Nerves are the conduit through which we are able to experience the world around us. There are two steps to transmitting information from our environment: (1) transduction of the stimulus from the environment to a cell of the nervous system or neuron. (2) transmission of the signal from the nerve ending to the brain through the axon of the neuron.
Sensory Transduction: Translation into the language of the brain
There are many sources of information that the brain receives from the world. Each source has a specialized receptor to be able to translate the state of something in the environment, be it a photon of light or an odor, to something the brain can understand: action potentials or spikes in a neuron or brain cell.
A. Vision: Rods and Cones in the Retina
The retina is a sheet of neural tissue at the back of the eye that detects incident light. The actual cells that detect light are called photoreceptors, which are subdivided into two classes, rods and cones, based on their shape. Rods are specialized to function at low light levels with no color information. Cones function at higher light levels and can detect short, medium and long wavelength of light (blue, green and red, respectively).
The actual conversion of photons of light occurs within opsin protein containing a photopigment such as retinal. A photon of light is absorbed by the photopigment, causing a conformational (shape) change of the pigments. This leads to the opening of a pore in the opsin channel, allowing a signaling molecule (cGMP) to leak from internal store into the cytoplasm of the cell. This signaling molecule opens another pore in a protein in the cell membrane, leading to electrical depolarization and the formation of an action potential, the electrical signal of the brain.
B. Olfaction: Smelling with Olfactory Receptor Neurons
Our sense of smell is actually simply the detection of chemicals (odorants if we’re talking about smell) floating around in the air around us. Each odor that we experience is the mix of many different odorants in the air. There can be an unlimited amount of molecules which we might experience, and although humans have around 400 different olfactory receptor genes, this is hardly enough! Each olfactory receptor protein is specialized to detect the shape of a group of atoms on an odorant (or moiety for you chemists). We can determine the exact odorant molecule by adding all of the moieties that are detected by olfactory receptor proteins. As we grow up we learn to associate the pattern of signals with a particular odorant with the source of that smell.
The conversion of the odorant signal into action potentials is accomplished similarly to in the retina by the formation of the second messenger molecule cGMP. The odorant receptor protein when it binds an odorant causes another protein enzyme to create cGMP from other molecules within the cell rather than releasing internal stores as in the retina.
C. Hearing, Balance and Acceleration: Hair Cells of the Inner Ear
There are two sensory organs in the inner ear, the cochlea for sensing sound waves and the three semicircular canals of the vestibular apparatus for sensing acceleration and aiding in balance. Both of these use the same sensory cells: hair cells. These cells are thusly named because of the hair-like protrusions called stereocillia. These stereocillia sense the motion of fluid (called endolymph in the inner ear) around them. Endolymph moves through the semicircular canals with head movement or acceleration of the body and vibrates in the cochlea when vibrations in the environment, which we call sound waves, causes oscillation of the tectorial membrane.
Mechanical motion of the stereocillia causes opening of mechanically-gated channels, allowing the hair cell to form an action potential to send to the brain. These channels are on the edges of the stereocillia and contain protein links between the hairs. When the hair moves channels are pulled open causing direct electrical depolarization of the hair cell.
D. Gustation: Tasting with Chemoreceptors on the Tongue
Taste is very related to smell in that it is chemical sensation of molecules in the environment. The receptors are present on the tongue, epiglottis and soft palate and detect chemicals present in the saliva in the mouth. The taste receptors are a combination of ion channels like in the ear and coupled channels like in the eye and nose. The main taste sensations are sour, sweet, salty, and bitter although other minor taste categories have been suggested including umami (savory), fattiness and the presence of calcium.
E. Touch: Mechanoreceptors in the Skin
Embedded in the tissue of the skin, mechanoreceptors allow us to feel objects we encounter in the environment. There are four major classes of mechanoreceptor (each named after the scientist who discovered them): Merkel’s disk for slow adaptation to pressure on the skin, Meissner’s corpuscles for fast adaptation to pressure on the skin, Pacinian corpuscles for fast adaptation to vibration, and Rufini endings for slow adaptation to stretching. In this context adaptation is how long the signal lasts after the initial exposure. A slow adapting receptor is like a friend that keeps talking and talking and won’t get off the phone, whereas a fast adapting receptor is like a friend that hangs up right after greeting you.
In addition to these major senses, there are also receptors for nociception (the sense of painful stimuli), proprioception (the sense of body posture and position), temperature, blood pressure as well as condition of specific aspects of organs, such as pressure in the bladder triggering the urge to urinate.
The Action Potential: The language of the brain
The action potential or spike is the electrical activity in a brain cell or neurons which allows it to transfer information over long distances. The action potential can be spread to other cells by chemical transmission across a synapse or by direct electrical transmission through a gap junction, which are rare in adulthood.
Actions potentials are not electrical in the sense that the computer that you are reading this article on is electrical. Computers function by moving around negatively charged particles called electrons. The neuron does it exactly opposite, by allowing charged atoms called ions (which can be either positively or negatively charged) move across the membrane. Essentially the neuron is a bubble in salt water which can rearrange the salt particles from inside to outside and vice verse, to create a small electrical charge.
The inside of cells is negatively charged relative to the outside, and has a high concentration of potassium, where the outside of the cell has a high concentration of sodium and chloride. An action potential, or a quick increase of the charge of the inside of the cell to be highly positive and then back to the normal negative charge, is started by a small increase in the charge caused by the action of sensory transduction (in the case of sensory neurons which we’ve been discussing). This small increase causes voltage-gated sodium channel to open, allowing sodium to rush in depolarizing the cell, which is reversed by allowing potassium to rush out. This change is then reversed by leaky channels and pumps rearranging the ions.
This begins at only a small part of the membrane of a cell and travels all along the rest of the neuron out to the end of the axon, the output appendage of a neuron. This is traditionally an “all-or-none” phenomenon, with the entire series of ion rearranging occurring once the voltage-gated sodium channels open. That being said, not every action potential looks the same, depending on the precise concentration of ions present inside and outside the cell.
Thanks to the action potential we are able to experience the many wonders around us everyday!