lab 6 - muscle contraction

Describe how the isolated muscle responded as the stimulus interval was decreased progressively (for summation experiement)

As the stimulus interval was decreased, the muscle was contracting or twitching closer. The graph shows that with each stimulus, the curves got closer until it almost became one whole curve. This represents summation

What do threshold stimulus voltage and maximum excitation voltage signify in terms of your muscle preparation

The threshold value signifies the smallest amount of stimulus needed for a contraction to occur. The maximum excitation value is when the stimulus continues to grow more and more beyond the threshold value, but the contractions remain constant

tetanus

a sustained muscular contraction resulting from a rapid series of nerve impulses; With increasing frequency of stimulation, there is less and less time for the muscle fiber to relax between stimuli and eventually the contractions fuse and a smooth powerful contraction - tetanus - is seen

recruitment of motor units

increase in number of motor units activated as stimulation intensity increases; As you increase the voltage in stimulator (that mimics activation of motor neurons), more muscle fibers will be electrically stimulated, causing increased contraction of the muscle

The strength of a muscle contraction can be increased in two ways

increasing the number of active motor units (recruitment) and stimulating existing active motor units more frequently

Failure of the muscle to contract may be due to

lack of a good connection between the muscle and the bipolar stimulator

summation experiment

stimulate the muscle with two pulses at different pulse intervals and observe the effect on muscle contractions

unfused tetanus

type of wave summation with partial relaxation observed between twitches

muscles are

variable activation via recruitment

To achieve maximal contraction (twitch recruitment)

we increased the stimulus amplitude; With greater stimulus amplitude, more motor units were recruited (increased activated), hence a stronger contraction, until a point was reached where every motor unit was recruited, and therefore every muscle fiber was contracting, and maximum contraction was reached

Gastrocnemius (frog)

#1

motor unit

A motor neuron and all of the muscle fibers it innervates; Most skeletal muscles, as the name suggests, are attached to bones, and their contractions are responsible for supporting and moving the skeleton. As the figure shows, each muscle is made up of many individual muscle fibers organized in fascicles. Each individual muscle fiber is innervated by a branch of an axon from a motor neuron.

neuromuscular junction

(1) The action potential reaches the axon terminal. (2) Voltage-dependent calcium gates open, allowing calcium to enter the axon terminal. (3) Neurotransmitter vesicles fuse with the presynaptic membrane and acetylcholine (ACh) is released into the synaptic cleft via exocytosis. (4) ACh binds to postsynaptic receptors on the sarcolemma. (5) This binding causes ion channels to open and allows sodium ions to flow across the membrane into the muscle cell. (6) The flow of sodium ions across the membrane into the muscle cell generates an action potential which travels to the myofibril and results in muscle contraction. Labels: A: Motor Neuron Axon B: Axon Terminal C. Synaptic Cleft D. Muscle Cell

causes of muscle fatigue

1. buildup of ADP, Pi, Mg2+, H+ (lactic acid buildup) 2. H+ competes w Ca2+ in binding to troponin (inhibition of cross bridge cycling) 3. Conduction failure (lack of power stroke)

Provide a possible mechanism for why the muscle was unable to maintain a prolonged contraction during fatigue

A possible mechanism to explain the inability to maintain a prolonged contraction during fatigue is that action potentials are constantly being fired, causing the t tubules to be depolarized since the K+ channels are left open. Because of the depolarization, action potentials cannot be generated leading to a conduction failure.

summation

A second stimulus arriving before the muscle has relaxed again, causes a second twitch on top of the first so that greater peak tension is developed

muscle fatigue

Inability of muscle to maintain its strength of contraction or tension

In a state of rigor mortis, the muscles are highly contracted and difficult to manipulate. Why does this phenomenon occur? a) Excess stimulation from motor neuron b) No stimulation from motor neuron c) Lack of ATP d) Lack of Calcium e) Nonsense, The Walking Dead proves corpses can move very well!

Lack of ATP in corpse leads to myosin heads to be firmly attached to the actin filament binding site. Why does rigor mortis go away? Decomposition of tissue (release of enzymes)

Describe how the isolated muscle responded as it was stretched progressively (for effects of stretch experiment)

Since the muscle was stretched, the contraction decreased due to the fact that there was less overlap of the contracting filaments

In light of the "all or none" law of muscle contraction, how can you explain the graded response

The all or none law is with action potentials being generated for a muscle to contract. If the stimulus is not strong enough and doesn't reach the threshold, an action potential cannot be generated and no contraction occurs. Graded response has to do with more stimulus intensity which allows recruitment of motor units

length-tension curve

The curve shows that the active tension is maximal when the muscle is near its rest length a & e - outside physiological range (e is stretched beyond normal physiological length) b & d - max physiological range (b is shortest physiological length, d is longest physiological length) c - optimal physiological range

What stretch resulted in the highest contraction force? What is happening in the muscle at the highest stretch levels

The highest contraction force was at 0 mm. Because the muscle wasn't stretched out as much as it was at 5mm, a full contraction wasn't able to occur

Describe how the isolated muscle responded to high frequency stimulation (for fatigue experiment)

The muscle stayed in a constant plateau and then it finally relaxed

a) Define Tetanus. b) At which stimulus interval did you observe tetanus? c) Explain the mechanism behind this phenomenon.

a) Tetanus is a sustained muscle contraction resulting from rapid series of nerve impulses. b) At the interval 0.1 there was an incomplete tetanus observed. At interval 0.04, the tetanus became complete as it plateaued. c) The incomplete tetanus demonstrates the stimulus starting to create a constant response and becomes even more defined when it becomes complete. This happens becuase the muscle has reached its maximum contraction.

motor units are

all or none

supramaximal stimulus (for twitch recruitment)

any stimulus stronger than a maximal stimulus. These stimuli cannot produce a greater frequency of action potentials than a maximal stimulus.

tetanus experiment

examine the muscle's response to a series of stimuli at different frequencies

fatigue experiment

examine the muscle's response to a sustained series of stimuli at a high frequency

twitch recruitment experiment

progressively increase the strength of stimulation and record the effects on the force of muscle contraction

effects of stretch experiment

progressively increase the stretch on the muscle and record the effects on the force of muscle contraction.

Length-tension relationship relates

relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs. Muscles operate with greatest active tension when close to an ideal length (their resting length). When stretched or shortened beyond this, the maximum active tension generated decreases. This decrease is minimal for small deviations, but the tension drops off rapidly as the length deviates further from the ideal

twitch

the brief contraction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron

To achieve tetanus

we incrementally decreased the interval between stimuli. In other words, increased the stimulus frequency. "With increasing frequency of stimulation, there is less and less time for the muscle fiber to relax between stimuli and eventually the contractions fuse and a smooth powerful contraction - tetanus - is seen

A typical threshold stimulus is

~ 0.5-1V