CHAPTER 2

THE BIOLOGY OF MIND

Neural and Hormonal Systems

2-1 Why are psychologists concerned with human biology?

Psychologists working from a biological perspective study the links between biology and behavior. We are biopsychosocial systems, in which biological, psychological, and social-cultural factors interact to influence behavior.

2-2 What are neurons, and how do they transmit information?

Neurons are the elementary components of the nervous system, the body’s speedy electrochemical information system. A neuron receives signals through its branching dendrites, and sends signals through its axons. Some axons are encased in a myelin sheath, which enables faster transmission. Glial cells provide myelin, and they support, nourish, and protect neurons; they may also play a role in learning and thinking.

If the combined signals received by a neuron exceed a minimum threshold, the neuron fires, transmitting an electrical impulse (the action potential) down its axon by means of a chemistry-to-electricity process. The neuron’s reaction is an all-or-none process.

2-3 How do nerve cells communicate with other nerve cells?

When action potentials reach the end of an axon (the axon terminals), they stimulate the release of neurotransmitters. These chemical messengers carry a message from the sending neuron across a synapse to receptor sites on a receiving neuron. The sending neuron, in a process called reuptake, then normally reabsorbs the excess neurotransmitter molecules in the synaptic gap. If incoming signals are strong enough, the receiving neuron generates its own action potential and relays the message to other cells.

2-4 How do neurotransmitters influence behavior, and how do drugs and other chemicals affect neurotransmission?

Neurotransmitters travel designated pathways in the brain and may influence specific behaviors and emotions. Acetylcholine (ACh) affects muscle action, learning, and memory. Endorphins are natural opiates released in response to pain and exercise.

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Drugs and other chemicals affect brain chemistry at synapses. Agonists increase a neurotransmitter’s action, and may do so in various ways. Antagonists decrease a neurotransmitter’s action by blocking production or release.

2-5 What are the functions of the nervous system’s main divisions, and what are the three main types of neurons?

The central nervous system (CNS)—the brain and the spinal cord—is the nervous system’s decision maker. The peripheral nervous system (PNS), which connects the CNS to the rest of the body by means of nerves, gathers information and transmits CNS decisions to the rest of the body. The two main PNS divisions are the somatic nervous system (which enables voluntary control of the skeletal muscles) and the autonomic nervous system (which controls involuntary muscles and glands by means of its sympathetic and parasympathetic divisions).

Neurons cluster into working networks. There are three types of neurons: (1) Sensory (afferent) neurons carry incoming information from sensory receptors to the brain and spinal cord. (2) Motor (efferent) neurons carry information from the brain and spinal cord out to the muscles and glands. (3) Interneurons communicate within the brain and spinal cord and between sensory and motor neurons.

2-6 How does the endocrine system transmit information and interact with the nervous system?

The endocrine system is a set of glands that secrete hormones into the bloodstream, where they travel through the body and affect other tissues, including the brain. The endocrine system’s master gland, the pituitary, influences hormone release by other glands, including the adrenal glands. In an intricate feedback system, the brain’s hypothalamus influences the pituitary gland, which influences other glands, which release hormones, which in turn influence the brain.

Tools of Discovery and Older Brain Structures

2-7 How do neuroscientists study the brain’s connections to behavior and mind?

Clinical observations and lesioning reveal the general effects of brain damage. Electrical, chemical, or magnetic stimulation can also reveal aspects of information processing in the brain. MRI scans show anatomy. EEG, PET, and fMRI (functional MRI) recordings reveal brain function.

2-8 What structures make up the brainstem, and what are the functions of the brainstem, thalamus, reticular formation, and cerebellum?

The brainstem, the oldest part of the brain, is responsible for automatic survival functions. Its components are the medulla (which controls heartbeat and breathing), the pons (which helps coordinate movements), and the reticular formation (which affects arousal).

The thalamus, sitting above the brainstem, acts as the brain’s sensory control center. The cerebellum, attached to the rear of the brainstem, coordinates muscle movement and balance and also helps process sensory information.

2-9 What are the limbic system’s structures and functions?

The limbic system is linked to emotions, memory, and drives. Its neural centers include the hippocampus (which processes conscious memories); the amygdala (involved in responses of aggression and fear); and the hypothalamus (involved in various bodily maintenance functions, pleasurable rewards, and the control of the endocrine system). The hypothalamus controls the pituitary (the “master gland”) by stimulating it to trigger the release of hormones.

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The Cerebral Cortex and Our Divided Brain

2-10 What are the functions of the various cerebral cortex regions?

The cerebral cortex has two hemispheres, and each hemisphere has four lobes: the frontal, parietal, occipital, and temporal. Each lobe performs many functions and interacts with other areas of the cortex.

The motor cortex, at the rear of the frontal lobes, controls voluntary movements. The somatosensory cortex, at the front of the parietal lobes, registers and processes body touch and movement sensations. Body parts requiring precise control (in the motor cortex) or those that are especially sensitive (in the somatosensory cortex) occupy the greatest amount of space.

Most of the brain’s cortex—the major portion of each of the four lobes—is devoted to uncommitted association areas, which integrate information involved in learning, remembering, thinking, and other higher-level functions. Our mental experiences arise from coordinated brain activity.

2-11 To what extent can a damaged brain reorganize itself, and what is neurogenesis?

If one hemisphere is damaged early in life, the other will pick up many of its functions by reorganizing or building new pathways. This plasticity diminishes later in life. The brain sometimes mends itself by forming new neurons, a process known as neurogenesis.

2-12 What do split brains reveal about the functions of our two brain hemispheres?

Split-brain research (experiments on people with a severed corpus callosum) has confirmed that in most people, the left hemisphere is the more verbal, and that the right hemisphere excels in visual perception and the recognition of emotion. Studies of healthy people with intact brains confirm that each hemisphere makes unique contributions to the integrated functioning of the brain.

2-13 What does research tell us about being left-handed? Is it advantageous to be right-handed?

Some 10 percent of us (somewhat more among males, somewhat less among females) are left-handed. Handedness appears to be influenced by genetic or prenatal factors. Most left-handers process speech in the left hemisphere, as right-handers do, but some do so in the right hemisphere or use both hemispheres. Left-handers are more likely to be among those with reading disabilities, allergies, and migraine headaches, but sometimes do better academically. Left-handedness is also more common among musicians, mathematicians, architects, artists, and in professional baseball and cricket players. The pros and cons of being left-handed seem roughly equal.