Key+Findings

1. The Discovery of the Nervous System
Claudius Galen was credited for the idea of a nervous system. He was the first to document that our senses initiate in the brain instead of the heart. It was once believed that "Animal Spirits", a slimy substance housed in the ventricles of out brains, controlled all of our actions. History shows that we can thank Johannes Purkinje for the first substantial description of a neuron, namely the Purkinje cell. Today, the understanding of our system goes much beyond Purkinje. Models of our nervous system depend on many important discoveries that stand against time, such as, the behavior of a neuron, types of neurons, division of the nervous system, and pathways to the cortex.

1. Distinction of the Visual Pathway
A major finding for the physiology of the visual system was the recognition of a determined pathway for the sensation of light from photo-receptors contained in the human eye. It begins with the reflection of light entering our pupils and leading to the activation of either rod or cone photo-receptors. The particular pattern these receptors fire are sent down the retinal axons and diverge at the optic chasm, depending on which side of each receptive field was activated. From there, the information from the left visual field is sent to the right lateral geniculate nucleus(LGN) and vice versa. The LGN then organizes the sensation and sends the signal to, mainly, designated areas of the visual cortex or the superior colliculus.

2. Anatomy of the Eye
The anatomy of the eye receives signals beginning from the cornea of the eye. The cornea is the dome-shaped lens that light is refracted through and into the body of the eye. The iris of the eye acts as shield to incoming light. It is able to enlarge the pupil to receive more light or shrink it to avoid an excess of light reaching the photo-receptors and resulting in over-activation. There are two types of receptors that detect light radiation and that our vision system rely on - rods and cones. Rods are receptors that are much more sensitive to elicit a response than cones. Rods are so sensitive to light because of the convergence of receptors to ganglion cells are approximately 126 million to 1 million (Humphrey & Saul, 1994). Convergence is a primary reason why rods are responsible for low-light vision. Cones, on the other hand, are recognized for their visual accuracy because of less convergence onto ganglion cells that travel directly down the optic nerve.



The location of receptors play an important role as well. The area of the eye that is most affected when we look directly at an object is the fovea. This area is dense with only cone receptors while rods are everywhere else on the retina along with low amounts of cones. The best way to experience this effect attempting to look at something in the dark then observing it in your peripheral vision. The image should be much clearer in the peripherals because of the dense rod population outside of the fovea.

3. Structure of the Lateral Geniculate Nucleus
There are two lateral geniculate nuclei, one for each hemisphere of the brain. The lateral geniculate nucleus is responsible for the regulation and organization of the information received from its corresponding receptive fields sent down the optic nerve. For every 10 electrical signals the LGN receives from the optic nerve, 4 are sent to the visual cortex. In addition to receiving the sensory response from the optic nerve, it also receives feedback information from the visual cortex. The layers of the LGN only receive signals from a single eye. That is why layers 2,3, and 5 only receive signals from the eye that is one the same side as that LGN, while layers 1,4, and 6 receive their signals from the opposite eye. These layers play an important role in the creation of a retinotopic map located in various parts of the LGN. Certain receptors in the retina of our eyes correspond to a certain areas on the LGN.



4. Understanding the Visual Cortex
The last step in the sensation of visual stimuli occurs in the visual cortex. The direction of the stimuli's signal depends on many factors that different receptive fields sense. The neurons used for this process are organized by three different types of qualities.
 * Simple cortical cells - These neurons play a specific role in receiving stimulation from excitatory and inhibitory areas of the retina that are side-by-side. Research has shown that these neurons respond best to bars of a particular orientation (Hubel, 1959).
 * Complex cortical cells - These neurons responds to particular orientation of bars similar to simple cortical cells but add the the variable of movement. Certain areas of complex cortical cells respond best to a certain orientations moving in a certain directions (Hubel, 1959).
 * End-stopped cortical cells - These types of neurons become progressively more complicated that the previous two types of neurons. Stimuli that cause end-stopped cortical cells to activate involve corners of shapes, the angles of the shapes, the movement of the shape, and in all relevance to each other (Hubel, 1959).

