Copyright © 2010 Pearson Education, Inc. Motor Unit: The Nerve-Muscle Functional Unit Motor unit = a motor neuron and all (four to several hundred) muscle

Copyright © 2010 Pearson Education, Inc.

Motor Unit: The Nerve-Muscle Functional Unit

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

Copyright © 2010 Pearson Education, Inc. Figure 9.13a

Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular 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.


Motor Units• Small motor units in muscles that control fine movements (fingers, eyes)

• Large motor units in large weight-bearing muscles (thighs, hips)


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


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


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


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


Isometric

Isotonic


Copyright © 2010 Pearson Education, Inc. Figure 9.18b


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?


Where does the body get the energy needed for

contraction?


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


Muscle Metabolism: Energy for Contraction

ATP is regenerated by:

1. Direct phosphorylation of ADP by creatine phosphate (CP)

2. Anaerobic pathway (glycolysis)

3. Aerobic respiration


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


Coupled reaction of creatinephosphate (CP) and ADP

Energy source: CP

(a) Direct phosphorylation

Oxygen use: NoneProducts: 1 ATP per CP, creatineDuration of energy provision:15 seconds

Creatinekinase

ADPCP

Creatine ATP


Anaerobic PathwayGlycolysis & 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

Copyright © 2010 Pearson Education, Inc. Figure 9.19b

Energy source: glucose

Glycolysis and lactic acid formation

(b) Anaerobic pathway

Oxygen use: NoneProducts: 2 ATP per glucose, lactic acidDuration of energy provision:60 seconds, or slightly more

Glucose (fromglycogen breakdown ordelivered from blood)

Glycolysisin cytosol

Pyruvic acid

Releasedto blood

net gain

2

Lactic acid

O2

O2ATP


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

Copyright © 2010 Pearson Education, Inc. Figure 9.19c

Energy source: glucose; pyruvic acid;free fatty acids from adipose tissue;amino acids from protein catabolism

(c) Aerobic pathway

Aerobic cellular respiration

Oxygen use: RequiredProducts: 32 ATP per glucose, CO2, H2ODuration of energy provision: Hours

Glucose (fromglycogen breakdown ordelivered from blood)

32

O2

O2

H2O

CO2

Pyruvic acidFattyacids

Aminoacids

Aerobic respirationin mitochondriaAerobic respirationin mitochondria

ATP

net gain perglucose

Copyright © 2010 Pearson Education, Inc. Figure 9.20

Short-duration exerciseProlonged-durationexercise

ATP stored inmuscles isused first.

ATP is formedfrom creatinePhosphateand ADP.

Glycogen stored in muscles is brokendown to glucose, which is oxidized togenerate ATP.

ATP is generated bybreakdown of severalnutrient energy fuels byaerobic pathway. Thispathway uses oxygenreleased from myoglobinor delivered in the bloodby hemoglobin. When itends, the oxygen deficit ispaid back.


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


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


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


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?

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Copyright © 2010 Pearson Education, Inc. Motor Unit: The Nerve-Muscle Functional Unit Motor unit = a motor neuron and all (four to several hundred) muscle