Muscles

Types of Muscle Three types of muscle: Cardiac- found only in the heart Striated, involuntary Smooth- surrounds tubular organs, such as intestines and esophagus. Non-striated, involuntary Skeletal- found attached to bones in the legs, back, shoulders and elsewhere. Striated, voluntary

Hyperplasia Individual muscle cells differentiate and divide before birth in a process called Hyperplasia. The Hyperplastic process is complete before birth, so an animal is born with all the muscle fibers it will ever possess.

Hypertrophy After birth as an animals skeletal system lengthens, muscles must also lengthen to keep up. Muscles must also grow in diameter as the animal matures. Both of these types of postnatal (after birth) growth are called Hypertrophy.

Hypertrophy Increase in bone length actually causes muscle cells to lengthen at each end. This type of hypertrophy is called stretch-induced hypertrophy. Muscle cells increase in width through exercise-induced hypertrophy, thus increasing size of whole muscle.

Muscles and Muscle Cells A single muscle cell is called a myofiber. Each myofiber is individually wrapped by a thin film of connective tissue called the endomysium. Bundles of myofibers are wrapped by a connective tissue sheath called perimysium. These bundles of myofibers make up whole muscles that are wrapped by a connective tissue sheath called epimysium.

Muscles and Muscle Cells All the connective tissues surrounding myofibers, bundles of myofibers, and whole muscles consist of a protein called collagen.

Arrangement of Whole Muscle

Sarcomere Each myofiber is divided into striped units called sarcomeres. The sarcomere is a tiny segment of the myofiber that shortens when muscles contract and elongate when muscles relax. Z-lines define the beginning and end of a single sarcomere.

Muscle Proteins Sarcomeres are predominately made of five muscle proteins: Myosin Actin Troponin Tropomyosin Titin

Muscle Proteins The thicker filament represents myosin and is referred to as the “A” band of the sarcomere. The thin filament represents the actin and is referred to as the “I” band of the sarcomere. Troponin and tropomyosin help regulate muscle contractions. Titin prevents overstretching of the muscle.

Review

Muscle Contractions As the sarcomere contracts, myosin stays stationary and attachments located at the ends of the myosin molecule, called “myosin feet”, pull the actin filaments closer together. This shortens the area called the “H” zone and then shortens the sarcomere.

Muscle Contractions/Relaxations Thousands of sarcomeres shortening in unison cause the entire muscle to contract and shorten. When the muscle relaxes, each sarcomere resumes its original position and the muscle resumes its normal shape.

Allowing Muscle Contractions The mineral Calcium allows muscle contraction to occur. Each myofiber is surrounded by a reservoir of calcium called the sarcoplasmic reticulum (SR). When the muscle receives a message to contract from a nerve, the SR releases calcium in to the myofiber. This causes the myosin feet to pull the actin filaments together.

Allowing Muscle Contractions When the nervous impulse ends, calcium flows back into the SR and the muscle relaxes. The relaxation process requires no energy; actin and myosin molecules simply slide back to their original positions.

Glucose or Glycogen The whole process of contraction is fueled by either glucose or glycogen. Glucose is blood sugar, which can be obtained directly from the blood stream. Glycogen is stored glucose found within the muscle. If the muscle runs out of its fuel, it goes into a state of exhaustion called muscle fatigue.

Types of Muscle Cells Two main types of myofibers distinguished by color and relative speed of contraction. Fast, white fibers Slow, red fibers

Fast, White Fibers Fibers are large in diameter and have few blood vessels running through them. Use stored glycogen as their primary energy source. Large muscle groups consist of mostly fast, white fibers. Ex.- Breast meat of poultry Anaerobic (no oxygen) exercise which is intense and short-term tends to develop this type of muscle fiber.

Slow, Red Fibers Fibers have many blood vessels running through them and are then red in appearance. Get most of their energy directly from glucose in the blood stream. ex.- Drumsticks of poultry Aerobic (with Oxygen) exercise, which is long term and less intense develops this type of muscle fiber.

Muscle Fibers in Large Animals The type of myofiber that predominates in an animals is largely controlled by genetics and varies by muscle group. Ex.- muscle fibers present in the loins of animals tend to be fast, white fibers while muscle fibers present in the legs of animals tend to be slow, red fibers.

Muscle Fibers in Large Animals Heavily muscled animals have more fast, white muscle fibers, which are larger in diameter and take up more space. Several intermediate types of myofibers also have properties of both fast, white fibers and slow, red fibers.

Turkey and Veal If an animal is subjected to stresses that favor a specific fiber type, fibers have the ability to slowly transform through the intermediate types toward the favored muscle type. Wild turkeys have mostly slow, red fibers because they have to run and fly away from predators. Issues with veal calf housing arose when calves were kept in small pens to restrict movement to keep meat color white and meat quality tender. These calves would have mostly fast, white fibers even in legs.

Muscle to Meat When an animal is slaughtered, blood stops flowing to muscle tissue. The energy supply of glucose is then cut off. Nerve pathways are severed and impulses still reach muscles. Muscles cells continue to attempt contraction using glycogen as an energy source. This process produces lactic acid as a waste product of anaerobic respiration.

Muscle to Meat The buildup of lactic acid cause the pH of the muscle to decline. Normal pH in a myofiber is 7.0 (neutral) When the pH drops to around 5.5 muscles cease to use glycogen. The sarcomeres stop moving and the muscle becomes stiff. This state of muscle stiffness is called rigor mortis.

What questions do you have?

Biology of Fat Development Fat = adipose tissue Fat cells = adipocytes Adipocytes are very large but that is due to being almost entirely stored energy