Chapters 48 and 49 Neurons and Nervous Systems

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1 Biology 120 J. Greg Doheny Chapters 48 and 49 Neurons and Nervous Systems NEURONS: Neurons are special cells that coordinate much of the body s actions. Most higher organisms have a central nervous system (including a brain) that processes information, and causes the body to react to situations with either simple or complicated behaviors. The brain is made mostly of neurons. The sensory cells that relay sensory information (from eyes, ears etc.) to the brain are also neurons (called afferent neurons), and the cells that then control muscle movements and other actions in response to the brain s commands are neurons, too (called efferent neurons; or motor neurons). Thus, neurons play a critical role in determining the actions of the body. A typical neuron (Figure 48.4) is comprised by a cell body (where the nucleus is located), a series of short projections from the body called dendrites, and a long axon. The axon is designed to send signals to other neurons, and the dendrites and cell body are designed to receive signals from other neurons. When a signal is sent down the axon of a neuron, it causes soluble macromolecules called neurotransmitters to be released from the end of the axon, stimulating other neurons. Neurotransmitters are similar to cytokines in that they will stimulate a nearby cell, provided that cell expresses the appropriate receptors. There are several classes of neurotransmitters (see below), and several types of receptors. One type of neuron generally only produces one type of neurotransmitter, but a neuron may have several different types of receptors. Unlike cytokines, however, neurotransmitters are only able to stimulate neurons that are located very close to the end of the axon. The short distance between the end of an axon, and the dendrite or cell body of the cell that it stimulates is called a synapse. Thus, a signal is sent down the axon, and a neurotransmitter is released into the synaptic space, stimulating a target neuron, which may go on to stimulate other neurons in the same way. Neurons literally conduct signals down their axons electronically, like an electrical wire. This electrical conductivity is made possible by charged ions, and a phenomenon called a resting membrane potential (see below). RESTING MEMBRANE POTENTIAL: The fluid just outside of a neuron is usually positively charged relative to the inside, due to active transport channels that transport sodium outside the cell while transporting potassium ions in (Figure 48.7). These channels are collectively called the sodium-potassium pump. The sodium-potassium pump transports three sodium ions outside for every two potassium ions it transports inside. The transport of greater numbers of positively charged ions outside the cell eventually leads to an electrical difference, where the inside of the cell has a net negative charge of about -70 mv (millivolts) relative to the outside. This is called a resting membrane potential, and the cell is said to be polarized. The cell membrane also contains sodium and potassium channels that would let sodium and potassium pass freely through the membrane if open, but these channels are kept closed unless the neuron is activated. 1

2 ACTION POTENTIAL: When a polarized neuron is stimulated by another neuron, the sodium and potassium channels open, allowing sodium and potassium to cross the membrane freely along their concentration gradients. Sodium rapidly flows in, and the inside of the cell quickly goes from being negative (relative to the outside) to positive (Figures and 48.11). This rapid depolarization is conducted down the axon, until it reaches the end of the axon where it causes the release of neurotransmitters (Figure 48.12). SCHWANN CELLS, AND NEURON INSULATION: Some neurons have short axons (like the ones in your brain), while others have extremely long axons (like the ones in your spinal cord). Extremely long axons are generally insulated (like electrical wires) with a layer of insulating material called myelin. Myelin is produced by cells called Schwann cells. Schwann cells literally wrap themselves around the axons of longer neurons (Figure 48.13). TYPES OF NEURONS: There are three basic types of neurons. 1. Sensory Neurons: Also known as afferent neurons. Sensory neurons are present in the sensory organs (eyes, nose, ears, skin). Sensory neurons receive signals from outside, and send the information to the brain. 2. Motor Neurons: Also known as efferent neurons. Motor neurons relay signals from the brain to stimulate muscles. 3. Interneurons: Interneurons neither send signals to, nor receive signals from the brain. They relay signals from one neuron to another. NEURONS vs. NERVES: When many neurons are bundled together into a string-like filament, the filament is called a nerve. This is particularly true of the longer neurons that run down your spinal cord, and to the peripheral parts of your body (ie- the nerves in the ends of your fingers). NEURONS DO NOT DIVIDE OR REGENERATE: Some types of cells are capable of an almost indefinite number of cell divisions. (Bone marrow stem cells, for example.) Neurons, by contrast, will never divide again after birth. We are born with a certain number of neurons, and we never make new ones. (On the contrary, we lose about 100,000 brain cells per day.) Thus, if a neuron is damaged or destroyed, we can t grow another one to replace it. This is why spinal cord injuries (damage to the nerves in the spinal cord) are very serious. Damage to the neurons in the brain may (in some cases) be less serious than damage to neurons in the spine, because the various duties of neurons in the brain can be re-assigned (see neural plasticity, below). TYPES OF NEUROTRANSMITTERS: There are several different types of neurotransmitters (Table 48.2), and several different types of receptors present on neurons to receive their signals. Most neurons only produce one type of neurotransmitter, but have several different types of receptors, and can thus be stimulated by several types of neurons. The different neurotransmitters are thought to mediate different functions, especially in the brain. The following are a few examples: 2

