10
Chapter 1: Neural Sciences
The skillful use of the hands is called
praxis,
and deficits in skilled
movements are termed
apraxias.
The three levels of apraxia are limb-
kinetic, ideomotor, and ideational.
Limb-kinetic apraxia
is the inability
to use the contralateral hand in the presence of preserved strength; it
results from isolated lesions in the supplementary motor area, which
contains neurons that stimulate functional sequences of neurons in the
motor strip.
Ideomotor apraxia
is the inability to perform an isolated motor act
on command, despite preserved comprehension, strength, and sponta-
neous performance of the same act. Ideomotor apraxia simultaneously
affects both limbs and involves functions so specialized that they are
localized to only one hemisphere. Conditions in two separate areas can
produce this apraxia. Disconnection of the language comprehension
area, Wernicke’s area, from the motor regions causes an inability to fol-
low spoken commands, and lesions to the left premotor area may impair
the actual motor program as it is generated by the higher-order motor
neurons. This program is transmitted across the corpus callosum to the
right premotor area, which directs the movements of the left hand. A
lesion in this callosal projection can also cause an isolated ideomotor
apraxia in the left hand. This syndrome implies the representation of
specific motor acts within discrete sections of the left premotor cortex.
Thus just as some cells respond selectively to specific environmental
features in the higher sensory cortices, some cells in the premotor cortex
direct specific complex motor tasks.
Ideational apraxia
occurs when the individual components of a
sequence of skilled acts can be performed in isolation, but the entire
series cannot be organized and executed as a whole. For example, the
sequence of opening an envelope, removing the letter, unfolding it, and
placing it on the table cannot be performed in order, even though the
individual acts can be performed in isolation. The representation of the
concept of a motor sequence may involve several areas, specifically
the left parietal cortex, but it likely also relies on the sequencing and
executive functions of the prefrontal cortex. This apraxia is a typical
finding of diffuse cortical degeneration, such as Alzheimer’s disease.
Autonomic Motor System
The
autonomic system
is divided into a sensory component
(described earlier) and a motor component. The
autonomic
motor system
is divided into two branches: the sympathetic and
the parasympathetic. As a rule, organs are innervated by both
types of fibers, which often serve antagonistic roles. The
para-
sympathetic system
slows the heart rate and begins the process
of digestion. In contrast, the
sympathetic system
mediates the
fight or flight response, with increased heart rate, shunting of
blood away from the viscera, and increased respiration. The
sympathetic system is highly activated by sympathomimetic
drugs, such as amphetamine and cocaine, and may also be
activated by withdrawal from sedating drugs such as alcohol,
benzodiazepines, and opioids. Investigators who have found an
increased risk of heart attacks in persons with high levels of
hostility have suggested that chronic activation of the sympa-
thetic fight or flight response, with elevated secretion of adrena-
line, may underlie this association.
The brain center that drives the autonomic motor system is the
hypo-
thalamus,
which houses a set of paired nuclei that appear to control
appetite, rage, temperature, blood pressure, perspiration, and sexual
drive. For example, lesions to the ventromedial nucleus, the satiety cen-
ter, produce a voracious appetite and rage. In contrast, lesions to the
upper region of the lateral nucleus, the hunger center, produce a pro-
found loss of appetite. Numerous research groups are making intense
efforts to define the biochemical regulation of appetite and obesity and
frequently target the role of the hypothalamus.
In the regulation of sexual attraction, the role of the hypothala-
mus has also become an area of active research. In the 1990s, three
groups independently reported neuroanatomical differences between
certain of the hypothalamic nuclei of heterosexual and homosexual
men. Researchers interpreted this finding to suggest that human sexual
orientation has a neuroanatomical basis, and this result has stimulated
several follow-up studies of the biological basis of sexual orientation. At
present, however, these controversial findings are not accepted without
question, and no clear consensus has emerged about whether the struc-
ture of the hypothalamus consistently correlates with sexual orientation.
In animal studies, early nurturing and sexual experiences consistently
alter the size of specific hypothalamic nuclei.
Primitive Reflex Circuit
Sensory pathways function as extractors of specific features
from the overwhelming multitude of environmental stimuli,
whereas motor pathways carry out the wishes of the organ-
ism. These pathways may be linked directly, for example, in the
spinal cord, where a primitive reflex arc may mediate the brisk
withdrawal of a limb from a painful stimulus, without immedi-
ate conscious awareness. In this loop, the peripheral stimulus
activates the sensory nerve, the sensory neuron synapses on
and directly activates the motor neuron, and the motor neuron
drives the muscle to contract. This response is strictly local and
all-or-none. Such primitive reflex arcs, however, rarely gener-
ate an organism’s behaviors. In most behaviors, sensory systems
project to association areas, where sensory information is inter-
preted in terms of internally determined memories, motivations,
and drives. The exhibited behavior results from a plan of action
determined by the association components and carried out by
the motor systems.
Localization of Brain Functions
Many theorists have subdivided the brain into functional sys-
tems. Brodmann defined 47 areas on the basis of cytoarchitec-
tonic distinctions, a cataloging that has been remarkably durable
as the functional anatomy of the brain has been elucidated. A
separate function, based on data from lesion studies and from
functional neuroimaging, has been assigned to nearly all
Brodmann’s areas. At the other extreme, certain experts have dis-
tinguished only three processing blocks: The brainstem and the
thalamic reticular activating system provide arousal and set up
attention; the posterior cortex integrates perceptions and gener-
ates language; and, at the highest level, the frontal cortex gener-
ates programs and executes plans like an orchestra conductor.
Hemispheric lateralization of function is a key feature of
higher cortical processing. The primary sensory cortices for
touch, vision, hearing, smell, and taste are represented bilat-
erally, and the first level of abstraction for these modalities is
also usually represented bilaterally. The highest levels of feature
extraction, however, are generally unified in one brain hemi-
sphere only. For example, recognition of familiar and unfamiliar
faces seems localized to the left inferior temporal cortex, and
cortical processing of olfaction occurs in the right frontal lobe.
Hypotheses about the flow of thought in the brain are based on few
experimental data, although this scarcity of findings has not impeded
numerous theoreticians from speculating about functional neuroanat-
omy. Several roles have been tentatively assigned to specific lobes of
the brain, on the basis of the functional deficits resulting from localized
