Muscles And Locomotion

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Musculoskeletal system

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Forms the basic internal framework of the vertebrate body. Muscles and bones work in close coordination to produce voluntary movement. Bone and muscle also perform a number of other independent functions. The skeletal system provides physical support and locomotion, while the muscular system generates force.

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Unicellular Locomotion

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Protozoans and primitive algae may move by beating cilia or flagella. These two possess the same basic structure. Each contains a cylindrical stalk of eleven microtubules – nine paired microtubules arranged in a circle with two single microtubules in the center. Flagella achieve movement by means of the power stroke, a thrusting movement generated by the sliding action of microtubules. Return of the cilium or flagellum to its original position is termed the recovery stroke. Amoeba extend pseudopodia for locomotion; the advancing cell membrane extends forward, allowing the cell to move.

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Hydrostatic Skeletons

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1. The muscles within the body wall of advanced flatworms (e.g. planaria) are arranged in two antagonistic layers, longitudinal and circular. The muscles contract against the resistance of the incompressible fluid within the animal’s tissues (this fluid is termed the hydrostatic skeleton). Contraction of the circular layer of muscles causes the incompressible interstitial fluid to flow longitudinally, lengthening the animal. Conversely, contraction of the longitudinal layer of muscles shortens the animal. The same type of hydrostatic skeleton assists in the locomotion of annelids, in which each segment of the animal can expand or contract independently. 2. Segmented worms (annelids) – earthworms advance principally by the action of muscles on the hydrostatic skeleton. Bristles in the lower part of each segment, called setae, anchor the earthworm temporarily in the earth while muscles push it ahead.

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Exoskeleton

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A hard skeleton that covers all muscles and organs of some invertebrates. They are found in arthropods (e.g. insects) and are composed of chitin. All are composed of noncellular material secreted by the epidermis. It offers the animal some protections, but impose limitations on growth. Thus, periodic molting and deposition of a new skeleton is necessary to permit body growth.

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Endoskeleton

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Serves as the framework within all vertebrate organisms. Muscles are attached to bones, permitting movement. It also provides protection by surrounding delicate vital organs in bone. The rib cage protects the thoracic organs (heart and lungs) while the skull and vertebral column protect the brain and spinal cord. The two major components are cartilage and bone.

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Cartilage

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A type of connective tissue that is softer and more flexible than bone. It is retained in adults in places where firmness and flexibility are needed (e.g. in humans, the external ear, the nose, the walls of the larynx and trachea, and the skeletal joints contain cartilage).

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Bone

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A specialized type of mineralized connective tissue that has the ability to withstand physical stress. It is hard and strong, but also elastic and lightweight and functions to provide physical support. There are two types: compact bone and spongy bone.

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Compact Bone

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Dense bone that does not appear to have any cavities. The bony matrix is deposited in structural units called osteons (Haversian systems). Each osteon consists of a central microscopic channel called a Haversian canal, surrounded by a number of concentric circles of bony matrix (calcium phosphate) called lamellae.

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Spongy Bone

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Less dense bone that consists of an interconnecting lattice of bony spicule (trabeculae); the cavities in between the spicules are filled with yellow and/or red bone marrow. Yellow marrow is inactive and infiltrated by adipose tissue; red marrow is involved in blood cell formation.

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Osteocytes

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Two types of cells found in bone tissue are osteoblasts and osteoclasts. Osteoblasts synthesize and secrete the organic constituents of the bone matrix; once they are surrounded by their matrix, they mature into osteocytes. They build the bone. Osteoclasts are large multinucleated cells involved in bone resorption. They break down the bone.

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Bone Formation

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Occurs by either endochondral ossification or by intramembranous ossification. In endochondral ossification, existing cartilage is replaced by bone. Long bones arise primarily through endochondral ossification. In intramembranous ossification, mesenchymal (embryonic, undifferentiated) connective tissue is transformed into, and replaced by, bone.

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Axial Skeleton

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The basic framework of the body, consisting of the skull, vertebral column, and the rib cage. It is the point of attachment of the appendicular skeleton, which includes the bones of the appendages and the pectoral and pelvic girdles.

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Sutures

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Hold the bones of the skull together accompanied by immovable joints.

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Movable Joints

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Hold together bones that move relative to one another.

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Ligaments

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Support and strengthen movable joints. They serve as bone-to-bone connectors.

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Tendons

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Attach skeletal muscle to bones and bend the skeleton and the movable joints.

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Origin

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The point of attachment of a muscle to a stationary bone (the proximal end in limb muscles).

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Insertion

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The point of attachment of a muscle to a bone that moves (distal end in limb muscles).

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Extension

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Indicates a straightening of a joint.

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Flexion

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Refers to a bending of a joint.

