Biology: An Interactive Tour
Biology: An Interactive Tour

Biology: An Interactive Tour

Lead Author(s): Robert Pozos

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Biology: An Interactive Tour is for a non-majors audience in technology-enhanced learning and makes the complex world of biological science approachable and relatable.

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Top Hat has reimagined the textbook – one that is designed to improve student readership through interactivity, is updated by a community of collaborating professors with the newest information, and accessed online from anywhere, at anytime.


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Key features in this textbook

Biology: An Interactive Tour introduces the content in a much more approachable way than traditional texts.

Includes homework sets with 30+ questions per chapter.

Embedded videos that apply biology concepts to the real world!

Comparison of Introduction to Biology Textbooks

Consider adding Biology: An Interactive Tour to your upcoming course. We’ve put together a textbook comparison to make it easy for you in your upcoming evaluation.

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Pricing

Average price of textbook across most common format

Up to 40-60% more affordable

Lifetime access on any device

$63

E-book

$126

Hardcover print text only

$117

E-book

$166.95

Hardcover print text only

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

In-Book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Only available with supplementary resources at additional cost

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Built-In Interactive Assessment Questions

Assessment questions with feedback embedded throughout textbook

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Pricing

Average price of textbook across most common format

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Up to 40-60% more affordable

Lifetime access on any device

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

$63

E-book

$126

Hardcover print text only

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

$117

E-book

$166.95

Hardcover print text only

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

$95.95

E-book

$173.85

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Macmillan

Shuster, Michele & Janet Vigna & Matthew Tontonoz, Biology for a Changing World

Only available with supplementary resources at additional cost

Hard Copy

Taylor, Martha R., et al., Campbell Biology: Concepts & Connections

Pearson

Colleen Belk, & Virginia Borden Maier, Biology: Science for Life, 6th Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Bob, Pozos, “Biology: An Interactive Tour”, Only One Edition needed

Pearson

Marieb & Hoehn – Human Anatomy and Physiology, 10th Edition

Wiley

Gerard Tortoria & Bryan Dickerson, Principles of Anatomy & Physiology, 14th Edition

McGraw-Hill

Kenneth Saladin, Anatomy and Physiology: The Unity of Form and Function, 7th Edition

About this textbook

Lead Author

Robert PozosUniversity of Minnesota-Duluth School of Medicine

Robert Pozos has extensive experience studying human response to environments resulting in hypothermia and hyperthermia. He established the hypothermia laboratory at University of Minnesota-Duluth School of Medicine and was a part of the chief civilian scientists at Naval Health Research Center in San Diego where he evaluated the thermal effectiveness of military garb for combat operations.

Contributing Authors

Christina AlevrasUniversity of Saint Joseph

Marion McClaryFairleigh Dickinson University

Explore this textbook

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Chapter 27: Central Nervous System

Figure 27.1: The nervous system is composed of specialized cells known as neurons. Neurons have dendrites and axons, projections that allow for communication in the nervous system. [1]

The Central Nervous System coordinates all our sensory and motor systems. In addition, it plays a major role in defining who we are.

Concept Map

Central Nervous System: This diagram demonstrates the four main concepts of the chapter.


27.1 The Neuron

Concept 27.1: The neuron is the functional unit of the Central Nervous System. It is because of communication among neurons that we are able to think, behave, perceive and be who we are as individuals.

The central nervous system serves as the integration center for the sensory information from the external and internal environment. It receives information from receptors throughout the body and coordinates the motor activity of all parts of the body. It not only coordinates, it thinks and plays a major role in terms of your self-identity. The CNS comprises the majority of the nervous system and consists of the brain and the spinal cord. However, it does not include the nerves and ganglia that are outside the brain or spinal cord—the peripheral nervous system (PNS). The PNS is divided into the somatic nervous system and the autonomic nervous system which will be discussed in a separate chapter. 


27.1.1 Introduction to the Neuron

Neurons vary in shape and size and have many of the same organelles as other cells. They have mitochondria that produce ATP, nuclei that contain DNA, and ribosomes that produce functional proteins. What makes neurons unique are their surface extensions which can be very long. There are two kinds of extensions: dendrites and axons.

CH29NeuronDetailed_V1.png
Figure 27.2: Anatomy of the Neuron: The neuron has the basic organelles integral to all cells. It also has fingerlike projections of its cell membrane known as dendrites and axons that allow the neuron to respond to stimuli. Myelin ensures efficient propagation of an action potential down the length of the axon.


27.1.1.1 Dendrites

The multiple “branches” that extend from the neuronal cell body are the dendrites, which are the “receivers” that gather signals from other neurons or from sensory stimuli. The dendrites serve as a large receiving station since they receive information from many (millions!) neurons and the axons serve as the command signal leaving the neuron to control muscle, glands or other neurons.

27.1.1.2 Axon

The single long branch that extends farther than the rest is known as the axon. Action potentials move down the axon to its terminal ends, which are called axon terminals.

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27.1.1.3 Myelin, Schwann, and Nodes of Ranvier

Neurons that are required for fast transmission of the action potential from the nerve cell body to the synapse have myelin sheath which is a specialized coating. Myelin is a lipid covering that is placed around the axon by Schwann cells. Schwann cells wrap the axon, but they also leave certain areas unwrapped which are called the node of Ranvier.

Figure 27.3: Myelin encloses the axon of the neuron and increases the speed of the action potential.


Question 27.03

Which neuron would you expect to have myelin wrapping around its axon?

A

A short neuron because myelin helps to propagate a message over short distances

B

A long neuron because myelin helps to propagate a message over significant distances.

C

All neurons are wrapped in myelin to help propagate messages over their distance.

Myelin's value is that it allows for the rapid movement of the action potential to activate the muscle. The action potential jumps from one node to another rather than just moving incrementally.

Non-myelinated fibers are also important. They are found in the gray matter of the brain and also in parts of the autonomic nervous system, which is responsible for controlling involuntary function.

