Study Brain 145G

Detail of optic tract neurons bending towards and into the population of neurons they will reconnect with.

Optic Tract

(244-1) cut by an incision anterior to the final lowering of the device, crosses this massively with thick bundles of unmyelinated axons. Its posterior cut portion faces the medial cut; next to the lateral cut it has reorganized in the AP direction as well as in a massive rostral bending towards and into the re-connecting population which is also shown on the anterior portion of the medial side. Anterior to the crossing is what seems like a crossing of a cut population of OT axons.

Medial Forebrain

(234-1) has a massive lateral detour around the caudal wire which had slipped laterally for 300 um, cutting the population of axons that now, within 50 um of the edge, curves in a concerted way towards the massive detour. Near the cut’s edges the tissue seems inert. (231&232-1) show fasciculi in the blood that contains the detour, as does 229, indicating that a cut here was massively crossed by axon bundles.

On Saturday July 1, 1972 at Hopkins a 358g rat was prepared to have two AP cuts with U-devices aimed at the optic tract by drilling burrholes over the selected locations with respect to bregma. Before the cut the pupil size was recorded for lights of different intensities. On the right the 1.7mm device, hand-held, was lowered ‘in well, smoothly, bottom felt and seen’ at 1:00, which means that under the dissecting microscope I saw the wires bowing out or in as I pressed down and felt the bottom of the skull. Both devices have nicely squared ends. There was an ‘initial dilatation then normal’. On the left the ~2.0mm device was pushed in at 1:15. When released it came up ~2mm and the left eye was ‘very dilated at first’. It must have been pushed in again because the holes in the brain are consistent with this. Left alive at 2:00, it was found dead art 2:30. Brain out and to fix at 2:50. Thus the 1.7mm cut is 1-1 1/2 hours; the 2mm is 45 min – 1h 15 min. Notes on the dissection say that there is a ‘slight gap rostral to loop beginning for the 2mm loop.

Cuts

Both of these non-cemented devices slipped when their wires were cut, creating extra incisions in the horizontal plane. In addition the 2mm device, when pushed down the second time, ended caudal to the first cut, creating an unmarked anterior incision 300 um long (244) and a corresponding new marked posterior incision. This cut is nearest to me on the section and will be described first.

wire separation, the two outlined holes, is clearly1.3mm in (237), above which the outlines were not so clear. It is 1.1mm above the most ventral section and has a blood-filled interruption for 225 um anterior to its rostral wire.