Lecture: Muscles and Muscle Tissue 2

1. Same principles apply to contraction of a single fiber and a whole muscle

2. Contraction produces tension, the force exerted on teh load or object to be moved (muscle tension)

• 3. Contraction does not always shorten a muscle
• *Isometric Contraction: no shortening; muscle tension increases but does not exceed load
• *Isotonic Contraction: muscle shortens because muscle tension exceeds the load

4. Force and duration of contraction vary in response to stimuli of different frequencies and intensities

• motor unit: a motor neuron and all (four to several hundred) muscle fibers it supplies
• 

• -Small motor units in muscles that control fine movements (fingers, eyes)
• -Large motor units in large weight bearing muscles (thighs, hips)

-muscle fibers from a motor unit are spread throughout muscle so that a single motor unit causes a weak contraction of the entire muscle

-motor units in a muscle ususally contract asynchronously; helps prevent fatigue

-Response of a muscle to a single, brief threshhold stimulus

-Simplest contraction observable in the lab (recorded by a myogram)

• Three phases of a Twitch
• 1. Latent period: events of E-C coupling
• 2. Period of Contraction: cross bridge formation; tension increases
• 3. Period of Relaxation: Ca²⁺ reentry into the SR; tension declines to zero
• 

Different strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles

• -variations in the degree of muscle contraction
• -required for proper control of skeletal movement

• Responses are graded by:
• 1. changing the frequency of the stimulation
• 2. changing the strength of the stimulation

• -A single stimulus results in a single contractile response-a muscle twitch
• 
• -increase in frequency of stimulus (muscle does not have time to relax between stimuli)

• -Ca2+ release stimulates further contraction--> temporal (wave) summation
• 
• -Further increase in stimulus frequency--> unfused (incomplete) tetanus

• -If stimuli are given quickly enough, fused (complete) tetany occurs
• *pretty rare, like if someone is lifting up a car
• 

• 
• twict, wave, unfused tetanus, fused tetany

• When frequency and strength are constant
• 

Threshold stimulus: stimulus strength at which that first observable muscle contraction occurs

-Muscle contracts more vigorously as stiimulus strength is increased above threshold

-Contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action

Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases

-Constant, slightly contracted state of all muscles

-Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles

-Keeps muscles firm, healthy, and ready to respond (helps posture and stabilize joints)

-muscle changes in length and moves the load

• -Isotonic contraction are either concentric or eccentric
• 1. Concentric: the muscle shortend and does the work
• 2. Eccentric: the muscle contracts as it lengthens (like your calf muscle when you walk uphill)

-The load is greater than the tension the muscle is able to develop

-Tension increases to the muscle's capacity, but the muscle neither shortens nor lengthens

*object is not lifted or something is being held in position

-ATP is the only source used directly for contractile activities

-Available stores of ATP are used within 4-6 seconds

• -ATP is regenerated by
• 1. Directy phosphorylation of ADP by creatine phosphate (CP)---> it gives enough ATP for 15-16 seconds and is replenished when resting
• 
• 2. Anaerobic Pathways
• 3. Aerobic respiration

-At 70% of maximum contractile activity:

• *bulging muscles compress blood bessels
• *oxygen delivery is impaired
• * pyruvic acid is converted into lactic acid

• Lactic acid:
• *diffuses into the bloodstream
• *used as fuel by the liver, kidneys, and heart
• *converted back into pyruvic acid by the liver
• 

-Produces 95% of ATP during rest and light to moderate exercise

-Fuels: stored glycogen, then bloodstream glucose, pyruvic acid, and free fatty acuds

-occurs int he mitochondria

-Physiological inability to contract

• Occurs when:
• *Ionic imbalances (K⁺, Ca²⁺, etc) interfere with E-C coupling
• *Prolonged exercise damages the SR and interferes with Ca²⁺ regulation and release

-Total lack of ATP is rare, during states of continuous contraction, and causes contractures (continuous contractions) like ragamortis

Extra O2 needed after exercise for:

• -The replenishment of:
• * oxygen reserves
• * glycogen stores
• *ATP and CP reserves

-Conversion of lactic acid to pyruvic acid, glucose, and glycogen

About 40% of the energy released in muscle activity is useful as work

The other 60% is given off as heat

-dangerous heat levels are prevented by radiation of heat from the skin and sweating

The force of the conraction is affected by:

*number of muscle fibers stimulated (Recruitment)

*Relative size of the fibers --hypertrophy of cells increases strength (increase size by regular resistence exercise)

*Frequency of Stimulation- higher frequency allows time for more effective transfer of tension to noncontractile components (summation of contractions)

• *Length-Tension Relationship: muscles contract most strongly when muscle fibers are 80-120% of their normal resting length (when cross bridges can happen)
• 
• 

-How fast they can contract and how long it takes for it to fatigue

• Influenced by:
• 1. Muscle fiber type
• 2. Load
• 3. Recruitment

Classified according to 2 different characteristics:

• 1. Speed of contraction: slow or fast, according to
• *speed at which myosin ATPases split ATP
• *Patter of electrical activity of the motor neurons

• 2. Metabolic oathways for ATP synthesis:
• *oxidative fibers--use aerobic pathways
• *Glycolytic fibers--use anaerbic glycolysis

• Split into 3 types:
• 1. Slow oxidative fibers (SO)
• 2. Fast oxidative fibers (FO)
• 3. Fast Glycolytic fibers (FG)

larger the load = the larger the latent period, smaller the contraction, and shorter duration of contration

Recruitment--> faster contraction and longer duration of contraction

Endurance exercise

• leads to increased:
• 1. Muscle capillaries
• 2. number of mitochondria
• 3. Myoglobin synthesis

results in greater endurance, strength, and resistance to fatigue

May convert fast glycolytic fibers into fast oxidative fibers

Typically Anaerobic

• Results in:
• 1. Hypertrophy (due to increase in fiber size)
• 2. Increased mitochondria, myofilaments, glycogen stores, and connective tissue

Forcing a muscle to work hard promotes increased muscle strength and endurance

Muscles adapt to increasing demands

Muscles can be overloaded to produce further gains