VM8054 Veterinary Histology

Example: Cerebellum

Author: Dr. Thomas Caceci
This is a low magnification image of most of the cerebellum. This part of the brain is concerned with "automatic" things like balance and unconscious coordination of movement. If you should trip and fall, your arms fly out in front of you immediately; this is the cerebellum in action. If you toss a cat in the air, invariably it will twist its body in "flight" and land on its feet. Again, the cerebellum is at work, integrating all the tens of thousands of visual and proprioceptive inputs, making some decisions about which muscles need to be contracted and by how much, and wiggling the falling feline into a proper landing configuration--or keeping you from breaking your nose.

The cerebellum has a cortex and a medulla, and what you see in this image is mostly cortex. It's also folded like a piece of cloth, so that if you could follow the outermost surface you would dip down between the leaf like folds (the foliae, F) and back up again. The medulla consists of nerve fibers (NF) leading out of the cortex.

The cerebellar cortex has three layers,only two of which can be made out at this magnification. The outermost molecular layer, the pink band along the surface, is a vast array of nerve fiber tracts, originating in brain nuclei outside the cerebellum, and forming parallel tracts. These interact with cells in the next layer, which isn't visible here. The innermost layer of the cerebellum, the granule cell layer is seen here as a darker-staining region, stippled with nuclei.

The cells of the next inmost layer, the Purkinje cell layer each send enormous fanlike dendritic arrays up into the molecular layer. At this magnification the Purkinje cells (Arrows) are visible as discrete "dots" between the molecular layer (ML) and the granule cell layer (GCL). The layer goes into the screen and out of it, too; it's not a "line" of cells, it's a three-dimensional field of regularly-spaced cells.

The Purkinje cell dendrites are arranged at right angles to the parallel fibers of the molecular layers, forming tens of millions of synapses.

The Purkinje cell is the classic example of an integrator neuron. Its function is to receive input from various sources, make a "decision" to fire or not, and to send a single integrated on/off signal from its own axon reflecting that decision. The arrangement of the Purkinje cell's dendrites and their relationship to the nerve fibers in the molecular layer are the important factors in making the system work.

In this field you get the merest hint of the fantastic array of Purkinje cell's (P) dendrites. The base of the "tree" where it's rooted to the cell soma is visible. Each "branch" of the tree is ramified into thousands and thousands of smaller branches, forming a fan-like array that extends up into the molecular layer. The latter consists of millions of fibers, passing through the serried ranks of Purkinje cell dendrites like telephone wires running through the trees of an orchard. At every point where a "wire" contacts a "branch" there is a synaptic connection. A single Purkinje cell may have tens of thousands of such inputs. Some are excitatory, others inhibitory, and the possible combinations on a given Purkinje cell are in the trillions. Multiply that by the tens of thousands of Purkinje cells, and you can readily see that the possibilities of integrating input and output are nearly infinite.

Equally important, the time required for the system to process millions of inputs and make a coordinated set of outputs take place is very short, thanks to what a computer engineer would call "massively parallel processing" of information. The final output of any given Purkinje cell is via a single axon, but all the Purkinje cells are doing this simultaneously, taking sensory information from all parts of the body and sending output to the appropriate parts of the CNS that controls the effectors. The Purkinje cell axon is passed through the granule cell layer (GCL), bundled with its neighbors to form the cerebellar medulla. Further integration takes place and final motor output is determined in other parts of the CNS. This colossal parallel processing capability allows a falling cat to carry out almost instantaneously the numerous decisions about postural control, muscle tensions, etc. before hitting the floor. The Purkinje cell axon is passed through the granule cell layer (GCL).

Cerebellar control of movement isn't really "automatic" in the same sense that breathing is. Breathing is truly automatic and a newborn baby can breathe properly. But animals have to learn coordination, "printing" the cerebellum with the appropriate paths for sensation and response. The time to do this varies considerably among species. Some animals are born with a fully-developed cerebellum, and the capability for coordinated movement is there from the time they hit the ground: the new-born calf romping around its mother is an example of such precocious development. The helplessness of kittens and puppies, who require weeks to learn to walk and run, is due to the fact that in those animals the development of the cerebellum is not complete until a few weeks after birth. In humans the process takes longer. But once the techniques of postural and movement control are in place, barring injury or disease, they become as "automatic" as breathing.

If the cerebellum doesn't develop fully, an animal may never develop proper coordination. A variety of causes may result in cerebellar hypoplasia, an underdeveloped cerebellum being the result. Animals like this are clumsy and uncoordinated and sometimes may actually fall into their food bowls.

Rat brain; Luxol fast blue/hematoxylin stain, paraffin section, 20x, 100x, and 400x

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