Comparison of the basal ganglia in humans and rats

Here is the first part of my neuroanatomy final, dealing with the basal ganglia. This is very dry material, with a lot of medial-this and dorsal-that. But the point was to describe anatomy, not physiology. Look here for part two and part three.

Also, if you see that I made a mistake, please don't tell me. I've already handed it in and I will find out soon enough. I would rather not know until then.
(picture via Brain Mind)

"Compare the anatomical organization of human and rodent basal ganglia and amygdala. Include a comment on the concept of extended amygdala"

To discuss this topic, I have broken down this essay into several sections. First, I will discuss the organization of the human basal ganglia, followed by a comparison with rat basal ganglia. Second, I will discuss the organization of the human amygdala with the same comparison to rat anatomy. In this section, I will comment on the concept of the extended amygdale. Lastly, I will discuss the functional aspects of both these areas in as much as that helps delineate the connections amongst the various nuclei.

The bulk of my research has come from a few sources. Unless otherwise stated, I have received my information from the following sources. For human basal ganglia, I used Haber and Gdowski (2007); for the human amygdala, de Olmos (2007); for comparison of human and rat anatomy, Paxinos and Watson (1997) and Kruger, et al. (1995). For ease of interspecies comparison, I have used the rostral-caudal and ventral-medial convention in referring to the orientation of the various structures. Also, I have not included any diagrams in this essay; however for my own reference I have used the online atlases available from Michigan State University.

Basal Ganglia

Human Basal Ganglia

The human basal ganglia is a telenchephalic group of subcortical structures consisting of three major parts: the caudate, putamen, and pallidum. The claustrum is an additional, minor telencephalic structure considered to be part of the basal ganglia. There are other structures that, while not anatomically part of the basal ganglia, are functionally considered part of it. These include the subthalamic nucleus, a diencephalic structure ventral to the dorsal thalamus; and the substantia nigra, a midbrain structure ventral to the red nucleus (i.e. nucleus ruber). In addition, as will be shown, cortical and thalamic areas are also functionally tied to the basal ganglia, but are considered to be separate structures.

These three major parts mentioned above can be further broken down; moreover, some of these components are referred to collectively by different names. The caudate and putamen are known collectively as the striatum. The dorsal striatum consists of just the dorsal sections of the caudate and putamen and is sometimes referred to as the neostriatum. The most rostro-ventral section or the caudate and putamen is called the nucleus accumbens, a circular nucleus when viewed coronally, which is further subdivided into core and shell components. Some refer to this nucleus, along with the rostro-ventral areas of the caudate and putamen, as the fundus striati. Further more, the nucleus accumbens, along with the substantia innominata, is referred to as the ventral striatum. Moving more caudally, the pallidum is broken down into three subdivisions: the ventral pallidum, the internal globus pallidus, and the external globus pallidus. The globus pallidus and the putatem are referred to collectively as the lentiform nucleus.

That being said, we can now proceed to the relative positions of these various nuclei. The caudate is the most medial structure of the rostral portion of the basal ganglia. In saggital sections, it is a ‘C’ shaped structure with a rostral head, more caudal body, and a tail ventral to the head and body. The caudate lines the lateral wall of the lateral ventricle and is for the most part contiguous with it. Lateral to the caudate is the putamen, a structure which, in saggital view, is ovular and appears to sit within the ‘C’ of the caudate. The caudate and putamen are separated by the internal capsule, a group of fibers that descend from the cortex to the spinal cord. Lateral to the putamen is the claustrum, separated by the external medullary capsule. The claustrum is in turn bordered laterally by the extreme capsule, which separates it from the insular cortex.

At the most rostral point of the basal ganglia is the nucleus accumbens. As we move caudally, the pallidal structures appear. The nucleus accumbens is replaced by the ventral pallidum. Dorsally, the anterior commissure divides the ventral pallidum from the globus pallidus. The globus pallidus is lateral to the internal capsule and medial to the putamen, separated here by the external medullary lamina. The globus pallidus exists as an internal (medial) and external (lateral) segment separated by the internal medullary lamina. The external segment is the larger of the two and extends more rostrally than the internal segment.

Rat Basal Ganglia

The most striking difference between the human and rat basal ganglia is the orientation of the caudate and putamen. Although both species have an internal capsule, in the rat these fibers lie medial to the caudate. Consequently, there is no clear distinction in the rat between the caudate and putamen. Neither Kruger nor Paxinos make this distinction, and Kruger even names the single structure the “caudoputamen”. Furthermore, the caudate/putamen in the rat is a linear structure, unlike the “C” shaped caudate seen in humans. Moving caudally through the rat brain, the structure moves more and more laterally, but does not have a ventrally extending tail.

Another striking difference is the terminology used for the globus pallidus. Paxinos refers to the internal and external segments as the medial and lateral segments. However, Kruger refers only to the globus pallidus, which is analogous to Paxinos’ lateral segment. Rather than identifying the internal segment as the medial segment, Kruger refers to this section as the entopeduncular nucleus, a structure that extends into the internal capsule.

There are several other minor differences. Paxinos refers to the fundus striate as the lateral accumbens shell. Kruger does not differentiate between nucleus accumbens shell and core, though Paxinos does. There is a structure lateral to the ventral pallidum called the nucleus basilus (alternatively called the basal nucleus or basal nucleus of Meynart) which is referred to by Paxinos; however, Kruger believes this structure, while prominent in primates, is not sufficiently delineated in rodents to warrant assignment. Lastly, the external and extreme capsules in the rat do not appear to extend as far ventrally as they do in the human. Consequently, the barrier separating the putamen from the claustrum from insular cortex in the rat appears less prominent.