MUSCLE (SKELETAL) PHYSIOLOGY

 

I   Structure of muscle

            A.  Organ

            B.  Muscle cell (fiber)

                        1.  myofibrils

                                    a.  filaments

                                                1.  thick (myosin)

                                                2.  thin (actin)

                                                        a.  tropomyosin and troponin

                                    b.  striations

                                                1.  A band

                                                2.  I  band

                                                3.  Z lines

                                                4.  H zone

                                    c.  sarcomeres

                                                1.  smallest contracting unit of muscle

 

II   Sliding Filament Mechanism of Contraction                        (animation of sliding filaments)

            A.  Components

                        1.  myosin “heads”

                        2.  actin strands; binding sites

                                    a.  tropomyosin strands

                                    b.  troponin molecules

            B.  Cross-bridge cycling   (CBC)                                 (animation of cross-bridge cycling)

                        1.  role of Ca+

                        2.  tropomyosin movement

                        3.  myosin binding and “power stroke”               

                        4.  ATP binding and release

            C.  Sarcomere shortened by                                       

                        1.  repeated cross-bridge cycling

                        2.  ratio of myosin to actin filaments

                       

III   Excitation-Contraction Coupling (ECC)

            A.  Neuromuscular junction                               

                        1.  axon terminal                                   

                        2.  motor end plate

                        3.  acetylcholine                                           

                        4.  one junction per muscle fiber

            B.  Action potential

            C.  Transverse (T) tubules

            D.  Sarcoplasmic reticulum

                        1.  Ca+ release

            E.  Muscle fiber shortened by                           

                        1.  all sarcomeres shortening at once

                        2.  all myofibrils shortening at once           (click here for tutorial on structures of muscle contraction)

 

IV   Myogram of muscle twitch

            A.  Action potential

            B.  Latent period

            C.  Contraction period

            D.  Relaxation period                                        

 

 

V   Factors Influencing the Strength of a Muscle Contraction

            A.  Number of muscle fibers involved

                        1.  big muscle vs. small muscle

                        2.  motor unit recruitment

                                    a.  motor unit definition

                                    b.  graded strength of contraction

                                                1.  gross motor control

                                                2.  fine motor control

            B.  Length of muscle fiber before contraction

                        1.  “length - tension”  relationship

                                    a.  myofilament overlap

                                    b.  optimal length

            C.  Frequency of stimulation

                        1.  summation

                        2.  tetanus

            D.  Fatigue

                        1.  definition

                        2.  causes

                                    a.  neural

                                    b.  muscular

                                    c.  psychological

                        3.  prevention:  asynchronous recruitment of motor units

                                                                                                                         

                                                                                                                

VI   Skeletal Muscle Fiber Types                                                               

            A.  Typical muscle

            B.  Slow twitch, slow to fatigue (red fibers)

                        1.  oxidation reactions for ATP

                        2.  resources to generate ATP

                                    a.  capillaries                        

                                    b.  mitochondria

                                    c.  myoglobin

            C.  Fast twitch, fast to fatigue (white fibers)

                        1.  glycolytic reactions for ATP

                        2.  fewer ATP resources

            D.  Muscle adaptation                                         

 

When you finish this unit you should be able to:

-  identify the structures making up a muscle, from the myofilaments to the muscle (organ)

-  draw a piece of a longitudinal section of a myofibril including the arrangement of sarcomeres, Z-lines, actin, myosin

-  describe the arrangement of troponin and tropomyosin within the sarcomere

-  explain what causes the striated appearance of skeletal muscle using A bands, I band, and H zones

-  explain how the structure and arrangement of the proteins in the sarcomere  permits them to shorten the sarcomere

-  describe the process of cross-bridge cycling and how this explains the "sliding filament" theory of muscle contraction

-  explain why a sarcomere is the smallest unit of muscle that can shorten

-  describe the neural innervation of a skeletal muscle fiber, including structures and events at the neuromuscular junction 

-  describe the pathway and propagation of an action potential through the muscle fiber

-  explain the source and role of Ca+ in muscle contraction

-  list the events  of excitation-contraction coupling

-  explain the events leading to relaxation of a muscle fiber

-  identify the events of muscle contraction and relaxation which require ATP

-  identify a myogram of a muscle twitch, the phases of the twitch and what is occurring in the muscle fiber during each phase

-  define a motor unit

-  list three ways to increase the strength of a muscle contraction

-  explain the terms "motor unit recruitment"

-  explain how the size of the motor units fit the function of the particular muscle (fine vs gross motor control)

-  describe the "optimal length"  of a muscle for generating tension, using actin and myosin arrangement

-  explain how increasing the frequency of APs affects the strength of a muscle contraction

-  diagram summation and tetanus and explain the difference

-  list the causes of fatigue

-  explain how postural muscles avoid fatigue

-  explain the structural and functional differences between red and white muscle fibers

-  describe the ways the muscle fibers are able adapt to demands of the body

 

 

 

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