Types of Muscle Contractions

00:01 Skeletal muscle.

00:04 There are a couple types of contractions that we need to consider.

00:07 The first is a concentric contraction.

00:10 A concentric contraction involves a muscle belly shortening and pulling on a bony lever and causing it to move towards the body, such as this.

00:21 As you’re contracting the bicep in this case, you’re pulling the muscle and it’s pulling on two bony levers to try to cause this reaction.

00:31 Opposite of this is an eccentric contraction.

00:35 And for this, the body is still trying to contract the bicep, but it is being overridden by the weight.

00:42 So both concentric and eccentric contractions involve the same crossbridge cycling that we’re going to discuss later on.

00:54 The structure of the thick and thin filament allow us to undergo muscle contraction.

01:00 So let’s talk about the thick filament first.

01:03 The thick filament has myosin heads.

01:07 These myosin heads are what's going to interact with actin.

01:11 They are looped together in this alpha helix, and so, they form kind of a multi-headed structure that’s going to be grouped together to form a lot of these myosin heads wrapped around in a circle.

01:25 The thin filament has a number of actin filaments tied together.

01:32 These actin filaments are kind of like small balls.

01:36 Grouping these together are tropomyosin rope-like structures.

01:41 These tropomyosin molecules also hold on to a troponin complex and a troponin complex involved three different subunits.

01:51 There’s a troponin T, which is bound with tropomycin, a troponin I, and a troponin C.

01:58 The troponin C is where calcium is going to bind.

02:03 As soon as calcium binds to this troponin C, you’re going to get a conformational change of the protein, and what it will do is move tropomyosin off of the active site on actin so that there can be an interaction between myosin and actin.

02:20 The thick and thin filaments are arranged in such a way that there are myosin heads and actin filaments on all sides, 360 degrees, around these filaments.

02:34 Interestingly, you can break down these units into smaller fields.

02:40 We call those smaller fields sarcomeres.

02:43 Sarcomere has a number of different components to it.

02:48 The A band is where the thick filaments are.

02:52 The H band is where the thin filaments are not, but the thick filaments are located.

02:59 Then, we have some structural proteins such as nebulin and titin that are attached to Z lines to help form the structural integrity of this dynamic unit.

03:12 Looking at it in a little bit more detail, you can see the close proximity of the myosin heads to the active sites on actin that are covered up in this case by troponin.