3.7 SUMMARY

• Neurons are the building blocks of the nervous system: They process information received from the outside world, communicate with one another, and send messages to the body’s muscles and organs.
• Neurons are composed of three major parts: the cell body (which contains the nucleus and houses the organism’s genetic material), the dendrites (which receive sensory signals from other neurons and transmit this information to the cell body), and the axon (which carries signals from the cell body to other neurons or to muscles and organs in the body).
• Neurons don’t actually touch. They are separated by a small gap, which is part of the synapse across which signals are transmitted from one neuron to another.
• Glial cells provide support for neurons, usually in the form of the myelin sheath, which coats the axon to facilitate the transmission of information. In demyelinating diseases, the myelin sheath deteriorates.

• The neuron’s resting potential is due to differences in the potassium (K+) concentrations inside and outside the cell membrane, resulting from open channels that allow K+ to flow outside the membrane while closed channels don’t allow sodium ions (Na+) and other ions to flow into the neuron.
• If electric signals reach a threshold, this event initiates an action potential, an all-or-none signal that moves down the entire length of the axon. After the action potential has reached its maximum, a chemical pump reverses the imbalance in ions, returning the neuron to its resting potential. For a brief refractory period, the action potential cannot be re-initiated.
• Communication between neurons takes place through synaptic transmission, where an action potential triggers release of neurotransmitters from the terminal buttons of the sending neuron’s axon, which travel across the synapse to bind with receptors in the receiving neuron’s dendrite.
• Neurotransmitters bind to dendrites on specific receptor sites. Neurotransmitters leave the synapse through reuptake, through enzyme deactivation, and by binding to autoreceptors.
• Some of the major neurotransmitters are acetylcholine (ACh), dopamine, glutamate, GABA, norepinephrine, serotonin, and endorphins.
• Drugs can affect behavior by acting as agonists, that is, by facilitating or increasing the actions of neurotransmitters, or as antagonists by blocking the action of neurotransmitters. Recreational drug use can have an effect on brain function.

• The nervous system is divided into the peripheral and the central nervous systems.
• The peripheral nervous system connects the central nervous system with the rest of the body, and it is itself divided into the somatic nervous system (which controls voluntary muscles) and the autonomic nervous system (which controls the body’s organs).
• The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system prepares the body for action in threatening situations, and the parasympathetic nervous system returns the body to its normal state.
• The central nervous system is composed of the spinal cord and the brain. The spinal cord can control some basic behaviors such as spinal reflexes without input from the brain.

• The brain can be divided into the hindbrain, midbrain, and forebrain.
• The hindbrain generally coordinates information coming into and out of the spinal cord with structures such as the medulla, the reticular formation, the cerebellum, and the pons. These structures respectively coordinate breathing and heart rate, regulate sleep and arousal levels, coordinate fine motor skills, and communicate this information to the cortex.
• The structures of the midbrain, the tectum and tegmentum, generally coordinate functions such as orientation to the environment and movement and arousal toward sensory stimuli.
• The forebrain generally coordinates higher-level functions, such as perceiving, feeling, and thinking. The forebrain houses subcortical structures, such as the thalamus, hypothalamus, hippocampus, amygdala, and basal ganglia; all of these structures perform a variety of functions related to motivation and emotion.
• Also in the forebrain, the cerebral cortex, composed of two hemispheres with four lobes each (occipital, parietal, temporal, and frontal), performs tasks that help make us fully human: thinking, planning, judging, perceiving, and behaving purposefully and voluntarily.
• Neurons in the brain can be shaped by experience and the environment, making the human brain amazingly plastic.

• The nervous system is the first system that forms in an embryo, starting as a neural tube, which forms the basis of the spinal cord, then expands on one end to form the hindbrain, midbrain, and forebrain. Then, within each of these areas, specific brain structures begin to differentiate.
• Nervous systems evolved from simple collections of sensory and motor neurons in simple animals, such as flatworms, to elaborate centralized nervous systems found in mammals.
• The gene, or the unit of hereditary transmission, is built from strands of DNA in a double-helix formation that is organized into chromosomes. Humans have 23 pairs of chromosomes; half come from each parent.
• The study of genetics indicates that both genes and the environment work together to influence behavior. Genes set the range of variation in populations within a given environment, but these genes do not predict individual characteristics; experience and other environmental factors play a crucial role as well.
• Epigenetics, environmental influences that determine whether or not genes are expressed even when not altering the basic DNA sequences that constitute the genes, have been shown to play a critical role in persisting effects of early experiences in rats and humans.

• The brain can be investigated by observing how perceptual, motor, intellectual, and emotional capacities are affected following brain damage; specific disruptions after damage in particular areas of the brain suggest that the brain area normally plays a role in producing those behaviors.
• Scientists can examine global electrical activity in large areas of the brain using the electroencephalograph (EEG), or they can examine the activity pattern of single neurons by recordings taken from specific neurons.
• Brain imaging, such as fMRI and PET, allow scientists to scan the brain as people perform different perceptual or intellectual tasks. Correlating energy consumption in particular brain areas with specific events suggests that those brain areas are involved in specific types of processing.