Find an interesting article (news or primary literature) about the nervous system and tie it into material from Chapter 45 or 46.


This article explores SPD, or sensory processing disorder. SPD is a condition where the person affected is either oversensitive or undersensitive to one or more environmental stimuli. As stated in Chapter 46, the process of perception starts with sensory receptors where a stimulus creates an action potential. The signal is then amplified and transmitted to the central nervous system where the information is processed. For someone to be oversensitive or undersensitive, there must be a functional problem in this system. The cause and cure for this disorder has not been identified.


Information Filters

Describe two ways that the nervous system filters information that is being transmitted to the brain.

Information from sensory receptors is coded in two ways. Firstly, each receptor is highly specific and will only fire in response to certain values of stimuli. For example, a receptor dealing with sound may respond to a small range of frequencies and may only be sensitive to low-pitched sounds. When this receptor fires, then, the signal indicates only a low-pitched sound.

Sensory receptors can also change the number and frequency of action potentials performed. A low number of action potentials could indicate a lower volume whereas a higher number of action potentials indicate a higher volume.

Sensory Receptors

What are sensory receptors and how do they limit or affect the kinds of things that an animal (or you) perceives in the world? Provide examples from the book, the news or your own experience.

Sensory receptors are cells that are specialized to sense environmental energies and convert them into action potentials. There are seven types of sensory receptors. Mechanoreceptors sense sound. Chemoreceptors detect specific molecules. Photoreceptors respond to light. Thermoreceptors perceive heat. Electroreceptors respond to electricity. Magnetoreceptors detect magnetic fields. Nociceptors sense pain.

Not all animals are equipped with each of these types of receptors. Humans cannot sense magnetic or electrical fields because we do not have the sensory receptors to do so. Animals also have limited ranges for perception. For example, humans are cannot hear every frequency of sound or see every wavelength of light.

Action Potentials

Do all stimuli produce an action potential? Why or why not?

Not all stimuli produce an action potential. For a neuron to transmit an electrical signal, its membrane must be depolarized beyond a specific potential. This specific potential is called the threshold potential. When this value is exceeded, an action potential is triggered. Any stimuli that depolarizes the membrane less than this value does not trigger an action potential.

Digestive Compartments

The digestive tract is highly compartmentalized. Describe the compartments and why it is important to separate different processes.

In general, there are six stages of a digestive tract. First, food is ingested and taken into the organism from the outside environment. Second, muscular contraction propels food through an alimentary canal. Third, food is broken down by physically crushing it into smaller pieces through mechanical digestion (this can actually take place at several locations throughout the system). Fourth, chemical digestion takes place and enzymes break down the food molecularly. Fifth, the product of this digestion is absorbed into the cells of the organism. Sixth, indigestible substances are eliminated through defecation.

The digestive tract is compartmentalized so that organs can specialize in carrying out these stages. The stomach specializes in chemical digestion because it is lined with mucus that protects its cells from the enzymes. If the cells were exposed to the enzymes, they would be broken down and digested along with the food particles. The small intestine specializes in absorption by having a high surface area. More nutrients can diffuse through the cell membranes when there is a higher surface area exposed.

Mammalian Digestion

How are mammals’ digestive tracts different from other animals?

Unlike some other animals, all mammals begin their digestive process by taking food into their mouth. The tissues in the mouth are protected with layers of keratin so that mechanical digestion can take place. Some chemical digestive fluids are secreted from salivary glands to break down starches. Mammals then use a tongue to propel food to the bolus. Food is propelled down the esophagus into the stomach where chemical and mechanical digestion continue to occur. Gastric pits release enzymes to break down the food. The broken down product moves to the small intestine where the enzymes are neutralized, chemical digestion finishes, and absorption occurs. Circular folds, villi and thin tissues allow mammals to absorb nutrients efficiently. The remaining fluid moves to the large intestine so electrolytes can be absorbed and bacterial digestion can occur. Water used in this process is reclaimed and all waste products move to the colon and is excreted through the anal canal.

Essential Nutrients

What are the main classes of nutrients that animals need? Where do you find them in your diet?

Nutrients are the substances that organisms need to ingest to survive. These include carbohydrates, fats, proteins, vitamins, electrolytes, and inorganic substances. Humans ingest simple carbohydrates in the form of sugars and complex carbohydrates in the form of breads. Animal fats or vegetable fats can be consumed in the form of butter, lard or oil. Proteins are present in meats, fish and dairy products. There are several types of vitamins which are found in different sources and serve different functions. Vitamin B1 is found in whole grains, vitamin B12 is located in red meat and eggs, niacin is found in meat, folate can be found in green vegetables, vitamin C is in citrus fruits, and vitamin D can be found in fortified milk. Similarly, electrolytes such as sodium, potassium, and chloride are found in different foods. Sodium is present in salt and meats, potassium can be found in fruits and vegetables, and chloride is located in salt and vegetables. Other essential inorganic compounds include calcium, phosphorus, iron and magnesium. Calcium and phosphorus can be found in dairy products, iron is located in meat and eggs, and magnesium is present in whole grains.

Body Tissue

What are the major types of tissue found in your body?

There are four major types of tissues found in the human body:

  1. Connective tissues secrete specific extracellular matrices for different purposes. Loose connective tissue provides padding, dense connective tissue creates a durable but flexible connection between muscles, bones and organs, supporting connective tissue make up bone and cartilage that provide support, and fluid connective tissue functions in circulation.
  2. Nervous tissues are the neurons and supporting cells that allow for the transmission of electrical signals.
  3. Muscle tissues function to allow movement. Skeletal muscles attach to bones and create body movement with different combinations of contractions. Cardiac muscles are responsible for pumping blood throughout the circulatory system. Smooth muscles line the gut and circulatory system to regulate the movement of particles throughout the body.
  4. Epithelial tissues are located on the surface of the body, all organs and are essential in forming glands, which secrete chemicals such as hormones.


How do you maintain homeostasis, in general? On a hot day? On a cold day?

Homeostasis occurs when chemical and physical conditions are stable within an animal’s cells, tissues and organs. There are two ways of maintaining this balance. These mechanisms are used for conditions such as pH, ion concentrations and pressure, but let’s look at them in respect to temperature. Some animals use conformation, which is when their body temperature is the same as the environment in which they live. Others utilize regulation, where they use mechanisms to maintain a set internal temperature. For regulation to occur, a negative feedback system with sensors, integrators and effectors bust be in place. Through this process, the environment is detected, the information is processed, and a response to changes (if necessary) is triggered.

Humans use regulation to maintain homeostasis, and are considered endothermic homeotherms. This means that we are able to warm our own tissues and that our body temperature remains relatively constant. On a hot day, we utilize mechanisms such as sweating to keep our body temperature from increasing too much. On cold days, we are able to keep warm by metabolizing. Heat is a by-product of metabolism and mammals have the ability to produce enough of it to warm to body.

Surface Area and Volume

How are surface area to volume ratios important to animal functions?

For animals to function, they must absorb oxygen and nutrients while emitting waste products such as urea and carbon dioxide. For this to occur, these molecules must defuse across cell membranes. The greater the surface area is, the faster this diffusion can occur. On the other hand, the rates of metabolism and other processes are dependent on cell volume.

In general, as a cell increases in size, the volume grows at a faster rate than its surface area. Similarly, as an organism increases in volume, the available surface area for diffusion increases at a lower rate. This decreased surface area affects the ability for molecules to enter and exit cells, and affects metabolic rate, in turn. This is why larger animals have slower metabolisms than smaller animals, in general.