FAQ Meds

L1 Eye

Question. You mention that dark acts as a stimulus for rods and cones. So my question is whether the ganglion cells that send info via the optic nerve are activated by dark rather than light?

Answer: The off centre cells (i.e. those activated by a black dot on a white page) are. The on center cells are not. We have not covered why. But FYI, the on center ganglion cells have an inhibitory connection between the center receptor and the bipolar cell. The off center ganglion cells have an excitatory connection.

L2 Visual Cortex

Question. In Session 2, the sentence reads: "The LGN is composed of 6 layers. 3 receive info from one eye, 3 from the other." I don't really understand how each LGN (one on each lobe of the thalamus) receives input from BOTH eyes, since the optic tracts from each eye aren't supposed to recombine until they reach the primary visual cortex.

Answer: They do not combine onto the same neuron until they reach the binocular cells in primary visual cortex. These cells are located above and below layer 4 and are responsible for stereopsis.

Question. Does the medial temporal motion area typically have more complex cells than the V1 area to allow for accurate detection of motion? How do the larger receptive fields of these cells in the MT allow for good detection of motion?

Answer: Try walking forward. Notice how the scene is expanding around the point that you are walking toward. That is a particular pattern of motion, one that fills the whole visual field. Some neurons in this area are good at recognizing this particular pattern, while other neurons specialize in other patterns.

L3 Association Cortex

Question. Does the what/where stream apply to the other senses?

Answer: Probably. But these pathways have as yet not been mapped.

Question. 1) Regarding neglect: with a lesion in the right PTO, I can appreciate how the patient would be unable to acknowledge the left hand side of an object because of the lack of ability to attend. However, given the example of "drawing the flower" in the notes, wouldn't the patient then realize, after the fact, that he is unable to "see" the LHS of things, when he reflects on how he has just drawn a half flower? I guess I don't fully understand the statement "the patient is unaware that one half is gone . . . "

Answer: Did you see the self portraits by the patient with a right parietal lesion? Go to

http://www.med.uwo.ca/physiology/courses/medsweb/L3AssMem/LinksM3.htm

This is the same problem. This smart patient, a neurologist, should have realized that he had missed drawing half his face. But he didn't. For him the picture was complete, nothing was missing. Had someone else added features to the other side, he would not have been aware of them.

Question. In your take home problems, you mention that a right PTO lesion would cause you to neglect/be unaware of objects to the left of you when looking at your bedroom. Would you also neglect the left side of objects on your right?

Answer: Yes. If you attend to the room, you neglect one side of the room. If you attend to an object in the room, you neglect one side of the object.

Question. Clinically, how would a PTO lesion be diagnosed, especially if there was a lesion of the left PTO (the ability to attend is unaffected, in this case)?

Answer: A left sided lesion would be harder to spot. But there are more sensitive tests, such as those that test for extinction. Perhaps you will learn these next year.

Question. Is neglect purely visual? Or are all the senses affected? If so, how would neglect manifest in a tactile, auditory manner?

Answer: Yes it applies to all senses. You neglect to put socks on one foot because of neglect of one side of the body.

L4 Spinal Reflexes and Muscles

Question. What exactly is the benefit of the gamma drive?

Answer: It makes the spindles more sensitive.

Question. How is the modulation of basic spinal reflexes of any benefit?

Answer: It lets the cortex adjust the sensitivity of the reflex; e.g. to tune them down when your hand is relaxed, doing nothing.

Question. Would it be fair to say that both the monosynaptic reflex and the golgi tendon organ reflex also work to maintain basic muscle tone?

Answer: One controls a limb’s position and movement. The other controls how much force is generated. True that both ultimately affect the muscle firing rate and thus tone, but their goals are different.

L5 Motor Cortex

Question. On pg 4., the notes show that the ventral corticospinal tract is "bilateral & polysynaptic," yet in our neuroanatomy class, our notes show that this spinal tract is monosynaptic, just like the lateral corticospinal tract. Which is correct (or are they both)?

Answer: I believe I am.

Question. The origin of the fibers for the lateral corticospinal tract shows 1/3 premotor, 1/3 motor, and 1/3 sensory cortex. Does this also apply to the ventral tract?

