what are muscle spindles and what do they do?
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A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. Its functions are to send proprioceptive information about the muscle to the central nervous system, and to respond to muscle stretching.\n\nMuscle spindles are found within the fleshy portions of muscles, embedded in so-called extrafusal muscle fibers. They are composed of 3-10 intrafusal muscle fibers, of which there are three types, dynamic nuclear bag fibers (bag1 fibres), static nuclear bag fibers (bag2 fibres) and nuclear chain fibers and the axons of sensory neurons. Axons of motoneurons also terminate in muscle spindles; they make synapses at either or both of the ends of the intrafusal muscle fibers and regulate spindle sensitivity. Muscle spindles are encapsulated by connective tissue, and are aligned parallel to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.\n\nThe muscle spindle has both sensory and motor components. Primary and secondary sensory fibers spiral around and terminate on the central portions of intrafusal fibers, providing the sensory component of the structure via stretch-sensitive ion-channels of the axons. The motor component is provided by gamma motoneurons and beta motoneurons that innervate the spindle and cause a slight contraction of the end portions of the intrafusal muscle fibers when activated. The gamma, or fusimotor, axons only innervate the intrafusal muscle fibres whereas the beta, or skeletofusimotor, axons innervate both extrafusal and intrafusal muscle fibres. These motornurons are classified as static or dynamic according to their pattern of innervation and their physiological effects. The static axons innervate the chain or bag2 fibres while the dynamic axons innervate the bag1 fibres and increase the velocity sensitivity of the Ia afferents.\n\n[edit]\nSensitivity Modification\nThe function of the gamma motoneuron neuromuscular junction is not to supplement the general muscle contraction provided by extrafusal fibers, but to modify the sensitivity of the muscle spindle to stretch. Upon release of acetylcholine by the gamma neuron, the end portions of the intrafusal muscle fibers contract, thus deliberately elongating the non-contractile central portions of intrafusal muscle fibers. This opens stretch-sensitive ion channels of the centrally-positioned sensory axons, leading to an influx of sodium ions. This raises the resting potential of these axons, thereby increasing the probability of action potential firing, thus increasing the sensitivity of the muscle spindle.\n\n[edit]\nStretch reflex\nWhen a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both the velocity and the degree of stretch, and send this information to the spinal cord. Likewise, secondary sensory fibers (Group II afferent neurons) detect and send information about the degree of stretch (but not the velocity thereof) to the CNS. This information is transmitted monosynaptically to an alpha efferent motor fiber, which activates extrafusal fibers of the muscle to contract, thereby reducing stretch, and polysynaptically through an interneuron to another alpha motoneuron, which inhibits contraction in opposing muscles.\n\nPNF stretching, or proprioceptive neuromuscular facilitation, is a method of flexibility training that reduces the automatic reflex action in order allow muscles to lengthen.\n\n[edit]\nDevelopment\nIt is also believed that muscle spindles play a critical role in sensorimotor development.
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http://en.wikipedia.org/wiki/Gracillis_muscle
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It involved training the propioceptive receptors to decrease antagonistic muscle contraction-propioceptice receptors sense stretch and tension on the muscle, and feed back signals to increase antagonistic muscular contraction (primarily to stop the muscle from tearing and protecting joints).
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for skeletal muscle - nerve impulse depolarises muscle cell surface, calcium released, ATP driven actin-myosin contraction cycle.
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They are pulled apart by the mitotic spindle fibers. This happens in anaphase of mitosis. :-)
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The muscles have a certain muscle tone (partial contraction) which keeps the body upright instead of limp. This is all your muscles, not just in your back. However, your back does have the long muscles running down your spinal chord (the name escapes me currently which makes me sad). Also, your leg muscles will partially contract to keep your legs upright.\n\nUnless you're dead, medicated, or paralyzed - your muscles will always have a partial contraction (aka tone).
