In order to survive - at least on the species level - we continually need to make decisions:
- "Should I cross the road?"
- "Should I run away from the creature in front of me?"
- "Should I eat the thing in front of me?"
- "Or should I try to mate it?"
To help us to make the right decision, and make that decision quickly, we have developed an elaborate system: a sensory system to notice what's going on around us; and a nervous system to handle all that information. And this system is big. VERY big! Our nervous system contains about nerve cells (or neurons), and about 10-50 times as many supporting cells. These supporting cells, called gliacells, include oligodendrocytes, Schwann cells, and astrocytes. But do we really need all these cells?
Keep it simple: Unicellular Creatures 
The answer is: "No!", we do not REALLY need that many cells in order to survive. Creatures existing of a single cell can be large, can respond to multiple stimuli, and can also be remarkably smart!
We often think of cells as really small things. But Xenophyophores (see image) are unicellular organisms that are found throughout the world's oceans and can get as large as 20 centimetres in diameter.
And even with this single cell, those organisms can respond to a number of stimuli. For example look at a creature from the group Paramecium: the paramecium is a group of unicellular ciliate protozoa formerly known as slipper animalcules, from their slipper shape. (The corresponding word in German is Pantoffeltierchen.) Despite the fact that these creatures consist of only one cell, they are able to respond to different environmental stimuli, e.g. to light or to touch.
And such unicellular organisms can be amazingly smart: the plasmodium of the slime mould Physarum polycephalum is a large amoebalike cell consisting of a dendritic network of tube-like structures. This single cell creature manages to connect sources finding the shortest connections (Nakagaki et al. 2000), and can even build efficient, robust and optimized network structures that resemble the Tokyo underground system (Tero et al. 2010). In addition, it has somehow developed the ability to read its tracks and tell if its been in a place before or not: this way it can save energy and not forage through locations where effort has already been put (Reid et al. 2012).
On the one hand, the approach used by the paramecium cannot be too bad, as they have been around for a long time. On the other hand, a single cell mechanism cannot be as flexible and as accurate in its responses as a more refined version of creatures, which use a dedicated, specialized system just for the registration of the environment: a Sensory System.
Not so simple: Three-hundred-and-two Neurons 
While humans have hundreds of millions of sensory nerve cells, and about nerve cells, other creatures get away with significantly less. A famous one is Caenorhabditis elegans, a nematode with a total of 302 neurons.
C. elegans is one of the simplest organisms with a nervous system, and it was the first multicellular organism to have its genome completely sequenced. (The sequence was published in 1998.) And not only do we know its complete genome, we also know the connectivity between all 302 of its neurons. In fact, the developmental fate of every single somatic cell (959 in the adult hermaphrodite; 1031 in the adult male) has been mapped out. We know, for example, that only 2 of the 302 neurons are responsible for chemotaxis (“movement guided by chemical cues”, i.e. essentially smelling). Nevertheless, there is still a lot of research conducted – also on its smelling - in order to understand how its nervous system works!
General principles of Sensory Systems 
Based on the example of the visual system, the general principle underlying our neuro-sensory system can be described as below:
All sensory systems are based on
- a Signal, i.e. a physical stimulus, provides information about our surrounding.
- the Collection of this signal, e.g. by using an ear or the lens of an eye.
- the Transduction of this stimulus into a nerve signal.
- the Processing of this information by our nervous system.
- And the generation of a resulting Action.
While the underlying physiology restricts the maximum frequency of our nerve-cells to about 1 kHz, more than one-million times slower than modern computers, our nervous system still manages to perform stunningly difficult tasks with apparent ease. The trick: there are lots of nerve cells (about ), and they are massively connected (one nerve cell can have up to 150'000 connections with other nerve cells).
The role of our "senses" is to transduce relevant information from the world surrounding us into a type of signal that is understood by the next cells receiving that signal: the "Nervous System". (The sensory system is often regarded as part of the nervous system. Here I will try to keep these two apart, with the expression Sensory System referring to the stimulus transduction, and the Nervous System referring to the subsequent signal processing.) Note here that only relevant information is to be transduced by the sensory system! The task of our senses is NOT to show us everything that is happening around us. Instead, their task is to filter out the important bits of the signals around us: electromagnetic signals, chemical signals, and mechanical ones. Our Sensory Systems transduce those environmental variables that are (probably) important to us. And the Nervous System propagates them in such a way that the responses that we take help us to survive, and to pass on our genes.
