Muscle Functions
Muscles are used in every body movement performed such as heart beat, food being digested, and all body movements. The muscles have 4 major functions:
- Production of movement
- External and Internal
- Muscles produce movement by the action of muscles crossing joints between the bones of the skeleton, the muscles are connected to the joints/bones via tendons
- The muscles define how well our bones and body are stabilized
- The muscles limit movement in a joint or provide balance the joint for a more stable joint
- Muscles also produce heat within the body when they contract. This heat causes blood vessels in the skin to dilate, which will increase the blood flow to the skin. The heat that the muscles produce is energy with only around 20-25% of this energy being efficient mechanical energy. The other 75-80% of the energy is lost as heat through the skin
Functional Characteristics of Muscles
- Excitability
- Can respond to chemical neurotransmitters
- Neurotransmitters - are the chemicals which allow the transmission of signals from one neuron to the next across synapses. They are also found at the axon endings of motor neurons, where they stimulate the muscle fibers (Boeree 2003)
- Muscle contracts when it is excited
- The ability of muscle to be stretched
- The ability of the muscle to resume a resting length after it has been stretched
The diagram to the left depicts how neurons from the brain communicate with other body cells using neurotransmitters and receptor sites. The neurotransmitter for the skeletal muscle is acetylcholine (ACh). Acetylcholine attaches to the receptors to the sarcolemma. Sarcolemma become permeable to sodium (Na+).
Muscle Types
Muscles are responsible for all types of body movement. There are three basic muscle types that are found in the body:
- Skeletal muscle
- Cardiac muscle
- Smooth muscle
Classification of Muscles
Skeletal Muscle
Skeletal muscle is attached to the skeleton and can be made to contract and relax under our control due to neurotransmitters. Skeletal muscles are the mechanism for powering human movement. While individual muscles are typically regarded as distinct organic structures, the skeletal muscles are the largest organ grouping in the body (Gale 2007). Also these fibers have a great ability to split and generate ATP and are made up from fibers and motor units. Skeletal muscle contracts from impulses from the brain. These muscles also fatigue rapidly. There is over 600 skeletal muscles found in the body and around 100 of these are involved when training and during movement. Skeletal muscle also has elastic properties and can return to its original position after being used and stretched.
- Large muscles
- Maintain posture
- Facilitate locomotion
- Move jointed bones
- Found in antagonistic pairs (a muscle that opposes the action of another, usually located in opposite sides of a joint or bone)
- Joined to boned by tendons
Cardiac Muscle
Cardiac muscles, as is evident from their name, make up the muscular portion of the heart. While almost all cardiac muscle is confined to the heart, some of these cells extend for a short distance into cardiac vessels before tapering off completely. The heart muscle is also called the myocardium. "The heart muscle is responsible for more than two billion beats in a lifetime. The myocardium has some properties similar to skeletal muscle tissue, but it is also unique. Like skeletal muscles, myocardium is striated; however, the cardiac muscle fibers are smaller and shorter than skeletal muscle fibers averaging 5-15 micrometers in diameter and 20-30 micrometers in length (Gale 2008)." In addition, cardiac muscles align lengthwise more than side-by-side compared to skeletal muscle fibers. The microscopic structure of cardiac muscle is also unique in that these cells are branched such that they can simultaneously communicate with multiple cardiac muscle fibers.
- Striations
- Usually has a single nucleus
- Branching cells
- Joined to another muscle cell at intercalated disks
- Involuntary
- Found only in the hard
Smooth Muscle
Smooth muscle falls into two general categories, visceral smooth muscle and multi-unit smooth muscle. "Visceral smooth muscle fibers line internal organs such as the intestines, stomach, and uterus. They also facilitate the movement of substances through tubular areas such as blood vessels and the small intestines. Multi-unit smooth muscles function in a highly localized way in areas such as the iris of the eye. Contrary to contractions in visceral smooth muscle, contractions in multi-unit smooth muscle fibers do not readily spread to neighboring muscle cells (Gale 2008)." Within hollow organs, such as the uterus, smooth muscle cells are arranged into two layers. The outer layer is usually arranged in a longitudinal fashion surrounding the inner layer which is arranged in a circular orientation.
