1. Chapter 49: Sensory and Motor Mechanism
Presented by: McQuade and Verpooten

2. Muscles move skeletal parts by contracting The action of a muscle
Is always to contract

3. Skeletal muscles are attached to the skeleton in antagonistic pairs
With each member of the pair working against each other
Human
Grasshopper
Biceps contracts
Triceps relaxes
Forearm flexes
Biceps relaxes
Triceps
contracts
Forearm extends
Extensor muscle relaxes
Flexor muscle contracts
Tibia flexes
Extensor muscle contracts
Flexor muscle relaxes
Tibia extends
Figure 49.27

4. Vertebrate Skeletal Muscle
Bundle of muscle fibers
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Light band
Dark band
Z line
Sarcomere
TEM
0.5 m
I band
A band
M line
Thick filaments (myosin)
Thin filaments (actin)
H zone
Nuclei
Vertebrate skeletal muscle
Is characterized by a hierarchy of smaller and smaller units
Figure 49.28

5. A skeletal muscle consists of a bundle of long fibers
Running parallel to the length of the muscle
A muscle fiber
Is itself a bundle of smaller myofibrils arranged longitudinally

6. The myofibrils are composed to two kinds of myofilaments
Thin filaments, consisting of two strands of actin and one strand of regulatory protein
Thick filaments, staggered arrays of myosin molecules
Skeletal muscle is also called striated muscle
Because the regular arrangement of the myofilaments creates a pattern of light and dark bands

7. Each repeating unit is a sarcomere
Bordered by Z lines
The areas that contain the myofilments
Are the I band, A band, and H zone

8. The Sliding-Filament Model of Muscle Contraction
According to the sliding-filament model of muscle contraction
The filaments slide past each other longitudinally, producing more overlap between the thin and thick filaments

9. As a result of this sliding
The I band and the H zone shrink
(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bands and H zone are relatively wide.
(b) Contracting muscle fiber. During contraction, the thick and thin filaments slide past each other, reducing the width of the I bands and H zone and shortening the sarcomere.
(c) Fully contracted muscle fiber. In a fully contracted muscle fiber, the sarcomere is shorter still. The thin filaments overlap, eliminating the H zone. The I bands disappear as the ends of the thick filaments contact the Z lines.
0.5 m Z H A
Sarcomere
Figure 49.29a–c

10. The sliding of filaments is based on
The interaction between the actin and myosin molecules of the thick and thin filaments
The “head” of a myosin molecule binds to an actin filament
Forming a cross-bridge and pulling the thin filament toward the center of the sarcomere

11. Myosin-actin interactions underlying muscle fiber contraction
Thick filament
Thin filaments
Thin filament
ATP
ADP
P i
Cross-bridge
Myosin head (low- energy configuration)
Myosin head (high- energy configuration) +
Thin filament moves toward center of sarcomere.
Thick filament
Actin
Cross-bridge binding site
1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.
5 Binding of a new mole-
cule of ATP releases the
myosin head from actin,
and a new cycle begins. 2
The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration. P
1 The myosin head binds to actin, forming a cross- bridge. 3 4
Releasing ADP and ( i), myosin relaxes to its low-energy configuration, sliding the thin filament. P
Figure 49.30

12. (a) Myosin-binding sites blocked
When a muscle is at rest
The myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin
Actin
Tropomyosin
Ca2+-binding sites
Troponin complex
(a) Myosin-binding sites blocked
Figure 49.31a

13. (b) Myosin-binding sites exposed
For a muscle fiber to contract
The myosin-binding sites must be uncovered
This occurs when calcium ions (Ca2+)
Bind to another set of regulatory proteins, the troponin complex
Ca2+
Myosin- binding site
(b) Myosin-binding sites exposed
Figure 49.31b

14. The stimulus leading to the contraction of a skeletal muscle fiber
Is an action potential in a motor neuron that makes a synapse with the muscle fiber
Motor neuron axon
Mitochondrion
Synaptic terminal
T tubule
Sarcoplasmic reticulum
Myofibril
Plasma membrane of muscle fiber
Sarcomere
Ca2+ released from sarcoplasmic reticulum
Figure 49.32

15. The synaptic terminal of the motor neuron
Releases the neurotransmitter acetylcholine, depolarizing the muscle and causing it to produce an action potential

16. Action potentials travel to the interior of the muscle fiber
Along infoldings of the plasma membrane called transverse (T) tubules
The action potential along the T tubules
Causes the sarcoplasmic reticulum to release Ca2+
The Ca2+ binds to the troponin-tropomyosin complex on the thin filaments
Exposing the myosin-binding sites and allowing the cross-bridge cycle to proceed

17. Review of contraction in a skeletal muscle fiber
ACh
Synaptic terminal of motor neuron
Synaptic cleft
T TUBULE
PLASMA MEMBRANE SR
ADP
CYTOSOL
Ca2 P2
Acetylcholine (ACh) released by synaptic terminal diffuses across synaptic cleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber. 1
Action potential is propa-
gated along plasma
membrane and down
T tubules. 2
Action potential
triggers Ca2+
release from sarco-
plasmic reticulum
(SR). 3
Tropomyosin blockage of myosin-
binding sites is restored; contraction
ends, and muscle fiber relaxes. 7
Calcium ions bind to troponin;
troponin changes shape,
removing blocking action
of tropomyosin; myosin-binding
sites exposed. 4
Cytosolic Ca2+ is
removed by active
transport into
SR after action
potential ends. 6
Myosin cross-bridges alternately attach
to actin and detach, pulling actin
filaments toward center of sarcomere;
ATP powers sliding of filaments. 5
Figure 49.33

18. Neural Control of Muscle Tension
Contraction of a whole muscle is graded
Which means that we can voluntarily alter the extent and strength of its contraction
There are two basic mechanisms by which the nervous system produces graded contractions of whole muscles
By varying the number of fibers that contract
By varying the rate at which muscle fibers are stimulated

19. In a vertebrate skeletal muscle
Each branched muscle fiber is innervated by only one motor neuron
Each motor neuron
May synapse with multiple muscle fibers
Spinal cord
Nerve
Motor neuron cell body
Motor unit 1
Motor unit 2
Motor neuron axon
Muscle
Tendon
Synaptic terminals
Muscle fibers
Figure 49.34

20. Recruitment of multiple motor neurons
A motor unit
Consists of a single motor neuron and all the muscle fibers it controls
Recruitment of multiple motor neurons
Results in stronger contractions

21. More rapidly delivered action potentials
A twitch
Results from a single action potential in a motor neuron
More rapidly delivered action potentials
Produce a graded contraction by summation
Action potential
Pair of action potentials
Series of action potentials at high frequency
Time
Tension
Single twitch
Summation of two twitches
Tetanus
Figure 49.35

22. Tetanus is a state of smooth and sustained contraction
Produced when motor neurons deliver a volley of action potentials

23. Types of Muscle Fibers
Skeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolytic
Based on their contraction speed and major pathway for producing ATP

24. Types of skeletal muscles

25. Other Types of Muscle Cardiac muscle, found only in the heart
Consists of striated cells that are electrically connected by intercalated discs
Can generate action potentials without neural input

26. In smooth muscle, found mainly in the walls of hollow organs
The contractions are relatively slow and may be initiated by the muscles themselves
In addition, contractions may be caused by
Stimulation from neurons in the autonomic nervous system