10/8/2009. Neural Communication. The Anatomy of the Neuron. Neuron. Myelin Degeneration

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1 Neural Communication The body s information system is built from billions of interconnected cells called neurons. Biological i l Bases of Behavior Bh Neural Communication Neurobiologists and other investigators understand that humans and animals operate similarly when processing information. Note the similarities in the above brain regions, which are all engaged in information processing. The Anatomy of the Neuron The job of the axon is to respond to the information received by the dendrites and soma, and pass it along to neighboring cells through the release of chemicals at the terminal button. Since axons and neighboring cells are not directly connected, these chemicals have to diffuse across a gap between cells called the synapse, before making contact with receptors on the neighboring dendrite. Bundles of axons are called nerves in the PNS. Neuron A nerve cell, or a neuron, consists of many different parts. Myelin Degeneration Myelin sheath speeds up neural transmission to more than 100 m per second, but degeneration of that sheath can have detrimental effect on the neural impulse. Loss of the myelin sheath, as in degenerative diseases like multiple sclerosis (MS), results in a tangled mess of the communication between cells. 1

2 Axon Diameter Thicker axons conduct the electrochemical signal, much faster than the thinner and shorter axons of the non myelinated type. So myelinated axons are found in neurons, which require a faster velocity of conduction such as in those which control the skeletal muscles, compared to those in the stomach muscles, which control digestion Threshold Threshold: Each neuron receives excitatory and inhibitory signals from many neurons. When the excitatory signals minus the inhibitory signals exceed a minimum intensity (threshold) the neuron fires an action potential. The Action Potential Communication within the neuron involves the balance of concentrations of positive and negative chemical particles, called ions, inside and outside of the neuron. When the concentration of these ions are equal across the membrane of the neuron, the neuron is said to be in a state of static equilibrium, or perfect balance. The Action Potential Ongoing electrical activity in the body always keeps these concentrations in flux. There will be no internal communication unless electrical charges are received and are sufficiently large enough to surpass the neurons at a minimum level of intensity required for response, called the threshold of excitation. Action Potential The specific threshold of excitation that is required for a given neuron s action potential differs in the various kinds of neurons. When the action potential occurs, the neuron fires fires. Action potentials are all or none. Either the electrical charge is strong enough to generate an action potential, or it is not. The Action Potential Once this threshold has been reached, a neuron responds with what is called an action potential in the following manner: At rest, the initial i i membrane potential lfor the neuron is 70 mv. During the action potential this potential gets more positive as sodium ions (NA+) enter the cell through special sodium gates. 2

3 The Action Potential As the potential peaks, the sodium gates close and the gates, that allow potassium ions (K+) to enter the cell, open. The exit of potassium ions from the cell causes the membrane potential to return back towards 70 mv. This process repeats many times down the length of the axon. Biological Basis 2 Action Potential A neural impulse. A brief electrical charge that travels down an axon and is generated by the movement of positively charged atoms in and out of channels in the axon s membrane. Action Potential Properties All or None Response: A strong stimulus can trigger more neurons to fire, and to fire more often, but it does not affect the action potentials strength or speed. Intensity of an action potential remains the same throughout the length of the axon. Refractory Period During this process, there are periods of time when the neuron cannot fire another action potential in rapid succession. This is called the refractory period. Refractory Period During the absolute refractory phase, no matter how strong the stimulus may be, the neuron cannot fire again. After the absolute refractory phase, some of the dynamic equilibrium of ions begins to return to normal. 3

4 Refractory Period During the relative refractory phase, the neuron can fire, but only in response to a stronger stimulus than would typically be necessary. Synapse Synapse [SIN aps] a junction between the axon tip of the sending neuron and the dendrite or cell body of the receiving neuron. This tiny gap is called the synaptic gap or cleft. Neurotransmitters (chemicals) released from the sending neuron travel across the synapse and bind to receptor sites on the receiving neuron, thereby influencing it to generate an action potential. Neurotransmitters Neurotransmitters in the synapse are reabsorbed into the sending neurons through the process of reuptake. This process applies the brakes on neurotransmitter action. Reuptake How Neurotransmitters Influence Us Dopamine Pathways Serotonin pathways are involved with mood regulation. From Mapping the Mind, Rita Carter, 1989 University of California Press Dopamine pathways influence movement, learning, attention and emotion and are involved with diseases such as schizophrenia and Parkinson s disease. From Mapping the Mind, Rita Carter, 1989 University of California Press 4

