Skeletal Muscle – The Body's Pulley System
Lever Design
Structural Organization
Contractile Apparatus
Motor Units
Thermogenesis & Energy
Starvation Source of Glucose
Lecture Notes:
..........Muscle Provides the Force to Move the Body's System of Levers - Muscle is truly an amazing tissue; it's composition and structure are designed to shorten when stimulated by a motor nerve impulse. Within the human body, muscle acts as a contractile “pulley” system connected to fixed, bony elements of the skeleton; hence, a contracting muscle provides the force necessary to move parts of the skeleton. In many instances as we'll see, the movement of our body parts depends on a mechanically-advantageous lever design. As you might know, a lever allows you to accomplish a task with a minimum of force (e.g., a crowbar used to separate pieces of wood nailed together). The same advantage is true for the many `levers' distributed within our body to do work. We can look at the basic elements of a lever and readily see the functional analogies existing in our body. For example, the `bar' within the lever design is the bone itself, while the pivot point or fulcrum is the `joint' where bones meet. However, what provides the force necessary to `move' the lever? This is where muscle comes in; the contracting muscle provides the force necessary to move the skeletal elements of the body. Whether it is standing on your tippy toes, or crossing your legs as you sit in a chair, a number of biological levers are at work to get the job done. To take a page from a Physics textbook, `work' can be defined as the ability to move a mass some distance; within the context of human movements, muscle is the key tissue in allowing our body to accomplish `work'.
..........General Functions of Muscle - There are about 640 muscles distributed within the human body; the primary functions of muscle are somewhat obvious, i.e., body movements and maintenance of an up-right posture. However it should also be noted that our muscles function in body heat production (shivering and non-shivering thermogenesis), as well as providing sources of glucose under fasting conditions. Each of these functions will be described later in this section.
Structural organization of muscle:
..........Connective Tissues of Muscle – Our muscles are wrapped, and held together by bundles of collagenous connective tissue fibers. The outer covering is called the epimysium; then there is the deeper perimysium which surrounds individual bundles of muscle fibers called fasicles, and lastly the endomysium which encapsulates individual muscle fibers. All of these layers of connective tissues come together to form tendons which connect the entire muscle to our bones. It should be noted that our bones are encapsulated by a connective tissue membrane called the periosteum, and it is this membrane which is continous with the connective tissue fibers of the tendon (and muscle), thereby forming a firm attachment of muscle to bone.
..........Structural Organization of Muscle - Each muscle is made up of several hundred fibers; it is the fiber surrounded by a plasma membrane which is the functional & structural unit of muscle. The muscle fibers are subsequently grouped together into bundles called fascicles; this is readily seen in a slab of meat where you see “marbling” in which connective tissues can be seen separating one fascicle from another. Hence a muscle is made up of several fascicles, and in turn each fascicle is made of several fibers; when fibers shorten, fascicles shorten, and consequently an entire muscle will shorten.
..........Organized within each muscle fiber is a contractile apparatus, or sarcomere, made up of thick and thin filaments (made up of large proteins called myosin and actin, respectively). The thick and thin filaments are arranged in such a way as to slide in between each other; when a muscle fiber is stimulated the filaments move or slide between each other, which in effect shortens the length of the entire fiber. Hence, the shortening of several fibers within a fascicle causes the entire muscle to shorten; as we'll see later the degree of muscle shortening is dependent upon the number of `motor units' which are stimulated within a muscle.
..........Besides actin and myosin, there are 100s of other proteins which make up muscle tissue; some of these will be identified later such as dystrophin which acts as a `scaffolding' tethering the contractile apparatus to the plasma membrane of the muscle fiber. Moreover as you'll learn in the `Kim Davis Case' (Lab Report #3), a genetic deficiency in dystrophin is the underlying cause Duchenne muscular dystrophy.
..........Muscle Contraction – What causes a fiber to shorten once stimulated by a motor nerve stimulus? Briefly, motor axons of the nervous system are connected to individual muscle fibers at a site called the neuromuscular synapse; once a nerve impulse reaches the pre-synaptic membrane of the axon, the neurotransmitter acetylcholine (Ach) is released within the synaptic space. Ach diffuses across the synapse and reaches the post-synaptic membrane of the target muscle fiber where Ach binds to a membrane receptor R forming a temporary [Ach-R] complex. Once the [Ach~R] complex is formed, the muscle membrane becomes depolarized (like the depolarization of the motor axon) which causes the release of calcium ions within the fiber; consequently calcium binds to an internal protein called troponin which thereby initiates the movement of the thick and thin filaments within the muscle fiber; once calcium binds to troponin, thin filaments slide along the thick filaments, thereby shortening the fiber; this process is dependent on energy provided in the form of ATP. Hence, as the filaments shorten, the fibers shorten, the fascicles shorten, and ultimately the entire muscle shortens.