3 Acetylcholine: Used for muscle stimulation, learning, and memory formation. Dopamine and Serotonin: Two neurotransmitters that effect mood, attention and learning. Serotonin in particular is thought to be involved in depression. Prozac (an anti-depressant) prevents the uptake of serotonin by neurons). Endorphins: A class of neurotransmitters that act as natural pain killers. They also produce a feeling of euphoria. (For example, when you ve been running for a long time, and it starts to feel good instead of bad, it s because your brain has started to release endorphins to compensate for the discomfort. Some people actually become addicted to running for this reason.) THE HUMAN NERVOUS SYSTEM The human nervous system is divided into a central nervous system (CNS) and a peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, and the PNS consists of everything else (Figure 49.4). THE CENTRAL NERVOUS SYSTEM (CNS) THE BRAIN THE BRAIN: The brain is composed of gray matter on the outside, and white matter on the inside (Figure 49.5). Neurons have a grayish colour, but when their axons are insulated by myelin they appear white. Gray matter is made of neurons that have little or no myelin insulation, and white matter is made of neurons that are heavily insulated by myelin, and generally have longer axons. White matter neurons are generally meant to transmit neural signals over long distances. The neurons in the spine, for example, are composed of white matter, as are the inner brain neurons that feed into them. Two large compartments in the brain, called ventricles, are filled with a fluid called cerebrospinal fluid, which is essentially just blood plasma. Cerebrospinal fluid is also present in the spinal cord. THE BRAIN HAS TWO HEMISPHERES: The brain is divided into two nearly-identical halves called the left and right hemispheres (Figure 49.9), which carry out nearly identical functions. The left and right hemispheres of the brain are able to communicate with each other via a large bundle of insulated neurons called the corpus callosum, which connects the two hemispheres. With regards to controlling the body, the left hemisphere controls the right side of the body, and vice versa. Thus, if you have a stroke (where blood flow to a part of the brain is cut off, causing necrosis) to the left side of your brain, you may lose movement on the right side of your body. 3

4 EACH HEMISPHERE OF THE BRAIN IS DIVIDED INTO FOUR SECTIONS (Figure 49.9): 1. CEREBRAL CORTEX The upper most part of the brain is called the cerebral cortex (or the cerebrum), and it is responsible for the highest levels of thinking, including perception, reason, emotions, decision making etc. 2. DIENCEPHALON: The central part of the brain is called the diencephalon. The diencephalon contains the thalamus, hypothalamus, pituitary gland and pineal gland. Many sensory neurons that detect how hot or cold we are, how high or low our blood pressure is, how high or low our blood sodium or calcium levels are etc., pass through the thalamus and hypothalamus. The hypothalamus then instructs the pituitary gland to release various hormones to modify these variables. 3. CEREBELLUM: A large structure located just below the posterior of the cerebral cortex is called the cerebellum. The cerebellum helps to coordinate complicated movements, like playing the piano, or walking. Such movements require the complicated coordination of hundreds of different muscles, but we do not actually have to think about moving them all. They just happen. The cerebellum is responsible for coordinating these movements. 4. BRAINSTEM: The brainstem is the lowest, and most primitive part of the brain. The brainstem connects directly to the spinal cord, and controls many of the body s basic functions, such as respiration, heart rate and digestion. We do not think about or control these functions, they just happen automatically. Basic functions, such as heart rate and digestion, which we do not control voluntarily are controlled automatically by what is called the autonomic nervous system (see below). THE CEREBRAL CORTEX IS RESPONSIBLE FOR HIGHER BRAIN FUNCTIONS: The outermost layer of the brain is called the cerebral cortex (or cerebrum ), is composed of grey matter, and is responsible for most of the higher functions of sensing, thinking, behavior, and decision making (Figures and 49.17). As mentioned above, the cerebral cortex is divided into two hemispheres, which communicate with each other via the corpus callosum. In addition, each hemisphere is divided into four lobes, each of which has a different function. 1. FRONTAL LOBE: The frontal lobe is responsible for concentration, decision making, self-restraint, control of emotions, and other executive functions. The prefrontal cortex (front part of the frontal lobe) is most important for this. It is believed that our sense of self-discipline and self-control is located there; and that people who have trouble controlling their emotions (ie-people who have trouble controlling their anger) have defects in this area. The LEFT frontal cortex is also the location of an area called Broca s Area, which gives us the ability to speak. People who have a stroke that damages Broca s Area lose the ability to speak. (But may regain it later. See plasticity below.) 2. PARIETAL LOBE: Located just posterior to the frontal lobe is responsible for integrating sensory information. 3. OCCIPITAL LOBE: Located posterior to the parietal lobe is responsible for processing visual information. 4. TEMPORAL LOBE: Located laterally (to the side of) and below the parietal and occipital lobes, is responsible for processing auditory (hearing) information. 4

