Basic neuroanatomy: Anatomy of the basal ganglia and the cerebellum
BASAL GANGLI A AND CEREBELLUM I NTRODUCTI ONThe basal ganglia and cerebellum are large collections of nuclei that modify movement on aminute-to-minute basis. M otor cortex sends information to both, and both structures sendinformation right back to cortex via the thalamus. (Remember, to get to cortex you must gothrough thalamus.) The output of the cerebellum is excitatory, while the basal ganglia areinhibitory. The balance between these two systems allows for smooth, coordinated movement, anda disturbance in either system will show up as movement disorders.A. The basal ganglia:What are the basal ganglia? The name is confusing, as generally a ganglion is a collection of cellbodies outside the central nervous system. Blame the early anatomists. The basal ganglia are acollection of nuclei deep to the white matter of cerebral cortex. The name includes: caudate,putamen, nucleus accumbens, globus pallidus, substantia nigra, subthalamic nucleus, andhistorically the claustrum and the amygdala. However, the claustrum and the amygdala do notreally deal with movement, nor are they interconnected with the rest of the basal ganglia, so theyhave been dropped from this section. Other groupings you may hear are the striatum (caudate +putamen + nucleus accumbens), the corpus striatum (striatum + globus pallidus), or the lenticularnucleus (putamen + globus pallidus), but these groupings obviously get confusing very quickly, sowe will try to avoid them.The anatomy of these structures should be a review from the "coronal and horizontal sections"lab. Here once again are the basal ganglia as they appear when stained for myelin:
caudal section:An alternate stain is the acetylcholinesterase (AChE) stain. This technique stains for the enzymethat degrades acetylcholine (ACh), a major neurotransmitter. Areas which use ACh generallystain darkly. Here is a section through monkey brain, stained for AChE.
You can see that the caudate and putamen are stained, while the globus pallidus remains fairlypale. This emphasizes their different functions and connections. And those are...?B. Different functions and connections:The relationships between the nuclei of the basal ganglia are by no means completely understood.When dealing with the brain, you may sometimes be tempted to think that everything is connectedto everything else. Take heart, some fairly simple generalizations and schematics can be drawn.The caudate and putamen receive most of the input from cerebral cortex; in this sense they are thedoorway into the basal ganglia. There are some regional differences: for example, medial caudateand nucleus accumbens receive their input from frontal cortex and limbic areas, and areimplicated more in thinking and schizophrenia than in moving and motion disorders. The caudateand putamen are reciprocally interconnected with the substantia nigra, but send most of theiroutput to the globus pallidus (see diagram below).The substantia nigra can be divided into two parts: the substantia nigra pars compacta (SNpc)and the substantia nigra pars reticulata (SNpr). The SNpc receives input from the caudate andputamen, and sends information right back. The SNpr also receives input from the caudate andputamen, but sends it outside the basal ganglia to control head and eye movements. The SNpc isthe more famous of the two, as it produces dopamine, which is critical for normal movement. TheSNpc degenerates in Parkinsons disease, but the condition can be treated by giving oral dopamineprecursors.The globus pallidus can also be divided into two parts: the globus pallidus externa (GPe) and theglobus pallidus interna (GPi). Both receive input from the caudate and putamen, and both are incommunication with the subthalamic nucleus. I t is the GPi, however, that sends the majorinhibitory output from the basal ganglia back to thalamus. The GPi also sends a few projections toan area of midbrain (the PPPA), presumably to assist in postural control.This schematic summarizes the connections of the basal ganglia as described above.Although there are many different neurotransmitters used within the basal ganglia (principallyACh, GABA, and dopamine), the overall effect on thalamus is inhibitory. The function of the basalganglia is often described in terms of a "brake hypothesis". To sit still, you must put the brakes onall movements except those reflexes that maintain an upright posture. To move, you must apply a
brake to some postural reflexes, and release the brake on voluntary movement. I n such acomplicated system, it is apparent that small disturbances can throw the whole system out ofwhack, often in unpredictable ways. The deficits tend to fall into one of two categories: thepresence of extraneous unwanted movements or an absence or difficulty with intendedmovements.C. Lesions of the basal ganglia:Lesions in specific nuclei tend to produce characteristic deficits. One well-known disorder isParkinsons disease, which is the slow and steady loss of dopaminergic neurons in SNpc. Aninstant Parkinson-like syndrome will result if these neurons are damaged. This happened severalyears ago to an unfortunate group of people who took some home-brewed Demerol in search of ahigh. I t was contaminated by a very nasty byproduct, M PTP ,which selectively zapped the SNpcneurons. The three symptoms usually associated with Parkinsons are tremor, rigidity, andbradykinesia. The tremor is most apparent at rest. Rigidity is a result of simultaneous contractionof flexors and extensors, which