Chapters 48, 49, and 50: Sensory and Motor Mechanisms Part II

Chapters 48, 49, and 50: Sensory and Motor Mechanisms Part II “I like nonsense; it wakes up the brain cells.” --Dr. Seuss

Muscle Cell Components • The function of muscle cells relies on microfilaments—the actin components of the cytoskeleton. • Microfilament movement is governed by chemical energy transformations that result in contraction.

Vertebrate Skeletal Muscle • Skeletal muscle (also called striated muscle) is attached to bones and is responsible for movement. • It is organized in a hierarchial scheme of smaller and smaller units. • Most skeletal muscles consist of a bundle of long fibers consisting of a single, multinucleated cell.

Vertebrate Skeletal Muscle • Muscle fibers contain bundles of small subunits called myofibrils. • The myofibrils are composed of both thin and thick filaments. • Thin filaments conatin two strands of actin and two strands of tropomyosin coiled around one another. • Thick filaments consist of myosin molecules.

The Sarcomere • The sarcomere is the contractile unit of the muscle. • Borders of each sarcomere line up with adjacent myofibrils. • Thin filaments are attached at the Z lines and extend inward toward the center of the contractile unit. Thick filaments, on the other hand, are attached to the M lines in the center of the sarcomere.

The Sarcomere • In resting muscle fibers, thick and thin fibers only partially overlap. At the edges of the sarcomere, you can see only thin filaments, whereas at the center only thick filaments overlap. Such an arrangement allows for contraction to occur.

The Sliding-Filament Model • This model explains muscle contraction and is based upon the interaction of actin and myosin which comprise the filaments.

The Sliding-Filament Model • Neither the thick nor the thin model change length during contraction. • Instead, the filaments slide past one another along their lengths increasing the amount of overlap.

The Sliding-Filament Model • The myosin molecues are comprised of a long tail and a globular head that extend out to the side of each thick filament. • The head is the main region where the reactions that power muscle contraction occur.

The Sliding-Filament Model • The head binds ATP and hydrolyzes it into ADP + Pi thus changing the myosin head into a high energy form capable of binding actin.

The Sliding-Filament Model • When actin and myosin bind, a cross-bridge is formed and the myosin acts to pull the actin filament toward the center of the sarcomere.

The Sliding-Filament Model • The myosin head ‘lets go’ when a new ATP molecule binds to its head. • These reactions repeat themselves very rapidly resulting in contraction.

Ca2+ and Regulatory Proteins • The regulatory protein, tropomyosin, and the troponin complex (additional regulatory proteins) bind to actin strands and are involved in muscle contraction and relaxation.

Ca2+ and Regulatory Proteins • At rest, tropomyosin covers the myosin binding sites along the actin filament—this prevents actin-myosin interactions. • As Ca2+ accumulates in the cytosol, it binds to the troponin complex causing proteins bound along the actin strand to move and expose myosin-binding sites.

Ca2+ and Regulatory Proteins • With myosin binding sites now available on the actin filaments, myosin can bind. • This binding is what contraction is—the sliding of actin and myosin fibers past one another.

Ca2+ and Regulatory Proteins • The increase in Ca2+ concentration results in contraction. • As the concentration of Ca2+ falls, the muscles relax as the binding sites are covered again.

Motor Neurons and Muscle Contaction • Signals from motor neurons stimulate the release of Ca2+ ions into the cytosol of muscle cells. • Ca2+ concentration within the muscle cell is a regulated, multistep process involving numerous cell structures.

Motor Neurons and Muscle Contaction • The first step in muscle contraction is the arrival of an action potential at the synaptic terminal of a motor neuron. • An action potential stimulates the release of acetylcholine which binds to receptors on the muscle fiber.

Motor Neurons and Muscle Contaction • This leads to a depolarization within plasma membrane of the muscle fiber and triggers an action potential within the cell. • The muscle cell contains transverse (T) tubules, and the action potential spreads to the interior of the cell through these structures.

