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Simple Muscle twitch By: Dr. Khaled Ibrahim

By the end of this section, you should : Objectives: By the end of this section, you should : 1- Describe the nerve-muscle preparation. 2- Define the stimulus and enumerate its various types. 3- Describe the procedure of the experiment. 4- Illustrate the simple muscle twitch (SMT). 5- Describe the various parts of SMT.

Dissection of the frog Aim of the experiment: To obtain the gastrocnemius muscle – sciatic nerve preparation. Steps: 1) Pith the frog by passing the dissecting needle through the space between the first cervical vertebra and the skull, and then destroy the brain and the spinal cord. 2) With a strong scissors divide the frog into two parts at the middle of the vertebral column, just below the forearms. 3) Hold the lower part of the vertebral column in one hand and the edge of the skin with the other. Pull the skin to remove it until the toes are exposed. 4) The anterior abdominal wall and the viscera are removed.

5) Remove the urostyle and cut with the scissors the backbone in the middle line without injuring the sciatic roots on either side. 6)By another cut in the middle line separate the two lower limbs from each other. 7) Place one limb in physiological saline of the frog as a reserve. 8) Hold the vertebral column of the other by the forceps and carefully dissect down the sciatic nerve, cutting through all the tissues around it. Cut the femur about 0.5 cm above the knee dividing the muscles at the same point. 9) Tie a thread around the tendo-achillis and then separate it from its attachment to the bone. Lift the thread and with it the muscle, breaking down connective tissue septa between it and the bone divide the tibia with scissors below the knee. Physiological saline of the frog: 0.6% sodium chloride solution for frog’s tissues.

The preparation consists now of three or four half vertebrae with nerve roots, the sciatic nerve, the knee joint and the gastrocnemius. 10) Mounting the preparation on the myograph: Place the preparation on the myograph, fix the knee joint strongly to the myograph by a pin, and attach the thread to the vertical limb of the lever. Slide the lever away from the myograph until the thread is stretched and the horizontal part of the lever is horizontal. Be sure that the lever writes evenly up and down on the drum. Apply the stimulating electrodes to the sciatic nerve when indirect stimulation is needed or on the muscle when direct stimulation is needed.

Stimulus Definition: It is any change in the surrounding environment. Types 1 Chemical 2 Physical 3 Electrical Chemical transmitters - Hormones. - Drugs. -Ions (Na+, K+, .... etc). - Gases (O2 and CO2). Thermal. e.g. cooling or warming. - Mechanical. e.g. stretch, touch, pressure and injury. - Electromagnetic. e.g. light rays - Galvanic Current: Low intensity Long Duration - Faradic Current: High intensity Short duration

Electrical stimuli are commonly used for stimulation in experimental work because they are: - Easily applied. - Accurately controlled as regard: strength & duration. - Similar to the physiological process of excitation. So, they cause no (or minimal) damage to the tissues & can be repeated.

Simple muscle twitch (SMT) Aim of the experiment: To study the different phases of muscle contraction. Steps of the experiment: Place the gastrocnemius sciatic preparation on the myograph. Apply the stimulating electrodes to the sciatic nerve (for indirect stimulation) or on the muscle (for direct stimulation). Connect the kymograph to the electrical current, the kymograph is included in the circuit to mark the point of stimulation. In this experiment we use the fast speed of the kymograph (625 mm/sec.).

Adjust the strength of the stimulus to give a maximal contraction. Draw a base line by allowing the drum to rotate without applying a stimulus. Put the projecting arm in contact with the spring key of the drum and them press the stimulator to record the response on a stationary drum and to identify the point of stimulation S. Let the drum to rotate by fast speed and then press the stimulator for a moment till you see a response then remove your finger from the stimulator otherwise a second contraction can by recorded on top of the first. Stop the drum and draw a line by the pointer in the middle of the twitch.

Simple muscle twitch Definition: A single brief contraction followed by rapid relaxation due stimulation of the muscle by a single maximum stimulus. It consists of the following: 1. Latent period (L.P): Def.: It is the time between application of the stimulus and beginning of contraction. Types: a) True L.P b) False L.P 2. Contraction phase: During this phase, the muscle shorten and performs work (move the lever). 3. Relaxation phase: During this phase, the muscle relaxes and returns to its original length. Its duration is longer than the contraction phase (as relaxation is an active process).

True L.P False L.P - Follows direct stimulation of the muscle. - Shorter in duration. - Represents time needed for: Steps 3 & 4. - Follows indirect stimulation of the muscle (stimulation of its motor nerve) - Longer in duration. 1- Conduction of the nerve impulse through the motor nerve fibers. 2- Production of the end plate potential. 3- Generation & propagation of the action potential along the muscle fibers. 4- Shortening of the muscle and movement of the recording lever.

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