Final answer:
In dissections, nerve fibers appear gray to white due to the presence of gray matter, composed of neuron cell bodies, and white matter, composed of myelinated axons. This distinction is especially visible in unstained tissue from the CNS such as the brain or spinal cord.
Step-by-step explanation:
In dissection, most nerve fibers appear gray to white because of the composition of the nervous tissue in the brain and the spinal cord, which consists of both gray matter and white matter. Gray matter contains mainly the cell bodies of neurons, and in living organisms, it appears more pink than gray, while the white matter consists mainly of myelinated axons, which give them their white color due to the presence of a myelin sheath. In the central nervous system (CNS), the distinction between these two types of matter is evident, such as in a frontal section of the brain or a cross-section of the spinal cord.
Fresh, unstained nervous tissue can be difficult to differentiate in detail, but generally, the gray matter is where the cell bodies, interneurons, and nuclei are found, while the white matter is primarily made up of axon tracts. In the peripheral nervous system (PNS), ganglia are essentially gray matter, and nerves correspond to white matter.
Most nerve dissections involve the CNS, where these distinctions are visible to the unaided eye, and this is why in a dissection setting without artificial staining, one would observe the gray to white appearance of nerve fibers.