Structural Biochemistry/Cell Signaling Pathways/Sensory System

Sensory Systems

There are five major sensory systems that all humans exhibit those being: olfaction, taste, vision, hearing, and touch. Our senses provide us with the ability to understand and detect the environment around us. Each of these primary sensory systems contains specialized sensory neurons that transmit information through the form of nerve impulses or action potentials to the central nervous system. The action potentials that reach the brain from sensory neurons are called sensations. As the brain receives the information of sensations, the brain interprets them into colors, smells, sound, and tastes, which are also called perceptions, and gives stimuli. Sensations and perceptions are detected through sensory receptors. In human body, there are different specialized sensory receptors for different stimuli coming from inside and outside of the body. The sensory receptors that detect stimuli coming from outside of the body are called exteroreceptors, where as those that detect stimuli coming from inside of the body are called interoreceptors.

Functions of Sensory Receptors

Sensory Transduction

Sensory Transduction is the conversion of stimulus energy into receptor potential, which is the change in membrane potential of a sensory receptor. As the ion channels in the sensory receptor's plasma membrane opens and closes, the ionic permeability of the membrane is changed, and results in receptor potentials. These sensory receptors can detect the smallest stimulus possible, which makes them really sensitive.

Amplification

Amplification is when the cells in sensory pathway strengthens the stimulus energy in order for humans to have according reactions.

Transmission

After amplification, action potentials are transmitted to the CNS.

Integration

As the information is received in the CNS, through summation the receptor potentials are then being delivered to sensory receptors at different parts of the body are integrated. One type of integration is called adaptation, where it decreases the responsiveness of stimuli so we don't feel every stimuli that is there.

Functions of Sensory Receptors

Sensory Transduction

Sensory Transduction is the conversion of stimulus energy into receptor potential, which is the change in membrane potential of a sensory receptor. As the ion channels in the sensory receptor's plasma membrane opens and closes, the ionic permeability of the membrane is changed, and results in receptor potentials. These sensory receptors can detect the smallest stimulus possible, which makes them really sensitive.

Amplification

Amplification is when the cells in sensory pathway strengthens the stimulus energy in order for humans to have according reactions.

Transmission

After amplification, action potentials are transmitted to the CNS.

Integration

As the information is received in the CNS, through summation the receptor potentials are then being delivered to sensory receptors at different parts of the body are integrated. One type of integration is called adaptation, where it decreases the responsiveness of stimuli so we don't feel every stimuli that is there.

Olfaction

Olfaction is very closely related to the sense of taste, since it has the ability to detect odors. A lot odorants are detected when they are carried as vapors into the nose. Many of these odorants are small organic molecules, which account for the smell of many familiar smells.

Odorants are detected in a specific region of the nose, called the main olfactory epithelium, which lies at the top of the nasal cavity. Approximately 1 million sensory neurons line the surface of this region. The property that is responsible for the smell of these molecules is the shape of the molecule and not necessarily the physical properties. The shape is the factor that matters the most because it determines how well it can bind to a specific surface, usually a protein receptor. Take for instance the molecule Carvone ((5R)-2-methyl-5-(prop-1-en-2-yl)cyclohex-2-en-1-one) which is chiral. Each enantiomer produces a different smell yet the only difference in the molecule is the way it is oriented, with no other physical differences. The (R) enantiomer produces the spearmint smell while the (S) enantiomer produces the smell for the plant caraway.

Biochemical research in olfactants suggested that G proteins played a role in detecting smells, and therefore it was determined that 7TM receptors were the cause. The seven-transmembrane-helix (7TM) receptors are responsible for transmitting information initiated by signals as diverse as photons, odorants, tastants, hormones, and neurotransmitters. Several thousand such receptors are known, and the list continues to grow. As the name indicates, these receptors contain seven helices that span the membrane bilayer, they are the largest class of cell receptors. The receptors are sometimes referred to as serpentine receptors because the single polypeptide chain “snakes” through the membrane seven times.

Olfaction Receptor Cells

There are neurons that line the upper part of the nasal cavity called olfaction receptor cells. When odorant molecules bind to these receptors, it triggers a signal transduction pathway that sends action potentials straight to the olfactory bulb of the brain. The odorant molecules only bind to specific receptor proteins. The different odors are determined by the binding of distinct odorants to selective receptors.

