Muscle Physiology Flashcards
Principales of whole muscle contraction/ Terminology
▪ Muscle tension – force exerted by a contracting muscle on an object.
▪ Load – force exerted on a muscle by the weight of the object to be moved
(force opposing muscle tension)
The motor unit
a single neuron and all of the muscle fibers it innervates.
very few (4) to several hundread
Motor unit size : 1
▪ Small motor units have few muscle fibers.
◦ produce precise movements
◦ highly excitable
◦ usually activated first.
- ex. muscles controlling eyes & fingers
Motor unit size - 2
▪ Large motor units have many muscle fibers.
◦ produce gross (large, strong) movements.
◦ are activated only as demand increases.
- ex. muscles controlling leg movements
Graded muscle contractions :
Our muscle responses can be adjusted or graded to:
◦ provide varying strengths of muscle contractions needed to meet different demands
◦ promote smoothness of muscle contractions
Graded muscle contractions - two factors
Two factors affect the force generated by muscular contraction:
1.Number of motor units activated
(Recruitment)
Graded Muscle contractions - two factors pt 2
- Frequency of the nerve impulse firing rate by the motor neuron (Wave summation)
Graded muscle contractions process pt 1
1 - The number of motor units activated
•The more motor units that are activated, the greater the force or muscle tension generated.
This is called recruitment or multiple motor unit summation.
Graded muscle contractions process pt 1
▪ Recruitment in the body is not random but is based on the size of motor units.
…Remember, smaller motor units are activated first.
▪ Most commonly, motor units are activated asynchronously*.
◦ allows for prolonged contraction & delays fatigue
◦ contributes to smoothness of contractions
Graded muscle contraction process pt 2
The frequency of the nerve impulse firing rate by the motor neuron
•An increase in the firing rate causes a greater force (muscle tension) to be generated.
This is called wave summation.
Graded muscle contractions process pt 2
Wave summation:
▪ When two identical impulses are delivered rapidly to a muscle, the force of the second contraction will be stronger than the first.
Helps produce smooth, continuous contractions.
(Second stimulus must arrive after absolute refractory period.)
Muscle tone
▪ Even “relaxed” muscles are almost always slightly contracted.
▪ Nerve stimuli are almost continually sent so that at any one time some fibers are contracting.
▪ Occurs via spinal reflexes (not conscious).
Importance of muscle tone:
▪ stabilizes joints and maintains posture
▪ keeps muscles healthy and ready to respond to stimulation
▪ generates body heat
▪ does not produce active movements
Types of contractions:
two types
Isotonic contractions:
▪ Contraction that causes movement
▪ Tension of the muscle stays the same but the length of the muscle changes
Isometric contractions :
▪ Contraction against a force that is too great for a muscle to move
▪ Length of muscle stays the same even though the tension changes
Energy sources
Main ATP
ATP fuels muscular contraction:
ATP provides muscle tissue with energy to:
- move and detach myosin cross bridges
- operate the calcium pump in the SR
- operate the Na+-K+ pump in the plasma membrane
▪ ATPase of myosin heads breaks ATP down:
ATP → ADP + P + energy
▪ Muscle fibers store a very small amount of ATP in sarcoplasm or bound to myosin heads.
▪ Energy for first 4-6 sec. of contraction.
Other sources of ATP for muscle :
3
1- creatine phosphate - can regenerate ATP creatine + ADP - creatine + ATP phosphate
▪ Creatine phosphate is a high energy molecule stored in muscle.
▪ Muscle cells store 4-5 times as much CP as ATP.
▪ Stored ATP and creatine phosphate provide energy for around 15 seconds of contraction.
2- Anaerobic glycolysis of glucose to form ATP
1 glucose- 2 lactic+ 2 ATP
▪ Does not require oxygen (anaerobic).
▪ Occurs when the blood cannot supply oxygen to the muscle fast enough.
▪ Used for short term exertion (sprints).
▪ Provides ATP very rapidly but is wasteful.
▪ Liver converts lactic acid in blood back to glucose or glycogen when oxygen is available again.
3 Aerobic breakdown of glucose to form ATP.
1 glucose+ O2 - pyruvic- CO2+H2O+ 32 ATP
▪ Requires oxygen (aerobic).
▪ Used for long term exertion (marathons).
▪ Is relatively slow but provides lots of ATP.
Fat metabolism:
▪ Fatty acids are the main fuel source for resting muscle.
Types of skeletal muscle fibers:
There are three different types of muscle fibers.
Each serve somewhat different functions.
Fibers are classified on the basis of:
1. Shortening velocity (fast or slow)
2. Major pathway used to form ATP (anaerobic or aerobic)
Fast glycolytic fibers ( white fibers)
▪ contract rapidly because myosin heads break down ATP fast
▪ rely on anaerobic glycolysis
▪ have high glycogen content which supplies plenty of glucose
▪ respond quickly but fatigue quickly
▪ have few capillaries (little incoming glucose & O2)
▪ are pale in color because they have little myoglobin and few capillaries
▪ best for rapid, short-term, intense movements
▪ ex. lifting a weight