1. Anatomy of the Ear Canal
Auditory sensory begins at the external open that is our visible ear. The cartilidge that forms the outer formation is called the "pinna". The pinna and ear canal make up the "outer ear". Sound waves enter through this external pathway and interact with the tympanic membrane to create a chain reaction to the middle and inner ear. The components inside the middle ear that are most responsible for auditory sensation are tiny bone structures called, ossicles. After the sound wave vibrates the eardrum it sends its wave energy to the hammer that vibrates onto the anvil and the anvil's energy is transferred to the stapes. The magic of the ossicles is that they are able to amplify the incoming sound wave to a higher energy level more suitable for the liquid-filled inner ear (Goldstein, 2010).

<span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: left;">As the energy travels to the stapes, it resonates onto an oval window that leads into the inner ear. The inner ear houses the thousands hair cells that are used for the transduction of wave energy into electrical energy that is interpreted by the brain. The system that hold the different hair cells is called the cochlea and houses the organ of Corti. The cochlea is a coiled tube that is split into two halves that connect at the apex to circulate the energy back out of the cochlea. While the energy passes through this, the vibrations are felt by two kinds of hair cells that are beneath the tectorial membrane. The cilia of the inner hair cell is what is responsible the translation into electrical signal (Moller, 2000). The outer hair cells are more responsible for increasing the vibration of the tectorial membrane. The higher frequencies are sensed at the beginning of the cochlea while lower frequencies affect the receptors at the apex of the cochlea.

<span style="font-family: Arial,Helvetica,sans-serif;">The Physiology of the Olfactory System
<span style="font-family: Arial,Helvetica,sans-serif;">

<span style="font-family: Arial,Helvetica,sans-serif;">1. Anatomy of the Olfactory System
<span style="font-family: Arial,Helvetica,sans-serif;">Along with taste and free nerve endings in our skin, the olfactory receptors consist of only chemo-receptors. These types of receptors respond to mechanical changes but primarily changes in the chemicals surrounding the particular receptor. When mammals inhale, chemicals that are suspended in the surrounding air enter the nasal cavity. In the nasal cavity there is a web of millions of receptors. There are 350 different types of receptors identified and over 10,000 of each type of receptor in the nasal cavity (Buck, 1991). The vast amount of receptors we possess are required for having such a variety of odors we are able to recognize. Each of the 350 types of receptors are stimulated by only a particular group of chemicals. When receptors are activated, the send signals to corresponding neurons in the olfactory bulb which is directly connected to the brain.



<span style="font-family: Arial,Helvetica,sans-serif;">2. Recognition Profiles
<span style="font-family: Arial,Helvetica,sans-serif;">Recognition profiles occur when multitudes of receptors fire but at different strengths. Many chemicals have the nearly the same structure but are perceived very differently. This is due to calcium ions being released from the activated receptors. The amount of calcium released contributes to the differences in profiles between similar molecules. Malnic (1999) found that octanoic acid and octanol acid (difference of only one oxygen molecule) had completely different recognition profiles. The first acid was perceived as "sour" and "repulsive" while the other was described as "sweet" and "fresh".

<span style="font-family: Arial,Helvetica,sans-serif;">1. Anatomy of the Gustatory System
<span style="font-family: Arial,Helvetica,sans-serif;">The sense of taste (or gustation) allows us to discriminate in terms of what we consume. Based on these evaluations we choose which substances to ingest, some are nutritious substances essential for survival and other just for satisfaction. The sense of taste can also prevent us from consuming potentially toxic or poisonous substances.

<span style="font-family: Arial,Helvetica,sans-serif;">The taste system includes the tongue, the papillae, taste buds, taste cells and receptor sites.The tongue is essentially the receptor sheet for taste and it contains papillae as well as the rest of the structures (taste buds, cells and receptor sites). The papillae are the structures that give the tongue its rough appearance and there a four specific types of them. Each papillae has a different shape location (Goldstein, 2010).


 * <span style="font-family: Arial,Helvetica,sans-serif;"> Filiform Papillae: Shaped like cones and are found over the entire surface of the tongue.
 * <span style="font-family: Arial,Helvetica,sans-serif;"> Fungiform Papillae: Shaped like mushrooms and are found at the tip and the sides of the tongue.
 * <span style="font-family: Arial,Helvetica,sans-serif;"> Foliate Papillae: A series of folds along the back of tongue.
 * <span style="font-family: Arial,Helvetica,sans-serif;"> Circumvilliate Papillae: Shaped like flat mounds surrounded by a trench and are found at the back of the tongue. [[image:papallia.jpg width="473" height="359" caption="Structures of the taste bud and papillae."]]

