“Walk-Along” Theory Figure 6-7; Guyton & Hall

Contraction of Skeletal Muscle Chapter 6: Contraction of Skeletal Muscle Slides by Thomas H. Adair, PhD

Anatomy of Skeletal Muscle Gross organization: Nuclei along length of muscle fiber come from satellite cells during formation of fiber. Figure 6-1; Guyton & Hall

Cellular Organization Muscle fibers single cells multinucleated surrounded by the sarcolemma Myofibrils contractile elements surrounded by the sarcoplasm Sarcolemma = true plasma membrane plus outer cost pf polysaccharide material with collagen fibers that fuses with tendon. The SR is the ER of skeletal muscle. Note error in book – calls this extracellular space in figure legend. Cellular organelles - lie between myofibrils (mitochondria, sarcoplasmic reticulum etc.) Figure 6-1; Guyton & Hall

Molecular Organization Figure 6-1; Guyton & Hall

The Sarcomere sarcomere A band M line H zone Z disc I band thick filament (myosin) thin filament (actin) titin (filamentous structural protein)

“Sliding Filament” Mechanism Contraction results from the sliding action of interdigitating actin and myosin filaments RELAXED: CONTRACTED:

The Actin Filament F-actin troponin tropomyosin the I band filament tethered at one end at the Z disc 1 mm long: v. uniform nebulin forms guide for synthesis Figure 6-6; Guyton & Hall F-actin double-stranded helix composed of polymerized G-actin ADP bound to each G-actin (active sites) myosin heads bind to active sites tropomyosin covers active sites prevents interaction with myosin almost exactly 1 mm long in vertebrate striated muscle (v. uniform). This is regulated by a thin filament associated protein, ‘nebulin’. Nebulin appears to form a guide that determines how long the actin filament will be. troponin I - binds actin T - binds tropomyosin C - binds Ca2+

The Myosin Molecule: two heavy chains (MW 200,000) four light chains (MW 20,000) “head” region - site of ATPase activity Figure 6-5; Guyton & Hall

Mechanism of Muscle Contraction Theory: Binding of Ca2+ to troponin results in a conformational change in tropomyosin that “uncovers” the active sites on the actin molecule, allowing for myosin to bind.

“Sliding Filament” Mechanism Contraction results from the sliding action of interdigitating actin and myosin filaments RELAXED: CONTRACTED:

Neuromuscular Transmission - The Neuromuscular Junction - Figure 7-1; Guyton & Hall Specialized synapse between a motoneuron and a muscle fiber Occurs at a structure on the muscle fiber called the motor end plate (usually only one per fiber)

Neuromuscular Junction (nmj) Synaptic trough: invagination in the motor endplate membrane Synaptic cleft: 20-30 nm wide contains large quantities of acetylcholinesterase (AChE) Subneural clefts: increase the surface area of the post-synaptic membrane Ach gated channels at tops Voltage gated Na+ channel in bottom half Figure 7-1; Guyton & Hall

The Motoneuron – vesicle formation Synaptic vesicles: are formed from budding Golgi and are transported to the terminal by axoplasm “streaming” (~300,000 per terminal) Acetylcholine (ACh) is formed in the cytoplasm and is transported into the vesicles (~10,000 per) Ach filled vesicles occasionally fuse with the post-synaptic membrane and release their contents. This causes miniature end-plate potentials in the post-synaptic membrane.

The Motoneuron - ACh Release 3 AP begins in the ventral horn of spinal cord. Local depolarization opens voltage-gated Ca2+ channels. An increase in cytosolic Ca2+ triggers the fusion of ~125 synaptic vesicles with the pre-synaptic membrane and release of ACh (exocytosis). 1 AP Ca2+ 2

ACh Release - details Ca2+ channels are localized around linear structures on the pre-synaptic membrane called dense bars. Vesicles fuse with the membrane in the region of the dense bars. Ach receptors located at top of subneural cleft. Voltage gated Na+ channels in bottom half of subneural cleft. Figure 7-2; Guyton & Hall

End Plate Potential and Action Potential - at the motor endplate - ACh released into the neuromuscular junction binds to, and opens, nicotinic ACh receptor channels on the muscle fiber membranes (Na+, K+, Ca2+). Opening of nACh receptor channels produces an end plate potential, which will normally initiate an AP if the local spread of current is sufficient to open voltage sodium channels. mV 40 -40 -80 What terminates the process? acetylcholinesterase nAChr Na channel 0 15 30 45 60 75 mSec

Drug Effects on End Plate Potential - Inhibitors - Curariform drugs (D-turbocurarine) block nicotinic ACh channels by competing for ACh binding site reduces amplitude of end plate potential therefore, no AP “normal” threshold curare botulinum toxin Botulinum toxin decreases the release of Ach from nerve terminals insufficient stimulus to initiate an AP Figure 7-4; Guyton & Hall

