skeletal muscle physiology

Question 1

muscle characteristics

Answer 1

1. elastic - return to original shape after contraction
2. contractile - can shorten and thicken
3. extensible - stretch when pulled
4. excitable - respond to stimuli

Question 2

muscle function

Answer 2

1. movement
2. posture, facial expression
3. heat production
4. protection of viscera

Question 3

neuromuscular junction

Answer 3

• each muscle fiber innervated by only one neuron

- a single motor neuron + all muscle fibers it innervates = a motor unit

Question 4

structure of motor unit

Answer 4

• presynaptic cell with ACh in vesicles
• postsynaptic cell (muscle) membrane (sarcolemma)- specialized region ACh receptors
• two membranes separated by synaptic cleft

Question 5

function of a motor unit

Answer 5

1. AP reaches axon terminal and synaptic end bulb of neuron
2. Ca++ enters via voltage gates - causes exocytosis of ACh
3. ACh binds to ACh receptors on motor end plate
4. chemical gates open and Na enters - end plate potential
5. EPP causes opening of Na+ voltage gates on adjacent sarcolemma

Question 6

what happens in a relaxed muscle

Answer 6

• tropomyosin covers myosin binding sites on the actin

- the myosin head is activated

Question 7

myosin head activation

Answer 7

ATP - ADP +Pi + energy

Question 8

steps of myosin head activation

Answer 8

1. excitation of muscle fibre
   a. sarcolemma depolarized
   b. AP propagates down T-tubules to deep within fibre
2. excitation-contraction coupling
   c. AP in T-tubules cause release of Ca from terminal cisternae of SR by mechanically gated channels
   d. Ca binds to troponin
   e. troponin-tropomyosin complex moves exposing myosin binding sites on actin
3. contraction (mechanical event)
   f. activated myosin heads attach to binding sites on actin
   g. energy stored in myosin head is released
   h. ATP attaches to myosin head, causing its release from actin
   I. myosin head reactivates
   j. if Ca in cytosol remains high, these steps repeat

Question 9

sliding filament mechanism

Answer 9

1. sarcomeres shorten
2. myofibrils shorten muscle shortens
3. thin (actin) and thick (myosin) myofilaments remain the same length

Question 10

relaxation

Answer 10

steps:
1. ACh broken down by AChE on motor end plate (facing cleft)
2. SR actively takes up Ca++
3. ATP binds to and releases myosin heads
4. tropomyosin moves back to cover myosin binding sites on actin

Question 11

what is ATP necessary for

Answer 11

1. cross bridge release (ATP not broken down)
2. activation of myosin
   3/ pump Ca into SR
3. fibre Na/K ATPase activity

Question 12

Rigor Mortis

Answer 12

• myosin heads still activated even after death can bind to actin
• ATP production gradually stops (no O2)
• starts 3 hours after death

Question 13

flaccid paralysis

Answer 13

a. myasthenia gravis

b. curare poisoning

Question 14

botulism

Answer 14

• improper canning
• prevents exocytosis of ACh
• used to control uncontrolled blinking , crossed eyes
• cosmetic - botox (wrinkles, sweating)

Question 15

substances resulting in muscle contractions

Answer 15

a. nicotine
- binds to receptors + mimics ACh effect
- muscle spasms
b. black widow spider venom
- massive release of ACh - stops breathing

Question 16

muscle tension

Answer 16

= force exerted by a muscle or muscle fibre determined by # of cross bridges formed
in a fibre

Question 17

what effects the number of cross bridges formed

Answer 17

1. frequency of stimulation
2. fibre length
3. size of fibre
4. fatigue

Question 18

periods of a single stimulus

Answer 18

a) 1 stimulus = 1AP (lasts 1-2 msec)
b) latent period (2 msec)
- processes associated with excitation and excitation contraction coupling
c) contraction period (10-100 msec) increase tension
- cross bridge formation + sliding filaments
- lots Ca++ released from SR on stimulation but taken back rapidly
d) relaxation - decrease in tension
- Ca++ pumped into SR; ATP releases myosin

Question 19

frequency of stimulation

Answer 19

a. single stimulus - produces a twitch (weak contraction and relaxation)
b. 2nd stim. arrives before complete relaxation from 1st
c. rapid sequence of stimuli
- tension increases further
- partial relaxation between contractions
d. high frequency of stimuli
- no relaxation between contractions
- highest tension, all troponin saturated with Ca++

Question 20

fibre length

Answer 20

a. resting fibre length is optimum

b. decrease tension if shorter or longer when stimulated

Question 21

size of fibre

Answer 21

• thicker = more myofibril
• thicker = more tension
• increase with e.g. exercise = testosterone

Question 22

fatigue

Answer 22

• muscle does not contract well

- reduced max tension

Question 23

how to fibre types in a muscle differ

Answer 23

1. fast - contract/ relax rapidly - white (little myoglobin)

2. slow - contract, relax slowly -red (more myoglobin)

Question 24

in a whole muscle how is tension affected

Answer 24

1. number of fibres contracting
   - - small motor units recruited first, then larger ones added when tension is needed
2. # fibres/motor unit
   • more fibres = increase tension
3. muscle size
   - larger = more fibres
4. fatigue

Question 25

muscle tone

Answer 25

- low level of tension in a few fibres that develops as different groups of motor units are alternately stimulated over time. - gives firmness to muscle

Question 26

types of whole muscle contraction ex: lifting a book

Answer 26

1. isotonic - muscle changes length - uses ATP - flexion at the elbow 2. isometric - muscle length constant - tension less than required to move load - uses ATP - tension increases

Question 27

what happens during resting conditions

Answer 27

a) fatty acids used to produce ATP b) storage of: I) glycogen ii) creatine phosphate iii) little ATP

Question 28

what happens during short term exercise ex: sprinting

Answer 28

- primarily anaerobic a) use available ATP b) creatine phosphate used to produce ATP c) creatine glycogen - glucose - pyruvic acid - anaerobic pathway - lactic acid

Question 29

what happens during long term exercise (1min to hours)

Answer 29

- ATP - from aerobic pathway - glucose (from liver) - fatty acids, used more as exercise continues - O2 sources: blood hemoglobin + muscle myoglobin`

Question 30

types of muscle fatigue

Answer 30

1) physiological fatigue - inability to maintain tension - fatigue decreases ATP use 2. psychological fatigue - failure of CNS to send commands to muscles - probably due to lactic acid

Question 31

why cant cross bridges not release ATP in physiological fatigue

Answer 31

a) depletion of energy supplies | b) build-up of end products

Question 32

EPOC

Answer 32

excess post-exercise O2 consumption - recovery O2 consumption (deep rapid breathing) - also increases in body temp from exercise = increases O2 demand

Question 33

what is O2 used for in EPOC

Answer 33

1. replenish stores of glycogen 2. convert lactic acid to: - pyruvic acid - krebs - glucose in liver