recap

994

46.2 recap

Information in the nervous system is processed by cellular interactions in neural networks. The opposing actions of the sympathetic and parasympathetic divisions of the ANS can be understood in terms of neural pathways consisting of just two neurons. Vision begins with retinal photoreceptors organized into circular retinal receptive fields that influence the activity of retinal ganglion cells. Information from ganglion cells is transmitted in the optic nerves to the thalamus and relayed from there to the visual cortex. The left visual field from both eyes is represented in the right visual cortex, and vice versa for the right visual field. Receptive fields of cortical cells represent input from multiple ganglion cells, and binocular, disparity-sensitive cortical cells enable depth perception.

learning outcomes

You should be able to:

  • Predict the outcome on effector tissues when specific peripheral nerves of the sympathetic or parasympathetic divisions of the autonomic nervous system are damaged or stimulated.

  • Explain how receptive fields are generated in the retina through the interactions between photoreceptors, bipolar cells, and ganglion cells, and through the modulation of their signaling by horizontal and amacrine cells.

  • Understand how neural connections between the eyes and the visual cortex produce three-dimensional vision.

Question 1

Explain how a knife wound to left side of the neck can result in pupillary constriction in the left eye.

A knife wound to the left side of the neck could sever the sympathetic chain of ganglia on that side. This could break the connection between the preganglionic and postganglionic sympathetic neurons that innervate the pupil of the left eye and cause it to dilate.

Question 2

Why can a small spot of light on the retina cause a larger response in a retinal ganglion cell than a large spot of light falling on the same area of retina?

A small spot of light on the retina can illuminate just the on-center of the receptive field of a ganglion cell and therefore activate it maximally. A larger spot of light would illuminate receptor cells around the on-center that have an inhibitory influence, and therefore decrease the activation caused by illuminating the center of that receptive field.

Question 3

What changes in vision would occur if the optic chiasm were cut right on the midline?

If the optic chiasm were cut right on the midline, the axons from the ganglion cells in the medial halves of the retina would not cross over to the opposite sides of the brain. As a result, the left eye would not see things in the left visual field and the right eye would not see things in the right visual field. Without input from both eyes, there would be no binocular cells and therefore no depth perception.

By studying the neural circuitry of the visual system and the ANS, you have gained some understanding of how information reaches the CNS and how the CNS controls various functions of the body. But what about the higher functions of the mammalian CNS—the complex functions between input and output, such as language, learning, memory, and dreams?