Total Number of Words in 1 Minute: ______

Reading Exercise 3: Maintaining Control

Functions such as breathing, thinking, reflexes and memory are all controlled by the central nervous system. It is comprised of highly specialized cells called neurones or nerves. These cells make electrical impulses and transmit them in a similar way to wire carrying electricity. The central nervous system comprises four parts. The brain and spinal cord are the main pathways by which an organism's cells communi- cate. The receptor cells detect a stimulus, and make up the five senses —sight, smell, touch, taste and hearing. The sensory nerves carry messages from the receptor cells to the spine and  brain. The motor nerves (/) carry messages to the effectors; these are glands or muscles that respond to a stimulus. A reflex, such as flinching at a pain, is a response to a danger.

Humans have a large brain compared to other mammals. In an adult the brain has about 100 billion neurones. Brain cells cannot be replaced once they are damaged; this is why brain damage is generally permanent. The cerebrum is the movement center of the brain. The area is highly folded, which increases the surface area of its cells and there- fore the number of messages that it can process simultaneously. The neurones  (/) do not touch each other and the gap between them is called the synapse. Impulses cross the gap via neurotransmitters, which are subsequently removed by biological catalysts called enzymes. Some toxins produced by snakes and spiders act by disrupting the enzymes. Over fifty neurotransmitters have been identified. Dopamine is one of them and is responsible for emo- tional behavior. The brain also produces pain killing neurotransmitters, such as encephalin and endorphins. Heroin and morphine are artificial forms of these neurotransmitters.

Some drugs act in ways similar to neurotransmitters and are called psy- choactive for this reason. Some are  legal, such as tobacco, alcohol and prescription drugs. Others, such as heroin and marijuana, are illegal. A person can come to believe that they need a particular drug to function and become psychologically dependent on a drug. The body itself can become physically dependent on a drug. This can occur when the neurotransmitters in the brain become conditioned to the presence of the drug and can’t function correctly if the drug is withdrawn. On the other hand, the brain may build up a tolerance to the drug and the user finds that a larger amount of it is required to get the same response.

 Another of the body's message net- works is the endocrine network. The endocrine glands include the pituitary glands thyroid gland, pancreas, adrenal glands, and the gonads. These glands produce hormones that lock onto the outside of a target cell and induce a change in that cell. While the pituitary gland produces some hormones, its main role is to direct the other endocrine glands. The thyroid gland produces two hormones involved in the homeostasis of calcium in the blood. Homeostasis is a constant level of a substance in the system; calcitonin removes soluble calcium and stores it in the insoluble form in  bones, while thyroxin removes it from the bones and returns it to the blood. Pheromones are the chemical messen- gers used by organisms of the same species to attract a mate. While insects and other mammals are known to use these, no human equivalent has been identified.

The regulation of cellular respiration is controlled by the supply of the simple sugar glucose. After eating, glucose levels in the blood rise. The pancreas excretes insulin to turn the excess glucose into glycogen, which is then stored by the liver. If glucose is con- sumed during exercise, the pancreas releases glucagons to turn glycogen  into glucose again. In this way the pancreas maintains homeostasis of glucose levels in the blood.

The regulation of cellular respiration is controlled by the supply of the simple sugar glucose. After eating, glucose levels in the blood rise. The pancreas excretes insulin to turn the excess glucose into glycogen, which is then stored by the liver. If glucose is con- sumed during exercise, the pancreas releases glucagons to turn glycogen  into glucose again. In this way the pancreas maintains homeostasis of glucose levels in the blood. In general, diabetes is caused by the destruction of the cells in the pancreas that produce insulin. Insulin-dependent diabetics must inject themselves daily with the hormone and follow a strict diet and exercise plan. This helps them avoid hypoglycemia, where the blood sugar level drops dramatically. The treatment used to avoid confusion, shaking or coma is to give the sufferer some sugar. Prior to 1980, the insulin used to treat diabetics was extracted from the pancreases of cows and pigs.  Today, the insulin is produced in bulk from genetically engineered bacteria. Other treatments include the implanta- tion of donor pancreases, or the pancreas of a genetically altered pig. The other option is to inject the cells responsible for insulin production. If another type of cell can be induced to produce insulin, they can be introduced then into the pancreas to replace the destroyed cells.

The kidneys are responsible for removing most of the waste produced by the body. Urea is the broken down amino acids produced by the liver. The kidney treats this to conserve water, salts, glucose, and complete amino  acids, while removing toxic waste. Kid- neys are involved in maintaining the homeostasis of water and salts in the body and can produce more concen- trate or dilute urine to do so. Illness, injury, or overuse of some drugs can cause kidney failure. While a human can survive with the use of one fully functioning kidney, anything less would require the person to undergo dialysis two to four times a week. Dialysis is when a machine is used to perform the same function as the kidneys. It takes approximately four hours, and most people on dialysis live in hope of a kidney transplant.

 Reptiles and insects are ectodermic— the external environment regulates their body temperature. Mammals, birds and fish are endotherms, maintaining a sta- ble body temperature through internal metabolism. The human body needs to stay around 37C, and hypothermia will result if the temperature drops dramatically. On the other hand, an elevated body temperature causes hyperther- mia. The body cools itself by sweating, or through vasodilatation. The blood vessels close to the skin expand to increase the volume of blood flowing through and so heat is lost to the environment. Heat is generated through shivering, vasorestriction or through goose bumps. Goose bumps raise the  hair on the skin, trapping warm air between fur and skin. On relatively hairless humans, this contributes little to heat generation.

Plants also respond to stimuli, but in a simpler fashion. These responses are called tropisms. Phototropism is the response to light—this helps the plant to measure the length of the day, and hence flower or die at the correct time of year. Gravitropism is the response to gravity—when a seed germinates it needs to point its roots down and its shoots up. The hormone responsible for this is called auxin. For example, it causes cells on the dark side of the  plant to elongate, so bending the plant to the light source. A plant will remove all food from a leaf, and transfer waste there before releasing a growth- inhibiting hormone to cause the leaf to drop off. The gas ethylene is involved in the ripening of fruit, so fruit can be picked before it ripens and is caused to do so at a more desirable time. (bbc.co.uk)

Total number of words in 1 minute: ______