Week Two:The Brain Essay One

WEEK TWO:THE BRAIN| ESSAY ONE

The Sensing Brain

By Dr. Rob DeSalle

Built into our nervous system from its evolutionary beginnings, the ability to make sense of our surroundings is essential to our survival.

We have five very easily defined senses – sight, hearing, touch, smell and taste—(and one not so obvious one, balance). Without them, we couldn’t cope with the day-to-day barrage of information inside and around our bodies. So how does our brain mediate this barrage? To answer this question, we need to examine how we take in the information, where it gets processed, and what happens at the molecular level to impress the information on our brains. Since the process differs for each of the senses, we’ll address each in turn.

Figure 1. Color Vision

The three types of cone cells sense colors from blue (S) to green (M) to red (L) in bright light, whereas the single peak of sensitivity for rod (R) cells does not allow us to distinguish colors in low light. ©Bowmaker JK & Dartnall HJA

Enlarge image »

Sight

Sight is essentially the interpretation of light that hits light-sensing organs. So at the molecular level light is the fuel for sight. Even very primitive organisms that can perceive light have what can be rudimentarily defined as sight. In humans, as in all vertebrates with sight, the process begins with the cells that interact with light. In our bodies the major light sensing cells are in the retina of our eyes and are called photoreceptors, which can be divided into two types:rodandconecells. Rod cells are more numerous and more sensitive, but can only sense wavelengths from blue to green and are used in low light situations. In comparison, the three types of cone cells allow us to process a wide range of wavelengths, as long as the light source is bright. The cone cells, which are found primarily in the center of the retina, enable us to distinguish between multiple wavelengths of light,(See Figure 1.)that then get interpreted as different colors by our brains.

Figure 2. Structure of the Mammalian Retina

This famous diagram of the retinal layer of the mammalian eye was drawn by neurobiologist pioneer Santiago Ramon y Cajal in 1900. The rod and cone cells are labeled at the top of the diagram (a=rods, b=cones). They are connected to the underlying nerve cells that transmit information about light to the brain through the giant and small ganglion cells (i=giant and j=small) at the bottom of the diagram. Nobel prizewinner Cajal was the first to document these and other structures in the brain. ©US National Library of Medicine

Enlarge image »

Photoreceptor cells contain proteins called opsins, which surround a small molecule that can change shape when it absorbs a specific wavelength of light. So, for instance, if an M-cone cell is hit with a photon in the green range, its opsins experience a change in shape and cause a chain reaction within the cone cell. This ultimately regulates neurotransmitter release by the cone cell, communicating what it has ‘seen’ to the next neuron in the chain and on to the brain to be interpreted.

Smell and Taste

These two senses work fundamentally the same way−our sensory experiences originate with small molecules with which our taste and smell organ cells interact. These interactions are actually physical at the protein level. The organs that we use to do most of our smelling and tasting are our noses and our tongues respectively. In the nose, at a microscopic level, little whip-like structures called cilia line the nasal area in a very thin layer of mucus. The mucus increases the efficiency with which odor molecules interact with our nervous system, which is necessary because the molecules that cause odors are very small. The cilia contain proteins that can bind to specific small odor molecules. Each cilia is connected to a nerve cell, and these nerve cells are bundled together to form what are called glomeruli. Groups of glomeruli converge in the olfactory bulb (the region of the brain that processes smell) into cells called mitral cells. These little relay centers allow the olfactory region to communicate with other parts of the brain.

Our tongues are a mass of taste pegs:fungiform papillaethat carpet the tongue like a field of daisies. They’re shaped somewhat like little mushrooms (hence the name fungiform). Each peg is made up of between 50 and 150 cells clumped into bunches resembling bananas. At the tip of the bunch is the taste pore. This is where little hairs protrude and come into contact with molecules in our mouths. Also called microvilli, these small hairs have proteins imbedded in their cell membranes that implement taste.

Cells that interact with the small molecules that we interpret as smell and taste are calledreceptor cells. Smell receptor cells have proteins embedded in their membranes called olfactory receptors. In humans, approximately 400 genes code for these olfactory proteins. Animals that rely more on smell to communicate with the outside world, like many rodents, have even more genes for olfactory receptors. Once a particular molecule that can be perceived by our olfactory receptors hits the membrane of the cell, it binds to the receptor, which changes the protein shape. This causes a chain reaction inside the cell, which gets converted to an action potential, which is transmitted to the olfactory bulb in the brain via the axons emanating from the olfaction organ’s receptor cells.