Motor Neurons and Muscle Contaction • From the T-tubules, the action potential spreads to the sarcoplasmic reticulum (SR), causing the release of Ca2+ • The Ca2+ enters the cytosol, binds to the troponin complex causing it to move and open myosin binding sites stimulating contraction of the muscle.

Motor Neurons and Muscle Contaction • Relaxation of the muscle occurs when the neural input stops. • During this phase, transport proteins within the SR pump Ca2+ out of the cytosol allowing regulatory proteins to bind to the thin filament and block the myosin binding sites. • Ca2+ accumulates in the SR and the muscle cell is ready for the next round of contraction.

Motor Neurons and Muscle Contaction • Obviously during muscle contractions, there is some control which you have over whole muscles. • There are two basic ways in which graded muscle contractions are controlled by the nervous system. • 1. By varying the number of muscle fibers that contract. • 2. By varying the rate at which the fibers are stimultated.

Motor Neurons and Muscle Contaction—Varying the Number • In vertebrate skeletal muscle, each muscle fiber is controlled by one motor neuron, however, each motor neuron may form a synapse with many muscle fibers. • The motor unit is the single motor neuron and all the muscle fibers it controls.

Motor Neurons and Muscle Contaction—Varying the Number • When an action potential is produced by a motor neuron, all the fibers within the motor unit contract as a group. • Motor units contain as many as a few muscle fibers or as many as a few hundred. • The strength of the contraction is governed by the number of muscle fibers the motor neuron controls.

Motor Neurons and Muscle Contaction—Varying the Number • The regulation of the strength of contraction is controlled by the nervous system. At any given instant, the nervous system can select a large or small numbe of motor neurons to activate. • The force of the contraction developed by the muscle is increased/decreased as the number of motor neurons controlling the motor unit increases or decreases.

Motor Neurons and Muscle Contaction—Recruitment • The process of activating an increased number of motor neurons is called recruitment. • Depending on the number of motor neurons recruited by your brain, you can lift something light or something heavy.

Motor Neurons and Muscle Contaction—Varying the Rate • By varying the rate at which the nervous system stimulates muscules to contract, it can vary the amount of tension created by the muscle. • For instance, if a second action potential arrives at the muscle fiber before it has completely relaxed, the two contractions will add together and create greater tension.

Motor Neurons and Muscle Contaction—Varying the Rate • This summation will continue to increase if the stimuli is frequent enough so as to not allow the muscle to relax. • When this is the case, a fused sustained contract called tetanus results.

Motor Neurons and Muscle Contaction • The increase in tension generated by tetanus occurs due to the fact that muscles are connected to bones. • The tension due to the contraction of the muscle fibers is transferred to the bones likely resulting in movement.

Skeletons • There are a number of different skeletons in the animal world. Three important ones are: • 1. Hydrostatic skeletons consist of fluid held under pressure within a body compartment. • Cnidarians, flatworms, annelids. http://arnica.csustan.edu/photos/animals/Jelly_Fish1.jpg

Skeletons • 2. Exoskeletons are comprised of a hard encasement on the animal’s surface. • Clams, insects, crustaceans. • Chitin, calcium carbonate http://blog.friendseat.com/clams-uses-recipes

Skeletons • 3. Endoskeletons consists of hard supporting elements like bones, cartilage, bony plates. • These are most familiar to us.

Joints • Within our body lies bones comprised of calcium carbonate. • The joints of our skeletons allow us move and there are three types. • 1. Ball and socket joints—allows for rotation around the joint.

Joints • The joints of our skeletons allow us move and there are three types. • 2. Hinge joints—restrict movement to a single plane.

Joints • The joints of our skeletons allow us move, and there are three types. • 3. Pivot joints—allow for rotation within a joint—usually a hinge joint. Think forearm.

Joint Movement • The movement in your body is due to the muscles attached to the bones which articulate about your joints. • The muscle always contracts toward the origin of the fiber. • For instance, your hamstring originates on the ischial tuberosity of your pelvis, and inserts on the fibula. Contaction of this muscle puls the lower leg up. http://orthoinfo.aaos.org/figures/A00408F01R.jpg

Chapters 48, 49, and 50: Sensory and Motor Mechanisms Part II