Muscle Structure and Function Flashcards
Composition of a Muscle Fiber
-Sarcolemma
-Myofibril
-Myofilaments
Sarcolemma
Cell membrane enclosing muscle fibers
Myofibril
-composed of numerous stacked myofilaments
Myofilaments
Myosin and actin
Which are the thick myofilaments?
Myosin
Which are the thin myofilaments?
Actin
What forms actin myofilaments?
-Two chainlike strings of actin molecules
-Troponin
-Tropomyosin molecules
What controls the binding of actin and myosin myofilaments?
-Troponin and tropomyosin molecules
What forms myosin myofilaments?
-Head groups that attach to specific binding sites on actin molecule
What is the critical role myosin head groups have?
-Muscle contraction and relaxation
Contractile Unit
-Sarcomere, distinguished by bands and zones
What portion of the myofibril is the sarcomere?
-Located between any two Z discs
What does the Z discs link together?
-The thin filaments
What is the A band?
-Portion of the sarcomere that extends over length of thick filaments and small portion of thin filaments
What is the I band?
-Areas that only include actin filaments
What is the H zone?
-A band area where there is no overlap with thin filaments
What is the M line?
-The center of the H zone; wide middle portion of thick filament
How is a muscle contraction initiated?
-By the interaction between thick and thin filaments within the sarcomere
Cross-Bridge Interaction
- Coupling
- Contraction
- Uncoupling
- Recharging
Coupling phase
-Calcium released makes troponin move to expose active sites of actin
-Myosin head will reach out + attach to actin forming the x-bridge
Contraction phase
-Cross-bridge will generate force (power stroke) to pull thin filament one notch to the Mline
Uncoupling phase
-Power stroke complete, myosin head will detach from actin and repeat cycle
Recharging phase
-ATP hydrolysis occurs, releasing ADP and Pi, bounding to myosin head, reattaching to actin binding site
Concentric (shortening) Contraction
-Sarcomere length is shorter (distance btw Z discs) than when fiber is at rest
-Closer to M line
Eccentric (lengthening) Contraction
-Sarcomere length is greater than a resting fiber
Isometric Contraction
-No change in length
What is happening in a Concentric contraction?
-Thin filaments overlap thick filaments
-Cross-bridge formed, broken, and re-formed
What happens in an Eccentric contraction?
-thin pulled away from thick filaments by an external force on muscle
-Cross-bridge broken, re-formed, and broken again
What happens to the Cros-bridge cycle when an eccentric contraction occurs?
-It is broken and re-formed as the muscle lengthens
What happens as the cross-bridge is re-formed?
-Tension is generated
What happens in an Isometric contraction?
-Active muscle fiber will not change length if the force created by the x-bridge cycling matches the external force
What initiates the muscle contraction?
Calcium influx
What fuels the cross-bridge cycle?
ATP hydrolysis
How is a Motor Unit organized?
-Cell body in Ventral Horn
-Motor axon extends to muscle
-Axon divides into a few or thousands of branches
-Each branch terminates in a motor endplate
Excitability
-Capacity of muscle fiber to react to stimulus
Contractibility
-Ability to create tension when stimulated from a motor neuron
Extensibility
-Capacity of a muscle tissue to stretch without damage
Elasticity
-Muscles tendency to return to original after being stretched
What affects the strength of the contraction?
-the changing # of motor units that are activated
-frequency at which they are activated
What affects the size of the motor unit?
-Depends of the function
-Controls fine movements
-Make small adjustments
How are motor neurons recruited?
-All or None Law
-Size Principle
All or None Law
-All muscle fiber will contract on the same stimulus
The Size Principle
-Small motor unit generate less tension than larger motor unit
-Will be stimulated first to save energy
When is the Size Principle in place?
-In an isometric contraction
Which affects the magnitude of the response to a stimulus?
-# of muscle fibers in a motor unit
Which determines the conduction velocity of the nerve impulse?
