Conducting Cells

00:01 Let's move on now to conducting cells, and I'm going to speak firstly about neurons.

00:09 On the right-hand side, you can see the brain and spinal cord represented in the image, and you see these long yellow strings of cords going down the upper limb and the lower limb.

00:23 They're peripheral nerves.

00:25 Well, on the left-hand side, you can see three representatives of what typical neurons look like.

00:36 The large circular structure with the smaller circle in the middle is the cell body of the neuron.

00:45 The small circle is the nucleus.

00:47 Those cell bodies are located in various regions in the spinal cord, the brain and the peripheral nervous system.

00:57 Let's look at the one on the right first, the multipolar motor neuron.

01:02 That cell body is located in the brain or the spinal cord.

01:09 The cell process that you see extending goes all the way down into the periphery, all the way down to our fingers, if necessary, or our toes, if necessary, forming a peripheral nerve that's going to innervate muscles in our extremities and bring about movement so conducting cells are extremely long in their processes.

01:35 The middle unipolar sensory neuron, its cell body lies just outside the spinal cord and it has two projections.

01:46 One projection goes all the way down to our extremities, our toes, our fingers for instance and detects sensation.

01:55 If we step on a nail, we feel it, and that pain passes up one of those cell processes past the cell body, near the spinal cord and through the other process you see there.

02:11 It extends into the spinal cord and also into the brain Again, very, very long processes are these conducting cells characterized by.

02:23 And here we see in the image on the right-hand side, you can see a very dark brown-stained cell.

02:30 That's the cell body of one of these large motor neurons sitting in the central nervous system and extending across to the left-hand side of the screen, you see a long cell process, the axon, and those axons from numbers of these motor neurons constitute a motor nerve or a peripheral nerve.

02:58 Other neurons coming into the central nervous system will be sensory, and together, the sensory neurons and the motor neurons constitute what we call a peripheral nerve.

03:12 Finally, when these neurons or axonal processes pass all the way down to skeletal muscle for instance, shown here, if you look carefully at the elongated pale brown structures, you can just make out some striations of the characteristic of skeletal muscle cells.

03:33 Well, those dark-stained areas are where the axons are coming down and they form a motor endplate, a junctional complex which with each skeletal muscle cell and the nerve impulse will pass down these axons, these long threads, dark-stained material you see here, and then that action potential passes over these motor endplates and brings about contraction of skeletal muscle.

04:05 Another example of conducting cells are the very specialized cardiac muscle fibers.

04:13 They're called Purkinje fibers.

04:16 Contraction of the heart chambers is initiated in the atria by the sinoatrial node and that wave of depolarization and contraction then gets and initiates a wave of depolarization at the atrioventricular node, the AV node shown here in the diagram.

04:42 And then the specialized cardiac muscle fibers transmit that wave of depolarization down into the other parts of the chambers of the heart to bring about contraction of cardiac muscle and therefore the chambers of the heart, and therefore move blood out of the heart.

05:02 While those conducting fibers again are not nerve cells, they're not nerve fibers.

05:09 They're very specialized cardiac muscle fibers called Purkinje fibers.

05:14 On the right-hand side, you can see a group of these clustered together.

05:18 They're very pale-staining pink compared to the normal deeper-staining real cardiac muscle cells that contract.

05:28 The Purkinje fibers are lighter-stained because they don't contain all that contractile proteins that the real cardiac muscle fibers contained that bring about contraction.

05:41 On the left-hand side, you can see just towards the surface that these very specialized conducting fibers are binucleate.

05:53 You can see a lot of binucleate cells there as if they're little eyes looking out at you from the screen.

06:01 Again, these cardiac muscle cells are very specialized for conduction, but here is some real cardiac muscle fibers that conduct the wave of impulse through their membrane system so that one cell passes its wave of depolarization to the next cell and so on and therefore, there's this wave of contraction brought about through the cardiac muscle fibers as the wave of depolarization travels through the cells.

06:36 So in a sense, they're both contractile but also have a role in conducting the wave of depolarization as well, once initiated by the Purkinje fibers.