5 THE PRIMARY MOTOR CORTEX, THE PRIMARY SOMATOSENSORY CORTEX, AND THE PRECENTRAL SULCUS (Figure 49.17): A sulcus is a large fissure or fold in the brain. The large sulcus that divides the frontal lobe from the parietal lobe is called the precentral sulcus. The primary motor cortex is located just anterior to the sulcus (on the frontal lobe), and is responsible for controlling our voluntary movements (movements we have to think about). (See the map in figure 49.17, and notice how much of our primary motor cortex is dedicated to controlling our face, tongue and hands, relative to how much is dedicated to controlling our feet, for example.) The primary somatosensory cortex is located just posterior to the sulcus (on the parietal lobe), and is dedicated to receiving sensory, touch information. (Again, see the map in Figure and notice how much of our sensory cortex is dedicated to receiving touch information from our face and hands, relative to our feet, for example.) THE FUNCTIONS OF THE CEREBRAL CORTEX ARE PLASTIC. : The term neural plasticity refers to the fact that if one part of the cerebral cortex is damaged (by a stroke, for example), other parts of the cerebral cortex can eventually be re-programmed to do the same job. (The word plastic means flexible. ) Thus, if a person has a stroke, and Broca s Area is damaged, they will initially lose the ability to speak. But they can eventually re-learn to speak. Similarly, if a right-handed person has a stroke that damages the left Primary Motor Cortex (which controls the right hand), they can often learn how to write again using their left hand. They may even be able to write with their right hand again, if other neurons in their left primary motor cortex are re-assigned. This is not the case with other areas of the brain or CNS. THE SPINAL CORD (Figures 49.4 and 49.8): The brain stem is contiguous (continuous) with the spinal cord, which emerges from the foramen magnum (large hole at the base of the skull). The spinal column is composed of 31 vertebrae, including eight cervical vertebrae (in the neck), 12 thoracic vertebrae (in the thorax), five lumbar vertebrae (in the small of the back), five sacral vertebrae (in the lower back), and the coccyx (the tail bone ). The cervical vertebrae are numbered C1 through C8, the thoracic vertebrae T1 through T12, the lumbar vertebrae L1 through L5, and the sacral vertebrae S1 through S5). A set of afferent and efferent nerves comes out of each vertebra, to service a slice or band of the body called a dermatome. The nerve sets that emerge from each vertebra are named after the vertebra they come out of (cervical nerves C1 through C8, thoracic nerves T1 through T12 and so on). The nerve emerging from the coccyx is called the coccygeal nerve. After these sets of nerves have emerged from the spinal cord, they are no longer considered to be part of the CNS, and are instead considered to be part of the Peripheral Nervous System (PNS). THE PERIPHERAL NERVOUS SYSTEM (PNS): The peripheral nervous system consists of all the nerves that are not part of the spinal cord or brain, and includes a set of sensory neurons (afferent neurons), and a set of motor neurons (efferent neurons) emerging from each vertebra. In addition to the sensory and motor neurons, other neurons emerge from the spinal cord which control involuntary bodily functions, like digestion rate, heart rate, bronchial dilation in the lungs, dilation of arteries etc. The nerves that control involuntary bodily functions (bodily 5