5 Neurotransmitters Lock & Key Mechanism Neurotransmitters bind to the receptors of the receiving neuron in a key lock mechanism. Agonists Antagonists Nervous System The Spinal Cord Central Nervous System (CNS) Peripheral Nervous System (PNS) One main function of the spinal cord, a slender cylindrical rope of interconnected fibers, is to collect information from the PNS and transmit them back to the brain and vice versa. In order to do this task, specialized types of neurons are required: sensory afferents and motor efferents 5

6 Sensory Afferents An afferent of any kind is a neuron that brings information into a structure. Sensory afferents received electro chemical information from outlying neurons in the eyes, ears, and the skin, and transmit it back through the spinal cord to the brain. Motor Efferents An efferent is a neuron that carries information away from a structure. Motor efferents transmit such information as movements of the large and small muscles either from the brain through the spinal cord to the muscles (for voluntary movements) or directly from the spinal cord to the muscles (in the case of reflexes). The Nervous System The Nervous System Nervous System: Consists of all the nerve cells. It is the body s speedy, electrochemical communication system. Central lnervous System (CNS): the brain and spinal cord. Peripheral Nervous System (PNS): the sensory and motor neurons that connect the central nervous system (CNS) to the rest of the body. Three Types of Neurons The cells in the brain, the neurons, are not one homogenous collection of cells. There are three main types: Kinds of Neurons Sensory Neurons carry incoming information from the sense receptors to the CNS. Motor Neurons carry outgoing information from the CNS to muscles and glands. Interneurons connect the two neurons. sensory neurons motor neurons interneurons Interneuron Neuron (Unipolar) Sensory Neuron (Bipolar) Motor Neuron (Multipolar) 6

7 Sensory Neurons Sensory neurons receive the information from the environment. They are connected to special cells, called receptor cells, in the body that can detect changes in the surrounding environment. Examples of these receptors are our eyes, nose, ears, skin, and the internal organs. Sensory neurons send their information towards the spinal cord and brain. Motor Neurons Motor neurons carry information away from the brain and spinal cord towards the muscles of the body. As mentioned earlier, sensory and motor neurons are members of the PNS and CNS, depending on if they are carrying their information outside of or within the spinal cord or brain. Interneurons Interneurons act as bridges between sensory and motor neurons within the brain or spinal cord. These neurons receive information from either sensory neurons or other interneurons, and send their information to motor neurons or other interneurons. For humans, a majority of the neurons in the brain are interneurons. Glial Cells Neurons are not the only kind nerve cells in the nervous system. In fact, they make up only about 10% of the cells in the CNS, where they are supposed to be supported by the all purpose glial cells (also termed neuroglia). Glial cells function as a kind of glue to hold the CNS together by keeping the neurons in their proper places, at optimal distances from one another and from other structures. Glial Cells Some of the glial cells also assist in forming the myelin sheath. In fact, the nodes of Ranvier in the sheath are actually the gaps between glial cells. Glial cells nourished and support healthy neurons. They also destroy and eliminate neurons that have died through injury or age. The dead neurons are then often replaced with new glial cells. The Peripheral Nervous System Peripheral nervous system (PNS) comprises all of the neurons except those in the brain and the spinal cord, even those of the head and face. There is a dual meaning of peripheral. g p p One is auxiliary, or supportive. The other is away from the center, because the neurons of the peripheral nervous system are outside of the CNS. 7

8 Peripheral Nervous System Somatic Nervous System: The division of the peripheral nervous system that controls the body s skeletal muscles. Autonomic Nervous System: Part of the PNS that controls the glands and other muscles. The Nerves Nerves consist of neural cables containing many axons. They are part of the peripheral nervous system and connect muscles, glands, and sense organs to the central nervous system. The Somatic and Autonomic Nervous Systems As with the CNS, the PNS is subdivided into the somatic and autonomic nervous systems. The somatic nervous system is in charge of quick deliberate movements of the skeletal muscles. These skeletal, or striated, muscles are so named because they look striped when viewed from under a microscope, and they allow us to walk, skip, write, or wave. The Somatic and Autonomic Nervous Systems We have voluntary control over these muscles controlled by the somatic nervous system and can quickly respond when the CNS activates them. Striated cardiac muscle Striated Muscle Tissue Biological Basis 1 The Somatic and Autonomic Nervous Systems The autonomic nervous system controls nonskeletal muscles, including the striated muscles of the heart, and the smooth muscles of the blood vessels and internal organs, muscles over which we have no voluntary control, and of whose functioning we are usually not aware; the function of this system is autonomic or selfregulating. Autonomic Nervous System (ANS) Sympathetic Nervous System: Division of the ANS that arouses the body, mobilizing its energy in stressful situations. Parasympathetic Nervous System: Division of the ANS that calms the body, conserving its energy. 8

