By W. Thorek. Northwest Christian College.

Unitary smooth muscle is composed of discrete cells purchase fluoxetine 10mg fast delivery menstruation gassy, usually thin (often as little as 2-5 mm in diameter) and spindle- shaped (ranging from 20 µm up to 100 µm in length) cheap fluoxetine 20 mg visa women's health controversial issues. Adjacent cells are often connected electrically by gap junctions like those in the heart. The gap junctions allow the movements of ions and small molecules and mediate the flow of ionic current and the spread of action potentials from one cell to the next. Cells are also mechanically connected to each other at specialized junctions analogous to desmosomes in heart muscle (see Fig. Vascular smooth muscle is sometimes classified as unitary, but often the properties are somewhat intermediate between the multi-unit and unitary extremes. Some regions are heavily innervated and come under strong control of the sympathetic nervous system (arterioles, as opposed to capillaries in the microcirculation -- see Fig. The smooth muscle in blood vessels may be only one or two cell layers thick in arterioles. The smooth muscle cells are oriented transversely to the long axis of the arteriole when the vessel is relaxed but change to a spiral orientation upon contraction. In large vessels like the aorta, the smooth muscle is many cells thick and lies in the media, a layer between the intima and the adventitia (Fig. The outermost cells of the media come into contact with sympathetic nerve axons with bead-like varicosities (Fig. The cells in the innermost layers of the media are not directly contacted by varicosities, but may receive stimuli from circulating factors, or from depolarizing current flow from the outer layers, mediated via gap junctions. The contractile machinery in smooth muscle is remarkably suited for its function in systems like blood vessels. Smooth muscle can generate large amounts of tension (force per unit cross-sectional area). During maximal activation, the tension can reach 6 kg/cm2, twice that found in skeletal muscle, even though smooth muscle has less contractile protein and much less myosin. Knowledge about the structure of the contractile machinery is incomplete, but gives some insights into how the remarkable performance is achieved. Force is generated in smooth muscle along multiple axes, running oblique to the fiber axis (Fig. Long actin filaments are attached to the surface membrane at membrane dense areas. This is because slanted force generators tend to act in parallel, like people pulling on a common rope in a tug-of-war (see idealized example in Fig. The parallel force generators add up to a total force with a large component along the long axis of the cell. This is unlike the situation in skeletal muscle, where the force generators are arranged in series. In that case, each force generator bears the same force, and it is their shortening that adds up. For a given amount of contractile protein per unit volume, the parallel arrangement gives more force, slower maximal velocity, and decreased energy consumption. Smooth muscle can produce force over a very wide range of muscle lengths, and shorten down to as little as 20% of the length where force is maximal. Skeletal and cardiac muscle work over a much narrower range of fiber lengths (Fig. As an explanation of the dynamic range of smooth muscle shortening, it has been suggested that the myosin in smooth muscle forms "side polar filaments" in contrast to the bipolar thick filaments found in skeletal or cardiac muscle. Side polar organization would allow myosin heads to move along the very long actin filaments in smooth muscle, without the limitation of unfavorable myosin-actin interactions that arise in the case of bipolar thick filaments. Ca2+ ions carry the key message that controls contraction in smooth muscle, as in striated muscles. However, there are some fundamental differences in the mechanism of activation by Ca2+ ions. You will recall that Ca2+ activates skeletal or cardiac muscle by binding to the Ca2+ -binding molecule troponin in a highly cooperative fashion. The Ca2+ -bound troponin in turn triggers conformational changes in tropomyosin, which normally blocks the active sites on the underlying actin filament.

That is order fluoxetine 20 mg without a prescription breast cancer jerseys, carrying a load 50% of the body weight increases their energy consumption by 30% (50%–20%) generic 10 mg fluoxetine visa pregnancy 30 weeks. What specific aspect of the movement or training that brings about these enhanced load carrying abilities is not yet understood. Explain why a runner is subject to a torque if she rounds a curve main- taining a vertical position. In the act of walking, the arms swing back and forth through an angle of 45◦ each second. Consider the carnival ride in which the riders stand against the wall inside a large cylinder. As the cylinder rotates, the floor of the cylinder drops and the passengers are pressed against the wall by the centrifugal force. Assuming that the coefficient of friction between a rider and the cylinder wall is 0. If a person stands on a rotating pedestal with his arms loose, the arms will rise toward a horizontal position. Assume that the length of the arm is 90 cm and the center of mass is at mid-length. Calculate the maximum velocity and acceleration of the foot of a runner who does a 100-m dash in 10 sec. Assume that the length of a step is 1 m and that the length of the leg is 90 cm and the center of mass is at mid-length. What is the most effortless walking speed for a person with 90-cm-long legs if the length of each step is 90 cm? Using the physical pendulum model for running described in the text, derive an expression for the amount of work done during each step. Compute the length of time for an erect human body without compen- sating movements to hit the floor once it looses its balance. Assume that the falling body behaves as a physical pendulum pivoted at the floor with the period given by Eq. Calculate the distance the center of mass is raised in the course of one step with parameters and assumptions as discussed in Section 4. Depending on how the force is applied, the body may be stretched, compressed, bent, or twisted. Elasticity is the property of a body that tends to return the body to its original shape after the force is removed. If the applied force is sufficiently large, however, the body is distorted beyond its elastic limit, and the original shape is not restored after removal of the force. We will review briefly the theory of deformation and then examine the damaging effects of forces on bones and tissue. The applied force is transmitted to every part of the body, and it tends to pull the material apart. This force, however, is resisted by the cohesive force that holds the material together. Similar considerations show that initially the compression is elastic, but a sufficiently large force will produce permanent deformation and then breakage. Stress S is the internal force per unit area acting on the material; it is defined as1 F S ≡ (5. The fractional change in length / is called the longitudinal strain St; that is, St ≡ (5. Young’s modulus has been measured for many materials, some of which are listed in Table 5. The force F required to stretch (or compress) the spring is directly proportional to the amount of stretch; that is, F K (5. A stretched (or compressed) spring contains potential energy; that is, work can be done by the stretched spring when the stretching force is removed. We shall first calculate the amount of energy required to break a bone of area A and length. As an example, consider the fracture of two leg bones that have a combined length of about 90 cm and an average area of about 6 cm2. The total energy absorbed by the bones of one leg at the point of compressive fracture is, from Eq.