1. THE NERVOUS SYSTEM
   1. Cells of the Nervous System
      • Cells of the nervous system share many characteristics with other cells in the body.
      • Many cells communicate with one another in a process whereby the internal activity of one cell changes in response to external stimuli (such as chemicals) released by another cell.
      • Nervous system cells, like other body cells, each have an outer membrane, a cell body, and a nucleus. Neurons respond to environmental changes by means of three special features: structures called axons and dendrites, "excitable" surface membranes, and synapses. Axons carry signals away from the neuron to points where communication occurs with other neurons, whereas dendrites detect and carry information from other nerve cells to the cell body. Other nervous-system cells, called glial cells, hold neurons in place, direct their growth and repair, and keep their chemical environment stable.
   2. Action Potentials
      • The selective permeability of the neuronal membrane keeps positively charged sodium and calcium ions from freely entering the axon through the gates or channels. As a result, the membrane becomes electrically polarized, such that the inside of the cell is more negatively charged than the outside. When an action potential occurs, some part of the axon membrane becomes depolarized, causing a sodium or calcium gate to open. Sodium rushes into the axon, causing the neighboring sodium gates to open as well, and more sodium rushes in. This chain of events occurs along the entire length of the axon.
      • The neuron either fires or does not fire in an all-or-none type of signal. The speed of the action potential is constant as it travels down the axon. If a neuron is larger or is coated with myelin, action potentials will be faster. The length of the pause between action potentials is known as the refractory period and determines the rate or number of action potentials that occur within a given time unit. Messages in the nervous system are coded by the speed and rate of action potentials.
   3. Synapses and Communication Between Neurons
      Communication between neurons occurs at the synapse.
      1. Neurotransmitters. Communication at the synapse is chemical in nature. Neurotransmitters are chemicals that carry the signal across the synapse to the postsynaptic cell (usually a dendrite). When neurotransmitters bind to receptors in the postsynaptic cell, a membrane potential is created.
      2. Excitatory and Inhibitory Signals. When a postsynaptic cell is reached by a neurotransmitter, the postsynaptic membrane becomes depolarized or hyperpolarized, creating a postsynaptic potential, and the signal once again becomes electric in nature. A depolarized membrane will cause an excitatory postsynaptic potential (EPSP), and a hyperpolarized membrane will cause an inhibitory postsynaptic potential (IPSP). The combined impact of the many EPSPs and IPSPs will determine whether or not an action potential will occur.
   4. Organization and Functions of the Nervous System
      • Neurons in the brain and spinal cord are organized into groups called neural networks. The sensory system provides input about the environment via the five senses. The motor system directs the response to the environment by influencing the muscles and other organs.
      • The peripheral nervous system is made up of the sensory and motor systems. The central nervous system (CNS) processes information and is encased in bone. The CNS is made up of the brain and spinal cord.
2. THE PERIPHERAL NERVOUS SYSTEM: KEEPING IN TOUCH WITH THE WORLD
   The peripheral nervous system has two components: the somatic and autonomic systems.
   1. The Somatic Nervous System
      The somatic system carries information from the senses to the CNS and sends movement instructions back to the muscles.
   2. The Autonomic Nervous System
      The autonomic system, via the parasympathetic and sympathetic branches, transmits messages between the CNS and the body's organs and glands. The sympathetic branch prepares the body for action through the "fight-or-flight" response. The parasympathetic branch does the opposite: it slows organ and gland activity to conserve the body's energy.
3. CENTRAL NERVOUS SYSTEM: MAKING SENSE OF THE WORLD
   The central nervous system (CNS) is made up of collections of neuronal cell bodies called nuclei and the fiber tracts or pathways that connect them.
   1. The Spinal Cord
      • The spinal cord carries messages to and from the brain. Reflexes--quick, involuntary muscular responses (through efferent neurons) that are initiated on the basis of incoming sensory information (through afferent neurons)--occur in the spinal cord without instruction from the brain. The brain is informed of each reflex after it occurs.
