Introduction to the Muscular System
Head and Neck Muscles
The muscular system is the biological system of humans that produces movement. The muscular system, in vertebrates, is controlled through the nervous system, although some muscles, like cardiac muscle, can be completely autonomous. Muscle is contractile tissue and is derived from the mesodermal layer of embryonic germ cells. Its function is to produce force and cause motion, either locomotion or movement within internal organs. Much of muscle contraction occurs without conscious thought and is necessary for survival, like the contraction of the heart or peristalsis, which pushes food through the digestive system. Voluntary muscle contraction is used to move the body and can be finely controlled, such as movements of the finger or gross movements that of the biceps and triceps.
Muscle is composed of muscle cells (sometimes known as “muscle fibers”). Within the cells are myofibrils; myofibrils contain sarcomeres which are composed of actin and myosin. Individual muscle cells are lined with endomysium. Muscle cells are bound together by perimysium into bundles called fascicles. These bundles are then grouped together to form muscle, and is lined by epimysium. Muscle spindles are distributed throughout the muscles, and provide sensory feedback information to the central nervous system. Skeletal muscle, which involves muscles from the skeletal tissue, is arranged in discrete groups. An example is the biceps brachii. It is connected by tendons to processes of the skeleton. In contrast, smooth muscle occurs at various scales in almost every organ, from the skin (in which it controls erection of body hair) to the blood vessels and digestive tract (in which it controls the caliber of a lumen and peristalsis, respectively).
A top-down view of skeletal muscle
There are approximately 640 skeletal muscles in the human body (see list of muscles of the human body). Contrary to popular belief, the number of muscle fibers cannot be increased through exercise; instead the muscle cells simply get bigger. It is however believed that myofibrils have a limited capacity for growth through hypertrophy and will split if subject to increased demand. There are three basic types of muscles in the body (smooth, cardiac, and skeletal). While they differ in many regards, they all use actin sliding against myosin
to create muscle contraction and relaxation. In skeletal muscle, contraction is stimulated at each cell by nervous impulses that releases acetylcholine at the neuromuscular junction, createing action potentials along the cell membrane. All skeletal muscle and many smooth muscle contractions are stimulated by the binding of the neurotransmitter acetylcholine. Muscular activity accounts for most of the body’s energy consumption. Muscles store energy for their own use in the form of glycogen, which represents about 1% of their mass. Glycogen can be rapidly converted to glucose when more energy is necessary.
There are three types of muscle:
• Smooth muscle or “involuntary muscle” consists of spindle shaped muscle cells found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, ureters, bladder, and blood vessels. Smooth muscle cells contain only one nucleus and no striations.
• Cardiac muscle is also an “involuntary muscle” but it is striated in structure and appearance. Like smooth muscle, cardiac muscle cells contain only one nucleus. Cardiac muscle is found only within the heart.
• Skeletal muscle or “voluntary muscle” is anchored by tendons to the bone and is used to effect skeletal movement such as locomotion. Skeletal muscle cells are multinucleated with the nuclei peripherally located.
Skeletal muscle is called ‘striated’ because of the longitudinally striped appearance under light microscopy.
Functions of the skeletal muscle include:
• Support of the body
• Aids in bone movement
• Helps maintain a constant temperature throughout the body
• Assists with the movement of cardiovascular and lymphatic vessels through contractions
• Protection of internal organs and contributing to joint stability
Cardiac and skeletal muscle are striated in that they contain sarcomere and are packed into highly-regular arrangements of bundles; smooth muscle has neither. Striated muscle is often used in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.
Skeletal muscle is further divided into several subtypes:
• Type I, slow oxidative, slow twitch, or “red” muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.
• Type II, fast twitch, muscle has three major kinds that are, in order of increasing contractile speed:
• a) Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
• b) Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin.
This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was,
confusingly, called type IIB
• c) Type IIb, which is anaerobic, glycolytic, “white” muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents or rabbits this is the major fast muscle type, explaining the pale color of their meat. For most muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber. However, some muscles (such as the heart) do not contract as a result of conscious effort. These are said to be autonomic. Also, it is not always necessary for the signals to originate from the brain. Reflexes are fast, unconscious muscular reactions that occur due to unexpected physical stimuli. The action potentials for reflexes originate in the spinal cord instead of the brain.
• The cross-bridge (which is already in a ready-state) binds to the newly uncovered binding sites. It then delivers a power stroke.
• ATP binds the cross-bridge, forcing it to conform in such a way as to break the actinmyosin bond. Another ATP is split to energize the cross bridge again.
• Steps 7 and 8 repeat as long as calcium is present on thin filament.
• Throughout this process, the calcium is actively pumped back into the sarcoplasmic reticulum. When no longer present on the thin filament, the tropomyosin changes back to its previous state, so as to block the binding sites again. The cross-bridge then ceases binding to the thin filament, and the contractions cease as well.
• Muscle contraction remains as long as Ca++ is abundant in sarcoplasm.
Types of Contractions:
• Isometric contraction–muscle does not shorten during contraction and does not require the sliding of myofibrils but muscles are stiff.
• Isotonic contraction–inertia is used to move or work. More energy is used by the muscle and contraction lasts longer than isometric contraction. Isotonic muscle contraction is divided into two categories: concentric, where the muscle fibers shorten as the muscle contracts (ie. biceps brachialis on the up phase of a biceps curl); and eccentric, where the muscle fibers lengthen as they contract (ie. biceps brachialis on the down phase of a biceps curl).
