Ganong's Review of Medical Physiology, 23rd Edition

230 SECTION III Central & Peripheral Neurophysiology
project to specific areas include the specific sensory relay
nuclei and the nuclei concerned with efferent control mechanisms.
The specific sensory relay nuclei include the medial
and lateral geniculate bodies, which relay auditory and visual
impulses to the auditory and visual cortices; and the ventral
posterior lateral (VPL) and ventral posteromedial, which relay
somatosensory information to the postcentral gyrus. The ventral
anterior and ventral lateral nuclei are concerned with
motor function. They receive input from the basal ganglia and
the cerebellum and project to the motor cortex. The anterior
nuclei receive afferents from the mamillary bodies and project
to the limbic cortex, which may be involved in memory and
emotion. Most of the thalamic nuclei described are excitatory
neurons that release glutamate. The thalamus also contains
inhibitory neurons in the thalamic reticular nucleus. These
neurons release GABA, and unlike the other thalamic neurons
just described, their axons do not project to the cortex. Rather,
they are thalamic interneurons and modulate the responses of
other thalamic neurons to input coming from the cortex.
Multiform
layer
CORTICAL ORGANIZATION
The neocortex is generally arranged in six layers (Figure 15–1).
The most common neuronal type is the pyramidal cell with an
extensive vertical dendritic tree (Figures 15–1 and 15–2) that
may reach to the cortical surface. Their cell bodies can be found
in all cortical layers except layer I. The axons of these cells usually
give off recurrent collaterals that turn back and synapse on
the superficial portions of the dendritic trees. Afferents from
the specific nuclei of the thalamus terminate primarily in cortical
layer IV, whereas the nonspecific afferents are distributed
to layers I–IV. Pyramidal neurons are the only projection neurons
of the cortex, and they are excitatory neurons that release
glutamate at their terminals. The other cortical cell types are local
circuit neurons (interneurons) which have been classified
based on their shape, pattern of projection, and neurotransmitter.
Inhibitory interneurons (basket cells and chandelier cells)
release GABA as their neurotransmitter. Basket cells have long
axonal endings that surround the soma of pyramidal neurons;
FIGURE 15–1 Structure of the cerebral cortex. The cortical layers are indicated by the numbers. Golgi stain shows neuronal cell bodies
and dendrites, Nissl stain shows cell bodies, and Weigert myelin sheath stain shows myelinated nerve fibers. (Modified from Ranson SW, Clark SL: The
Anatomy of the Nervous System, 10th ed. Saunders, 1959.)
I
II
III
IV
V
VI
Pial surface Golgi stain
Nissl stain Weigert stain
Molecular
layer
External
granule
cell layer
External
pyramidal
cell layer
Internal
granule
cell layer
Internal
pyramidal
cell layer
White matter