Tendon

Tendon, in anatomy, extremely strong cord that is flexible but does not stretch, made of large bundles of white, fibrous protein known as collagen. A tendon joins a muscle with a bone or another muscle.

A tendon delivers the pulling force of a muscle to a bone, which makes body motion possible. A tendon attached to a bone is cylindrical while one connected to a flat muscle on the wall of the abdomen is a wide sheet of fibers. The tendons in the hands, wrists, and feet are enclosed in protective membranes and lubricated by a fluid to prevent excessive friction. A larger tendon includes a system of nerves that registers pain when the tendon is squeezed, and triggers a reflex contraction in the adjoining muscle when the tendon is stretched.

The Achilles tendon, which extends from midcalf to the heel, is the thickest and strongest tendon in the human body. Rupture of this tendon is a serious injury that most commonly affects athletes participating in events involving sprinting and jumping. Rupture of a tendon in the finger can result in deformity of the hand.

Tears

Tears, fluid produced in the lacrimal glands above the outside corner of each eye. The fluid lubricates the eyes and protects them from foreign matter and infection—the latter because it contains salt and lysozyme, an enzyme that kills microorganisms. During normal flow, tears constantly wash over the exterior of the eye and collect at its inside corner, where they drain through two small lacrimal ducts into the nasal cavity. During heavy tear flow, such as that caused by irritation, the excess fluid that is not taken up by the lacrimal ducts spills over the eyelids, carrying off foreign bodies too large to be flushed through the lacrimal ducts.

Musculoskeletal System

The human skeleton consists of more than 200 bones bound together by tough and relatively inelastic connective tissues called ligaments. The different parts of the body vary greatly in their degree of movement. Thus, the arm at the shoulder is freely movable, whereas the knee joint is definitely limited to a hingelike action. The movements of individual vertebrae are extremely limited; the bones composing the skull are immovable. Movements of the bones of the skeleton are effected by contractions of the skeletal muscles, to which the bones are attached by tendons. These muscular contractions are controlled by the nervous system.

Menopause

Menopause, permanent ending of menstruation in women. Menopause marks the end of a woman’s natural ability to bear children. Menopause is usually preceded by 10 to 15 years during which the ovaries gradually stop producing eggs and sex hormones, a period called the climacteric. Perimenopause encompasses this period of changing ovarian activity and also the first few years without menstrual cycling, typically characterized by hormonal and physical changes and sometimes emotional and psychological changes as well.

Melatonin

Melatonin, naturally occurring hormone that is released into the bloodstream during the hours of darkness. While scientists continue to study the function of melatonin in humans, they believe the hormone plays a role in the body’s circadian rhythm. This rhythm regulates the physiological functions that occur in the body within a 24-hour period, such as sleep-wake cycles, fluctuations in body temperature, heart rate, and blood pressure. More recently melatonin has become a popular nonprescription dietary supplement with the alleged ability to combat a variety of health problems.

Melatonin is produced by the pineal body, a cone-shaped gland about the size of a pea that is situated near the center of the brain. The release of melatonin from the pineal body into the bloodstream is orchestrated by the hypothalamus, a region of the brain that governs the body’s internal environment to maintain temperature, water balance, and hormone balance. The hypothalamus receives cues about the amount of sunlight that is absorbed by the eye—darkness causes the hypothalamus to stimulate melatonin release while light suppresses its release.

Meiosis

Meiosis, process of cell division in which the cell’s genetic information, contained in chromosomes, is mixed and divided into sex cells with half the normal number of chromosomes. The sex cells can later combine to form offspring with the full number of chromosomes. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance.

Meiosis differs from normal cell division, or mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell.

Lysosome

Lysosome, membrane-bound sac found in nucleated cells that contains digestive enzymes that break down complex molecules in the body. Lysosomes are numerous in disease-fighting cells, such as white blood cells, that destroy harmful invaders or cell debris.

Lysosomes vary greatly in size, typically ranging from 0.05 to 0.5 micrometers in diameter. Each lysosome is surrounded by a membrane that protects the cell from the lysosome’s digestive enzymes—if the lysosome breaks open, the enzymes would destroy the cell. Proteins embeded in the lysosome membrane protect the activity of the enzymes by maintaining the proper internal acidity. Membrane proteins also transport digested products out of the lysosome.

Lysosome enzymes are manufactured in the rough endoplasmic reticulum and processed in the Golgi apparatus. They are delivered by sacs known as transport vesicles to fuse with three types of membrane-bound structures: endosomes, phagosomes, and autophagosomes. Endosomes form when the cell membrane surrounds nutritional molecules like polysaccharides, complex lipids, nucleic acids, or proteins. In a process called endocytosis, these molecules are broken down for reuse. Phagosomes form when the cell membrane engulfs large objects, like debris from sites of injury or inflammation or disease-causing bacteria, in a process called phagocytosis. Autophagosomes form when the endoplasmic reticulum wraps around spent cell structures, such as mitochondria, that are destined for recycling. In all cases the digestive enzymes supplied by the lysosomes digest the membrane-bound objects into simple compounds that are delivered to the cytoplasm as new cell-building materials.