Innervation of a Skeletal Muscle Fiber

00:00 Well, let us have a look at the neuromuscular junction. The neuromuscular junction consists of an axon that passes down from the nerve, the nerve fibre, coming down from the spinal cord reaches the muscle. It then branches into a number of different sorts of branches, maybe two, three maybe many. And then those axons interact with individual skeletal muscle fibres through the neuromuscular junction.

00:37 Sometimes it is called a motor endplate. And what happens then, is that the wave of depolarization passing down or the action potential passing down through the axon, then releases neurotransmitter from the synaptic cleft. And the synaptic cleft and the muscle have very specialized structures, some of which you can see here. Maybe not so clearly, but let me just explain to you what they are. The axon terminal has lots and lots of mitochondria there. That energy is required to retake in or reabsorb back neurotransmitter and has also lots and lots of vesicles there, synaptic vesicles. These synaptic vesicles contain the neurotransmitter substance and when the axon potential passes down to this terminal, those synaptic vesicles then release that neurotransmitter at the synaptic cleft. And that neurotransmitter diffuses across the synaptic cleft or the gap between the axon terminal and the sarcolemma of the muscle cell. And then the muscle fibre itself has enormous junctional folds you can see a lot folded appearance of the sarcolemma of the muscle fibre here. That fold appearance is to increase the surface area of the cell membrane of the muscle fibre, the sarcolemma so that it can interact with a lot more neurotransmitter and make the transmission of the axon potential to a wave of depolarization very efficient and bring about contraction.

02:33 Let us continue with the motor innervation of a skeletal muscle fibre and let me explain to you what a motor unit is. Motor unit, is the number of muscle fibres that is innervated by a single neuron, a single axon. In this picture, you can see there are a number of muscle fibres that have been tears down and put into a dish and the nerve fibre has been stained.

03:07 Here is the dark stain. And this nerve fibre, this axon is coming down and it branches.

03:12 And you can see the motor endplates or neuromuscular junctions shown on some of the individual muscle fibres as a round black stained region. Well, the motor unit, again is the number of muscle fibres innervated by one single axon or one single neurone. In ocular muscles that motor unit is very small. Also maybe in your fingertips, where you need very precise control of muscle movement. But in postural muscles, we do not need that precise movement. The motor unit can be very very large, one axon can innervate several hundred different muscle fibres. So make sure you are aware of what the motor unit is. Now, I said earlier that muscle has a special sensory role as well. It has sensory receptors within it called the muscle spindle. And again I am not going to go through the physiology of the muscle spindle, but I just want to point out a couple of features about the muscle spindle shown in this diagram. That you can see, if you look at a histologic section through a muscle spindle within the muscle mass when viewed with an electron or a light microscope. First of all, the spindle cells are of two types. These are the special sensory cells. They're wrapped up by an internal capsule and filled with fluid. And these spindle cells are called nuclei bag fibres or nuclear chain fibres. And if you look at the diagram in more detail yourself, you will find that the nuclear bag fibre is named just because the nuclei is filled or appeared to be clumped together like a bag of nuclei. Whereas the nuclear chain fiber, the nuclei are elongated along the length of the muscle fibre like a chain. So make sure you see the two in the diagram. Each of those bag fibres or chain fibres receives innervation from both the sensory and motorneurons. And together these special spindle cells perceive the change in muscle length or stretch. They are called intrafusal fibres, intrafusal meaning within the spindle.

05:52 Normal skeletal muscle fibres are termed extrafusal fibres. So again these special muscle spindles, measure length or gives the information to the central nervous system about length or stretch of the muscle fibre. There is also the Golgi tendon organ. These Golgi tendon organs are found at the location of where the muscles starts to form with the tendon.

06:21 And those Golgi tendon organs measure tension in the muscle. So the muscle is continuously sending information to the central nervous system about change in length or stretch and also tension. And the central nervous system can then use that information to tell us about our position in space, to tell us about the positions of our limbs. When move my arm around all that information coming from the muscle spindle and the Golgi tendon organ gives the central nervous system the information to be able to locate our position in space.

07:01 Here is a section of the muscle spindle on the right hand side and the diagram on the left just shows you the rather elaborate way in which the bag fibres and the chain fibres are innervated. But the section shows you a section through the spindle, you can see an outer capsule and you can see some muscle fibres wrapped together. They are going to be intrafusal fibres wrapped up by fluid and they have peripheral nuclei you can see also in the section.