At the most basic level, the function of the nervous system is to send signals
from one cell to others, or from one part of the body to others. There are
multiple ways that a cell can send signals to other cells. One is by releasing
chemicals called hormones into the internal circulation, so that they can
diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the
nervous system provides "point-to-point" signals where neurons project their axons to
specific target areas and make synaptic connections with specific target cells.
Thus, neural signaling is capable of a much higher level of specificity than
hormonal signaling. It's also much faster: the fastest nerve signals travel at
speeds that exceed 100 meters per second.
At a more integrative level, the primary function of the nervous system is to
control the body. It does this by extracting information from the environment
using sensory receptors, sending signals that encode this information into the
central nervous system (CNS), processing the information to determine an appropriate
response, and sending output signals to muscles or glands to activate the
response. The evolution of a complex nervous system has made it possible for
various animal species to have advanced perception abilities such as vision,
complex social interactions, rapid coordination of organ systems, and integrated
processing of concurrent signals. In humans, the sophistication of the nervous
system makes it possible to have language, abstract representation of concepts,
transmission of culture, and many other features of human society that would not
exist without the human brain.
Neurons and Synapses
Most neurons send signals via their axons, although some types are capable of
dendrite-to-dendrite communication. The types of neurons called amacrine cells
have no axons, and communicate only via their dendrites. Neural signals
propagate along an axon in the form of electrochemical waves called action
potentials, which produce cell-to-cell signals at points where axon terminals
make synaptic contact with other cells.
Synapses may be electrical or chemical. Electrical synapses make direct
electrical connections between neurons, but chemical synapses are much more
common, and much more diverse in function. At a chemical synapse, the cell that
sends signals is called presynaptic, and the cell that receives signals is
called postsynaptic. Both the presynaptic and postsynaptic areas are full of
molecular machinery that carries out the signaling process.
The presynaptic area contains large numbers of tiny spherical vessels called
synaptic vesicles, packed with neurotransmitter chemicals. When the presynaptic
terminal is electrically stimulated, an array of molecules embedded in the membrane
are activated, and cause the contents of the vesicles to be released into the narrow
space between the presynaptic and postsynaptic membranes, called the synaptic
cleft. The neurotransmitter then binds to receptors embedded in the postsynaptic
membrane, causing them to enter an activated state.
Depending on the type of receptor, the resulting effect on the postsynaptic cell
may be excitatory, inhibitory, or modulatory in more complex ways. For example,
release of the neurotransmitter acetylcholine at a synaptic contact between a motor
neuron and a muscle cell induces rapid contraction of the muscle cell. The entire
synaptic transmission process takes only a fraction of a millisecond, although the
effects on the postsynaptic cell may last much longer (even indefinitely, in
cases where the synaptic signal leads to the formation of a memory trace).
There are literally hundreds of different types of synapses. There are over a
hundred known neurotransmitters, and many of them have multiple types of
receptor. Many synapses use more than one neurotransmitter—a common arrangement
is for a synapse to use one fast-acting small-molecule neurotransmitter such as
glutamate or GABA, along with one or more peptide neurotransmitters that play
slower-acting modulatory roles.
Molecular neuroscientists generally divide receptors into two broad groups: chemically
gated ion channels and second messenger systems. When a chemically gated ion channel
is activated, it forms a passage that allow specific types of ion to flow across the
membrane. Depending on the type of ion, the effect on the target cell may be excitatory
or inhibitory. When a second messenger system is activated, it starts a cascade of
molecular interactions inside the target cell, which may ultimately produce a
wide variety of complex effects, such as increasing or decreasing the
sensitivity of the cell to stimuli, or even altering gene transcription.
According to a rule called Dale's principle, which has only a few known
exceptions, a neuron releases the same neurotransmitters at all of its synapses.
This doesn't mean, though, that a neuron exerts the same effect on all of its
targets, because the effect of a synapse depends not on the neurotransmitter,
but on the receptors that it activates. Because different targets can (and
frequently do) use different types of receptors, it's possible for a neuron to
have excitatory effects on one set of target cells, inhibitory effects on
others, and complex modulatory effects on others still. Nevertheless, it happens
that the two most widely used neurotransmitters, glutamate and GABA, each have
largely consistent effects.