The taste buds are contained on the papillae and the whole tongue contains around 10,000 taste buds. Each taste bud contains between 50 to 100 taste cells. Taste cells make up a taste bud. There are a number of cells for each bud, and the tip of each one sticks out into a taste pore. One or more nerve fibers are associated with each cell. Receptor sites are located on the tips of the taste cells. There are different types of sites for different chemicals. Chemicals contacting the sites cause transduction by affecting ion low flow across the membrane of the taste cell (Goldstein, 2010).

Food is dissolved in the saliva in the mouth and this solution is then able to slip into the pores where it stimulates the receptors of the taste cell microvilli. The microvilli are the receptor surface of the taste cells and responsible for detecting tastes.

Bowen (2006) describes that mixed within the taste cells in a taste bud is a network of dendrites of sensory nerves called taste nerves. When the chemicals bind to their receptors stimulates taste cells, they depolarize. This depolarization is transmitted to the taste nerve fibers resulting in an action potential that is ultimately transmitted to the brain. Once taste signals are transmitted to the brain, several efferent neural pathways are activated that are important to digestive function. In addition to signal transduction by taste receptor cells, it is also clear that the sense of smell profoundly affects the sensation of taste (Physiology of Taste section, para. 5).

Furthermore Bowen (2006) goes on to explain that the sense of taste is equivalent to excitation of taste receptors, and receptors for a large number of specific chemicals have been identified that contribute to the reception of taste.

The five types of tastes are commonly recognized by humans are:
 * Sweet - usually indicates energy rich nutrients.
 * Umami - the taste of amino acids (savory).
 * Salty - allows modulating diet for electrolyte balance.
 * Sour - typically the taste of acids.
 * Bitter - allows sensing of diverse natural toxins.

1. Anatomy of the Somatosensory System
The somatosensory system is composed of different receptors that interpret various types of stimuli to produce sensory modalities such as touch. These sensory receptors cover the skin and epithelial, skeletal muscles, bones and joints, internal organs, and the cardiovascular system.The system reacts to diverse stimuli using different receptors:
 * Thermoreceptors: sensory receptor that responds to heat and cold.
 * Nociceptors: a group of cells that acts as a receptor for painful stimuli.
 * Mechanoreceptors: A specialized sensory end organ that responds to mechanical stimuli such as tension or pressure.
 * Chemoreceptors: A sensory nerve cell or sense organ, as of smell or taste, that responds to chemical stimuli.



For the sense of touch as we know it, begins with the skin. Many of the tactile perceptions that we feel from stimulation of the skin can be traced to mechanoreceptors located in the epidermis and the dermis. The Merkel receptor and the Meissner corpuscle are the ones located closest to the surface of the skin. The perception associated with the Merkel receptor is sensing fine details and the ability to control handgrip with the Meissner corpuscle. Two additional mechanoreceptors in the skin are the Ruffini cylinder and the Pacinian corpuscle (both are located deeper in the skin). The Ruffini cylinder is associated with perceiving stretching of the skin and the Pacinian corpuscle with sensing rapid vibrations and fine textures (Goldstein, 2010).

Goldstein (2010) explains that nerve fibers from receptors in the skin travel in bundles called peripheral nerves that enter the spinal cord through the dorsal root. The nerve fibers then go up the spinal cord along two major pathways: the medial lemniscal pathway and the spinothalamic pathway. The lemniscal pathway has large fibers that carry signals related to sensing the positions of the limbs and perceiving touch. The spinothalamic pathway consists of smaller fibers that transmit signals related to temperature and pain. Fibers from both pathways cross over to the other side of the body during their upward journey to the thalamus. Most of these fibers synapse in the ventrolateral nucleus in the thalamus, but some synapse in the thalamic nuclei. From the thalamus, signals travel to the somatosensory receiving area in the parietal lobe of the cortex and possibly also to the secondary somatosensory cortex.



An important characteristic of the somatosensory cortex is that it is organized into a map that corresponds to specific locations in the body. Dr. Wilder Penfield, a neurosurgeon, while operating on patients drafted the Homunculus. He drafted this body map by stimulating points on the s1 or somatosensory receiving area and asked patients while awake during surgery to report what they perceived (Goldstein, 2010, p.332).