excretion
The process of removing wastes from the body.
excretory system
The organ system that maintains homeostasis by keeping the correct balance of water and salts in your body; also helps to release wastes from the body.
homeostasis
The ability to maintain a stable internal environment despite external changes.
kidney
Organ that filters and cleans the blood and forms urine; also maintains the volume of body fluids, maintains the balance of salt ions in body fluids, and excretes harmful metabolic by-products such as urea, ammonia, and uric acid.
kidney dialysis
The process of artificially filtering the blood of wastes; a patient’s blood is sent through a filter that removes waste products and the clean blood is returned to the body.
kidney failure
When the kidneys are not able to regulate water and chemicals in the body or remove waste products from the blood.
kidney stone
"Stones" formed when certain mineral wastes in urine crystallize; may be found anywhere in the urinary system.
nephron
Tiny, tube-shaped filtering unit found inside each kidney.
urea
A nitrogen-containing molecule that is made when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body.
ureter
Tube-shaped structure that brings urine from the kidneys to the urinary bladder.
urethra
Structure through which urine leaves the body.
urinary bladder
Organ that collects the urine which comes from the kidneys.
urinary system
The organ system that makes, stores, and gets rid of urine.
urinary tract infection (UTI)
Bacterial infections of any part of the urinary tract.
urination
The process of releasing urine from the body.
urine
A liquid that is formed by the kidneys when they filter wastes from the blood; contains mostly water and also dissolved salts and nitrogen-containing molecules.
Points to Consider
Next we turn our attention to the nervous system. What do you think the nervous system is? What do you think it does?
Chapter 20: Controlling the Body
Lesson 20.1: Nervous System
Lesson Objectives
Identify the functions of the nervous system.
Describe neurons and explain how they carry nerve impulses.
Describe the structures of the central nervous system.
Outline the divisions of the peripheral nervous system.
Check Your Understanding
If groups of cells are called tissues and groups of tissues are called organs, what are groups of organs called?
What are examples of human organ systems?
Which organ system controls all the others?
Introduction
Groups of organs are called organ systems. Examples of human organ systems are skeletal, digestive, and respiratory systems. The nervous system controls all the others.
Michael was riding his scooter when he hit a hole in the sidewalk and started to lose control. He thought he would fall, but in the blink of an eye, he shifted his weight and regained his balance. His heart was pounding, but at least he didn’t get hurt. How was he able to react so quickly? Michael can thank his nervous system for that (Figure below).
Figure 20.1
Staying balanced when riding a scooter requires control over the bodys muscles; the nervous system controls the muscles and maintains balance.
What Does the Nervous System Do?
The nervous system controls all the other systems of the body. Controlling muscles and maintaining balance are just two of its roles. The nervous system also lets you:
Sense your surroundings with your eyes and other sense organs.
Sense your internal environment, including temperature and pH.
Control your internal body systems and keeps them in balance.
Prepare your body to fight or flee in emergency situations.
Think, learn, remember, and use language.
The nervous system works by sending and receiving electrical signals. The signals are carried by nerves throughout the body. For example, when Michael started to fall off his scooter, his nervous system sensed that he was losing his balance. It responded by sending messages to muscles throughout his body. Some muscles tightened while others relaxed. As a result, Michael’s body became balanced again. How did his nervous system do all that in just a split second? To answer this question, you need to know how the nervous system carries messages.
Neurons and Nerve Impulses
The nervous system is made up of nerves. A nerve is a bundle of individual nerve cells. A nerve cell that carries messages is called a neuron (Figure below). The messages carried by neurons are referred to as nerve impulses. Nerve impulses are able to travel very quickly because they are electrical impulses. Think about flipping on a light switch when you enter a room. When you flip the switch, it closes an electrical circuit. With the circuit closed, electricity can flow to the light through wires inside the walls. The electricity may have to travel many meters to reach the light, but the light still comes on as soon as you flip the switch. Nerve impulses travel equally fast through the network of nerves inside the body.
Figure 20.2
The axons of many neurons, like the one shown here, are covered with a fatty layer called myelin sheath that insulates the axon like the plastic covering on an electrical wire, and allows nerve impulses to travel faster along the axon.
What Does a Neuron Look Like?
A neuron has a special shape that lets it pass signals from one cell to another. As shown in Figure 2, a neuron has three main parts: cell body, dendrites, and axons. The cell body contains the nucleus and other organelles. Dendrites and axons project from the cell body. Dendrites receive nerve impulses from other cells, and axons pass the nerve impulses on to other cells. A single neuron may have thousands of dendrites and axons, so it can communicate with thousands of other cells.
