The spinal cord is an 18 inch, fibrous, rope-like structure, which is surrounded by rings of bone or vertebrae (backbone), as illustrated in Figure 4. Nerves reach every millimeter of the surface of the skin. They send signals, such as pain, heat, cold or pressure to the spinal cord, which then takes it to the brain to be processed. Reflexes are when a signal is just sent to the spinal cord (i.e., does not involve the brain), which immediately responds by sending another signal to the appropriate muscle group (e.g., when your hand touches a hot stove, you have a reflex that immediately jerks your hand away to minimize bodily damage).

Figure 4. Cross section of the human spinal cord. click for copyright

If someone suffers from a spinal cord injury, they can become paralyzed form the waste or even the neck down. They lose all ability to feel or move their limbs (paralysis) and are confined to a wheelchair for the rest of their lives. Biomedical engineers and doctors are diligently working on a treatment for paralysis, but currently, it is a permanent condition. In the mean time, they have designed assistive technology to give them back quality of life: respirators, wheelchairs, and other technology to help keep patients alive and give them the ability to move around.

The eyes, often called the "windows to the soul," take in light, focus the images of the world on the retina, and transfer the information to the brain where it is interpreted into what we actually see. The eye sits in the eye socket, which is a cavity of bone that protects this delicate organ. The eyelids also protect the eye. They wash the eye with tears produced in lacrimal glands, and excess tears are drained away through tear ducts.

Figure 5. The human eye. click for copyright

The middle layer contains the iris, ciliary body and choroid. The iris controls how much light enters the eye. It dilates to let more light in through the pupil and contracts to let in less light. The iris is pigmented and is what gives the eye its color. The ciliary body produces the aqueous humor and the vitreous humor, which are the jelly-like substances that fill the two cavities of the eye. The aqueous humor fills the front cavity behind the cornea and in front of the iris. The vitreous humor fills the main cavity of the eyeball. The ciliary muscles transform the shape of the lens to focus images. The choroid, which makes up most of the middle layer of the eye, is spongy and soft and contains the blood vessels which supply the eye.

The innermost layer of the eye is the retina which is where the images are focused. The retina contains photoreceptor cells. Rods perceive light, dark, movement and shape, while cones perceive color. The lens , held in place by the ciliary ligaments, focuses the light entering the eye. The ciliary muscles alter the shape of the lens automatically, depending on how close objects are to the eye. At the back of the eye is the optic nerve, which branches in order to bring messages from the left eye to the right visual cortex of the brain and from the right eye to the left visual cortex of the brain.

Engineers have created eye devices for people who have trouble seeing or cannot see at all (glasses, contacts, Lasik eye surgery equipment, etc.). Electrical Engineers have used their understanding of the eye to create a microchip that can be implanted into the back of the eye to work as a light sensor for people whose natural sensor does not work properly or at all.

The ear consists of three parts: the outer ear, middle ear and inner ear. The outer ear, or pinna, collects sound waves and directs them into the ear canal. The ear canal is a tube that connects the outer ear to the middle ear. Sound waves bounce back and forth along its length until they reach the middle ear.

The middle ear contains the eardrum, which vibrates when sound hits it after traveling through the ear canal. Three small bones in the middle ear conduct the sound from the eardrum to the inner ear. These bones are called the malleus, incus and stapes. The malleus acts like a mallet, hitting the incus, which is like an anvil. The incus vibrates in turn and causes the stapes to vibrate as well. The stapes is shaped like a stirrup and is connected to the incus by a ball and socket joint. The Eustachian tube, or auditory tube, connects the middle ear with the back of the throat (see Figure 6). Excess fluid is drained away to equalize pressure on either side of the eardrum. As the stapes vibrates, it touches the oval window, which is the beginning of the inner ear.

Figure 6. The human ear. click for copyright

Behind the oval window are a series of canals. Two main structures are located here, the semicircular canals and the cochlea. Both of these structures are filled with fluid. When the stapes from the middle ear vibrates the oval window, the fluid in the inner ear also vibrates. Sensory hairs in the cochlea, which is shaped like a snail shell, pick up the sound vibrations and transmit signals to the brain, which receives the message as sound. The semicircular canals in the inner ear are arranged at right angles to each other. When fluid in these canals shift position, as when someone bends over, sensory hairs inside the canals are bent by the fluid, and messages are sent from the hairs to nerves which tell the brain that there has been a shift in position.

Deafness is caused by blockage of the ear canal, perforation of the eardrum, mechanical difficulties in the bones of the middle ear (fusion of the bones), or breaking the hairs in the cochlea. Disease, infection, birth defects and loud noise are also causes of deafness. Bioengineers have invented devices to treat deafness, including hearing aids, cochlear implants, placing tubes in the eardrum, and replacing part of the bones in the middle ear with wires. Engineers must have a great understanding of the ear and its physiology is necessary to design these devices.