      • The spinal cord is an example of a feedback system, a process in which information about an action's results are conveyed back to the source of the action so that further adjustments to the activity can be made.
   2. The Brain
      1. The Hindbrain.Hindbrain structures such as the medulla control vital functions (for example, blood pressure, heart rate, and breathing). The reticular formation is a network of cells running throughout the hindbrain that alters the activity of other brain structures. For example, the locus coeruleus, an area thought to be involved in the state of vigilance, is activated by the reticular formation. The cerebellum controls finely coordinated movements, including speech.
      2. The Midbrain. Located between the hindbrain and forebrain, the midbrain controls certain automatic behaviors. The substantia nigra is a midbrain structure that, together with the striatum, is involved in initiating smooth movement.
      3. The Forebrain. The forebrain is the most highly developed part of the human brain. The diencephalon includes the thalamus, which relays sensory signals, and the hypothalamus, which regulates basic drives. The suprachiasmatic nuclei, part of the hypothalamus, determines our biological rhythms. The cerebrum includes the amygdala and hippocampus. The limbic system plays an important role in regulating emotion and is involved in memory and other thought processes.
   3. Focus on Research Methods: Manipulating Genes in Animal Models of Human Disease
      To test hypotheses about the cause of Alzheimer's disease, experimenters implanted a gene into mice and found that it created brain damage similar to Alzheimer's. If the damage creates similar memory impairments, scientists may have succeeded in making an animal model of the disease that will allow more specific research on Alzheimer's.
   4. The Cerebral Cortex
      The cerebral cortex is the outer surface of the cerebrum or cerebral hemispheres. The frontal, parietal, occipital, and temporal lobes are used as physical landmarks for describing the cortex. The functional areas of the cortex include the sensory, motor, and association cortex.
      1. Sensory Cortex. The sensory cortex receives sensory information.
      2. Motor Cortex. The motor cortex neurons control the onset of voluntary movement.
      3. Association Cortex. The association cortex receives information from more than one sense and combines sensory and motor information. Aphasia, a deficit in understanding and producing language, is caused by damage to Broca's area or Wernicke's area. Many areas of the brain are related to language; several are associated with specific semantic abilities.
   5. The Divided Brain in a Unified Self
      1. Split-Brain Studies. Split-brain (severed corpus callosum) data demonstrate that each hemisphere is superior in certain abilities. The left hemisphere controls spoken language, and the right controls recognition of faces and tasks dealing with spatial relations, such as drawing three-dimensional shapes. In addition, the left hemisphere controls the right side of the body and the right hemisphere controls the left side.
      2. Lateralization of Normal Brains. Data collected from people with intact brains demonstrate the superiority of the left hemisphere in logical thinking and language abilities. The right hemisphere exhibits better spatial, artistic, and musical abilities. But keep in mind that, while the two halves of the brain may have lateralized abilities, the cerebral hemispheres work closely together.
   6. Plasticity: Repairing Damage in the Central Nervous System
      • Synaptic plasticity is the brain's ability to strengthen neural connections and establish new connections. Unfortunately, synaptic plasticity is somewhat limited. New neurons cannot be generated, and exact replication of the many synaptic connections prior to brain damage is almost impossible. However, old neurons do produce new axons and dendrites, which make new connections.
      • Scientists are continually trying new methods to reproduce and grow nerve cells. Several methods for enhancing brain-damage recovery are under study, such as tissue transplants into the damaged area from another brain, nerve growth factor for stimulation, and guidance of axon growth.
   7. Linkages: Human Development and the Changing Brain
      Patterns of behavioral development in infants are correlated with plastic changes in activity and structure in the developing brain. During development, the brain overproduces neural connections and, based on experience, establishes which connections are needed and then eliminates the extras. Even in adulthood, the number of connections is affected by experience: stimulating environments produce greater numbers of connections.