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The facial muscles are like elastic sheets that are stretched in layers over the cranium, facial bones, the openings they form, and the cartilage, fat, and other tissues of the head. These are the muscles of facial expression, acting singly and in combination. The diagram below shows a simplified view of how the facial muscles are actually arranged. It has a cut-away view of some of the muscles on the right side of the image to show the layers. Move the mouse cursor over the diagram of facial muscles below and click on a facial muscle of interest to see more information. Most of the muscles have a short video that illustrates the appearances that are produced when the muscle acts. Innervation and blood supply are also indicated. (MPEG 1 plugin is needed to view the videos.) For more detailed depictions of facial anatomy, see the Anatomy pages.
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Muscle fibers become torn after an intense exercise. They then need time to repair the damage, and prepare for another round of intense exercise by making a little bit more muscle. As a bodybuilder, I will not exercise my muscles till they are not sore, and I know that my muscle fibers are repaired and prepared for another round of even heavier weights! So, I would hold off a few days till they feel not sore (sore means that the cells that you broke in your muscle fibers have not been cleaned up by the body and are causing inflammation at the sight. This is so the blood can keep coming in and cleaning up the broken cells as your body repairs them).
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Hip flexor. It's the muscle you use to lift your legs. I pulled it once and it hurt so bad and I was limping the rest of the day.
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D) cremaster reflex\n\nThe cremaster muscle is a skeletal muscle and is controlled by the somatic motor neurons.\n\nAll other reflexes are mediated by the visceral motor neurons of the autonomic nervous system.
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muscle = yes\nmuscle fiber = yes\nmyofibril = yes\nactin filaments = no\nmyosin filaments = no\nsarcomere = yes\nZ line = doesn't shorten...they get closer together (the two z lines get closer)\n\n\nI'm not sure this is all 100% correct...but I hope it helped a little.  You can check our the websites in my sources for more info
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The muscles in my lower body and torso
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Some terms to help:\n\n"rectus" means "straight", so "rectus abdominus" are straight muscles in your abdomen, "rectus femoris" are straight muscles near your femur (upper leg).\n\nMany muscles are named for their shape:  "deltoid" is the triangular-shaped (like the Greek letter Delta) muscle on the shoulder.  The "trapezius" is the trapezoid-shaped muscle in your upper back and neck.\n\n"Latissiumus dorsi" muscles would be on your back (dorsal).\n\nSome muscles are named for the bones they attach to.  The "sternocleidomastoid" attaches from the sternum (breastbone) to the mastoid process of the jaw.\n\nJust a few examples to get you started.
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Animal muscle tissue is fascinating, when you learn how it works or "clenches". The process by which muscle clenche and release seems rather complicated to learn about.  But, it's worth it.  I believe there's a possibility for alternative energy stemming from the principles for how muscles work.  I wish all engineers had to learn about anatomy and physiology.  Here are the three easiest sites I could find. You might buy a used entry level human anatomy and physiology textbook and just start at the beginning, don't try to remember all the names of everything.  But start with cells and move up to muscle tissue early in the text. It's amazing stuff. Excellent question and sense of curiosity:\n\n\nhttp://en.wikipedia.org/wiki/Muscle_contraction\nhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Muscles.html
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vibration of the spincter muscle there, which allows, well, stuff to pass out at will and not uncontrollably! a fart is a pretty powerful thing!
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I have heard the term, but had to 'look it up' to find the answer:\n\n"reflexive muscle contraction elicited by electrically stimulating the skin over a peripheral nerve.  Used to assess the degree of excitation of alpha motoneurons." \nNote: this text spells it "motoneurons" not motorneurons.
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Muscle is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Its function is to produce force and cause motion, either locomotion or movement within internal organs. Much of muscle contraction occurs without conscious thought and is necessary for survival, like the contraction of the heart or peristalsis, which pushes food through the digestive system. Voluntary muscle contraction is used to move the body and can be finely controlled, such as movements of the finger or gross movements like the quadriceps muscle of the thigh.\n\nthe masseter or jaw muscle is the strongest human muscle