Psychology, 5/e   Wortman, Loftus & Weaver
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CONCEPT I: The Human Nervous System

A great deal of human behavior is determined by neurobiological functions; that is, the way we feel and the way we act is often a function of, as Francis Crick noted, "our nerve cells and their associated molecules." Scientists have learned a great deal about this relationship between the nervous system and behavior by studying individuals who have suffered severe physical damage to their nervous systems. Thus, psychologists can learn a great deal about the normal by studying the abnormal (Theme 2).

The nervous system is composed of hundreds of billions of interconnected specialized cells. Receptor cells are embedded in the sense organs and receive stimulation from the environment. Effector cells contract muscles and cause glandular secretions. Neurons, or nerve cells, connect receptor and effector cells and conduct the signals that coordinate activity. Nerves are long, fibrous parts of many neurons bundled together. Glia function to hold neurons in place, carry nutrients and remove wastes, help repair damaged neurons, protect neurons from harmful substances, play a role in the propagation of nerve impulses, and may be involved in assembling and disassembling neural circuits.

The nervous system can be divided into the central nervous system (CNS), composed of the brain and spinal cord, and the peripheral nervous system (PNS), which extends to all parts of the body. Within the peripheral nervous system, the neurons in the afferent pathways carry information from the sensory receptor cells to the CNS, whereas the neurons in the efferent pathways transmit messages from the CNS to effector cells in the muscles and glands.

Efferent pathways are divided into the somatic nervous system, which controls the skeletal muscles, and the autonomic nervous system, which regulates the organism’s internal environment. The autonomic division has two subdivisions: The sympathetic nervous system, which mobilizes the body’s resources, and the parasympathetic nervous system, which relaxes the body and conserves its energy. The two subdivisions often have opposing effects on body functions.

CONCEPT II: The Process of Neural Transmission

Most neurons have a cell body, which is the life-support center for the cell, and two types of branching fibers: the numerous short dendrites (which receive stimulation) and the long axon (which transmits the stimulation).

Cells can transmit information down the axon through a process known as the action potential. Information may also be passed from one neuron to another through synaptic transmission.

Neural impulses are conducted by an electrochemical process: The nerve cell membrane selectively regulates which ions, or electrically charged particles, are able to move across it. Organic ions (An-) are kept inside the cell while the smaller chloride (Cl-), potassium (K+), and sodium (Na +) ions can pass through the membrane.

In the resting state, potassium ions move more freely outward than sodium ions move inward. This causes the cell membrane to become polarized, or negatively charged inside, relative to the outside of the cell. This electrical imbalance is called the resting potential. When an axon is stimulated, its membrane becomes completely permeable to sodium, allowing the positively-charged sodium ions to rush into the cell, causing the inside of the cell to become positively charged relative to the exterior. The cell has become depolarized; that is, its inside is less negative. Inputs causing this depolarization are called excitatory potentials. When other inputs cause the cell membrane to allow in more chlorine ions, the inside becomes less negative, a process known as hyperpolarization. Inputs which cause the inside of the cell to become more negative are called inhibitory potentials. Graded potentials are changes in the electrical potential of dend rites in proportion to the amount of stimulation being received.

The presence of a myelin sheath (a fatty, whitish substance made up of certain glial cells) around the axon speeds neural conduction and forms the white matter of the nervous system. (Gray matter is composed of nonmyelinated axons, dendrites, and cell bodies.) Between the glial cells in the myelin sheath are gaps, called nodes of Ranvier. Action potentials can jump from one node to the next, thereby increasing their speed. The disease called multiple sclerosis is caused by the progressive destruction of myelin and results in progressive loss of muscle control.

The axon fires in an all-or-none fashion, reaching the same action potential regardless of level of stimulation beyond the threshold. Dendrites, however, work differently. When a dendrite is stimulated, the electrical charge across its membrane changes in proportion to the amount of stimulation, a change called a graded potential. Thus, excitatory stimuli can be added together until they are strong enough to trigger an action potential in the axon. Inhibitory stimulation can diminish the graded potential and prevent the depolarization of the axon.

CONCEPT III: Neurotransmitters: Chemical Messengers

The axon of one cell comes close to but never touches the dendrites of the next cell. The area surrounding this space, as well as the tip of the axon on one side and the cell membrane on the other, is called the synapse. Conduction of the neural impulse across this space (called a synaptic cleft) is accomplished by the axon’s release of neurotransmitters, which diffuse across the synaptic cleft and activate receptor sites on the adjacent cell.