- Lacks striations
- Spindle-shaped cells
- Single nucleus
- Involuntary
- Found mainly in the walls of hollow organs
Muscle Contraction
Muscle contraction is the response a muscle has to any kind of stimuli where the result is shortening in length and development of force. Each of the different types of muscles, skeletal, cardiac, and smooth, react differently and preform different actions when they contract. For example, as for skeletal muscles, they are connected to muscles and receive electrical impulses from the brain which then results in the movement of the skeletal system. This is certainly true of many of the muscles of respiration (breathing), "which are automatically programmed to continue contracting and relaxing, so that an individual does not have to decide to take each breath. There are more than 650 different voluntary muscles in the human body (Gale 2007)." Additionally, the second type of muscle is the cardiac muscle which is only found in the heart and is responsible for the forceful contraction of the heart beat. Due to its ability to beat for ones entire lifetime gives this muscle a name of being described as extremely strong and resilient (Gale 2007). Furthermore, third type of muscle is called smooth muscle. Smooth muscle makes up the muscles of the intestine, the blood vessels, the uterus, the eyes, and mainly the hollow organs of the body. "The intestine contracts and relaxes without an individual even being aware of its actions, allowing food to be churned up and moved along its length. An individual cannot exert control over these muscles; one cannot, for example, decide to speed up the amount of time it takes for a meal to travel down the length of intestine (Gale 2007)."
Skeletal Muscle Contraction:
Cardiac Muscle Contraction:
Smooth Muscle Contraction:
Skeletal Muscle Contraction:
- Excitability (also called responsiveness or irritability) - ability to receive and respond to a stimulus
- Contractility - ability to shorten when an adequate stimulus is received
- Extensibility - ability of the muscle cells to be stretched
- Elasticity - ability to recoil and resume resting length after stretching
- Sequence of skeletal muscle contraction:
- Nerves excite the muscle. (Acetylcholine neurotransmitter)
- Muscle excitation --> calcium release from terminal cisternae
- Increase in intercellular calcium causes changes in thin filaments that allow think filaments to attach.
- ATP is used to contract
- When the excitation is over, calcium levels in the cell go back to normal and contraction ends with the muscle relaxing
Cardiac Muscle Contraction:
- Serves as a pump, propelling blood throughout the body
- Cardiac fibers cushioned by small amounts of connective tissue (endomysium)
- Arranged in spiral shaped bundles
- Heart contract, internal chambers become smaller
- Forces blood into the large arteries leaving the heart
- Intercalated disks and spiral arrangement allow the heart activity to be closely coordinated
Smooth Muscle Contraction:
- There are two layers of smooth muscle
- One runs laterally, and the other longitudinal
- They alternately contract and relax: changing the shape of the organ
- Moving food through the digestive system, emptying of bowls and bladder
- Slow and sustained contractions
Muscle Tissue
Damage, Repair, and Regeneration:
Of the three types of muscle tissue, smooth muscle tissue has the greatest capacity for regeneration. Pericytes are the stem cells of smooth muscle and are capable of regenerating small blood vessels that consist of smooth muscle. Pericytes are basically a connective tissue cell that occurs about small blood vessels. However, muscle tissue damage can occur in all three types of muscle tissue: skeletal, cardiac, and smooth muscle. Skeletal muscle tissue is susceptible to damage from strenuous exercise. In particular, eccentric exercises (muscle lengthening) cause fatigue more rapidly than concentric exercises (muscle shortening), leading to muscle damage. “Blood vessels and nerves are important factors in the regeneration of muscle tissue. If vascularization is absent, macrophages will be unable to reach the damaged area, and the healing process will be halted. Additionally, any living cells will die within hours if blood is absent from the injured region. In the case of severe skeletal muscle damage, fibrosis can occur, interrupting contraction of skeletal muscle. Fibrosis is the development of scar tissue in place of muscle fibers; it limits the ability of muscle tissue to regenerate (Gale 2007).” Macrophages release a hormone called prostaglandin that is theorized to stimulate the sensory nerves in muscles, thus creating the characteristic pain. Muscles are usually most sore two to three days following exercise and are not fully healed until eight to ten days post exercise. During repair, muscle tissue becomes resistant to damage from further strenuous exercise.