Types of sensory transducers 
- Mechanical receptors
- Balance system (vestibular system)
- Hearing (auditory system)
- Fast adaptation (Meissner’s corpuscle, Pacinian corpuscle) ? movement
- Slow adaptation (Merkel disks, Ruffini endings) ? shape Comment: these signals are transferred fast
- Muscle spindles
- Golgi organs: in the tendons
- Chemical receptors
- Smell (olfactory system)
- Light-receptors (visual system): here we have light-dark receptors (rods), and three different color receptors (cones)
- Heat-sensors (maximum sensitivity at ~ 45°C, signal temperatures < 50°C)
- Cold-sensors (maximum sensitivity at ~ 25°C, signal temperatures > 5°C)
- Comment: The information processing of these signals is similar to those of visual color signals, and is based on differential activity of the two sensors; these signals are slow
- Electro-receptors: for example in the bill of the platypus
- Pain receptors (nocioceptors): pain receptors are also responsible for itching; these signals are passed on slowly.
Now what distinguishes neurons from other cells in the human body, like liver cells or fat cells? Neurons are unique, in that they:
- can switch quickly between two states (which can also be done by muscle cells).
- That they can propagate this change into a specified direction and over longer distances (which cannot also be done by muscle cells).
- And that this state-change can be signalled effectively to other connected neurons.
While there are more than 50 distinctly different types of neurons, they all share the same structure:
- An input stage, often called dendrites, as the input-area often spreads out like the branches of a tree. Input can come from sensory cells or from other neurons; it can come from a single cell (e.g. a bipolar cell in the retina receives input from a single cone), or from up to 150’000 other neurons (e.g. Purkinje cells in the Cerebellum); and it can be positive (excitatory) or negative (inhibitory).
- An integrative stage: the cell body does the household chores (generating the energy, cleaning up, generating the required chemical substances, etc), combines the incoming signals, and determines when to pass a signal on down the line.
- A conductile stage, the axon: once the cell body has decided to send out a signal, an action potential propagates along the axon, away from the cell body. An action potential is a quick change in the state of a neuron, which lasts for about 1 msec. Note that this defines a clear direction in the signal propagation, from the cell body, to the:
- output Stage: The output is provided by synapses, i.e. the points where a neuron contacts the next neuron down the line, most often by the emission of neurotransmitters (i.e. chemicals that affect other neurons) which then provide an input to the next neuron.
Principles of Information Processing in the Nervous System 
Parallel processing 
An important principle in the processing of neural signals is parallelism. Signals from different locations have different meaning. This feature, sometimes also referred to as line labeling, is used by the
- Auditory system - to signal frequency
- Olfactory system - to signal sweet or sour
- Visual system - to signal the location of a visual signal
- Vestibular system - to signal different orientations and movements
Population Coding 
Sensory information is rarely based on the signal nerve. It is typically coded by different patterns of activity in a population of neurons. This principle can be found in all our sensory systems.
The structure of the connections between nerve cells is not static. Instead it can be modified, to incorporate experiences that we have made. Thereby nature walks a thin line:
- If we learn too slowly, we might not make it. One example is the "Eskimo curlew", an American bird which may be extinct by now. In the last century (and the one before), this bird was shot in large numbers. The mistake of the bird was: when some of them were shot, the others turned around, maybe to see what's up. So they were shot in turn - until the birds were essentially gone. The lesson: if you learn too slowly (i.e. to run away when all your mates are killed), your species might not make it.
- On the other hand, we must not learn too fast, either. For example, the monarch butterfly migrates. But it takes them so long to get from "start" to "finish", that the migration cannot be done by one butterfly alone. In other words, no single butterfly makes the whole journey. Nevertheless, the genetic disposition still tells the butterflies where to go, and when they are there. If they would learn any faster - they could never store the necessary information in their genes. In contrast to other cells in the human body, nerve cells are not re-generated in the human body.