Muscle Contraction Theory - Lecture 7

Chapter Links and Quizzes - Review of Muscle Physiology

Quantal release of ACH

Summation

Muscle Contraction

Structure

Contraction Mechanism

Characteristics of muscle contraction

Audio - Lecture - - Review 

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Muscle are important in Homeostasis; heart; blood vessels; digestive tract; movement-if cold-walk inside.

Muscles - forces generated by muscle cells contribute not only to the regulation of the internal environment but also to the movement of the organism; control objects in external environment.

Three types of muscle - 40-45% of body weight

1. skeletal - attached to bones - movement - somatic nerves voluntary

2. smooth - surrounds hollow organs - autonomic nerves, no direct conscious control

3. cardiac - heart - autonomic, no direct control

Although there are differences in the three muscle types - force generation mechanisms are similar.

A. Structure of Skeletal Muscle Fibers

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1. Single muscle cell is a muscle fiber - cell

Put many muscle fibers together get a muscle similar to many nerve fibers (axons) make a nerve.

single muscle cell (fiber)

The muscles appear to be striped or banded in skeletal and cardiac muscle; called striated muscle.

Smooth muscle-no striations

2. Myofibril - cylindrical element 1-2 mm diameter

200-2000 myofibrils/muscle fiber

3. Myofibril is made up of filaments arranged in a repeating fashion

One unit of repeating pattern is a sarcomere

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A) Filaments - proteins

1. thick filaments - myosin - has a protein head

a. heads have site which will bind to site on actin (thin filament)

b. heads have ATPase enzyme to release energy from ATP

c. heads can pivot

2. thin filament - actin - composed of 2 spiral chains

a. in grove between chains is a site where myosin can bind - normally covered by a molecule called tropomyosin

b. attached to tropomyosin is troponin which has a binding site for Ca⁺⁺

B. Molecular Mechanism of Contraction

-Sliding filament theory of muscle contraction

-Excitation contraction coupling

1. AP travels down motor neuron to muscle fiber

2. Ach is released from neuron -pre synaptic. terminal

3. Ach diffuses to motor end plate

4. binds to motor end plate receptors -depolarization - graded potential - end plate potential

5. if threshold is reached, AP spreads in all directions rapidly across fiber.

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down t-tubule to sarcoplasmic reticulum

6. membrane depolarization - release of Ca⁺⁺  stored in sarcoplasmic reticulum

7. Ca⁺⁺ diffuses into myofibril and binds to troponin

8. troponin changes conformation and pulls tropomyosin away from the active sites on actin chains.

9. myosin heads now attach to actin and pivot - slides actin filaments inward - form cross bridges; energy for pivot has been stored in myosin from earlier splitting of ATP

process is staggered - not all heads release at same time

10. ATP attaches to myosin and myosin ATPase splits the ATP, heads release and move back to original position - this energy is stored in myosin for next contraction -- ATP binding breaks the bond while ATP splitting resets the pivot

11. if actin sites are still open, mechanism will repeat: rate of ATPase determines rate of contraction.

12. if no additional action potentials - Ca⁺⁺ no longer released - Ca⁺⁺ pumped back into sarcoplasmic reticulum - requires ATP

13. due to low Ca⁺⁺, Ca⁺⁺ is released from troponin and tropomyosin springs back and covers the active site; relaxation

Note - ATP energy is not used directly for contraction, but is used for relaxation; used when myosin releases from actin, used to store energy in myosin head, used to pump calcium out of myofibril.

Rigor mortis - no ATP - muscle becomes rigid - can not break bond

C. Characteristics of Muscle Contraction

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1. Latent period

a. AP activates release of Ca⁺⁺ -10-7 M

b. latent period is time required to activate contractile machinery and begin shortening

2. Innervation Pattern

a. in skeletal muscle, each fiber requires its own innervation: depolarization can not spread from fiber to fiber; no gap junctions like cardiac.

b. nerve that innervates muscle is a motor neuron - it has its cell body in the spinal cord.

c. axon branches and innervates more than 1 fiber.

muscles for fine movement: 1 axon - 10-20 fibers; i.e. eye

muscle for strong coordinated movement: 1 axon -1000-2000 fibers; i.e. leg

d. motor unit: single motor neuron axon and all the fibers it innervates; each AP - stimulates all the connected fibers in the motor unit.

REVIEW QUESTIONS

1. If a muscle is stimulated to contract, allowed to shorten but not allowed to develop force, then the contraction is called:
   1. spasmodic
   2. isometric
   3. tetanic
   4. isotonic
   5. Schwartzentonic
2. The contractile response in skeletal muscle:
   1. starts after the depolarization phase of the action potential
   2. is longer than the action potential
   3. produces more tension when the muscle contracts isometrially than when the muscle contracts isotonically
   4. All of the above are correct.
3. The amount of force produced by a non-fatigued muscle during contraction can be increased by
   1. decreasing saltatory conduction to the motor units.
   2. increasing the frequency of stimulation.
   3. making sure the actin and the myosin do not have maximum overlap.
   4. All of the above are correct.
   5. None of the above are correct because external force is all-or-none
4. An isotonic contraction is possible when:
   1. muscle length is changing.
   2. muscle length is not changing.
   3. muscle tension is changing.
   4. all series elastic elements in a muscle are totally stretched out.
5. During contraction, the lengths of the thick and thin myofilaments:
   1. increase
   2. decrease
   3. first increase, then decrease
   4. remain the same
6. The thick filament of a muscle is
   1. Actin
   2. Tropomyosin
   3. Troponin
   4. Myosin
7. A sarcomere is the area between:
   1. two Z lines
   2. two I bands
   3. two A bands
   4. two H bands
8. The nerve and all the muscle fibers under its control is called a ____ unit.
   1. motor
   2. movement
   3. muscle
   4. end
   5. coordination

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