|< Prev||Next >|
| || |
The two principal functions of muscle are to produce movement and to maintain posture. These functions are achieved by adjusting the length and tension of muscle. In other words, muscle works by contracting. Therefore, when we look at the structure and function of muscle, we see that muscle is designed to contract.
There are three basic types of muscle in the human body: skeletal muscle (also called striated muscle), cardiac muscle (the muscle of the heart) and visceral muscle (also called smooth muscle).
Skeletal muscle is the most abundant type of muscle. It attaches to bone and so is important in producing movements in our joints and maintaining our posture. Under the microscope, skeletal muscle cells look like long fibres and have a striped (striated) appearance (figure 1). Skeletal muscle cells also have many mitochondria and more than one nucleus. Sometimes special terms are applied to the organelles in muscle cells. For example, the cell membrane is called the sarcolemma, and the cytoplasm is called the sarcoplasm. A network that looks like the endoplasmic reticulum is called the sarcoplasmic reticulum. We generally think of skeletal muscle as being under voluntary control.
Cardiac muscle is the muscle of the heart. Under the microscope, cardiac muscle cells also appear striated, however the striations are not as well organized as in skeletal muscle. Cardiac muscle cells are often branched, so that one cell forms connections with several of its neighbours (figure 2). The activity of cardiac muscle is controlled to a large degree by the autonomic nervous system. We generally think of cardiac muscle as being involuntary.
Visceral muscle is the muscle which lines our blood vessels and internal organs. Under the microscope, visceral muscle cells do not have the obvious striations of skeletal or cardiac muscle. Hence, it is called smooth muscle. Visceral muscle cells are often relatively short, and usually they each have only one nucleus. We generally think of visceral muscle as being involuntary.
The ability of muscle to contract depends on the presence of two protein molecules: actin and myosin. Bundles of actin and myosin molecules are arranged end to end in muscle cells. It is these bundles that give skeletal and cardiac muscle their striated appearance. When a muscle is properly stimulated, the actin and myosin bundles slide together and so the muscle cell gets shorter. This process requires calcium and ATP for energy.
Muscles, especially skeletal muscle, may be stimulated to contract by motor nerves (figure 3). Depolarization from a motor nerve causes calcium to move from the sarcoplasmic reticulum into the sarcoplasm. This release of calcium causes actin and myosin to slide together and shorten the muscle.The length and tension of muscles is measured by two special types of sensory receptors. These are muscle spindles (figure 4) and Golgi tendon organs (figure 5).
actin: アクチン (akuchin); ATP: アデノシン三リン酸 (adenoshinsanrinsan); autonomic nervous system: 自律神経系統 (jiritsushinkeikeitou); calcium: カルシウム (karushiumu); cardiac muscle: 心筋 (shinkin); contract: 収縮する(shuushuku suru); depolarization: 脱分極化 (datsubunkyokuka); Golgi tendon organs: ゴルジケン器官 (gorujikenkikan); involuntary: 不随意的な (fuauiitekina); motor nerve: 運動神経 (undoushinkei); movement: 運動 (undou); muscle spindle: 筋紡錘 (kinbousui); myosin: ミオシン (mioshin); posture: 姿勢 (shisei); sensory receptors: 感覚受容体 (kankakujuyoutai); sarcolemma: 筋細胞膜 (kinsaiboumaku); sarcoplasm: 筋形質 (kinkeishitsu); sarcoplasmic reticulum: 筋小胞体 (kinshouhoutai); smooth muscle: 平滑筋 (heikatsukin); skeletal muscle: 骨格筋 (kokkakukin); striated muscle: 横紋筋 (oumonkin); visceral muscle: 内臓筋 (naizoukin); voluntary: 随意的な (zuiitekina)
|< Prev||Next >|