Lecture 8: Muscle, Bone, and Skin
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There are three types of muscle tissue:
1) skeletal muscle
2) cardiac muscle
3) smooth muscle
Muscle contraction has four possible functions:
1) body movement
2) stabilization of body position
3) movement of substances through the body
4) generating heat to maintain body temperature
Skeletal Muscle is voluntary muscle tissue. It can be consciously controlled.
Skeletal muscle connects one bone to another. The muscle does not attach directly to the bone, but instead is attached via a tendon. A tendon connects muscle to bone. A ligament connects bone to bone.
Muscles work in groups. The agonist (the muscle responsible for movement) contracts, while a second muscle, the antagonist stretches. In addition to antagonistic muscles, there are usually synergistic muscles. Synergistic muscles assist the agonist by stabilizing the origin bone or by positioning the insertion bone during the movement.
Contraction of skeletal muscle may squeeze blood and lymph vessels aiding circulation.
Contraction of skeletal muscle provides large amounts of heat. Shivering is the rapid contraction of skeletal muscle to warm the body.
Physiology of Skeletal Muscle Contraction
The smallest functional unit of skeletal muscle is the sacromere
. A sacromere is composed of many strands of two protein filaments, the thick and the thin filament
, laid side by side to form a cylindrical segment.
Skeletal muscle is multinucleate
- The thick filament of a sacromere is made of the protein myosin. The thin filament is co mposed mainly of a polymer of the globular protein actin. Myosin and actin work together sliding alongside each other to create the contrctile force of skeletal muscle. Each myosin head crawls along the actin in a 5 stage cycle.
- -First, tropomyosin covers an active site on the actin preventing the myosin head from binding. The myosin head remains cocked in a high energy position with a phosphate and ADP group attached. Second, in the presence of Ca2+ ions, troponin pulls the tropomyosin back, exposing the active site, allowing the myosin head to bind to the actin. Third the myosin head expels a phosphate and ADP and bends into a lower energy position. This is called the power stoke because it causes the shortening of the sacromere and the muscle contraction. Fourth, ATP attaches to the myosin head, releasing the myosin head from the active site. Fifth, ATP splits to inorganic phosphate and ADP causing the myosin head to cock into the high energy position.
A muscle contraction begins with an action potential. A neuron attaches to a muscle cell forming a neuromuscular synapse
. The action potential of the neuron releases acetylcholine
into the synaptic cleft, creating an action potential. The action potential moves deep into the muscle cell via small tunnels in the membrane called T-tubules
. T-tubules allow for a uniform contraction of the muscle by allowing the action potential to spread through the muscle cell more rapidly. The Ca2+ ions begin the 5 stage cycle above. At the end of each cycle, Ca2+ is actively pumped back into the sacromere.
A Motor Unit
The muscle fibers of a single muscle do not all contract at once. The neuron and the muscle fibers that it inneraves are called a motor unit. Motor units are independent of one another. The force of contracting muscle depends upon the number and size of the active motor units, and the frequency of action potentials in each neuron of the motor unit.
Muscles requiring intricate movements (the finger) have smaller motor units.
Skeletal Muscle Type
There are 3 types of skeletal muscle fibers:
1) slow oxidative (type I) fibers
2) fast oxidative (type IIA) fibers
3) fast glycolytic (type II B) fibers
Type I or slow-twitch muscle fibers are red from large amounts of myoglobin. Myoglobin is an oxygen storing protein similar to hemoglobin, but having only one protein unit. They split ATP at a slow rate.
Type II A or fast-twitch A fibers ae also red, but they split ATP at a high rate. Type II A fibers contract rapidly, are resistant to fatigue, but as resistant as Type I fibers.
Type II B or fast-twitch B fibers have a low myoglobin content, appear white under a microscope, and contract very rapidly. They contain large amounts of glycogen.
Adult human skeletal muscle does not generally undergo mitosis to create new muscle cells. Instead, a number of changes occur over time when the muscles are exposed to forceful, repetitive, contractions.
The human heart is composed mainly of cardiac muscle. Cardiac muscle is striated, which means that it is composed of sacromeres. Each cardiac muscle cell contains only one nucleus, and is separated from its neighbor by an intercalated disc. The intercalated discs contain gap junctions which allow an action potential to spread from one cardiac cell to the next via electrical synapses.
Skeletal muscle connects bone to bone via tendons; cardiac muscles on the other hand, is not connected to bone. Cardiac muscle forms a net which contracts in upon itself like a squeezing fist. Cardiac muscle is involuntary.
The action potential of cardiac muscle exhibits a plateau after depolarization. The plateau is created by slow voltage-gated calcium channels which allow calcium to enter and hold the inside of the membrane at a positive potential difference.
