Medical brain notes 27

  The Muscular System   D o you like to dance? Most of us do, or we may   simply enjoy watching good dancers. The grace and coordination involved in dancing result from the interaction of many of the organ systems, but the one you think of first is probably the muscular system.   There are more than 600 muscles in the human body. Most of these muscles are attached to the bones of the skeleton by tendons, although a few muscles are attached to the undersurface of the skin. The primary function of the  muscular system  is to move the skeleton. The muscle contractions required for movement also produce heat, which contributes to the maintenance of a constant body temperature. The other body systems directly involved in movement are the nervous, respiratory, and circulatory systems. The nervous system transmits the electrochemical impulses that cause muscle cells to contract. The respiratory system exchanges oxygen and carbon dioxide between the air and blood. ...

  MUSCLE STRUCTURE   All muscle cells are specialized for contraction. When these cells contract, they shorten and pull a bone to produce movement. Each skeletal muscle is made of thousands of individual muscle cells, which also may be called  muscle fibers  (see Fig. 7–3). Depending on the work a muscle is required to do, variable numbers of muscle fibers contract. When picking up a pencil, for example, only a small portion of the muscle fibers in each finger muscle will contract. If the muscle has more work to do, such as picking up a book, more muscle fibers will contract to accomplish the task.   Muscles are anchored firmly to bones by  tendons . Most tendons are rope-like, but some are flat; a flat tendon is called an  aponeurosis . Tendons are made of fibrous connective tissue, which, you may remember, is very strong and merges with the  fascia  that covers the muscle and with the  periosteum , the fibrous connective tissue membran...

  MUSCLE TONE   Except during certain stages of sleep, most of our muscles are in a state of slight contraction; this is what is known as muscle tone . When sitting upright, for example, the tone of your neck muscles keeps your head up, and the tone of your back muscles keeps your back straight. This is an important function of muscle tone for human beings, because it helps us to maintain an upright posture. For a muscle to remain slightly contracted, only a few of the muscle fibers in that muscle must contract. Alternate fibers contract so that the muscle as a whole does not become fatigued. This is similar to a pianist continuously rippling her fingers over the keys of the piano—some notes are always sounding at any given moment, but the notes that are sounding are always changing. This contraction of alternate fibers, muscle tone, is also regulated by the cerebellum of the brain.   Muscle fibers need the energy of  ATP  (adenosine triphosphate) in order to co...

  MUSCLE SENSE   When you walk up a flight of stairs, do you have to look at your feet to be sure each will get to the next step? Most of us don’t (an occasional stumble doesn’t count), and for this freedom we can thank our muscle sense.  Muscle sense  ( proprioception ) is the brain’s ability to know where our muscles are and what they are doing, without our having to consciously look at them.   Within muscles are receptors called  stretch recep-tors  (proprioceptors or muscle spindles). The general   function of all sensory receptors is to detect changes. The function of stretch receptors is to detect changes in the length of a muscle as it is stretched. The sen-sory impulses generated by these receptors are inter-preted by the brain as a mental “picture” of where the muscle is.   We can be aware of muscle sense if we choose to be, but usually we can safely take it for granted. In fact, that is what we are meant to do. Imagine what life wou...

  ENERGY SOURCES FOR MUSCLE CONTRACTION   Before discussing the contraction process itself, let us look first at how muscle fibers obtain the energy they need to contract. The direct source of energy for mus-cle contraction is ATP. ATP, however, is not stored in large amounts in muscle fibers and is depleted in a few seconds.   The secondary energy sources are creatine phos-phate and glycogen.  Creatine phosphate  is, like ATP, an energy-transferring molecule. When it is bro-ken down (by an enzyme) to creatine, phosphate, and energy, the energy is used to synthesize more ATP. Most of the creatine formed is used to resynthesize creatine phosphate, but some is converted to  creati-nine , a nitrogenous waste product that is excreted by   the kidneys.   The most abundant energy source in muscle fibers is  glycogen . When glycogen is needed to provide energy for sustained contractions (more than a few seconds), it is first broken down into the...

  MUSCLE FIBER - MICROSCOPIC STRUCTURE   We will now look more closely at a muscle fiber, keep-ing in mind that there are thousands of these cylindri-cal cells in one muscle. Each muscle fiber has its own motor nerve ending; the  neuromuscular junction  is where the motor neuron terminates on the muscle fiber (Fig. 7–2). The  axon terminal  is the enlarged tip of the motor neuron; it contains sacs of the neuro-transmitter  acetylcholine (ACh). The membrane of the muscle fiber is the  sarcolemma , which contains receptor sites for acetylcholine, and an inactivator called  cholinesterase . The  synapse  (or synaptic cleft) is the small space between the axon terminal and the sarcolemma.   Figure 7–2.  Structure of the neuromuscular junction, showing an axon terminal adja-cent to the sarcolemma of a muscle fiber. Schematic of events: ( 1 ) Acetylcholine is about to bond to the ACh receptor in the sarcolemma. ( 2 ) Channel ope...

  CONTRACTION - THE SLIDING FILAMENT MECHANISM   All of the parts of a muscle fiber and the electrical changes described earlier are involved in the contrac-tion process, which is a precise sequence of events called the sliding filament mechanism. In summary, a nerve impulse causes depolarization of a muscle fiber, and this electrical change enables the myosin filaments to pull the actin filaments toward the cen ter of the sarcomere, making the sarcomere shorter. All of the sarcomeres shorten and the muscle fiber contracts. A more detailed description of this process is the following:   1.             A nerve impulse arrives at the axon terminal; acetylcholine is released and diffuses across the synapse.   2.             Acetylcholine makes the sarcolemma more per-meable to Na +  ions, which rush into the cell.   3.      ...