CHAPTER OUTLINE
Biological psychology studies the cells, genes, and organs of the body, and the physical and chemical changes involved in behavior and mental processes.
I. CELLS OF THE NERVOUS SYSTEM
What are neurons and what do they do?
A. Neurons
1. There are two main cell types in the nervous system.
a) Neurons are specialized to respond rapidly to signals and send signals of their own.
b) Glial cells hold neurons together, guide their growth, maintain a stable chemical environment, provide energy, help restore damage, and respond to signals from neurons.
2. All cells have some features in common.
a) An outer membrane selectively allows only some substances to pass in and out.
b) The cell body contains the nucleus.
c) Mitochondria turn oxygen and glucose into energy.
3. Neurons have special features that permit effective signal communication.
a) An axon is a cell fiber that carries signals away from the cell body. Most neurons have just one axon.
b) A dendrite is a cell fiber that receives signals from other neurons and carries information toward the neuron’s cell body. Most neurons have many dendrites.
B. Action Potentials
1. Action potentials are electrochemical pulses that shoot down the neuron’s axon. They are “all-or-none”: A neuron either fires an action potential at full strength or does not fire at all.
2. After an action potential, there is a brief recovery time called a refractory period, during which a neuron cannot fire another action potential.
3. The speed of an action potential depends on the thickness of the axon and on the presence of myelin, a white, fatty substance that speeds up action potentials.
C. Synapses and Communication Between Neurons
1. At the axon end, the action potential causes baglike vesicles to release stored chemicals called neurotransmitters into a space between the two neurons.
2. This space is called a synapse, a connection that is a narrow gap separating the axon of one neuron from the dendrites of another. It is the means by which two neurons communicate.
3. Released neurotransmitters “float” across the synapse to “bind” with receptors, proteins on a dendrite of a receiving neuron.
4. The interaction between neurotransmitters and receptors is very specific, like a lock and key. A specific receptor (a “lock”) can only be stimulated by a specific neurotransmitter (a “key”).
5. This interaction creates a signal called a postsynaptic potential (PSP) that might make action potentials in the receiving, or postsynaptic, neuron either more or less likely. A number of PSPs sum together at the junction of the cell body and the axon. Whether or not an action potential “fires” depends on the kind of signals that are most numerous.
D. Organization of the Nervous System
The nervous system is organized into two main parts:
1. The central nervous system (CNS), encased in bone, consists of the brain and spinal cord. The CNS is the nervous system’s central executive.
2. The peripheral nervous system extends throughout the body and relays information to and from the brain.
II. THE PERIPHERAL NERVOUS SYSTEM: KEEPING IN TOUCH WITH THE WORLD
How do sights and sounds reach my brain?
The peripheral nervous system has two subsystems:
A. The Somatic Nervous System
The somatic nervous system carries signals between the senses and CNS and between the CNS and skeletal muscles. Sensory neurons bring information to the brain, and motor neurons send information from the brain to the muscles.
B. The Autonomic Nervous System
The autonomic nervous system (ANS) carries messages between the CNS and the heart, lungs, and other organs and glands. The ANS has two divisions:
1. The sympathetic system directs the body to spend energy (e.g., increased heart rate, faster breathing, sweating; sometimes called the “fight-or-flight” response) to react to stress.
2. The parasympathetic system directs the body’s functions to conserve energy (e.g., slower heart rate, increased digestive activity). Parasympathetic activity helps “calm” a person after increased sympathetic arousal.
3. Both systems may act on the same body areas, with their relative “balance” regulating the state of the targeted organs.
III. THE CENTRAL NERVOUS SYSTEM: MAKING SENSE OF THE WORLD
How is my brain "wired"?
In the CNS different functions are performed by different networks of neurons. Clusters of neurons are called nuclei, and pathways that connect the networks are bundles of axons called fiber tracts.
A. The Spinal Cord
The spinal cord receives and sends signals to and from the brain.
1. Reflexes are simple, involuntary behaviors controlled by spinal cord neurons, without requiring instructions from the brain.
2. Reflexes are controlled by a feedback system. Information about the consequences of an action goes back to the source of the action for further adjustment, if necessary.
