Muscle Physiology Twitch Contraction of muscle in response

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Muscle Physiology

Muscle Physiology

Twitch • Contraction of muscle in response to stimuli – Action potential in one

Twitch • Contraction of muscle in response to stimuli – Action potential in one or more muscle fibers • Phases – Lag/latent • Interval between stimulus application to the motor neuron to actual contraction – Contraction – Relaxation

 • Relationship between strength of stimulation and contraction – All or none law

• Relationship between strength of stimulation and contraction – All or none law of contraction • Threshold must be reached – Generation of action potential • Stimuli stronger than threshold level – Identical contraction to the stimuli @ threshold level – No difference in level of action potential being generated

Motor unit

Motor unit

 • Role of motor unit – Respond as a single unit to stimulation

• Role of motor unit – Respond as a single unit to stimulation – Generation of action potential by muscle fibers by neural action potential

 • Muscle response to a neural action potential – Graded • Different strength

• Muscle response to a neural action potential – Graded • Different strength of contraction • Number of motor units being stimulated by an action potential

Concepts of summation • Muscle – Many motor units – Innervation • Combination of

Concepts of summation • Muscle – Many motor units – Innervation • Combination of axons that branch from a neuron • Force of contraction – Number of motor units being stimulated • Multifiber/multiple motor unit summation

 • Relationship between level of stimulation and contraction – Subthreshold • No action

• Relationship between level of stimulation and contraction – Subthreshold • No action potential generated = no contraction – Threshold – Submaximal stimuli • Progressively increasing stimulation – Increased number of motor units being stimulated – Maximal stimuli • All motor units stimulated – Supramaximal stimuli • No additional effects

 • Increased level of stimuli – Increased number of motor units being stimulated

• Increased level of stimuli – Increased number of motor units being stimulated • Motor units being “recruited” • Number of muscle fibers in a motor unit – Different in different muscle – Depends on types of motion/contraction being generated • Delicate movement =fewer fibers per motor unit

 • Frequency of stimulation and contraction – Relationship between action potential and muscle

• Frequency of stimulation and contraction – Relationship between action potential and muscle contraction • Action potential completed before muscle completes its contraction phase • No relaxation of muscle required before stimulation of subsequent contraction by subsequent action potentials • Increased frequency of stimulation, increased frequency of contraction

 • Tetanus – Incomplete • Partial relaxation of muscle between contraction – Complete

• Tetanus – Incomplete • Partial relaxation of muscle between contraction – Complete • Rapid generation of action potential • No muscle relaxation – Increased force of contraction in response to increased action potential frequency • Frequency/multiple wave summation

 • Frequency of action potential – Increased frequency • Additional amount of Ca

• Frequency of action potential – Increased frequency • Additional amount of Ca ions – No/partial removal – Additional release in response to AP • Results – Increased force of contraction

 • Elasticity of connective tissue and sacroplasm • Treppe – Staircase effects of

• Elasticity of connective tissue and sacroplasm • Treppe – Staircase effects of increased frequency of stimulation • Second contraction generates greater force than the first in response to the same level of stimulation • Eventually reach the maximum level

 • Importance of treppe – Proper warm-up before exercise • Increased muscle efficiency

• Importance of treppe – Proper warm-up before exercise • Increased muscle efficiency – Increased blood flow – Increased muscle metabolism

Types of muscle contraction • Isometric – No change in length of muscle –

Types of muscle contraction • Isometric – No change in length of muscle – Change in force generated by muscle during contraction – Postural muscle • Maintenance of posture without altering the length of muscle

 • Isotonic contraction – No change in tension generated – Change in length

• Isotonic contraction – No change in tension generated – Change in length of the muscle • Movement of upper limbs and fingers • Concentric contraction – Tension of muscle great enough to overcome the load/resistance – Changes in length – Many movements

 • Eccentric contraction – Increased length of muscle in response to the resistance/load

• Eccentric contraction – Increased length of muscle in response to the resistance/load – Tension being maintained against the load

 • Muscle tone – Constant tension • Generated by non-synchronous contraction of a

• Muscle tone – Constant tension • Generated by non-synchronous contraction of a small percent of motor units – Maintenance of the posture

Fatigue • Decreased capacity – Work – Efficiency of performance • Psychological – Muscles

Fatigue • Decreased capacity – Work – Efficiency of performance • Psychological – Muscles can function – Mental perception • Muscular – Loss of ATP • Synaptic – Loss of acetylchorine

 • Physiological contracture – Inability to contract or relax – Loss of ATP

• Physiological contracture – Inability to contract or relax – Loss of ATP • No release of cross-bridges • Rigor mortis – Rigidity of muscle after death • No removal of Ca from sacroplasm due to loss of ATP • No release of cross-bridge

Energy source • ATP (1 -2 second full contraction) • Creatine phosphate/phosphocreatine (8 -10

Energy source • ATP (1 -2 second full contraction) • Creatine phosphate/phosphocreatine (8 -10 seconds full contraction) • Anaerobic metabolism (2 -3 min) – Glycolysis – Production of lactic acid – Less efficient • Aerobic metabolism (80 -90 % total ATP) – Requires oxygen – Slow process

 • Source of ATP – Glucose • Glycogen in the muscle – Fatty

• Source of ATP – Glucose • Glycogen in the muscle – Fatty acids • More energetic than glucose