In order to smell something, molecules from that object must travel through the air and enter your nose. If an object does not give off an odor (e.g., steel), it is because nothing evaporates from it, or it is a non-volatile solid. Air molecules enter the nose through the two nostrils, which are separated by a wall called the septum. Behind the nose is a space called the nasal cavity, which is separated from the mouth by the palate, or the roof of the mouth. At the top of the nasal passage are a group of neurons ( olfactory epithelium) that are out in the open in order for them to come into contact with air passing in and out of the nose. This patch of neurons is about the size of a postage stamp and has hair-like projections ( cilia) that bind to odor molecules. Smell receptors then send a signal to the olfactory bulb, which passes the signal to the brain to be interpreted. If smell receptors are damaged, than that person will be unable to detect certain odors.

Figure 7. The nose and throat. click for copyright

Your sense of taste is strongly tied to your sense of smell. You may have even noticed that foods taste differently if you have a cold and your sense of smell is impaired. Engineers even design food packaging to maintain or enhance the smell of the food inside. The tongue also gathers information about taste. The top of it is covered with a layer of bumps called papillae. These bumps help grip the food and contain your taste buds. People are born with 10,000 taste buds, but may only have 5,000 at the end of their life. There are four types of taste buds: sweet (tip of the tongue), sour (side), bitter (back), and salty (all over the tongue). Each taste bud has tiny, sensitive hairs (microvilli), which send messages to the brain to be interpreted.

The space environment is completely different than that of Earth, and the sensory data that your body receives can be confusing. Your senses work together in order to provide your brain with information about your environment. For example, your eyes and inner ear both send signals to your brain about your body's position. Your eyes gather information by seeing the orientation of the objects around you (floor, ceiling, etc.), while your inner ear depends on the orientation of the hairs in the semicircular canals. These hair respond to gravity to determine which way is up and which way is down; without gravity present then, they are not able to do their job. In space, your eyes are still able to tell which direction is up and down, but your inner ear no longer has a vertical reference since there is little or no gravity in space. This causes your brain to receive conflicting signals and often results in headache, nausea and vomiting. Because of this problem, astronauts are not allowed to leave the spacecraft for the first three days or so that they are in space. Why would you not want to be in a spacesuit (required to leave their spacecraft) when you are motion sick? Well, if you got sick inside the spacesuit, your vomit would have nowhere to go and you would end up inhaling it. Your vomit is very acidic due to the juices in you stomach, and inhaling it would cause the acid to eat through your lung tissue, which would kill you, quickly and painfully. Although this quite a graphic explanation, it is a good example of why it is important for engineers to understand how the nervous system works, especially how the senses work together. Eventually, the body does adapt to this new environment by relying on your eyes for its position and ignoring most of the signals from the inner ear. Once the body adjusts, astronauts are able to leave the spacecraft in their spacesuit.

Another less serious affect that spaceflight has on your nervous system is suppressing your sense of smell. As mentioned in previous lessons, your bodily fluids shift up to your head without gravity. This causes nasal congestion, kind of like a cold, and decreases your sense of smell in space. Your taste buds are not affected in space, but since rich flavors are largely dependent on smell, food tastes much blander. Of course, the lack of variety of food in space does not help the situation. Another result of the fluid shift is memory changes. This extra pressure on the brain ─ due to the excess fluid ─ is thought to be the cause of astronauts' memory problems in space. They often forget even simple instructions that another crewmember or mission control just told them seconds ago. Astronauts compensate for this by writing down notes when they are talking to someone else. Engineers have designed clothing with magnets, Velcro®, extra pockets, and elastic so they can easily carry pens and pads of paper with them.

Brainstorming: As a class, have the students engage in open discussion. Remind students that in brainstorming, no idea or suggestion is "silly." All ideas should be respectfully heard. Take an uncritical position, encourage wild ideas and discourage criticism of ideas. Have them raise their hands to respond. Write their ideas on the board. Ask the students:

• Think of things that would be hard to do without your hearing, sight, etc.

Discussion Question: Solicit, integrate and summarize student responses.

• What does your nervous system do?
• Which of your five senses would you rather lose? Why?
• How do your five senses work together?

Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Count the number of true and false and write the number on the board. Give the right answer.

• True or False: The brain helps us interpret all the signals from our nervous system. (Answer: True, the brain coordinates all the different sensory signals, or provides you with a complete picture of your surroundings, and controls your body's movement.)
• True or False: Electrical Engineers are creating a microchip that can be implanted into the back of the eye for people who have trouble seeing. (Answer: True, the microchip works as a light sensor for people when their natural light sensor does not work.)
• True or False: Astronaut's motion sickness in space has nothing to do with the nervous system. (Answer: False, in space, an astronaut's eyes are still able to tell which direction is up and down, but their inner ear no longer has a vertical reference (because of little or no gravity). This causes their brain to receive conflicting signals in space and often results in headache, nausea and vomiting.)
• True or False: Bicycle and motorcycle helmets are good ways to help save the brain from injury. (Answer: True, biomedical engineers and doctors work side by side to develop better ways to save the brain from trauma and disease, such as bicycle and motorcycle helmets.)

One and Done: Ask the students to think of something that involves using one of their five senses, and raise their hands (or indicate thumbs up) when they have an example. (Possible answers: eating a meal, smelling a flower, touching a sticky substance, listening to their favorite song, etc.). Call on students at random to state their answer (the activity and which of their senses it uses). Students put their hands down once they have contributed an answer. No repeat answers permitted.