9 Autonomic Nervous System (ANS) Sympathetic NS Arouses (fight or flight) flight) Central Nervous System The Brain and Neural Networks Interconnected neurons form networks in the brain. Theses networks are complex and modify with growth and experience. Parasympathetic NS Calms (rest and digest) Complex Neural Network Central Nervous System The Spinal Cord and Reflexes Spinal Reflexes It takes only 50 ms for your foot to jerk up after your patellar tendon is lightly tapped, while it takes hundreds of milliseconds for you to move your foot in response to being told to do so. The reflex response shows that the spinal cord has the power to act alone, but the connection to the brain is required for us to be consciously aware of the sensations in the body. Simple Reflex The Endocrine System Neurons and neural communication aren t the only way information is communicated by our bodies. Sometimes, an alternate system communication called the endocrine system, is used. The endocrine system operates by use of specialized groups of cells called glands, which secrete chemicals, called hormones. The Endocrine System While the neural communication is fast, to allow us to react quickly to environmental stimuli, the endocrine system is slower and represents more long term information transfer. 9

10 Hormones and Glands Hormones and Glands Hormones foster growth and proliferation of cells, as well as growth and development of the body in general. Hormones perform their work by interacting with receptors on the surface of target cells, or by entering the cells and interacting with specialized molecules inside them. Hormones are similar to neurotransmitters in that they are chemicals which interact with receptors on one cell after being secreted by another, and like neurotransmitters, hormones are key aspects of internal communication; some acting as neurotransmitters when secreted by neurons. Hormonal control is largely an unconscious process, by which the body monitors the levels of the given hormone and the activities it affects through a negative feedback loop. The Endocrine System The Endocrine System is the body s slow chemical communication system. Communication is carried out by hormones synthesized by a set of glands. Hormones Hormones are chemicals synthesized by the endocrine glands that are secreted in the bloodstream. Hormones affect the brain and many other tissues of the body. For example, epinephrine (adrenaline) increases heart rate, blood pressure, blood sugar, and feelings of excitement during emergency situations. Pituitary Gland Is called the master gland. The anterior pituitary lobe releases hormones that regulate other glands. The posterior lobe regulates water and salt balance. Thyroid & Parathyroid Glands Regulate metabolic and calcium rate. 10

11 Adrenal Glands Adrenal glands consist of the adrenal medulla and the cortex. The medulla secretes hormones (epinephrine and norepinephrine) during stressful and emotional situations, while the adrenal cortex regulates salt and carbohydrate metabolism. Gonads Sex glands are located in different places in men and women. They regulate bodily development and maintain reproductive organs in adults. The Brain: Older Brain Structures The Brainstem is the oldest part of the brain, beginning where the spinal cord swells and enters the skull. It is responsible for automatic survival functions. The Medulla [muh DUL uh] is the base of the brainstem that controls heartbeat and breathing. Brainstem The Midbrain The midbrain is less important in mammals than in the non mammals, where it is the main source of control for visual and auditory information. In mammals these functions are mostly taken over by the forebrain. But the midbrain does help to control eye movements and coordination. The Midbrain Midbrain also contains components of the reticular activating system (RAS), which is a network of neurons that regulate arousal, sleep, and attention. 11

12 Pons The Pons, or bridge is a relay station for fibers going to and from the cerebral cortex and cerebellum. Additional fibers cross over the midline in this region as they innervate the head and face. The little brain attached to the rear of the brainstem. It helps coordinate voluntary movements and balance. Cerebellum The Brain Techniques to Study the Brain A brain lesion experimentally destroys brain tissue to study animal behaviors after such destruction. Hubel (1990) Clinical Observation Clinical observations have shed light on a number of brain disorders. Alterations in brain morphology due to neurological and psychiatric diseases are now being catalogued. Tom Landers/ Boston Globe Electroencephalogram (EEG) An amplified recording of the electrical waves sweeping across the brain s surface, measured by electrodes placed on the scalp. AJ Photo/ Photo Researchers, Inc. PET (positron emission tomography) Scan is a visual display of brain activity that detects a radioactive form of glucose while the brain performs a given task. PET Scan Courtesy of National Brookhaven National Laboratories 12

