Page 627 - Fundamentals of anatomy physiology

614 Unit 3 Control and Regulation

a receptor and a sensory neuron that delivers information to Figure 17–19 Convergence and Ganglion Cell Function.
the CNS. In the visual pathways, the message must cross two
synapses (photoreceptor to bipolar cell, and bipolar cell to gan- Photoreceptors are organized in groups within a receptive field.
glion cell). Only then does it head toward the brain. The extra Each ganglion cell monitors a well-defined portion of that field. Some
synapse increases the synaptic delay, but it provides an oppor- ganglion cells (on-center neurons, labeled A) respond strongly to light
tunity for the processing and integration of visual information arriving at the center of their receptive field. Others (off-center neu-
before it leaves the retina. rons, labeled B) respond most strongly to illumination of the edges
of their receptive field.

Processing by the Retina Retinal surface
(contacts pigment epithelium)
Each photoreceptor in the retina monitors a specific receptive
field. The retina contains about 130 million photoreceptors, Receptive field BB
6 million bipolar cells, and 1 million ganglion cells. Thus, a of ganglion cell
considerable amount of convergence takes place at the start of BA B
the visual pathway. The degree of convergence differs between
rods and cones. Regardless of the amount of convergence, BB
each ganglion cell monitors a specific portion of the field of
vision. Receptive field

17 As many as a thousand rods may pass information by their Photoreceptors
bipolar cells to a single ganglion cell. The fairly large ganglion
cells that monitor rods are called M cells (magnocells; magnus, Horizontal Bipolar cell
great). They provide information about the general form of cell Amacrine
an object, motion, and shadows in dim lighting. Because so cell
much convergence occurs, the activation of an M cell indicates
that light has arrived in a general area rather than at a specific Ganglion cell
location.
by rods have a coarse, grainy, pixelated appearance that blurs
The loss of specificity due to convergence is partially details. By contrast, images produced by cones are sharp, clear,
overcome by the fact that the activity of ganglion cells varies and of high resolution.
according to the pattern of activity in their receptive field,
which is usually shaped like a circle. Typically, a ganglion Central Processing of Visual Information
cell responds differently to stimuli that arrive in the center
of its receptive field than to stimuli that arrive at the edges Axons from the entire population of ganglion cells converge
(Figure 17–19). Some ganglion cells, called on-center neu- on the optic disc, penetrate the wall of the eye, and proceed
rons, are excited by light arriving in the center of their sensory toward the diencephalon as the optic nerve (II). The two optic
field and are inhibited when light strikes the edges of their
receptive field. Others, known as off-center neurons, are
inhibited by light in the central zone, but are stimulated by
illumination at the edges. On-center and off-center neurons
provide information about which portion of their receptive
field is illuminated. This kind of retinal processing within
ganglion receptive fields improves the detection of the edges
of objects within the visual field.

Cones typically show very little convergence. In the fovea,
the ratio of cones to ganglion cells is 1:1. The ganglion cells
that monitor cones, called P cells (parvo cells; parvus, small),
are smaller and more numerous than M cells. P cells are active
in bright light, and they provide information about edges, fine
detail, and color. Because little convergence occurs, the activa-
tion of a P cell means that light has arrived at one specific loca-
tion. As a result, cones provide more precise information about
a visual image than do rods. We could say that images formed