B. The Brain
A number of tools have been developed for monitoring the brain’s structure and activity:
1. The earliest technique is called an electroencephalogram (EEG), which measures general electrical activity through electrodes on the scalp.
2. A newer technique is a positron emission tomography (PET) scan, which records the location of radioactive substances that were injected into the bloodstream. These show the location of brain activity during specific tasks.
3. Magnetic resonance imaging (MRI) records radio frequency waves after exposure to a magnetic field providing clear pictures of the anatomical structure of the brain.
4. Functional MRI (fMRI) detects changes in blood flow to provide a “moving picture” of neuronal activities.
5. The newest techniques provide even more information about brain activity, structure, and functioning. These include diffusion tensor imaging (DTI) and transcranial magnetic stimulation (TMS).
C. Thinking Critically: What Can fMRI Tell Us about Behavior and Mental Processes?
fMRI scans show where brain activity occurs as people think and experience emotion. Like phrenology in the nineteenth century, which claimed that personality traits and other mental features could be determined from bumps on people’s skulls, some feel that people will uncritically accept the claim that fMRI scans also indicate how the mind works.
1. What am I being asked to believe or accept?
fMRI scans cannot indicate the anatomical locations—in other words, the biological causes—of particular thoughts and emotions.
2. Is there evidence available to support the claim?
Although brain areas do “light up” when a person thinks or feels something, fMRI scans of these areas are not precise. First, they do not directly measure brain cell activities but just reflect blood flow and oxygen in the brain that are related in some unknown way to neuron firings. Second, an fMRI scan may miss brain cell activities that do not create simple increases in blood flow. Third, coordinated changes in millions of neurons are necessary before a detectable fMRI signal occurs. Fourth, many of the results of fMRI research depend on how the researchers decide to interpret them—they depend on judgments. And, finally, no one knows what it really means when certain brain areas appear to be activated during certain experiences.
3. Can that evidence be interpreted another way?
Supporters point to important fMRI research on brain mechanisms involved with experiencing empathy and learning by watching others. Mirror neuron mechanisms were found in parts of the brain including Broca’s area. Neurons in these areas become activated not only when a person actually experiences something, but also when he/she watches someone else do or feel the same thing. Some fMRI studies have found malfunctioning mirror mechanisms in people diagnosed with autism, a disorder that includes problems with language development, imitative skills, and empathy.
4. What evidence would help to evaluate the alternatives?
The fMRI technology will continue to improve, but knowledge about correlation and causation in fMRI must also grow. Transcranial magnetic stimulation (TMS) procedures might help identify causal versus correlational relationships in the brain by temporarily disrupting neural activity in brain regions identified by fMRI as related to a particular kind of thought or feeling. Sharing information from fMRI experiments will help to better interpret the available data. An fMRI data center has been established to store research data for review.
5. What conclusions are most reasonable?
The fMRI is an exciting tool that offers images of the structure and functioning of the brain. However, by itself, fMRI probably will not be able to explain exactly how the brain creates behavior and mental processes. Critical thinking must always underlie analysis of results of fMRI research.
D. The Hindbrain
The hindbrain is found just above the spinal cord and is composed of the following structures:
1. The medulla controls vital life functions (e.g., blood pressure, heart rate, breathing).
2. The reticular formation is a web of neurons involved in arousal and attention.
3. The locus coeruleus is a small nucleus within the reticular formation involved in directing attention.
4. The cerebellum coordinates fine motor movements, stores a memory code for well-rehearsed behaviors, and participates in cognitive tasks such as reading.
E. The Midbrain
The tiny midbrain relays information from the eyes, ears, and skin and controls certain types of automatic behaviors. The midbrain and its connections to the forebrain permit the smooth initiation of movement.
F. The Forebrain
The forebrain, the largest part of the brain, regulates many complex aspects of behavior and mental phenomena. Interior structures include the following:
1. The thalamus processes inputs from sense organs (except for smell) and then relays sensory information to appropriate “higher” forebrain areas. It is the primary sensory relay into the rest of the brain.
