Muscle contraction in the context of "Orgasm"

Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy of approximate 30.5kJ/mol to drive and support many processes in living cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known forms of life, it is often referred to as the "molecular unit of currency" for intracellular energy transfer.

When consumed in a metabolic process, ATP converts either to adenosine diphosphate (ADP) or to adenosine monophosphate (AMP). Other processes regenerate ATP. It is also a precursor to DNA and RNA, and is used as a coenzyme. An average adult human processes around 50 kilograms (about 100 moles) daily.

Adenine (symbol A, or Ade) is a purine nucleotide base that is found in DNA, RNA, and ATP. Usually a white crystalline subtance. The shape of adenine is complementary and pairs to either thymine in DNA or uracil in RNA. In cells adenine, as an independent molecule, is rare. It is almost always covalently bound to become a part of a larger biomolecule.

Adenine has a central role in cellular respiration. It is part of adenosine triphosphate which provides the energy that drives and supports most activities in living cells, such as protein synthesis, chemical synthesis, muscle contraction, and nerve impulse propagation. In respiration it also participates as part of the cofactors nicotinamide adenine dinucleotide, flavin adenine dinucleotide, and Coenzyme A.

Soft-bodied organisms are organisms that lack rigid physical skeletons or frame, roughly corresponds to the group Vermes as proposed by Carl von Linné. The term typically refers to non-panarthropod invertebrates from the kingdom Animalia, although many non-vascular plants (mosses and algae), fungi (such as jelly fungus), lichens and slime molds can also be seen as soft-bodied organisms by definition.

All animals have a muscular system of some sort but, since myocytes are tensile actuator units that can only contract and pull but never push, some animals evolved rigid body parts upon which the muscles can attach and act as levers/cantilevers to redirect force and produce locomotive propulsion. These rigid parts also serve as structural elements to resist gravity and ambient pressure, as well as sometimes provide protective surfaces shielding internal structures from trauma and exposure to external thermal, chemical and pathogenic insults. Such physical structures are the commonly referred "skeletons", which may be internal (as in vertebrates, echinoderms and sponges) or external (as in arthropods and non-coleoid molluscs). However, many soft-bodied animals do still have a functional skeleton maintained by body fluid hydrostatics known as a hydroskeleton, such as that of earthworms, jellyfish, tapeworms, squids and an enormous variety of invertebrates from almost every phyla of the animal kingdom; and many have hardened teeth that allow them to chew, bite and burrow despite the rest of body being soft.

Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to contract. Muscle tissue contains special contractile proteins called actin and myosin which interact to cause movement. Among many other muscle proteins present are two regulatory proteins, troponin and tropomyosin. Muscle is formed during embryonic development, in a process known as myogenesis.

Skeletal muscle tissue is striated, consisting of elongated, multinucleate muscle cells called muscle fibers, and is responsible for movements of the body. Other tissues in skeletal muscle include tendons and perimysium. Smooth and cardiac muscle contract involuntarily, without conscious intervention. These muscle types may be activated both through the interaction of the central nervous system as well as by innervation from peripheral plexus or endocrine (hormonal) activation. Skeletal muscle only contracts voluntarily, under the influence of the central nervous system. Reflexes are a form of non-conscious activation of skeletal muscles, but nonetheless arise through activation of the central nervous system, albeit not engaging cortical structures until after the contraction has occurred.

A tendon or sinew is a tough band of dense fibrous connective tissue that connects muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tension.

Tendons, like ligaments, are made of collagen. The difference is that ligaments connect bone to bone, while tendons connect muscle to bone. There are about 4,000 tendons in the adult human body.

A cramp is a sudden, involuntary, painful contraction of one or more skeletal muscles, or an overshortening of such associated with electrical activity. While generally temporary and non-damaging, they can cause significant pain and a paralysis-like immobility of the affected muscle. A cramp usually goes away on its own over several seconds or (sometimes) minutes. Cramps are common and tend to occur at rest, usually at night (nocturnal leg cramps). They are also often associated with pregnancy, physical exercise or overexertion, and age (common in older adults); in such cases, cramps are called idiopathic because there is no underlying pathology. In addition to those benign conditions, cramps are also associated with many pathological conditions.

Cramp definition is narrower than the definition of muscle spasm: spasms include any involuntary abnormal muscle contractions, while cramps are sustained and painful. True cramps can be distinguished from other cramp-like conditions. Cramps are different from muscle contracture, which is also painful and involuntary, but which is electrically silent. The main distinguishing features of cramps from dystonia are suddenness with acute onset of pain, involvement of only one muscle, and spontaneous resolution of cramps or their resolution after stretching the affected muscle. Restless leg syndrome is not considered the same as muscle cramps and should not be confused with rest cramps. Cramps are sometimes also called a "Charley horse", which is the term used by the lay people.

Flaccid paralysis is a neurological condition characterized by weakness or paralysis and reduced muscle tone without other obvious cause (e.g., trauma). This abnormal condition may be caused by disease or by trauma affecting the nerves associated with the involved muscles. For example, if the somatic nerves to a skeletal muscle are severed, then the muscle will exhibit flaccid paralysis. When muscles enter this state, they become limp and cannot contract. This condition can become fatal if it affects the respiratory muscles, posing the threat of suffocation. It also occurs in the spinal shock stage in complete transection of the spinal cord occurring in injuries such as gunshot wounds.