Hello, in this video, we will talk about muscle spindle and stretch reflex. Welcome back to Nonstop Neuron.com where learning medical concepts is as easy as watching cartoons. Let's get started with the muscle spindle first. In the video on the physiological anatomy of the skeletal muscle, we studied the muscle cells which are also called muscle fibers. These cells are the main force-generating cells. They are also referred to as extrafusal fibers. Among them are special types of other muscle cells referred to as intrafusal fibers. These fibers combined with their nerve supply form a complex called the muscle spindle. It is a sensory structure. Such spindles are distributed throughout the belly of the muscle. They send information about muscle length and changes in length. Now let's see the structure of the muscle spindle in more detail. The muscle spindles are 3 to 10 mm long. As I already said, the main components of the muscle spindle are the intrafusal muscle fibers and their nerve supply. Let's study intrafusal muscle fibers first. Each muscle spindle has 3 to 12 intrafusal fibers. They are basically modified muscle fibers. At the ends, they are pointed and the ends are attached to the glycocalyx of the surrounding large extrafusal fibers. They have contractile elements like actin and myosin. So they can contract when stimulated. This stimulation comes from gamma motor neurons. In simple words, the ends are innervated by gamma neurons. So these were the ends. Now let's talk about the central region. They have few or no contractile elements. So this portion does not contract. Rather it serves the sensory function. Now, this central portion contains the nuclei of the fiber. Based on the arrangement of the nuclei, the intrafusal fibers are divided into two types. Nuclear bag fibers and nuclear chain fibers. Nuclear bag fibers have nuclei concentrated in a wide central portion like a bag. There are one to three such nuclear bag fibers in a muscle spindle. Nuclear chain fibers have nuclei aligned in a chain. There are three to nine such nuclear chain fibers in a muscle spindle. So these were the intrafusal muscle fibers. Now let's talk about the nerve supply of the muscle spindle. The muscle spindle has both, sensory as well as motor nerve supplies. The sensory innervation is seen in the central region as its the main sensory area. There are two types of sensory endings: primary endings and secondary endings. The primary ending encircles the central portion of each intrafusal fiber. Thus it receives information from both, nuclear bag fibers as well as nuclear chain fibers. This is a type Ia fiber. Now the secondary endings. They mainly innervate nuclear chain fibers on one or both sides of the primary ending. They are formed by type II fibers. All these sensory endings sense the length of the muscle and changes in the length. They are stimulated by stretching of the mid-portion of the muscle spindle. This stretching might result either from the lengthening of the entire muscle or contraction of only the end portion of the muscle spindle. When the muscle spindle is stretched, the firing rate of the sensory neuron increases. And on shortening, the firing decreases. So this was the sensory supply of the muscle spindle. Apart from that the spindle also receives a motor supply. It innervates the contractile ends of the intrafusal fibers. This is a gamma motor neuron. Here I want to remind the motor innervation of extrafusal fibers. They are innervated by alpha motor neurons. Thus, the contractile ends of the muscle spindle and extrafusal fibers receive different motor innervation. So this was the structure of the muscle spindle. Now let's move toward its function. The main role of the muscle spindle is in the stretch reflex. So we see the stretch reflex now. In stretch reflex when a muscle is stretched suddenly, excitation of the spindles causes reflex contraction of the same muscle. The circuit of the stretch reflex includes the muscle spindle, sensory neuron, motor neuron, and the same muscle from where the sensory neuron originated. You see, there is only one synapse in this circuit. This allows the shortest pathway for the reflex. What happens is, that when the muscle is stretched suddenly, the central region of the muscle spindle is also stretched. Due to this stretching, the firing rate of sensory nerve endings increases. By type Ia fibers, these signals are carried to the spinal cord. Here they make synapse directly with the α motor neuron of the same muscle. Upon getting excited by the sensory neuron, these α motor neuron takes impulse to the extrafusal fibers of the same muscle from where the stimulus first came. Stimulation of extrafusal fibers causes muscle contraction. This prevents further stretching of the muscle. This is what happens on sudden stretching of the muscle. On sudden unstretching, opposite things happen and there will be reflex relaxation. Thus the stretch reflex opposes a sudden change in muscle length. This prevents the damage from a sudden change in the length of the muscle. So this was the basic about stretch reflex. But before closing, I want to cover one more point. It is about reciprocal innervation. We have seen that the sensory neuron directly stimulates the motor neuron of the same muscle. But that is not it. A branch from the same sensory neuron also innervate an inhibitory interneuron. This interneuron ends on a motor neuron that supplies the antagonistic muscles. For example, if the first muscle is causing flexion of the elbow, its antagonistic muscle would be the one that causes extension of the elbow. Now, we have already seen that stretching of a muscle increases the firing of sensory neurons. Now that neuron stimulates the α motor neuron of the same muscle to cause muscle contraction. Now at the same time, the sensory neuron also activates the inhibitory interneuron. This neuron releases inhibitor neurotransmitters that decrease the excitation of the motor neurons to antagonistic muscle. Or in simple words, this neuron is inhibited. This causes relaxation of the antagonistic muscle. So it won't provide any resistance to the movement by the main muscle. Thus during stretch reflex, the opposite type of movement occurs in the antagonistic muscles. This reciprocal innervation minimizes the antagonistic forces and increases the effectiveness of the stretch reflex. This completes all the points that we wanted to see in this video. We will understand the importance of motor nerve supply to the muscle spindle in a separate video. Now let's have a quick summary. The muscle spindle is a sensory organ to sense the length of the muscle. It is made up of intrafusal fibers which are of two types: nuclear bag fibers and nuclear chain fibers. Their mid-portion is sensory and is innervated primarily by type Ia fiber. The ends are contractile and are innervated by gamma motor neurons. Upon sudden stretching of the muscle, the spindle sends a signal to the spinal cord. Here the sensory neuron directly stimulates the α motor neuron resulting in reflex contraction of the same muscle. This stretch reflex prevents sudden changes in muscle length. That's it for this video. If you feel this video will help your friends and colleagues, please share it with them too. And don't forget to subscribe because lots more to come. At nonstop neuron, learning medical concepts is as easy as watching cartoons. Thanks for watching, see you in the next video.