tends to lock up the limbs. Bradykinesia, or "slow movement", is adifficulty initiating voluntary movement, as though the brake cannot be released.Huntingtons disease, or chorea, is a hereditary disease of unwanted movements. I t results fromdegeneration of the caudate and putamen, and produces continuous dance-like movements of theface and limbs. A related disorder is hemiballismus, flailing movements of one arm and leg, whichis caused by damage (i.e., stroke) of the subthalamic nucleus.D. The cerebellum:The cerebellum is involved in the coordination of movement. A simple way to look at its purpose isthat it compares what you thought you were going to do (according to motor cortex) with what isactually happening down in the limbs (according to proprioceptive feedback), and corrects themovement if there is a problem. The cerebellum is also partly responsible for motor learning, suchas riding a bicycle. Unlike the cerebrum, which works entirely on a contralateral basis, thecerebellum works ipsilaterally.The cerebellum ("little brain") has convolutions similar to those of cerebral cortex, only the foldsare much smaller. Like the cerebrum, the cerebellum has an outer cortex, an inner white matter,and deep nuclei below the white matter. Single folium, enlarged
Cat cerebellum, sagittal sectionI f we enlarge a single fold of cerebellum, or a folium, we can begin to see the organization of celltypes. The outermost layer of the cortex is called the molecular layer, and is nearly cell-free.I nstead it is occupied mostly by axons and dendrites. The layer below that is a monolayer of largecells called Purkinje cells, central players in the circuitry of the cerebellum. Below the Purkinjecells is a dense layer of tiny neurons called granule cells. Finally, in the center of each folium is thewhite matter, all of the axons traveling into and out of the folia.These cell types are hooked together in stereotypical ways throughout the cerebellum.M ossy fibers are one of two main sources of input to the cerebellar cortex. A mossy fiber is anaxon terminal that ends in a large, bulbous swelling. These mossy fibers enter the granule celllayer and synapse on the dendrites of granule cells (right); in fact the granule cells reach out withlittle "claws" to grasp the terminals. The granule cells then send their axons up to the molecularlayer, where they end in a T and run parallel to the surface. For this reason these axons are calledparallel fibers. The parallel fibers synapse on the huge dendritic arrays of the Purkinje cells.However, the individual parallel fibers are not a strong drive to the Purkinje cells. The Purkinjecell dendrites fan out within a plane, like the splayed fingers of one hand. I f you were to turn aPurkinje cell to the side, it would have almost no width at all. The parallel fibers runperpendicular to the Purkinje cells, so that they only make contact once as they pass through thedendrites.
Although each parallel fiber touches each Purkinje cell only once, the thousands of parallel fibersworking together can drive the Purkinje cells to fire like mad.The second main type of input to the folium is the climbing fiber. The climbing fibers go straightto the Purkinje cell layer and snake up the Purkinje dendrites, like ivy climbing a trellis. Eachclimbing fiber associates with only one Purkinje cell, but when the climbing fiber fires, it provokesa large response in the Purkinje cell.The Purkinje cell (left) compares and processes the varying inputs it gets, and finally sends its ownaxons out through the white matter and down to the deep nuclei. Although the inhibitory Purkinjecells are the main output of the cerebellar cortex, the output from the cerebellum as a whole comesfrom the deep nuclei. The three deep nuclei are responsible for sending excitatory output back tothe thalamus, as well as to postural and vestibular centers.There are a few other cell types in cerebellar cortex, which can all be lumped into the category ofinhibitory interneuron. The Golgi cell is found among the granule cells. The stellate and basketcells live in the molecular layer. The basket cell (right) drops axon branches down into thePurkinje cell layer where the branches wrap around the cell bodies like baskets.E. I nputs and outputs of the cerebellum:The cerebellum operates in 3s: there are 3 highways leading in and out of the cerebellum, thereare 3 main inputs, and there are 3 main outputs from 3 deep nuclei. They are:The 3 highways are the peduncles, or "stalks". There are 3 pairs: the inferior, middle, andsuperior peduncles.
The 3 inputs are: M ossy fibers from the spinocerebellar pathways, climbing fibers from theinferior olive, and more mossy fibers from the pons, which are carrying information from cerebralcortex. The mossy fibers from the spinal cord have come up ipsilaterally, so they do not need tocross. The fibers coming down from cerebral cortex, however, DO need to cross (remember thecerebrum is concerned with the opposite side of the body, unlike the cerebellum). These fiberssynapse in the pons (hence the huge block of fibers in the cerebral peduncles labeled"corticopontine"), cross, and enter the cerebellum as mossy fibers.The 3 deep nuclei are the fastigial, interposed, and dentate nuclei. The fastigial nucleus isprimarily concerned with balance, and sends information mainly to vestibular and reticularnuclei. The dentate and interposed nuclei are concerned more with voluntary movement, and sendaxons mainly to thalamus and the red nucleus. Professor Yasser Metwally www.yassermetwally.com