0:00 Hello, in this video, we're going to talk about reflexes, the physiology of a 0:11 reflex. 0:11 A reflex is a subconscious stimulus response mechanism. 0:15 Clinically, many reflexes are tested to identify any abnormalities in the 0:19 reflex pathway, which 0:20 may indicate problems within the central nervous system or the peripheral 0:25 nervous system. 0:26 Diseases tested include superficial and deep tendon reflexes, such as triceps, 0:32 biceps, 0:33 brachioradiolis, patella, and the Achilles reflex. 0:37 What you are looking for in these reflexes are not the movement of the limbs, 0:41 but rather 0:41 the contraction of the actual muscles. 0:45 A reflex involves sensory nerve fibers delivering information to the central 0:49 nervous system, 0:50 and motor fibers carrying commands to the effectors via the peripheral nervous 0:54 system. 0:55 The reflex arc is a neural wiring of a single reflex, and involves five steps. 1:01 The spinal cord here corresponds with a central nervous system. 1:05 The reflex arcs include a receptor reacting to a stimulus, such as this nail. 1:12 The afferent neuron, the sensory neuron, transmits the impulse through the 1:16 peripheral 1:16 nerve to the central nervous system. 1:19 The central nervous system is where information processing occurs. 1:24 The afferent neuron, the sensory neuron, synapses with an e-ferent neuron, or 1:29 an intranuron, which 1:30 will then relay information to the e-ferent neuron. 1:34 The e-ferent neuron is the motor neuron, which exits the spinal cord and 1:38 delivers the signal 1:39 to an effector, which is the muscle or the gland. 1:42 In this case, it is the muscle of the flexes of the hand causing the hand to 1:46 make a fist 1:47 to withdraw from the stimulus. 1:51 Exerces are classified according to development, response, complexity, and 1:58 processing side. 1:59 Development includes whether the reflex is innate or it is acquired. 2:04 The reflex responds in the nature of the resulting motor response, whether it 2:08 is somatic or visceral. 2:11 Complexity means the complexity of the neural circuit involved, whether it is a 2:15 monosynaptic 2:15 reflex or a polysynaptic reflex. 2:19 And finally, the processing side, where the information is being processed, 2:23 whether it 2:23 is the spinal cord or the brain, both of which make up the central nervous 2:28 system. 2:29 In this video, we will talk about monosynaptic reflex and polysynaptic reflexes 2:34 , and also 2:35 mainly focus on the spinal cord as a site of information processing. 2:40 Because really, most of the reflexes we test are spinal reflexes. 2:45 The monosynaptic reflexes range from simple monosynaptic reflexes to more 2:49 complex polysynaptic. 2:51 Let's first talk about monosynaptic reflexes, the stretch reflex. 2:56 The monosynaptic reflex is the most rapid, simple reflex with a single synapse 3:00 between 3:00 the afferent neuron and the efferent neuron. 3:04 An example of a monosynaptic reflex is the patellar reflex. 3:07 The patellar reflex is a spinal reflex involving the knee joint. 3:12 In this neural circuit, the patellar tendon is hit with a tendon hammer. 3:16 The resulting effect is a contraction of the quadriceps and then the knee 3:20 kicking out. 3:22 There are sensory neurons innervating muscles and tendons, which attach to the 3:26 patellar or 3:26 the kneecap. 3:28 When the patellar tendon is hit, this will stimulate receptors within the 3:33 muscle, which 3:34 will then activate sensory nerve fibers. 3:37 The receptors in the muscle fibers I'm talking about are called muscle spindles 3:41 , which are 3:42 extremely important in the reflex arc. 3:46 Muscle spindles are receptors within muscle. 3:49 Here is the tendon of the muscle. 3:51 And here are the muscle fibers, which are called extrafusal muscle fibers, 3:55 because these muscle 3:56 fibers are responsible for muscle tone, muscle contraction and then relaxation. 4:02 The red surrounded by the extrafusal muscle fibers are the intrafusal muscle 4:07 fibers, which 4:08 are the muscle spindles. 4:10 Muscle spindles are essentially receptors which respond to stretch and are inn 4:14 ervated 4:15 by sensory neurons, which are your afferent neurons in blue here. 4:22 The afferent neurons, which have been activated or stimulated, will carry 4:27 information to the 4:28 spinal cord. 4:30 The afferent neurons will synapse with the motor neuron, the e-ferent neuron. 4:34 This is a monosynatic reflex, because it only involves one synapse. 4:39 The e-ferent neuron will supply the quadriceps muscle and will innervate the 4:44 extrafusal muscle 4:45 fibers. 