Drug Effects on End Plate Potential - Stimulants - ACh-like drugs (methacholine, carbachol, nicotine) bind and activate nicotinic ACh receptors not destroyed by AChE – prolonged effect Anti-AChE (neostigmine, physostigmine, diisopropyl fluorophosphate or “nerve gas”) block the degradation of ACh prolong its effect

Myasthenia Gravis Incidence / symptoms: Cause: Treatment: paralysis - lethal in extreme cases when respiratory muscles are involved 2 per 1,000,000 people / year Cause: autoimmune disease characterized by the presence of antibodies against the nicotinic ACh receptor which destroys them weak end plate potentials Treatment: usually ameliorated by anti-AChE (neostigmine) increases amount of ACh in nmj

Lambert-Eaton Myasthenic Syndrome Incidence / symptoms: 1 per 100,000 people / year 40% also have small cell lung cancer muscle weakness/paralysis Cause: LEMS results from an autoimmune attack against voltage-gated calcium channels on the presynaptic motor nerve terminal. weak end plate potentials Treatment: can be treated with anti-AChE (neostigmine) increases amount of ACh in nmj

Excitation-Contraction Coupling Transverse tubule / SR System T-tubules: Invaginations of the sarcolemma filled with extracellular fluid Penetrate the muscle fiber, branch and form networks Transmit AP’s deep into the muscle fiber Sarcoplasmic Reticulum: terminal cisternae and longitudinal tubules terminal cisternae form junctional “feet” adjacent to the T- tubule membrane intracellular storage compartment for Ca2+ Figure 7-5; Guyton & Hall

Arrangement of T-tubules to Myofibrils - Skeletal muscle vs cardiac muscle - Vertebrate skeletal muscle: Two T-tubule networks per sarcomere Located near the ends of the myosin filaments (zone of overlap) Cardiac muscle (and lower animals): Single T-tubule network per sarcomere Located at the level of the Z disc Figure 7-5; Guyton & Hall

EC Coupling - the “Triad” the junction between two terminal cisternae and a T-tubule dihydropyridine receptor: it’s a voltage sensor T-tubule Terminal cisterne of SR ryanodine Ca2+ release channel

EC Coupling – how it works (skeletal muscle) AP Sequence of Events: AP moves along T-tubule The voltage change is sensed by the DHP receptor. Is communicated to the ryanodine receptor which opens. (VACR) Contraction occurs. Calcium is pumped back into SR. Calcium binds to calsequestrin to facilitate storage. Contraction is terminated. AP Ca2+ pump calsequestrin

Clinical Oddity: Malignant Hyperthermia Symptoms: spontaneous combustion skeletal muscle rigidity lactic acidosis (hypermetabolism) Cause: triggered by anesthetics (halothane) familial tendency - can be tested for by muscle biopsy constant leak of SR Ca2+ through ryanodine receptor Why is so much heat generated? Ca pump Our bodies are only about 45% energy efficient. 55% of the energy appears as heat. ATP

Muscle Mechanics

Tension as a Function of Sarcomere Length sarcomere length (mm) 1 2 3 4 0.5 Stress is used to compare tension (force) generated by different sized muscles stress = force/cross-sectional area of muscle; units kg/cm2) In skeletal muscle, maximal active stress is developed at normal resting length ~ 2 mm At longer lengths, stress declines - At shorter lengths stress also declines - Cardiac muscle normally operates at lengths below optimal length - Normal operating range active stress (tension)

Frequency Summation of Twitches and Tetanus Myoplasmic [Ca2+] Force AP Time (1 second) Fused tetanus Myoplasmic Ca2+ falls (initiating relaxation) before development of maximal contractile force If the muscle is stimulated before complete relaxation has occurred the new twitch will sum with the previous one etc. If action potential frequency is sufficiently high, the individual contractions are not resolved and a ‘fused tetanus’ contraction is recorded.

Motor Unit: A collection of muscle fibers innervated by a single motor neuron large diameter All fibers are same type (fast or slow) in a given motor unit Small motor units (eg,larnyx, extraocular) as few as 10 fibers/unit precise control rapid reacting Large motor units (eg, quadriceps muscles) as many as 1000 fibers/unit coarse control slower reacting Motor units overlap, which provides coordination Not a good relation between fiber type and size of motor unit Size of motor unit does not seem to dictate fiber type – extraocular muscle is fast but motor unit is small. Soleus muscle is slow but motor unit is large. Large, fast muscle for sprinting etc have large motor units. Smaller motor neurones tend to be more tonically active, and to innervate fewer fibres (smaller motor units); the fibres tend to be slow oxidative types. Large motor neurones tend to be phasically active (silent mostly, with brief intense bursts of activity), and have large motor units, with fast, glycolytic fibres. Numbers of fibres per motor unit vary from 10 for small muscles with finely graded action (extraocular muscles) to 1000 for large postural muscles.