..........Motor Unit – Muscle is sub-divided into a number of motor units; these units can be defined as the number of muscle fibers innervated (i.e., making synaptic connections) by a single motor axon. As we'll see later in the Physical Fitness section of this course, muscle contractions which involve the movement of body parts involve a varying range of motor units; for example, it should not come as a surprise that a relatively small number of motor units are involved in lifting a pencil from a desk-top, but a much greater number are involved if you are lifting several textbooks all at one time. In addition we'll learn later on that weight-bearing isometric muscular contractions recruit a great number of motor units within a muscle, whereby repetitious isotonic contractions recruit motor units in proportion to the muscle force needed.
..........Thermogenesis – Muscle needs ATP energy to contract; this energy is made from the metabolic breakdown of glucose and fat .As you know the metabolic breakdown of glucose and fat yields energy in the form of heat and ATP (as well as producing water and carbon dioxide by-products). Hence, muscle is a major source of internal heat production, especially in cases of strenuous, continuous activity. If the body needs heat above and beyond what is made from normal muscle activities, involuntary shivering can be initiated by the hypothalamus; hence muscle plays a pivotal role in body temperature homeostasis.
..........Energy for muscle activity – As mentioned above, energy in the form of ATP is necessary for muscle to function. However to better appreciate the role of ATP, as well as its metabolic generation to sustain muscular activity, lets have you raise your arm over your head. What's the source of energy for your arm muscles to raise your arm? Easy, right? ATP! But what if you were told that the amount of ATP.available at any given moment is only enough to keep your arm above your head for 4 seconds! What then? There is a back-up supply of energy in the form of creatine phosphate (CP); once ATP is depleted, CP donates energy to form more ATP. However the amount of energy from CP will only give you another 45 seconds worth of ATP. What then? This is where glucose comes in; glucose is metabolically broken down to yield more ATP – about 1 or 2 minutes worth of ATP. But as you look at your watch, you notice that you've been able to keep your arm over your head now for 3, 4, 5 minutes and counting. What then? This is where fat comes in (as well as oxygen). To be able to generate sufficient ATP to keep your arm over your head for several minutes, fat also needs to be burned; however this breakdown of fat requires oxygen. Consequently, for enduring muscular activity (like keeping your arm over your head for more than 2-3 minutes), the oxidative breakdown of fat is required in order to generate sufficient levels of ATP. This enduring physical activity is often called `aerobics' (a subject we will discuss later in April 16 Lecture Notes).
..........Muscle Protein as Starvation Sources of Glucose – Normally the human body is equipped to store 2-3 days worth of glucose (stored as glycogen mainly in the liver, muscle, and kidneys). As you know glucose is critical for the survival and functioning of our vital reflex centers in our brain stem; however, what happens during a 2-3 day fast when there is no glucose available from the diet to support brain stem functioning? This where the `gluconeogenesis' pathway' comes into play. During starvation (when glucose stores are used up), the pituitary is activated to release ACTH which targets the adrenal cortex to release the `stress hormone' glucocorticoids (ie: cortisol); subsequently the glucocorticoids activate protease enzymes within muscles which causes the degradation of muscle protein into amino acids. These amino acids are carried by the blood stream to the liver which has enzymes necessary to convert amino acids into glucose molecules; hence during starvation, our brain stem is kept alive and supplied with sugar from the conversion of muscle protein into glucose. Obviously, this is not an ideal situation, but it is a biological stress in which the human body is equipped to handle. Remember thousands of years ago we human organisms once roamed the earth as itinerant hunters, and there were several days between meals; the ability for `gluconeogenesis' was in many ways critical for our early survival as a human species.
Related Links on muscle:
www.ultranet.com/~jkimball/BiologyPages/M/Muscles.html
www.ptcentral.com/muscles/ - very technical but complete
www.healthweb.org/ - good web search vehicle for health sites including
muscle
http://dante.med.utoronto.ca/skeletalmuscle/index.htm - history of muscle discovery
www.shockfamily.net/skeleton/muscles.html - good figures of human muscles
www.accessexcellence.org - site of the National Health Museum where `graphics
gallery' has some revealing animations on muscle function
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