Chapter 3 Biological Psychology

Transcription

1 Chapter 3 Biological Psychology Introduction Reductionism? Scientists in many fields use a strategy called reductionism; they attempt to explain complex phenomena by reducing them to combinations of simpler components. Chemists use atoms and molecules; physicists reduce the subatomic world to the interactions of a few fundamental forces. Nervous System Cells Neurons You experience yourself as a unitary entity. Neuroscientists have demonstrated that experience is the product of a nervous system made up of an enormous number of discrete cells. The cells that make up your nervous system are called neurons. Glia Glia support the neurons in many ways. They provide insulation, and remove waste products and foreign bodies. They are 1/10th the size of the neurons, but about 10 times as numerous. Neurons and communication Neurons are a unique type of cell that can receive and transmit information electrochemically. Sensory neurons (Afferent) carry information from sense organs to the central nervous system. Inter-Neurons in the central nervous system process that information, interpret it, and then send commands to muscles, glands and organs. Motor neurons (Efferent) carry information from the central nervous system to the muscles. Anatomy of a neuron Neurons have a variety of shapes, but they all have 3 basic parts. A cell body that contains the nucleus and most of the organelles. The dendrites, widely branching structures that receive transmissions from other neurons. The axon, which is a single, long, thin fiber with branches near its tip. Axons The function of the axon is to send the electrochemical message on to the next cell. Most axons transmit information to the dendrites or cell bodies of neighboring neurons. Many axons in vertebrates (backboned animals) have a coating of myelin, which speeds up transmission. Nerve Impulses

2 The electrochemical messages carried by neurons either increase or decrease the likelihood that the next cell will continue to transmit. Excitatory messages increase the probability that the next cell will fire - continue to carry the transmission. Inhibitory messages decrease the likelihood that transmission will continue to travel as in the case of the brain sending a message to inhibit pain in an injured extremity. Nerve cell growth Neurons do not have a fixed anatomy. Researchers have discovered that neurons are constantly growing and losing branches to dendrites and axons. This growth seems to be related to new experiences and learning. Nerve cell generation Neurons can be generated later in life (to a limited extent.) It was once thought that all neurons developed well before birth. Researchers have discovered stem cells - undifferentiated cells growing in some brain areas that are capable of developing into neurons in older organisms. Neuronal generation is generally very limited in scope. The action of stem cells seems to be stimulated after some types of brain damage, so their purpose may be in part compensatory. The growth of new neurons is much more limited than that which occurs in skin and hair cells. Action Potentials Axons convey information by a combination of electrical and chemical processes. This combination is called an action potential. An action potential is an excitation that travels along the axon at a constant strength regardless of the distance it must travel. The all-or-none law An action potential is an all-or-nothing process it s either happening or not; there s no sort of action potential. This allows the message to reach the brain at full strength, but does slow it down compared to regular electrical conduction. How an action potential works: An unstimulated axon has resting potential. Resting potential is an electrical polarization across the membrane covering the axon. A polarized axon has an inside charge that is negative (-70 millivolts) relative to the outside. Resting potential is maintained by the mechanism called the sodium-potassium pump.

3 Sodium is mostly concentrated outside the neuron, and potassium mostly inside, and they are held in place by special gates while the polarization is maintained by the action of the pump. The sodium-potassium pump sends positively charged (+1) sodium ions out of the cell and brings in a smaller number positively charged (+1) potassium ions. The result is that the outside has more positive charges than the inside. When a message from a neighboring cell excites part of the axon s membrane, some of the sodium gates are opened and sodium can enter the axon. This makes the charge inside the cell positive. Depolarization has taken place. The charge is now briefly the same inside and outside the cell. This is the action potential. The sodium gates shut very quickly and potassium gates open to allow potassium ions to leave the cell. These ions take positive charge out with them, and bring the axon back to a polarized state. Eventually the action of the sodium-potassium pump removes the excess sodium ions and recaptures the exiled potassium ions. Synapses Communication between neurons occurs at the synapses. A synapse is a specialized junction between two neurons where chemical messages cross from one to the other. The chemicals released by one will either excite or inhibit the other, making it either more or less likely to produce an action potential. This activity at the synapses is crucial to everything the brain does. Synaptic communication: Each axon has a bulge at the end called a presynaptic ending or a terminal bouton (alternately spelled button. ) When the action potential reaches the terminal bouton, molecules of a neurotransmitter are released. A neurotransmitter is a chemical that is stored in the neuron. It activates special receptors of other neurons. Neurons use a variety of neurotransmitters, but each individual neuron always uses a particular neurotransmitter or combination of them. The neurotransmitter diffuses over the synapse to the surface of the receiving neuron (called the postsynaptic neuron.) The neurotransmitter attaches to receptors on the dendrite or cell body of the receiving neuron and either excites or inhibits it.

