Ganong's Review of Medical Physiology, 23rd Edition

A
CHAPTER 15 Electrical Activity of the Brain, Sleep–Wake States, & Circadian Rhythms 231
FIGURE 15–2 Neocortical pyramidal cell, showing the
distribution of neurons that terminate on it. A denotes nonspecific
afferents from the reticular formation and the thalamus; B denotes recurrent
collaterals of pyramidal cell axons; C denotes commissural fibers
from mirror image sites in the contralateral hemisphere; D denotes specific
afferents from thalamic sensory relay nuclei. (Modified from Chow KL,
Leiman AL: The structural and functional organization of the neocortex. Neurosci Res
Program Bull 1970;8:157.)
they account for most inhibitory synapses on the pyramidal
soma and dendrites. Chandelier cells are a powerful source of
inhibition of pyramidal neurons because they have axonal endings
that terminate exclusively on the initial segment of the pyramidal
cell axon. Their terminal boutons form short vertical
rows that resemble candlesticks, thus accounting for their
name. Spiny stellate cells are excitatory interneurons that release
glutamate as a neurotransmitter. These cells are located
primarily in layer IV and are a major recipient of sensory information
arising from the thalamus; they are an example of a
multipolar neuron (Chapter 4) with local dendritic and axonal
arborizations.
In addition to being organized into layers, the cortex is also
organized into columns. Neurons within a column have similar
response properties, suggesting they comprise a local processing
network (eg, orientation and ocular dominance
columns in the visual cortex).
RETICULAR FORMATION & RETICULAR
ACTIVATING SYSTEM
The reticular formation, the phylogenetically old reticular
core of the brain, occupies the midventral portion of the medulla
and midbrain. It is primarily an anatomic area made up
of various neural clusters and fibers with discrete functions.
For example, it contains the cell bodies and fibers of many of
the serotonergic, noradrenergic, adrenergic, and cholinergic
C
D
Axon
B
B
Cortex
Intralaminar nuclei
of thalamus
Midbrain reticular formation
FIGURE 15–3 Diagram showing the ascending reticular
system in the human midbrain, its projections to the intralaminar
nuclei of the thalamus, and the output from the intralaminar
nuclei to many parts of the cerebral cortex. Activation of these areas
is shown by PET scans when subjects shift from a relaxed awake
state to an attention-demanding task.
systems. It also contains many of the areas concerned with
regulation of heart rate, blood pressure, and respiration. Some
of the descending fibers in it inhibit transmission in sensory
and motor pathways in the spinal cord; various reticular areas
and the pathways from them are concerned with spasticity and
adjustment of stretch reflexes. The reticular activating system
(RAS) and related components of the brain concerned
with consciousness and sleep are considered in this chapter.
The RAS is a complex polysynaptic pathway arising from
the brain stem reticular formation with projections to the
intralaminar and reticular nuclei of the thalamus which, in
turn, project diffusely and nonspecifically to wide regions of
the cortex (Figure 15–3). Collaterals funnel into it not only
from the long ascending sensory tracts but also from the trigeminal,
auditory, visual, and olfactory systems. The complexity
of the neuron net and the degree of convergence in it
abolish modality specificity, and most reticular neurons are
activated with equal facility by different sensory stimuli. The
system is therefore nonspecific, whereas the classic sensory
pathways are specific in that the fibers in them are activated
by only one type of sensory stimulation.
EVOKED CORTICAL POTENTIALS
The electrical events that occur in the cortex after stimulation
of a sense organ can be monitored with an exploring electrode
connected to another electrode at an indifferent point some
distance away. A characteristic response is seen in animals under
barbiturate anesthesia, which eliminates much of the
background electrical activity. If the exploring electrode is
over the primary receiving area for a particular sense, a surface-positive
wave appears with a latency of 5 to 12 ms. This is
followed by a small negative wave, and then a larger, more
prolonged positive deflection frequently occurs with a latency
of 20 to 80 ms. The first positive–negative wave sequence is the