Neurophysiology
Q1. Are we responsible to know the six eye muscles, or just lateral rectus and medial rectus eye muscles?
A1. You must know which cranial nerve innervates each of the six eye muscles. For the eye movements themselves, all you have to know is that the lateral rectus moves the eye laterally (abduction) and the medial rectus moves the eye medially (adduction); you don't have to know how the other four eye muscles move the eyeball.
Q2. I have a question about tolerance on page 47. In the table it has high degree of tolerance, does that mean that the more you use opoids for those things listed, the less effective it becomes?
A2. The short answer is "Yes." Tolerance means that the same dose of a drug produces less of a response over time: you become tolerant to the drug. You can overcome tolerance by administering a larger dose of the drug. For example, if you have chronic pain, at first you will receive the "standard" dose of an opiate to control the pain. After a period of time, the pain will return. Then you will take larger doses of the drug to control the pain. This process can continue indefinitely. There are people who have to take massive doses of opiates, doses that would kill anyone who did not have the tolerance. On the other hand, there is no tolerance for the miosis effect of opiates. This means that no matter how long you take the drug, you will always experience miosis; this effect never diminishes.
Q3. 1) Does the high concentration of sodium in the perilymph play an important role? It seems like calcium and potassium play the most important roles.
A. No. That is what is unique about the hair cells in the cochlea. It is the potassium that enters the cell that produces depolarization, rather than sodium as in nerve or muscle cells. Calcium enters as a result of depolarization, comparable to what it does in nerve cells.
2) On page 76 in the syllabus, it seems like the paragraph explanation doesn't match the figures. On part B, it looks like both the excitation and inhibition are exacty the same picture. Same for part C.
3) What is an example of a tendon reflex?
A. The stretch reflex is the major tendon reflex. Clinician's term the stretch reflex a "deep tendon reflex," or DTR. Despite the name, the stretch reflex is elicited by stretching muscle spindles, not Golgi tendon organs.
4) When you say that a cranial nerve "exits" at a region in the brain. What does "exit" mean -- go to the appropriate motor/sensory organ?
A. Cranial nerves by definition are attached to the brain. I term the attachment point the location that the nerve exits the brain.
5) In our TBL, how did you decide that the clinical problem involved the pons (and not the medulla)?
A. The nucleus of the abducens nerve, and the course of the nerve itself as it goes from the nucleus to its "exit" point on the ventral surface, are located in the pons. The nerve is located at the junction of the pons and medulla when you look at the ventral surface of the brainstem, but essentially all of the nerve and nucleus within the brain is located in the pons.
6) I have a question about the direction of endolymph force when turning your head to the left. The text part of they syllabus (pg 75, 76) says that it would be opposite to the direction of head rotation, but the figure on pg. 76 shows that when the head turns left, the endolymph flow is also to the left, so I'm a little confused about which it is.
A. I agree that the figure initially looks like it says exactly opposite what I mentioned in class. Actually both of us agree. Bottom line is that if the head turns left, the initial movement of endolymph is to the right. Let me try to convince you that the figure shows this. First, look at the head diagram in the middle of panel B, and the semicircular canal. The arrow in the semicircular canal points toward the right, or opposite the head movement. Now, if you took that semicircular canal out of the picture and rotated it so that the ampulla is pointing up rather than pointing a little "southwest" as it is in the head diagram, you will find that the arrows are pointing in the same direction as they were in the head diagram, but now they appear to be moving left instead of right. This is simply because of the rotation of the diagram. If this is still unclear, then just remember the statement at the top of p. 76.
7) Dr. Blair, I've got a question about the anesthetics. I know that the anesthetics are usually identified with the suffix 'caine' and I was under the assumption that cocaine was also an anesthetic. But I've noticed that some of the adverse effects associated with anesthetics that are mentioned in the notes do not correlate with reactions to cocaine, e.g. cardiovascular depression in notes when cocaine causes an increase in heart rate and blood pressure. I was wondering if maybe cocaine was a different class of anesthetic or not one at all, or if perhaps the different use of the drug caused different reactions. I was also wondering about miosis and mydriasis associated with anesthetics. I think cocaine causes mydriasis, but I don't think the others were listed in the notes.
A. The 'caine' drugs are local anesthetics, and cocaine does have local anesthetic effects. One of the reasons that cocaine has its effects on the heart is because of this action: cocaine disrupts the electrical activity of the heart due to its local anesthetic action. The trouble is that cocaine also has numerous other effects that are not definitively linked to its local anesthetic action, such as its euphoric effect. Cocaine is what can be called a messy drug because of numerous effects.
To give you an indication of how things change over time, when I was in graduate school we were taught that cocaine was a relatively safe drug of abuse because it had no addictive qualities. This undoubtedly was due to the relatively impure forms of the drug that were available to users. With the advent of crack cocaine, which is much more potent, we know that cocaine actually is highly addictive.
Make sure that you know the difference between analgesic drugs and anesthetic drugs. Analgesic drugs relieve pain but do not put you to sleep (at least not with the normal dose). Anesthetic drugs can be local or general, the former to block nerve conduction at a localized point, and the latter to put you to sleep. Most general anesthetics are poor analgesics. You'll learn much more about this in your future courses.
Some of the side effects of drugs can be predicted from their normal mechanism of action, but that is not always the case. Most drugs are not nearly as specific in their effects as you are sometimes led to believe. The effects on pupillary diameter can fall into this category. A drug that affects the autonomic nervous system could have an effect on pupillary diameter by altering sympathetic or parasympathetic activity. This does not readily explain the effects of opiates on pupillary diameter. I do not know the rationales for the effects of different drugs on pupil size.