Figure 27.4: Myelinated vs. non-myelinated neurons. Myelinated neurons generate action potentials that travel faster than non-myelinated neurons since they "jump" from node to node.


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27.1.1.4 Axon Terminals (Axon Tip)

The axon terminals interact with other neurons or cells such as muscle to transmit chemical or electrical signals to those cells. The branches at the axon terminals allow one neuron to simultaneously communicate with many other neurons, or with other cell types such as muscle cells or gland cells.

27.1.1.5 Neuron Classifications

Neurons are classified into three types:

  • Sensory neurons
  • Interneurons
  • Motor neurons

27.1.1.6 Sensory Neurons

Sensory neurons transmit information from external stimuli, like sound, touch, smell, sight, and taste, by converting the signal from the physical stimuli into electrical and chemical signals. They can also respond to internal stimuli such as blood pressure, the position of a joint, or the orientation of the head.

27.1.1.7 Interneurons

Interneurons exchange information between neurons and perform complex computations that produce thought, complex motor movements and behavior. All interneurons are stationed within the CNS.

27.1.1.8 Motor Neurons

Motor neurons carry specific instructions to effector cells such as skeletal muscles. For example, they cause muscles to contract or gland cells to secrete various factors. The words, “information” and “instructions” refer to electrical and chemical signals produced by these three kinds of neurons. You should be aware that there are subsets to these classifications and that only the general categories have been presented.

27.1.2 Glial Cells

Figure 27.5: Glial cells are non-neuronal cells that help maintain neurons.

There is another set of cells that interact with the neurons, called Glial cells, which outnumber neurons approximately 10 to 1. There are three types of glia:

  • Astrocytes
  • Microglia
  • Oligodendrocytes. 

27.1.2.1 Astrocytes

Astrocytes help maintain a proper chemical environment for neurons and help maintain the blood-brain barrier which protects the neurons from different chemicals.

27.1.2.2 Microglia

The microglia function like immune cells and play a major role in terms of inflammation. They also remove debris after an immune system response.

27.1.2.3 Ependymal Cells

Ependymal cells are involved in the production of the cerebrospinal fluid.

27.1.2.4 Oligodendrocytes

Oligodendrocytes lay down myelin, which is a lipid structure that surrounds the axon of a neuron to insulate it and to increase the transmission speed of electrical signals. Patients with multiple sclerosis have areas in their nervous system where the myelin is not uniformly spread, resulting in a multitude of problems depending on the location of the depleted myelin.

Question 27.05

Which of the following supporting cells would best be characterized as chemical "recyclers" of the brain?

A

Astrocytes

B

Microglia

C

Ependymal cells

D

Oligodendrocytes


Question 27.06

Which of the following supporting cells would you expect to multiply in response to a brain injury?

A

Astrocytes

B

Microglia

C

Ependymal cells

D

Oligodendrocytes


27.1.3 Blood Flow to the Neurons: The Blood-Brain Barrier

The brain requires 15-20% of the blood ejected from each pulse of the heart. However, the neurons have a protective barrier, called the blood-brain barrier that separates the blood from the neurons. It is composed of specialized brain cells called astrocytes that wrap around the capillaries. Although certain important chemicals are allowed through, such as oxygen and glucose, others are excluded.

The blood-brain barrier is a mixed blessing. When new chemicals are developed to help patients who suffer from different brain diseases, one of the challenges is to pass through the blood-brain barrier!

Figure 27.6: The blood-brain barrier selectively allows in certain chemicals from the blood vessels to brain tissue.
Question 27.07

A group of scientists in your company is making a completely artificial, non-organic brain that will drive robots. Which property would the artificial brain not have relative to a natural brain?

A

Chemical synapses

B

Electrical synapses

C

Protein production in synthetic neurons


27.2 Neuronal Communication

Concept 27.2: Neurons communicate by using electrical and chemical signals. Electrical signals known as action potentials are generated by neurons and then communicated to neighboring neurons via chemical messengers known as neurotransmitters.

27.2.1 The Synapse

There exist billions of interconnection sites between neurons. These sites of interconnections are called synapses, and the synapses allow signals to travel from neuron to neuron in a matter of milliseconds. The synapse consists of a presynaptic neuron, the postsynaptic neuron, and the neurotransmitters.

27.2.1.1 The Presynaptic and Postsynaptic Neuron

The presynaptic neuron is the cell that will send a signal (in the form of chemicals) to the postsynaptic neuron which is the receiving cell.

Figure 27.7: The synapse is a space between neurons in which chemical signals are transmitted for cellular communication.

27.2.1.2 The Neurotransmitters

The chemicals that are released at the synapse are called neurotransmitters. There are two types of neurotransmitters: inhibitory and excitatory. The inhibitory neurotransmitters are those that cause the postsynaptic membrane to become more negative, whereas the excitatory neurotransmitter will cause the postsynaptic membrane to become more positive. There are approximately 51 transmitters and they are categorized as either rapid-acting or slow acting. They are produced by neurons and can either affect other neurons or can be sent into the bloodstream. These transmitters are called hormones and will be discussed in the chapter on hormones.


Question 27.08

What must happen to the electrical message, aka action potential, for it to cross the synapse and communicate with the post-synaptic cell?

A

The electrical message is translated into a chemical message

B

The electrical message remains in the form of an electrical message as it crosses the synapse

C

The presynaptic and postsynaptic cells do not communicate


27.2.2 Membrane Potentials (Voltages): Resting, Inhibitory, Excitatory and Action

The membrane potential is an electrical signal that is recorded when an electrode is placed inside the cell close to the membrane. The units are measured in millivolts. Hence the name: membrane potential which means membrane voltage. There are two main types of membrane potentials: resting membrane potential and action potential.