4. THE CHEMISTRY OF PSYCHOLOGY
   A group of neurons that communicates with the same neurotransmitter is called a neurotransmitter system. Some neurotransmitter systems are responsible for certain behaviors or problems.
   1. Three Classes of Neurotransmitters
      1. Small Molecules are found in both the CNS and PNS. Below are six examples of small molecules that function as neurotransmitters.
         • Acetylcholine controls the contraction of muscles and is used by neurons in the parasympathetic nervous system. In the brain, cholinergic neurons are involved in movement and memory. A loss of cholinergic neurons is linked to Alzheimer's disease.
         • Arousal is the main task of norepinephrine, which is also known as noradrenaline. Noradrenaline is used by neurons in the sympathetic nervous system and in the locus coeruleus.
         • Sleep, moods, and appetite are influenced by serotonin.
         • Problems with the dopamine system in the substantia nigra and striatum contribute to Parkinson's disease, which leads to difficulty in the initiation of movement. Dopamine is also involved in the experience of pleasure. Malfunctioning dopaminergic neurons may be partly responsible for schizophrenia, a psychological disorder characterized by distorted perception, emotion, and thought.
         • Gamma-amino butyric acid or GABA is the major inhibitory transmitter of the central nervous system. Huntington's disease causes a loss of GABA-using neurons and results in uncontrollable movement of the arms and legs.
         • The major excitatory neurotransmitter in the CNS is glutamate. Glutamate helps strengthen synaptic connections, which may be the origin of learning and memory.
      2. Peptides were discovered as scientists investigated opiates, substances derived from the poppy flower.
         • Endorphins, natural opiate-like compounds, can reduce pain and cause sleep.
      3. Gases were most recently discovered to act as neurotransmitters.
         • Nitric oxide and carbon monoxide are gases that function as neurotransmitters.
   2. Thinking Critically: Are There Drugs That Can Make You Smarter?
      What am I being asked to believe or accept?
      Certain drugs, called nootropics, improve cognitive functioning.
      What evidence is available to support the assertion?
      Some research results are positive; subjects showed significant improvement on memory and general cognitive tasks.
      Are there alternative ways of interpreting the evidence?
      Research studies using a double-blind design show weak and temporary results in a limited number of cognitive areas.
      What additional evidence would help to evaluate the alternatives?
      Much research remains to be done to determine which drugs have positive effects and on what type of cognitive functions with little or no side effects.
      What conclusions are most reasonable?
      Eventually, drugs that improve memory and other cognitive functions will be found, but those currently on the market have very limited effect.
5. THE ENDOCRINE SYSTEM: COORDINATING THE INTERNAL WORLD
   The endocrine system, like the nervous system, influences a wide variety of behaviors. Glands secrete hormones, which travel via the bloodstream and affect coordinated systems of target tissues and organs, producing such responses as the fight-or-flight syndrome. A negative feedback system involving the brain regulates the amount of hormone released.
6. THE IMMUNE SYSTEM: LINKING THE BRAIN AND THE BODY'S DEFENSE SYSTEM
   • The immune system is very similar to the nervous and endocrine systems: all three systems are capable of responding to their environment, and all have cells that communicate with one another. The function of the immune system is to monitor the internal body for the presence of foreign or harmful material and to eliminate it. In autoimmune disorders the immune system attacks normal body cells.
   • Five lines of evidence demonstrate the interaction of the immune, nervous, and endocrine systems.
     1. Stress has a negative impact on the functioning of the immune system.
     2. Immune responses can be learned. Learning involves the nervous system.
     3. Stimulating or damaging certain parts of the nervous system, such as the hypothalamus and portions of the cortex and brainstem, cause changes in the ways in which the immune system functions.
     4. Immune system activity causes changes in neurotransmitter activity, hormonal secretion, and behavior.
     5. Some of the same chemical messengers are found in the brain and in the immune system.