4:49 The e-ferent neuron will cause the extrafusal muscles of the quadriceps to 4:54 contract increasing 4:55 muscle tone. 5:00 This is a contraction of the quadriceps. 5:02 When the quadriceps contract, the knee will kick out. 5:08 Looking at the patellar reflex arc, step by step, the stimulus is a tendon 5:12 hammer hitting 5:13 the patellar tendon, which will stimulate the intrafusal muscle fibers, the 5:17 muscle spindles, 5:18 the receptors within the muscle. 5:21 The muscle spindles react to stretching and movement of the surrounding area. 5:25 This in turn will stimulate the afferent neurons, the sensory neurons, which 5:28 will carry the information 5:29 to the spinal cord. 5:31 The spinal cord is the information processing site, and so this is an example 5:36 of a spinal 5:36 reflex. 5:38 The afferent neuron will synapse with an e-ferent neuron in the spinal cord. 5:42 The e-ferent neuron, which is a motor neuron, is stimulated and will target the 5:46 extrafusal 5:47 muscles of the quadriceps. 5:50 The extrafusal muscles of the quadriceps will contract, completing the patellar 5:54 reflex 5:54 or the knee jerk. 5:57 Now when we talk about muscle tone, we are essentially talking about the 6:00 resistance of 6:01 muscle to stretch. 6:03 It's a good idea to get a grasp on muscle tone, to understand some pathology 6:07 that can 6:08 be going on when testing knee reflexes. 6:11 Here are two examples of a spinal reflex. 6:13 The afferent sensory neuron in blue is bringing information to the spinal cord, 6:17 synapses with 6:17 an e-ferent neuron, which will leave the spinal cord to supply a muscle, 6:21 causing it to contract. 6:23 If this area is severed, the e-ferent neuron there will be unable to deliver 6:26 signals to 6:27 the muscle, because the muscle has no motor neuron supplying it. 6:31 You expect the muscle to be hypotonic, low tone, it will be flaccid, because 6:37 there is 6:38 a disruption in the motor neuron supplying it. 6:41 If for example the e-ferent neuron, the motor neuron is continuously firing 6:46 signals, or there 6:47 is no inhibition to that e-ferent neuron, the effect of organ being the muscle 6:52 will also 6:53 get continuously stimulated and become hypertonic, spastic. 6:59 The patellar reflex was an easy example of a monosynaptic reflex, a single syn 7:03 apse, the 7:04 simplest reflex arc. 7:06 A polysynaptic reflex produces a more complex response. 7:11 There can be anywhere from two to hundreds of synapses within a polysynaptic 7:16 reflex arc. 7:22 All synaptic reflexes involve intranurons, intersegmental distribution along 7:30 different 7:30 areas of the central nervous system, and it also involves reciprocal inhibition 7:37 . 7:37 Here is the right leg and the knee joint, the anterior muscles of the thigh, 7:42 other quadriceps, 7:44 and the posterior compartment of the hamstrings. 7:46 The withdrawal reflex is a polysynaptic reflex that is initiated by no-ce 7:51 ceptive stimuli. 7:52 It can serve as a protective mechanism to prevent further injury. 7:56 The first part of the reflex pathway is a stimulation of receptors in the area. 8:00 In this case due to a painful stimulus to the foot. 8:04 The receptors will in turn activate the e-ferent neurons, which are your 8:09 sensory neurons, which 8:10 will bring this information to a specific spinal cord level, the processing 8:15 center in 8:15 the spinal cord. 8:17 The e-ferent neuron will synapse with one or multiple intranurons. 8:23 Intranurons here, drawn in black, are important in the reflex pathway as they 8:26 enable the sensory 8:27 neurons to communicate to many other neurons in the area. 8:31 In this case, the intranuron will relay the e-ferent neuron's information to 8:35 the e-ferent 8:36 neuron. 8:38 The e-ferent neuron, which is the motor neuron, will stimulate the hamstring 8:42 muscles, which 8:43 are your flecks of muscles. 8:45 The flecks of muscles is the effect of organ, and then will contract. 8:51 With polysynaptic reflex, as mentioned, there are many intranurons involved, 8:55 and also the 8:56 reflex involves different segments of the spinal cord. 9:00 Here the e-ferent neuron communicates with another segment of the spinal cord 9:04 above, 9:04 and stimulates the intranuron there. 9:07 Interestingly, the intranurons have another ability. 9:11 They can be excitatory intranurons, or they can be inhibitory intranurons. 9:16 Excitatory intranurons mean they will stimulate the e-ferent neuron. 