Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

NEURAL DEVELOPMENT
1209
motor
neuron
generation
2.5 day chick
embryo
developing
spinal cord
9 day chick
embryo
motor
neuron
death
section of spinal cord
Figure 21–81 The survival of motor
neurons depends on signals provided
by the target muscles. (A) Removal of
the limb bud shortly after arrival of motor
axons results in the death of motor neurons
in the spinal cord on the amputated side.
(B) Transplantation of an extra limb bud
increases the survival of motor neurons.
(After E. Kandel et al., Principles of Neural
Science, 5th ed., New York: McGraw Hill
Medical, 2012.)
(A)
REMOVAL OF
ONE DEVELOPING
LIMB BUD
missing limb
10% survive
normal limb
50% survive
section of spinal cord
developing
spinal cord
motor
neuron
generation
motor
neuron
death
(B)
TRANSPLANTATION
OF EXTRA
LIMB BUD
extra limb
75% survive
normal limb
50% survive
NGF is produced by the tissues that these neurons innervate. When extra NGF is
provided, extra sensory and sympathetic neurons survive, just as if extra target
tissue were present. Conversely, in a mouse with a mutation that inactivates the
gene for NGF or for its receptor MBoC6 n22.239/22.82
(a receptor tyrosine kinase called TrkA), almost
all sympathetic neurons and the NGF-dependent sensory neurons are lost. There
are many neurotrophic factors, only a few of which belong to the neurotrophin
family, and they act in different combinations to promote the survival and growth
of different classes of neurons.
Formation of Synapses Depends on Two-Way Communication
Between Neurons and Their Target Cells
At journey’s end, the task of a growth cone is to halt its travels and make synapses
with specific target cells. Synapses were introduced in Chapter 11, where we discussed
channels and the electrical properties of membranes. Two main classes
of synapses are found in vertebrates; those made with muscle cells and those
made with other neurons. Synapse formation is best understood in the case of
the highly specialized connections between motor neurons and skeletal muscle
cells—so-called neuromuscular junctions (see Figure 11–38). During synapse
formation, the axonal growth cone differentiates into a nerve terminal that contains
synaptic vesicles filled with the neurotransmitter acetylcholine, while acetylcholine
receptors become clustered in the muscle cell plasma membrane at
the site of synapse formation. A synaptic cleft separates the pre- and postsynaptic
plasma membranes, and a thin sheet of basal lamina lies in this space between
them (Figure 21–82).