Nervous System
Answers
134.The answer is a.(Sadler, pp 296, 300–303. Moore and Persaud,
Developing, pp 67–69, 86–87, 442–443.)Abnormalities in the anatomy
and connectivity of cortical regions and the so-called social brain regions
(limbic–striatal systems) contribute to the behavioral and brain metabolic
differences in autism. In the cerebral cortex, neurons originate from the
ventricular and subventricular zones. Before neural tube closure, cell pro-
liferation is the predominant process. After neural tube closure, neurons
differentiate. Three layers differentiate from the wall of the neural tube.
Mitotic activity occurs in the ventricular zone, closest to the lumen. The
mantle (intermediate) zone (answers c and d),is where cell bodies of dif-
ferentiating motoneurons are located. The most peripheral zone is the mar-
ginal zone (answer b), which contains the myelinated axons of the
developing motoneurons (adult white matter). In the spinal cord, the lay-
ers differentiate into peripheral white matter with a central H-shaped
region of gray matter from the marginal and mantle zones, respectively. The
sulcus limitans separates the alar (sensory) and basal (motor) plates in the
developing brainstem (answer e).The macroglia (astrocytes and oligoden-
drocytes), arise from the neural epithelium. Microglia (the macrophages of
the brain) are bone marrow–derived, arising from monocytes. Cerebral and
cerebellar cortex are areas of peripheral gray matter formed through a sec-
ond wave of cell proliferation. In cerebellar development, the second wave
comes from the external granular layer. In the cerebral cortex, layers I to VI
form by waves of proliferation and migration from deep to superficial lay-
ers with new cells arising from the subventricular zone.
135.The answer is c.(Alberts, 4/e, pp 638–643. Junqueira, pp 154, 159.
Kandel, pp 29–39, 147–148.)In the resting state, the presence of Na+/K+
ATPase builds up high ionic gradients across the axolemma. There are
many more Na+on the outside of the axon; the inside of the axon is nega-
tive relative to the outside (−70 mV resting potential). An action potential
is initiated by an exchange of ions across the axonal membrane that will
displace the membrane potential toward zero. The first step is the presence
of a stimulus that causes Na+-gated channels to open, Na+ions flow
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