1. Motor Unit: The Nerve-Muscle Functional Unit
Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies

2. neuromuscular junctions
Spinal cord
Motor neuron
cell body
Muscle
Nerve
Motor
unit 1
unit 2
fibers
neuron
axon
Axon terminals at
neuromuscular junctions
Axons of motor neurons extend from the spinal cord to the
muscle. There each axon divides into a number of axon
terminals that form neuromuscular junctions with muscle
fibers scattered throughout the muscle.
Figure 9.13a

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

4. Motor Unit
Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle
If a muscle has more than one motor unit…
Motor units in a muscle usually contract asynchronously; helps prevent fatigue

5. Muscle Tone 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
Does not produce movement, but keeps muscles firm, healthy, and ready to respond
Helps maintain posture and stabilize joints

6. Muscle changes in length and moves the load
2 Main Categories of Contraction
Isotonic
Muscle changes in length and moves the load
Isometric
The load is greater than the tension the muscle is able to develop
The muscle neither shortens nor lengthens

7. Isotonic Contractions
Isotonic contractions are either concentric or eccentric:
Concentric contractions—the muscle shortens and does work
Eccentric contractions—the muscle generates force as it lengthens

8. Isometric
Isotonic

9. Figure 9.18a

10. Figure 9.18b

11. Check Point!!!!!! What is a motor unit?
Betty White just signed up for a chin up competition to raise money for a local animal shelter. What type of muscle contractions are occurring in her biceps muscles immediately after she grabs the bar? As her body begins to move upward toward the bar? When her body begins to approach the mat?

12. Where does the body get the energy needed for contraction?

13. Muscle Metabolism: Energy for Contraction
ATP is the only source used for muscle contraction
Muscles store only 4–6 seconds worth of ATP at any given time.
ATP must therefore be created as quickly as it is broken down for a contraction to continue

14. Muscle Metabolism: Energy for Contraction
ATP is regenerated by:
Direct phosphorylation of ADP by creatine phosphate (CP)
2. Anaerobic pathway (glycolysis)
3. Aerobic respiration

15. Direct Phosphorylation
Fastest form of ATP regeneration
Creatine phosphate (CP) is stored in the muscles
After stored ATP is used, CP combines with ADP to make more ATP
Creatine phospate + ADP  Creatine + ATP
Can provide enough energy for 15 seconds of vigorous activity

16. Coupled reaction of creatine phosphate (CP) and ADP
Energy source: CP
(a) Direct phosphorylation
Oxygen use: None
Products: 1 ATP per CP, creatine
Duration of energy provision:
15 seconds
Creatine
kinase
ADP CP
ATP
Figure 9.19a

17. Anaerobic Pathway Glycolysis & Lactic Acid Formation
Makes ATP by breaking down glucose
Glucose breaks down into 2 pyruvic acid molecules, releasing energy to make ATP
2 ATP are produced per glucose molecule
Pyruvic acid is then converted into lactic acid
Produces less ATP than aerobic but it’s FAST!
Can support strenuous activity for about a minute

18. Glucose (from glycogen breakdown or delivered from blood)
Energy source: glucose
Glycolysis and lactic acid formation
(b) Anaerobic pathway
Oxygen use: None
Products: 2 ATP per glucose, lactic acid
Duration of energy provision:
60 seconds, or slightly more
Glucose (from
glycogen breakdown or
delivered from blood)
Glycolysis
in cytosol
Pyruvic acid
Released
to blood
net gain 2
Lactic acid O2
ATP
Figure 9.19b

19. Aerobic Pathway
Produces 95% of ATP during rest and light to moderate exercise
Occurs in the mitochondria and requires Oxygen
Turns fuels (stored glycogen, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids) into ATP
Produces 32 ATP per glucose, CO2, & H20

20. Glucose (from glycogen breakdown or delivered from blood)
Energy source: glucose; pyruvic acid;
free fatty acids from adipose tissue;
amino acids from protein catabolism
(c) Aerobic pathway
Aerobic cellular respiration
Oxygen use: Required
Products: 32 ATP per glucose, CO2, H2O
Duration of energy provision: Hours
Glucose (from
glycogen breakdown or
delivered from blood) 32 O2
H2O
CO2
Pyruvic acid
Fatty
acids
Amino
Aerobic respiration
in mitochondria
ATP
net gain per
glucose
Figure 9.19c

21. Short-duration exercise
Prolonged-duration
exercise
ATP stored in
muscles is
used first.
ATP is formed
from creatine
Phosphate
and ADP.
Glycogen stored in muscles is broken
down to glucose, which is oxidized to
generate ATP.
ATP is generated by
breakdown of several
nutrient energy fuels by
aerobic pathway. This
pathway uses oxygen
released from myoglobin
or delivered in the blood
by hemoglobin. When it
ends, the oxygen deficit is
paid back.
Figure 9.20

22. Muscle Fatigue
Physiological inability to contract even with a neural impulse
Occurs when:
ATP production fails to keep pace with ATP usage
When ATP use exceeds its production
Total lack of ATP occurs rarely, results in states of continuous contraction (contracture) Example: Writer’s Cramp

23. Oxygen Deficit Exercise uses oxygen stores in the muscle
Oxygen demand rises so extra Oxygen must be taken in by the body to restore normal levels of Oxygen
Vigorous exercise  heavy breathing  restores balance

24. Heat Production During Muscle Activity
Only about 40% of the energy released in muscle activity is useful as work
Remaining energy (60%) given off as heat
Dangerous heat levels are prevented by radiation of heat from the skin and sweating
Conversely, shivering will increase body temperature when cold

25. Critical Thinking!!!
When Eric returned from jogging, he was breathing heavily, sweating profusely, and complained that his legs ached and felt weak. His wife poured him a sports drink and urged him to take it easy until he could “catch his breath.” On the basis of what you have learned about muscle energy metabolism, respond to the following questions: Why is Eric breathing heavily? What ATP-generating pathway have his working muscles been using that leads to such breathlessness? What metabolic pathway might account for his sore muscles and his feeling of muscle weakness?