Of the three types of muscle tissue, smooth muscle tissue has the greatest capacity for regeneration. Pericytes are the stem cells of smooth muscle and are capable of regenerating small blood vessels that consist of smooth muscle. Pericytes are basically a connective tissue cell that occurs about small blood vessels. However, muscle tissue damage can occur in all three types of muscle tissue: skeletal, cardiac, and smooth muscle. Skeletal muscle tissue is susceptible to damage from strenuous exercise. In particular, eccentric exercises (muscle lengthening) cause fatigue more rapidly than concentric exercises (muscle shortening), leading to muscle damage. “Blood vessels and nerves are important factors in the regeneration of muscle tissue. If vascularization is absent, macrophages will be unable to reach the damaged area, and the healing process will be halted. Additionally, any living cells will die within hours if blood is absent from the injured region. In the case of severe skeletal muscle damage, fibrosis can occur, interrupting contraction of skeletal muscle. Fibrosis is the development of scar tissue in place of muscle fibers; it limits the ability of muscle tissue to regenerate (Gale 2007).” Macrophages release a hormone called prostaglandin that is theorized to stimulate the sensory nerves in muscles, thus creating the characteristic pain. Muscles are usually most sore two to three days following exercise and are not fully healed until eight to ten days post exercise. During repair, muscle tissue becomes resistant to damage from further strenuous exercise.
Energy Metabolism
Skeletal muscles are the mechanism for powering human movement. While individual muscles are typically regarded as distinct organic structures, the skeletal muscles are the largest organ grouping in the body (the skin is the largest contiguous organ). Virtually all joints are moved by pairs of muscles working in contrasting but complementary ways. “The energy metabolism characteristics of each type of fiber also contribute to the function of each type. Glycogen, the storage form of the carbohydrate product glucose, is then utilized at the muscle in the cycle of electrochemical reactions that produce adenosine triphosphate (ATP), the source of energy within the muscle (Gale 2007).” The mitochondria is the portion of the muscle cell where the energy production occurs. It is within the mitochondrial membrane that, depending on the energy sources available, either fatty acids are reduced for energy production, or glucose is ultimately converted to lactate as a part of the ATP energy cycle (Gale 2007).
Muscle Response and Stimuli
A stimulus is any event that triggers a response in an organism. When an organism reacts, or changes its behavior because of some environmental change, it is responding to a stimulus. Organisms also have rapid or immediate responses to certain stimuli that are built into their nervous systems. These are called reflex actions and are usually geared to matters of well-being and survival. As a result, a person instantly pulls his or her hand away from a hot stove without thinking. The heat sensed by the person's skin is the stimulus and the hand jerking back is the response.
- Muscle force depends on the number of fibers stimulated
- More fibers contracting results in greater muscle tension
- Muscles can continue to contract unless they run out of energy (ATP)
Muscle Fatigue and Oxygen Deficit
- When a muscle is fatigued, it is unable to contract even with a stimulus
- Common causes for muscle fatigue are oxygen debt
- Oxygen must be "repaid" to tissue to remove oxygen debt
- Oxygen is required to get rid of accumulated lactic acid
- Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less
Effect of Exercise on Muscles
- Exercise increases strength, muscle size, and endurance
- Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue
- Makes body metabolism more efficient
- Improves digestion, coordination
- Resistance (isometric) exercise (weight lifting) increased muscle size and strength