One group of neurotransmitters includes the monoamines. Examples of monoamines are dopamine, norepinephrine, and serotonin. Dopamine is often found in the basal ganglia, a part of the forebrain that controls fine motor activity. It plays an important role in thought disorders and movement disorders. Schizophrenia and the motor disorder Parkinson’s disease are related to dopamine. Norepinephrine is important in arousal and mood, especially depression. Serotonin, often found in the brain stem and thalamus, has an important influence on sleep, hallucinations, and emotional behavior.

Acetylcholine is a prominent neurotransmitter at neuromuscular junctions, and has been implicated in Alzheimer’s disease. Gamma-aminobuturic acid (GABA), the most prevalent inhibitory neurotransmitter, plays a role in arousal and sleep. It is often found in the brain and spinal cord. Endorphins are a group of neurotransmitter-like substances, which regulate pain, pleasure, eating, and drinking. Opiates such as morphine and heroin can mimic the pain-reducing effects of endorphins.

CONCEPT IV: The Brain: Structure and Function

The human brain is very similar to the brains of other mammals, having been shaped in structure and function by millions of years of evolution. Yet, differences do exist. Humans have considerably larger forebrain structures, which are important in thought and reason.

The major regions of the brain are the hindbrain, midbrain, and forebrain. The hindbrain is the top of the spinal cord and includes the medulla (which controls aspects of circulation, breathing, chewing, salivation, and facial movements), the pons (which integrates movements from the right and left body halves, transmits information to other brain parts, and may serve some function in sleep control), and the reticular formation (which affects attention and the sleep-waking cycle). The hindbrain also includes the cerebellum (which coordinates voluntary movement, regulates balance, and is involved in the remembering of simple motor tasks).

The midbrain contains important centers for visual and auditory reflexes and conveys information between the brain and spinal cord. The forebrain is extremely important for mammals. Its outer surface is called the cerebral cortex.

Beneath the cerebral cortex is a highly interconnected group of structures known as the limbic system. The thalamus acts as a relay and processing station between sensory receptors and the higher brain and integrates various areas of the brain. The hypothalamus is an important structure that regulates much of the body’s internal environment, such as temperature and metabolism. It is also involved in many basic behaviors, such as eating, drinking, sexual activity, fear, and aggression. It plays an important role in regulating the pituitary gland. This gland regulates hormonal secretions from other glands throughout the body. Another important limbic structure is the hippocampus, associated with learning and memory. The amygdala are associated with learning and memory, especially as related to emotion. The septum is another limbic structure and is associated with emotion and pleasure.

The cerebral hemispheres, also known as the forebrain, are the most recent in the brain’s evolutionary development. The cerebral hemispheres are the large twin structures at the top of the brain; they are involved with the higher-level functions of learning, speech, reasoning, and memory. These hemispheres are covered by the cortex. They can be separated into four lobes: the frontal lobes (front), parietal lobes (top back), temporal lobes (side), and occipital lobes (lower back). The central fissure separates the frontal from the parietal lobe; the lateral fissure marks the top boundary of the temporal lobe.

The area of the frontal lobe next to the central fissure is called the motor cortex and is responsible for regulation of voluntary movements. The same area in the parietal lobe is the somatosensory cortex, and it registers body sensations. Control in the motor cortex and somatosensory cortex is called contralateral control--that is, the left side of the brain controls the right half of the body, and vice versa. The temporal lobe of each hemisphere involves integration of sensory information, especially auditory signals. Visual information is primarily processed in the occipital lobe.

CONCEPT V: Split Brain Research: Studying the Two Hemispheres

The two cerebral hemispheres each have certain special abilities to control, a phenomenon called lateralized function. For example, speech centers, including Wernicke’s area and Broca’s area, are generally housed in the left hemisphere. Studies with patients who have had the fibers connecting the two hemispheres, called the corpus callosum, severed to treat severe epilepsy show further evidence of the lateralized functions of the brain. Studies of these split-brain patients have shown that language, mathematics, and analytical thinking are generally left-brain activities; perception of spatial relationships and artistic abilities are generally attributes of the right hemisphere. It is important to remember, however, that in normal people with intact corpus callosums the hemispheres are in constant communication with each other. The two hemispheres have much in common, and complex activity involves many locations in both hemispheres.

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