• Twitch–exciting the nerve to a muscle or by passing electrical stimulus through muscle itself. Some fibers contract quickly while others contract slowly.
• Tonic -maintaining postural tone against the force of gravity. The Efficiency of Muscle Contraction:
• Only about 20% of input energy converts into muscular work. The rest of the energy is heat.
• 50% of energy from food is used in ATP formation.
• If a muscle contraction is slow or without movement, energy is lost as maintenance heat.
• If muscle contraction is rapid, energy is used to overcome friction. Summation of Muscle Contraction: It is the adding together of individual muscle twitches to make strong muscle movements.
• Multiple motor unit summation–increasing number of motor units contracting simultaneously.
• Wave summation–increasing rapidity of contraction of individual motor units.
• Tetanization–higher frequency successive contractions fuse together and cannot be distinguished from one another.
There are three general types of muscle contractions, skeletal muscle contractions, heart
muscle contractions, and smooth muscle contractions.
Muscular System Working With Other Body Systems
• 1. Homeostasis
• 2. Protection
• 3. Calcium Metabolism
• 4. Maintaining Body Temperature
Skeletal Muscle Contractions
Steps of a skeletal muscle contraction:
• An action potential reaches the axon of the motor neuron.
• The action potential activates voltage gated calcium ion channels on the axon, and calcium rushes in.
• The calcium causes acetylcholine vesicles in the axon to fuse with the membrane, releasing the acetylcholine into the cleft between the axon and the motor end plate of the muscle fiber.
• The skeletal muscle fiber is excited by large mylenated nerve fibers which attach to the neuromuscular junction. There is one neuromuscular junction for each fiber.
• The acetylcholine diffuses across the cleft and binds to nicotinic receptors on the motor end plate, opening channels in the membrane for sodium and potassium. Sodium rushes in, and potassium rushes out. However, because sodium is more permeable, the muscle fiber membrane becomes more positively charged, triggering an action potential.
• The action potential on the muscle fiber causes the sarcoplasmic reticulum to release calcium ions(Ca++).
• The calcium binds to the troponin present on the thin filaments of the myofibrils. The troponin then allosterically modulates the tropomyosin. Normally the tropomyosin physically obstructs binding sites for cross bridge; once calcium binds to the troponin, the troponin forces the tropomyosin to move out of the way, unblocking the binding sites.
Sliding Filament theory
When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible (as in the bottom diagram, left). Note that the actin and myosin filaments themselves do not change length, but instead slide past each other.
Cellular Action of Skeletal Muscles
During cellular respiration the mitochondria, within skeletal muscle cells, convert glucose from the blood to carbon dioxide and water in the process of producing ATP (see cell physiology1). ATP is needed for all muscular movement. When the need of ATP in the muscle is higher than the cells can produce with aerobic respiration, the cells will produce extra ATP in a process called anaerobic respiration. The first step of aerobic respiration(glycolysis) produces two ATP per glucose molecule. When the rest of the aerobic respiration pathway is occupied the pyruvate molecule can be converted to lactic acid. This method produces much less ATP than the aerobic method, but it does it faster and allows the muscles to do a bit more than if they relied solely on ATP production from aerobic respiration. The drawback to this method is that lactic acid accumulates and causes the muscles to fatigue. They will eventually stop contracting until the breakdown of lactic acid is sufficient to allow for movement once again. People experience this most noticeably when they repeatedly lift heavy things such as weights or sprint for a long distance. Muscle soreness sometimes occurs after vigorous activity, and is often misunderstood by the general public to be the result of lactic acid buildup. This is a misconception because the muscle does fatigue from lactic acid buildup, but it does not stay in the muscle tissue long enough to cause tissue breakdown or soreness. During heavy breathing, following exercise, the cells are converting the lactic acid either back into glucose or converting it to pyruvate and sending it through the additional steps of aerobic respiration. By the time a person is breathing normally again the lactic acid has been removed. The soreness is actually from small tears in the fibers themselves. After the fibers heal they will increase in size. The number of mitochondria will also increase if there is continued demand for additional ATP. Hence, through exercise the muscles can
increase in both strength and endurance. Another misconception is that as the muscle increases in size it also gains more fibers. This is not true. The fibers themselves increase in size rather than in quantity. The same holds
true for adipose tissue–fat cells do not increase in number, but rather the amount of lipids (oil) in the cells increase. Muscle fibers are also genetically programmed to reach a certain size and stop growing from there, so after awhile even the hardest working weightlifter will only reach a certain level of
strength and endurance. Some people will get around this by taking steroids. The artificial steroids cause all sorts of trouble for the person. They can cause the adrenal glands to stop producing corticosteroids and glucosteroids. This leads to the atrophy of the gland’s medulla and causes permanent loss of the production of these hormones. The testicles may also atrophy in response to steroids. Eventually the testes will stop making testosterone and sperm, rendering the male infertile.
One of the more serious problems associated with abnormal gain of muscle mass is heart failure. While for most people gaining muscle and losing fat is desirable, a body builder is at risk of producing more muscle mass than the heart can handle. One pound of fat
1 Chapter 1.5 on page 12
Source: Cray MI, Ch. 5 The Muscular System,Textbook of Human Physiology and Biophysics, V#1. Atlanta Ga: IVMS 2014:141-47