365
CHAPTER TEN
Nervous System I
tribution of positive and negative ions on either side of the
membrane. It is important in the conduction of muscle and
nerve impulses.
Distribution of Ions
Potassium ions (K
+
) are the major intracellular positive ion
(cation), and sodium ions (Na
+
) are the major extracellular
cation. The distribution is created largely by the sodium–
potassium pump (Na
+
/K
+
pump), which actively transports
sodium ions out of the cell and potassium ions into the cell.
It is also in part due to channels in the cell membrane that
determine membrane permeability to these ions. These
channels, formed by membrane proteins, can be selective;
that is, a particular channel may allow only one type of ion
to pass through and exclude all other ions of different size
and charge. Thus, even though concentration gradients are
present for sodium and potassium, the ability of these ions
to diffuse across the cell membrane depends on the presence
of channels.
RECONNECT
To Chapter 3, Cell Membrane, page 80.
Neurons do not divide. New neural tissue arises from neural stem cells,
which give rise to neural progenitor cells that can give rise to neurons
or neuroglia. In the adult brain, the rare neural stem cells are in a region
called the dentate gyrus and near F
uid-±
lled cavities called ventricles.
Neural stem cells were discovered in the 1980s, in songbirds—
the cells were inferred to exist because the numbers of neurons
waxed and waned with the seasons, peaking when the birds learned
songs. Moving songbirds far from food, forcing them to sing longer,
resulted in more brain neurons, thanks to the stem cells. In the 1990s,
researchers identi±
ed the cells in brain slices from marmosets and tree
shrews given a drug that marks dividing cells. Then they were discov-
ered in humans when a researcher learned that patients with tongue
and larynx cancer were taking the drug to mark their cancer cells.
²ive patients donated their brains after their deaths, and researchers
identi±
ed the cells. Today, human neural stem and progenitor cells
are being used to screen drugs and are being delivered as implants to
experimentally treat a variety of brain disorders. One day, a person’s
neural stem cells may be coaxed to help heal from within.
10.5
THE SYNAPSE
Nerve impulses pass from neuron to neuron (or to other
cells) at synapses
(f
g. 10.11)
. A
presynaptic neuron
brings
the impulse to the synapse and, as a result, stimulates or
inhibits a
postsynaptic neuron
(or a muscle or gland). A
synaptic cleft,
or gap, separates the two cells, which are con-
nected functionally, not physically
(f g. 10.12)
. The process
by which the impulse in the presynaptic neuron signals the
postsynaptic cell is called
synaptic transmission.
A nerve impulse travels along the axon to the axon ter-
minal. Axons usually have several rounded synaptic knobs at
their terminals, which dendrites lack. These knobs have arrays
of membranous sacs, called synaptic vesicles, that contain
neurotransmitter molecules. When a nerve impulse reaches
a synaptic knob, voltage-sensitive calcium channels open
and calcium diffuses inward from the extracellular fl uid. The
increased calcium concentration inside the cell initiates a series
of events that fuses the synaptic vesicles with the cell mem-
brane, where they release their neurotransmitter by exocytosis.
Once the neurotransmitter binds to receptors on a post-
synaptic cell, the action of neurotransmitter on the post-
synaptic cell is either excitatory (turning a process on) or
inhibitory (turning a process off). The net effect on the post-
synaptic cell depends on the combined effect of the excit-
atory and inhibitory inputs from as few as 1 to 100,000 or
more presynaptic neurons.
10.6
CELL MEMBRANE POTENTIAL
A cell membrane is usually electrically charged, or
polar-
ized,
so that the inside is negatively charged with respect
to the outside. This polarization is due to an unequal dis-
Dendrites
Axon of
postsynaptic
neuron
Cell body of
postsynaptic
neuron
Impulse
Impulse
Synaptic
cleft
Impulse
Axon of
presynaptic
neuron
Axon of
presynaptic
neuron
FIGURE 10.11
²or an impulse to continue from one neuron to
another, it must cross the synaptic cleft. A synapse usually separates an
axon and a dendrite or an axon and a cell body.
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