Muscles:
A skinny, 22 year-old male fat visits the local medical clinic complaining of severe cramping in his calves, thighs, buttocks, back, and shoulders, as well as increased fatigue. He tells you that for the last 3 weeks he has been trying very hard to get in shape. He is doing a new, intense weight-training program for at least an hour almost every day, and running on a treadmill for 45 minutes four days per week. He says that despite his intense workout regimen he seems to be losing, not gaining, muscle mass. This has prompted him to work out even harder. He says that he is drinking at least a gallon of water a day to stay hydrated, and that he avoids sports drinks because they are “full of chemicals.” He has adopted a “vegan” diet that you learn in his case consists mainly of fruit juices. He tells you that he doesn’t like soy or tofu. He also tells you that he skips meals on weight training days to try to reduce body fat. He wants to know why he is cramping so much and losing muscle mass.
-
-
Background Information
The anatomy of a muscle from chemical/organelle level to the organ level,
3 Types of muscle tissue:
All muscles share four main characteristics:
- Excitability (responsiveness): ability to receive and respond to stimuli
- Contractility: ability to shorten forcibly when stimulated
- Extensibility: ability to be stretched
- Elasticity: ability to recoil to resting length
Cardiac muscle
- Cardiac muscle tissue occurs only in the heart, where it constitutes the bulk of the heart walls. Like skeletal muscle cells, cardiac muscle cells are striated, but cardiac muscle is not voluntary. Indeed, it can and does contract without being stimulated by the nervous system. Most of us have no conscious control over how fast our heart beats.
- Key words to remember for cardiac muscle are cardiac, striated, and involuntary.
- Cardiac muscle usually contracts at a fairly steady rate set by the heart’s pacemaker, but neural controls allow the heart to speed up for brief periods, as when you race across the tennis court to make that overhead smash.
Smooth muscle
- Smooth muscle tissue is found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages. Its role is to force fluids and other substances through internal body channels. Smooth muscle also forms valves to regulate the passage of substances through internal body openings, dilates and constricts the pupils of your eyes, and forms the arrector pili muscles attached to hair follicles.
- Like skeletal muscle, smooth muscle consists of elongated cells, but smooth muscle has no striations. Like cardiac muscle, smooth muscle is not subject to voluntary control. Its contractions are slow and sustained.
- We can describe smooth muscle tissue as visceral, non-striated, and involuntary.
Skeletal muscle
- Skeletal muscle tissue is packaged into the skeletal muscles, organs that attach to and cover the skeleton. Skeletal muscle fibers are the longest muscle cells and have obvious stripes called striations. Although it is often activated by reflexes, skeletal muscle is called voluntary muscle because it is the only type subject to conscious control.
- When you think of skeletal muscle tissue, the key words to keep in mind are skeletal, striated, and voluntary.
- Skeletal muscle is responsible for overall body mobility. It can contract rapidly, but it tires easily and must rest after short periods of activity. Nevertheless, it can exert tremendous power. Skeletal muscle is also remarkably adaptable. For example, your forearm muscles can exert a force of a fraction of an ounce to pick up a paper clip—or a force of about 6 pounds to pick up this book!
- Makes up nearly half of body’s mass
- Can transform chemical energy (ATP) into directed mechanical energy, which is capable of exerting force
-
Four important functions:
- Produce movement: responsible for all locomotion and manipulation
-Example: walking, digesting, pumping blood
- Maintain posture and body position
- Stabilize joints
- Generate heat as they contract
-Each skeletal muscle is a discrete organ, made up of several kinds of tissues. Skeletal muscle fibers predominate, but blood vessels, nerve fibers, and substantial amounts of connective tissue are also present. We can easily examine a skeletal muscle’s shape and its attachments in the body without a microscope.
Nerve and Blood Supply
- In general, one nerve, one artery, and one or more veins serve each muscle. These structures all enter or exit near the central part of the muscle and branch profusely through its connective tissue sheaths (described below). Unlike cells of cardiac and smooth muscle tissues, which can contract without nerve stimulation, every skeletal muscle fiber is supplied with a nerve ending that controls its activity.
- Skeletal muscle has a rich blood supply. This is understandable because contracting muscle fibers use huge amounts of energy and require almost continuous delivery of oxygen and nutrients via the arteries. Muscle cells also give off large amounts of metabolic wastes that must be removed through veins if contraction is to remain efficient. Capillaries, the smallest of the body’s blood vessels, take a long and winding path through muscle, and have numerous cross-links, features that accommodate changes in muscle length. They straighten when the muscle stretches and contort when the muscle contracts.
-
Attachments
Recall from Chapter 8 that most skeletal muscles span joints and attach to bones (or other structures) in at least two places. When a muscle contracts, the movable bone, the muscle’s insertion, moves toward the immovable or less movable bone, the muscle’s origin. In the muscles of the limbs, the origin typically lies proximal to the insertion.
Muscle attachments, whether origin or insertion, may be direct or indirect.
- In direct, or fleshy, attachments, the epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage.
- In indirect attachments, the muscle’s connective tissue wrappings extend beyond the muscle either as a ropelike tendon or as a sheetlike aponeurosis (ap˝o-nu-ro´sis). The tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles.
Indirect attachments are much more common because of their durability and small size. Tendons are mostly tough collagen fibers, which can withstand the abrasion of rough bony projections that would tear apart the more delicate muscle tissues. Because of their relatively small size, more tendons than fleshy muscles can pass over a joint—so tendons also conserve space.
Structure
Muscle (organ): A muscle consists of hundreds of thousands of muscle cells, plus connective tissue wrappings, blood vessels, and nerve fibers.
wrappings -> Covered Externally by the epimysium
Fascicle(a portion of the muscle): A fascicle is a discrete bundle of muscle cells, segregated from the rest of the muscle by a connective tissue sheath.
wrappings -> Surrounded by perimysium
Muscle fiber(cell): A muscle fiber is an enlongated multinucleate cell; it has a banded (striated) appearance.
wrappings -> Surrounded by endomysium
Myofibril (complex organelle composed of bundles of myofilaments): Myofibrils are rodlike contractile elements that occupy most of the muscle cell volume. Composed of sarcomeres arranged end to end, they appear banded, and bands of adjacent myofibrils are aligned.
Myofilament, or a filament (extended macromolecular structure): Contractile myofilaments re of two types- thick and thin. Thick filaments contain bundled myosin molecules ; thin filaments contain actin molecules(plus other proteins). The sliding of the thin filaments past the thick filaments produces muscle shortening. Elastic filaments provide elastic recoil when tension is released and help maintain myofilament organization.
Sarcomere (A segment of a myofibril): A sarcomere is the contractile unit, composed of myofilaments made up of contractile proteins
Steps involved in excitation-contraction coupling,
Fuels and other compounds needed by muscles and how those needs change with intense activity,
-
