Question

1. We visually are not aware of a “blind spot” when both eyes are open because____?

1. the other eye is redundant so there is no need for it.
2. your eyes have different focal points and thus would give different answers.
3. light stimuli excite the same point on both retinas.
4. the blind spots of our eyes are not on corresponding points.
5. None of the answers are correct.

2.Sound, light, taste, touch and smell do not come in one level. We are able to describe the intensity of a stimulus (i.e. the intensity of ammonia salts or the brightness of a camera’s flash). What are the mechanisms used by the nervous system to communicate intensity via graded and action potentials?

Answer 1

Answer :

1. c. light stimuli excite the same point on both retinas. With both eyes open , the blind spots are not preceived because the visual fields of the two eyes overlap. Indeed , even with one eye closed, the blind spot can be difficult to detect subjectively because of the ability of the brain to "fill in" or ignore the missing portion of the image. Blind spot, small portion of the visual field of each that corresponds to the position of the optic disk( also known as the optic nerve head) within the retina.There are no photoreceptors in the optic disk and therefore there is no image detection in this area.

2. Graded potential and action potential :

Graded potentials are initiated by a stimulus that vary in magnitude depending on the strength of the stimulus.Graded potential dessipate with distance from stimulus.

Action potential occur when graded potential reach the axon hillock at a certain level of depolarisation ( around -55mV).

During depolarisation voltage gated sodium ion channels open due to an electrical stimulus. As the sodium rushes back into the cell the positive sodium ions raise the charge inside the cell from negative to positive ,if a threshhold is reached , then action potential is produced.

A graded potential is produced when a ligand opens a ligand-gated channel in the dendrites, allowing ions to enter ( or exit) the cell. For example N+ will enter the cell and K+ will exit, until they both reach equilibrium.

Stimulus intensity is encoded in two ways.

1. Frequency coding : where the firing rate of sensory neurons increases with increased intensity
2. Popoulation coding : where the number of primary afferents responding increases ( also called RECRUITMENT).

So if the number of the stimulus is less, then the less amount of neurons activated and thus amound of action potential are transmitted through neurons and vice versa. By this mechanism neurons can identify the intensity of a stimulus, for Eg. whether a light is dim or bright.