2. The hypothalamus regulates many physiological feedback systems, coordinating hunger, thirst, temperature regulation, and sexual behavior. It directly influences both the autonomic and the endocrine systems. It contains the suprachiasmatic nuclei, the brain’s “clock” that sets biological rhythms for the body.
3. The limbic system includes the amygdala and the hippocampus.
a) The amygdala is involved in memory and emotion. It links different kinds of sensory information together in memory. The amygdala also plays a role in fear and other emotions, linking emotions to sensations.
b) The hippocampus is critical to the ability to form new memories.
G. The Cerebral Cortex
1. The forebrain’s outer surface, the cerebral cortex, is a thin sheet of neurons. In humans, the sheet folds in on itself, giving the brain its characteristic wrinkled appearance.
2. The cerebral cortex is divided down the middle, creating two halves called the left and right cerebral hemispheres. The corpus callosum connects the two halves.
3. The folds of cortex produce gyri (ridges) and sulci, or fissures (valleys or wrinkles), on the brain’s outer surface. Several deep sulci make convenient markers for dividing the cortex of each hemisphere into four anatomical areas: the frontal, parietal, occipital, and temporal lobes.
H. Sensory and Motor Cortex
The sensory cortex and the motor cortex are two of the functional areas of the cortex.
1. Each region of the sensory cortex receives and processes input from a single sensory organ.
a) Inputs from the eyes are sent to the visual cortex in the occipital lobe.
b) Inputs from the ears are sent to the auditory cortex in the temporal lobe.
c) Inputs from the skin sensory organs connect to the somatosensory cortex in the parietal lobe. Information about skin sensations from neighboring parts of the body comes to neighboring parts of the somatosensory cortex.. This pattern on the somatosensory cortex is called a homunculus, because it is an outline of an upside-down little person.
2. Neurons in the motor cortex, in the frontal lobe, initiate voluntary movements of specific body parts. These neurons are organized so that the combined activity of neighboring groups of neurons controls movements of neighboring body regions.
I. Focus on Research: The Case of the Disembodied Woman
1. What was the researcher’s question?
Why was an apparently healthy woman falling and dropping things?
2. How did the researcher answer the question?
Dr. Sacks conducted a case study of Christina to check her sensory feedback from her joints and muscles.
3. What did the researcher find?
Christina’s sensory neurons that usually supply kinesthetic information had stopped working.
4. What do the results mean?
Our sense of our bodies comes not just from seeing them, but also from proprioception.
5. What do we still need to know?
Dr. Sack’s research provided detailed information on what neurological problem Christina experienced, but it did not show what caused her condition. Did megadoses of vitamin B6 contribute to Christina’s problem and, if so, why? Are there other causes of this kinesthetic disorder?
J. Association Cortex
1. Most of the cortex in each lobe is association cortex, with no specific sensory inputs or direct motor outputs. Rather, the association cortex combines inputs from various senses and is involved in many different mental processes.
a) Some regions of the association cortex are specifically involved in language processing.
(1) Broca’s area is a region of association cortex, usually in the left frontal lobe. Damage to this region causes difficulty speaking smoothly and grammatically, a condition called Broca’s aphasia.
(2) Wernicke’s area is a region of the association cortex, usually in the left temporal lobe. Damage to this region leaves fluency intact but makes it difficult to understand the meaning of words or to speak understandably.
b) Other association areas in the front of the brain called the prefrontal cortex are involved in complex, higher-level thought processes.
K. The Divided Brain: Lateralization
1. The physically separate left and right hemispheres perform different functions.
2. Most sensory and motor pathways cross as they enter or leave the brain. As a result, the left hemisphere receives information from and controls movements of the right side of the body, and the right hemisphere does the same for the left side of the body.
3. Studies of split-brain patients highlight the different functions of the two hemispheres.
a) The left and right hemispheres communicate through the corpus callosum, a bundle of over a million fibers. To relieve seizures in some epilepsy patients, a “split-brain” operation cuts the corpus callosum. In such patients, the two hemispheres operate somewhat independently of each other.
b) Special techniques were used to present information to only the left or right hemisphere of split brains. Patients could verbally name only those objects shown to the left hemisphere; they could use their hands to recognize objects shown to either hemisphere. This suggested that the left hemisphere, more than the right, is specialized for language.