9:21 Here there is another e-ferent neuron supplying the hamstring, and so it is 9:25 stimulated to 9:26 propagate the response that we want. 9:31 The inhibitory intranuron will inhibit the e-ferent neurons. 9:35 In this case, it will inhibit e-ferent neurons, which are supplying the ext 9:39 ensor muscles, 9:40 preventing the extensor muscles to contract. 9:43 This is an example of a reciprocal inhibition we talked about earlier in the 9:47 definition 9:48 of polysynaptic reflexes. 9:51 In summary, polysynotic reflexes involve multiple intranurons, many spinal cord 9:56 segments, and 9:57 reciprocal inhibition. 10:00 The brain can also affect spinal cord-based reflexes. 10:03 The brain can facilitate or inhibit reflex motor patterns based in the spinal 10:08 cord. 10:09 Here is a coronal section of the brain, cross section of the brainstem and the 10:13 spinal cord. 10:15 Let's take a look at the patellar reflexes, an example of how the brain can 10:18 affect the 10:19 reflexes. 10:20 Remember, the patellar reflex involves striking the patellar tendon with a 10:24 tendon hammer. 10:25 This will stimulate sensory nerve fibers, which will carry information to a 10:28 specific 10:28 level of the spinal cord, where it will synapse with a motor neuron. 10:32 The motor neuron will stimulate the quadriceps muscles, the quadriceps muscles, 10:36 which are 10:37 your extensor muscles, will contract and cause a need to jerk up. 10:43 We learned this reflex to be a monosynaptic reflex, which it is. 10:46 However, reflexes are also more complicated and usually involve more than one 10:51 single synapse. 10:52 In the patellar reflex, it is understandable to say that the afferent neurons 10:56 can also 10:56 synapse with inhibitory intranurons, which will inhibit efferent neurons which 11:02 supply 11:02 the flexes of the leg, the hamstring muscles here, for example. 11:07 This will allow for a more exaggerated kick. 11:11 The brain can facilitate or inhibit reflex motor patterns based in the spinal 11:15 cord. 11:16 An example is a brain's power over the knee jerk reflex, because consciously we 11:20 can inhibit 11:21 the reflex. 11:23 We can control whether our leg kicks out or not. 11:26 This is because voluntary upper motor neurons can travel to that spinal level 11:31 and stimulate 11:32 or inhibit the lower motor neuron. 11:35 We are able to tell our legs to not move, or we can even tell our legs to kick 11:40 out on 11:40 command. 11:42 But this will be a true reflex, because we are actually voluntarily doing it. 11:48 The planter reflex or Babinski reflex is another reflex elicited during 11:52 examination. 11:53 It is a superficial reflex. 11:56 The planter reflex is performed by scraping the sole of the foot from its 12:01 lateral aspect 12:01 up and medial below the toes. 12:04 This will stimulate sensory nerve fibers in the area. 12:07 The afferent nerve will carry information to the spinal cord and synapse with 12:10 an efferent 12:11 neuron. 12:12 The efferent neuron will tug at muscles involved in plantar flexion. 12:17 And so a normal response is plantar flexion, the toes curl down. 12:24 Interestingly in a normal planter reflex you have upper motor neurons 12:28 continuously inhibiting 12:29 efferent neurons responsible for stimulating the plantar extensor muscles, so 12:35 thus they 12:36 will inhibit dorsiflexion. 12:40 A Babinski positive or Babinski sign is typically an abnormal sign. 12:45 The reason is that when the sole of the foot is scraped, the efferent neuron is 12:51 stimulated 12:52 and will carry information to the spinal cord as per normal. 12:55 It will synapse with an efferent neuron which will tug at the plantar reflex. 13:02 However, in Babinski positive there is upper motor neuron lesions somewhere. 13:15 And this is known as a peramidal lesion, as a peramidal system is part of the 13:19 voluntary 13:20 control. 13:23 As a result, the upper motor neurons do not inhibit the extensor plantar 13:28 muscles. 13:29 And so the extensor muscles can contract during this reflex, and the toes curl 13:34 up and fan out. 13:37 The cause of Babinski sign is either peramidal lesions as mentioned, or an 13:41 underdeveloped 13:43 peramidal system. 13:44 For example, when you are an infant, your peramidal system is still developing. 13:49 Thus when you elicit a plantar reflex, in an infant it will show a positive Bab 13:54 inski sign. 13:55 This is a primitive reflex and infants. 13:58 The positive Babinski sign will eventually go away once the peramidal system 14:01 develops. 14:02 It is very important to know that there is no such thing as a negative Babinski 14:06 .
