Properties of Muscle Tissue Excitability: respond to stimuli Conductivity: propagate signals over membrane Contractility: shorten and generate force Extensibility: stretch without damage Elasticity: return to their original length after being shortened or stretched

Functions of Muscular Tissue Body Movements Walking and running Stabilize Body Positions Posture Support Soft Tissue Move Substances Within the Body Blood (cardiac m.) Digestive tract (smooth m.; sphincters and oropharynx skeletal m.) Generate heat Shivering: contraction of muscle produces heat (ATP)

Organization of Muscle Epimysium The outermost layer Separates 10-100 muscle fibers into bundles called fascicles Perimysium Surrounds numerous bundles of fascicles Endomysium Separates individual muscle fibers (cells) from one another Epi-, Peri-, Endomysium converge into tendon CT that attaches a muscle to periosteum of bone Aponeurosis = Broad, flattened tendon

Functional Organization Muscle Fascicle Myofiber Myofibril Sarcomere Myofilaments Speak to the myofilaments and structure of the sarcomere here. When skeletal muscles are viewed under a microscope, they have distinct bands called striations They are formed by the arrangement of myofibrils within the muscle cell Each myofibril contains groups of long myofilaments Each myofilament is composed of myosin and actin filaments

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Sarcomere From Z to Z is a functional unit: the sarcomere This is looking at one myofibril in the myofiber. It is misleading in that there are several myofibrils in each myofiber and the Z line is actually a disc. The M line indicates areas of protein filaments that join the thick myosin filaments together. The titan filaments are large, elastic proteins that run through the myosin filaments,beginning at the M lines and ending at the Z discs. Their function is to stabilize myosin during contraction and help return the myofiber to its resting length after contraction. Realize that there are numerous sarcomeres in a myofibril, and many myofibrils in one myofiber.

Motor Neuron Motor neuron How do we get the sarcomere to contract? Motor neurons. Neuromuscular junction Skeletal muscle cells Motor unit

Neuromuscular Junction (Motor End Plate) Each myofiber receives a single axon terminal Neuromuscular junction (a.k.a. motor end plate) Specialized region of sarcolemma Neurotransmitter: Acetylcholine Each somatic neuron can branch. These branches innervate the myofibers at specialized areas called motor end plates. When acetylcholine is released at this site, the muscle fiber (the post-synaptic cell) undergoes membrane depolarization and a change in permeability (muscle cells, like neurons, are "excitable cells"). The result is a change in electrical charge on the plasma membrane that spreads to cover the entire surface. This in turn results in the release of calcium ions from the myofiber's sarcoplasmic reticulum, triggering the "ratcheting" of actin and myosin filaments past each other, and the overall shortening of that specific myofiber. This is called "excitation-contraction coupling". Note carefully that as with neurons, depolarization and the subsequent contraction it induces is an all or nothing response; when depolarization takes place, all the myofibrils inside that myofiber contract simultaneously and maximally.

SR and T Tubules: Excitation Contraction Coupling Sarcoplasmic reticulum (SR) Terminal cisternae Ca2+ is normally sequestered in the SR; released by stimulation from the motor end plate transferred from T Tubules Indentations of sarcolemma to transfer depolarization from motor end plate Triads

Nerve and Blood Supply Nerves Blood Supply Neurons stimulate skeletal muscle to contract Neuromuscular junction The axon of a somatic motor neuron typically branches many times Each branch extends to a different skeletal muscle fiber Blood Supply Each muscle fiber is in close contact with one or more capillaries

Organization of Muscle Fibers Power, range of movement, and speed of contraction all vary with the arrangement of fascicles in a muscle Four different arrangements Parallel Convergent Pennate Circular Parallel: fascicles are parallel to the long axis of the muscle (most of the muscles in the body are of this type). Muscle acts like individual muscle fiber: As muscle contracts, body increases in diameter. Decrease in length is dramatic, but power is not as strong as some following. Convergent muscles: come together at a single attachment. Direction of pull can be changed, but if the entire muscle is contracted the power is not as great since some fibers will be in opposition to eachother. Pennate: fascicles are oblique to tendon: powerful, but decrease in ability to shorten length. Unipennate (extensor digitorum) vs. bipennate (powerful: rectus femoris) vs. multipennate (really powerful: deltoid). Circular: sphincters, fibers arranged concentrically around orifice.

Actions of Muscles Review movement of joints Prime mover (agonist) Contraction of the muscle is the main reason for producing a movement Synergist Assists prime mover Antagonist Act in opposition to the prime mover Fixator Immobilize origin of prime mover

Muscle Nomenclature Directions: Rectus (straight), Transversus (across), Oblique (at an angle) Number of heads: Biceps (2 origins), Triceps (3 origins), Quadriceps (4 origins) Shapes: trapezoid, deltoid, rhomboid, orbicular Length: Longus or longissimus (long), teres (long and round), brevis (short) Size: magnus (big), major (bigger), maximus (biggest), minor (smaller), minimus (smallest) Externus (superficialis) vs. Internus (profundus) Extrinsic (outside) vs. Intrinsic (inside) Origins and insertions (first name is origin, second insertion) Primary function: flexor, extensor, retractor Don’t spend a lot of time here…they should already be familiar. Just remind them… Trapezoid: (trapezius); deltoid (triangle), orbicular (circle)