13 MRI Scan The Forebrain MRI (magnetic resonance imaging) uses magnetic fields and radio waves to produce computergenerated images that distinguish among different types of brain tissue. Top images show ventricular enlargement in a schizophrenic patient. Bottom image shows brain regions when a participants lies. Both photos from Daniel Weinberger, M.D., CBDB, NIMH James Salzano/ Salzano Photo Lucy Reading/ Lucy Illustrations The forebrain, at the front of the head is the largest area of the brain, and consists of four parts: The limbic system The thalamus the hypothalamus The cerebral cortex The Limbic System The Limbic System is a doughnut shaped system of neural structures at the border of the brainstem and cerebrum, associated with emotions such as fear, aggression and drives for food and sex. It includes the hippocampus, amygdala, and hypothalamus. The Thalamus [THALuh muss] is the brain s sensory switchboard, located on top of the brainstem. It directs messages to the sensory areas in the cortex and transmits replies to the cerebellum and medulla. Brainstem The Hypothalamus lies below (hypo) the thalamus. It directs several maintenance activities like eating, drinking, body temperature, and control of emotions. It helps govern the endocrine system via the pituitary gland. Hypothalamus The Amygdala [ah MIGdah la] consists of two lima bean sized neural clusters linked to the emotions of fear and anger. Amygdala 13

14 Reward Center Rats cross an electrified grid for self stimulation when electrodes are placed in the reward (hypothalamus) center (top picture). When the limbic system is manipulated, a rat will navigate fields or climb up a tree (bottom picture). Sanjiv Talwar, SUNY Downstate The Cerebral Cortex The cerebral cortex layer is 2 mm thick and enfolds the surface of the brain. In humans, this layer is highly convoluted and comprises about 80% of the human brain. This layer is responsible for most of our higher functions, such as our ability to plan, coordinate thoughts and actions, perceived sensory information, use language, and, in general, think. The Cerebral Cortex The cerebral cortex is comprised of gray and white tissue, with a gray tissue being the cell bodies of neurons and the white tissue being the axons of those neurons covered in myelin. Both white matter and gray matter are important for human intelligence. The Cerebral Cortex The cerebral cortex is the outer covering of both the left and right cerebral hemispheres, which are similar in appearance, and are connected by a dense fiber pathway, called the corpus callosum. Although similar looking, the two hemispheres actually process different types of information. The Cerebral Cortex Fibers coming from, or going to, the left side of the body are processed by the right hemisphere, while the left hemisphere processes information pertaining to the right side of the body. The Cerebral Cortex Not all information transmission is contralateral in this fashion, some information is ipsilateral, or stays on the same side of the brain from which the pathway originated. The corpus callosum allows for the easy transfer of information between the two hemispheres 14

15 The Cerebral Cortex The intricate fabric of interconnected neural cells that covers the cerebral hemispheres. It is the body s ultimate control and information processing center. Structure of the Cortex Each brain hemisphere is divided into four lobes that are separated by prominent fissures. These lobes are the frontal lobe (forehead), parietal lobe (top to rear head), occipital lobe (back head) and temporal lobe (side of head). Functions of the Cortex The Motor Cortex is the area at the rear of the frontal lobes that control voluntary movements. The Sensory Cortex (parietal cortex) receives information from skin surface and sense organs. Visual Function The functional MRI scan shows the visual cortex is active as the subject looks at faces. Courtesy of V.P. Clark, K. Keill, J. Ma. Maisog, S. Courtney, L.G. Ungerleider, and J.V. Haxby, National Institute of Mental Health The functional MRI scan shows the auditory cortex is active in patients who hallucinate. Auditory Function Association Areas More intelligent animals have increased uncommitted or association areas of the cortex. 15

16 Language Aphasia is an impairment of language, usually caused by left hemisphere damage either to Broca s area (impaired speaking) or to Wernicke s area (impaired understanding). Specialization & Integration Brain activity when hearing, seeing, and speaking words Today, it appears that about 90% of the adult population have language ability localized within the left hemisphere. Over 95% of righthanders have left hemisphere language dominance, and about 70% of left handers. The right hemisphere processes spatial relationships and is largely mute. It can, however, understand the meaning of words and is involved in practical language use. Damage to the right hemisphere tends to cause people to have trouble following a conversation or story, and understand metaphorical speech. The Brain s Plasticity The brain is sculpted by our genes but also by our experiences. Plasticity refers to the brain s ability to modify itself after some types of injury or illness. Our Divided Brain Our brain is divided into two hemispheres. The left hemisphere processes reading, writing, speaking, mathematics, and comprehension skills. In the 1960s, it was termed as the dominant brain. 16

