Porth's Essentials of Pathophysiology, 4e

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Nervous System

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proximal limbs. The basal ganglia are thought to be par- ticularly important in starting, stopping, and monitor- ing movements executed by the cortex, especially those that are relatively slow and sustained, or stereotyped. They also help to regulate the intensity of these move- ments, and they act to inhibit antagonistic or unneces- sary movements. The function of the basal ganglia is not limited to motor functions; they also are involved in cognitive and perceptual functions. Functional Properties of The Basal Ganglia The structural components of the basal ganglia include the caudate nucleus, putamen, and globus pallidus. 3,32 They are located lateral and caudal to the thalamus, occupying a large portion of the interior of both cerebral hemispheres. The caudate and putamen are collectively referred to as the striatum, and the putamen and globus pallidus form a wedge-shaped region called the lentiform nucleus. Two other structures, the substantia nigra of the midbrain and the subthalamic nucleus of the dien- cephalon, are considered part of the basal ganglia (Fig. 36-11). The dorsal part of the substantia nigra contains cells that synthesize dopamine and are rich in a black pigment called melanin. The high concentration of mela- nin gives the structure a black color, hence the name sub- stantia nigra. The axons of the substantia nigra form the nigrostriatal pathway, which supplies dopamine to the striatum. The subthalamic nucleus lies just below the thalamus and above the anterior portion of the sub- stantia nigra. The glutaminergic cells of this nucleus are the only excitatory projections to the basal ganglia. The basal ganglia have input structures that receive afferent information from the cerebral cortex and

thalamus, internal circuits that connect the various struc- tures of the basal ganglia, and output structures that deliver information to other brain centers. The neostria- tum represents the major input structure for the basal ganglia. Virtually all areas of the cortex and afferents from the thalamus project to the neostriatum. The output areas of the basal ganglia, including the lateral globus pal- lidus, have both ascending and descending components. The major ascending output is transmitted to thalamic nuclei, which process all incoming information that is transmitted to the cerebral cortex. Descending output is directed to the midbrain, brain stem, and spinal cord. The output functions of the basal ganglia are mainly inhibitory. Looping circuits from specific cortical areas pass through the basal ganglia to modulate the excit- ability of specific thalamic nuclei and control the corti- cal activity involved in highly learned, automatic, and stereotyped motor functions. The most is known about the inhibitory basal ganglia loop that is involved in modulating cortical motor control. This loop regulates the release of stereotyped movement patterns that add efficiency and gracefulness to cortically controlled move- ments. These movements include inherited patterns that add precision, efficiency, and balance to motion, such as the swinging of the arms during walking and running and the highly learned automatic postural and follow- through movements of throwing a ball or swinging a bat. The basal ganglia also have a cognitive function in that they monitor sensory information coming into the brain and apply it to information stored in memory as a means of planning and sequencing motor movements. 3 The cognitive control of motor activities determines, subconsciously and within seconds, which patterns of movement will be needed to achieve a goal. The caudate nucleus, which receives large amounts of input from the association areas of the brain, plays a major role in the cognitive control of motor activity. Disorders of the basal ganglia comprise a complex group of motor disturbances characterized by tremor and other involuntary movements, changes in posture and muscle tone, and poverty and slowness of move- ment. They include tremors and tics, hypokinetic disor- ders, and hyperkinetic disorders 1 (Table 36-1). Unlike disorders of the motor cortex and corticospi- nal (pyramidal) tract, lesions of the basal ganglia disrupt movement but do not cause paralysis. The various types of involuntary movements often occur in combination and appear to have a common underlying cause. Recent studies indicate that hypokinetic and hyperkinetic dis- orders can be explained as specific disturbances in the indirect and direct pathways that link the basal ganglia with the thalamocortical motor circuit. 32 Accordingly, overactivity of the indirect pathway relative to the direct pathway would result in hypokinetic disorders such as Parkinson disease, and underactivity of the indirect pathway would result in hyperkinetic disorders such as chorea and ballismus. Basal Ganglia–Associated Movement Disorders

Caudate nucleus

Putamen

A

Lateral ventricle

Cortex

Caudate nucleus

Putamen

Basal ganglia

Globus pallidus Subthalamic nucleus Substantia nigra

Thalamus

B FIGURE 36-11. (A) Lateral view of the basal ganglia. (B) Coronal section showing the basal ganglia in relation to surrounding structures.