Environmental Exposure
Any substance that interferes with cellular function and causes cellular malfunction is a cellular toxin. There are many different sources of toxins, for example, natural or synthetic drugs, plants, and animal bites. Air pollution, another form of environmental exposure to toxins is shown in Figure below. A commonly seen example of an exposure to cellular toxins is by a drug overdose. When a person takes too much of a drug that affects the central nervous system, basic life functions such as breathing and heartbeat are disrupted. Such disruptions can results in coma, brain damage, and even death.
Figure 19.10
Air pollution can cause environmental exposure to cellular toxins such as mercury.
The six factors described above have their effects at the cellular level. A deficiency or lack of beneficial pathways, whether caused by an internal or external influence, will almost always result in a harmful change in homeostasis. Too much toxicity also causes homeostatic imbalance, resulting in cellular malfunction. By removing negative health influences and providing adequate positive health influences, your body is better able to self-regulate and self-repair, which maintains homeostasis.
Lesson Summary
Homeostasis is an organism’s ability to maintain a stable internal environment. Homeostasis is an important characteristic of living things. Keeping a stable internal environment requires constant adjustments as conditions change inside and outside the cell.
Feedback regulation mechanisms are important to homeostasis. Feedback regulation occurs when the response to a stimulus has an effect of some kind on the original stimulus. The type of response (increase or decrease in the stimulus) determines what the feedback is called.
Negative feedback occurs when the response to a stimulus reduces the original stimulus. Positive feedback occurs when the response to a stimulus increases the original stimulus.
No system of the body works in isolation, and the well-being of a person depends upon the well-being of all the interacting body systems. A disruption within one system generally has consequences for several additional body systems.
The homeostatic balance of most organs and organ systems is controlled by hormones secreted from the pituitary gland, a part of the endocrine system.
When the cells in your body do not work correctly, homeostatic balance is disrupted. Homeostatic imbalance may lead to a state of disease. Type 2 diabetes is a disease in which homeostasis of the blood glucose level is disturbed, leading to an imbalance that affect many other body systems, including the cardiovascular system.
Review Questions
Outline the importance of homeostasis to an organism.
How do feedback mechanisms help maintain homeostasis?
What is the difference between negative and positive feedback?
Identify and give an example of two organ systems working together to maintain homeostasis.
Summarize the role of the endocrine system in homeostasis.
Name two diseases that can result from an imbalance in body homeostasis.
Why is positive feedback not an effective way of controlling hormone levels?
Further Reading / Supplemental Links
Ostrom, K.M., Department of Nutritional Sciences, University of Connecticut, Storrs.
A review of the hormone prolactin during lactation.Prog Food Nutr Sci. 1990; 14(1):1-43.
Anatomy and Physiology © 2002 by Elaine N. Marieb. Published by Benjamin Cummings.
http://www.medscape.com/medline/abstract/2092340
http://www.cotc.edu/ebrisker/PDF%20files/49%206%20endocrine%20LECTURE.pdf
http://en.wikibooks.org/wiki/Human_Physiology
http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html
http://www.health.gov/dietaryguidelines/
http://en.wikipedia.org
Vocabulary
homeostasis
Stability, balance, or equilibrium within the cell or a body; an organism’s ability to keep a constant internal environment.
negative feedback
Occurs when the response to a stimulus reduces the original stimulus.
positive feedback
Occurs when the response to a stimulus increases the original stimulus.
Points to Consider
Negative feedback is most common feedback loop in biological systems. The system acts to reverse the direction of change. Positive feedback is less common in biological systems. The system acts to speed up the direction of change. Consider how your social interactions with teachers, parents and other students may be classified as either positive or negative feedback.
When homeostasis is interrupted, your body can correct or worsen the problem, based on certain influences. In addition to genetic influences, there are external influences that are based on lifestyle choices and environmental exposures. Describe how your lifestyle may positively or negatively affect your body's ability to maintain homeostasis
Chapter 20: Nervous and Endocrine Systems
Lesson 20.1: The Nervous System
Lesson Objectives
Identify the type of cells that make up nervous tissue.
Describe the structure of a neuron.
Relate membrane potential to action potential.
Outline the role of neurotransmitters in neuron communication.
Distinguish between the sensory and motor divisions of the peripheral nervous system.
Describe the structure of the eye and identify the roles of rods and cones in vision.
Describe the structure of the ear and identify the structures that are important to hearing and balance.
Distinguish between the receptors for pain, pressure, and temperature.
Identify the main effect of psychoactive drugs on the CNS.
Summarize the mechanism of addiction.
