Major Functions Movement Support frame & joints Heat production

Properties of Muscle Cells Excitability – respond to stimulus by changing membrane potential Contractibility - shorten Extensibility – extend or stretch Elasticity – resume resting length after stretching

Types of Muscle Tissues

Connective Tissues Tendon – connects muscle to bone Ligament – connects bone to bone Strain vs. Sprain

I heard an ESPN commentator once say that an athlete strained his ACL (anterior cruciate ligament). Is that possible? I’ve also heard someone complain of a sprained bicep. Is that possible?

Connective Tissues Fascia Epimysium – around entire muscle Perimysium – divides muscle into bundles Endomysium – around each muscle fiber or cell

Microscopic Structure Muscle made up of… Muscle fibers = muscle cells Lots of nuclei & mitochondria Made of… Myofibrils – cords of proteins Thick filaments – Myosin Thin filaments – Actin, Troponin, Tropomyosin

UNDERSTANDING CHECK Endomysium Myofibrils Perimysium Thin Epimysium Thick

Microscopic Structure A band – thick & thin filaments H zone – thick only M line – vertical protein linking thick filaments I band – thin filaments only Z line – vertical protein connecting thin filaments Sarcomere – Z line to Z line

Microscopic Structure

The Sliding Filament Mechanism

At Rest Calcium ions within sarcoplasmic reticulum ATP bound to myosin head Tropomyosin blocking myosin binding sites

To begin the process… ACh (acetylcholine) released by neuron across synapse of neuromuscular junction Action potential generated opening Ca channels in SR membrane Calcium ions released into sarcoplasm

Contraction Calcium binds to troponin Binding sites exposed Cross bridges from myosin bind to new sites ATP breakdown – Energy as heat & to move cross bridges

Contraction Movement of cross bridges – “Power Stroke” Thin filaments drawn to center of sarcomere ATP reattaches to myosin head causing release Repeating cycle of cross bridge formation, movement, & release

Contraction Z lines drawn together Sarcomere shortens As action potential continues, more sarcomeres shorten, leading to muscle fiber contraction & ultimately muscle contraction

Rest/Relaxation of Muscle ACh release halted & any remaining is inactivated Ca ions pumped back to SR by active transport Tropomysoin blocking is restored – no binding sites No place for myosin to reconnect Thin filaments slide back to original position

Energy Needed For… Movement of cross bridges Detachment of cross bridges Return of calcium to SR

Rigor Mortis Starts 3-4 hours after death and peaks at 12 hours Dying cells cannot keep calcium out so it flows in, allowing formation of cross bridges ATP synthesis stops so none available for detachment Relaxation after 4-5 days when muscle proteins break down

Sources of Energy Muscles MUST have ATP But only store 4-6 seconds worth So they turn to other sources to regenerate ATP

Sources of Energy 1. ATP storage – 4-6 seconds 2. Creatine phosphate CP + ADP  Creatine + ATP Twice as much stored so lasts 10 seconds past ATP Replenished during rest

Sources of Energy 3. Glycogen stored in muscles Lasts seconds past CP Anaerobic – no oxygen – produces few ATP & lactic acid Aerobic – with oxygen – lots of ATP 4. Fatty acids Start around a minute; primary fuel after about 30 minutes

Energy Systems/Pathways Anaerobic ATP, CP, & glycolysis Activities that require surge of power but last only a few seconds Aerobic Glycolysis & fatty acids ATP generated at same rate you use it Prolonged, steady activities

Oxygen Debt In order to get the most ATP from glycogen, you need LOTS of oxygen Aerobic endurance Where do we get oxygen? So how do we get more oxygen when we exercise?

Oxygen Debt All debts must be re-paid. Therefore, we keep breathing hard after we stop exercising even though we no longer need the oxygen for muscle contraction Now called EPOC (Excess Postexercise Oxygen Consumption) Helps deal with lactic acid, replenish ATP, CP, & glycogen

Other muscle “issues” Lactic acid “burn” from anaerobic respiration DOMS – Delayed Onset Muscle Soreness Microscopic muscle tears & inflammation

Other muscle “issues” Muscle fatigue – Physiological inability to contract even if receiving a stimulus Ionic imbalance? Cramp/Spasm Hyperactive nerve-muscle reflex arc?

Muscular Responses All-or-none response Threshold stimulus – minimum stimulus to create a contraction If over threshold, action potential occurs & you get a FULL STRENGTH CONTRACTION But can’t we vary contraction strength?

Recruitment Motor unit – one motor neuron connecting to several muscle fibers Motor units have different thresholds Recruitment – adding motor units to vary the strength of contraction

Types of contractions Twitch – single stimulus Wave summation – receive second stimulus before first relaxes

Types of Contractions Tetanus – bombardment of stimuli at maximum intensity Incomplete/unfused tetanus – relaxation between stimuli Complete/fused tetanus – no relaxation; forceful, sustained contraction

Types of contractions Isometric – same length (no motion) Isokinetic – same speed Isotonic – same weight Concentric – shorten muscle Eccentric – controlled lengthening

Muscle Fiber Variations Every muscle has mix of all Genetically initiated proportions but can be modified by exercise “Slow Twitch” or Type I Highly dependent on aerobic pathways Slow contraction, slow fatigue Endurance, posture

Muscle Fiber Variations “Fast Twitch” or Type IIb Highly dependent on glycogen Fast contraction, fast fatigue Short & powerful motions Type IIa – Fast oxidative Intermediate Sprinting, walking