Biology Lesson 07

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Biology Lesson 15: The Nervous and Endocrine Systems
A spider web? Some sort of exotic bacteria? Maybe an illustration of a new species of jellyfish. This is actually a nerve cell the cell of the nervous system. This cell sends electrical sparks that transmit signals throughout your body. In this chapter you will learn more about nerve cells such as this one and their impressive abilities.
Section 1: The Nervous System
Section Objectives
Vocabulary
Introduction
A small child darts in front of your bike as you race down the street. You see the child and immediately react. You put on the brakes steer away from the child and yell out a warningall in just a split second. How do you respond so quickly? Such rapid responses are controlled by your nervous system. The nervous system is a complex network of nervous tissue that carries electrical messages throughout the body (see Figure below). To understand how nervous messages can travel so quickly you need to know more about nerve cells.
The human nervous system includes the brain and spinal cord (central nervous system) and nerves that run throughout the body (peripheral nervous system).
(image in .pdf file)
Nerve Cells
Although the nervous system is very complex nervous tissue consists of just two basic types of nerve cells: neurons and glial cells. Neurons are the structural and functional units of the nervous system. They transmit electrical signals called nerve impulses. Glial cells provide support for neurons. For example they provide neurons with nutrients and other materials.
Neuron Structure
As shown in Figure below a neuron consists of three basic parts: the cell body dendrites and axon. You can watch an animation of the parts of a neuron at this link: http://www.garyfisk.com/anim/neuronparts.swf.
The structure of a neuron allows it to rapidly transmit nerve impulses to other cells.
(image in .pdf file)
The neuron is discussed at:

Myelin Sheath
The axon of many neurons has an outer layer called a myelin sheath (see Figureabove). Myelin is a lipid produced by a type of a glial cell known as a Schwann cell. The myelin sheath acts like a layer of insulation similar to the plastic that encases an electrical cord. Regularly spaced nodes or gaps in the myelin sheath allow nerve impulses to skip along the axon very rapidly.
Types of Neurons
Neurons are classified based on the direction in which they carry nerve impulses.
(image in .pdf file).
This axon is part of a motor neuron. It transmits nerve impulses to a skeletal muscle causing the muscle to contract.
(image in .pdf file)
Nerve Impulses
Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions which are electrically charged atoms or molecules.
Resting Potential
When a neuron is not actively transmitting a nerve impulse it is in a resting state ready to transmit a nerve impulse. During the resting state the sodium-potassium pump maintains a difference in charge across the cell membrane (see Figure below). It uses energy in ATP to pump positive sodium ions (Na+) out of the cell and negative potassium ions (K-) into the cell. As a result the inside of the neuron is negatively charged while the extracellular fluid surrounding the neuron is positively charged. This difference in electrical charge is called the resting potential.
The sodium-potassium pump maintains the resting potential of a neuron.
(image in .pdf file)
Action Potential
A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential.It begins when the neuron receives a chemical signal from another cell. The signal causes gates in the sodium-potassium pump to open allowing positive sodium ions to flow back into the cell. As a result the inside of the cell becomes positively charged and the outside becomes negatively charged. This reversal of charge ripples down the axon very rapidly as an electric current (see Figurebelow).
An action potential speeds along an axon in milliseconds.
(image in .pdf file)
In neurons with myelin sheaths ions flow across the membrane only at the nodes between sections of myelin. As a result the action potential jumps along the axon membrane from node to node rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.
The action potential is discussed at:

and
https://www.youtube.com/watch?v=7wgb7ggzFNs
You may choose to review the sodium-potassium pump prior to watching the action potential videos.

The Synapse
The place where an axon terminal meets another cell is called a synapse. The axon terminal and other cell are separated by a narrow space known as a synaptic cleft (see Figure below). When an action potential reaches the axon terminal the axon terminal releases molecules of a chemical called a neurotransmitter. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron this starts an action potential in the other cell. You can view animations of neurotransmission at a synapse at the following links:
http://outreach.mcb.harvard.edu/animations/synaptic.swf
http://www.garyfisk.com/anim/neurotransmission.swf
At a synapse neurotransmitters are released by the axon terminal. They bind with receptors on the other cell.
The synapse is further discussed at:

Central Nervous System
The nervous system has two main divisions: the central nervous system and the peripheral nervous system (see Figure below) (image in .pdf file). The central nervous system (CNS) includes the brain and spinal cord (see Figure below) (image in .pdf file)
You can see an overview of the central nervous system at this link:
http://vimeo.com/2024719.
