Lecture 11: Muscle 3, 4, 5 Flashcards
Muscle twitch
a single action potential in a muscle fiber causes a brief, weak contraction called a twitch
Why does a twitch normally not take place in the body
It is too short and too weak to be useful
Whole muscles
Muscle fibers are arranged into these where they can function cooperatively to produce contractions of variable grades of strength, stronger than a twitch
Two primary factors can be adjusted to accomplish gradation of whole-muscle tension
- The number of muscle fibers contracting within a muscle
2. The tension developed by each contracting fiber
The greater the number of fibers contracting,
the greater the total muscle tension
Larger muscles consisting of more muscle fibers are capable of
generating more tension than the smaller muscles with fewer fibers
Each whole muscle is innervated by
a number of different motor neurons
When a motor neuron enters a muscle,
it branches with each axon terminal supplying a single muscle fiber
One motor neuron innervates a number of muscle fibers, but
each muscle fiber is supplied by only one motor neuron
When a motor neuron is activated, all of the muscle fibers it supplies are
stimulated to contract
Motor unit
One motor neuron plus all of the muscle fibers it innervates
Each muscle consists of a number of intermingled
motor units
Activation of only one or a few motor units lead to
A weak contraction of muscle
Motor unit recruitment
more and more motor units are recruited (stimulated) to contract
How much stronger the contraction will be with recruitment depends on
the size of the motor unit (the number of muscle fibers controlled by a single motor neuron)
The number of muscle fibers per motor unit and number of motor units per muscle depends on
the specific function of the muscle
For muscles that produce precise, delicate movements (ex. the eye), a single motor unit may contain
as few as 12 muscle fibers
Small motor units allow
a fine degree of control over muscle tension
For muscles that are designed for powerful, coarsely controlled movement, a single motor unit may contain
1000 or more muscle fibers
More powerful contractions occur at the expense of
precisely controlled movements
Asynchronous recruitment of motor units
Delays or prevents fatigue during sustained contraction involving only a portion of the motor units of the muscles
Important for supporting weight of body against force of gravity
Asynchronous recruitment of motor units is possible for ________ contractions but not for _______ contraction
submaximal
maximal
The type of muscle fiber recruited varies with
the type of tension required of the whole muscle
Muscles consist of a mixture of fiber types that differ ______, with some
metabolically
being more resistant to fatigue than others
During endurance activities
- Motor units most resistant to fatigue are recruited first
- The last fibers to be recruited when there is further demand for increases in tension are those that fatigue rapidly
Three factors that influence the extent to which tension can be developed
- Frequency of stimulation
- Extent of fatigue
- Length of the fiber at the onset of contraction
A single stimulation produces
a single twitch
All normal activities involve
sustained muscle contractions
Although an action potential triggers contraction of a muscle fiber, the action potential and its refractory period
are completed long before the contraction and relaxation phases are completed
Contractile mechanism in a muscle fiber exhibits no
refractory period
_____ of a muscle is not required before a second action potential can stimulate a second contraction
relaxation
Frequency of action potentials in a skeletal muscle increases
frequency of contraction
If a muscle fiber is restimulated after it has completely relaxed, the second twitch is
the same magnitude as the first twitch
If a muscle fiber is completely stimulated before it has completely relaxed, the second twitch
is added to the first which results in Wave (Twitch) Summation
There is continued stimulation of a muscle, never allowing it to relax completely and tension
rises to a peak
Incomplete tension
the state of a muscle producing peak tension during rapid cycles of contraction and relaxation
Tetanus
Muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between stimuli.
A smooth sustained contraction of maximal strength
Mechanism of Tetanus
- First action potential leads to Ca release from SR which leads to a contraction
- Relaxation begin as Ca is pumped back into SR
- If next action potential occurs before relaxation is complete then:
- Not enough time for all Ca to re-eter SR. Ca levels around the myofibrils remains elevated
- The magnitude of cross-bridge cycling and tension development increase correspondingly
Tetanus
As frequency of action potentials increases, the duration of elevated _______ ____ concentration increases, as well as ____ ______ until
cystolic Ca
contractile activity
a maximum contraction is reached
Virtually all normal muscular contractions involve complete
tetanus of the participating muscle fibers
Tetanus disease
- caused by Clostridium tetani
- bacteria release Tetanus toxin which prevents release of GABA from inhibitory presynaptic vesicles from the terminals of the neurons that supply skeletal muscles that result in uncontrolled spasm
Length-tension relationship
In a muscle fiber, the amount of tension generated during a contraction depends on the number of cross-bridge interactions that occur in all of the sarcomeres along the myofibrils
The number of cross bridge interactions is determined by
the degree of overlap between thick and thin filaments
When the muscle is stimulated to contract, what myosin heads can bind to active sites and produce tension
Those within the zone of overlap
Tension produced by the intact muscle fiber can be related to the structure of the
individual sarcomere
Length A
- Optimal range of sarcomere length
- Maximum number of cross bridges formed
- Tension produced is highest
- Muscles of the body are positioned that their relaxed length is approx. their optimal length, thus are capable of achieving near maximal tetanic contraction when stimulated
Length B
- Increase in sarcomere length
- Reduced overlap of potential cross-bridge overlap
- Reduced tension developed
Length C
- Zone of overlap disappears
- Thick and thin filaments cannot interact
- As a result, muscle fiber cannot actively produce any tension and a contraction cannot occur
Length D
- Sarcomeres are as short as they can be
- Thick and thin filaments are jammed against the Z-lines
- Cross-bridge can occur, but the myosin heads cannot pivot
- Less tension is produced
For every muscle there is an optimum length at which
maximal force can be achieved
Muscle fibers can contract forcefully when stimulated over
a relatively narrow range of resting length
