Biological Psychology • In this chapter we will examine: • What are the components of the nervous system? • How does the brain create mental processes and behavior? “What we understand least is why brain activity produces experience at all.” -- James W. Kalat
Module 3.1 • Neurons and Behavior
Introduction • Reductionism? • Scientists in many fields use a strategy calledreductionism; 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.
Introduction • Reductionism? • Does reductionism work in the science of psychology? • Let’s find out as we try to explain behavior in terms of the activity of the cells that comprise the nervous system.
Nervous System Cells • Neurons • You experience yourself as a unitary entity. • Neuroscientists have demonstrated that 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.
Figure 3.1 Distribution of the estimated 100 billion neurons in the adult human central nervous system. (Based on data of R. W. Williams & Herrup, 1988)
Nervous System Cells • Neurons and communication • Neurons are a unique type of cell that can receive and transmit information electrochemically. • Sensory neurons carry information from sense organs to the central nervous system. • Neurons in the central nervous system process that information, interpret it, and then send commands to muscles, glands and organs.
Nervous System Cells • The best current estimate is that the human nervous system has nearly 100 billion neurons. • And they aren’t the only type of cell in the system.
Nervous System Cells • 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.
Nervous System Cells • 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.
Nervous System Cells • 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 have a coating of myelin, which speeds up transmission.
Nervous System Cells • 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.
Nervous System 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.
Nervous System Cells • Action potentials • 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.
Nervous System Cells • Action potentials • How an action potential works: • An un-stimulated 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.
Nervous System Cells • Action potentials • How an action potential works: • Resting potential is maintained by the mechanism called the sodium-potassium pump. • 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.
Figure 3.4 The sodium and potassium gradients for a resting membrane. Sodium ions are concentrated outside the neuron; potassium ions are concentrated inside. Because of the negatively charged protein ions inside the neuron, the inside of the cell is negatively charged relative to the outside of the cell. Protein and chloride ions (not shown) bear negative charges inside the cell. At rest, very few sodium ions cross the membrane except by the sodium-potassium pump. Potassium tends to flow into the cell because of an electrical gradient, and tends to flow out because of the concentration gradient.
Nervous System Cells • Action potentials • How an action potential works: • The sodium-potassium pump sends positively charged (+1) sodium ions out of the cell and brings in a smaller number of positively charged (+1) potassium ions. • The result is that the outside of the cell has more positive charges than the inside.
Nervous System Cells • Action potentials • How an action potential works: • 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.
Figure 3.6 (a) During an action potential, sodium gates in the neuron membrane open, and sodium ions enter the axon, bringing a positive charge with them. (b) After an action potential occurs at one point along the axon, the sodium gates close at that point and open at the next point along the axon. When the sodium gates close, potassium gates open, and potassium ions flow out of the axon, carrying a positive charge with them. (Modified from Starr & Taggart, 1992)
Figure 3.6 (a) During an action potential, sodium gates in the neuron membrane open, and sodium ions enter the axon, bringing a positive charge with them.
Figure 3.6 (b) After an action potential occurs at one point along the axon, the sodium gates close at that point and open at the next point along the axon. When the sodium gates close, potassium gates open, and potassium ions flow out of the axon, carrying a positive charge with them. (Modified from Starr & Taggart, 1992)
Nervous System Cells • Action potentials • How an action potential works: • The sodium gates shut very quickly and potassium gates open to allow potassium ions to leave the cell. • These ions take the 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.
Concept Check If a hamster and a seven-foot-tall human step on a sharp object, which will respond faster? Why? The hamster, because the action potential has a shorter distance to travel.
Nervous System Cells • 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.
Nervous System Cells • Synaptic transmission • 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.
Figure 3.8 The synapse is the junction of the presynaptic (message-sending) cell and the postsynaptic (message-receiving) cell. At the end of the presynaptic axon is the terminal bouton (or button), which contains many molecules of the neurotransmitter, ready for release.
Nervous System Cells • Synapses • Synaptic communication: • Each axon has a bulge at the end called a pre-synaptic 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.
Nervous System Cells • Synapses • Synaptic communication: • 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.
Figure 3.9 The complex process of neural communication actually takes only 1–2 milliseconds.
Nervous System Cells • Synapses • Synaptic communication: • 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.
Concept Check Learning and environmental challenges sometimes produce branching in axons and dendrites of an organism’s neurons. How would that affect the number of synapses? It would increase the number of synapses.
Concept Check • Dopamine is a neurotransmitter that excites postsynaptic neurons. If a drug were injected into an animal that blocked dopamine from attaching to its receptors, what would happen to the postsynaptic neurons? They would be less likely to produce further action potentials.
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.
Neurotransmitters and Behavior • 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.
Neurotransmitters and Behavior • Parkinson’s disease • 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.
Figure 3.11With Parkinson’s disease, axons from the substantia nigra gradually die. (a) Normal brain (b) Brain of person with Parkinson’s disease. Green = excitatory path; red = inhibitory.
Neurotransmitters and Behavior • The link is not always so clear though. • The symptoms of a disorder such as attention-deficit disorder or ADDinclude 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 between dopamine and ADD.
Concept Check People suffering from schizophrenia are given haloperidol, a drug that blocks activity at dopamine synapses. How would haloperidol affect a person with Parkinson’s Disease? It would make the symptoms worse.
Neurotransmitters and Behavior • 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.
Module 3.2 • The Nervous System and Behavior
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.
Figure 3.12 The nervous system has two major divisions: the central nervous system and the peripheral nervous system. Each of these has major subdivisions, as shown.
The Major Divisions of the Nervous System • The central nervous system and 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 Peripheral Nervous System • The somatic nervous system and autonomic nervous system • 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: • Theforebrain that becomes the cerebral cortex and other higher structures. • The midbrain and hindbrain become the brainstem. • The forebrain is especially dominant in human beings.
Figure 3.13 The human brain begins development as three lumps. By birth the forebrain has grown much larger than either the midbrain or the hindbrain, although all three structures perform essential functions.
The Forebrain • 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. • The hemispheres of the brain communicate with each other through a thick bundle of axons crossing between them, called the corpus callosum.
The Forebrain • 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.