4 After the neurotransmitter has excited or inhibited the receiving cell, it detaches from the receptor site, ending the message. The neurotransmitter may be reabsorbed by the axon that released (a process called reuptake) or diffuse away, be metabolized and removed from the body as a waste product, or remain the synapse and reattach to the receptor. system Neurotransmitters and Behavior Our understanding of the role of neurotransmitters has revolutionized medicine, particularly psychiatry. A drug that can be designed to act on a particular kind of receptor in the nervous system can also have specific effects on an organism s functioning and behavior. It can be hypothesized that unusual behavior or problems in functioning may be due to lack or excess of a particular neurotransmitter. Parkinson s Disease Parkinson s Disease is a condition in which the individual has trouble executing voluntary movements, and has tremors, rigidity and a depressed mood. This condition has been linked to a gradual decay in a system of axons that release the neurotransmitter dopamine. Dopamine is a neurotransmitter that promotes activity levels and facilitated movement. Symptoms of Parkinson s Disease can be managed in mild cases with a drug called L-dopa, which is synthesized into dopamine by the neurons. The link is not always so clear though. The symptoms of a disorder such as attention-deficit disorder or ADD include impulsive, agitated behavior and a short attention span. These symptoms would suggest an oversupply of dopamine. But there doesn t seem to be any relationship The neurotransmitter, whether it is in over, under or normal supply, is just one part of a complex system. What alleviates the problem may not necessarily tell us what originally caused the problem. The Major Divisions of the Nervous System The central nervous system and the peripheral nervous system The central nervous system consists of the brain and the spinal cord. The central nervous system communicates with the rest of the body via the peripheral nervous system. The peripheral nervous system is composed of bundles of axons between the spinal cord and the rest of the body. There are two sets of subdivisions of the peripheral nervous system. The somatic nervous system and autonomic nervous

5 The somatic nervous system is made up of the peripheral nerves that communicate with the skin and muscles. The autonomic nervous system controls the involuntary actions of the heart, stomach and other organs. The Central Nervous System Embryological development During the embryonic stage, the vertebrate nervous system forms out of a simple tube with three lumps. The forebrain that becomes the cerebral cortex and other higher structures. The midbrain and hindbrain become the brainstem. The forebrain is especially dominant in human beings. The Spinal Cord The spinal cord communicates with the body below the head by means of sensory and motor neurons. The sensory neurons carry information received by the senses from the extremities of the body to the spinal cord. The motor neurons transmit messages from the central nervous system to the muscles and glands. Both reflex and voluntary responses are conducted through the spinal cord. A reflex is a rapid, automatic response to a stimulus. The spinal cord is usually the origination point of these responses. A voluntary response originates in the brain and travels through the spinal cord to the muscles needed to carry out the movements. The Peripheral Nervous System The Autonomic Nervous System The Autonomic Nervous System A division of the peripheral nervous system that is closely associated with the spinal cord is the autonomic nervous system. The individual has very little control over the responses in this division, thus the name, autonomic. The autonomic nervous system has two subdivisions. The Divisions of the Autonomic Nervous System The sympathetic nervous system is the crisis management center. It increases heart and respiration rate and prepares the body for fight or flight. A chain of neurons lying just outside the spinal cord controls it. The parasympathetic nervous system is in charge of long-term survival related functions, nutrition and energy conservation. It decreases heart rate, increases digestive activities and promotes processes in the body that take place during rest.

