Transcript Chapter 9 PowerPoint B

Motor Unit: The Nerve-Muscle
Functional Unit
• Motor unit = a motor neuron and all (four to
several hundred) muscle fibers it supplies
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Spinal cord
Motor Motor
unit 1 unit 2
Axon terminals at
neuromuscular junctions
Nerve
Motor neuron
cell body
Motor
Muscle
neuron
axon
Muscle
fibers
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.
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Figure 9.13a
Motor Units
• Small motor units in muscles that control
fine movements (fingers, eyes)
• Large motor units in large weight-bearing
muscles (thighs, hips)
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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
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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
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2 Main Categories of Contraction
Isotonic
Isometric
The load is greater than
the tension the muscle is
able to develop
Muscle changes in
length and moves the The muscle neither
load
shortens nor lengthens
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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
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Isometric
Isotonic
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Figure 9.18a
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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?
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Where does the body get
the energy needed for
contraction?
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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
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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
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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
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(a)
Direct phosphorylation
Coupled reaction of creatine
phosphate (CP) and ADP
Energy source: CP
CP
ADP
Creatine
kinase
Creatine
ATP
Oxygen use: None
Products: 1 ATP per CP, creatine
Duration of energy provision:
15 seconds
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Figure 9.19a
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
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(b)
Anaerobic pathway
Glycolysis and lactic acid formation
Energy source: glucose
Glucose (from
glycogen breakdown or
delivered from blood)
Glycolysis
in cytosol
2
O2
ATP
Pyruvic acid
net gain
O2
Released
to blood
Lactic acid
Oxygen use: None
Products: 2 ATP per glucose, lactic acid
Duration of energy provision:
60 seconds, or slightly more
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Figure 9.19b
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
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(c)
Aerobic pathway
Aerobic cellular respiration
Energy source: glucose; pyruvic acid;
free fatty acids from adipose tissue;
amino acids from protein catabolism
Glucose (from
glycogen breakdown or
delivered from blood)
O2
Pyruvic acid
Fatty
acids
O2
Aerobic
respiration
Aerobic respiration
in mitochondria
mitochondria
Amino
acids
32
CO2
H2O
ATP
net gain per
glucose
Oxygen use: Required
Products: 32 ATP per glucose, CO2, H2O
Duration of energy provision: Hours
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Figure 9.19c
Short-duration exercise
ATP stored in
muscles is
used first.
ATP is formed
from creatine
Phosphate
and ADP.
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Glycogen stored in muscles is broken
down to glucose, which is oxidized to
generate ATP.
Prolonged-duration
exercise
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
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
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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
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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
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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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