Laboratory Exercise 9: Skeletal Muscle Physiology

Laboratory Exercise 9: Skeletal Muscle Physiology Skeletal muscle will only contract when stimulated extrinsically (from outside itself) either by a nerve impulse or by electrical stimuli. Smooth and cardiac muscle can contract by intrinsic (from inside itself) stimulation. Applying electrical stimuli a muscle elicits muscle contractions and demonstrates stimulus-response relationships. To observe the stimulus-muscle response in the laboratory, the following equipment is required:

1. Stimulator - supplies controlled electrical stimuli in term of intensity, frequency and duration. 2. Transducer - converts mechanical energy of contraction to electrical energy. 3. Amplifier - magnifies the electrical energy. 4. Recorder - provides a graphic record, myogram of the muscle contraction.

Twitch Contraction The basic unit of mechanical action of skeletal muscle is the twitch. A twitch is a rapid, jerky response of a skeletal muscle to a single stimulus of sufficient intensity. The individual twitch is not significant; a series of twitches forms the basis of normal skeletal muscle contraction.

Three stages to a twitch: 1. Latent period (2-4 msec) - time lag between stimulation of the muscle and its contraction. Upon application of a threshold (liminal or minimal) or greater stimulus, the latent period is the time for electrical, chemical and physical events to occur that cause the contraction.

2. Contraction period (10-100msec) - muscle tension increases, contraction occurs as represented by an upward sweep of the myogram recording from the baseline.

3. Relaxation period (100-200msec) - muscle tension decreases, relaxation occurs as represented by a downward sweep of the myogram recording back to the baseline.

A twitch myogram. Key 1.: time of stimulation; 1-2: latent period; 2-3: contraction period; 3-4: relaxation period. A contraction-relaxation cycle lasts 100-200 msec.

1 Muscle Recruitment - Strength of Stimulus vs. Strength of Contraction

Individual muscle cells are grouped together into functional units called motor units. The motor unit is the functional unit of all types of muscles. The motor units consist of a motor neuron that supplies all the muscle cells in the motor unit. When the motor unit is stimulated, its muscles will obey the all or none law. The all or none law states, when a threshold stimulus (least strength of stimulus that can cause a contraction) is applied the muscle cells in the motor unit will contract to their fullest extent or not at all. When stimuli above threshold (sub-maximal stimuli) are applied to the motor neuron and its cells, the contraction is no more forceful than that of a threshold stimulus. As increased strength of stimulus (sub-maximal stimulus) is applied to the whole muscle, more and more motor units reach threshold are recruited and increases the force of contraction of the whole muscle. The total contractile force generated in a whole muscle is increased due to activation of more motor units. When a maximal and supra-maximal stimulus is given all motor units are contracting and no further contractile force is generated. This is known as muscle recruitment, multiple motor unit summation or spatial summation, due to the increase in number of motor units taking up space and contracting.

Wave Summation and Tetanus - Frequency of Stimulus vs. Strength of Contraction

The force of muscle contraction also depends on frequency and duration of stimulus, besides strength of stimulus.

Incomplete Tetanus

If stimulate the muscle at a frequency when the muscle does not have time to relax completely, i.e. during the relaxation period, a second more powerful contraction will occur. The adding of the twitches together is known as summation of twitches, wave summation or temporal summation. Temporal summation of the contraction is caused by frequency (rate) of stimuli going to the same contracting muscle cells before they have a chance to completely relax. The contractions are summed in term of time and the next contraction is greater than the previous one. This type of contraction is known as incomplete tetanus.

Complete Tetanus

If the frequency of the stimuli is very high, so the muscle is stimulated during the contraction period, the contractions fuse as the myogram appears as a smooth line and the contraction is greater that the single twitch. This sustained summed smooth contraction is known as tetanus.

In the intact body, skeletal muscle experiences tetanic contractions. The muscle receives stimuli at a frequency that causes the muscle to contract during the contraction period. This allows a summed smooth contraction. However, at any one time all the motor units and their muscle cells are not functioning together, but operate asynchronously. Thus when one motor unit is contracting, another is relaxing, but overall the muscle contraction is sustained and smooth.

Temporal summation may seem to violate the all or none law. The explanation for temporal summation and tetanus is due to the muscle's elastic connective tissue elements. During tetanus the muscle's elastic connective tissue is stretched. Once these elements are stretched and do not have time to relax, successive stimuli will cause a greater contraction of the muscle, because more of the energy is used for the contractile elements to lift the load and less energy is used to stretch out the elastic elements as they are already stretched out. Although the same number of motor units may be contracting during an isolated twitch or a tetanic contraction, the muscle generates a greater force when stimulated during tetanus.

Single Muscle Twitch

1. Latent Period

Upon observing the muscle twitch recording, the muscle does not respond immediately to a given stimulus of sufficient strength as the tracing does not leave the baseline. During this time the muscle appears not to be reacting after the stimulus has been applied is the latent period. This time period allows events to occur leading to the contraction of the muscle takes time. During the latent period, there is depolarization of the sarcolemma and its transverse (T) tubules, the migration of Ca ions from the SR and its cisternae into the sarcoplasm around the myofilaments to bind the troponin so as to free the actin’s active site for the myosins cross bridges and the attachment of the myosins cross bridges to the actin’s active site.

The time of the latent period for most mammalian muscles is about 4 msec, but can vary from muscle to muscle. The latent period is longer than 4 msec probably due to the viscosity of the muscle and the surrounding body fluids around the muscle.