6 It is controlled by neurons at the upper and lower levels of the spinal cord. The Endocrine System The Endocrine System is under the control of the nervous system. The endocrine system is a system of glands that release hormones into the bloodstream. Hormones are chemicals that affect mood, behavior and even anatomy. Some neurotransmitters act as hormones when released into the bloodstream. An example of one of these is epinphrenine, which is called adrenaline when it is acting as a hormone. Between the Spinal Cord and the Forebrain The hindbrain The medulla oblongata and the pons are two important structures in the hindbrain. They contain the axons that control breathing and heart rate. They are also in charge of relaying sensory information from the head and sending motor messages back to it. The cerebellum is important for coordination and timing. It is also in charge of tasks that requiring shifting of attention and discrimination between stimuli. The hindbrain & midbrain The medulla, pons and midbrain contain the reticular activating system (or reticular formation.) This structure regulates levels of arousal in the brain. The Forebrain The limbic system The limbic system is comprised of three major structures. The hippocampus is crucial for memory consolidation. The hypothalamus, which drives the endocrine system and regulates hunger, thirst and sexual desire. The amygdala, which is important for generating emotional and motivated behaviors. The thalamus is comprised of three major structures. Motor Control Receives Auditory, Somatosensory and Visual Sensory Signals Relays Sensory Signals to the Cerebral Cortex General structure The Forebrain The forebrain has two separate hemispheres, left and right. Each hemisphere controls sensation and motor functioning on the opposite side of the body.

7 The hemispheres of the brain communicate with each other through a thick bundle of axons crossing between them, called the corpus callosum. Cerebral Cortex The cerebral cortex The outer covering of the forebrain is known as the cerebral cortex. It is made up of the gray matter, the cell bodies of the cortical neurons. The interior of the forebrain is made up of white matter or axons of cortical neurons. It is white because of the myelin that coats axons. The four lobes of the cerebral cortex It s customary to represent the areas of the cerebral cortex as four lobes: occipital, parietal, temporal, and frontal. The occipital lobe is at the rear of the head, and contains many specialized areas for interpreting visual sensory information. There are areas both inside and outside the occipital lobes for shape, color and motion vision. The four lobes of the cerebral cortex The parietal lobe is directly in front of the occipital lobe. It contains the primary somatosensory cortex, the area of the brain that is specialized for body senses and awareness of the location of body parts. The temporal lobes are located on the sides of the head, near the ears. They are the main processing areas for hearing and complex aspects of vision. The hippocampus and amygdala are deep inside the temporal lobes. The left temporal lobe contains important language processing areas. The frontal lobes are at the front of the brain. They contain the primary motor cortex, and area that is important for control of fine movements. The foremost part of the frontal lobes, the prefrontal cortex, is responsible for organization, planning of action, and aspects of memory. Imaging the brain Methods for looking at and mapping the brain include: Computerized axial tomography (CT or CAT scanning), passes x-ray through the head while dye is present in the blood stream. This allows viewing of anatomical structures. CAT scans do not allow the viewing of brain activity. Positron emission tomography (PET) provides a high-resolution picture of brain activity using radioactivity from chemicals injected into the bloodstream.

8 The color of the image indicates the level of activity, red areas are most active, followed by yellow, green and blue for the least active areas. PET scans provide fascinating information, but are expensive and can be risky to the subject. Functional magnetic resonance imaging (fmri) uses magnetic detectors outside the head to measure the amounts of hemoglobin and oxygen in different areas of the brain. Highly active areas of the brain appear to use more oxygen in fmri images. Experience and the brain Learning changes the brain We now know, because we can see the brain, and its activity that practicing behaviors (learning to play a musical instrument, for example) can change the structure of the brain by altering the cortical neurons. Experience and the brain (The binding problem ) We still don t understand precisely how all the different parts of the brain allow us to have a unified experience of objects or events, since the areas of the brain that help us analyze our experience are often not directly connected to each other. It is amazing that people can lose just one aspect of vision, for example, color, motion, or the ability to recognize faces. The two halves of the brain Work with individuals who have had the split-brain operation (severing the corpus callosum) to control seizures provides evidence that the two hemispheres are highly specialized. The right hemisphere needs to communicate with the left in order to name the objects in its visual field. The left hemisphere needs the right in order to synthesize details into a whole picture (the parts of a face into a whole recognizable image.) The brain and the self We are still learning about the brain, but we now understand that your brain is composed of many separate areas with separate abilities. If you lose part of the brain, you lose part of your unique experience. Brain activity and mind are inseparable, one is the other.

9