27.2.2.1 Resting Membrane Potential (RMP)

A neuron at rest is said to maintain a resting membrane potential. The resting membrane potential is generated by the Sodium/Potassium ATPase molecule. This molecule pumps three Sodium Ions out of the neuron for every two Potassium Ions into the neuron. Given that these ions are pumped against their concentration gradients, the ATPase molecule uses ATP hydrolysis to power this distribution. Furthermore, there is an unequal distribution of charge across the neuronal cell: three positive charges out of the neuron, carried by the sodium for every two positive charges into the neuron, carried by the potassium ion. Hence, there is a membrane potential that is generated across the neuronal cell. This membrane potential is negative - with respect to the exterior environment and is known as the resting membrane potential.

The neurotransmitters released by the presynaptic neuron trigger an electrical signal in the postsynaptic neuron. However, each membrane has a resting membrane potential(RMP), which refers to the fact that the membrane has an electrical signal that is approximately minus 30 to minus 60 millivolts. This value is constantly changing, becoming more slightly more positive or negative depending on the neurotransmitters that are being secreted onto the membrane. The RMP is approximately -60 mv which means that the inside of the neuron is minus relative to the outside. This negativity is determined by the concentration of potassium ion inside the cell relative to the outside. 


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27.2.2.2 Inhibitory and Excitatory Postsynaptic Membrane Potentials

The neurons have millions of synapses and each synapse is presenting different chemicals to the postsynaptic membrane. The electrical changes that occur at the membrane are localized. Thus you can have areas of the postsynaptic membrane that are positive (positive postsynaptic potentials) and others that are negative (negative postsynaptic potential). Only if there are enough positive charges will the postsynaptic membrane produce an action potential. Conversely, if there is a summation of negative charges, the postsynaptic membrane will not produce an action potential. The sequence is as follows:

  • Electrical signal from the presynaptic neuron
  • Release of a neurochemical from the presynaptic site
  • Attachment of the neurotransmitters to the receptor on postsynaptic site
  • Production of an electrical signal that is dependent on the type (inhibitory/excitatory) and the number of synapses.

If the neurotransmitter is excitatory, the potential on the postsynaptic site will be positive. If there are enough positive charges they will summate and you have an action potential. If on the other hand, the neurochemical causes a negative charge on the postsynaptic site, the postsynaptic neuron will not produce an action potential.

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27.2.2.3 Action Potential (AP)

The term “potential” is always confusing. It has nothing to do with the ability to do anything, it refers to voltage. Whenever you see the word “potential” relative to neurons it refers to an electrical voltage recorded at the inside of the membrane of the neuron.

Imagine an electrical signal like a bolt of lightning being generated on a high power line. It then travels from that initial site to other parts of the wire simultaneously. That is what an AP does. The voltage travels from one part to another. The end result of the AP is to release a chemical at the end of the axon. This chemical can be either an inhibitory or excitatory neurotransmitter.

Figure 27.8: The presynaptic neuron (green) releases excitatory neurotransmitters that cause the postsynaptic potential to become more positive.
Figure 27.9: The presynaptic neuron (pink) releases inhibitory neurotransmitters that cause the postsynaptic potential to become more negative.

27.2.3 Fluctuating Membrane Potentials

Once the RMP reaches a certain electrical threshold, an action potential is born. As the RMP has a minus electrical value, the AP has a positive value. When an AP is generated, it goes from the -60mv of the RMP to +60 mv and then returns back to a value of -60mv. However, the change that occurs at the membrane to cause the AP to be generated continues in the segments before and after the AP.

Figure 27.10: Postsynaptic potential recordings of excitatory (A) and inhibitory (B) neurotransmitters. Excitatory neurotransmitters cause a slight depolarization (positive), while inhibitory neurotransmitters cause a slight hyperpolarization (negative). If there are enough excitatory neurotransmitters released at the synapse, the postsynaptic potential will cause an action potential.

There are two ways in which postsynaptic potentials can trigger an action potential. They can either do it through spatial or temporal summation.

27.2.3.1 Spatial Summation

The electrical signals produced by the various chemicals will cause the postsynaptic membrane to be either positive or negative at that specific area. The signal will not spread. There are millions of neurons that secrete neurotransmitters causing positive and negative charges. If enough presynaptic signals occur at the same time with the same voltage, they can summate and cause an action potential.

Figure 27.11: Spatial summation of the release of neurotransmitters of many presynaptic terminals

27.2.3.2 Temporal Summation

In addition, if one presynaptic neuron fires continuously, rather than once or twice, it can also trigger an action potential. This process is called temporal summation.

Figure 27.12: Temporal summation of the repeated release of a neurotransmitter

27.2.3.3 Ion Movements Across the Membrane Determine Neuronal Voltage Levels

Ions, such as sodium (Na+) and potassium (K+) determine all the electrical events you see in neurons. Depending on the ion, its concentration on either side of the neuronal membrane will determine whether the inside is positive or negative. All cells in your body have an electrical potential due to the difference of concentration of ions. Neurons are unique in that this difference can be dramatically reversed by changes in the neuronal membrane, which allow different ions to move in and out of the cell causing different voltage levels.

Sodium ions are responsible for action potential in that the membrane permeability to sodium changes, and as a result, the negative signal of the neuronal membrane becomes positive. Once the action potential reaches a certain value, its permeability is altered and potassium ions will cause the signal to decrease back to a negative value. This process repeats itself all the way down the axon.

Let's review membrane potential. The membrane potential is a voltage—measured in mV—and is a comparison of charge as it is distributed on the interior of the cell in comparison to that of the exterior environment. Furthermore, this membrane potential is generated by the influx or efflux of a charged particle (i.e., an ion, such as sodium or potassium). For example, the unequal distribution of sodium and potassium ions across the neuronal cell membrane generate the resting membrane potential. Once the neuron responds to a stimulus, voltage-gated sodium channels open, allowing for sodium ion to enter the cell. The sodium carries its positive charge into the cell thereby depolarizing the cell. If enough sodium channels open and enough sodium enters the cell, the cell may reach a threshold beyond which an action potential will be generated. Finally, the late responding potassium channels will eventually open causing an efflux of potassium and its positive charge along with it. This will eventually enable to the cell to regain its resting membrane potential.

