Notes on the Basal Ganglia

I. Introduction:

1. Diseases produce three characteristic types of motor disturbances: (1) involuntary movements, including tremor (particularly resting tremor); (2) poverty or slowness of movement without paralysis; and (3) changes in posture and muscle tone.

2. Terminology: The terminology is very confusing and has changed with time. It is now used to designate those structures which when damaged produce "extrapyramidal syndrome" (This too is a confusing term). Today the list includes: caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra.

a. Lenticular (lentiform) nucleus = putamen + globus pallidus (pallidum)

b. Striatum (neostriatum) = caudate nucleus + putamen (common embryology and similar connections and identical histology, a functional term; anterior part of putamen fuses with the head of the caudate above the orbital surface of the frontal lobe)

c. Corpus striatum = caudate nucleus + putamen + globus pallidus

II. Connections:

All of the afferent input to the basal ganglia terminates in the striatum. The internal (medial) segment of the globus pallidus and the substantia nigra (pars reticulata) generate all of its major efferent systems.

1. Afferents:

The striatum receives afferent input form three major sources: the cerebral cortex, the intralaminar nuclei of the thalamus, and the substantia nigra.

a. Corticostriate projection: The putamen receives input form the sensory and motor cortices. The Caudate receives input largely from the association areas. The putamen is largely associated with movement in general and the caudate nucleus with eye movements and cognitive functions.

b. Intralaminar nuclei of the thalamus: mainly from the centromedian nucleus (which in turn receives afferents from the motor cortex) and terminates in the putamen; particularly well developed in primates.

c. dopaminergic projection from the pars compacta of the substantia nigra; also topographically organized; fine unmyelinated fibers.

2. Intranuclear connections:

a. Caudate and putamen project to both regions of the globus pallidus and the pars reticulata of the substantia nigra. These projections are also topographically organized so that each part of the striatum projects to circumscribed parts of the globus pallidus and substantia nigra. Because the corticostriatal pathway and the striatopallidal and striatonigral pathways are topographically organized, specific parts of the cortex act on specific parts of the globus pallidus and substantia nigra via the striatum.

b. The globus pallidus and substantia nigra are intimately connected with the subthalamic nucleus. In fact the entire efferent output of the (lateral) external portion of the globus pallidus goes to the subthalamic nucleus (subthalamic fasciculus). The subthalamic nucleus projects back to both portions of the globus pallidus and to the substantia nigra pars reticulata. Topographic input to the subthalamic nucleus also arises in the premotor and motor cortices.

3. Efferent Connections:

Major efferents arise in the internal segment of the globus pallidus and the substantia nigra pars reticulata. These efferents terminate in the VL and VA nuclei of the thalamus. The centromedian nucleus in addition receives input from the internal segment of the globus pallidus.

Regions of VA and VL that receive basal ganglia input project to the premotor and prefrontal cortices.

Additional efferents: i. globus pallidus to the brainstem and habenula ii. substantia nigra pars reticulata to the superior colliculus and reticular formation.

III. Diseases of the basal ganglia:

1. Parkinson's Disease:

a. Symptoms: rhythmic tremors at rest; a unique kind of increased muscle ton or rigidity: cogwheel-like; slowness in the initiation of movement (akinesia) as well as the execution of movement (bradykinesia); often show a shuffling gait.

b. Presumptive cause: lesions in the dopaminergic projection form the substantia nigra to the striatum (Patients also show loss of locus ceruleus neurons [noradenergic neurons]: depigmentation of the pigmented nuclei of the brain stem).

c. Tremor and rigidity: Loss of inhibition and abnormal outflow from the internal segment of the globus pallidus to VA and VL and to the cortex.

Surgical treatment (ablation of VA\VL) may alleviate tremor and rigidity but not slowness to act. Tremor and rigidity may return after 1-3 yrs.

d. L-DOPA treatment: pars compacta of substantia nigra contain most of the dopaminergic neurons.

e. Complicating factors: loss of noradenergic neurons of locus ceruleus and serotonergic neurons of raphe nuclei: both systems have wide spread connections through out the brain including cerebellum and neocortex.

Dopaminergic projections to parts of the limbic system (nucleus accumbens, olfactory tubercle and amygdala) frontal neocortex may be important. Some experimental evidence suggests that akinesia may result with dopamine depletion in the nucleus accumbens.

2. Huntington's Chorea

a. Genetic, autosomal dominant:

i. symptoms include: personality and behavioral changes (including irritability); uncontrolled movements (choreiform movements); speech at first slurred then incomprehensible. Mental functions undergo deterioration.

ii. Loss of cholinergic neurons and GABA-ergic neurons in the striatum. Thought to cause impaired movements and choreic movements (lack of inhibition). Impaired cognitive functions are thought to result from simultaneous loss of cortical neurons. An imbalance is set up in the relevant biochemical pathways. Thus if dopamine is excitatory to the striatal neurons, the loss of GABA-ergic neurons would lead to an increased firing (disinhibition) of the dopaminergic pathway producing an abnormal outflow from the globus pallidus etc., producing choreic movements. L-DOPA makes the choreic movements worse.

iii. Chromosome 4 has been implicated: There is now a genetic screening test.

3. Ballismus (generally hemiballismus):

i. Flailing movements of an arm or leg

ii. Cause: lesion to contralateral subthalamic nucleus; most often due to a stroke involving the small ganglionic branch of the posterior cerebral artery.

iii. Most often seen in older people.

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