Actions ofAcetylcholine and Norepinephrine on Cardiac Muscle Cells

Each muscle fiber of a skeletal muscle receives a direct synaptic input from a particular motor neuron without this synaptic input, a skeletal fiber does not contract unless stimulated directly by artificial means. Nevertheless, we have seen that cardiac muscle fibers generate spontaneous contractions that are coordinated into a functional heartbeat by the electrical conduction mechanisms inherent in the heart itself. This does not mean, however, that the activity of the heart is not influenced...

Asynchronous Activation ofMotor Units During Maintained Contraction

As anyone who has done prolonged physical labor or exercised vigorously can attest, muscle contractions cannot be maintained indefinitely muscles fatigue and must be rested. Thus, the state of tetanus in Figure 11-6 could not be maintained in a single motor unit for very long without allowing the muscle fibers in the motor unit to relax. However, some muscles such as those involved in maintaining body posture are required to contract for prolonged periods. What mechanism helps prevent muscle...

The Cardiac Action Potential

Cardiac Action Potential Time

In Chapters 6 and 7, we discussed the ionic mechanisms underlying the action potential of nerve membrane. The action potential of skeletal muscle fibers is fundamentally the same as that of neurons. The cardiac action potential, however, is different from these other action potentials in several important ways. Figure 12-5 compares the characteristics of action potentials of skeletal and cardiac muscle cells. One striking difference is the difference in time-scale cardiac action potentials can...

Interaction between Myosin and Actin

Myosin Actin Interaction Cardiac Muscle

When actin combines with energized myosin, the stored energy in the myosin molecule is released. This causes the myosin molecule to return to its resting state, and the globular head pivots about its hinged attachment point to the thick filament. The pivoting motion requires that the thick and thin filaments move longitudinally with respect to each other. This mechanical analog is illustrated schematically in Figure 10-6. The exact nature of the chemical changes in a myosin molecule during the...

Factors Affecting the Speed of Action Potential Propagation

The speed with which an action potential moves down an axon varies considerably from one axon to another the range is from about 0.1 m sec to 100 m sec. What characteristics of an axon are important in the determining the action potential propagation velocity Examine Figure 6-9b again. Clearly, if the rate at which the depolarization falls off with distance is less, the region of axon brought above threshold by an action potential at point 1 will be larger. If the region above threshold is...

Coordination of Contraction Across Cardiac Muscle Fibers

Cardiac Cell Current Flow Directional

In order for the contraction of a heart chamber to be able to propel the expulsion of fluid, all the individual muscle fibers making up the walls of that chamber must contract together. It is this unified contraction that constricts the cavity of the chamber and drives out the blood into the blood vessels of the circulation. In skeletal muscles, an action potential in one muscle fiber is confined to that fiber and does not influence neighboring fibers therefore, contraction is restricted to the...

Temporal and Spatial Summation of Synaptic Potentials

Motor Neuron Summation

Figure 9-la shows an experimental arrangement for recording the change in membrane potential of a motor neuron in response to action potentials in a single presynaptic sensory neuron. An intracellular microelectrode is placed inside the postsynaptic motor neuron, and presynaptic action potentials are triggered by electrical stimuli applied to the sensory nerve fiber. Figure 9-lb illustrates responses ofthe motor neuron to a single action potential in the sensory neuron and to a series of four...