27.2.3.4 The Steps

  • Step 1: Resting Membrane Potential is determined by the difference in concentration of sodium and potassium ions across the membrane. A recording will show a negative voltage of -80 millivolts.
  • Step 2: Depolarization Phase One refers to the fact that the negative charge inside the membrane will change and become positive. This is caused by the sodium ion rushing in with its positive charge.
  • Step 3: Depolarization Phase Two, or the rising phase, is the change in polarity going from a negative value to a maximum positive value. The positive value or the depolarization will be +30 mv.
  • Step 4: Repolarization occurs, meaning the value of the electrical voltage will now begin to return to a negative value. It is determined by the efflux of potassium ion from inside the membrane to the outside.
  • Step 5: Hyperpolarization occurs. Oops! Too much potassium has left the inside of the neuron causing the electrical voltage to become more negative than normal. Instead of -80 mv it is -100 mv. (it is also called the undershoot).
  • Step 6: Back to Resting Membrane Potential. All is ok. The resting membrane potential returns to a -80 mv ready for the next neurochemical stimulus.


Question 27.12

Match the stage to its correct description.

Premise
Response
1

Resting membrane potential

A

Negative membrane potential or voltage determined by the flow of sodium and potassium ions across the neuron.

2

Depolarization

B

Influx of positive charge carried by sodium ion. If enough sodium ions enter the neuron an action potential will be on its way.

3

Repolarization

C

Potassium channels close and the cell is prepared to respond to a new stimulus.

4

Hyperpolarization

D

Efflux of positive charge carried by the potassium ion. The neuron is now working to get itself back to its resting state.

5

Return to rest

E

Efflux of an excess of potassium charge thereby leaving the internal world of the neuron a bit too negative.


Steps 1-5 are thought to require no energy, in the form of ATP, for the ionic changes to occur. However, there are scientists who contend that activation of “gates”, protein structures involved in ionic transport, require ATP. Step 6 does require ATP to reconstitute the final ionic balance between both sides of the neuronal membrane.

Figure 27.13: Ionic components of the Action Potential. (See text for details)


Figure 27.14: Flow diagram demonstrating the ionic and electrical sequences of an action potential.

An action potential triggers the release of chemicals at the synapse. Like it does at the neuromuscular junction, the action potential will release a chemical called a neurotransmitter. The release of chemicals from the neuron goes through the following steps:

1. Neurotransmitters are produced in the presynaptic neuron

2. Neurotransmitters are sent to the axon terminal with further modifications

3. Neurotransmitters are stored in vesicles

4. An action potential is generated from the presynaptic neuron

5. Calcium flows into the presynaptic neuron

6. Vesicles combine with the presynaptic membrane of the axon terminal

7. Neurotransmitters are released by way of exocytosis

8. Neurotransmitters diffuse to the synaptic site

9. Neurotransmitters combine with the receptor of the postsynaptic neuron

10. The receptor of the postsynaptic neuron is activated 

11. Membrane permeability of postsynaptic neuron is changed for certain ions

12. The postsynaptic membrane becomes more positive or negative depending upon the neurotransmitters.

13. Neurotransmitters return to the membrane of the presynaptic neuron at the axon terminal

14. Neurotransmitters are metabolized within the presynaptic neuron

15. Metabolites of the neurotransmitter are recycled into the presynaptic neuron


Question 27.13

What must happen at the synapse for the action potential to transfer from the presynaptic to the postsynaptic cell? Place the steps in the correct order.

A

Influx of calcium will cause vesicles to associate with the presynaptic membrane of the axon terminal.

B

Neurotransmitters leave the presynaptic cell via exocytosis.

C

Neurotransmitters are removed from the synapse.

D

Neurotransmitters are recycled back into the presynaptic cell through endocytosis

E

Neurotransmitters are produced in the presynaptic neuron. Once modified they will be stored in vesicles.

F

Neurotransmitters will associate with the receptor of the postsynaptic neuron.

G

Membrane permeability of postsynaptic neuron is changed becoming more positive or negative depending upon the neurotransmitters.

H

Action potential generated from the presynaptic neuron will travel to the axon terminal where it motivated influx of calcium.

I

Receptor of postsynaptic neuron is activated.


Figure 27.15: Neurotransmitter release: Sequence of events in which electrical Signals trigger the release and uptake of neurotransmitters.


27.2.4 Neuronal Communication Interference

Numerous chemicals can interfere with the release of neurotransmitters. In the case of a synapse that releases acetylcholine, the poison botulinum (poison from spoiled food) will block the release of acetylcholine leading to paralysis and death. However, in low amounts, the same deadly poison can be used to minimize wrinkles. Hemicholinium inhibits the synthesis of the neurotransmitter, acetylcholine by interfering with the transport of choline used in the production of acetylcholine.

Another example of interference with synaptic transmission is spider venom which will cause a massive release of acetylcholine leading to tonic contractions of skeletal muscle.

Figure 27.16: Neurotoxins (botulinum, spider venom) affect the release of acetylcholine.


Question 27.14

As a businesswoman, you want to change people’s drinking behavior. You want them to drink more of your product. Your scientists have discovered that certain neurons inhibit people’s appetite for your product. How do these inhibitory neurons work?

A

Cause action potentials to be activated to cause you to drink more of product

B

Cause inhibitory neurons to have an increase in threshold when you drink the product

C

Cause inhibitory neurons to have a decrease in threshold when you drink the product

D

Cause excitatory neurons to have an increased in threshold when you drink the product


27.3 The Brain

Concept 27.3: The brain has many subdivisions, each consisting of neurons and supporting cells. Each subdivision is specialized in both anatomy and physiology. Neurons allow for communication among the subdivisions of the CNS.
Figure 27.17: The central nervous system consists of the brain and the spinal cord.

