Contraction cycle/channel gating/action potetial

what is the contraction cycle?

the contraction cycle is the repeating sequence of events that causes filaments to slide and it occurs in four steps, which repeats as the myosin ATPase hydrolyzes the newly bound molecule ATP until ATP runs out and the Ca levels are high.

1: ATP hydrolysis

a myosin head includes an ATP binding site that functions as an ATPase-an enzyme that hydrolyzes ATP into ADP and a phosphate group. The energy that is generated from this hydrolysis reaction is stored in the myosin head for later use during the contraction cycle. The myosin head is said to be energized when it contains stored energy. The head is perpendicular to the thick and thin filaments and has the proper orientation t bind with an actin molecule.

2\.) attachment of myosin to actin

the myosin head binds to actin via a myosin binding site o, forming a cross bridge. Although myosin is double headed, only one head attaches to the actin at a time.

3\.) Power stroke

after the cross bridge forms, the myosin head pivots, changing its position from a 90 degree angle to a 45 degree angle relative to the thick and thin filaments. It pulls the thin filament past the thick filament and towards the center of the sarcomere, which generates tension in the process. This event is known as a power stroke, which energy is acquired from the hydrolysis of the ATP from the myosin head. ADP is then released from the myosin head.

4\.) detachment of myosin for actin

after the power stroke, the cross bridge remains firmly attached to the actin until it binds with another molecule of ATP. As ATP binds with the ATP bdinign site on the myosin head, the head detaches itself.

how is RMP established?

The RMP is established through the action of ion channels in the cell membrane, particularly the Na+/K+ ATPase pump and leak channels for potassium ions. The Na+/K+ ATPase pump actively transports three sodium ions out of the cell for every two potassium ions brought into the cell, creating an electrochemical gradient that contributes to the RMP. Leak channels for potassium allow potassium ions to move down their concentration gradient out of the cell, also contributing to the RMP.

describe the resting membrane potential

The resting membrane potential (RMP) is the electrical potential difference between the inside and outside of an excitable cell when the cell is at rest. It is maintained by the balance of ion concentration gradients and ion channels in the cell membrane. The RMP is an important aspect of excitability because it determines the threshold at which an action potential can be generated.

How does the cell become less negative?

When an excitatory stimulus reaches the cell, it can cause depolarization of the membrane potential, meaning the inside of the cell becomes less negative. If this depolarization reaches a certain threshold, it triggers the opening of voltage-gated ion channels, leading to an action potential and further electrical signaling. This process is fundamental to the function of excitable cells in the nervous and muscular system

 **Compare and contrast voltage vs. chemical/ligand**

 Both voltage- and ligand-gated channels can be further regulated by various intracellular and extracellular factors. For example, some channels may be phosphorylated by specific kinases, leading to changes in their gating properties. In some cases, molecules such as neurotransmitters or hormones can modulate channel activity by binding to specific sites on the channel or by activating intracellular signaling pathways. In general, voltage-gated channels tend to be more rapidly responding and are involved in generating and propagating action potentials, while ligand-gated channels tend to be slower and are involved in synaptic transmission and other signaling processes. However, there is a significant overlap between these two types of channels, and many channels can be regulated by both voltage and ligand-gating mechanisms.

termination of muscle contraction

1. Cessation of action potentials: The generation of action potentials in the muscle fibers by the nervous system is required for muscle contraction. Once the action potentials cease, the muscle fibers stop contracting.
2. Calcium reuptake into the sarcoplasmic reticulum: The presence of calcium ions in the cytosol is required for muscle contraction. Once the action potentials cease, the sarcoplasmic reticulum begins to actively transport calcium back into its lumen, reducing the concentration of calcium ions in the cytosol and leading to muscle relaxation.
3. ATP-dependent myosin detachment: Myosin, one of the proteins involved in muscle contraction, binds to actin to generate force during muscle contraction. To relax the muscle, ATP is required to detach the myosin heads from the actin filaments.
4. Reduction in cytosolic calcium concentration: Once the sarcoplasmic reticulum begins to reuptake calcium, the concentration of calcium ions in the cytosol decreases. This reduction in calcium concentration leads to the dissociation of calcium from the troponin complex on the actin filament, causing the tropomyosin to block the binding sites on the actin filaments and preventing further interaction with myosin.