6 functions that happen without us having to think about it) are called the Autonomic Nervous System (ANS). THE SYMPATHETIC AND PARASYMPATHETIC DIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM (Figure 49.8): The neurons controlling involuntary bodily functions are further divided into what are known as the Sympathetic and Parasympathetic Divisions of the ANS. The sympathetic division is sometimes called the Fight-or-Flight division, because it modifies bodily functions in a way that are best suited for body activity. (Examples: dilation of pupils, acceleration of heart rate, vasoconstriction [increasing blood pressure], stimulation of release of glucose from liver, inhibition of pancreas, gallbladder [digestive functions].) The parasympathetic division is sometimes called the rest-and-digest division, because it modifies bodily functions in a way that are best suited for body relaxation and recharging. (Examples: constriction of pupils, lowering of heart and breathing rate, stimulation of intestines to digest food, stimulation of pancreas and gallbladder which produce components needed for digestion, vasodilation [lowering blood pressure].) PRACTICE QUESTIONS Short Answer Questions: 1. What do you call sensory neurons that relay sensory information to the brain? 2. What do you call motor neurons that relay signals from the brain to muscles? What is the long shaft that transmits a signal from one neuron to another called? 3. What are the short projections which extend from the body of a neuron, and which are designed to receive signals from other cells called? 4. What do you call the short distance that separates the end of one neuron s axon from the dendrite or cell body of another neuron? 5. For a resting, polarized neuron, is the concentration of potassium higher inside or outside of the cell? 6. For a resting, polarized neuron, is the inside of the cell positively charged or negatively charged relative to the outside? 7. What is the name of the material that insulates the axons of long neurons, and which cells produce it? (2 points) 8. What are the two components of the human CNS? (2 points) 9. What is the area of the brain that gives us the ability to speak called, and where is it located? (2 points) 10. What is the name of the fluid that fills the ventricles of the brain, and is also present in the spinal cord, and what is it derived from? (2 points) 11. Does the left side of the brain control the left or the right side of the body? 12. Where is the speech centre (also known as Broca s Area) located in the brain? 13. Which two parts of the body take up the largest area on the Primary Motor Cortex? (2 points) 6

7 14. Which two parts of the body take up the largest area on the Primary Somatosensory Cortex? (2 points) 15. Some cells in the body are capable of regenerating (ie-dividing) throughout our entire lives. Other cells will divide during development in the womb, but will never divide again once we are born. Give one example of a cell type that is capable of an indefinite number of divisions, and one example of a cell type that never divides again after birth. (2 points) Neurotransmitters: Match the neurotransmitter to the question. A. Acetylcholine B. Dopamine C. Endorphins D. Serotonin 1. Which is blocked by Prozac (an anti-depressant)? 2. Which act anesthetics (pain killers)? 3. Which two are through to be involved in mood and attention? 4. Which generates a sense of euphoria? 5. Which is used by most motor neurons to control muscle movements (among other things)? 6. Which is used in memory formation? PARTS OF THE BRAIN: The brain is divided into the four general sections listed below. In addition, the Cerebrum is further divided into a number of sections. Answer the following questions regarding the various parts of the brain. A. The Cerebrum B. The Frontal Lobe (Cerebrum) C. The Parietal Lobe (Cerebrum) D. The Occipital Lobe (Cerebrum) E. The Temporal Lobe (Cerebrum) F. The Cerebellum G. The Diencephalon H. The Brainstem 1. What is the function of the Pineal Gland, and which part of the brain (listed above) is it located in? 2. Which two parts of the brain (listed above) does the Central Sulcus separate? 3. Where is visual information processed in the brain (from the list above)? 4. We take walking for granted, and it seems easy to us. In reality, it is actually a very complicated activity, requiring a great deal of coordination. (They are only now beginning to build robots that can do it!) Which part of the brain is critical to your ability to walk? 5. Where is auditory information (hearing) processed in the brain (from the list above)? 7

8 6. The hypothalamus and thalamus are critical information centers that are used to determine if we are too hot or too cold, or if our blood pressure is too high or too low, and then correcting these deviations. In which part of the brain (listed above) are the thalamus and hypothalamus located? 7. What is the function of Broca s Area, and which part of the brain is it located in (from the above list)? 8. What is the function of the Medulla Oblongata, and which part of the brain (listed above) is it located in? 9. What is the function of the Pituitary Gland, and which part of the brain (listed above) is it located in? 10. What is the function of the Primary Motor Cortex, and in which part of the brain (listed above) is it located? 11. What is the function of the Primary Somatosensory Cortex, and in which part of the brain (listed above) is it located? 12. In which part of the brain (listed above) is our sense of self-control, and emotional control believed to reside? 13. Does the left hemisphere of the brain control the left or right side of the body? Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System: For each bodily function, state whether it is part of the sympathetic or parasympathetic division of the ANS. A. Dilation of Pupils B. Inhibition of salivary gland activity C. Vasoconstriction D. Constriction of pupils E. Acceleration of heart rate F. Vasodilation G. Inhibition of the pancreas H. Stimulates gall bladder to release bile salts into duodenum I. Stimulation of glucose release from the liver J. Restriction of blood flow to the intestines K. Lowering of heart rate L. Increase activity of salivary glands M. Increases blood flow to the intestines N. Stimulates pancreas O. Inhibition of release of bile salts from the gall bladder P. Stimulation of adrenal medulla Definition Questions: 1. What is a polarized neuron? (5 points) 2. What is a membrane potential? (5 points) 3. What happens when a neuron depolarizes? (5 points) 4. What is Grey Matter? 5. What is White Matter? 8

