1. Lindsey Bily Anatomy & Physiology Austin High School

2.  Have certain characteristics that allow them to do their job. ◦ Excitability (aka irritability): the ability to be stimulated ◦ Contractility: they can contract or shorten which allows them to pull on bones and produce movement ◦ Extensibility: they than extend or stretch letting them return back to their original shape after contracting.

3.  Remember a muscle is composed of bundles of muscle fibers (cells).  Can range in length from 1mm to 40 mm long!  They share many of the same structural parts as other types of cells, but of course, have will have different names since they are in a muscle fiber.

4.  Sarcolemma: plasma membrane of a muscle fiber.  Sarcoplasm: cytoplasm of a muscle fiber.  Sarcoplasmic Reticulum (SR): network of tubules and sacs similar to the ER of other cells.  They contain many mitochondria and have multiple nuclei.

5.  Fibers have structures that are only present in muscle fibers.  Myofibrils: very thin fibers within the cell that extend lengthwise and almost fill up the sarcoplasm.  Sarcomere: basic contractile unit of the muscle fiber.  T Tubules (transverse tubules): they are tubes that run perpendicular to the myofibrils. ◦ Formed by extensions of the sarcolemma ◦ Allow nerve impulses (electrical signals) move deeper into the cell.

8.  Each muscle fiber contains about a thousand myofibrils. Shockingly, these myofibrils are composed of even smaller fibers called myofilaments.  Myofilaments can be either thick or thin.  Proteins that make up myofilaments… ◦ Myosin ◦ Actin ◦ Tropomyosin ◦ Troponin

9.  Thin Filaments are made up of actin, tropomyosin, and troponin.  Thick Filaments are up of myosin. They are shaped like golf clubs so that they can “grab” the actin on the thin filament.

10.  Contraction of a muscle requires several processes to happen in a sequence.

11.  Excitation of the Sarcolemma ◦ A motor neuron is a specialized nerve cell that connects to the sarcolemma of a muscle fiber at the motor endplate. However, they don’t touch completely, there is a gap. ◦ This connection is called a neuromuscular junction (also a synapse). ◦ Neurotransmitters are chemicals that transmit signals. ◦ When a nerve impulses reaches the end of a neuron, it releases the neurotransmitter, acetylcholine into the synaptic cleft (gap between the muscle fiber and the neuron).

12.  Acetylcholine diffuses across the synapse and binds to receptors on the sarcolemma on the muscle fiber.  Binding to the receptors causes them to sent an electrical impulse into the sarcolemma.

13.  The electrical impulse travels across the sarcolemma and down the T tubules.  The impulse triggers the sarcoplasmic reticulum (SR) to release stores of Ca 2+.  The Ca 2+ combine with the troponin on the thin filaments and cause the active sites of actin to become exposed.  The myosin heads on the thick filament can now bind to the actin and pull the thin filaments past them.  The myosin then releases itself and binds to another actin active site, there by shortening the muscle.  This is the SLIDING FILAMENT THEORY.

14.  Click here to see an animation.  http://msjensen.cehd.umn.edu/1135/Links/ Animations/Flash/0011- swf_breakdown_of_a.swf http://msjensen.cehd.umn.edu/1135/Links/ Animations/Flash/0011- swf_breakdown_of_a.swf

15.  Almost immediately after the SR releases Ca 2+ into the sarcoplasm, it actively pumps it back into its sacs.  Within a couple of milliseconds, the Ca 2+ has been recovered.  The SR steals the Ca 2+ from the troponin which then causes the active sites of the actin to become blocked again.  The contraction process automatically shuts itself off within a small fraction of a second after initial stimulation.

16.  Muscles use ATP for energy.  The myosin head will bind to the ATP and it is “at rest” waiting for actin to be able to bind. Imagine pulling back on a slingshot.  Once the myosin has bound to the actin, a new ATP molecule comes in and the myosin goes back in the “rest” position therefore pulling the actin towards it.

17.  Remember, in order for ATP to be used, the muscle must have an ample supply of glucose and oxygen.  Some muscle fibers store glycogen so they have enough glucose readily available.  Muscle fibers also store oxygen in a molecule called myoglobin (similar to hemoglobin).  Fibers with lots of myoglobin are called red fibers and those with few myoglobin are called white fibers.  Most muscles have a mixture of the two.

19.  If there is not enough oxygen present, then the body will undergo anaerobic respiration.  This will produce a buildup of lactic acid in the muscles and they might start to cramp or fatigue.  The lactic acid will gradually get back into the blood and travel to the liver where it will be converted back to glucose.  Remember the ULTRAMARATHON MAN?

20.  A functional unit that includes a motor neuron and all of the muscle fibers it attaches to.  So, each muscle fiber DOES NOT have to have their own neuron. One neuron could stimulate hundreds of fibers.

21.  “muscle graphing”  This is done to see the contraction of a muscle.  The force of the contraction is shown as a line on a graph.

22.  Threshold stimulus is the minimum amount of electrical stimulus that needs to be applied to get a muscle to contract.  Applying a single, brief threshold stimulus to a muscle will produce a quick jerk of the muscle (twitch contraction).

24.  The entire process takes 1/10 of a second!  1. Latent Period: the nerve impulse travels through the sarcolemma and T tubules and causes the release of Ca 2+ by the SR.  2. Contraction: The Ca 2+ binds to the troponin, so the myosin can bind to the actin.  3. Relaxation: the Ca 2+ is reabsorbed by the SR and the contraction stops.

25.  Our muscles rarely have twitch contractions. The nervous system makes sure that we move in fluid, smooth movements.  However, if we ever get an electrical shock or for some reason there is overactivity of the nervous system, we may experience twitch contractions.

26.  If a series of twitches occur, you will see a gradual increase in the strength of each contraction.  Due to several factors, but mainly Ca 2+ diffuses through muscle fibers that are warm.  This is why athletes “warm up” before they exercise.

27.  Occurs when there is repeated stimulation of the muscle. Over time, the muscle responds less and less to the same stimulation.  Usually doesn’t occur over the whole body, just an isolated muscle.  Physiological muscle fatigue is due to low levels of ATP, oxygen and glycogen, high levels of lactic acid.  We physically cannot contract our muscles.

28.  Different from psychological fatigue, where we feel exhausted or tired. During psychological fatigue, we will not contract our muscles as opposed to cannot.

29.  This is normal.  Tetanus is sustained contractions. This is how we move.  Multiple motor units will stagger their stimuli so that the muscle never has time to relax before it gets stimulated again.  This produces nice, smooth movements.

30.  A continual, partial contraction of a muscle.  When people go unconscious, they become limp, or lose their muscle tone.  Muscles that don’t have enough tone are called flaccid.  Muscles with too much tone are called spastic.

31.  If you don’t use your muscles, you’ll lose them (atrophy).  Strength training will make the muscles bigger (hypertrophy) by increasing the number of myofilaments within each muscle fiber. You do not get more muscle fibers!  Aerobic training will not make your muscles bigger, you will however, increase the number of mitochondria in each fiber as well as the number of blood vessels in the muscle and it will be more efficient at using oxygen.