What are the components of the neuromuscular junction, and the events involved in the neural control

1
What are the components of the neuromuscular
junction, and the events involved in the neural
control of skeletal muscles?
2

• Neural stimulation of sarcolemma
• causes excitationcontraction coupling
• Cisternae of SR release Ca2
• which triggers interaction of thick and thin
  filaments
• consuming ATP and producing tension
• Active force applied to pull muscle fibers

3
Skeletal Muscle Contraction
Figure 109 (Navigator)
4

• The neuromuscular junction
• location of neural stimulation
• Action potential (electrical signal)
• travels along nerve axon
• ends at synaptic terminal

5

• Synaptic terminal
• releases neurotransmitter (acetylcholine or ACh)
• Into synaptic cleft (gap between synaptic
  terminal and motor end plate)

6

• Acetylcholine or ACh
• travels across the synaptic cleft
• binds to membrane receptors on sarcolemma (motor
  end plate)
• causes sodiumion rush into sarcoplasm
• is quickly broken down by enzyme
  (acetylcholinesterase or AChE)

7
Skeletal Muscle Innervation
Figure 1010a, b (Navigator)
8

• Action potential generated by increase in sodium
  ions in sarcolemma
• Travels along the T tubules
• Leads to excitationcontraction coupling

9

• Action potential reaches a triad
• releasing Ca2
• triggering contraction
• Requires myosin heads to be in cocked position
• loaded by ATP energy

10
What are the key steps involved in the
contraction of a skeletal muscle fiber?
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Sliding Filaments
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Skeletal Muscle Contraction

• Sliding filament theory
• thin filaments of sarcomere slide toward M line
  between thick filaments
• the width of A zone stays the same
• Z lines move closer together

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Steps of Muscle Contraction

• Action potential (AP) arrives causes change in
  TMP of nerve fiber
• Release of acetylcholine (ACh) binds to
  receptors on motor end plate
• Change in sarcolemma permeability to sodium.
  Influx of sodium causes an AP
• AP moves across the sarcolemma and down T-tubules
  
• Acetylcholinesterase (AChE) removes ACh from
  receptors
• Excitation-contraction coupling

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Figure 1010c
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Steps of the Contraction Cycle

• Exposure of active sites
• Formation of cross-bridges
• Pivoting of myosin heads
• Detachment of cross-bridges
• Reactivation of myosin

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Figure 1011
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Fiber Shortening

• As sarcomeres shorten, muscle pulls together,
  producing tension

Figure 1013
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• Contraction duration depends on
• duration of neural stimulus
• number of free calcium ions in sarcoplasm
• availability of ATP
• Contraction is an active process

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• Relaxation
• Ca2 concentrations fall
• Ca2 detaches from troponin
• Active sites are recovered by tropomyosin
• Sarcomeres remain contracted
• Relaxation and return to resting length is a
  passive process

20

• Rigor Mortis
• fixed muscular contraction after death
• Caused when
• ion pumps cease to function
• calcium builds up in the sarcoplasm

21
Table 101 (1 of 2)
22
What is the mechanism responsible for tension
production in a muscle fiber?
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• The allornone principal
• as a whole, a muscle fiber is either contracted
  or relaxed
• will produce the same tension at any given
  resting length
• tension is equal to the number of crossbridge
  attachments

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• Variation in force and duration of contractions
  depends on
• Number of muscle fibers stimulated
• the frequency of stimulation

25

• Motor Units
• single motor neuron and few to hundreds of muscle
  fibers that contract when stimulated
• Fibers are distributed through out muscle
• Recruitment
• Increase in tension produced by increasing the
  number of active motor units

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Motor Units in a Skeletal Muscle
Figure 1017
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Muscle Relaxation

• After contraction, a muscle fiber returns to
  resting length by
• elastic forces
• Pull of tendons and ligaments expands the
  sarcomeres to resting length
• opposing muscle contractions
• Reverse the direction of the original motion
• gravity

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Muscle Tone

• The normal tension and firmness of a muscle at
  rest
• Muscle units actively maintain body position,
  without motion
• Increasing muscle tone increases metabolic energy
  used, even at rest

29
What are the types of muscle contractions, and
how do they differ?
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2 Types of Skeletal Muscle Tension

• Isotonic contraction
• Skeletal muscle changes length
• resulting in motion
• Isometric contraction
• Skeletal muscle develops tension, but is
  prevented from changing length

31
What are the mechanisms by which muscle fibers
obtain energy to power contractions?
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ATP and Muscle Contraction

• Sustained muscle contraction uses a lot of ATP
  energy
• Muscles store enough energy to start contraction
• Muscle fibers must manufacture more ATP as needed

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Energy Stores

• Adenosine triphosphate (ATP)
• the active energy molecule
• Creatine phosphate (CP)
• the storage molecule for excess ATP energy in
  resting muscle
• Glycogen

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ATP Re-generation

• CP recharges ADP to ATP
• using the enzyme creatine phosphokinase (CPK)
• Aerobic respiration
• Anaerobic respiration

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Aerobic Metabolism

• Is the primary energy source of resting muscles
• Produces 36 ATP molecules per glucose molecule

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Anaerobic Glycolysis

• Is the primary energy source for peak muscular
  activity
• Produces 2 ATP molecules per molecule of glucose
• Breaks down glucose from glycogen stored in
  skeletal muscles

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Energy Use and Muscle Activity

• At peak exertion
• muscles lack oxygen to support mitochondria
• muscles rely on glycolysis for ATP
• pyruvic acid builds up, is converted to lactic
  acid

38
What factors contribute to muscle fatigue, and
what are the stages and mechanisms involved in
muscle recovery?
39

• Muscle Fatigue
• When muscles can no longer perform a required
  activity, they are fatigued
• Due to
• Depletion of metabolic reserves
• Damage to sarcolemma and sarcoplasmic reticulum
• Low pH (lactic acid)

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• Recovery Period
• The time required after exertion for muscles to
  return to normal
• Oxygen debt repaid
• Mitochondrial activity resumes

41

• Skeletal muscles at rest metabolize fatty acids
  and store glycogen
• During light activity, muscles generate ATP
  through anaerobic breakdown of carbohydrates,
  lipids or amino acids
• At peak activity, energy is provided by anaerobic
  reactions that generate lactic acid as a byproduct

42
What role does smooth muscle play in systems
throughout the body?
43

• In blood vessels
• regulates blood pressure and flow
• In reproductive and glandular systems
• produces movements
• In digestive and urinary systems
• forms sphincters and produces contractions
• In integumentary system
• arrector pili muscles cause goose bumps

44
What are the structural differences between
skeletal muscle fibers and smooth muscle cells?
45
Structure of Smooth Muscle

• Nonstriated tissue different internal
  organization of actin and myosin

Figure 1023
46
Characteristics of Smooth Muscle Cells

• Long, slender, and spindle shaped
• Have a single, central nucleus
• Have no T tubules, myofibrils, or sarcomeres
• Have no tendons or aponeuroses

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• Have scattered myosin fibers
• Thin filaments arranged in a mesh-like network
• Contraction causes muscle cell to twist