excitation-contraction coupling

Examples of excitation-contraction coupling in the following topics:

• Excitation–Contraction Coupling

  • Excitation–contraction coupling is the connection between the electrical action potential and the mechanical muscle contraction.
  • Excitation–contraction coupling is the physiological process of converting an electrical stimulus to a mechanical response.
  • It is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction .
  • This diagram shows excitation-contraction coupling in a skeletal muscle contraction.
  • Explain the process of excitation-contraction coupling and the role of neurotransmitters

• Peripheral Motor Endings

  • The highly excitable region of muscle fiber plasma membrane is responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract.
  • This depolarization spreads across the surface of the muscle fiber and continues the excitation–contraction coupling to contract the muscle.
  • The affects of myasthenia gravis illustrate the importance of effective and functioning neuromuscular junctions for communication between neurons and muscles to allow contraction and relaxation of muscle fibers.
  • Skeletal muscle contracts following activation by an action potential.
  • The binding of acetylcholine at the motor end plate leads to intracellular calcium release and interactions between myofibrils to elicit contraction.

• Mechanism and Contraction Events of Cardiac Muscle Fibers

  • The gap junctions spread action potentials to support the synchronized contraction of the myocardium.
  • In cardiac, skeletal, and some smooth muscle tissue, contraction occurs through a phenomenon known as excitation contraction coupling (ECC).
  • The actual mechanical contraction response in cardiac muscle occurs via the sliding filament model of contraction.
  • The pathway of contraction can be described in five steps:
  • This removal of the troponin complex frees the actin to be bound by myosin and initiates contraction.

• Interactions of Skeletal Muscles

  • Skeletal muscle contractions can be grouped based on the length and frequency of contraction.
  • The time between the stimulus and the initiation of contraction is termed the latent period, which is followed by the contraction period.
  • When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first.
  • As the strength of the signal increases, more (and larger) motor units are excited.
  • If the frequency of these contractions increases to the point where maximum tension is generated and no relaxation is observed then the contraction is termed a tetanus.

• Adrenergic Neurons and Receptors

  • α1 couples to Gq, which results in increased intracellular Ca2+ that results in smooth muscle contraction.
  • α2, on the other hand, couples to Gi, which causes a decrease of cAMP activity, that results in smooth muscle contraction.
  • β receptors couple to Gs, and increases intracellular cAMP activity, resulting in heart muscle contraction, smooth muscle relaxation, and glycogenolysis.
  • Other areas of smooth muscle contraction are:
  • Adrenaline and noradrenaline are ligands to α1, α2, or β-adrenergic receptors. α1-receptors couple to Gq, resulting in increased intracellular Ca2+ and causing smooth muscle contraction. α2 receptors couple to Gi, causing a decrease in cAMP activity and resulting in smooth muscle contraction. β-receptors couple to Gs, increasing intracellular cAMP activity and resulting in heart muscle contraction, smooth muscle relaxation, and glycogenolysis.

• Electrical Events

  • Cardiac contraction is initiated in the excitable cells of the sinoatrial (SA) node by both spontaneous depolarization and sympathetic activity.
  • The SA and AV nodes initiate the electrical impulses that cause contraction within the atria and ventricles of the heart.
  • The SA node nerve impulses travel through the atria and cause direct muscle cell depolarization and contraction of the atria.
  • The SA node impulses also travel to the AV node, which stimulates ventricular contraction.
  • Without autonomic nervous stimulation, it sets the rate of ventricular contraction at 40-60 bpm.

• The Nature of Marriage

  • Marriage is a social union or legal contract between people called spouses that creates kinship.
  • In some jurisdictions, such as Brazil, New Zealand, Uruguay, France and many U.S. states, civil unions are also open to opposite-sex couples.
  • Marriage is a social union or legal contract between spouses that creates kinship.
  • Outside of the traditional marriage between monogamous heterosexual couples, other forms of marriage exist.
  • In many Western cultures, marriage usually leads to the formation of a new household comprising the married couple, with the married couple living together in the same home, often sharing the same bed, but in some other cultures this is not the tradition.

• Rigor Mortis

  • Physiologically, rigor mortis is caused a release of calcium facilitating crossbridges in the sarcomeres; the coupling between myosin and actin cannot be broken, creating a constant state of muscle contraction until enzymatic decomposition eventually removes the crossbridges.
  • Unlike muscular contractions during life, the body after death is unable to complete the cycle and release the coupling between the myosin and actin, creating a state of muscular contraction until the breakdown of muscle tissue by enzymes (endogenous or bacterial) during decomposition .
  • As part of the process of decomposition, the myosin heads are degraded by the enzymes, allowing the muscle contraction to release and the body to relax.

• Thermal Distributions of Atoms

  • In thermal equilibrium the number of atoms in a particular state is proportional to $ge^{-\beta E}$ where $\beta=1/kT$ and $g$ is the statistical weight or degeneracy of the state (for $L-S-$ coupling $g=2(2J+1)$), so we find that
  • The size of the highly excited states of atoms increases as $n^2$ so we only have to sum over the states until we reach

• Hypotonia and Hypertonia

  • The loss of motor neuron control leads to increased excitability of muscle fibers.
  • Effects of hypertonia include spasticity dystonia (a state of prolonged muscle contractions) and rigidity (a state of muscle stiffness and decreased flexibility).
  • Spastic hypertonia is the general condition of muscle spasms caused by random contractions of the muscles, and is typical in cerebral palsy and spinal cord injuries; it can also occur from stroke.
  • Dystonic hypertonia is the resistance to passive stretching in muscles, and the return of limbs to fixed positions after contraction.
  • Hypotonia is the state of reduced muscle tone and tension, resulting in lessened ability to generate force from muscle contractions.