Muscle Physiology (Exam 3, 1 of 4)

Tetanus

(lockjaw) spastic paralysis caused by toxin from Clostridium tetani; blocks release of inhibitory neurotransmitter in spinal cord, resulting in overstimulation of muscles. vaccination prevents life-threatening condition

Botulism

muscular paralysis caused by toxin from Clostridium botulinum, prevents release of ACh at synaptic knobs

Muscle tension

force generated when a muscle is stimulated to contract

Threshold

minimum voltage that triggers a twitch

Muscle twitch

a brief contraction to a single stimulus

Periods of a muscle twitch

latent period
contraction period
relaxation period

Latent period

time after stimulus but before contraction begins; no change in tension

Contraction period

time when tension is increasing; begins as power strokes pull thin filaments

Relaxation period

time when tension is decreasing to baseline; begins with release of crossbridges

the relaxation period generally lasts longer than the…

contraction period

Recruitment

as voltage increases, more units are recruited to contract; also called multiple motor unit summary

wave summation (temporal)

if stimulus frequency set at about 20 per second, relaxation is not completed between twitches, contractile forces add up to produce higher tensions

Incomplete tetany

if frequency is increased further (than 20 per second), tension increases and twitches partially fuse

Tetany

if frequency is increased further still (40-50 per second), tension trace is a smooth line without relaxation, high frequency stimuli lead to fatigue

Muscle tone

resting tension in a muscle generated by involuntary nervous stimulation; do not generate enough tension for movement, decreases during deep sleep

Isometric contraction

although tension is increased, it is insufficient to overcome resistance, muscle length stays the same (ex: holding a weight while arm doesn’t move)

Isotonic contraction

muscle tension overcomes resistance by resulting in movement; tone stays constant but length changes

Types of isotonic contractions

concentric contraction and eccentric contraction

Concentric contraction

muscle shortens as it contracts

Eccentric contraction

muscle lengthens as it contracts

Muscle cells have a little __ in storage

ATP

Additional ATP rapidly produced via…

myokinase

3 ways to generate additional ATP in skeletal muscle fiber

creatine phosphate
glycolysis
aerobic cellular respiration

Creatine phosphate

contains a high-energy bond between creatine and phosphate
10-15 seconds of additional energy

Phosphate can be transferred to __ to form _

ADP; ATP

Creatine phosphate is catalyzed by…

creatine kinase

Glycolysis

does not require oxygen, glucose is converted into 2 pyruvate molecules, 2 ATP released per glucose molecule

Glycolysis occurs in…

cytosol

Aerobic respiration

requires oxygen, pyruvate oxidized to carbon dioxide

Aerobic respiration occurs within…

mitochondria

Lactate formation

occurs under conditions of low oxygen availability

Pyruvate converted to lactate by…

lactate dehydration

Lactic Acid Cycle

cycling of lactate to liver where it is converted to glucose, and transport of glucose back to muscle

Skeletal muscle fibers are classified based on…

type of contraction generated and means of supplying for ATP

Types of contraction generated

differences in power, speed, and duration

Fast-twitch fibers vs slow-twitch fibers

fast-twitch fibers are more powerful and have quicker and briefer contractions that slow-twitch fibers

Primary means for supplying ATP

oxidative fibers and glycotic fibers

Oxidative fibers

(fatigue-resistant) use aerobic cellular respiration; extensive capillaries, many mitochondria, large supply of myoglobin. red fibers

Glycotic fibers

(fatiguable) use anaerobic cellular respiration; fewer capillaries, fewer mitochondria, small supply of myoglobin, large glycogen reserves. white fibers

3 types of skeletal muscle fibers

slow oxidative fibers, fast oxidative fibers, fast glycotic fibers

Slow oxidative fibers

contractions are slower and less powerful
high endurance since ATP supplied aerobically
about half the diameter of other fibers, red in color due to myoglobin

Fast oxidative fibers

contractions are fast and powerful
primarily aerobic respiration, but delivery of oxygen lower
intermediate size, light red in color

Fast glycotic fibers

contractions are fast and powerful

contractions are brief, as ATP production primarily anaerobic
largest size, white in color due to lack of myoglobin

A single muscle contains a mixture of…

fiber types

Long distance runner have higher percent of…

slow oxidative fibers in legs

Sprinters have higher percent of…

fast glycotic fibers

Sustained isometric contractions may be a concern for those…

with baseline high blood pressure

Length-tension relationship

the tension a muscle produces based on its length at the time of stimulation

Fiber at resting length…

generates maximum contractile force

Fiber at shortened length…

generates weaker contractile force

Fiber at extended length…

generates weaker contractile force

Endurance exercise

leads to better ATP production

Resistance exercise

leads to hypertrophy

Hypertrophy

muscle increases in size due to increases in synthesis of contractile proteins; also increases glycogen reserves and mitochondria. stimulates limited amount of hyperplasia

Hyperplasia

increase in number of fibers

Atrophy

decrease in muscle size due to lack of use (ex: someone wearing a cast)

Slow muscle mass loss begins in person’s mid-…

30s due to decrease in activity

Loss of muscle mass decreases…

fiber number and diameter
oxygen storage capacity
circulatory supply to muscles with exercise

Reduced capacity to recover from injury is due to decreased number of…

satellite cells

Fibrosis

muscle mass often replaced by dense regular connective (decreased flexibility)

Myasthenia Gravis

autoimmune disease (mostly in women), Antibodies attack ACh receptors at the neuromuscular junction, Receptors are removed by endocytosis

Anabolic steroids

synthetic substances that mimic testosterone, stimulate manufacture of muscle proteins (popular performance enhancers)

Flaccid paralysis

a state in which the muscles are limp and cannot contract

Curare

bind to ACh receptors but do not stimulate the muscles

Tetrodotoxin

block voltage-gated Na+ channels

Botulism

blocks release of ACh causing flaccid paralysis; botox

Cardiovascular system smooth muscle

found in blood vessels

Respiratory system smooth muscle

found in brachioles

Digestive system smooth muscle

found in intestines

Like skeletal muscle, smooth muscle filaments have…

actin, myosin, and troponin

unlike skeleton muscle, smooth muscle:

have myosin heads, can perform latchbridge mechanism, has calmodulin, has myosin light-chain kinase (MLCK) and myosin light-chain phosphatase (MLCP), lack sarcomeres and z discs

Latchbridge mechanism

myosin attaches to actin for extended time without using extra ATP

Calmodulin

protein that binds Ca2+ to trigger contraction

Myosin light-chain kinase (MLCK)

enzyme that phosphorylates myosin heads when activated by calmodulin

Myosin light-chain phosphatase (MLCP)

enzyme that dephosphorylates myosin head for relaxation

Relaxation of smooth muscle

cessation of stimulation
removal of ca2+ from sarcoplasm
dephosphorylation of myosin head by MLCP

Smooth muscle contraction characteristics

long latent period, long duration, fatigue-resistant, broad length-tension curve

Long latent period

takes time to phosphorylate myosin head; slow ATPase activity

Long duration

slow calcium pumps; need for dephosphorylation of myosin head; latchbridge mechanism

Fatigue resistant

energy requirements low compared to skeletal muscle
can maintain contraction without ATP through latchbridge mechanism

Broad length-tension curve

lacks limitations because no z discs
myosin heads present in center of thick filaments
can contract forcefully, even when 50-200% of resting length