Figure 3. Olfactory System

1:Olfactory bulb2:Mitral cells3:Bone4: Nasalepithelium5:Glomerulus6:Olfactory receptor cells. ©Patrick J. Lynch

Enlarge image »

Taste has far fewer receptor types in humans. There are five major kinds of taste: salt, sweet, bitter, umami and sour. When molecules from items that taste bitter, sweet, or umami interact with receptors imbedded in taste organ cells, the response is similar to the mechanism for smell. The small molecule that conveys the taste interacts with the receptor on the cell surface, which changes the shape of the receptor protein, which in turn causes a chain reaction in the interior of the cell that ultimately causes neurotransmitter release. This in turn gets transmitted to the brain via nerve cells that contact the taste receptor cell. Salt and sour tastes are our response to small ions that can diffuse through ion channels on the surface of the cells, stimulating the cell directly.

Figure 4. Tongue Anatomy

Left shows the gross anatomy of the tongue. Middle shows the anatomy of the papillae or pegs. Right shows the structure of the taste pore. ©Art for Science/Science Source

Enlarge image »

Hearing

Sound waves provide the raw material for auditory sensing. Our ears collect and transform ambient sound waves into neural impulses. Ears are very intricate organs indeed, so we’ll divide their anatomy into three parts: the inner, middle, and outer ear. Sound waves are collected by cartilaginous structures in the outer ear. They are diverted to the middle ear, where our eardrums andossiclesare found. The three ossicles and their structures are known as the hammer (malleus), the anvil (incus), and the stirrup (stapes). The stapes is connected to the inner ear, which, unlike the outer and middle ear, is filled with liquid.

Figure 5. Ear Anatomy

Diagram of the outer, middle and inner ear. ©Amplifon

Enlarge image »

The inner ear contains several structures: thecochlea, theutricle, thesaccule, and the semicircular canals. The saccule, utricle, and the three semicircular canals make up the vestibular system, which is responsible for that sixth sense–balance. The cochlea has on the order of 15,000 specialized mechanoreceptors, called hair cells, lining membranes along the length of the cochlea. In comparison to the millions of photoreceptors used to process visual stimuli, the astonishing dynamic range of our hearing, from whisper to thunderclap, rests on this small number of fixed and fragile hair cells. The whole contraption is then connected to nerve cells that lead to the auditory region of the brain.

This Rube Goldberg contraption works in the following way. Sound is collected by the outer ear and diverted to the ear canal. The sound waves then hit the eardrum which move it back and forth. This mechanical energy then travels through the hammer, anvil and stirrup. It is then transmitted to the inner ear membrane, where it stimulates the cochlear hair cells. Because the hairs contain mechanically gated ion channels, the stimulation causes an electrical response that triggers neurotransmitter release. We hear sounds as different pitches because they stimulate cochlear hair cells on different locations along the inner ear membrane. The resulting nerve impulses, which carry information that is directly connected back to the original sound wave, are sent to the brain via the auditory nerve to the auditory region of the brain for initial processing.

Touch

The largest organ in our body is our skin. It is where the sense of touch begins, and is also our first line of defense from the environment. Our outer layer of skin is called the epidermis and helps us perceive temperature changes, pain, and pressure. There is a menagerie of touch receptor cell types, all of which are imbedded in epidermal tissues and connected to the nervous system. These are shown inFigure 6.

Figure 6. The Skin

Pressure, vibration, and whether something is smooth or rough are all perceived by mechanoreceptors variously called Merkel’s disks, Meissner’s corpuscles, Ruffini’s corpuscles, and Pacinian corpuscles. Merkel’s and Meissner’s organs are the most sensitive and are found in the outer layers of the dermis and epidermis in skin that doesn’t bear hair. The other two receptors— Pacinian and Ruffini’s corpuscles— are found deeper in the dermis, in joints, tendons, and muscles. For the most part they sense vibrations. ©Midlands Technical College

Enlarge image »

For all of these receptor cell types mechanical energy is transformed into an electrical potential and transmitted to the brain. How? Let’s look closely at one specific kind of mechanoreceptor, calledtransient receptor potential channels(TRP). TRPs react to pressure changes in the environment (as when something comes into contact with the skin) or to temperature changes (such as coming near a flame). Most likely the TRP changes shape, which alters the flow or concentration of ions in the nerve cell. These changes in ionic milieu produce a cascade of cellular responses that are converted to action potentials and sent to the brain.

Our senses translate stimuli from the environment to the language of the brain, in the form of action potentials that then course through our nervous system. Intimately entwined, taste and smell use chemosensory mechanisms to detect chemicals in the air or in our food. The other senses convert non-chemical information into neuronal impulses: light into sight, air pressure into sound, and physical pressure into touch. Different regions of the brain then interpret this information in ways that vary widely according to the senses involved. In darkness touch grows more acute and hearing sharpens, and even if it’s too dark to see our hands, we know where they are—remarkable sensory abilities that enable us to fully experience the world around us.