9 6. What is the Corpus Callosum? 7. What is the Central Sulcus? (5 points) 8. What is the Primary Motor Cortex? (5 points) 9. What is the Primary Somatosensory Cortex? (5 points) Essay Questions: 1. What does the term neural plasticity mean, and which types of neurons does this term refer to, and which types of neurons are not plastic? (20 points) 2. What is the difference between gray matter and white matter? (Be specific, listing more than one cell type.) (10 points) 3. What is the difference between an Afferent and an Efferent neuron? (10 points) 4. What is the Corpus Callosum, and what is its function? (5 points) 5. The words Cerebrum and Cerebellum sound similar, but actually describe very different parts of the brain. What is the difference between the Cerebrum and the Cerebellum? Where is each located, and what does each do? (20 points) 6. What are some examples of executive functions of the brain, and where are they thought to take place? (10 points) 7. What is Broca s Area, where is it located, and what is its function? (10 points) 8. People who run or jog every day often claim that they eventually become addicted to jogging. Can you explain why this might happen? (10 points) Extended Matching: Match the term to the definition. A. Acetylcholine B. Action Potential C. Afferent D. Axon E. Central Sulcus F. Cerebellum G. CNS H. Corpus Callosum I. Dendrite J. Dermatome K. Diencephalon L. Dopamine M. Efferent N. Medulla Oblongata O. Myelin P. Nerve Q. Neurotransmitter R. Occipital S. Parasympathetic T. Resting Potential U. Schwann V. Serotonin W. Sympathetic X. Synapse 1. A short projection from the cell body of a neuron, which is designed to receive signals from other neurons. 2. A division of the autonomic nervous system responsible for the fight-or-flight response. 3. The lobe of the cerebral cortex in which visual information in interpreted. 9

10 4. A central structure in the brain which contains the thalamus, hypothalamus, pituitary gland and pineal gland. 5. A horizontal slice of the body, which is serviced by a set of afferent and efferent neurons emerging from a spinal vertebra. 6. The division of the nervous system consisting of the brain and spinal column. 7. Part of the brain stem that regulates respiration and heart rate. 8. A large fissure (groove) in the brain that separates the frontal lobe of the cerebral cortex from the parietal lobe. 9. A large bundle of insulated neurons that allows the left and right hemispheres of the brain to communicate. 10. A division of the autonomic nervous system responsible for the rest-and-digest response. 11. Name for a bunch of neurons bundled together into a string-like filament. 12. Word that describes a sensory neuron that relays sensory information to the brain. (An neuron ). 13. A large fissure in the brain that separates the primary motor cortex from the primary somatosensory cortex. 14. A neurotransmitter thought to be involved in mood and attention (two possible answers). 15. Term used to describe a neuron that is resting in a polarized state, with a higher concentration of sodium ions outside the cell, causing the inside of the cell to be negatively charged relative to the outside. 16. Word that describes a motor neuron that carries signals from the brain to muscles. (An neuron ) 17. A neurotransmitter whose uptake is blocked by the antidepressant drug Prozac. 18. A neurotransmitter used in muscle stimulation, learning and memory. 19. A membrane that insulates the axons of long neurons. 20. Term used to describe what happens when the sodium and potassium channels of a polarized neuron open up, allowing sodium ions to flow into the cell, and potassium ions to flow out; thus equalizing the charge on each side of the membrane. 21. The name for the short distance between the axon of one neuron and the body or dendrite of another. 22. A brain structure that helps coordinate complicated movements, like walking or playing the piano. 23. Name for the cells that produce myelin. 24. A soluble macromolecule that stimulates neurons. 25. The long shaft of a neuron, down which a signal can be sent to stimulate another neuron. J. Greg Doheny