Neurons are the cells of nerve tissue, and basic working units of the brain in vertebrates and many invertebrates ranging in size and complexity from simple corals and jellyfish, most insects, rodents, primates, birds, and upward to humans. Light, sound, temperature, pressure, and chemical stimuli bind to receptors in our eyes, ears, nose, mouth, and skin. The brain processes this environmental information, and sends it to the central nervous system for further action from organs, glands, and muscles. It's a fact that neurons connect us to the external world, and like internet nodes linked by fiber-optic cables, “stream” reality to us at every moment.
Corals are especially fascinating, and rely on a “nerve net” which combines functions—sensory, motor, even pacemaker-like cells regulating muscles for swimming—not structurally feasible in organisms possessing prosaic nervous systems. Nerve fibers in higher-order vertebrates can be quite long, with the axons of the human sciatic nerve starting at the lumbar and sacral plexuses in the spine and ending in the big toes.
Some insects, such as hoverflies, suffer blurred vision when weather turns cold and exemplify what is meant by “novel neurons.” The hoverfly has neuronal regulators that draw heat from muscle activity into the head and eyes. Equally as science-fictional, the female of the species uses a class of neurons with unique small-target motion detectors enabling her to pick out a male against any background, including rushing water, trees, and clouds.
Sensory neurons use receptors to gather stimuli from the environment and send this information to the central nervous system. The input is transmitted by dendrites, branching extensions of the cell, to afferent (or directed toward) neurons for processing by the brain to signal our body into an appropriate response, such as pausing to observe traffic before crossing a street. Information-rich impulses pass through axons, the core of nerve fibers which, like electrical wires, require insulation known as a myelin sheath. The terminal buttons, present at the neuron's end, transmit signals to other neurons. Unlike sensory neurons, motor neurons in the spinal cord transmit signals that enter and exit the vertebral column respectively using the dorsal and ventral roots, ending at an effector (or activator), usually a muscle or gland, to perform some appropriate mechanical process.
A useful example of how this works can be found by recalling the soft taps of our doctor's rubber-headed reflex hammer against knees, biceps, and Achilles tendons to check the integrity of sensory and motor pathways. Stretch receptors in the tendons signal sensory nerves to fire an impulse to the spinal cord, where the message jumps across a synapse to the applicable motor neuron. A motor neuron in the brain also transmits input to the same synapse, and triggers the target muscle—flexing your knee, biceps, or foot.
- Matheson, Thomas. “Invertebrate Nervous Systems,” University of Cambridge; Cambridge, UK
- Dantzker, Jami Milton. “How Females Keep Male Hoverfly Visual Neurons from Distraction,” http://dx.doi.org/10.1371/journal.pbio.0040081