8) when reading over the notes over the part about the three major systems consisted in the somatosensory system i began to get confused as to which system controls what. i know that the spinothalamic system helps to localize pain, but what does the trigeminal sys and dorsal column-medial lemniscal sys do?
A. We did talk about this extensively, particularly last Friday and today. I urge you to look over your syllabus and notes. Here's a brief summary.
Spinothalamic: pain and temperature from body
Dorsal column/medial lemniscal: touch from body
Spinal trigeminal nucleus: pain and temperature from head
Main sensory trigeminal nucleus: touch from head
Trigeminal motor nucleus: Chewing muscles
Mesencephalic trigeminal nucleus: Cell bodies of sensory neurons from chewing muscles
9) another question arose while studying. on the diagrams of the locations of various parts of the medulla middle, pons middle...all of that, are there certain ones we need to concentrate on other than the trigeminal system, medial lemniscus? i guess i'm trying to ask which ones do we need to know the location of in each drawing?
A. The other major one is the corticospinal tract.
10) I have a question about the TBL. You were talking about "mediation" concerning the first question. Could you explain this to me a little more thoroughly?
A. You can think of this in several ways. One way is to think of structures or pathways that are involved in the response, and if they are removed then the response won't occur. For blinking, sensory receptors in the cornea, trigeminal nerve, spinal trigeminal nucleus, facial nucleus, and facial nerve are involved. If any of these structures is damaged, then the response (in this case blinking) won't occur. Therefore, each of these structures mediates blinking.
Another way to think of it is to think of which structures can be stimulated (or activated) to produce the response. If you stimulate any of the structures in the list above, then you will cause (elicit) a blink. Therefore, each of the structures mediates the blink.
To take another example, what structures would mediate the stretch reflex? Spindle, Ia and group II fibers, and alpha motoneurons.
11) I wanted to ask what the is between the vibrational detector and the velocity receptors. It seems that both generate action potentials with a frequency proportional to the velocity of the stimulus and both have a frequency of 0 when the velocity of the stimulus is not changing. They are not the same. The velocity detectors discharge in proportion to speed. That is, when there is something moving these receptors fire. An example is the time period that you push an eraser against you skin. When you touch the skin the receptors do not fire because there is no speed involved. Then you push the eraser against the skin at some speed; now the receptors will fire. If you pushed the eraser against your skin faster, then the receptors would fire at a higher discharge rate. Once you stopped pushing the eraser (so that speed is now 0), the receptors would stop firing.
The vibration detectors respond to acceleration, or changes in speed. In the previous example, the receptors do not fire during the indentation of the skin; they only fire when you start pushing the eraser against the skin (in other words, going from a speed of zero to a speed greater than zero), and when you stop pushing the eraser against the skin (going from a speed greater than zero to a speed of zero). The greater the difference in the two speeds, the greater the discharge rate. For example, comparing the discharge rate of going from a speed of zero to a speed of 1 mm/s with going from a speed of zero to 10 mm/s, the discharge rate during the change of speed would be higher in the second instance. Importantly, the velocity detectors do not respond when the speed is constant, such as when the skin is being indented by the eraser.
Re-examine Fig. 2 and Fig. 3 on pp. N-29 and N-30 to see these different patterns of responses.
Q4. was studying the neuroanatomy of the spinal cord and just wanted to make sure I understood you correctly during class. I was questioning which parts that we would need to know for the test. Do we only need to know those tracts that you mentioned specifically: Gracile Fasciculus, Lateral Corticospinal tract, Spinothalamic tract, Cuneate fasciculus, Trigeminal, and Medial lemniscus. Just making sure I don't need to know the other parts that were listed in the syllabus.
A. The structures that you may have to identify on the cross sections are any of the structures that are part of the dorsal column/medial lemniscal system, spinothalamic system, corticospinal system, and trigeminal system. Many of the structures are located not just in the spinal cord, but also in the brainstem and thalamus.
Dorsal column/medial lemniscal system:
gracile fasciculus
gracile nucleus
cuneate fasciculus
cuneate nucleus
medial lemniscus
ventral posterior lateral nucleus (in thalamus)Spinothalamic system:
spinothalamic tract
ventral posterior lateral nucleus (same as above)Corticospinal system:
corticospinal tract (in brainstem)
lateral corticospinal tract (in spinal cord)Trigeminal system:
spinal trigeminal nucleus
main sensory trigeminal nucleus
trigeminal motor nucleus
mesencephalic nucleus
ventral posterior medial nucleus (in thalamus)
Q4A. Question #6 of the Neurophysiology example questions in the syllabus requires that we are able identify the location of the facial nucleus on cross-section to correctly answer the question. Was it a mistake to include this question as an example, or do we actually need to be able identify much more than the 4 aforementioned systems on cross-section?
A4. No, you do not need to know the locations of cranial nerve nuclei. I did mean to require that this year, but I neglected to state it. Therefore, you don't have to know the locations of the cranial nerve nuclei for the exam.
Q5. If you have a lesion in the area of the medial lemnicus in the caudal medulla, will you have a lose of touch on BOTH sides since this is where the cross over occurs?
A5. If you damage the area where the crossing of fibers occurs, then you will produce bilateral deficits. This could occur as you state it, it could occur where the corticospinal tract crosses, and it can occur for the spinothalamic tract in the spinal cord (we discussed this last one in class). In reality, it is rare that a brainstem lesion produces a bilateral deficit.