BSCI353 Final Exam 3 Material

The figure illustrates the receptive field of an ON-center ganglion cell at various location along a light-dark border. In which locations (A-E) would the action potential rate of the ganglion cell be: (A) the highest, why? (B) the lowest, why? (C) What is the mechanism for the antagonistic surround receptive field?

(A) D. It has the entire center receptive field excited with light, and with less inhibition (compared to E) in the surround. (B) B. While the center receptive field is not excited with light at all, there is more inhibition (compared to A) coming from the light shining on the surround. (C) Horizontal cells provide negative feedback (through GABA neurotransmission) back to many photoreceptors in the vicinity that form the antagonistic surround.The photoreceptors in the center are hyperpolarized in response to the light which decreases the amount of glutamate released which means that fewer of the metabotropic glutamate receptors on the bipolar cell are activated so more cGMP is porduced so more cGMP gated Na+ channels are opened and the cell is depolarized causing it to release glutamate and activate the AMPA, kainate, and NMDA receptors on the ganglion cell, causing depolarzation of the ganglion cell. The photoreceptors on the surround are alse hyperpolarized by the light however they then hyperpolarize the horizontal cells they synapse with which then (through gap junctions with other horizontal cells) depolarize the photoreceptor cells in the center, reducing their response to light and consequently (through reversal of the mechanism described above) decreasing the firing rate of the ganglion cells.

Which of the following could a human ear transduce most optimally?

A conversation

In which scenario would an OFF-center ganglion cell fire the most robustly?

A light filling only the surround of the receptor field

A) Explain how Hubel and Wiesel mapped visual receptive fields. B) How do receptive field characteristics of neurons in V1 compare with those in the lateral geniculate nucleus?

A) Hubel and Wiesel mapped visual receptive fields in anesthetized animals by recording individual neurons in the lateral geniculate nucleus and the visual cortex during retinal stimulation (they projected visual stimuli with a projector on a screen in front of the animal). B) Neurons in the lateral geniculate nucleus were selective for luminance increases and decreases with center-surround receptive field organization like the retina. Neurons in the V1 cortex selectively responded to bars or edges with specific orientation.

Assume you are recording from a CA1 hippocampal neuron and stimulating the CA3 neuron (Schaffer collateral inputs) presynaptic to the CA1 neuron. (A) Describe one method to induce long-term potentiation (LTP). Be sure to describe the relevant details of stimulation and if you manipulate the postsynaptic cell in any way. (B) Describe one way to induce long-term depression (LTD). Be sure to describe the relevant details of stimulation and if you manipulate the postsynaptic cell in any way. (C) Why does the strength of synaptic input increase after LTP (i.e. what is different about the synapse that explains why the same presynaptic input will result in a larger postsynaptic response)? (D) Why does the strength of synaptic input decrease after LTD?

A) Two methods we talked about in lecture (only need one): 1. Tetanus to the presynaptic neuron (just passive recording/nothing on postsynaptic side). 2. Pairing protocol: presynaptic stimulation at 1 Hz while postsynaptic cell is depolarized (to - 40 mV). 3. (we didn't cover) Spike-time dependent plasticity: 1 Hz presynaptic stimulation while the postsynaptic cell is briefly depolarized also at 1 Hz to cause precisely timed spikes shortly after the presynaptic spikes. (B) 1 Hz stimulation for 15 min to the presynaptic neuron (just passive recording/nothing on postsynaptic side). Other methods we didn't cover: 2. Pairing protocol: presynaptic stimulation at 1 Hz, while postsynaptic cell is in voltage clamp and depolarized to -55 mV. 3. Spike-time dependent plasticity: 1 Hz presynaptic stimulation while the postsynaptic cell is briefly depolarized also at 1 Hz to cause precisely timed spikes shortly before the presynaptic spikes. (C) Insertion of additional AMPA receptors into the postsynaptic membrane (D) Deletion of AMPA receptors on the postsynaptic membrane

A) How do auditory nerves code for sound frequencies? B) How do auditory nerves code for different sound intensities?