Vision

The sense of vision is based on the absorption of light by photoreceptor cells in the eye. Photoreceptor cells are sensitive to light in the region between 300-850nm in the electromagnetic spectrum. There are two types of photoreceptors, they are called rods and cones. A person’s retina contains around 3 million cones and 100 million rods cone receptors are responsible for colors and only function in bright light. Rods function in dim light and do not perceive color. The human retina contains two types of photoreceptors called rods and cones. Rods are light sensitive but cannot distinguish color. Cones, on the other hand, are able to distinguish color but are not as light sensitive as rods. Rods and cones both have visual pigments that contain the retinal, a light-absorbing molecule. Rods specifically contain rhodopsin, a pigment that is made up of the retinal attached to a particular opsin. When rods are exposed to light, the retinal molecule absorbs the light and it changes shape. In the retinal inactive state, the rod stays in the cis isomer. After the retinal of the rod absorbs light, it changes from cis isomer to the trans isomer. In the trans isomer, it also becomes unbound from the opsin. The outer segment of the rod is made up of disks that are stacked together. The rhodopsin is located within these disks. Once retinal absorbs light, this triggers a signal transduction pathway. First the rhodopsin gets activates by the absorption of light. The active rhodopsin activates transducing, a G protein. This G protein then activates the enzyme, phosphodiesterase. The activated phosphodiesterase hydrolyzes cGMP to GMP, which allows the cGMP to become removed from the sodium channel. Once the cGMP is removed, the sodium channels close and the rod hyperpolarizes as the membrane’s permeability to sodium decreases.

In the Dark

Rhodopsin is inactive
Sodium channels are open
Rod is depolarized
Rods and cones release glutamate, a neurotransmitter into synapses with neurons called bipolar cells
Bipolar cell is either depolarized and hyperpolarized

In Light

Rhodopsin is active
Sodium chanels are closed
Rod is hyperpolarized
No glutamate is released
Bipolar cell is either depolarized and hyperpolarized

Information Processing

There are three types of neurons that aid in information processing in the retina. Ganglion cells are neurons that send signals from bipolar cells to the brain and contribute in long range signaling. Horizontal cells and amacrine cells help integrate information before it gets transmitted to the brain. Horizontal cells and amacrine cells participate in local signaling. The horizontal cells receive information from rods or cones and will reduce the effect of surrounding rods and cones, which helps to sharpen our image by increasing the contrast. Amacrine cells are activated by the bipolar cells and reduce the effect of surrounding bipolar and ganglia cells. Lateral inhibition is a specific type of integration that aids in creating a greater contrast in image. The optic chiasm is located near the cerebral cortex and is the area where the optic nerves meet and axons cross. The axons from the left visual field, in both the left and right eye, join and travel to the right side of the brain. Vice versa, the axons from the right visual field, join and travel to the left side of the brian.

Taste

Usually when one has nasal congestion one can have difficulty tasting food. This is due to the fact that taste is a combination of senses that function by different mechanisms. The sense of taste can therefore be called the sister sense to olfaction. They are however very different in many ways. Tastants fall into five groups: sweet, salty, umami, bitter, and sour.

The simplest of the tastants, the sodium ion, is perceived as salty while the hydrogen ion is considered sour. The salty taste is not caused by 7TM receptors, it is actually detected by the passage of sodium ion through channels expressed on the surface of cells in the tongue. Like salty tastes, sour tastes arise from the effect of hydrogen ions on channels. The taste called umami is caused by the amino acids glutamate and aspartate. Usually one encounters glutamate as a salt called monosodium glutamate (MSG), which is used as a flavor enhancer. Glutamate and aspartate are the only amino acids that cause taste. The umami taste they emit is mediated by a heterodimeric receptor related to the sweet receptor. On the other hand, bitter and sweet tastants are caused by a wide range of different molecules. Many bitter tasting compounds are associated with being toxic. Most sweet compounds are carbohydrates, which are rich in energy and easily digestible. Some non-carbohydrate compounds such as saccharin and aspartame also taste sweet. Just as in olfaction, research pointed to the involvement of G proteins which, also meant that 7TM receptors detect the bitter and sweet taste.

Hearing

Hearing is based on the detection of mechanical stimuli. People can detect frequencies ranging from 200 to 20,000Hz, corresponding to times of 5 to 0.05ms. Our ability to hear noise is enhanced by the ability to detect where the noise is coming from. It the ability to detect time delay between the arrival of sound in one ear to the time it takes to reach the other. Sound waves are detected inside the cochlea, which is a fluid-filled membranous sac that is coiled like a snail. It is located in the inner part of the ear. Each cochlea has approximately 16,000 hair cells, which are responsible for sound detection. Within each hair cell there is approximately 20 to 300 hair-like projections called stereocilia.

Equilibrium

Not only do human ears can detect sound, the inner ear of humans can also detect body position and balance. hair cells in the utricle in inner ear respond when there is changes in head position due to gravity or movement. Different body positions can cause different hair cells and sensory neurons to be stimulated and send different message to the body. This is why some people get motion seasickness with the sudden change in movement or gravity.

Touch

The sense of touch like other senses is a combination of many factors. Touch, detected by skin, senses pressure, temperature, and pain. Although currently biochemist do not necessarily know what exactly causes the sense of touch, it has been determined that specialized neurons, termed nociceptors transmit signals from skin to pain processing centers in the spinal cord and brain. It was determined that nociceptors play a role when studying spicy food. The molecule capsaicin activates nociceptors and is the molecule responsible for the “hot” taste. The capsaicin receptor, also called VR1, functions as a cation channel that initiates a nerve impulse.

Reference

Campbell, Neil A., and Jane B. Reece. Biology. Menlo Park, CA: Benjamin Cummings, 1999. Print.