Smooth muscle is mainly involuntary, so it is innervated by the autonomic nervous system. Smooth muscle cells contain only one nucleus. Smooth muscles also contain thick and thin filaments, but they are not organized into sacromeres. Smooth muscle cells contain intermediate filaments, which are attached to dense bodies spread throughout the cell. They cause the intermediate filaments to pull the dense bodies together. Upon contraction, the smooth muscle cell shrinks length-wise.
In addition to responding to neutral stimulus, smooth muscle also contracts or relaxes in the pesence of hormones, or to changes in pH, O2, and CO2 levels, temperature, and ion concentrations.
Bone is living tissue. Its functions are support of soft tissue, protection of internal organsa, assistance in movement of the body, mineral storage, blood cell production, and energy storage in the form of adipose cells in bone marrow.
Bone tissue contains four types of cells surrounded by an extensive matrix:
1) Osteogenic cells differentiate into osetoblasts
2) Osteoblasts secrete collagen and organic compounds upon which bone is formed. Osteoblasts are incapable of mitosis. As osteoblasts release matrix materials around themselves, they become enveloped by the matrix and differentiate into osteocytes.
3) Osteocytes are also incapable of mitosis. Osteocytes exchange nutrients and waste materials with the blood.
4) Osteoclasts reabsorb bone matrix, releasing minerals back into the blood. Osteoclasts are believed to develop from the white blood cells called monocytes.
Spongy bone contains red bone marrow or red blood cell development.
Compact bone holds yellow bone marrow.
In a continuous remodeling process, osteoclasts burrow tunnels, called Haversian (or central) canals, through compact bone. The osteoclasts are followed by osteoblasts, which lay down a new matrix onto the tunnel walls forming concentric rings called lamellae. Osteocytes trapped between the lamellae exchange nutrients via canaliculi. The entire system of lamellae and Haversian canal is called an osteon (haversian system).
Bone Function in Mineral Homeostasis
Most calcium in the blood is not in the form of free calcium ions, but is bound mainly by proteins and, to a much lesser extent, by phosphate and other ions. It is the concentration of free calcium ions (Ca+2) in the blood that is important physiologically. Too much Ca+2 results in membranes becoming hypo-excitable producing lethargy, fatigue, and memory loss; too little produces cramps and convulsions.
Most of the Ca2+ in the body is stored in the bone matrix as hydroxyapatite.
Bone Types and Structures
Long bones have a shaft that is curved for strength. They are composed of compact and spongy bone.
Short bones are cuboidal. They are the ankle and wrist bones.
Flat bones are made from spongy bone surrounded by compact bone. They provide large areas for muscle attachment, and organ protection. The skull, sternum, ribs, and shoulder blades are flat bone.
Cartilage is flexible, resilient connective tissue. It is composed primarily of collagen, and has great tensile strength.
Hyaline cartilage is the most common. Hyaline cartilage reduces friction and absorbs shock in bones.
Joints can be classified by structure into three types:
1) Fibrous joints occur between two bones held closely and tightly together by fibrous tissue permitting little or no movement.
2) Cartilaginous joints also allow little or no movement. They occur between two bones tightly connected by cartilage, such as the ribs.
3) Synovial joints are not bound directly by the intervening cartilage. Instead, they are separated by a capsule filled with synovial fluid. Synovial fluid provides lubrication and nourishment to the cartilage. Allow for a wide range of motion.
The skin is an organ, which means that it is a group of tissues working together to perform a specific function.
Thermoregulation: the skin helps to regulate the body temperature.
Protection: the skin is a physical barrier to abrasion, bacteria, dehydration, chemicals, and UV radiation.
Environmental sensory input: the skin gathersw information from the environment by sensing temperature, pressure, pain, and touch.
Excretion: water and salts are secreted through the skin..
Immunity: besides being a physical barrier to bacteria, specialized cells of the epidermis are components of the immune system
Blood reservoir: vessels in the dermis hold up to 10% of the blood of a resting adult.
Vitamin D synthesis:
The skin has two principal parts 1) the epidermis and 2) the dermis
The fat of this subcutaneous layer is an important heat insulator for the body. The fat helps maintain normal core body temperatures.
The epidermis is avascular. There are five strata or layers of the epidermis. The deepest layer contains Merkel cells and stem cells. Exposure to friction or pressure stimulates the epidermis to thicken forming a callus.
The dermis is connective tissue derived from mesodermal cells. The dermis is embedded by blood vessels, nerves, glands, and hair follicles. Collagen and elastic fibers in the dermis provide skin with strength, extensibility, and elasticity.
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