The brain is protected by the skull, and the spinal cord by the vertebral column. The spinal cord does not extend the entire duration of the vertebral column.

The brain can be divided into three main areas: cerebral cortex, brainstem, and cerebellum. The average human brain weighs approximately three pounds and it has the consistency of Jello. It floats in a fluid called the cerebrospinal fluid (CSF) which is produced by cells in the brain within the choroid villi.

The brain is not a completely solid structure since it has a number of interconnecting cavities called ventricles. The ventricles play a major role in the production and transmission of the cerebrospinal fluid. Without cerebral spinal fluid, the weight of the brain would collapse upon itself. Therefore, CSF is the cushion for the brain. The fluid is produced in the lateral ventricles, flows through the third and fourth ventricle to the space in the brain called the subarachnoid space and then to the spinal cord.


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Figure 27.18: The cerebrospinal fluid flows through the ventricles of the brain and surrounds the spinal cord. The Blood-brain barrier controls which solutes enter the cerebrospinal fluid.

27.3.1 The Cerebral Cortex

The cerebral cortex is the control center and it is made up of four different lobes: frontal, parietal, occipital, and temporal. In humans, these lobes work together so that responses to the environment are processed based on intelligence, previous experiences, memory, sensation, and emotions. The cerebral cortex is also divided into two hemispheres: right and left, which further compartmentalizes and specializes the behaviors coordinated by the brain. The two hemispheres are connected by commissural fibers which are designed to promote communication between the two hemispheres. The cortex has rounded elevations called gyrus and valleys called sulcus. The gyrus and sulcus increase the area inside the brain allowing for more neurons. In addition, there are groups of myelinated fibers that tie different areas together called association areas. Thus the brain can coordinate information from different groups of neurons.

Figure 27.19: All parts of the body are represented in an organized fashion in both the motor and sensory cortices.

27.3.2 The Hemispheres

The two hemispheres communicate with each other so that the function of one side of the body is coordinated with the other by way of commissural fibers. In other words, your right hand knows what your left hand is doing. The connection between the hemispheres is called the cerebral commissure and is much more developed in females than males. Females can process data from both sides faster than males. This might explain the commonly observed ability of females to listen and coordinate different activities much faster than males. The various lobes of the cerebral cortex can be viewed as specialized areas that are responsible for certain major function. 

27.3.3 The Lobes

Figure 27.20: Lobes of the Brain

The frontal lobe is involved in processing “higher functions”: recognizing future consequences resulting from current actions, choosing between good and bad actions (or better and best), overriding and suppressing unacceptable social responses, and determining similarities and differences between things or events. Ever notice that intoxicated persons may demonstrate inappropriate behaviors such as dancing with a lampshade or taking risks? Blame it on the alcohol that inhibits the frontal lobes.

The parietal lobe plays important roles in integrating sensory information from various parts of the body, knowledge of numbers and their relations, and in the manipulation of objects. Portions of the parietal lobe are involved with visuospatial processing, which involves visual representation of images in space. It also includes tasks involving mental rotation of three-dimensional visualizations. Is there a gender difference between males and females in the parietal lobe? In tests, males are overall more adept at performing mental rotation tasks. From a parietal lobe perspective, males have greater surface area than do females which might help them in their mental rotation tests.

The occipital lobe is the primary visual cortex, which means that the visual signals from your retina are ultimately processed by the occipital lobe. The occipital data is then transferred to the parietal lobe. The occipital lobe has connections with motor centers in the brain. Certain frequencies of light flashes, or images with multiple colors, can trigger seizures, suggesting connections between the image processed in the occipital and parietal lobe, leading ultimately to motor centers that produce the seizures.

The temporal lobe is involved in auditory perception. The tunes that you hear are processed by the temporal lobe. Interestingly, the temporal lobe houses another group of neurons, called the hippocampus, which is involved in long-term memory. As we age, what happens to the temporal lobe? Males show a larger decrease in brain volume suggesting that they are losing more neurons than their female counterparts. Possibly hormones play a role in producing this difference.

Figure 27.21: Functions of the brain are localized to specific areas. Major damage to any area will have an effect on more than one function. The auditory cortex is involved in both word formation as well as language comprehension. Damage to the auditory cortex will cause problems with understanding and speaking.​


Figure 27.22: Wernicke’s area coordinates the signals arriving from auditory, visual and somatic areas of the brain. These signals may be “interpreted” previously to being sent to Wernicke’s area.


Question 27.16

Match the lobes of the cerebrum to their correct functions.

Premise
Response
1

Frontal Lobe

A

This lobe is active when you hear a familiar song and then again when you begin singing along to it.

2

Parietal Lobe

B

This lobe is responsible for the integration of sensory information incoming from the body or the external environment.

3

Temporal Lobe

C

Considered the decision maker. This lobe will help you decide to make good choices and override the bad ones.

4

Occipital Lobe

D

Your ability to recognize your friends' faces is due to the activity of this lobe.


27.3.4 The Brainstem

The brainstem, or “lower brain”, is the life support system of the CNS because it regulates breathing, heart rate, and blood pressure. It is the lower extension of the brain and connects it to the spinal cord. The brainstem is the pathway for nerves traveling to the highest parts of the brain. It consists of three parts: the midbrain, pons, and medulla.


Figure 27.23: The brainstem controls many vital functions such as heart rate and breathing.


The midbrain serves to initially integrate sensory input and project it to regions within the cerebrum. It contains auditory and visual reflex centers so when you get scared by a loud noise and respond it is the midbrain that controls the response.

The medulla helps control the body's autonomic functions (things you don't need to think about to perform) like respiration, digestion and heart rate. Damage to this site will cause the patient to have abnormal breathing patterns. Motor neurons in the medulla also regulate swallowing, coughing, and vomiting. The medulla acts as a relay station for nerve signals going to/from the brain.

