Unit V: MovementMuscle Contraction - Part II

Chapter 9 – pg 293-307

Review

1. What are the four stages of muscular activity?

2. Before a muscle fiber can contract, ATP must bind to: a.) a Z disc, b.) the myosin head, c.) sarcomere, d.) motor end plate

3. Before a muscle fiber can contract, Ca2+ must bind to: a.) calsequestrin, b.) the myosin head, c.) tropomysin, d.) troponin, e.) G-actin

4. The __________ portion of the thin filament houses the active site to make muscles contract.

5. Whereas the ______________ is the portion of the thin filament that changes shape and allows the muscles to relax.

Neural Control

Muscle Activity

A skeletal muscle fiber contracts when stimulated by a motor neuron

The action potential causes the release of ACh which leads to the production of an action potential in the sarcolemma.

Release of calcium ions from thesarcoplasmic reticulum.

Contraction cycle begins.

The sarcomeres shorten, pulling the ends of the muscle fiber closer together.

Skeletal muscle produces tension on the tendons at either end.

Tensionproduction

Muscle fibercontraction

leads to

Thick-thinfilament interaction

triggers

Calciumrelease

Excitation

Muscle Twitch in Frogs

• Threshold =

– Twitch: cycle of contraction and relaxation at threshold (lasting less than 1/10 second)

• Phases of a twitch contraction

– latent period (2 msec delay)

– contraction phase

– relaxation phase

• Myogram

Contraction Strength of Twitches

Intensity:

• Multiple motor unit summation (recruitment)

– lift a glass of milk versus a whole gallon of milk

Varies with:•Muscle temperature•Muscle pH•Hydration•Concentration of Ca+

•Stretch of muscle•Intensity•Frequency

Contraction Strength of Twitches

Frequency:

• Higher frequency = stronger contractions

– sustained fluttering contractions

• Maximum frequency stimulation (40-50 stimuli/second)

– Tetanus

Twitch

Muscle twitches

Stimuli(a)

High Highest-Tetanus

LowComplete tetanus

Fatigue

Tension in tendon

Motor unit 1

Motor unit 2

Motor unit 3

Ten

sion

Time

Asynchronous motor unit summation during a sustained contraction

Contraction Strength

Decrease in the resting sarcomere length reducestension.

Optimal range = optimal tension

Increase in sarcomere length, reduced tension

When overlap equals zero, fibers cannot produce tension.Normal

range

Decreased length Increased sarcomere length

Optimal resting length:75 to 130% of the optimal length.

With complete overlap, tension equals zero.

Ten

sion

(%

of

max

imu

m)

Sarcomere Length vs. Tension

Isometric and Isotonic Contractions

• Isometric muscle contraction– prelude to muscle movement

• Isotonic muscle contraction– Concentric

– Eccentric

6 kg 6 kg

6 kg

6 kg

Tendon

2 kg2 kg

Muscle Contraction Phases

Immediate Energy Needs

• Phosphagen system– myokinase– creatine kinase

• Short, intense exercise (100 m dash)

• Result is power enough for 1 minute brisk walk or 6 seconds of sprinting

Energy NeedsShort-term

• Glycogen-lactic acid system

– produces ATP for 30-40 seconds of maximum activity

• playing basketball or running around baseball diamonds Long-term

• Aerobic respiration

–Produces 36 ATPs/glucose molecule

Aerobic respirationusing oxygen frommyoglobin

Glycogen–lactic acidSystem (anaerobicfermentation)

Phosphagensystem

Duration of exercise

0 10 seconds 40 seconds

Aerobic respirationsupported bycardiopulmonaryfunction

Repayment ofoxygen debt

Mode of ATP synthesis

Muscle Fatigue

• Progressive weakness from use– ATP synthesis declines– Na+ and K+ pumps slow– lactic acid inhibits enzyme function– accumulation of extracellular K+– motor nerve fibers use up their ACh

Oxygen Debt

• Difference between resting rate of O2 consumption and elevated rate following exercise.

• Purposes for extra oxygen– replace oxygen reserves– replenishing the phosphagen system– oxidizing lactic acid– serving the elevated metabolic rate

Slow- and Fast-Twitch Fibers

• Slow oxidative, slow-twitch fibers, red

– adapted for aerobic respiration and resistant to fatigue

– postural muscles of the back (100msec/twitch)

• Fast glycolytic, fast-twitch fibers, white

– adapted for anaerobic fermentation

– sarcoplasmic reticulum releases calcium quickly so contractions are quicker (7.5 msec/twitch)

– extrinsic eye muscles, gastrocnemius and biceps brachii

• Proportions genetically determined

Cardiac Muscle

• Autorhythmic due to pacemaker cells

• aerobic respiration

– resistant to fatigue

– very vulnerable to interruptions in oxygen supply

Types of Smooth Muscle

Functional categories:

• Multiunit smooth muscle

– terminal nerve branches synapse on myocytes

– large arteries, iris, arrector pili muscles

– independent contraction

• Single-unit smooth muscle

– blood vessel walls, digestive, respiratory, urinary, and reproductive tracts

– coupled by gap junctions

– large number of cells contract as a unit

Synapses

Autonomicnerve fibers

(a) Multiunit smooth muscle

Gap junctions

Autonomicnerve fibers

Smooth Muscle Contraction

• Ca2+ binds to calmodulin

• activates myosin light-chain kinase

• which activates myosin ATPase

• power stroke occurs

• Thin filaments pull on plasma membrane

– shortens the entire cell in a twisting fashion

Repsonses to Stretch

• Ex. – esophagus distended by food brings on peristalsis

• Stress-relaxation response

– Ex. - urinary bladder

The process of peristalsis

Foodbolus

Towardanus

Longitudinalmuscle

Circular muscle

Circular musclescontract behindbolus.

Bolus of foodarrives indigestivesystem.

Longitudinalmuscles aheadof boluscontract.

Contraction incircular musclelayer forcesbolus forward.