Walk along theory

Question 1

What is the sliding filament mechanism?

Answer 1

• how a muscle contraction occurs
• forces generated by interaction of cross bridges from the myosin filaments w/ actin filaments
• inactive when resting
• AP travels along muscle fiber, Ca released from SR activates forces between myosin and actin filaments.

Question 2

Describe constitution of thin and thick filaments.

Answer 2

• myosin binding sites on actin of relaxed muscle are inhibited or physically covered by troponin-tropomyosin complex
• sites cannot attach to heads of myosin filaments
• Ca+ binding exposes this site

Question 3

Describe walk along theory of muscle contraction.

Answer 3

1. prior to muscle contraction, myosin heads bind w/ ATP ( low energy configuration)
   –> ATPase activity cleaves ATP ( ADP and phosphate ion)
   –> cleavage products are kept bound to head
   –> head becomes energised in “ cocked position “
2. troponin-tropomyosin complex bind to Ca ions, active sites of actin are uncovered
   –> myosin heads bind to sites
   –> cross bridge is formed
   –> phosphate ion and ADP released
3. cross bridge causes conformational change in head
   –> power stroke = myosin heads bend toward centre of sarcomere, causes actin to slide toward M line
   –> energy activates come from stored ADP
   –> ATP molecule will take place
4. binding of new ATP causes detachment of myosin head from actin filament
   –> new molecule of ATP is cleaved to begin a new cycle

Question 4

Describe how muscle changes its strength of contraction.

Answer 4

• strength of contraction is controlled by changing number of motor units recruited and/or frequency of stimulation of muscle
• contracts completely when exposed to threshold stimulus
  –> always contracts maximally
• strength is separated into 3 :
  –> twitch
  –> summation
  –> tetanus
• latent period = time between stimulus to motor neuron and subsequent contraction of innervated muscle

Question 5

What is twitch?

Answer 5

• a single contraction and relaxation cycle produced by AP w/in muscle fiber.

Question 6

What is summation? What are the 2 types?

Answer 6

• when another AP comes before complete relaxation of muscle twitch, the next will sum onto previous one.
  -2 ways summation occurs :
  1. multiple fiber
  2. Frequency

Question 7

What is Multiple Fiber Summation?

Answer 7

• increasing in # of motor units contracting simultaneously
  –> size principle = more and larger motor units are activated the stronger the force of muscle contraction
• CNS sends weak signal to contract muscle, smaller motor unit are stimulated first due to higher excitability

Question 8

What is frequency summation?

Answer 8

• increasing frequency of contraction
  –> force exerted by skeletal muscle is controlled by varying frequency at which AP are sent to muscle fibers
  –> AP do not arrive at muscle synchronously
• at lower frequency, contraction occurs one after another
  –> if muscle fiber is re-stimulated after completely relaxing, second twitch is same magnitude as first.
• frequency increases, each new contraction occurs before preceding one is over
  –> second contraction is partially added to first
  –> total strength of contraction rises progressively w/ increasing frequency

Question 9

What is tetanization?

Answer 9

-muscle fiber is stimulated so rapidly that it does not have a chance to relaxation between stimuli, smooth muscle sustained contraction of maximal strength.

Question 10

Describe length tension relationship.

Answer 10

• tension = pulling force
• relation between length of muscle before onset of contraction and tension that each contracting fiber can develop at that length
• # of cross bridge interactions is determined by degree of overlap between thin and thick filaments
• for every muscle, there is an optimum length at which max force can be achieved
• muscle fibers can contract forcefully when stimulated over relatively narrow range of resting lengths

Question 11

What is optimal resting length?

Answer 11

• maximal tension
• when sarcomere is in optimal resting length, there is optimal rate of cross bridges formed
• contraction = optimal

Question 12

What is decreased length?

Answer 12

• when sarcomere is overly contracted, high degree of overlap between thin and think filaments
  –> decreased length
• muscle contraction cannot progress
• tension decreases
• muscle fiber can not actively produce any tension, contraction cannot occur.

Question 13

What is increased length?

Answer 13

• when sarcomere is pulled away
  –> increased length
• either little degree interaction between filaments or no interaction between filaments

Question 14

List sources of energy for muscle contraction.

Answer 14

• muscle tissue needs energy for
  –> walk along mechanism
  –> calcium pump in SR
  –> sodium potassium pump
• there are 3 sources of energy in muscle contraction :
  1. phosphocreatine/ creatine phosphate
  2. glycolysis
  3. oxidative metabolism

Question 15

What is phosphocreatine?

Answer 15

• creatine produced from AA in liver are phosphorylated in muscle by creatine phosphokinase = produce phosphocreatine
• used to reconstitute ATP molecule
  –> cleavage of phosphocreatine releases energy, used to bond of new phosphate ions to ADP to reconstitute ATP
• amount of muscle phosphocreatine is small

Question 16

What is glycolysis?

Answer 16

• enzymatic breakdown of carbs as glucose and glycogen then w/ release of energy and production of pyruvate acid and lactic acid
• liberated energy is used to reconstitute both ATP and phosphocreatine
  –> can be done in absence of oxygen
  –> muscle contraction can be sustained for more than a minute w/out O2 from blood
• rate of ATP formation by glycolysis is 2.5x as rapid as ATP formation by oxidative metabolism
  –> end product accumulate = lactate

Question 17

What is oxidative metabolism?

Answer 17

• responsible for 95% of energy used for contraction
• combining of O2 w/ end products of glycolysis and w/ other sources of energy to produce ATP
  –> short periods = carbs
  –> long periods = fatty acids