Exam 2: Lab 4 Contraction of Motor Units & Contraction of Whole Muscle Flashcards
What are the groups of muscle cells called, within a muscle, that are innervated by one motor neuron and contracted together and stimulated by that motor neuron?
motor unit
What important factors determine the strength of contraction of a muscle?
the size and number of motor units being stimulated
What is a motor unit composed of?
motor neuron and all of the muscle cells it stimulates
What is the junction between the branch of a neuron and a muscle cell called?
Neuromuscular junction
When a motor neuron fires, how many cells are stimulated?
all of muscle cells
What is the relationship between the strength of the muscle contraction and the number of motor units which are stimulated?
stronger contraction more number of motor units to be stimulated
What is recruitment
the stimulation of additional motor units for increased strength of contraction
What is the general function of motor units that contain only a few muscle cells?
generates precise movement
what two factors effect the strength and degree o f muscle movement?
number of motor units firing and the number of cells per motor unit
Describe the motor units in large muscles which exhibit gross movements.
They have a large motor units in which a single neuron is connected to a large number of muscle cells
Tell if the following muscles would have a small or large motor units:
a. muscles of the fingers
b. biceps brachii mucle
c. gastrocnemius muscle in speech
d. muscles of the throat involved
a. small
b. large
c. small
d. large
Muscle A contains 12,000 muscle cells and 30 motor neurons which innervate these cells. Muscle B contains 2,000 muscle cells and 400 motor neurons which innervate these cells.:
a. On average, how many muscle cells are there in each motor unit for both of these muscles?
b. Which of these muscles would you expect to contract with precise control?
a. Muscle A: 400 muscle cells
Muscle B: 5 muscle cells
b. B
what is muscle tone?
resistance to stretch and (low level tension)
What causes muscle tone?
random asynchronous motor unit contraction (provide a nearly constant state of low level tension)
What happens to a muscle if all the motor neurons to that muscle are cut?
it will become flaccid
What is a motor unit?
groups of muscle cells, within a muscle, that are innervated by one motor neuron and contracted together.
What is recruitment?
the stimulation of additional motor units for increased strength of contraction
Describe the different types of contraction that occur in motor units of different sizes.
small motor unit - precise movement
large motor unit - gross movement
what is muscle tone and how is it produced?
Muscle tone: resistance to stretch and produced by random asynchronous motor unit contraction
The level of muscle tension that can be developed within a given muscle is always constant.
a. true
b. false
b. false
What are the three factors which affect muscle tension in a whole muscle?
- Frequency of stimulation
- Number of motor units recruited
- Degree of muscle stretch
Muscle twitch
a muscle contraction in response to a single stimulus of adequate strength
What are the three phases of a muscle twitch?
- latent period
- contraction period
- relaxation period
latent period
- no visible shortening of the muscle
- the sarcolemma and the T tubules depolarize
Contraction period
myosin cross bridge cycling causes sarcomeres to shorten
relaxation period
- calcium ions are actively transported back into the terminal cisternae
- cross bridge cycling decreases and ends
- muscle to return to its original length
What two factors will cause the speed of the contraction phase of a muscle twitch to vary?
- slow-twitch fibers
2. fast-twitch fibers
Temporal (wave) summation
when a second stimulus of the same intensity is applied to a muscle before the completion of the relaxation period of the first stimulus resulting in increased muscle tension
In temporal summation, why is the second peak is higher than the first?
Additional influx of Ca+ promotes a second contraction, which is added to the first contraction.
If two stimuli are given to the same muscle, which will result in a more intense second contraction?
a. if the stimuli are given 50 milliseconds apart
b. if the stimuli are given 70 milliseconds apart
a.
If you wait until relaxation is complete from the first stimulus, then give a second stimulus to the same muscle, why wont temporal summation occur?
- cross bridge cycling has stopped; there is no activity to be summed
- The Ca+ ions from the first contraction have already been actively transported back into the terminal cisternae
complete tetanus
Muscle stay contracted, with no apparent relaxation
treppe
the muscle relax totally between stimulations
temporal summation
strength of contraction increase with each stimulation because of a gradual accumulation of calcium ions in the cytosol
incomplete tetanus
strength of contraction decrease with each stimulation
reasons for muscle fatigue
- ionic imbalance
- relative but not total lack of ATP
- a build-up of acidic compounds (lactic acid)
Factors that affects the development of muscle tension
a. number of motor units recruited
b. degree of muscle stretch
c. frequency of stimulation
a given muscle always uses the same number of motor units, no matter what the muscle is trying to accomplish.
a. true
b. false
b. false
subthreshold stimulus
the minimum stimulus (voltage) which can evoke a response is applied to a muscle
maximal stimulus
the stimulus (voltage) is applied to a muscle. when another, more intense stimulus does not evoke greater tension the original stimulus would be called maximal stimulus.
Temporal summation:
How does the time interval between stimuli affect he muscle contraction?
Does relaxation occur?
- smaller time frequency = increase contraction
- no relaxation occur
What happens to the graph of muscle contraction when the frequency of stimulation is increased?
reaches maximal stimulus
How does the number of motor units recruited affect the strength of contraction?
increase number of motor units = greater strength of contraction
multiple motor unit summation:
- how is threshold determined?
- how is this different than the threshold in vivo?
- in vivo: happen naturally
- pick up something/or move hit threshold
multiple motor unit summation:
- how are motor units recruited?
- how is this different from recruitment in vivo?
- varied voltage - varied wt pick up
- brain estimates number of motor unit needed and activates it.
