Muscle Physiology

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 of contraction Threshold must be reached – Generation of action potential Stimuli stronger than threshold level – Identical contraction to the threshold level – No difference in level of action potential being generated

Motor unit

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 of contraction Number of motor units being stimulated by an action potential

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 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 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 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 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 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 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 – Increased blood flow – Increased muscle metabolism

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 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 – Tension being maintained against the load

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 can function – Mental perception Muscular – Loss of ATP Synaptic – Loss of acetylchorine

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 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 acids More energetic than glucose

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 – 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 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 – 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 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 – 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 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 – 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 for contraction

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

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 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 – 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 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 – 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 Ca ions – Neural Neurotransmitters from autonomic nerve fibers – Acetylchorine and norepinephrine – Hormones

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