A) When a sound comes, an auditory nerve fires more action potentials in the upward phase of the sound wave and less action potentials in the downward phase. In other words, the fluctuation of an auditory nerve firing rates follows the periodicity of a sound wave (i.e. the phase-locked response), and therefore the higher the sound frequency is reflected by the faster the action potential rate fluctuating in the auditory nerves. B) They fire more action potentials overall to code for a louder sound. *This is the temporal coding mechanism in the auditory nerves for coding sound frequencies below 3KHz. When the frequency is above 3KHz, labeled-line coding is used. This 2nd mechanism directly refers to the auditory tonotopic map: auditory nerve fibers related to the apical end of the cochlea respond to low frequencies, and fibers that are related to the basal end respond to high frequencies (see textbook p.294).

How would replacing the endolymph solution in the cochlea with normal perilymph solution change auditory transduction in the cochlea? Why?

Auditory transduction in hair cells would fail. This is because the perilymph has low K+ and high Na+ (the opposite of endolymph), therefore when sound comes and opens the mechanical-gated K+ channels on the tips of the hair cells (between stereocilia), K+ flows out into the perilymph instead of flowing in. Without K+ influx the hair cells can no longer depolarize, and no neurotransmitters would release to transmit the sound stimuli.

How would an OFF-center neuron's firing rate change when a light was turned on, turned off, and then turned on again?

Decrease, increase, decrease

In the images below why does the same middle bar appear to be progressively darker from left to right (A) while in reality it has the same gray level throughout (B)? (be sure to include retinal ganglion cells and their action potential firing rate in your answer).

It is because in (A) the background changes progressively brighter from left to right, and the antagonistic center-surround organization of our retina processing neurons (i.e. the bipolar and ganglion cells) enhances the luminance differences between the object (the middle bar) and the background. In the six ON-center RGC cells illustrated in the left figure, for example, all the center receptive fields receive the same light within the middle bar, but their surround receptive fields receive progressively more light from the background, and therefore are progressively more inhibited and fire less action potentials. The differences in action potential firing rates (highest in RGC1 and lowest in RGC6) thus cause our perception of the middle bar lightest on the left and progressively darker to the right.

What would happen if Mg2+ was not expelled from NMDA channels?

LTP would not occur.

A student who is working on a cure hearing loss develops a hair cell mechanoelectrical transduction (MET) channel agonist. Would you expect this drug to be effective? Why?

No; the MET channel can only be altered by mechanical means.

Which event is involved among those in the presynaptic enhancement underlying short-term behavioral sensitization? 1) Serotonin is released from facilitatory interneuron. 2) Ca2+ influx to the presynaptic terminal is enhanced. 3) More neurotransmitter is released. 4) Synaptic transmission is enhanced.

PKA signaling keeps presynaptic K+ channels closed.

What is the most direct path that light information travels on its way to brain in the retina?

Photoreceptor cell; bipolar cell; ganglion cell; optic nerve

Which statement about sensory transduction by hair cells is false?

The electrical activity initiated by the tip links is transmitted to the vesicular release sites along microtubules that undergo voltage-dependent rearrangements. True: Hair cells are presynaptic to second-order sensory neurons. The hair cell body (basal end) is bathed in perilymph, while the hair cell cilia are bathed in endolymph. The firing of action potentials in second-order sensory neurons can be either up- or down-regulated, depending on the direction in which the bundle of cilia (of the afferent hair cell) is bent. Bending of the cilia toward the longest cilium produces depolarization.

Describe the tonotopy of the basilar membrane.

The structure (i.e. width and texture) of the basilar membrane allows a traveling wave from a sound stimulus to reach a point of maximum displacement based on frequency. High frequencies are best tuned to the base of the basilar membrane, while low frequencies are best tuned to the apex. This gives rise to the topographical mapping of frequency.

Refer to the figure below. Repeated stimulation of the siphon results in habituation. Which synaptic change occurs during habituation?

The synapse between the sensory and motor neurons is depressed.

In relation to other retinal ganglion cells, a P ganglion cell has

finer spatial resolution and better color sensitivity.