2. Contraction Period

The contraction period follows the latent period. The muscle at first contracts isometric before it has developed enough tension to overcome the resistance. The muscle then contract isotonic as the tracing spikes upward. It is during the contraction period that the myosins’s cross bridges pivot and cause the actin myofilaments to slide pass the myosin myofilaments.

3. Relaxation Period

The relaxation period follows the contraction period as the tracing returns to the baseline. During the relaxation period the muscle components (muscle cells and connective tissues) return to their non- contracted state due to the force of gravity and /or contraction of the antagonist muscle(s).

The contraction period is a powered event as the energy from the high energy ~P of the ATP causes the attached myosin’s cross bridges to the actin’s active sites to pivot. However, for the muscle relaxation ATP is also needed. The ATP is needed to detach the myosin cross bridge from the actin active site. 3 Furthermore, the ATP is also needed to power the Ca ion pump in order to pump Ca ions back into the SR. The ATP also powers the Na-K ion pump to restore the proper Na to K ion concentrations across the sarcolemma after the action potential has occurred.

Thus upon examination of a muscle twitch you will see that the contraction period of the twitch is faster than the relaxation period of the twitch. Why? This is because the ATP used in the contraction period must be sufficiently regenerated to carry out the functions necessary for relaxation to occur and this takes time before the relaxation can occur.

4 5 Threshold Stimulus: Strength of Stimulus and Height of Contraction

Muscles are made up of functional units called motor units. Each motor unit consists of a motor neuron that supplies one or more muscle cells. When a motor unit is stimulated, its muscle component(s) will contract with all of the force it (they) can develop. In other words, all of the muscle cells in an active motor unit contract with all of the force they (it) can generate, or they (it) are not contracting at all. This is known as the All or None Principal. In a given muscle there are many motor units, which vary in their level of irritability. That is, some will respond to a weak stimulus, while others may require a much stronger stimulus. Often the smaller units are less irritable than the larger units. Therefore we often have better control when the task requires only slight strength and we have much poorer control when greater strength is needed. The overall strength of a contraction of a muscle is the summation of the contractions of the motor units operating at any one time.

There are two types of summation, spatial and temporal, to increase the strength of the overall muscle contraction. Increasing the strength of a stimulus increases the number of motor units that are operating this brings about increased contraction due to spatial summation. Remember that each muscle cells is contracting with all of its strength.

Temporal summation is brought about by increasing the rate of stimulus so that the same muscle cells are contracting again before they have had a chance to completely relax. Each individual cell can contract more powerfully when the stimuli arrive close together in this fashion than it can in response to a single stimulus. This may at first seem to violate the all or nothing principle. The explanation for temporal summation involves the elastic properties of muscles. Tension developed is transmitted through many structures. It is transmitted from the cross bridges through thick and thin filaments, across Z lines of the muscle cells, extra cellular connective tissue, and bone. Each of these components has a certain amount of elasticity. Only when all of these elastic structures are taut can increasing contraction by the muscle occur. When a second stimulus occurs very close to the first, the elastic structures are not yet slack. The result of this is the contraction is stronger than the simple muscle twitch.

6 In this experiment we will establish the liminal or threshold stimulus required to cause the most sensitive motor unit to contract, and see the consequences of increasing the stimulus strength to demonstrate that the increased force of contraction is due solely to spatial summation.

Example of threshold stimulus and spatial summation.

Notice the stair stepping of the response as the stimulus voltage was increased; also notice that when the maximal stimulus was reached, increasing the voltage further did not increase the response. In this case the threshold stimulus was 43 volts and the maximal stimulus was 92 volts.

7 This myogram illustrates the stimulus-response characteristics of a single muscle cell. Note that the response ("square wave”) is an "all or none" type of response. Compare this response to the one illustrated in the myogram below. Why is this response not seen in the experiments where the entire muscle is stimulated?

This myogram illustrates the stimulus-response behavior of a bundle of muscle cells (or a whole muscle). Note that the response is a gradual one or "graded". Compare this response to the one illustrated in the above myogram. Why is this response not observed in the single muscle cell?

8 Temporal (Wave) Summation and Tetanizing Contractions

When the frequency of stimulation becomes great enough, the muscle will not have time to relax completely before the next contraction begins. At this point the degree of contraction will increase due to temporal summation.

If the muscle is stimulated at still higher frequencies, the contractions will finally fuse, and a steady state of contraction called tetany will ensue. Once the frequency for a tetanic contraction has been reached, increasing the rate of stimulus any further will only increase the force of the contraction slightly.

Under normal circumstances active muscle fibers experience tetanic contractions. In other words whenever a motor unit fires, the firing is a rapid multiple fire. However at any one time the motor units of a given muscle would not be functioning together, but rather the motor units would be operating asynchronously so that when one motor unit is contracting, another is relaxing, but the overall muscle contraction would be very smooth (spatial summation).

The maximum strength of a tetanizing contraction is about 50 lbs. per square inch of cross sectional area of the muscle.

The experiment demonstrates an increase in the strength of contraction due solely to temporal summation caused by a tetanizing contraction.

Myogram of temporal summation and tetanic contraction. Temporal summation (incomplete tetanus): Stimulus period - 0.22 sec. Tetanizing contraction (complete tetanus): Stimulus period - 0.10 sec.