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Muscular System

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Muscle tissue consists of bundles of specialized contractile fibers held together by connective tissue. There are three morphologically and functionally distinct types of muscle in mammals: skeletal muscle, smooth muscle, and cardiac muscle.

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Skeletal Muscle

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Muscle responsible for voluntary movements and is innervated by the somatic nervous system. Each fiber is a multinucleated cell created by the fusion of several mononucleated embryonic cells. Embedded in the fibers are filaments called myofibrils, which are further divided into contractile units called sarcomeres. The microfibrils are enveloped by a modified ER that stores calcium ions and is called the sarcoplasmic reticulum. The cytoplasm of a muscle fiber is called sarcoplasm, and the cell membrane is called the sarcolemma. The sarcolemma is capable of propagating an action potential, and is connected to a system of transverse tubules (T system) oriented perpendicularly to the myofibrils. The T system provides channels for ion flow throughout the muscle fibers, and can also propagate an action potential. Because of the high-energy requirements of contraction, mitochondria are very abundant in muscle cells, distributed along the myofibrils. Skeletal muscle has striations of light and dark bands, and is the

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The Sarcomere

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Composed of thin and thick filaments. The thin filaments are chains of actin molecules. The thick filaments are composed of organized bundles of myosin molecules. It is organized as follows: Z lines define the boundaries of a single sarcomere and anchor the thin filaments. The M line runs down the center of the sarcomere. The I band is the region containing thin filaments only. The H zone is the region containing only thick filaments. The A band spans the entire length of the thick filaments and any overlapping portions of the thin filaments. During contraction, the A band is not reduced in size, while the H zone and I band are reduced.

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Contraction

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Stimulated by a message from the somatic nervous system sent via motor neuron. The link between the nerve terminal (synaptic bouton) and the sarcolemma of the muscle fiber is called the neuromuscular junction. The space between the two is known as the synapse, or synaptic cleft. Depolarization of the motor neuron results in the release of neurotransmitters (e.g. acetylcholine) from the nerve terminal. The neurotransmitter diffuses across the synaptic cleft and binds to special receptor sites on the sarcolemma. If enough of these receptors are stimulated, the permeability of the sarcolemma is altered and an action potential is generated. Once an action potential is generated, it is conducted along the sarcolemma and the T system, and into the interior of the muscle fiber. This causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm. Ca2+ initiate the contraction of the sarcomere. Actin and myosin slide past each other and the sarcomere contracts.

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Stimulus/Muscle Response

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Individual muscle fibers generally exhibit an all- or-none response; only a stimulus above a minimal value called the threshold value can elicit contraction. The strength of the contraction of a single muscle fiber cannot be increased, regardless of the strength of the stimulus. However, the strength of contraction of the entire muscle can be increased by recruiting more muscle fibers.

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Simple Twitch

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The response of a single muscle fiber to a brief stimulus at or above the threshold stimulus, and consists of a latent period, a contraction period, and a relaxation period. The latent period is the time between stimulation and the onset of contraction. During this time lag, the action potential spreads along the sarcolemma and Ca2+ ions are released. Following the contraction period, there is a brief relaxation period in which the muscle is unresponsive to a stimulus – this period is known as the absolute refractory period.

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Temporal Summation

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When fibers of a muscle are exposed to very frequent stimuli, the muscle cannot fully relax. The contractions begin to combine, becoming stronger and more prolonged.

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Tetanus

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When contractions become continuous when the stimuli are so frequent that the muscle cannot relax. It is stronger than a simple twitch of a single fiber. If it is maintained, the muscle will fatigue and the contraction will weaken.

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Tonus

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A state of partial contraction. Muscles are never completely relaxed and maintain a partially contracted state at all times.

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Smooth Muscle

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Muscle responsible for involuntary actions and is innervated by the autonomic nervous system. It is found in the digestive tract, bladder, uterus, and blood vessel walls, among other places. It possesses one centrally located nucleus and lacks the striations of skeletal muscle.

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Cardiac Muscle

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Muscle that makes up the muscle tissue of the heart. Its muscle fibers possess characteristics of both skeletal and smooth muscle fibers. Like skeletal muscle, actin and myosin filaments are arranged in sarcomeres, giving cardiac muscle a striated appearance. It typically has only one or two centrally located nuclei.

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Energy Reserves

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ATP is the primary source of energy for muscle contraction. Very little ATP is actually stored in the muscles, and other forms of energy must be stored and rapidly converted to ATP (e.g. creatine phosphate/argenine phosphate and myoglobin).

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Creatine phosphate

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High-energy compound where energy can be temporarily stored (particularly in echinoderms). Many invertebrates utilize a similar compound called argenine phosphate.

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Myoglobin

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A hemoglobin-like protein found in muscle tissue. It has a high oxygen affinity and maintains the oxygen supply in muscles by binding oxygen tightly.