Muscular Response to Stimuli
Threshold stimulus – is the stimulus required for an action to occur. The amount of stimulus must reach and cross the threshold for an action to happen. If a not enough stimulus is produced then the action does not happen. Any process that is based on the stimulus occurs once threshold is met, it does not matter how much the stimulus was above threshold. It only matters that it crossed the threshold line. (Think all or none).
Threshold stimulus is important at the motor end plate. If stimulus at the motor end plate is reached by acetylcholine binding to receptors, then a signal is sent down the sarcolemma and sarcoplasmic reticulum for release of calcium. If there are not enough acetylcholine molecules binding to receptors on the motor end plate, then a muscle contraction does not occur.
Questions
What neurotransmitter does the axon release to cross the synaptic cleft? _________________
Where and what does the neurotransmitter bind to? __________________________________
Example. Let’s pretend it requires 20 acetylcholine molecules binding to receptors on the motor end plate to reach threshold.
What is threshold in this example? ________________________________________________
Does a reaction happen if the stimulus does not cross threshold? ________________________
Does the reaction occur if the stimulus crosses threshold by a little? ______________________
Does the reaction occur if the stimulus crosses threshold by a lot? _______________________
Is threshold reached if 10 molecules of acetylcholine bind? ______________________________
Does the muscle contract? _______________________________________________________
Is threshold reached if 18 molecules of acetylcholine bind? ______________________________
Does the muscle contract? _______________________________________________________
Is threshold reached if 23 molecules of acetylcholine bind? ______________________________
Does the muscle contract? _______________________________________________________
Is threshold reached if 25 molecules of acetylcholine bind? ______________________________
Does the muscle contract? _______________________________________________________
Is the contraction different than 23 molecules binding? __________________________________
Would the contraction be different if 35 molecules of acetylcholine bound to the motor end plate? ____________
Muscular responses
Muscles can be studied and removed by removing a single muscle fiber from a skeletal muscle and connecting it to an electric device and recording the findings.
Threshold stimulus – the muscle fiber remains unresponsive until a certain strength of stimulation is produced resulting in a threshold stimulus. Once a threshold is reached an electric impulses generated throughout the muscle fiber releasing calcium ions from the sarcoplasmic reticulum and into the fiber. A single impulse from a motor neuron releases enough acetylcholine for a muscle fiber to reach threshold.
Thresholds
Thresholds are common in the body. If the stimulus does not reach threshold the reaction does not occur. If enough stimulation is present to cross threshold then the reaction will occur.
For a muscle contraction to occur a stimulus must be strong enough to reach threshold. A single muscle fiber contraction from a single impulse is called a twitch. The latent period is the time prior to a muscle being stimulated and before the first contraction force can be measured. A latent period is the time required for the impulse to travel down the sarcoplasmic reticulum and calcium to be released into the muscle fiber. A small amount of time is required for calcium to diffuse into the fiber and begin binding to troponin and tropomyosin complex before the first myosin heads can pull actin and generate a measurable force. A human muscle has a latent period of less than 2 milliseconds.The initial contraction phase is called the period of contraction, followed by a period of relaxation where the pulling forces decrease.
Muscles tend to contract completely such that each twitch generates the same amount of force. This is referred to as a all-or-none response of each muscle fiber.
In the body different amounts of muscle fibers and motor units are recruited to contract at different times. The body will use a different amount of muscle fibers to pick a pen off the desk compared to a bowling ball. A bowling ball is going to require significantly more motor units to lift a bowling ball. Recruitment – is the increased number of motor units that the body stimulates for a desired action.
Fast twitch muscles – contract fast and fatigue quickly. Think of an Olympic hundred meter dash sprinter. Muscles that contract rapidly and produce a tremendous amount of force over a short period of time.
Slow twitch muscles – contracts slower and are fatigue resistant. Think of an Olympic marathon runner who is required to sustain a steady amount of contraction over long periods of time.
Individual people have different amounts of fast and slow twitch muscle. The average person has about half fast twitch and half slow twist. Olympic and professional athletes have a greater concentration of specific muscle fibers that benefit their sport.
Summation
With a twitch a muscle fiber is allowed to contract and then completely relax before the next stimulus causes a contraction. By allowing the muscle to completely relax the next stimulus will produce the exact same amount of force for each contraction.
In summation the force of contraction is increased because the next stimulus reaches the motor end plate before the fiber has completely relaxed. Summation results in a greater force of contraction in the muscle fiber. With a twitch the electric impulse causes release of calcium inside the muscle fiber causing a contraction. Then calcium is completely removed and contraction ceases. With summation calcium is released into the cell and contraction occurs, but before all the calcium can be removed and next stimulus causes an increased amount of calcium to be released into the muscle fiber. Continued stimulation results in more calcium being able to bind to troponin and tropomyosin complex, which increases the amount of myosin and actin binding.
The more calcium that is available inside the muscle fiber results in a greater force contraction because more myosin heads can bind and pull the actin fibers. The following the stimulus further increases the amount of calcium released into the fiber. Eventually the stimulus will stop and calcium will be eventually removed from the muscle fiber and contraction will stop.
Tetanus, or tetanic contraction, is the result of forceful and sustain contraction that lacks even partial relaxation. This is the maximal amount of force the muscle fiber is able to produce. In this situation calcium has flooded into the muscle fiber and increased amounts of calcium will not increase the contraction force. In tetanus the force of contraction is maximized because the myosin and actin binding is already maximized.
Recruitment of Motor Units
Summation increases the force of contraction of a single muscle fiber. A whole muscle can generate greater force if more muscle fibers are stimulated to contract. A motor unit is a motor neuron and the muscle fibers it controls. If more motor units are stimulated an increased number of fibers will contract and a greater force will be produced.