Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 16

Control of Posture & Movement 251

The main feature of the connections of the basal ganglia is
that the cerebral cortex projects to the striatum, the striatum
to GPi, GPi to the thalamus, and the thalamus back to the cor-
tex, completing a loop. The output from GPi to the thalamus
is inhibitory, whereas the output from the thalamus to the
cerebral cortex is excitatory.

The striatum is made up of two parts that differ histologi-

cally: a unique mosaic of

patches

or

striosomes

(an area with

little acetylcholinesterase) and a

matrix

(an area high in ace-

tylcholinesterase). The neurons of the corticostriate projec-

tion that originate in the deep portion of layer V of the cortex

terminate in the patches, whereas the neurons that originate

in layers II and III and the superficial part of layer V end pri-

marily in the matrix. Neurons with their cell bodies in patches

project in large part to dopaminergic neurons in the substan-

tia nigra pars compacta, whereas many of the neurons with

their cell bodies in the matrix project to GABAergic neurons

in the substantia nigra pars reticulata.

FUNCTION

Neurons in the basal ganglia, like those in the lateral portions

of the cerebellar hemispheres, discharge before movements

begin. This observation, plus careful analysis of the effects of

diseases of the basal ganglion in humans and the effects of

drugs that destroy dopaminergic neurons in animals, have led

to the idea that the basal ganglia are involved in the planning

and programming of movement or, more broadly, in the pro-

cesses by which an abstract thought is converted into volun-

tary action (Figure 16–1). They influence the motor cortex via

the thalamus, and the corticospinal pathways provide the final

common pathway to motor neurons. In addition, GPi projects

to nuclei in the brain stem, and from there to motor neurons

in the brain stem and spinal cord. The basal ganglia, particularly

FIGURE 16–9
The basal ganglia.
The basal ganglia are composed of the caudate nucleus, putamen, and globus pallidus and the function-
ally related subthalamic nucleus and substantia nigra. The frontal (coronal) section shows the location of the basal ganglia in relation to surround-
ing structures.

Caudate nucleus

Thalamus

Putamen and

globus pallidus

Amygdaloid

nucleus

Lateral view

Horizontal section

Caudate nucleus

Globus

pallidus

Thalamus

Tail of

Putamen caudate nucleus

Amygdala

Substantia

nigra

Subthalamic

nucleus

Internal segment

External segment

Putamen

Globus pallidus:

Caudate nucleus

Lateral ventricle

Internal

capsule

Thalamus

Frontal section

Internal capsule

FIGURE 16–10 Diagrammatic representation of the principal
connections of the basal ganglia. Solid lines indicate excitatory path-
ways, dashed lines inhibitory pathways. The transmitters are indicated
in the pathways, where they are known. Glu, glutamate; DA, dopamine.
Acetylcholine is the transmitter produced by interneurons in the stria-
tum. SNPR, substantia nigra, pars reticulata; SNPC, substantia nigra, pars
compacta; ES, external segment; IS, internal segment; PPN, peduncu-
lopontine nuclei. The subthalamic nucleus also projects to the pars com-
pacta of the substantia nigra; this pathway has been omitted for clarity.

Cerebral

cortex

Striatum

(acetylcholine)

Globus

pallidus, ES

Subthalamic

nucleus

Brain stem

and

spinal cord

GABA

GABA

GABA GABA

GABA GABA GABA

Glu

Glu

Glu

DA

Globus

pallidus, IS

Thalamus

SNPR SNPC

PPN