so let's examine some interesting features of smooth muscle so we've established in previous videos that smooth muscle doesn't have a classical neuromuscular junction like skeletal muscle it can undergo various different types of stimuli it could be electrical it could be chemical but in smooth muscle in many cases they also can exhibit dual innervation where they can be controlled by both the sympathetic and parasympathetic nervous system so we talked about this in module six the communication module where you have this antagonistic relationship between the sympathetic and parasympathetic nervous systems and that can help regulate contraction versus relaxation in smooth muscle you could also have stretch activated or mechanically activated channels and this is responsible for a type of contraction called myogenic contraction so mild for muscle but really this is stretch dependent contraction so this is an intrinsic ability of smooth muscle and we'll also learn in cardiac muscle a little bit later on that is hard wired into the smooth muscle cell itself so the more you stretch the smooth muscle the stronger the contraction is going to be now certain hormones and local signals called paracrine signals can also alter smooth muscle contraction for example histamine this is the compound usually associated with allergic reactions uh released by a specific group of cells cells called basophil cells but in any case histamine can initiate the contraction of the smooth muscle that is in the bronchioles and this restricts airflow through the lungs and obviously makes it more difficult to breathe nitric oxide for example this is a chemical that can cause relaxation in smooth muscle and therefore increase the diameter of blood vessels so for example you might have a parent or grandparent that might have angina or chest pains and the physician might have given them a compound called nitroglycerin right nitroglycerin is actually an explo an explosive but in small quantities your body will convert nitroglycerin into nitric oxide nitric oxide is also what causes the dilation in the blood vessels of the penis that leads to an erection so there are many different ways of initiating contraction in smooth muscle so what's outlined over here on the left is just to show a small amount you can have what are called slow wave potentials where multiple stimuli are sub threshold until you reach threshold and then you initiate an action potential in the smooth muscle probably the most important one is known as a pacemaker potential so instead of having the leaky channels and the sodium potassium pump to establish a constant resting membrane potential you have different proteins here that lead to in essence an unstable resting membrane potential so there's a periodicity that allows us to reach threshold and when threshold is reached you have your wave of depolarization and repolarization so this is very important in smooth muscle to initiate a series of contractions like in the gi tract but you also see pacemaker potentials in the heart specifically in the pacemaker or autorhythmic cells to initiate contraction in the heart independent of the nervous system you could have various chemicals that initiate contractions so pharmaceutical chemicals that can initiate contraction without changing the resting membrane potential okay so smooth muscle also and we stated this earlier in skeletal muscle section smooth muscle is capable of hyperplasia you're able to through the actions of various hormones increase the number of smooth muscle cells this does not happen with skeletal muscle so just to give you an example during pregnancy the level of estrogen causes the growth of smooth muscle cells so the myometrium which is the smooth muscle lining of the uterus will increase the number of smooth muscle cells to increase the force of contraction that you will need during the birthing process after the third trimester this would not be sufficiently covered with hypertrophy right you cannot make the diameter of the smooth muscle cells that you already have cause enough tension for the birthing process so you need to add more smooth muscle cells so if we compare skeletal smooth and cardiac cardiac has attributes that overlap with both skeletal and smooth but there are some subtle differences so cardiac muscle like skeletal is striated so it has that sarcomere structure the way the thin and the thick filaments align the use of troponin the mechanism for contraction is going to be the same however unlike skeletal instead of being multinucleated cardiac muscle cells tend to have one maybe two nuclei they are branched so when they contract they're contracting in three dimensions courtesy of these branches and most importantly they have graded contraction so it's not just you're contracting or not there are levels of contraction so there's a much higher fine tuning that is needed to properly address the metabolic needs the oxygen demand the nutrient demand of the tissues that the cardiovascular system is serving cardiac muscle also has attributes that are similar to smooth the cells are linked with gap junction so they're electrically linked to allow for rapid communication some cardiac muscle cells will also exhibit this pacemaker potential and we talked about them they were the auto autorhythmic hold on the auto rhythmic cells aka the pacemaker cells and cardiac muscle is also capable of being controlled by both the sympathetic and parasympathetic to cause an increase in the pacemaker potential and have your heart rate go up that's what the sympathetic does or to cause a decrease in the pacemaker potential and consequently slower the heart rate down