Muscular System: Histology and Physiology Chapter 9

Muscular System Functions  Body movement  Maintenance of posture  Respiration  Production of body heat  Communication  Constriction of organs and vessels  Heart beat

Criteria for Naming Muscles  Shape: romboideus, trapezius, biceps  Location: pectoralis (chest) intercostal (ribs)  Attachment: zygomaticus, sternocleidomastoid  Size: maximus, minimus, brevis, longis  Orientation of fibers: rectus (straight), oblique (slanting)  Relative position (lateral, medial, internal, external)  Function: adductor, flexor, extensor, pronator

Properties of Muscle  Contractility Ability of a muscle to shorten with force  Excitability Capacity of muscle to respond to a stimulus  Extensibility Muscle can be stretched to its normal resting length and beyond to a limited degree  Elasticity Ability of muscle to recoil to original resting length after stretched

Skeletal Muscle Smooth Muscle Cardiac Muscle

Features Skeletal MuscleSmooth MuscleCardiac Muscle LocationAttached to bonewalls of hollow organs, blood vessels, eyes, glands and skin heart Cell shapevery long, cylindrical Spindle shapedCylindrical and branched NucleusMultiple, peripherally located Single centrally located single centrally located Special features Gap junctions join visceral smooth muscle Intercalated disks join cells Controlvoluntary and involuntary reflexes Involuntary Spontaneous contraction NoYes FunctionBody movementFood movement, urinary bladder, blood vessels, glands and duct pumps blood

Skeletal Muscle Structure  Muscle fibers or cells Develop from myoblasts Numbers remain constant Hypertrophy – increase in the size of each fiber.  Connective tissue  Nerve and blood vessels

Connective Tissue, Nerve, Blood Vessels  Connective tissue External lamina Endomysium Perimysium Fasciculus Epimysium  Fascia Binds adjacent muscles or overlying skin.  Nerve and blood vessels Abundant

Parts of a Muscle

Structure of Actin and Myosin

Components of Sarcomeres

Sliding Filament Model  Actin myofilaments sliding over myosin to shorten sarcomeres Actin and myosin do not change length Shortening sarcomeres responsible for skeletal muscle contraction  During relaxation, sarcomeres lengthen

Sarcomere Shortening

Physiology of Skeletal Muscle  Nervous system Controls muscle contractions through action potentials  Resting membrane potentials Membrane voltage difference across membranes (polarized) Inside cell more negative and more K + Outside cell more positive and more Na + Must exist for action potential to occur

Ion Channels  Types Ligand-gated Example: neurotransmitters Voltage-gated Open and close in response to small voltage changes across plasma membrane

Action Potentials  Phases Depolarization Inside plasma membrane becomes less negative Repolarization Return of resting membrane potential  All-or-none principle Like camera flash system  Propagate Spread from one location to another  Frequency Number of action potential produced per unit of time

Action Potential Propagation

Neuromuscular Junction  Synapse or NMJ Presynaptic terminal Synaptic cleft Postsynaptic membrane or motor end-plate  Synaptic vesicles Acetylcholine: Neurotransmitter Acetylcholinesterase: A degrading enzyme in synaptic cleft

Function of Neuromuscular Junction

Excitation-Contraction Coupling  Mechanism by which an action potential causes muscle fiber contraction  Involves Sarcolemma Transverse or T tubules Terminal cisternae Sarcoplasmic reticulum Ca 2+ Troponin

Action Potentials and Muscle Contraction

Cross-Bridge Movement

Muscle Twitch  Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers  Phases Lag or latent Contraction Relaxation

Stimulus Strength and Muscle Contraction  All-or-none law for muscle fibers A motor unit contracts with a consistent force in response to each action potential Sub-threshold stimulus Threshold stimulus Stronger than threshold  Motor units Single motor neuron and all muscle fibers that it innervates  Graded for whole muscles Strength of contractions range from weak to strong depending on stimulus strength

Multiple Motor Unit Summation  A whole muscle contracts with a small or large force depending on number of motor units stimulated to contract  Muscle performing delicate and precise movements have motor units with smaller numbers of fibers

Multiple-Wave Summation  As frequency of action potentials increase, frequency of contraction increases Incomplete tetanus Muscle fibers partially relax between contraction Complete tetanus No relaxation between contractions  Multiple-wave summation Muscle tension increases as contraction frequencies increase Due to increased calcium concentration around myofibrils and more complete stretching of muscle elastic elements

Treppe  Increase in the force of contraction during the first few contractions of a rested muscle.  Occurs in muscle rested for prolonged period  Each subsequent contraction is stronger than previous until all equal after few stimuli  Due to Ca++ ion levels around myofibrils and increased temperature of muscle Enzymes for muscle contraction respond more effectively at higher temperature.