Glutamate has several widely-occurring types of receptors, but all of them are
excitatory or modulatory. Similarly, GABA has several widely occurring receptor types,
but all of them are inhibitory. Because of this consistency, glutamatergic cells are
frequently referred to as "excitatory neurons", and GABAergic cells as
"inhibitory neurons". It is the receptors that are excitatory and inhibitory,
not the neurons.
One very important subset of synapses are capable of forming memory traces by
means of long-lasting activity-dependent changes in synaptic strength. The
best-known form of neural memory is a process called long-term potentiation
(LTP), which operates at synapses that use the neurotransmitter glutamate acting
on a special type of receptor known as the NMDA receptor.
The NMDA receptor has an "associative" property: if the two cells
involved in the synapse are both activated at approximately the same time, a
channel opens that permits calcium to flow into the target cell. The calcium
entry initiates a second messenger cascade that ultimately leads to an increase
in the number of glutamate receptors in the target cell, thereby increasing the
effective strength of the synapse. This change in strength can last for weeks or
longer. Since the discovery of LTP in 1973, many other types of synaptic memory
traces have been found, involving increases or decreases in synaptic strength that
are induced by varying conditions, and last for variable periods of time.
Reward learning, for example, depends on a variant form of LTP that is conditioned
on an extra input coming from a reward-signaling pathway that uses dopamine as
neurotransmitter. All these forms of synaptic modifiability, taken collectively,
give rise to neural plasticity, that is, to a capability for the nervous system
to adapt itself to variations in the environment.
Neural Circuits and Systems
The basic neuronal function of sending signals to other cells includes a
capability for neurons to exchange signals with each other. Networks formed by
interconnected groups of neurons are capable of a wide variety of functions,
including feature detection, pattern generation, and timing. In fact, it is
difficult to assign limits to the types of information processing that can be
carried out by neural networks: Warren McCulloch and Walter Pitts showed in 1943
that even networks formed from a greatly simplified mathematical abstraction of
a neuron are capable of universal computation. Given that individual neurons can
generate complex temporal patterns of activity all by themselves, the range of
capabilities possible for even small groups of interconnected neurons are beyond
current understanding.
Historically, for many years the predominant view of the function of the nervous
system was as a stimulus-response associator. In this conception, neural
processing begins with stimuli that activate sensory neurons, producing signals
that propagate through chains of connections in the spinal cord and brain,
giving rise eventually to activation of motor neurons and thereby to muscle
contraction, i.e., to overt responses. Descartes believed that all of the
behaviors of animals, and most of the behaviors of humans, could be explained in
terms of stimulus-response circuits, although he also believed that higher
cognitive functions such as language were not capable of being explained
mechanistically.
Charles Sherrington, in his influential 1906 book The
Integrative Action of the Nervous System, developed the concept of
stimulus-response mechanisms in much more detail, and Behaviorism, the school of
thought that dominated Psychology through the middle of the 20th century,
attempted to explain every aspect of human behavior in stimulus-response terms.
However, experimental studies of electrophysiology, beginning in the early 20th
century and reaching high productivity by the 1940s, showed that the nervous
system contains many mechanisms for generating patterns of activity
intrinsically, without requiring an external stimulus. Neurons were found to be
capable of producing regular sequences of action potentials, or sequences of
bursts, even in complete isolation. When intrinsically active neurons are
connected to each other in complex circuits, the possibilities for generating
intricate temporal patterns become far more extensive. A modern conception views
the function of the nervous system partly in terms of stimulus-response chains,
and partly in terms of intrinsically generated activity patterns—both types of
activity interact with each other to generate the full repertoire of behavior.
Reflexes and Other Stimulus-Response Circuits
Simplified schema of basic nervous system function: signals are picked up by
sensory receptors and sent to the spinal cord and brain, where processing occurs
that results in signals sent back to the spinal cord and then out to motor neurons.
The simplest type of neural circuit is a reflex arc,
which begins with a sensory input and ends with a motor output, passing through a
sequence of neurons in between.
