140.The answer is c.(Kierszenbaum, pp 216–217. Junqueira, pp 154, 171,
- Kandel, pp 21–22, 148, 160.)The nodes of Ranvier increase the effi-
ciency of nodal conduction because of restriction (sequestration) of
energy–dependent Na+influx to the node. The nodes of Ranvier represent
the space between adjacent units of myelination. This area is bare in the
CNS, whereas in the PNS the axons in the nodes are partially covered by
the cytoplasmic tongues of adjacent Schwann cells. Most of the Na+-gated
channels are located in the bare areas. Therefore, spread of depolarization
from the nodal region along the axon occurs until it reaches the next node.
This is often described as a series of jumps from node to node, or saltatory
conduction.
141.The answer is b.(Kierszenbaum, pp 216–217. Junqueira, p 168.
Kandel, pp 1288–1295.)The barrier function of the blood-brain barrier is
formed by occluding junctions (zonulae occludens) between endothelial
cells that comprise the lining of brain capillaries. Adding to the imperme-
ability are the nonfenestrated nature of the capillary endothelium and the
paucity or absence of pinocytotic vesicles that represent the physiological
pores seen in other endothelia. Astrocytes form foot processes around the
brain capillaries that induce and maintain the blood-brain barrier. Sur-
rounding the CNS is a basement membrane with a lining of astrocyte foot
processes; this forms the glia limitans. Oligodendrocytes function in myeli-
nation of CNS axons. Microglia function as brain macrophages and are
involved in antigen presentation and phagocytosis.
142.The answer is a.(Alberts, pp 645–648. Junqueira, pp 194–195.
Kierszenbaum, pp 183–184. Kandel, pp 43, 175–177, 183, 210–211.)Ca^2 +
entry through specific channels results in fusion of acetylcholine-contain-
ing synaptic vesicles with the presynaptic membrane and ultimately the
release of neurotransmitter. Neuromuscular (myoneural), junctions repre-
sent the site at which end feet (boutons terminaux) approximate the sur-
face of skeletal muscle cells. The arrangement is similar to the synapse; a
neuromuscular junction can be considered the best-studied synapse. Na+,
K+, and Cl−voltage–gated channels (answers b, c, and d)are involved in
transmission of nerve impulses, but do not couple action potentials (an
electrical signal) to neurotransmitter release (a chemical alteration). Ca^2 +
influx into the end feet may have a direct effect on phosphorylation of
synapsin I, a vesicular membrane protein, which in its nonphosphorylated
Nervous System Answers 245