Introduction
Your body has two systems that help you maintain homeostasis: the nervous system and the endocrine system. The nervous system is a complex network of nervous tissue that sends electrical and chemical signals. The nervous system includes the central nervous system (CNS) and the peripheral nervous system (PNS) together. The central nervous system is made up of the brain and spinal cord, and the peripheral nervous system is made up of the nervous tissue that lies outside the CNS, such as the nerves in the legs, arms, hands, feet and organs of the body. The nervous system mediates communication between different parts of the body as well as the body’s interactions with the environment.
The endocrine system is a system of glands around the body that release chemical signal molecules into the bloodstream. The electrical signals of the nervous system move very rapidly along nervous tissue, while the chemical signals of the endocrine system act slowly in comparison and over a longer period of time. Working together, the nervous and endocrine systems allow your body to respond to short or long term changes in your environment, such as a pedestrian suddenly stepping out in front of your bike, or your body adapting to cycling in a warm, humid summer evening, as shown in Figure below.
Figure 20.1
Cycling home in rush-hour traffic demands a lot of your nervous and endocrine systems. Your nervous systemmostly through your eyes and earsconstantly monitors your surroundings, alerting you instantly at any sign of change or danger. Your endocrine system gears up your muscles and cardiovascular system for the ride, by flooding your body with metabolism-boosting hormones.
Nerve Cells
Although the nervous system is very complex, there are only two main types of nerve cells in nervous tissue. All parts of the nervous system are made of nervous tissue. The neuron is the "conducting" cell that transmits electrical signals, and it is the structural unit of the nervous system. The other type of cell is a glial cell. Glial cells provide a support system for the neurons, and recent research has discovered they are involved in synapse formation. A type of glial cell in the brain, called astrocytes, is important for the maturation of neurons and may be involved in repairing damaged nervous tis
sue. Neurons and glial cells make up most of the brain, the spinal cord and the nerves that branch out to every part of the body. Both neurons and glial cells are sometimes referred to as nerve cells.
Structure of a Neuron
The special shape of a neuron allows it to pass an electrical signal to another neuron, and to other cells. Electrical signals move rapidly along neurons so that they can quickly pass “messages” from one part of the body to another. These electrical signals are called nerve impulses.
Neurons are typically made up of a cell body (or soma), dendrites, and an axon, as shown in Figure below. The cell body contains the nucleus and other organelles similar to other body cells. The dendrites extend from the cell body and receive a nerve impulse from another cell. The cell body collects information from the dendrites and passes it along to the axon. The axon is a long, membrane-bound extension of the cell body that passes the nerve impulse onto the next cell. The end of the axon is called the axon terminal. The axon terminal is the point at which the neuron communicates with the next cell. You can say the dendrites of the neuron receive the information, the cell body gathers it, and the axons pass the information onto another cell.
Figure 20.2
The general structure of a neuron. Neurons come in many different shapes and sizes, but they all have a cell body, dendrites, and an axon. The cell body contains a nucleus and other organelles. However, not all neurons have a myelin sheath.
The axons of many neurons are covered with an electrically insulating phospholipid layer called a myelin sheath. The myelin speeds up the transmission of a nerve impulse along the axon. It acts like a layer of insulation, like the plastic you would see around an electrical cord.
The myelin is an outgrowth of glial cells. Schwann cells which are shown wrapped around the neuron in Figure above, are a type of glial cell. Schwann cells are flat and thin, and like other cells, contain a nucleus and other organelles. Schwann cells supply the myelin for neurons that are not part of the brain or spinal cord, while another type of glial cell, called oligodendrocytes, supply myelin to those of the brain and spinal cord. Myelinated neurons are white in appearance, and they are what makes up the "white matter" of the brain. A cross section of a myelinated neuron is shown in Figure below. Myelin is not continuous along the axon. The regularly spaced gaps between the myelin are called Nodes of Ranvier. The nodes are the only points at which ions can move across the axon membrane, through ion channels. In this way the nodes act to strengthen the nerve impulse by concentrating the flow of ions at the nodes of Ranvier along the axon.
Figure 20.3
A transmission electron microscope (TEM), image of a cross section of a myelinated axon. The rings around the axon are made up of Schwann cell membrane, which is wrapped many times around the axon.
Neurons are specialized for the passing of cell signals. Given the many functions carried out by neurons in different parts of the nervous system, there are many different of shapes and sizes of neurons. For example, the cell body of a neuron can vary from 4 to 100 micrometers in diameter. Some neurons can have over 1,000 dendrite branches, which make connections with tens of thousands of other cells. Other neurons have only 1 or 2 dendrites, each of which has thousands of synapses. A synapse is a specialized junction at which neurons communicate with each other, and is shown in Figure below. Also, a neuron may have 1 or many axons. The longest axon of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the spinal cord, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks.