An example of the reflex arc could be the "withdrawal
reflex" causing the hand to jerk back after a hot stove is touched. The circuit
begins with sensory receptors in the skin that are activated by harmful levels of
heat: a special type of molecular structure embedded in the membrane causes heat to
generate an electrical field across the membrane. If the electrical potential change
is large enough, it evokes an action potential, which is transmitted along the axon
of the receptor cell, into the spinal cord. There the axon makes excitatory
synaptic contacts with other cells, some of which project to the same region of
the spinal cord, others projecting into the brain. One target is a set of spinal
interneurons that project to motor neurons controlling the arm muscles. The
interneurons excite the motor neurons, and if the excitation is strong enough,
some of the motor neurons generate action potentials, which travel down their
axons to the point where they make excitatory synaptic contacts with muscle
cells. The excitatory signals induce contraction of the muscle cells, which
causes the joint angles in the arm to change, pulling the arm away.
In reality, this straightforward schema is subject to numerous complications.
Although for the simplest reflexes there are short neural paths from sensory
neuron to motor neuron, there are also other nearby neurons that participate in
the circuit and modulate the response. Furthermore, there are projections from
the brain to the spinal cord that are capable of enhancing or inhibiting the
reflex.
Although the simplest reflexes may be mediated by circuits lying entirely within
the spinal cord, more complex responses rely on signal processing in the brain.
Consider, for example, what happens when an object in the periphery of the
visual field moves, and a person looks toward it. The initial sensory response,
in the retina of the eye, and the final motor response, in the oculomotor nuclei
of the brain stem, are not all that different from those in a simple reflex, but
the intermediate stages are completely different. Instead of a one or two step
chain of processing, the visual signals pass through perhaps a dozen stages of
integration, involving the thalamus, cerebral cortex, basal ganglia, superior
colliculus, cerebellum, and several brainstem nuclei. These areas perform
signal-processing functions that include feature detection, perceptual analysis,
memory recall, decision-making, and motor planning.
Feature detection is the ability to extract biologically relevant information
from combinations of sensory signals. In the visual system, for example, sensory
receptors in the retina of the eye are only individually capable of detecting
"points of light" in the outside world. Second-level visual neurons receive
input from groups of primary receptors, higher-level neurons receive input from
groups of second-level neurons, and so on, forming a hierarchy of processing
stages. At each stage, important information is extracted from the signal
ensemble and unimportant information is discarded. By the end of the process,
input signals representing "points of light" have been transformed into a neural
representation of objects in the surrounding world and their properties. The
most sophisticated sensory processing occurs inside the brain, but complex
feature extraction also takes place in the spinal cord and in peripheral sensory
organs such as the retina.
Intrinsic Pattern Generation
Although stimulus-response mechanisms are the easiest to understand, the nervous
system is also capable of controlling the body in ways that do not require an
external stimulus, by means of internally generated rhythms of activity. Because
of the variety of voltage-sensitive ion channels that can be embedded in the
membrane of a neuron, many types of neurons are capable, even in isolation, of
generating rhythmic sequences of action potentials, or rhythmic alternations
between high-rate bursting and quiescence. When neurons that are intrinsically
rhythmic are connected to each other by excitatory or inhibitory synapses, the
resulting networks are capable of a wide variety of dynamical behaviors,
including attractor dynamics, periodicity, and even chaos. A network of neurons
that uses its internal structure to generate temporally structured output,
without requiring a corresponding temporally structured stimulus, is called a
central pattern generator.
Internal pattern generation operates on a wide range of time scales, from
milliseconds to hours or longer. One of the most important types of temporal
pattern is circadian rhythmicity—that is, rhythmicity with a period of
approximately 24 hours. All animals that have been studied show circadian
fluctuations in neural activity, which control circadian alternations in
behavior such as the sleep-wake cycle. Experimental studies dating from the
1990s have shown that circadian rhythms are generated by a "genetic clock"
consisting of a special set of genes whose expression level rises and falls over
the course of the day. Animals as diverse as insects and vertebrates share a
similar genetic clock system. The circadian clock is influenced by light but
continues to operate even when light levels are held constant and no other
external time-of-day cues are available. The clock genes are expressed in many
parts of the nervous system as well as many peripheral organs, but in mammals
all of these "tissue clocks" are kept in synchrony by signals that emanate from
a master timekeeper in a tiny part of the brain called the suprachiasmatic
nucleus.
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