-The diameter of the axon innervating a motor unit
Which affects the total force response of the muscle?
-# of motor units that are firing at any one time
Which affects the total response of the muscle?
-The frequency of motor unit firing
Wave summation
-Pace at which nerve signals are fired through the motor unit
What does the wave summation say?
-In sighting a motor unit before fully relaxing, the second contraction won’t reach a stronger force compared to the first one
What are the characteristics of muscle fibers?
-Diameter
-Muscle color
-Capillarity
-Myoglobin content
-Speed of contraction
-Rate of fatigue
Type I (slow) fiber
-Small
Slow contraction but greater efficiency
-Longer duration/ endurance
What activity will the Type I fiber be ideal for?
-Sustained activities
-Example: Marathon running
Type IIA (intermediate) Fiber
-Larger
-Faster contraction
-Recover slowly
-Fatigue rapidly
What activity will the Type IIA fiber be ideal for?
-High intensity workouts
Type IIX or Type IIB (Fast) Fibers
-Large and powerful
-Fatigues the quickest
-Longer periods of rest to recover and replenish energy
What activity will the Type IIX/B fiber be ideal for?
-Heavy lifting
Postural / Stability
-High proportion of Type I fibers in relationship to Type II fibers
-Sustained activity due to slow fatigue
-Ex: Soleus
Nonpostural / Mobility
-High proportion of Type II fibers
-Produce large ROM
-More force, able to contract faster, higher power output
-Ex: Hamstrings and Gastroc.
Muscle Architecture
-arrangement of the fibers in relation to the axis of force
-size, arrangement, and length
Muscle Fiber Length
-Determined by the # of sarcomeres along the fiber
The more __________, the more a fiber can shorten.
-Sarcomeres
Muscle Length
-The longer the muscle, the more sarcomeres, shortening more
What is the advantage of having a long muscle?
-Able to move bony lever to which it is attached through a greater distance
Physiological Cross-Sectional Area (PCSA)
-a measure of the cross-sectional area of the muscle perpendicular to the orientation of the muscle fibers
Fusiform (strap) Muscles
-Parallel fibers to the long axis of the muscle and to e/other
-Ex: sternocleidomastoid/sartorius
Pennate Muscles
-Fiber arrangement oblique to muscle’s long axis
-shorter and numerous fascicles
Unipennate Muscles
-Oblique fascicles fan out on only one side of a central muscle tendon
-Ex: Flexor pollicis longus
Bipennate Muscle
-Fascicles are obliquely set on both sides of a central tendon
-Ex: Biceps femoris/tibialis anterior
Multipennate Muscle
-Oblique fascicles converge on several tendons
-Ex: Soleus/subscapularis
Building a Muscle
-Endomysium surrounds individual fibers and their sarcolemmas
-Perimysium surrounds groups of fibers (fascicles)
-Epimysium on the outer layer of connective tissue
Superficial Fasciae
-Loose tissue, located directly under dermis (skin)
What does the Superficial fascia zone help with?
-mobility of the skin, insulator
Deep Fascia
-Compacted and regularly arranged collagenous fibers
What do Deep fasciae form?
-Tracts, bands, or retinaculae
What does the Retinaculae help with?