The two main divisions of the human nervous system are the central nervous system and the peripheral nervous system. The peripheral nervous system has additional divisions.
This diagram shows the components of the central nervous system.
(image in .pdf file)
The Brain
The brain is the most complex organ of the human body and the control center of the nervous system. It contains an astonishing 100 billion neurons! The brain controls such mental processes as reasoning imagination memory and language. It also interprets information from the senses. In addition it controls basic physical processes such as breathing and heartbeat. The brain has three major parts: the cerebrum cerebellum and brain stem. These parts are shown in Figure below (image in .pdf file) and described in this section.
You can also take interactive animated tours of the brain at these links:
http://www.pbs.org/wnet/brain/3d/index.html
http://www.garyfisk.com/anim/neuroanatomy.swf
In this drawing assume you are looking at the left side of the head. This is how the brain would appear if you could look underneath the skull.
Each hemisphere of the cerebrum consists of four parts called lobes. Each lobe is associated with particular brain functions. Just one function of each lobe is listed here (image in .pdf file).
Spinal Cord
The spinal cord is a thin tubular bundle of nervous tissue that extends from the brainstem and continues down the center of the back to the pelvis. It is protected by the vertebrae which encase it. The spinal cord serves as an information superhighway passing messages from the body to the brain and from the brain to the body.
Peripheral Nervous System
The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside the central nervous system. It is shown in blue in Figure below. It is connected to the central nervous system by nerves. A nerve is a cable-like bundle of axons. Some nerves are very long. The longest human nerve is the sciatic nerve. It runs from the spinal cord in the lower back down the left leg all the way to the toes of the left foot. Like the nervous system as a whole the peripheral nervous system also has two divisions: the sensory division and the motor division.
The nerves of the peripheral nervous system are shown in yellow in this image. Can you identify the sciatic nerve?
(image in .pdf file)
Somatic Nervous System
The somatic nervous system (SNS) controls mainly voluntary activities that are under conscious control. It is made up of nerves that are connected to skeletal muscles. Whenever you perform a conscious movementfrom signing your name to riding your bikeyour somatic nervous system is responsible. The somatic nervous system also controls some unconscious movements called reflexes. A reflex is a very rapid motor response that is not directed by the brain. In a reflex nerve impulses travel to and from the spinal cord in a reflex arc like the one in Figure below (image in .pdf file). In this example the person jerks his hand away from the flame without any conscious thought. It happens unconsciously because the nerve impulses bypass the brain.
A reflex arc like this one enables involuntary actions. How might reflex responses be beneficial to the organism?
(image in .pdf file)
Autonomic Nervous System
All other involuntary activities not under conscious control are the responsibility of the autonomic nervous system (ANS). Nerves of the ANS are connected to glands and internal organs. They control basic physical functions such as heart rate breathing digestion and sweat production. The autonomic nervous system also has two subdivisions: the sympathetic division and the parasympathetic division. You can watch an animation comparing these two subdivisions at this link:
http://www.garyfisk.com/anim/autonomicns.swf.
The Senses
The sensory division of the PNS includes several sense organsthe eyes ears mouth nose and skin. Each sense organ has special cells called sensory receptors that respond to a particular type of stimulus. For example the nose has sensory receptors that respond to chemicals which we perceive as odors. Sensory receptors send nerve impulses to sensory nerves which carry the nerve impulses to the central nervous system. The brain then interprets the nerve impulses to form a response.
Sight
Sight is the ability to sense light and the eye is the organ that senses light. Light first passes through the cornea of the eye which is a clear outer layer that protects the eye (see Figure below)(image in .pdf file). Light enters the eye through an opening called the pupil. The light then passes through the lens which focuses it on the retina at the back of the eye. The retina contains light receptor cells like those in the photograph on the first page of this chapter. These cells send nerve impulses to the optic nerve which carries the impulses to the brain. The brain interprets the impulses and tells us what we are seeing. To learn more about the eye and the sense of sight you can go to the link below. Be sure to take the quick quiz at the end of the animation.
http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP14304
The eye is the organ that senses light and allows us to see.
(image in .pdf file)
Hearing
Hearing is the ability to sense sound waves and the ear is the organ that senses sound. Sound waves enter the auditory canal and travel to the eardrum (see Figure below)(image in .pdf file). They strike the eardrum and make it vibrate. The vibrations then travel through several other structures inside the ear and reach the cochlea. The cochlea is a coiled tube filled with liquid. The liquid moves in response to the vibrations causing tiny hair cells lining the cochlea to bend. In response the hair cells send nerve impulses to the auditory nerve which carries the impulses to the brain. The brain interprets the impulses and tells us what we are hearing.
