Fiber Dissection Technique for Demonstrating the Lateral Lemniscus of the Human Brain

Okajimas Folia Anat. Jpn., 80(5–6): 115–118, March, 2004

Harumichi SHINOHARA*1,SatoruYASUTAKA1, Yutaka TANIGUCHI1, Rieko KOMINAMI1 and Seiichi KAWAMATA2

1Department of Anatomy 2, Kanazawa Medical University, Uchinada 1-1, Ishikawa 920-0293, Japan 2Institute of Health Sciences, Hiroshima University, Hiroshima

– Received for Publication, November 25, 2003 –

Key Words: Fiber dissection, Human brain, Lateral lemniscus

Summary: This report describes fiber dissection technique for tracing the auditory pathway from the cochlear nerve to the medial geniculate body via the lateral lemniscus, inferior colliculus and inferior brachium. Some fibers of the lateral leminiscus appear to reach the thalamus in conjunction with fibers of the medial lemniscus.

Fiber dissection is a classical gross anatomical Recently, we discovered that the lateral lemnis- technique that has been used since the 17th century cus can be easily exposed by crumbling from the to reveal fiber connections between various parts entry of the cochlear nerve to the medulla oblon- of the human brain (Tu¨ re et al., 1997). In this gata to the inferior colliculus. Using this method, technique, the gray matter is bluntly removed with we were able to determine the relative positions scalpels and the white matter is peeled away using of the lateral and medial lemnisci from the upper simple instruments like scalpels, forceps and knives. medulla oblongata to the level of the superior col- Three-dimensional comprehension of the brain’s liculus. The technique is very easy to perform, and structure is one of the great benefits of this tech- anyone who uses the technique will obtain a clear nique. Researchers can obtain an understanding of understanding of the course of the lateral lemniscus the human brain’s organization by reconstructing and its relative position to the lateral lemniscus and the nuclei and ventricles as they are encountered other structures. during the fiber dissection. The lateral lemniscus is a fiber bundle that starts from the ventral and dorsal cochlear nuclei and Materials and Methods extends to the, predominantly, ipsilateral inferior colliculus. This fiber bundle forms an important Four human brains obtained from four organ part of the auditory pathway; however, its course donors, two males and two females, ranging in age through the midbrain is often hard to discern. Sev- from 66 to 76 years at the time of their death, were eral structures described by confusing anatomical examined during a dissection practice by medical names, such as the medial lemniscus, superior col- students of the Kanazawa Medical University. The liculus, medial and lateral geniculate body, and the brains were perfused with 10% formalin via the superior and inferior brachium, are all located close femoral artery, removed from the skull, and stored to the lateral lemniscus. Some of these structures in 5% formalin solution for at least six months be- are related to the visual pathway, while others are fore use. Two brains were frozen in a refrigerator at also related to the auditory pathway. Confusion re- À5C for three days and then thawed at room tem- garding both the structures and their functions is perature. This freezing and thawing procedure was common. repeated twice.

* Person for future correspondence. E-mail: [email protected]

115 116 H. Shinohara et al.

Results of Fiber Dissection Discussion

The brains were cut into left and right hemi- Although a technical description of fiber dissec- spheres, producing eight sides where the mid- tion was not included in De Homine, written by the brain could be observed. The cerebellar cortex was famous philosopher Rene Descartes (1662), the il- crumbled and removed to expose the inferior, mid- lustrations of the visual pathway suggest that the dle and superior cerebellar peduncles. The corpora pathway was revealed by blunt dissection of the quadrigemina, medial and lateral geniculate bodies neuronal fibers along the optic nerve, optic chiasm as well as the superior and inferior brachium were and optic tract to a point near the lateral geniculate also located. body. Thus, fiber dissection seems to have been First, the fiber bundle of the inferior brachium performed as early as the age of Descartes, in the connecting the inferior colliculus and medial lem- latter half of the 17th century. This technique was niscus was crumbled (Fig. 1). Then, the fiber bundle the only method of examining neuronal pathways adjacent to the cochlear nerve was crumbled dor- in the brain for about two centuries, but histological solaterally to the inferior colliculus. This bundle, sections gradually began to be used in the latter the lateral lemniscus, was observed as a narrow half of the 19th century. Fiber dissection was rap- bundle at the level of the cochlear nerve that wid- idly abandoned as a research technique at the be- ened when it reached a position parallel to the lat- ginning of the 20th century, when Golgi and Cajal eral wall of the midbrain. Most of the fibers reached were awarded Nobel prizes for their research on the inferior colliculus, but some fibers obviously the nervous system that established staining tech- passed deep under the fiber bundle of the inferior niques for neurons. In 1956, Klingler and Ludwig brachium. When the fibers of the inferior brachium reported that the freezing and thawing of formalin- were cut and removed, some of the lateral lemnis- fixed human brains was advantageous for the dis- cus fibers appeared to reach the pulvina thalami section of the association, commissural and projec- (Fig. 2). tion fibers. In the latter half of the 20th century, Cutting and removing the longitudinal and however, fiber dissection came to be regarded as transverse fibers of the pons and some gray matter a technique for demonstrating the structure and buried in the fibers, a wide group of fibers running location of neuronal bundles to medical students cranially was bluntly dissected. These fibers were (Curran, 1909; Klingler and Gloor, 1960). Recently, from the medial lemniscus (Figs. 1 and 2). After the importance of fiber dissection has been re- crossing at the level of the medulla oblongata, they evaluated in terms of its practical utility in the clin- formed a wall of fibers facing the frontal plane of ical field. The development of magnetic resonance the pons; eventually, the fibers reached a position imaging (MRI) has resulted in remarkable ad- where they faced the lateral wall of the midbrain. vances, revealing functional neuronal pathways The medial lemniscus fibers ran cranially, parallel in the human brain, using diffusion tensor imaging to the lateral lemniscus, and appeared to reach the (Mori et al., 1999; Wiegell et al., 2000). Also, the thalamus. The most dorsal part of the fibers of the introduction of navigation systems for neurosurgery medial lemniscus ran parallel to the most ventral has enhanced the necessity of a thorough under- part of the fibers of the lateral lemniscus, and the staning of the 3-dimensional structure of the human two fiber types appeared to merge with one another brain. during their courses. For most medical students, the auditory pathway

