Muscles contract through the sliding filament mechanism, where actin filaments slide past myosin filaments. Contraction is regulated by motor neurons at the neuromuscular junction releasing acetylcholine, which triggers excitation-contraction coupling in the muscle fiber. Skeletal muscle contractions can be graded, tetanic, or isometric depending on stimulus frequency. Muscles require ATP generated through aerobic and anaerobic pathways to fuel contraction and fatigue results from an inability to continue contracting.

1. Muscles

2. Contraction of a skeletal muscle
fiber

8. Sliding filament mechanisms of
contraction
The sliding filament theory of contraction
states that during contraction, the actin
filaments slide past the myosin filaments so
that they overlap to a greater degree.

13. Events leading to muscle
contraction
Cross bridge attachment
The working stroke
Cross bridge detachment
Return of myosin head to high energy
position

15. Regulation of contraction
The neuromuscular junction is formed from
the axon of each motor neuron as each
divides profusely as it enters the muscle.
The synaptic cleft separates the axonal
ending from the muscle, and is filled with a
gel-like substance.

19. Within axonal ending are synaptic vesicles,
which are small membranous sacs
containing a neurotransmitter called
acetylcholine.
The motor end plate is the trough-like part
of the muscle fiber’s sarcolemma that helps
form the neuromuscular junction, which
provide surface area for the ACh receptors.

20. Depolarization is an event that occurs when
there is a change in membrane potential
such that the muscle cell interior becomes
slightly less negative.
The action potential is a large transient
depolarization event that is conducted along
the membrane of a muscle cell or a nerve
fiber.

23. Repolarization is movement of the
membrane potential to the initial resting
state.
Refractory period is the period during
which an excitable cell is not responsive to
a threshold stimulus.

24. All-or-none response refers to the fact that
muscle fibers contract to the full extent of
their ability or not at all.
Acetylcholinesterase is an enzyme present
at the neuromuscular junction that prevents
continued muscle contraction in the absence
of additional stimulation.

25. Excitation-contraction coupling is a
sequence of events by which transmission
of an action potential along the sarcolemma
leads to the sliding of myofilaments.

28. Contraction of a skeletal muscle

29. The motor unit
A motor unit is a motor neuron and all the
muscle cells it stimulates.

31. The muscle twitch and
development of muscle tension
A myogram is a graphic recording of
mechanical contractile activity produced by
an apparatus that measures muscle
contraction.
A muscle twitch is the response of a muscle
to a single brief threshold stimulus.

34. Phases of a muscle twitch
Latent period is the period of time between
stimulation and the onset of muscle
contraction.
Period of contraction is the time from the
onset of shortening to the peak of tension
development.
Period of relaxation is initiated by reentry
of Ca+2 into the SR.

35. Graded muscle responses
A graded response is a variation in the
degree of muscle contraction by changing
the frequency or strength of the stimulus.
Wave summation occurs if two identical
stimuli are delivered to a muscle in rapid
succession, and the second twitch is
stronger than the first.

37. Tetanus is a sustained muscle contraction
resulting from high-frequency stimulation.
Recruitment is achieved by delivering
shocks of increasing voltage to the muscle,
increasing the number of muscle fibers
associated with the contraction.

38. Threshold stimulus is the weakest stimulus
capable of producing a response in a
irritable tissue.
Maximal stimulus is the strongest stimulus
that produces increased contractile force.

39. Treppe: The staircase effect
Treppe refers to the increasing force of
muscle contractions as they occur later in
response to stimuli of the same strength.

41. Muscle tone
Muscle tone refers to a sustained partial
contraction of a muscle in response to
stretch receptor inputs.

42. Isotonic and isometric
contractions
Muscle tension is the force exerted by a
contracting muscle on some object.
Load is the weight exerted by the object on
the muscle.

43. Isotonic contraction is a contraction in
which muscle tension remains constant and
the muscle shortens.
Concentric contractions are isotonic
contractions in which the muscle shortens
and does work.

45. Eccentric contractions are isotonic
contractions in which the muscle contracts
as it lengthens.
Isometric contractions cause the tension to
increase, but the muscle neither shortens
nor lengthens.

47. Muscle metabolism

48. Providing energy for contraction
Creatine phosphate is a compound that
serves as an alternative energy source for
muscle tissue.
Creatine kinase is an enzyme that catalyzes
the transfer of phosphate from
phosphocreatine to ADP, forming creatine
and ATP.

49. Aerobic endurance is the length of time a
muscle can continue to contract using
aerobic pathways.
The anaerobic threshold is the point at
which muscle metabolism converts to
anaerobic glycolysis.

53. Muscle fatigue
Muscle fatigue is a state of physiological
inability to contract.
Contractures are states of continuous
contraction.

54. Oxygen debt
Oxygen debt is the volume of oxygen
required after exercise to oxidize the lactic
acid formed during exercise.

55. Force, velocity, and duration of
muscle contraction

56. Force of contraction
Series-elastic elements refers to all the non-
contractile structures of muscles.
Internal tension is generated by the
myofibrils, and stretches the series-elastic
elements.

57. External tension is transferred from the
series-elastic elements to the load.
Length-tension relationship refers to the
optimal resting length for muscle fibers at
which they can generate maximum force,
which is when a muscle is slightly stretched
and the filaments barely overlap.

65. Velocity and duration of
contraction
Slow oxidative fibers contract slowly and
depend on plentiful oxygen delivery.
Fast oxidative fibers contract quickly and
depend on plentiful oxygen delivery.
Fast glycolytic fibers contract quickly and
depend on plentiful glycogen reserves, but
do not use oxygen.

66. Effect of exercise on muscles

67. Adaptations to exercise
Aerobic exercise results in greater synthesis
of myoglobin, and increased capillaries and
mitochondria surrounding the muscle fibers.
Resistance exercise results in muscle
hypertrophy, and occurs under anaerobic
conditions.

68. Arrangement and microscopic
structure of smooth muscle fibers

69. Peristalsis is the alternating contraction and
relaxation of opposing layers of smooth
muscles to mix substances in the alimentary
canal.

70. Bulbous swellings called varicosities
release neurotransmitter into a wide
synaptic cleft in junctions in the smooth
muscle called diffuse junctions.
The plasma membrane of the smooth
muscle fiber has multiple pouch-like
infoldings called caveoli.

71. Dense bodies attach to non contractile
filaments that resist tension.
Dense bands act as anchoring points for
groups of thin filaments, and correspond to
the Z discs of the skeletal muscles.

72. Contraction of smooth muscle

73. Special features of smooth
muscle contraction
Stress-relaxation response, in which
stretching of the smooth muscle moves
substances along an internal tract, allows a
hollow organ to fill or expand slowly to
accommodate a greater volume without
promoting contractions to expel their
contents.