-Maintains relationship between tendon and their respective joints and joint axes
Passive Elastic Components
-Include the Parallel and Series Components
Parallel Elastic Components
-Function in parallel with the muscle contractile unit; when a muscle lengthens or shortens, the tissues do the same
Series Elastic Component
-Tendon functions in series with the contractile elements; when at rest, tendon relaxed and may be crimped (slack)
-tendon under tension when muscle actively produces tension
Total Tension
-includes both active or passive components
Passive tension
-tension created by lengthening the muscle beyond the slack length of the tissues (stretching muscle) at rest
Isometric Length-Tension Relationship
-direct relationship between isometric tension + length of the sarcomeres in muscle fibers
Optimal Sarcomere Length
Plateau of active tension curve
Decrease of active tension
-when muscle is lengthened because fewer x-bridges can be formed
Decrease of active tension
-when muscle is shortened; x-bridges already connected, losing capacity to shorten any further
Passive tension Increases
-with elongation of the muscles as it continues to be stretched
Active Insufficiency
-2 joint muscles have been maximally SHORTENED across 2 joints and can no longer generate additional or maximal torque
Passive Insufficiency
-2 joint muscles have been maximally LENGTHENED across 2 joints and can no longer generate maximal torque in that position
Force-Velocity Relationship
-the ability of a muscle to generate force is affected by the speed at which a concentric / eccentric contraction happens
Maximum velocity of muscle shortening (concentric contraction)
-No force is produced; only when there is no load on the muscle
Shortening velocity decreases
-force during concentric contraction increases
Zero velocity
-isometric contraction
Force increases dramatically and then plateaus
-Muscle is lengthened actively (eccentric contraction)
The greater the # of cross-bridges that are formed,
-the greater the tension
Are large or small cross-sections better in the capacity of producing more tension?
-Large cross sections
When does tension increase?
-When velocity of active shortening decreases and velocity of active lengthening increases
Recruitment order of the motor units
-Units with slow conduction velocities are generally recruited first
Type of muscle fibers in the motor units
-Type II muscle fibers develop maximum tension more rapidly than Type I; rate of cross-bridge formation, breaking, and re-formation may vary
Length of the muscle fibers
-Long fibers have a higher shortening velocity than shorter fibers
Magnitude of the resistance
-A given muscle force production, greater resistance to the muscle action will result in a slower contraction
Size of motor units
-Larger units produce greater tension
and Size of muscle fibers in cross section of muscle
-Larger cross section, greater amount of tension produced
Frequency of firing of motor units
-Higher the frequency of firing, greater tension
Fiber arrangement
-Pennate fiber = > # muscle fibers = large x-sectional area = > tension
Type of muscle contraction
Greater Tension:
Isometric > concentric
Eccentric > isometric
Velocity
-Speed of short increases < tension in concentric
-Speed of active length increases > tension in eccentric
Golgi Tendon Organ (GTO)
-sensitive to tension and may be activated either by an active muscle contraction or by an excessive passive stretch
Muscle Spindle
-sensitive to length and the velocity of lengthening of the muscle fibers
What does the GTO adjust?
-Muscle tension by sending message to the CNS via Ib afferent neurons when excited
What are muscle spindles concerned about?
-State of stretch of a muscle; message sent to spinal cord and CNS via the group Ia afferent neurons
-movement will be affected
Immobilization Effects
-Depend on immobilization position (short or lengthened), % of fiber types w/in muscle, length of immobilization period
Shortened Position Changes
-Decrease in # of sarcomeres, increase length
-increase in perimysium
-Thick endomysium
-Collagen fibril orientation more circumferential
-increase in ratio of connective tissue to muscle fiber tissue
-decreased muscle mass and muscle atrophy
Lengthened Position Changes
-Increase in # sarcomeres, decrease in length
-increased endomyseal + perimyseal connective tissue
-muscle hypertrophy, can be followed by atrophy
Overuse
-causes injury to tendons, ligaments, bursae, nerves, cartilage, and muscle
What causes overuse?
-repetitive trauma that does not allow time for complete repair of tissue
Muscle Strain
-Injuries from single high-force contraction of the muscle while the muscle is lengthened by external forces
What does the muscle fail at during Muscle Strain?
-The interface between muscle and tendon
Eccentric Exercise-Induced
-injuries may occur with a single bout of eccentric exercise
Sarcopenia
loss of muscle mass with age due to loss of fibers and decrease in size of existing fibers
Connective Tissue Changes with Age
-Increases within a muscle; causes decreased ROM and increased muscle stiffness