Relationship of Contraction Velocity to Load no afterload: maximum velocity at minimum load A increased afterload: contraction velocity decreases B contraction velocity is zero when afterload = max force of contraction A: larger, faster muscle (white muscle) B: smaller, slower muscle (red muscle)

Types of Skeletal Muscle - speed of twitch contraction - Speed of contraction determined by Vmax of myosin ATPase. High Vmax (fast, white) rapid cross bridge cycling rapid rate of shortening (fast fiber) Low Vmax (slow, red) slow cross bridge cycling slow rate of shortening (slow fiber) Most muscles contain both types of fiber but proportions differ All fibers in a particular motor unit will be of the same type i.e., fast or slow. Figure 6-12; Guyton & Hall

- resistance to fatigue - Types of Skeletal Muscle - resistance to fatigue - Slow (red muscle) fast and slow fibers show different resistance to fatigue slow fibers oxidative small diameter high myoglobin content high capillary density many mitochondria low glycolytic enzyme content fast fibers glycolytic force (% initial) Fast (white muscle) Fast fibres that are resistant to fatigue are sometimes called ‘intermediate’ or type Iia. Sometimes also FOG for Fast Oxidative Glycolytic or even FR for Fast ResistantThese are quite rare in humans. Type Iia fibres have high concentrations of myoglobin and oxidative enzymes cardiac has quite slow myosin ATPase rates and has other properties that make it more similar to type I fibres. Type IIB or just type II fibres can be called FF for Fast Fatiguing or FG for Fast Glycolytic. The root cause of experimental (in vitro) fatigue is not known. It appears to be a metabolic limitation but it is not ATP since this is not drastically reduced in exercise exhausted (fatigued) muscle. In life the muscle is never metabolically fatigued. Exercise is halted before this occurs and this is a central nervous phenomenon. There is pain associated with severe exercise and this and other homeostatic markers (temperature, plasma metabolyte levels) are probably the ‘sensed’ variables. 5 60 time (min)

What do the different types do? Fast-twitch slow-twitch Fast, slow and intermediate twitch type muscle can be identified by histochemistry. In any muscle there will be a mixture of slow and fast fibers. Motor units containing slow fibers will be recruited first to power normal contractions. Fast fibers help out when particularly forceful contraction is required. Different people have different proportions of these types. There is little evidence that training alters these proportions in humans. Marathon 18% 82% Runners Swimmers 26 74 Average 55 45 man Weight 55 45 Lifters Sprinters 64 37 Jumpers 63 37 In animals the relative proportions of fibres that are type I can be altered by exercise regimes. There is little evidence of this in man. Exercise can however influence the biochemical capacities of muscle. For example, aerobic exercise can increase capillary density, myoglobin content and the number of mitochondria. This creates improvement is performance and fatigue resistance. There will also be similar changes to the capacity of the cardiovascular system. In trained atheletes it appears that performance is ultimately limited by the pulmonary system - supply of oxygen to the muscles is the limiting factor.

Conversion of Fiber Type - fast to slow - left AT Anterior tibialis – Predominantly fast twitch (upper) Stains light: few mitochondria Few, small capillaries Large fibers Electrical stimulation (10 Hz) via motor nerve (60 days) Stimulating fast muscle at the pace of a slow muscle converts fast twitch fibers to predominantly slow twitch fibers (lower) Stains dark: more mitochondria Many, large capillaries Larger fibers right AT

Muscle Contraction - force summation Force summation: increase in contraction intensity as a result of the additive effect of individual twitch contractions (1) Multiple fiber summation: results from an increase in the number of motor units contracting simultaneously (fiber recruitment) Figure 6-13; Guyton & Hall (2) Frequency summation: results from an increase in the frequency of contraction of a single motor unit

Muscle Remodeling - growth Hypertrophy (common, weeks) Caused by near maximal force development (eg. weight lifting) Increase in actin and myosin Myofibrils split Hyperplasia (rare) Formation of new muscle fibers Can be caused by endurance training Hypertrophy and hyperplasia Increased force generation No change in shortening capacity or velocity of contraction Lengthening (normal) Occurs with normal growth No change in force development Increased shortening capacity Increased contraction velocity hypertrophy lengthening hyperplasia

Muscle Remodeling - atrophy Causes of atrophy Denervation/neuropathy Tenotomy Sedentary life style Plaster cast Space flight (zero gravity) Muscle performance Degeneration of contractile proteins Decreased max force of contraction Decreased velocity of contraction Atrophy with fiber loss Disuse for 1-2 years Very difficult to replace lost fibers weeks atrophy with fiber loss months/ years