 Understanding deep-tended reflexes and localizing problems based on physical exam findings is a key practical skill and understanding this topic is going to be critical not only for step one, but also for your future practice. You're likely to need some part of this knowledge no matter what specialty area you end up in. I can't count the number of times I've had questions about localizing lesions from abnormal reflexes on step one, step two, and shelf exams, not to mention in the clinical setting. So pay close attention to this topic. So to start this discussion, let's talk about the concept of a reflex arc. This is an important foundational concept when talking about deep-tended reflexes. If you remember nothing else, just know that reflexes are handled purely by the peripheral nerves and spinal cord. They have nothing to do with the brain. The brain can be permanently damaged and the patient can still have intact deep-tended reflexes. This is why peripheral reflexes are not considered in the evaluation of brain death. Patients can be completely brain dead and still have normal deep-tended reflexes. Brainstem reflexes, however, will likely be abnormal, but that's a topic for another video. So if we take a look at this reflex arc, we can visualize the pathway of a reflex. It starts by stimulation of the reflex, which is when the hammer strikes the tendon. And in the case of the patella reflex, which you're observing here, that's when the hammer strikes the patella tendon. It's going to cause a stretch of that tendon, which causes an efferent signal to travel back towards the spinal cord by the dorsal root ganglion. That signal enters the posterior horn of the spinal cord by the spinal nerve of the corresponding level of the spinal cord. An efferent signal then travels from the anterior horn of the spinal cord back towards the muscle via alpha motor neurons. This efferent signal causes the muscle attached to the tendon that we struck, which in this case is the quadriceps tendon to contract. This is going to cause the leg to kick out and that completes the patella reflex. Once again, I want you to realize that this entire circuit is contained within the afferent nerve. The spinal cord and the efferent nerve. So, damage to the spinal cord above or below the spinal nerve that's responsible for the reflex should not affect that reflex. As long as the efferent nerve, posterior horn, anterior horn, and efferent nerve are all intact. This is also why the reflex seems so involuntary to the patient. The reflex arc is complete before any sensory or proprioceptive signals have a chance to reach the brain and it feels very involuntary to the patient. And that's how reflexes work at a high level. It should be enough for you to answer a lot of what's asked on Step 1, along with what reflexes correspond to what spinal nerves. So let's go through all that now. Just like dermatomes, deep tendon reflexes correspond with particular spinal nerves. So the efferent and efferent nerves I was mentioning are contained within that particular spinal nerve. They travel through the posterior and anterior horn at that level. There are six main reflexes you should be able to localize to spinal nerves for Step 1. The first one is the biceps reflex. This corresponds to spinal nerves C5 and C6. This is the reflex you get when you strike the biceps tendon with the reflex hammer. It'll cause the biceps to contract and the arm to flex. So if there's a problem with the biceps reflex on physical exam and all other reflexes in the body are normal, there could be a problem somewhere in the reflex arc. So either the spinal nerves or the spinal cord itself on that side. The next one down is the triceps reflex, which causes the arm to straighten when you hit the triceps tendon. This corresponds to the C7 and C8 level of the spinal cord. Next is the cremesteric reflex. The cremesteric reflex is observed when you stroke the inner part of the thigh and the testicle on that side is pulled upwards and towards the anguinal canal. It traces back to the L1 and L2 level of the spinal cord so we can use it to assess the nerves of the lumbar spine. But of course, it can only be performed in a male patient. The patella reflex, which you saw in the intro of the video, is one of the classic reflexes that we see tested in movies and on TV. It's when you strike the patella tendon with the reflex hammer and it causes the leg to straighten. It corresponds to the levels L3 and L4. The Achilles reflex, also known as the ankle jerk reflex, tests the sacral nerves S1 and S2. When you strike the Achilles tendon with the reflex hammer, it should cause the foot to plantar flex. And that's the Achilles reflex. And finally, we have the anal wink reflex. The anal wink reflex is one which is observed when you stroke the skin outside the external anal sphincter and you see it contract. This is a test of the S3 and S4 spinal level. We hope you found this discussion of deep tendon reflexes and reflex arcs useful. It truly is one of the highest yield topics relating to CNS for USM Elite Step 1.