The pons has roles in your level of arousal, consciousness, and sleep. The pons also relays sensory information to/from the brain and is involved in controlling autonomic body functions. Axons from the medulla and pons signal to areas of the cerebral cortex and cerebellum to cause changes in attention, alertness, appetite, and motivation. Injury to the brainstem can result in death. It is extremely important since it houses the cardiac center to control heart rate, vasomotor center to control blood pressure, and the respiratory center in conjunction with the pons.


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27.3.5 The Cerebellum

The cerebellum controls balance and the refinement of movements. It is also likely responsible for learning and remembering motor responses. The cerebellum receives and integrates information about the position of joints, length of muscles, visual and auditory stimuli to coordinate fine motor movements. These include learning new dance steps, hand-eye coordination, and playing the piano.


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Figure 27.24: Cerebellum; a major motor part of the central nervous system


Figure 27.25: The cerebellum has different projections (shaded areas) that control specific parts of the body.

Injury to the cerebellum will result in the loss of fine motor control and tremors. The tremors or shaking of the hands in persons with cerebellar injuries is interesting. It is called an intention tremor, meaning that the patient does not show any signs of tremor when he is completely rested. Once he moves his hand to get a glass of water, for example, the hand begins to demonstrate a pronounced shaking of the hands.

Figure 27.26: The cerebellum is adjacent to the brain stem.

27.3.6 The Spinal Cord

Lastly, we have the spinal cord, a long stretch of axons and neurons that course from the brain stem to the lumbar level of the vertebral column, the bone that encases and protects the spine. Each segment of the spinal cord contains two spinal nerves that flank the spinal cord.

Figure 27.27: Spinal cord functions to receive sensory signals and deliver motor signals. In addition, it transmits the sensory and motor signals to higher centers. It also receives input from higher centers and does processing of sensory and motor signals.

Each individual spinal nerve, containing sensory neurons (afferents), enters at a dorsal root ganglion (DRG) and threads into dorsal roots before entering the spinal cord. The side on which afferents enter the cord is the dorsal side and the opposite is called ventral. Many cell bodies in the ventral horn of the spinal cord send axons through the ventral root to muscles to control movement. Some fibers make synapses with other neurons in the dorsal horn, while others continue up to the brain. Within the spinal cord, nerve cell bodies are located in the gray matter. Surrounding the gray mater is white matter, (lighter color shading) where the axons are located.

Figure 27.28: Cross section of the spinal cord. Sensory signals enter via the dorsal part of spinal cord and motor signals exit via the ventral part. Both fibers are found in the spinal nerve. (Soma= cell body)

27.3.7 The Segments

The spinal cord has segments that innervate distinct areas of the body. These are the cervical, thoracic, lumbar, and sacral spinal cord segments. Within the vertebral column, the spinal cord actually only extends through the thoracic segment. The roots from the lumbar and sacral segments thread down into the base of the vertebral column. They make up the cauda equina (or horse's tail). This area is the place where spinal taps are performed and epidural injections are made, because no spinal cord neuronal cell bodies are here, only axons.

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Question 27.20

Action potentials have an all or nothing behavior. When the motor neurons in the brain cause your fingers to hit the keyboard, where are the action potentials occurring?

A

Just in the brain

B

Brain and Spinal cord

C

Brain, spinal cord, blood vessels

D

Brain spinal cord and neuroglial cells


27.4 The Brain

Concept 27.4: The brain has many functions. In addition to these many functions, it is the brain that generates our memories and also is responsible for sleep and wakefulness thereby allowing for consolidation of those memories.

The central nervous system has multiple functions which are beyond the scope of this course. However, to kindle your curiosity, let us consider memory. Who you are, what you memorize, where to go are all manifestations of memory.

Memory refers to the storage, retention, and recall of information including past experiences, knowledge, and thoughts. Most memories are reconstructions. Memories are not stored in our brains like books on a shelf. Whenever we want to remember something, we have to reconstruct it from elements scattered throughout various areas of our brains, similar to how a computer stores and accesses files before it is defragmented. This is one of the reasons that over time, our memories can “fade” and change. For example, two people can watch the same movie, and a year, a month or even a day later remember or forget different parts of the same movie.

27.4.1 Short-term and Long-term Memory

Memory can be categorized into two major categories: short-term and long-term.

Short-term memory is the ability to store information for seconds to minutes after the present moment has passed. Short-term memory, as the name suggests, is of limited capacity, usually 3-4 items. A common example of short-term memory is the ability to remember a phone number until it is dialed, after which the number is forgotten. Interestingly, new information can "bump" out other items from short-term memory.

Short-term memory occurs in the part of the brain called the hippocampus. Recent studies also suggest that short-term memory also requires a part of the brain adjacent to the hippocampus called the subiculum. Both areas of the brain are required for short-term memory but work at different times. If a stroke (e.g., decrease in blood supply to the brain) damages these areas, the person cannot remember current events even if they saw them minutes earlier.

Working memory is a special kind of short-term memory that refers to the ability to hold information in mind long enough to carry out sequential actions. An example is searching for the lost cell phone! Working memory allows the search to proceed efficiently, avoiding places already inspected. However, working memory is influenced by stress. If you are too anxious about the lost cell phone you will not be able to focus and will not see the cell phone in the car where you last left it.

Figure 27.29: Processes involved in the development of short and long-term memory.


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Long-term memory refers to information that is retained for months to years and includes information such as facts, semantics (words, their symbols, and their meanings), and autobiographical information. In general, long-term memory is organized so that it is easy to reach a stored item by a number of routes. It is as if the brain is searching for the word “cookie” and can access it by either thinking of chocolate chips, oatmeal, milk, sweet, forbidden (!) etc. Retrieval of an item also facilitates other related items so that retrieving information about a cookie can lead to retrieval of information about the taste of chocolate, the smell of cookies baking, or watching the Cookie Monster on TV as a kid.