The ear is the organ that senses sound waves and allows us to hear. It also senses body position so we can keep our balance.
(image in .pdf file)
Balance
The ears are also responsible for the sense of balance. Balance is the ability to sense and maintain body position. The semicircular canals inside the ear (see Figure above)(image in .pdf file) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response they send nerve impulses to the vestibular nerve which carries the impulses to the brain. The brain interprets the impulses and sends messages to the peripheral nervous system. This system maintains the bodys balance by triggering contractions of skeletal muscles as needed.
Taste and Smell
Taste and smell are both abilities to sense chemicals. Like other sense receptors both taste and odor receptors send nerve impulses to the brain and the brain tells use what we are tasting or smelling. Taste receptors are found in tiny bumps on the tongue called taste buds (see Figure below)(image in .pdf file). There are separate taste receptors for sweet salty sour bitter and meaty tastes. The meaty taste is called umami. You can learn more about taste receptors and the sense of taste by watching the animation at the following link:
http://www.bbc.co.uk/science/humanbody/body/factfiles/taste/taste_ani_f5.swf.
Taste buds on the tongue contain taste receptor cells.
(image in .pdf file)
Odor receptors line the passages of the nose (see Figure below)(image in .pdf file). They sense chemicals in the air. In fact odor receptors can sense hundreds of different chemicals. Did you ever notice that food seems to have less taste when you have a stuffy nose? This occurs because the sense of smell contributes to the sense of taste and a stuffy nose interferes with the ability to smell.
Odor receptors. Odor receptors and their associated nerves (in yellow) line the top of the nasal passages.
(image in .pdf file)
Touch
Touch is the ability to sense of pressure. Pressure receptors are found mainly in the skin. They are especially concentrated on the tongue lips face palms of the hands and soles of the feet. Some touch receptors sense differences in temperature or pain. How do pain receptors help maintain homeostasis? (Hint: What might happen if we couldnt feel pain?)
See the link below for a summary.

Drugs and the Nervous System
A drug is any chemical that affects the bodys structure or function. Many drugs including both legal and illegal drugs are psychoactive drugs. This means that they affect the central nervous system generally by influencing the transmission of nerve impulses. For example some psychoactive drugs mimic neurotransmitters.
Examples of Psychoactive Drugs
Caffeine is an example of a psychoactive drug. It is found in coffee and many other products (see Table below). Caffeine is a central nervous system stimulant. Like other stimulant drugs it makes you feel more awake and alert. Other psychoactive drugs include alcohol nicotine and marijuana. Each has a different effect on the central nervous system. Alcohol for example is a depressant. It has the opposite effects of a stimulant like caffeine.
Caffeine Content of Popular Products
Product
Caffeine Content (mg)
Coffee (8 oz)
130
Tea (8 oz)
55
Cola (8 oz)
25
Coffee ice cream (8 oz)
60
Hot cocoa (8 oz)
10
Dark chocolate candy (1.5 oz)
30
Table 22.2 Many commonly consumed products contain caffeine.
Drug Abuse and Addiction
Psychoactive drugs may bring about changes in mood that users find desirable so the drugs may be abused. Drug abuse is use of a drug without the advice of a medical professional and for reasons not originally intended. Continued use of a psychoactive drug may lead to drug addiction in which the drug user is unable to stop using the drug. Over time a drug user may need more of the drug to get the desired effect. This can lead to drug overdose and death.
Disorders of the Nervous System
There are several different types of problems that can affect the nervous system.
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Section Summary
Extra Practice
1. Tonys dad was in a car accident in which his neck was broken. He survived the injury but is now paralyzed from the neck down. Explain why.
2. Multiple sclerosis is a disease in which the myelin sheaths of neurons in the central nervous system break down. What symptoms might this cause? Why?
3. Explain how resting potential is maintained and how an action potential occurs.
4. Compare and contrast the somatic and autonomic nervous systems.
Points to Consider
In this Section you learned that the nervous system enables electrical messages to be sent through the body very rapidly.
