Nervous system
NERVOUS SYSTEM
Most of
us can probably remember being told,
when we were children,
not to touch the stove or some
other source of
potential harm. Because children are
curious, such warnings
often go unheeded. The
result? Touching a hot
stove brings about an immediate
response of pulling away
and a vivid memory of
painful fingers. This
simple and familiar experience
illustrates the
functions of the nervous system:
1. To detect changes and
feel sensations
2. To initiate
appropriate responses to changes
3. To organize
information for immediate use and
store it for future use
The nervous system is
one of the regulating systems
.
Electrochemical impulses of
the nervous system make
it possible to obtain information
about the external or
internal environment
and do whatever is
necessary to maintain homeostasis.
Some of this activity is
conscious, but much of it happens
without our awareness.
NERVOUS
SYSTEM DIVISIONS
The nervous system has
two divisions. The central
nervous
system (CNS) consists of the brain and
spinal cord. The peripheral
nervous system (PNS)
consists of cranial
nerves and spinal nerves. The PNS
includes the autonomic
nervous system (ANS).
The peripheral nervous
system relays information
to and from the central
nervous system, and the brain
is the center of
activity that integrates this information,
initiates responses, and
makes us the individuals we are.
NERVE
TISSUE
Nerve cells are called neurons,
or nerve fibers.
Whatever their specific
functions, all neurons have the
same physical parts. The
cell body contains the
nucleus and
is essential for the continued life of the neuron. As
you will see, neuron cell bodies
are found in the central
nervous system or close to it in the trunk of the
body. In these locations, cell bodies are protected by bone.
There are no cell bodies in the
arms and legs, which are
much more subject to injury.
Dendrites
are processes (extensions) that transmit
impulses toward the cell
body. The one axon of a neuron
transmits impulses away
from the cell body. It is
the cell membrane of the
dendrites, cell body, and
axon that carries the
electrical nerve impulse.
In the peripheral
nervous system, axons and dendrites
are “wrapped” in
specialized cells called
Schwann
cells . During embryonic
development, Schwann
cells grow to surround the neuron processes,
enclosing them in several layers of Schwann cell membrane.
These layers are the myelin sheath;
myelin is a phospholipid that electrically insulates neurons from one
another. Without the myelin
sheath, neurons would
short-circuit, just as electrical
wires would if they were
not insulated
The spaces between
adjacent Schwann cells, or segments
of the myelin sheath,
are called nodes of
Ranvier (neurofibril
nodes). These nodes are the parts
of the neuron cell
membrane that depolarize when an
electrical impulse is
transmitted
The nuclei and cytoplasm
of the Schwann cells are
wrapped around the
outside of the myelin sheath and
are called the neurolemma,
which becomes very
important if nerves are
damaged. If a peripheral nerve
is severed and
reattached precisely by microsurgery,
the axons and dendrites
may regenerate through the
tunnels formed by the
neurolemmas. The Schwann
cells are also believed
to produce a chemical growth
factor that stimulates
regeneration. Although this regeneration
may take months, the
nerves may eventually
reestablish their proper
connections, and the
person may regain some
sensation and movement in
the once-severed limb.
In the central nervous
system, the myelin sheaths
are formed by oligodendrocytes,
one of the neuroglia
(glial cells),
the specialized cells found only in
the brain and spinal
cord. Because no Schwann cells
are present, however,
there is no neurolemma, and
regeneration of neurons
does not occur. This is why
severing of the spinal
cord, for example, results in permanent
loss of function.
Another kind of neuroglia are
microglia,which
are constantly moving, phagocytizing
cellular debris, damaged
cells, and pathogens.