Most types of long-term memory appear to be stored in the frontal cortex. Different areas of the cortex specialize in different kinds of information. Visual information of a cookie, for example, may be stored in one location (e.g., the inferior temporal cortex), while information about its associations to Santa Claus and milk might be stored in another (e.g., the frontal cortex). The linkage between these two areas means that seeing a picture of a cookie can retrieve a memory about receiving a special gift from Santa at Christmas.

figure 27.30 (2).png
Figure 27.30: Long-Term Memory


Short-term memories can be transferred to long-term memories (i.e. from the hippocampus to the cortex) by retaining the new piece of information long enough either by repetition or association. For example, if you are memorizing the fact that active transport of water requires ATP, you could repeat it to yourself over and over again. However, you would retain the information much better in your long-term memory if you associated the information with something you already know, such as a water pump in a fish tank that requires electricity (an energy source). Because you are associating the new information with old information (i.e., an existing network), it strengthens the connections of the old network and provides another route for your brain to access the information. Association is, therefore, a more efficient means of storing new information in your long-term memory.

Figure 27.31: Long-term memory is explained on the basis of Long-Term Potentiation (LTP)

The molecular basis of creating of long-term memories is dependent on long-term potentiation (LTP), a process which strengthens the connection between two neurons. LTP is the cellular equivalent of a conditioned response. During LTP, signaling between the pre- and postsynaptic neurons are patterned (the signal is repeated). The response of the postsynaptic neuron is subsequently stronger than the original signal (the response is potentiated) and lasts for a longer period of time (long-term). The more you do something, the greater are the connections.


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Figure 27.32: A synapse may be strengthened by specificity and associativity. This may result in reinforcing experiences and consolidating memories.

However, this chemical connection does not completely explain memory. LTP explains that persons or animals can more quickly identify an object but does not specifically state how it does it. Keep in mind that short and long-term memory are not the sole properties of humans since many vertebrates and invertebrates have short and long-term memory.

During LTP, both neurons are activated at the same time resulting in more effective communication. That is, if the presynaptic neuron releases a chemical message, the message will be much better received by the postsynaptic neuron if that neuron is activated to receive the message. It is an analogous feeling that you are much better friends with someone you have a conversation with on a daily basis (someone you can have a dialogue with) vs. someone you only write e-mails to (someone you have one-sided conversations with). LTP does not occur with inactive neurons and is therefore specific to the activated neurons.

27.4.2 Memory and Sleep

As you may know from your own experiences, sleep has a big effect on memory. If you are feeling tired, or have not gotten enough sleep the night before, it is harder to learn new material in class. Conversely, you can perform certain tasks better after a good night’s sleep. All mammals require sleep and spend much of their lives doing so (think of your pet cat or dog, who spends most of their days sleeping!). Humans also require a lot of sleep, and researchers are still not sure why. One of the reasons may be to help us learn new information and strengthen our memories.

Figure 27.33: All mammals require sleep. [2]

Some studies have shown that your brain actually undergoes remodeling when you sleep. Studies in fruit flies show that the number of synapses increases during social interactions, while the number of synapses decreases during sleep. This decline in synapses during sleep is thought to be a consolidation of memories. This consolidation subsequently strengthens the connection of the remaining synapses.

A good example is when you are faced with several choices and need to make a decision. If you “sleep” on it, your brain has time to process the information and you may find that in the morning, you can more easily make a decision after your brain has consolidated all of the information contained in those choices. In addition to information consolidation, it also appears that your mind replays the newly learned tasks during sleep. This replay is just like when you repeat something to yourself over and over to help transfer the information from your short to your long-term memory. The same patterns of neuron activation that occur while learning the activity while awake also occur during sleep. This reinforcement of new information (and the reinforcement of the connection between the neurons) that happens while we sleep may also help us in the learning process.

Just as sleep can help us learn new tasks and process information, the lack of sleep can also affect our ability to learn or perform a newly learned task. Many studies have shown that sleep deprivation for both animals and humans negatively affects both learning and performing a newly learned task. 

27.4.3 Memory and Caffeine

Many people know (including myself!) that if you are feeling tired, a cup of coffee or an energy drink can give you a little mental boost. While studies have shown that some caffeine (e.g., 100mg, about the amount you find in an 8oz. cup of brewed coffee) can improve alertness and even short-term memory functions, one study showed that a lot of coffee (e.g., 2-3 cups of coffee, or one tall coffee from Starbucks) has been shown to impair certain functions such as motor tasks and recall tasks.

Furthermore, the people who had caffeine performed these tasks more poorly than people who had taken a nap instead. Researchers are not exactly sure of all of the effects of caffeine from the brain, but one effect might be on the levels of acetylcholine (the neurotransmitter involved in learning, discussed earlier). Levels of acetylcholine usually decrease while you sleep, just like the number of synapses decreases. Caffeine boosts the levels of acetylcholine in your basal forebrain, and may therefore negatively impact the learning process by not allowing information consolidation to occur.

Figure 27.34: How much caffeine is in your favorite drink?

27.4.4 Damaged Memory

Memory can be impaired by various injuries and diseases. A common outcome of a brain injury due to concussions as observed in sports or a serious accident is amnesia. The brain is physically damaged and as a result, the person does not remember what occurred before, during, or after the trauma. Amnesia is a severe disruption of memory without deficits in intelligence, attention, perception, or judgment. It may occur following damage to any of several brain structures that are critical for memory. There are three major classes of amnesia: anterograde amnesia, an impairment in storing new memories, retrograde amnesia, a loss of old memories, and psychogenic amnesia, a temporary loss of identity. Anterograde and retrograde amnesia usually result from brain injury or disease, while psychogenic amnesia is a psychological condition that occurs in the absence of brain injury and usually involves both short and long-term memory loss. 

27.4.5 Alzheimer’s Disease

Alzheimer’s disease is a common disease that causes memory loss. The disease is characterized by the presence of plaques and tangles in the brain. Plaques build up between nerve cells, containing deposits of a protein fragment called beta-amyloid. Tangles are twisted fibers of another protein called tau.

