Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 15

Electrical Activity of the Brain, Sleep–Wake States, & Circadian Rhythms 231

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 end-
ings that terminate exclusively on the initial segment of the py-
ramidal cell axon. Their terminal boutons form short vertical
rows that resemble candlesticks, thus accounting for their
name. Spiny stellate cells are excitatory interneurons that re-
lease glutamate as a neurotransmitter. These cells are located
primarily in layer IV and are a major recipient of sensory infor-
mation 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 simi-
lar response properties, suggesting they comprise a local pro-
cessing 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 me-
dulla 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

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 sys-

tem (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 tri-

geminal, auditory, visual, and olfactory systems. The com-

plexity 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 un-

der 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 sur-

face-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

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 re-
current collaterals of pyramidal cell axons;
C
denotes commissural fibers
from mirror image sites in the contralateral hemisphere;
D
denotes spe-
cific 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.)

A

B

B

C

D

Axon

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 ar-

eas is shown by PET scans when subjects shift from a relaxed awake

state to an attention-demanding task.

Intralaminar nuclei

of thalamus

Midbrain reticular formation

Cortex