Section 2: The Endocrine System
Section Objectives
Vocabulary
Introduction
The nervous system isnt the only message-relaying system of the human body. The endocrine system also carries messages. The endocrine system is a system of glands that release chemical messenger molecules into the bloodstream. The messenger molecules are hormones. Hormones act slowly compared with the rapid transmission of electrical messages by the nervous system. They must travel through the bloodstream to the cells they affect and this takes time. On the other hand because endocrine hormones are released into the bloodstream they travel throughout the body. As a result endocrine hormones can affect many cells and have body-wide effects.
Glands of the Endocrine System
The major glands of the endocrine system are shown in Figure below (image in .pdf file).
You can access an a similar animated endocrine system chart at:
http://www.abpischools.org.uk/page/modules/hormones/horm2.cfm
The glands of the endocrine system are the same in males and females except for the testes which are found only in males and ovaries which are found only in females.
Hypothalamus
The hypothalamus is actually part of the brain (see Figure below)(image in .pdf file) but it also secretes hormones. Some of its hormones that tell the pituitary gland to either secrete or stop secreting its hormones. In this way the hypothalamus provides a link between the nervous and endocrine systems. The hypothalamus also produces hormones that directly regulate body processes. These hormones travel to the pituitary gland which stores them until they are needed. The hormones include antidiuretic hormone and oxytocin.
The hypothalamus and pituitary gland are located close together at the base of the brain.
(image in .pdf file)
Pituitary Gland
The pea-sized pituitary gland is attached to the hypothalamus by a thin stalk (see Figure above)(image in .pdf file). It consists of two bulb-like lobes. The posterior (back) lobe stores hormones from the hypothalamus. The anterior (front) lobe secretes pituitary hormones. Several pituitary hormones and their effects are listed in Table below. Most pituitary hormones control other endocrine glands. Thats why the pituitary is often called the master gland of the endocrine system.
Pituitary Hormones
Hormone
Target
Effect(s)
Adrenocorticotropic hormone (ACTH)
Adrenal glands
Stimulates the cortex of each adrenal gland to secrete its hormones
Thyroid-stimulating hormone (TSH)
Thyroid gland
Stimulates the thyroid gland to secrete thyroid hormone
Growth hormone (GH)
Body cells
Stimulates body cells to synthesize proteins and grow
Follicle-stimulating hormone (FSH)
Ovaries testes
Stimulates the ovaries to develop mature eggs; stimulates the testes to produce sperm
Luteinizing hormone (LH)
Ovaries testes
Stimulates the ovaries and testes to secrete sex hormones; stimulates the ovaries to release eggs
Prolactin (PRL)
Mammary glands
Stimulates the mammary glands to produce milk
Table 22.4 Hormones secreted by the pituitary gland control many body processes often by regulating other endocrine glands.
Other Endocrine Glands
Other glands of the endocrine system are described below. You can refer to Figure above to see where they are located.
http://www.abpischools.org.uk/page/modules/hormones/horm8.cfm?coSiteNavigation_allTopic=1
How Hormones Work
Endocrine hormones travel throughout the body in the blood. However each hormone affects only certain cells called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins that are specific to that hormone. A hormone travels through the bloodstream until it finds a target cell with a matching receptor it can bind to. When the hormone binds to a receptor it causes a change within the cell. Exactly how this works depends on whether the hormone is a steroid hormone or a non-steroid hormone.
You can watch an animation that shows how both types of hormones work at:
http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP13704
Hormones are discussed at:

Steroid Hormones
Steroid hormones are made of lipids such as phospholipids and cholesterol. They are fat soluble so they can diffuse across the plasma membrane of target cells and bind with receptors in the cytoplasm of the cell (see Figure below)(image in .pdf file). The steroid hormone and receptor form a complex that moves into the nucleus and influences the expression of genes. Examples of steroid hormones include cortisol and sex hormones.
A steroid hormone crosses the plasma membrane of a target cell and binds with a receptor inside the cell.
(image in .pdf file)
Non-Steroid Hormones
Non-steroid hormones are made of amino acids. They are not fat soluble so they cannot diffuse across the plasma membrane of target cells. Instead a non-steroid hormone binds to a receptor on the cell membrane (see Figure below)(image in .pdf file). The binding of the hormone triggers an enzyme inside the cell membrane. The enzyme activates another molecule called the second messenger which influences processes inside the cell. Most endocrine hormones are non-steroid hormones including insulin and thyroid hormones.
A non-steroid hormone binds with a receptor on the plasma membrane of a target cell. Then a secondary messenger affects cell processes.