Figure 20.4
The location of synapses. Synapses are found at the end of the axons (called axon terminals) and help connect a single neuron to thousands of other neurons. Chemical messages called neurotransmitters are released at the synapse and pass the message onto the next neuron or other type of cell.
Nerve Impulses
In the late 18th century, the Italian doctor and physicist Luigi Galvani first recorded the action of electricity on the muscle tissue of frogs. He noted that an electrical charge applied to a nerve in the legs of a dead frog made the legs move. Galvani attributed the movement of the frog’s muscles to an electrical current that was carried by the nerves. Galvani coined the term "animal electricity" to describe this vital force for life.
Galvani believed that animal electricity came from the muscle and was unique to living creatures. However, his fellow Italian, and physicist, Alessandro Volta disagreed with him and reasoned that animal electricity was a physical phenomenon, that occurred between metals. Volta disproved Galvani’s claim by building the first battery, which showed that a current could flow outside an organism’s body. Since then scientists have learned much about electrical charges in living systems.
Ion Channels and Nerve Impulses
Ion transport proteins have a special role in the nervous systems because voltage-gated ion channels and ion pumps are essential for forming a nerve impulse. Ion channels use energy to build and maintain a concentration gradient of ions between the extracellular fluid and the cell’s cytosol, as shown in Figure below. This concentration gradient results in a net negative charge on the inside of the membrane and a positive charge on the outside. Ion channels and ion pumps are very specific; they allow only certain ions through the cell membrane. For example, potassium channels will allow only potassium ions through, and the sodium-potassium pump acts only on sodium and potassium ions.
All cells have an electrical charge which is due to the concentration gradient of ions that exists across the membrane. The number of positively charged ions outside the cell membrane is greater than the number of positively charged ions in the cytosol. This difference causes a voltage difference across the membrane. Voltage is electrical potential energy that is caused by a separation of opposite charges, in this case across the membrane. The voltage across a membrane is called membrane potential. Membrane potential is the basis for the conduction of nerve impulses along the cell membrane of neurons. Ions that are important in the formation of a nerve impulse include sodium (Na+) and potassium (K+).
Figure 20.5
Channel proteins in the plasma membrane. Membrane channel proteins (or channel proteins), allow the movement of specific ions across the cell membrane, in this case the hypothetical X ion. The concentration gradient results in electrical potential energy building up across the membrane, the basis for the conduction of a nerve impulse.
Resting Potential
When a neuron is not conducting a nerve impulse, it is said to be at rest. The resting potential is the resting state of the neuron, during which the neuron has an overall negative charge. In neurons the resting potential is approximately -70 milliVolts (mV). The negative sign indicates the negative charge inside the cell relative to the outside.
The reasons for the overall negative charge of the cell include:
The sodium-potassium pump removes Na+ ions from the cell by active transport. A net negative charge inside the cell is due to the higher concentration of Na+ ions outside the cell than inside the cell.
Most cells have potassium-selective ion channel proteins that remain open all the time. The K+ ions move down the concentration gradient (passively) through these potassium channels and out of the cell, which results in a build-up of excess positive charge outside of the cell.
There are a number of large, negatively charged molecules, such as proteins, inside the cell.
Action Potential
An action potential is an electrical charge that travels along the membrane of a neuron. It can be generated when a neuron’s membrane potential is changed by chemical signals from a nearby cell. In an action potential, the cell membrane potential changes quickly from negative to positive as sodium ions flow into and potassium ions flow out of the cell through ion channels, as shown in Figure below.
The change in membrane potential results in the cell becoming d
epolarized. An action potential works on an all-or-nothing basis. That is, the membrane potential has to reach a certain level of depolarization, called the threshold, otherwise an action potential will not start. This threshold potential varies, but is generally about 15 millivolts (mV) more positive than the cell's resting membrane potential. If a membrane depolarization does not reach the threshold level, an action potential will not happen. You can see in Figure below how two depolarizations did not reach the threshold level of -55mV. The first channels to open are the sodium ion-channels, which allow sodium ions to enter the cell. The resulting increase in positive charge inside the cell (up to about +40 mV) starts the action potential. Potassium ion-channels then open up, allowing potassium ions out of the cell, which ends the action potential. Both of the ion channels then close, and the sodium-potassium pump restores the resting potential of -70 mV. The action potential will move down the axon toward the synapse like a wave would move along the surface of water.
In myelinated neurons, ion flows occur only at the nodes of Ranvier. As a result, the action potential signal "jumps" along the axon membrane, from node to node, rather than spreading smoothly along the membrane, as they do in axons that do not have a myelin sheath. This is due to clustering of Na+ and K+ ion channels at the Nodes of Ranvier. Unmyelinated axons do not have Nodes of Ranvier; and ion channels in these axons are spread over the entire membrane surface.
CK-12 Biology I - Honors Page 92