Answer: As indicated in the notes, the ventral tract originates from premotor and motor. The precise ratio is not important.

Question. Regarding the columnar organization of cells that influence a particular muscle, I don't really understand how 1 "motor cell" in the cortex can be distributed to many cells (top figure, p.5), when I thought that each muscle has 1 dedicated cell in each column (thus stipulating that each motor cell affects only 1 lower motor neuron path - direct to that 1 muscle).

Answer: The axon divides and each branch makes contact with several spinal motor neurons (as is shown in the diagram).

Question. What happens to the spinal monosynaptic stretch reflex response when the cortical long loop response is not available (i.e. in the case of a stroke)?

Answer: It becomes more sensitive to Ia input. That is why the reflexes become more pronounced; hyper-reflexive.

L6 Cerebellum and Basal Ganglia

Question. How are the resting tremors generated in the patient with Parkinson's?

Answer: No one knows for sure, but the prevailing view is that a group of neurons in the basal ganglia produce a rhythmic activity.

Question. When you indicated that the "deficits were ipsilateral" on page 5 of your notes, were you referring to all of the deficits discussed on that page?

Answer: Yes. All

Question. Why do the symptoms of cerebellar disease improve with time if the cerebellum is the "repair shop" and it is being affected? (p. 13 of the notes)

Answer: Good question. Perhaps the cerebellum is itself flexible. When one part is damaged other parts can do the repair

Question. Given the deficits in the basal ganglia, I figured that the patient would have intention tremors, because of difficulty in initiating/halting movements.

Answer: I agree, it would seem so. But they do not.

L7 Auditory

Question. On page 5, for the figure at the bottom of the page, I understand what is included in the chart (frequency of sound heard vs. loudness heard), but I don't understand what the arrows represent.

Answer: The arrows represent that the high frequency end is to the left on the basilar membrane and on the right in the chart. It is the opposite for low frequencies.

Question. On page 7, for the figure, will there also be a pathway activated (ipsilaterally) to the input from the right side (only the decussation to the left side is shown)?

Answer: Yes there is a mirror symmetric circuit on the opposite side.

L8 Vestibular

Question. In one of your lectures you asked a question: What semicircular canals change their activity when you tilt your head forward?

I thought your otolith organs would change when you tilt your head forward because they sense gravity.

Answer: You are right. The otolith will be activated because they do sense the direction of gravity.

But tilting you head is a rotation. This will also activate the canals.

Question. What are you referring to as the pulse and step in your wave forms charts with the saccade and VOR movements.

Answer: The pulse is what turns your eyes, the phasic activity.

The step is what holds your eyes in their new position, the tonic activity from the PPH.

Question. You mention that a lesion in the PPH can lead to nystagmus in both directions. What if only the left PPH is lesioned? Then in the case of saccading to the right, would there still be drifting back to centre?

Answer: Don't worry about that. It won't be on the exam. But FYI the answer is that the PPH is a bilateral structure. Lesioning one side affects the operation of the other. There is however a change in the settling point. In a bilateral lesion the eye drifts towards centre. In a unilateral lesion the eye drifts to a settling point to one side.

Question. With regards to an imbalance in the VOR (resulting in nystagmus), the notes read that "normally, vestibular afferents have a TONIC drive". I thought they had a phasic drive, hence the necessity for the PPH to convert this to a tonic drive?

Answer: The hair cells fire even when stationary (i.e. tonic). Moving will increase or decrease this firing rate (the phasic response). PPH converts the latter to a tonic drive (i.e. a big phasic response results in a big tonic response).

In short there are two tonic drives. Each is responsible for a different type of nystagmus.

Question. Does a blind person become dizzy?

Answer: Yes. Suppose a blind person turned round and round until his cupola sprang back with normal upright position. Now his vestibular system sensed that he was stationary. If then he suddenly stopped moving, his cupola would be displaced by the fluid and he would incorrectly sense that he was turning in the opposite direction. He would have a sense of imbalance. He would feel dizzy.

Question. Is a blind person susceptible to motion sickness?

Answer: I am not sure. Certainly there would be no visual to conflict with vestibular signals. There are reports that if one closes one’s eyes, one becomes less susceptible to motion sickness. But I am not aware of any scientific evidence for this.

L9 Touch