(image in .pdf file)
Hormone Regulation: Feedback Mechanisms
Hormones control many cell activities so they are very important for homeostasis. But what controls the hormones themselves? Most hormones are regulated by feedback mechanisms. A feedback mechanism is a loop in which a product feeds back to control its own production. Most hormone feedback mechanisms involve negative feedback loops. Negative feedback keeps the concentration of a hormone within a narrow range.
Negative Feedback
Negative feedback occurs when a product feeds back to decrease its own production. This type of feedback brings things back to normal whenever they start to become too extreme. The thyroid gland is a good example of this type of regulation. It is controlled by the negative feedback loop shown in Figurebelow(image in .pdf file).
You can also watch an animation of this process at:
http://biologyinmotion.com/thyroid/
The thyroid gland is regulated by a negative feedback loop. The loop includes the hypothalamus and pituitary gland in addition to the thyroid.
(image in .pdf file)
Heres how thyroid regulation works. The hypothalamus secretes thyrotropin-releasing hormone or TRH. TRH stimulates the pituitary gland to produce thyroid-stimulating hormone or TSH. TSH in turn stimulates the thyroid gland to secrete its hormones. When the level of thyroid hormones is high enough the hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH the thyroid gland stops secreting its hormones. Soon the level of thyroid hormone starts to fall too low. What do you think happens next?
This process is discussed at:

Negative feedback also controls insulin secretion by the pancreas. You can interact with a feedback loop of this process at:
http://www.abpischools.org.uk/page/modules/hormones/horm6.cfm?coSiteNavigation_allTopic=1
Positive feedback
Positive feedback occurs when a product feeds back to increase its own production. This causes conditions to become increasingly extreme. An example of positive feedback is milk production by a mother for her baby. As the baby suckles nerve messages from the nipple cause the pituitary gland to secrete prolactin. Prolactin in turn stimulates the mammary glands to produce milk so the baby suckles more. This causes more prolactin to be secreted and more milk to be produced. This example is one of the few positive feedback mechanisms in the human body. What do you think would happen if milk production by the mammary glands was controlled by negative feedback instead?
Endocrine System Disorders
Diseases of the endocrine system are relatively common. An endocrine disease usually involves the secretion of too much or not enough hormone. When too much hormone is secreted it is called hypersecretion. When not enough hormone is secreted it is called hyposecretion.
Hypersecretion
Hypersecretion by an endocrine gland is often caused by a tumor. For example a tumor of the pituitary gland can cause hypersecretion of growth hormone. If this occurs in childhood it results in very long arms and legs and abnormally tall stature by adulthood. The condition is commonly known as gigantism (see Figurebelow) (image in .pdf file).
Hyposecretion
Destruction of hormone-secreting cells of a gland may result in not enough of a hormone being secreted. This occurs in Type 1 diabetes. In this case the bodys own immune system attacks and destroys cells of the pancreas that secrete insulin. A person with type 1 diabetes must frequently monitor the level of glucose in the blood (see Figure below) (image in .pdf file). If the level of blood glucose is too high insulin is injected to bring it under control. If it is too low a small amount of sugar is consumed. To measure the level of glucose in the blood a drop of blood is placed on a test strip which is read by a meter.
Hormone Resistance
In some cases an endocrine gland secretes a normal amount of hormone but target cells do not respond to the hormone. Often this is because target cells have because resistant to the hormone. Type 2 diabetes is an example of this type of endocrine disorder. In Type 2 diabetes body cells do not respond to normal amounts of insulin. As a result cells do not take up glucose and the amount of glucose in the blood becomes too high. This type of diabetes cannot be treated by insulin injections. Instead it is usually treated with medication and diet.
Section Summary
Points to Consider
In this Section you learned that endocrine hormones can affect cells throughout the body because they travel in the blood through the circulatory system.
Opening image copyright by Sebastian Kaulitzki 2010. Used under license from Shutterstock.com.
Lesson 15 Review Questions
Directions: Answer each of the following questions.
1. List and describe the parts of a neuron.
2. What do motor neurons do?
3. Define resting potential.
4. Name the organs of the central nervous system.
5. Which part of the brain controls conscious functions such as reasoning?
6. What are the roles of the brain stem?
7. Identify the two major divisions of the peripheral nervous system.
8. List five human senses.
9. What is a psychoactive drug? Give two examples.
10. Define drug abuse. When does drug addiction occur?
11. Identify three nervous system disorders.
12. Define hormone.
13. List the major glands of the endocrine system.
Essay submission: Select 1 Biology topic from this lesson and submit a 3-5 paragraph essay about the topic. Remember to cite your sources!