Types of Neurons
Neurons are usually classified based on the role they play in the body. Two types of neurons are sensory neurons and motor neurons.
Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous system (see below).
Motor neurons carry nerve impulses from the central nervous system to internal organs, glands, and muscles.
Both types of neurons work together. Sensory neurons carry information about conditions inside or outside the body to the central nervous system. The central nervous system processes the information and sends messages through motor neurons to tell the body how to respond to the information.
The Synapse
The place where the axon of one neuron meets the dendrite of another is called a synapse. Synapses are also found between neurons and other type of cells, such as muscle cells. The axon of the sending neuron doesn’t actually touch the dendrite of the receiving neuron. There is a tiny gap between them, as shown in Figure below.
Figure 20.3
This diagram shows a synapse between neurons; when a nerve impulse arrives at the tip of the axon, neurotransmitters are released and travel to the receiving dendrite, carrying the nerve impulse from one neuron to the next.
When a nerve impulse reaches the tip of an axon, the axon releases chemicals called neurotransmitters. These chemicals travel across the gap between the axon and the dendrite of the next neuron. They bind to the membrane of the dendrite. This triggers a nerve impulse in the receiving neuron. Did you ever watch a relay race? After the first runner races, she passes the baton to the next runner, who takes over. Neurons are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron. Examples of neurotransmitters include serotonin, dopamine, and adrenaline.
Some people have low levels of the neurotransmitter serotonin in their brain. Scientists think that this is one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives.
Central Nervous System
The central nervous system (CNS) is the largest part of the nervous system. As shown in Figure below, it includes the brain and the spinal cord. The brain is protected within the bony skull. The spinal cord is protected within the bones of the spine, which are called vertebrae.
Figure 20.4
The brain and spinal cord make up the central nervous system.
The Brain
What weighs about 3 pounds (1.5 kilograms) and contains up to 100 billion cells? The answer is the human brain. The brain is the control center of the nervous system. It’s like the pilot of a plane. It tells other parts of the nervous system what to do. The brain is also the most complex organ in the body. Each of its 100 billion neurons has synapses connecting it with thousands of other neurons. All those neurons use a lot of energy. In fact, the adult brain uses almost a quarter of the total energy used by the body. The developing brain of a baby uses an even greater percentage of the body’s total energy.
Figure 20.5
Side view of the brain; find the location of the three major parts of the brain, noting that the cerebrum is divided into four lobes at the upper portion of the brain: the frontal, parietal, temporal, and occipital lobes(Left).Top view of the brain and cerebrum; divided from front to back into two halves, these are the right and left hemispheres(Right).
The brain is the organ that lets us interpret what we see, hear, or sense in other ways. It also allows us to learn, think, remember, and use language. The brain controls all of our internal body processes and external movements, as well. As shown in Figure above, the brain consists of three main parts:
The cerebrum is the largest part of the brain. It lies on top of the brainstem (discussed below). The cerebrum controls functions that we are aware of, such as problem-solving and speech. It also controls voluntary movements, like waving to a friend. Whether you are doing your homework or jumping hurdles, you are using your cerebrum.
The cerebellum is the next largest part of the brain. It lies under the cerebrum and behind the brain stem. The cerebellum controls body position, coordination, and balance. Whether you are riding a bicycle or writing with a pen, you are using your cerebellum.
The brain stem is the smallest of the three main parts of the brain. It lies directly under the cerebrum. The brain stem controls basic body functions such as breathing, heartbeat, and digestion. The brain stem also carries information back and forth between the cerebrum and spinal cord.
The cerebrum is divided into a right and left half, as shown in Figure above. Each half of the cerebrum is called a hemisphere. The two hemispheres are connected by a thick bundle of axons called the corpus callosum. It lies deep inside the brain and carries messages back and forth between the two hemispheres. The right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body. This would be impossible without the corpus callosum.
Dr. Jill Bolte Taylor is a brain scientist. At the age of 37, she suffered massive brain damage when blood vessels burst inside her brain. It took Dr. Taylor almost ten years to recover from the damage to her brain. She had to relearn even basic skills, like walking and talking. To share her story of recovery with others, Dr. Taylor wrote a popular book describing what she went through. Her story gave other people so much inspiration that Time Magazine named her one of the world’s 100 most influential people in 2008.