SYNAPSES
Neurons that transmit impulses to other neurons donot actually touch one another. The small gap or space between the axon of one neuron and the dendrites or cell body of the next neuron is called the synapse.Within the synaptic knob (terminal end) of the presynaptic axon is a chemical neurotransmitter that is released into the synapse by the arrival of an electrical nerve impulse . The neurotransmitter diffuses across the synapse, combines with specific receptor sites on the cell membrane of the postsynaptic neuron, and there generates an electrical impulse that is, in turn, carried by this neuron’s axon to the next synapse, and so forth. A chemical inactivator at the
cell body or dendrite of the postsynaptic neuron
quickly inactivates the neurotransmitter. This prevents unwanted, continuous impulses, unless a new impulse from the first neuron releases more neurotransmitter.Many synapses are termed excitatory, because theneurotransmitter causes the postsynaptic neuron to depolarize (become more negative outside as Na_ ions enter the cell) and transmit an electrical impulse to another neuron, muscle cell, or gland. Some synapses,however, are inhibitory, meaning that the
TYPES
OF NEURONS
Neurons may be
classified into three groups: sensory
neurons, motor neurons,
and interneurons (Fig. 8–3).
Sensory
neurons (or afferent neurons) carry impulses
from receptors to the
central nervous system.
Receptors
detect external or internal changes and
send the information to
the CNS in the form of
impulses by way of the
afferent neurons. The central
nervous system
interprets these impulses as a sensation.
Sensory neurons from
receptors in skin, skeletal
muscles, and joints are
called somatic; those from
receptors in internal
organs are called visceral sensory
neurons.
Motor
neurons (or efferent neurons) carry
impulses from the
central nervous system to effectors.
The two types of
effectors are muscles and glands. In
response to impulses,
muscles contract or relax and
glands secrete. Motor
neurons linked to skeletal muscle
are called somatic;
those to smooth muscle, cardiac
muscle, and glands are
called visceral.
Interneurons
are found entirely within the central
nervous system. They are
arranged so as to carry only
sensory or motor
impulses, or to integrate these functions.
Some interneurons in the
brain are concerned
with thinking, learning,
and memory.
A neuron carries
impulses in only one direction.
This is the result of
the neuron’s structure and location,
as well as its physical
arrangement with other
neurons and the
resulting pattern of synapses. The
functioning nervous
system, therefore, is an enormous
network of “one-way
streets,” and there is no danger
of impulses running into
and canceling one another
out.
THE
NERVE IMPULSE
The events of an
electrical nerve impulse are the same
as those of the
electrical impulse generated in muscle
fibers, which is
discussed in Chapter 7. Stated simply,
a neuron not carrying an
impulse is in a state of polarization,
with Na_ ions more
abundant outside the
cell, and K_ ions and
negative ions more abundant
inside the cell. The
neuron has a positive charge on
the outside of the cell
membrane and a relative negative
charge inside. A
stimulus (such as a neurotransmitter)
makes the membrane very
permeable to Na_
ions, which rush into
the cell. This brings about
depolarization, a
reversal of charges on the membrane.
The outside now has a
negative charge, and the
inside has a positive
charge.
As soon as
depolarization takes place, the neuron
membrane becomes very
permeable to K_ ions, which
rush out of the cell.
This restores the positive charge
outside and the negative
charge inside, and is called
repolarization. (The
term action potential refers to
depolarization followed
by repolarization.) Then the
sodium and potassium
pumps return Na_ ions outside
and K_ ions inside, and
the neuron is ready to respond
to another stimulus and
transmit another impulse. An
action potential in
response to a stimulus takes place
very rapidly and is
measured in milliseconds. An individual
neuron is capable of
transmitting hundreds of
action potentials
(impulses) each second.
Transmission of
electrical impulses is very rapid.
The presence of an
insulating myelin sheath increases
the velocity of
impulses, since only the nodes of
Ranvier depolarize. This
is called saltatory conduction.
Many of our neurons are
capable of transmitting
impulses at a speed of
many meters per second.
Imagine a person 6 feet
(about 2 meters) tall who stubs
his toe; sensory
impulses travel from the toe to the
brain in less than a
second (crossing a few synapses
along the way). You can
see how the nervous system
can communicate so
rapidly with all parts of the body,
and why it is such an
important regulatory system.
At synapses, nerve
impulse transmission changes
from electrical to
chemical and depends on the release
of neurotransmitters.
Although diffusion across
synapses is slow, the
synapses are so small that this
does not significantly
affect the velocity of impulses in
a living person.
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