Types of Muscle Contractions  Isometric: No change in length but tension increases Postural muscles of body  Isotonic: Change in length but tension constant Concentric: Overcomes opposing resistance and muscle shortens Eccentric: Tension maintained but muscle lengthens  Muscle tone: Constant tension by muscles for long periods of time

Muscle Length and Tension

Fatigue  Decreased capacity to work and reduced efficiency of performance Usually follows a period of activity  Types Psychological (in CNS) Depends on emotional state of individual Perception that muscle is too tired ( Home court advantage Muscular Results from ATP depletion in muscle Synaptic Occurs in NMJ due to lack of acetylcholine

Energy Sources  ATP provides immediate energy for muscle contractions from 3 sources Creatine phosphate During resting conditions stores energy to synthesize ATP Exhausted quickly (10-15 sec.) Anaerobic respiration Occurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acid Aerobic respiration Requires oxygen and breaks down glucose to produce ATP, carbon dioxide and water More efficient than anaerobic  Oxygen Debt After anaerobic respiration, aerobic respiration is higher than normal to replace creatine phosphate and convert lactic acid to glucose.

Slow and Fast Fibers  Slow-twitch or high-oxidative Contract more slowly, smaller in diameter, well developed blood supply, more mitochondria and high myoglobin content, more fatigue-resistant than fast- twitch  Fast-twitch or low-oxidative Respond rapidly to nervous stimulation, less blood supply, fewer and smaller mitochondria, lower myoglobin content than slow-twitch, fatigue easily. Two types: Fast twitch fatigable fibers Fast twitch fatigue resistant (highly trained muscle)  Distribution of fast-twitch and slow twitch Most muscles have both but varies for each muscle

Effects of Exercise  Training muscle increases muscular size and strength (Hypertrophy). Aerobic exercise can convert fast-twitch easily fatigued muscle into fatigue-resistant fast-twitch muscle. Change in myosin type, increase size and number of mitochondria and increased blood supply  Muscles that are not used Atrophy or decreases in muscle size.  Atrophy or hypertrophy are the result of changes in the size of individual muscle cells not the number of muscle cells. Number of myofibrils and sacromeres changes. Blood vessels, mitochondria and connective tissues increase.  Trained athletes: Have the ability to recruit large numbers of motor units simultaneously improving coordination. Have a greater capacity for nutrient uptake and ATP production (increased metabolism) Have improved circulation and more efficient respiration.

Heat Production  Heat is a biproduct of the chemical reactions that occur in the body.  As muscles are worked they produce excess heat that must be disipated by other body systems (circulatory and integument)  When body temperature drops muscle shiver to generate more heat (up to 18 times that of resting muscle).

Smooth Muscle  Characteristics Spindle shaped Fewer actin and myosin Organized in loose bundles. Not striated. Dense bodies hold actin filaments together and are attached to noncontractile intermediate filaments Ca 2+ required to initiate contractions Sarcoplamic reticulum is not well developed.

Smooth Muscle Contraction

Types of Smooth Muscle  Visceral or Unitary Smooth Muscle Found in digestive, urinary and reproductive tracts. Contains gap junctions, contracts in waves and often has autorhythmicity.  Multiunit smooth muscle Found in iris, blood vessels, arrector pili. Fewer gap junctions, groups of cells act as independent units, only contracts when stimulated by nerves or hormones.

Electrical Properties of Smooth Muscle

Functional Properties of Smooth Muscle  Some visceral muscle exhibits autorhythmic contractions  Tends to contract in response to sudden stretch but not to slow increase in length  Exhibits relatively constant tension: Smooth muscle tone  Amplitude of contraction remains constant although muscle length varies

Smooth Muscle Regulation  Innervated by autonomic nervous system  Neurotransmitter are acetylcholine and norepinephrine  Hormones important as epinephrine and oxytocin  Receptors present on plasma membrane which neurotransmitters or hormones bind determines response

Cardiac Muscle  Found only in heart  Striated  Each cell usually has one nucleus  Has intercalated disks and gap junctions  Autorhythmic cells  Action potentials of longer duration and longer refractory period  Ca 2+ regulates contraction

Types of Muscle Contraction  Isometric Increase in tension with no change in length during the contraction process (postural muscles)  Isotonic Tension produced by muscle remains constant while length changes.  Note - Both Isometric and Isotonic contractions are used in most body movements  Concentric contractions  Eccentric contractions

Effects of Aging on Skeletal Muscle  Reduced muscle mass  Increased time for muscle to contract in response to nervous stimuli  Reduced stamina  Increased recovery time  Loss of muscle fibers  Decreased density of capillaries in muscle