through the channels into the neuron. The positive charge of Na+makes
the axon more positive and depolarized. There is a delay in the opening of
K+-channels. The opening of K+-channels causes the flow of K+out of the
cell, reversing the depolarization. Na+-channels start to close at about this
same time. That causes the action potential to return toward −70 mV, repo-
larizing the membrane. The membrane actually hyperpolarizes (answer a)
as the potential passes −70 mV. This is due to the fact that the K+-channels
remain open longer than required to return to −70 mV. Gradually, the ion
concentrations return to resting levels and the membrane returns to −70 mV.
K+channels(answer b)serve to bring the membrane potential to the hyper-
polarized state. Inward flux of K+(answer d) combined with the closing of
Na+-channels(answer e),is important in return to the resting membrane
potential. The action potential is an all-or-none phenomenon and occurs
with constant amplitude and duration for a given axon. Myelination results
in a much more rapid conduction of the action potential.
136.The answer is d.(Young, pp 121, 123–125, 137. Junqueira, pp
170–171. Kandel, pp 20, 147–148.)The Babinski sign or extensor plantar
reflex is an infantile reflex present in children until the age of two years, but
abnormal in older children or adults. The Babinski sign is an indication of
immaturity of the corticospinal tract which is myelinated after birth
(answers a and e);the process of myelination is completed by oligoden-
drocytes in the CNS (answer c).Cerebellar cortical development (answer b)
begins in the second trimester and continues until several years after birth,
but is not involved in the Babinski reflex.
A short review of myelination:
Myelination in the CNS and PNS occurs by similar methods, although
there are differences in the supportive cells responsible. In the CNS, the
oligodendrocytes myelinate axons, whereas Schwann cells myelinate axons
in the PNS. In the PNS, formation of myelin is initiated by the invagination
of an axon into a Schwann cell. A mesaxon is formed as the outer leaflets of
the cell membrane fuse. Subsequently, the mesaxon of the Schwann cell
wraps itself around the fiber. In the CNS, oligodendrocytes form myelin
around multiple axon segments compared with the 1:1 relationship
between Schwann cells and axon segments in the PNS. Myelination occurs
in both pre- and postnatal development. CNS myelin is the target for attack
by components of the immune system in multiple sclerosis, and PNS
Nervous System Answers 241