Each hemisphere of the cerebrum is divided into four parts called lobes. The four lobes are the frontal, parietal, temporal, and occipital lobes (Figure below). Each lobe has different functions. Some of the functions are listed in Table (below).
Cerebral Lobes and Their Functions Lobe Main Function(s)
Frontal Speech, thinking, touch
Parietal Speech, taste, reading
Temporal Hearing, smell
Occipital Sight
The Spinal Cord
The spinal cord is a long, tube-shaped bundle of neurons. It runs from the brain stem to the lower back. The main job of the spinal cord is to carry nerve impulses back and forth between the body and brain. The spinal cord is like a two-way highway. Messages about the body, both inside and out, pass through the spinal cord to the brain. Messages from the brain instructing the body how to respond pass through the spinal cord to the body.
Peripheral Nervous System
The peripheral nervous system (PNS) consists of all the nerves of the body that lie outside the central nervous system. The network of nerves that make up the peripheral system is shown in Figure below. They include nerves of the hands, arms, feet, legs, and trunk. They also include nerves of the scalp, neck, and face. Nerves that supply the internal organs and glands are part of the peripheral nervous system, as well.
Figure 20.6
The blue lines in this drawing represent nerves of the peripheral nervous system; every peripheral nerve is connected directly or indirectly to the spinal cord.
The peripheral nervous system is divided into two parts: the sensory division and the motor division. How these divisions of the peripheral nervous system are related to the rest of the nervous system is shown in Figure below. Refer to the figure as you read more about the peripheral nervous system below.
Figure 20.7
The central nervous system interprets messages from sense organs and internal organs and the motor division sends messages to internal organs, glands, and muscles.
The sensory division carries messages from sense organs and internal organs to the central nervous system. Human beings have several senses. They include sight, hearing, balance, touch, taste, and smell. We have special sense organs for each of these senses. Sensory neurons in each sense organ detect a certain type of stimulus, or input. For example, sensory neurons in the eyes detect light, and sensory neurons in the skin detect touch.
Other animals have senses that humans don’t have. For example, sharks and some other fish can detect weak electric currents. Many animals can detect magnetism. Detecting magnetism is like having an internal compass. It helps the animals find their way from place to place. For example, birds use their sense of magnetism to guide their seasonal migrations.
Our sense organs detect sensations, but they don’t tell us what we are sensing. For example, when you inhale chemicals given off by baking cookies, your nose doesn’t tell you that you are smelling cookies. That’s your brain’s job. The sense organs send messages about sights, smells, and other stimuli to the brain (Figure below). The brain then interprets the messages. A particular area of the brain interprets information from each sense organ (Figure below). For example, information from the nose is interpreted by the temporal lobe of the cerebrum.
Figure 20.8
Remember which lobes of the cerebrum interpret messages from each of the senses; decide which senses would be stimulated by these raspberries or look back at () for clues.
The motor division of the peripheral system carries messages from the central nervous system to internal organs and muscles. As shown in Figure below, the motor division is also divided into two parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system carries messages that control body movements. It’s responsible for activities that are under your control, such as waving your hand or kicking a ball. The girl in Figure below is using her somatic nervous system to control the muscles needed to play the violin. Her brain sends commands to motor neurons that move her hands so she can play. Without the commands from her brain, she wouldn’t be able to move her hands and play the viol
in.
Figure 20.9
This girls central nervous is controlling the movements of her hands and arms as she plays the violin; her brain is sending commands to her somatic nervous system, which controls the muscles of her hands and arms.
The autonomic nervous system carries nerve impulses to internal organs. It is responsible for activities that are not under your control, such as sweating and digesting food. The autonomic nervous system has two divisions:
The sympathetic division controls internal organs and glands during emergencies. It prepares the body for fight or flight (Figure below). For example, it increases the heart rate and the flow of blood to the legs.
The parasympathetic division controls internal organs and glands the rest of the time. It manages routine functions such as digestion, heartbeat, and breathing under normal conditions.
Figure 20.10
The woman pictured here is just pretending to be frightened, but assuming that she really was scared, think of which division of the autonomic nervous system would prepare her body for an emergency.
Remember Michael on his scooter at the start of this lesson? Why was his heart pounding after he regained his balance? The answer is his autonomic nervous system. The sympathetic division prepared him to deal with the emergency by increasing his heart rate. The fact that this happened in the blink of an eye shows how amazing the nervous system is.
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