Ch. 6 The Muscular System Muscle tissue has 4 important properties: 1. Excitability = it can respond to stimuli 2. Contractility = it can shorten 3. Extensibility = it can stretch 4. Elasticity = it can return to its original length after shortening or stretching
A review of the 3 types of muscle tissue 1. Cardiac muscle is striated (striped), and is involuntarily (subconsciously) controlled It is found in the walls of the heart It functions to pump blood 2. Smooth muscle is not striated, and is involuntarily controlled It is found in the walls of hollow organs It functions to move eggs, sperm, urine, food, hairs, etc. 3. Skeletal muscle is striated, and is voluntarily controlled It is usually attached to bones (via tendons) It is the major focus of this chapter It has many functions (see the next slide)
Skeletal muscles = organs made up mainly of skeletal muscle tissue, but also CTs, nerves, and blood vessels The cells of skeletal muscle tissue can be very long (up to 1 foot long!) and thus are often called muscle fibers or myofibers Functions: 1. Produce skeletal movement 2. Maintain posture and body position 3. Support soft tissues E.g. the floor of the pelvic cavity 4. Guard entrances and exits E.g. the upper part of the esophagus, the external anal sphincter, and the external urethral sphincter 5. Produce heat to help maintain body temperature 6. Store nutrient (protein) reserves
Skeletal muscles work in pairs or groups Skeletal muscles cause movement (actions) by contracting/ shortening, bringing the insertion closer to the origin Antagonists are muscles that perform opposite actions When one is contracting, the other is relaxing E.g. the biceps and triceps in flexion and extension (respectively) of the forearm Synergists are muscles that work together to perform the same action E.g. the 4 quadriceps in extension of the lower leg Figure 6.1
Some examples of skeletal muscles FYI only do not memorize! Figure 6.2
The organization of skeletal muscle Part 1 Myofibril = a bundle of myofilaments Myofilaments = special long contractile proteins (actin and myosin) Figure 6.3
The organization of skeletal muscle Part 2 (a higher magnification view) Figure 6.3
The organization of skeletal muscle Part 3 (the highest magnification view) (thin filament) (thick filament) Figure 6.3
The sliding filament model of skeletal muscle contraction Sarcomeres = the short, repeating functional units of myofibrils A myofibril is made up of thousands of sarcomeres arranged end-to-end The model: The sarcomere shortens as thin and thick filaments slide past one another Sarcomeres shorten myofibrils shorten muscle cells shorten the whole muscle shortens! Figure 6.4
Muscle contraction 1: at rest ATP has attached to the myosin heads and split into ADP (shown as the yellow circle below) and P i (shown as the blue circle below), releasing energy to cock the myosin heads into a high-energy position Figure 6.5
Muscle contraction 2: the stimulus Figure 6.6
Muscle contraction 3: cross-bridge formation Figure 6.5
Muscle contraction 4 The power stroke : the myosin heads bend ( uncock ), pulling the actin toward the center of the sarcomere, as the ADP and P i are released Cross-bridge detachment: new ATP binds to the myosin heads, causing them to release the actin Myosin reactivation: the ATP splits into ADP and P i, re-energizing the myosin heads again This cycle of events is repeated over and over (as long as [Ca 2+ ] and [ATP] are high enough), shortening the sarcomeres and thus the entire muscle, creating a pulling force (tension) on the attached bone
Properties of skeletal muscle fiber contraction All-or-none = all the sarcomeres in a single muscle fiber contract together maximally when the fiber is stimulated I.e., a muscle fiber is either on (producing tension) or off (relaxed) The more frequently a muscle fiber is stimulated, the stronger the contraction (see figure shown here) Figure 6.8
Motor units and recruitment Motor unit = one motor neuron and all of the muscle fibers that it innervates The size of a motor unit is inversely related to the precision of control (e.g. eye muscles are innervated by smaller motor units than leg muscles are) Recruitment = the stimulation of additional motor units When a stronger contraction of the whole muscle is needed Figure 6.7
Muscle tone = a low level of tension in skeletal muscle that is present at all times (but NOT enough tension to produce movement), causing healthy muscle to feel solid and firm to the touch, even at rest Some motor units are active at any given point in time Which motor units are active varies constantly Functions of muscle tone: Stabilizes bones and joints Maintains body position (posture) Allows more rapid activation of the whole muscle (i.e., it accelerates recruitment) when needed Energy usage when muscles are at rest (i.e., resting metabolic rate) Lookin good!
Energy sources for muscle contraction Maximal intensity (e.g. sprinting) Submaximal intensity (e.g. jogging) (anaerobic) modified from Figure 6.9 Longer duration (minutes to hours) ATP generated aerobically in mitochondria from glucose, fats, or proteins Rate of oxygen delivery meets demand Very efficient, and no acid buildup
Slow- vs. fasttwitch muscle fibers The particular mix of fiber types in muscles is genetically determined and varies among people Figure 6.10