Muscle fiber Neuron membrane Action potential potential from CNS in mV
−70
Time
Motor Recording end plate electrodes Axon Muscle Fiber Action Potential terminal
+30 Muscle fiber Muscle action membrane potential potential in mV
−70 2 msec
N A V I G A T O R Time
Neuro- muscular junction Development of Tension During One Muscle Twitch (NMJ)
E-C Latent Contraction Relaxation coupling period phase phase Tension
Muscle twitch 10–100 msec FIGURE QUESTIONS Time Movement of what ion(s) in What’s causing thiswhat direction(s) delay creates between electrical activity (a) the neuronal action potential? and force(b) the generation muscle action potential? in the muscle? Figure 12.10 ESSENTIALS – Excitation-Contraction Coupling and Relaxation
Excitation
Contraction
Lots of steps in between excitation and contraction leads to ~2-3msec delay 2. Energy storage/availability Figure 12.12
Phosphocreatine
Resting muscle stores energy from ATP in the high-energy bonds of phosphocreatine. Working muscle then uses that stored energy.
Muscle at rest
creatine ATP from metabolism + creatine ADP + phosphocreatine kinase
Working muscle
creatine Phosphocreatine + ADP Creatine + ATP kinase
needed for When these reserves are depleted, ATP is made through aerobic and • Myosin ATPase (contraction)
anaerobic glycolysis. • Ca2+-ATPase (relaxation)
• Na+-K+-ATPase (restores ions that cross cell membrane during action potential to their original compartments) 3. Ideal muscle length Figure 12.15 Adapted from A. M. Gordon et al., J Physiol 184: 170–192, 1966.
LENGTH-TENSION RELATIONSHIPS
Too much or too little overlap of thick and thin filaments in resting muscle results in decreased tension.
C
B D
100
80
60
40 A E
20
0 1.3 µ m 2.0 µ m 2.3 µ m 3.7 µ m Tension/Force of max) percentage (as Decreased Increased length length Optimal resting length
Sarco mere l en g th 4. Motor unit recruitment can help a whole muscle increase force generated Figure 12.17 MOTOR UNITS
A motor unit consists of one motor neuron and all the muscle fibers it innervates. A muscle may have many motor units of different types.
One muscle may have SPINAL CORD many motor units of different fiber types.
Neuron 1 Neuron 2 Neuron 3 Motor nerve
KEY
Muscle Motor unit 1 fibers Motor unit 2
Motor unit 3
How do we get fine motor movement (e.g. hands): only a few muscle fibers per motor unit. Allows for more precise control of movement/fiber activation