Smooth Muscles: Structure Function

by Thad Wilson, PhD

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    00:01 You can see smooth muscle is named because it’s non-striated appearance.

    00:07 Striations will be lines that occur.

    00:10 And when we went through the skeletal muscle, you noticed that there were varying components that you can see individual lines in the muscle itself.

    00:19 Why this occurs is because there are a number of different processes.

    00:24 Smooth muscle has a larger degree in which it can shorten.

    00:28 So a smooth muscle can shorten to a greater degree than skeletal muscle.

    00:34 They also have a large number of intermediate filaments that help maintain its structural integrity.

    00:40 They have thick filaments and thin filaments just like skeletal muscle.

    00:46 They also have these dense bodies and these dense bodies help maintain that structural relationship of thick and thin filaments.

    00:56 The other thing that’s different is they are mechanically coupled at least in the single unit or phasic smooth muscle via gap junctions.

    01:05 It can be seen here.

    01:08 Now, let’s talk through the various ways in which smooth muscle develop force as which there are a number.

    01:15 The first we’ll talk through is in this type of graph where we have force on the Y-axis and time on the X-axis.

    01:26 This is one type of force production in which force production is high almost all the time until you see a relaxation and then it returns back to a high amount of force.

    01:39 The good example for this are sphincters in which they are normally contracted and then relaxed for a very short period of time and contract again.

    01:51 Other types of smooth muscle work almost in the opposite way in which they are normally relaxed and then they have bursts of activity and then they go back to a relaxed state again.

    02:03 A good example of that is urinary bladder tissue in which it’s relaxed most of the time until one wants to urinate.

    02:12 Other types of smooth muscle have a partially contracted state almost all the time, and therefore, they have a tonic tone.

    02:22 They can constrict to a greater degree, but then will relax only a small amount.

    02:28 Good examples of these are blood vessels, where there always will be a tonic amount of vasoconstriction and then you can still vasoconstrict more, but there will be a tonic kind of contracted state.

    02:41 And then finally, we have very active type of smooth muscle and where there are lots of oscillations.

    02:48 These oscillations are oftentimes driven by such things as slow waves in the GI system.

    02:55 These can be phasically active and intestinal muscle is a great example in which you get waves of contraction and relaxation, contraction and relaxation, but the relative force is pretty low.

    03:09 So combining all of these together, you can see that smooth muscle has a wide range of functions and that’s probably why it’s present in so many different tissues because it could be contracted normally, partially contracted, phasically active, or normally relaxed.

    03:29 How do you get a muscle to contract in terms of smooth muscle? There are two primary ways.

    03:37 The first way is via causing an electrical potential across the membrane, which opens up calcium channels to cause calcium-induced calcium release.

    03:48 So calcium enters in through the cell from the extracellular fluid into the cell, which engages a calcium channel to release more calcium.

    03:58 So that’s done via an electrical mean.

    04:02 The other way to do it is by using a ligand or a particular substance to bind to a receptor.

    04:08 So good examples here are things like hormones and neurotransmitters that can bind to specific receptors to cause an enzymatic second messenger signal pathway to release calcium into the cell.

    04:23 So let’s go through those two in a little bit more detail.

    04:26 So let’s go through calcium-induced calcium release.

    04:30 Oftentimes, these L-type calcium channels are located in small invaginations in smooth muscle.

    04:36 These are located in close proximity to calcium-induced calcium release channels of the SR.

    04:43 SR is the sarcoplasmic reticulum.

    04:45 Therefore, once there’s a membrane potential change, these calcium channels open to allow little calcium influx into the cell.

    04:55 Once the small amount of calcium enters the cell, it activates this calcium-induced calcium release channel to spill out lots of calcium.

    05:05 So the calcium that comes into the cell isn’t really involved with the contraction to a great degree.

    05:11 The most of the contraction is induced from what is released from the SR.

    05:16 That can be seen here right below the caveolae.

    05:24 Now, a ligand-gated contraction, seen up here, can be done via again a hormone or a neurotransmitter activating let’s say a G-coupled protein receptor, and the example that we have here activates phospholipase C, which is an enzyme, and that converts a number of substances, but it creates IP3 and DAG, and it’s IP3 that then goes to the SR to bind to an IP3-gated channel to open up to allow calcium afflux into the cytosol.

    05:58 So both of these two mechanisms both involve calcium in the cytosol but how they get it out is via a different mechanism: one through calcium-induced calcium release and the other one through an enzymatic process involving phospholipase C and IP3.

    06:19 There are a number of calcium-binding proteins that are located in smooth muscle that help regulate its function.

    06:26 These smooth muscle-binding proteins will help activate to cause contraction to occur, as well as there are calcium-binding proteins that are there to help sequester calcium or to bind it up so it’s not in its free form.

    06:42 Graded contractions are a telltale sign of what smooth muscle does.

    06:48 You can have a little calcium release and get a little contraction or a lot of calcium release and get a lot of contraction.

    06:55 So it’s all graded by the amount of calcium release, whether it’s done via calcium-induced calcium release or whether it’s done via an IP3 mechanism.

    About the Lecture

    The lecture Smooth Muscles: Structure Function by Thad Wilson, PhD is from the course Musculoskeletal Physiology.

    Included Quiz Questions

    1. Sarcoplasmic reticulum
    2. Plasma membrane
    3. Caveolus
    4. Peroxisome
    5. Mitochondria
    1. Striations
    2. Increased intermediate filaments
    3. Increased gap junctions
    4. Membrane dense bodies
    5. Thick and thin filaments
    1. Hormonal activation of a GPCR
    2. Calcium influx
    3. Release of DAG
    4. Increase in ATP
    5. Use of PLC
    1. A sphincter
    2. Large intestine
    3. Urinary bladder
    4. Small intestine
    5. Stomach
    1. Calcium-binding proteins are found on the outer surface of the smooth muscle cell membrane.
    2. Either calcium-induced calcium release or ligand-mediated calcium release can be used by smooth muscle for contraction.
    3. The amount of calcium released dictates the amount of contraction.
    4. Smooth muscle can be in a contracted state, partially contracted, phasically active, or relaxed.
    5. L- type calcium channels are commonly located in caveolae.

    Author of lecture Smooth Muscles: Structure Function

     Thad Wilson, PhD

    Thad Wilson, PhD

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