Researchers are not absolutely sure what role plaques and tangles play in Alzheimer’s disease, but most people believe they block communication among nerve cells and disrupt activities that cells need to survive. Interestingly, plaques and tangles appear in a predictable pattern in the brains of Alzheimer patients.

Early stages of Alzheimer's disease show memory impairments due to neuronal cell death in the hippocampus and the basal forebrain, the area that produces acetylcholine, the neurotransmitter essential for learning. Damage to the neurons in these areas results in a kind of anterograde amnesia, where new memories are not formed while old memories are retained. As the disease progresses and more plaques and tangles appear in other parts of the brain, other impairments in memory and motor function occur. However, the presence of the tangles is by themselves not sufficient to cause Alzheimer's or to explain Alzheimer’s memory loss.

Figure 27.34: Plaques and tangles

27.4.6 Cerebrovascular Accidents (CVA): The Stroke

A stroke, while not a disease, is another common cause of memory loss. A stroke is commonly caused either by a sudden loss of blood flow to the brain or by bleeding inside the head. Large ischemic strokes are usually caused by narrowing of the large arteries in the neck and brain or blockage of arteries by blood clots or pieces of atherosclerotic plaque (buildup of cell debris on artery walls). People with uncontrolled high blood pressure and diabetes often have small ischemic strokes that involve very small arteries in the brain. Strokes can cause memory loss due to neuronal cell death when the blood supply is cut off. Remember, the decrease in oxygen causes a decrease in ATP which leads to cell death. Consequently, there is no specific medical treatment to help reverse the memory loss that occurs after a stroke. The amount of recovery is dependent on the degree of injury that the patient suffers.

figure 27.35 (2).png
Figure 27.35: Ischemic stroke


27.4.7 Alcohol-Induced Memory Impairment

Another kind of impairment of memory can occur with alcohol use and is frequently referred to as a “blackout”. Some of the factors involved in alcohol-induced blackouts are blood alcohol level, the rate of consumption (the faster you drink, the more likely you will have a blackout), your prior history with blackouts (some researchers believe that having a blackout damages your brain which makes you more susceptible to having another blackout), and your gender (studies suggest that females are more susceptible to blackouts than males).

Females are more susceptible than males since they do not have to drink as much as their male counterparts to overwhelm the liver which detoxifies the ethanol drink. Research data suggests that alcohol disrupts electrical and chemical activity in the hippocampus, thus impairing the brain’s ability to transfer information from short-term memory to long-term memory, and resulting in a form of anterograde amnesia.

Recall that the molecular basis of creating long-term memories is dependent on long-term potentiation. LTP is a process that strengthens the connection between two neurons. A person who is heavily intoxicated may remember an incident 2-3 minutes after it has occurred, but not the next day. Alcohol also interferes with the establishment of LTP, and this impairment can occur by consuming just one or two standard drinks (e.g., a 12–oz. beer, 1.5–oz. of liquor in a shot or mixed drink, or a 5–oz. glass of wine). For chronic drinkers, alcohol may also damage the frontal lobe (where the frontal cortex is involved in the storage of long-term memories) by causing shrinkage in brain volume, changes in gene expression in brain cells and changes in blood flow in the brain.

Question 27.23

Memory in the brain can be severely disrupted by which of the following?

A

Chronic sleep deprivation

B

Caffeine intake

C

Alcohol intake

D

All of the above factors can disrupt consolidation of memories in the brain


Question 27.24

You are attempting to design a new helmet to minimize head concussions associated with sports. You study the human brain and want to mimic how the brain minimizes trauma to the head. Which of the following is the most important in minimizing head concussions?

A

Neurons themselves since they can withstand the shock waves when the skull is hit

B

Cerebral spinal fluid since it floats the brain and minimizes the shock waves

C

Blood vessels that support the brain so that they will support the brain to minimize shock waves

D

Neuroglial cells since they transmit oxygen directly to the neurons

Figure 27.36: Chapter overview of the central nervous system.​


27.5 Vocabulary Questions


Vocabulary Question 27.01

Refers to your brain and your spinal cord.


Vocabulary Question 27.02

The storage, retention, and recall of information including past experiences, knowledge, and thoughts.


Vocabulary Question 27.03

The ability to store information for seconds to minutes after the present moment has passed.


Vocabulary Question 27.04

Special kind of short-term memory that refers to the ability to hold information in mind long enough to carry out sequential actions.


Vocabulary Question 27.05

Information that is retained for months to years and includes information such as facts, semantics, and autobiographical information.


Vocabulary Question 27.06

Area of the brain where most types of long-term memory appear to be stored.


Vocabulary Question 27.07

Also called nerve cell; an excitable cell in the nervous system that processes and transmits information by electrochemical signaling.


Vocabulary Question 27.08

Long projections that send signals from the neurons.


Vocabulary Question 27.09

Much shorter projections and receive signals from other neurons.


Vocabulary Question 27.10

Junctions between axons and dendrite, and where neurons communicate.


Vocabulary Question 27.11

Chemical messages which relay, amplify, and modulate signals between a neuron and another cell.


Vocabulary Question 27.12

Neuron sending the message before the synapse.


Vocabulary Question 27.13

The neuron receiving the message after the synapse.


Vocabulary Question 27.14

Membrane-bound organelles that hold neurotransmitters.


Vocabulary Question 27.15

A process which strengthens the connection between two neurons. The cellular equivalent of a conditioned response.


Vocabulary Question 27.16

A severe disruption of memory without deficits in intelligence, attention, perception, or judgment.


Vocabulary Question 27.17

Impairment in storing new memories.


Vocabulary Question 27.18

Loss of old memories.


Vocabulary Question 27.19

Temporary loss of identity.


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27.6 Image Credits

[1] Image courtesy of geralt in the Public Domain.

[2] Image courtesy of Rafal Jedrzejek in the Public Domain.