Skeletal Muscle Physiology Exercise 2 PDF

02_017_028_PhyEx8_HP_Ch02 1/11/08 7:58 AM Page 17 E X E R C I S E 2...

02_017_028_PhyEx8_HP_Ch02 1/11/08 7:58 AM Page 17 E X E R C I S E 2 Skeletal Muscle Physiology O B J E C T I V E S 1. To define motor unit, twitch, latent period, contraction phase, relaxation phase, threshold, summation, tetanus, fatigue, isometric contraction, and isotonic contraction 2. To understand how nerve impulses trigger muscle movement 3. To describe the phases of a muscle twitch 4. To identify threshold and maximal stimuli 5. To understand the effect of increases in stimulus intensity on a muscle 6. To understand the effect of increases in stimulus frequency on a muscle 7. To demonstrate muscle fatigue 8. To explain the differences between isometric and isotonic muscle contractions umans make voluntary decisions to talk, walk, stand up, or sit down. The H muscles that make these actions possible are skeletal muscles. Skeletal muscle is generally muscle that is attached to the skeleton of the body, al- though there are some exceptions—for example, the obicularis oris, a muscle around the mouth, never attaches to any skeletal element. Skeletal mus- cles characteristically span two joints and attach to the skeleton via tendons con- necting to the periosteum of the bone. The Motor Unit and Muscle Contraction A motor unit consists of a motor neuron and all of the muscle fibers it innervates. Motor neurons direct muscles when and when not to contract. A motor neuron and a muscle cell intersect at what is called the neuromuscular junction. Specifically, the neuromuscular junction is where the axon terminal of the neuron meets a spe- cialized region of the muscle cell’s plasma membrane. This specialized region is called the motor end-plate. An action potential (depolarization) in a motor neuron triggers the release of acetylcholine, which diffuses into the muscle plasma mem- brane (also known as the sarcolemma). The acetylcholine binds to receptors on the muscle cell, initiating a change in ion permeability that results in depolarization of the muscle plasma membrane, called an end-plate potential. The end-plate poten- tial, in turn, triggers a series of events that results in the contraction of a muscle cell. This entire process is called excitation-contraction coupling. We will be simulating this process in the following activities, only instead of using acetylcholine to trigger action potentials, we will be using electrical shocks. The shocks will be administered by an electrical stimulator that can be set for the precise voltage, frequency, and duration of shock desired. When applied to a muscle that has been surgically removed from an animal, a single electrical stimulus will result in a muscle twitch—the mechanical response to a single ac- tion potential. A twitch has three phases: the latent period, which is the period of time that elapses between the generation of an action potential in a muscle cell and the start of muscle contraction; the contraction phase, which starts at the 17 02_017_028_PhyEx8_HP_Ch02 1/11/08 7:58 AM Page 18 18 Exercise 2 end of the latent period and ends when muscle tension peaks; 3. Click on the Measure button on the stimulator. Note that and the relaxation phase, which is the period of time from a thin, vertical yellow line appears at the far left side of the peak tension until the end of the muscle contraction (Figure oscilloscope screen. 2.1b). 4. Click on the (>) button underneath Time (msec). You will see the vertical yellow line start to move across the Single Stimulus screen. Watch what happens in the Time (msec) display as Follow the instructions in the Getting Started section at the the line moves across the screen. Keep clicking the (>) button front of this manual. From the drop-down menu, select Exer- until the yellow line reaches the point in the tracing where the cise 2: Skeletal Muscle Physiology and click GO. Before graph stops being a flat line and begins to rise (this is the you perform the activities, watch the Skeletal Muscle video point at which muscle contraction starts.) If the yellow line to gain an appreciation for the preparation required for this moves past the desired point, you can use the (