17 Splitting the Brain A procedure in which the two hemispheres of the brain are isolated by cutting the connecting fibers (mainly those of the corpus callosum) between them. Courtesy of Terence Williams, University of Iowa Corpus Callosum Martin M. Rother Split Brain Patients With the corpus callosum severed, objects (apple) presented in the right visual field can be named. Objects (pencil) in the left visual field cannot. Divided Consciousness Try This! Try drawing one shape with your left hand and one with your right hand, simultaneously. BBC Biological Basis 3 17

18 Right Left Differences in the Intact Brain People with intact brains also show left right hemispheric differences in mental abilities. A number of brain scan studies show normal individuals engage their right brain when completing a perceptual task and their left brain when carrying out a linguistic task. Natural Selection Evolutionary theory, developed by English naturalist Charles Darwin ( ) in 1859, describes the way our bodies and behaviors change across many generations. Darwin propose the notion of natural selection is a mechanism by which organisms have developed and changed based on the survival of the fittest. Natural Selection The individuals who can adapt to their environment will have an advantage for survival and will eventually produce more offspring. These individuals have been selected by nature for survival. Over time, an increasing percentage of our behavior has gone from being controlled by instincts to more voluntary control by our brains. Natural Selection Thus, biopsychologists used evolutionary theory to study the nervous system to pinpoint its influence on our mood, feelings, drives, thought processes, and behavior. Evolutionary theory provides unified framework for integrating a variety of psychological phenomena, but does not provide all answers for psychological questions. 18

19 Genetics Inside each cell, our genes provide the basic physiological building blocks for the hereditary transmission of our distinctive characteristics or behavior patterns, called our biological traits. The degree by which genetics plays a role in controlling behavior is a debate that has been going on for years and is the research question that the branch of psychology, called behavioral genetics, aims to study. Mendel s Peas The Austrian monk and botanist Gregor Mendel ( ) is considered the father of the modern genetics due to his breeding experiments on common varieties of the garden pea. He found that if true breeding tall pea plants are crossed with true breeding dwarf pea plants, the offspring will always be tall. Biological Basis 5 Mendel s Peas Mendel s Peas He referred to the stronger attribute that appeared in the first generation of offspring as the dominant trait (tallness), and the weaker trait, a recessive trait (dwarfism). Mendel s Peas Whenever the dominant gene is paired with a recessive gene, Mendel found that the expression of this genotype called the phenotype, is of the dominant trait. Therefore we have a 3:1 ratio of tallness versus dwarfism Human Genotypes & Phenotypes Human genetics are a bit more complex than Mendels peas, since the expression of a single genotype can give rise to a wide range of phenotypes. For example, human height is largely genetically controlled, but a number of environmental factors, such as nutrition, hormones, and immune system deficiency, can lead to a range of phenotypic heights. 19

20 The Human Genome Project Efforts are underway to map the entire human genome. The 15 year Human Genome Project has recently preliminarily mapped the entire human genome, and it is hoped that this mapping will allow scientists to better understand how genes cause or contribute to disease and behavior. Heritability Heritability is the technical term that refers to the extent to which variation among individuals is due to genetic causes. For example, height and hair color are highly heritable because both can be predicted on the basis of inheritance from both parents. Heritability Heritability is always estimated with respect to a given trait in a given population at a given time. It does not apply to single individuals. Chromosomes Genes are parts of chromosomes, rod shaped bodies that contain many genes that are found inside the nucleus of cells. Each species has different numbers of chromosomes; humans have 46 (23 pairs) in each cell. One of each pair came from each parent at conception. Chromosomes are comprised of deoxyribonucleic acid (DNA)and govern everything from eye color to blood type and sex. Chromosomes Sex is determined by special chromosomes, known as X and Y chromosomes. Females are XX and males XY. Selective Breeding Through selective breeding, animals have been bred to possess and express various traits. Experiments have also shown that other, psychological, traits, such as intelligence can be influenced through selective breeding. 20

21 Nature and Nurture Genes and the environment interact in various ways to give various results. Through twin studies researchers can begin to uncover the contributions of both, since identical twins show similarities in intelligence and other characteristics, even if they have been raised separately in different environments. Nature and Nurture Our genes do not determine everything about us; our environment plays a definite role on who we become, but the relative contribution of each for our traits such as intelligence is still under debate. Mendel s Peas 21