Oxygen requirement • Deficit – Initiation of exercise – Lack of oxygen compared to

Oxygen requirement • Deficit – Initiation of exercise – Lack of oxygen compared to the demand – Delayed generation of ATP via aerobic metabolism • Recovery – Generation of ATP via aerobic metabolism – Restoration of homeostasis • Body temperature • Ion concentrations • Metabolite/hormone levels

Muscle fiber types and contraction • Slow twitch muscle/type I – Slower contraction –

Muscle fiber types and contraction • Slow twitch muscle/type I – Slower contraction – Smaller fiber diameter – Extensive vasculature – Higher mitochondria and myoglobulin concentrations • Dark appearance – Slow ATPase on myosin heads – More fatigue resistant

 • Fast twitching muscle/type II – Faster response to nervous stimulation – Fast

• Fast twitching muscle/type II – Faster response to nervous stimulation – Fast ATPase on myosin heads – Less vascurature – Less myoglobulin and mitochondria • Lighter color – Higher glycogen content – More susceptible to fatigue

Muscle growth • Exercise – Increased size and capacity • Both muscle types –

Muscle growth • Exercise – Increased size and capacity • Both muscle types – Increased anaerobic metabolism (weight lifting) • Enlargement of fast-twitch muscle – Increased aerobic metabolism (running) • Enlargement of slow-twitch muscle – No conversion of muscle type

 • Increased force of contraction – Increased recruitment (neuromascular coordination) • Enlargement of

• Increased force of contraction – Increased recruitment (neuromascular coordination) • Enlargement of muscle – Increase accumulation of myofilaments – Increased mitochondria and blood supply • Increased metabolism (endurance) – Results • Hypertrophy (majority of muscle growth) • Hyperplasia (rare)

 • Atrophy – Decreased muscle size and function due to lack of use

• Atrophy – Decreased muscle size and function due to lack of use – Irreversible – Prevention • Exercises (contraction and extension) – Prevention of contracture formation in people with spinal cord damage

Heat production • ATP metabolism during muscle contraction – Release of heat • Normal

Heat production • ATP metabolism during muscle contraction – Release of heat • Normal body temperature – Rate of heat production • Related to rate of contraction – Shivering • Uncoordinated contraction of skeletal muscle • Generation of heat in response to cold air temperature – Reduced body core temperature

Smooth muscle • More variable functions • Shaped differently – Spindle shaped cells –

Smooth muscle • More variable functions • Shaped differently – Spindle shaped cells – No cell fusion • Single cell per fiber – Arrangement of actin and myosin • Bundle – Action to myosin ratio • > 2: 1

 • Less developed SR – Caveolae – Ca ions from the extracellular fluid

• Less developed SR – Caveolae – Ca ions from the extracellular fluid for contraction

Types • Unitary smooth muscles – More common – Sheet • Digestive tracts •

Types • Unitary smooth muscles – More common – Sheet • Digestive tracts • Reproductive tracts • Urinary tracts – Cells connected via gap junction • Transfer of action potentials between cells – Often autorythmic

 • Multiunit – Sheet – Contracts only when stimulated • Neural • Hormonal

• Multiunit – Sheet – Contracts only when stimulated • Neural • Hormonal

Smooth muscle contraction • No tropomyosin-troponin complex associated with actin • Ca ion binds

Smooth muscle contraction • No tropomyosin-troponin complex associated with actin • Ca ion binds to calmodulin – Activates myosin kinase • Adds phosphate ion to myosin head for cross-bridge formation and cycling • Relaxation – Removal of phosphate group by myosin phosphatase

 • Latching – Sustained tension during contraction due to slow release of myosin

• Latching – Sustained tension during contraction due to slow release of myosin heads from actin • Removal of phosphate ions while cross-bridges are formed • Role of Ca ions – Same as the skeletal muscle • Extracellular source • Movement between sacroplasm and the ECF regulates contraction and relaxation

 • Electrical potential – Higher resting potential – Slow wave • Slow depolarization

• Electrical potential – Higher resting potential – Slow wave • Slow depolarization – Spike potential • Action potential – Some muscles have action potential with plateau • Prolonged depolarization

 • Action potential in smooth muscles – Not all-or-nothing potential • Series of

• Action potential in smooth muscles – Not all-or-nothing potential • Series of slow waves resulting in prolonged contraction and slow relaxation • Pacemakers – Generation of action potential

Unique functional properties of smooth muscles • Autorythmicity • Contraction in response to stretching

Unique functional properties of smooth muscles • Autorythmicity • Contraction in response to stretching – Action potential generated by physical stimulation • Smooth muscle tones – Relatively constant tension • Constant tension amplitude

Regulation of contraction • Various factors – Change in membrane permeability • Na and

Regulation of contraction • Various factors – Change in membrane permeability • Na and Ca ions – Neural • Neurotransmitters from autonomic nerve fibers – Acetylchorine and norepinephrine – Hormones

 • Response to neurotransmitters – Presence of specific receptor – Reciprocal relationship •

• Response to neurotransmitters – Presence of specific receptor – Reciprocal relationship • If one is stimulatory, then the other is inhibitory • One depolarizes the membrane but the other hyperpolarizes the membrane