Fig. 1. Left lateral view (1A) and posterior view (1B) of a fiber dissection revealing the lateral lemniscus. The superior cerebellar peduncle (1), the inferior brachium from the inferior colliculus to the medial geniculate body (2), the lateral lemniscus fiber bundle from the entry of the cochlear nerve (VIII) to the brain stem to the inferior colliculus (3), and the medial lemniscus fibers (4) are crumbled in this order. Crumbling towards the arrowhead is preferable. IC, inferior colliculus; P, pons; SC, superior colliculus.

Fig. 2. Left lateral view (2A) and posterior view (2B) of a fiber dissection after the removal of the inferior brachium. Note that some of the lateral lemniscus (LL) fibers run upwards to the pulvina thalami (PT) in association with the fibers of the medial lemniscus (ML). PB, pineal body; SB, superior brachium; SCP, superior cerebellar peduncle. Fiber Dissection of the Lateral Lemniscus 117 118 H. Shinohara et al. is one of the most difficult neuronal pathway to nucleus for all kinds of sensory neurons, except memorize, in spite of its neurological importance. for olfactory neurons. Thus, it is not surprising that However, this difficulty can be easily eliminated auditory input reaches the thalamus directly via the through an exercise in which the fiber connections fibers of the lateral lemniscus. are dissected starting from the root of the cochlear nerve and extending to the medial geniculate body via the lateral lemniscus, inferior colliculus and in- References ferior brachium. Furthermore, fiber dissection tracing enables the three-dimensional winding of the 1) Mori S, Crain BJ and Chacko VP. Three-dimensional track- lateral lemniscus and its positional relation to sur- ing of axonal projections in the brain by magnetic resonance rounding structures to be visualized. imaging. Ann Neurol 1999; 45:265–269. 2) Wiegell MR, Larsson HBW and Wedeen VJ. Fiber crossing Unfortunately, only a few descriptions of the in human brain depicted with diffusion tensor MR imaging. course of the lateral lemniscus traced using fiber Radiology 2000; 217:897–903. dissection have been made. Hoeve dissected fibers 3) Ture U, Yasargil MG and Pait TG. Is there a superior of the lateral lemniscus from the inferior colliculus occipitofrontal fasciculus? A microsurgical anatomic study. downward to the level of the pons (1909). He de- Neurosurgery 1997; 40:1226–1232. 4) Klingler J and Gloor P. The connections of the amygdala scribed that the lateral lemniscus was continuous and of the anterior temporal cortex in the human brain. J with a broad sheet of fibers (medial lemniscus), Comp Neurol 1960; 115:333–369. which extends upward in the same plane. This 5) Ludwig E and Klingler J. Atlas Cerebri Humani, S. Karger, observation was confirmed by the present study. Basel, 1956. However, Hoeve did not mention that the fibers 6) Curran EJ. A new association fiber tract in the cerebrum. With remarks on the fiber tract dissection method of study- of the lateral lemniscus extend to the thalamus in ing the brain. J Comp Neurol 1909; Vol. XIX:645–656. association with those of the medial lemniscus, as 7) Hoeve HJH. A modern method of teaching the anatomy of described in this report. The thalamus is a principal the brain. Anat Rec 1909; April: pp. 247–253.