Concept 34.1 Nervous Systems Consist of Neurons and Glia

• The cells of the nervous system are either neurons or glia. Neurons generally receive information via their dendrites and transmit information via their axon. Review Figure 34.1
   
• Glia include Schwann cells and oligodendrocytes, both of which generate myelin sheets on axons. Glia also include astrocytes, which contribute to the blood–brain barrier. Review Figure 34.2
   
• Neurons are organized in networks with sensory inputs (afferent neurons), outputs (efferent neurons), and integration (interneurons). These networks may be simple or complex. Review Figure 34.3
   

Concept 34.2 Neurons Generate and Transmit Electrical Signals

• Neurons have an electric charge difference across their plasma membranes, called the membrane potential. The membrane potential is created by ion transporters and channels. In inactive neurons, the membrane potential is called the resting potential. Review Figures 34.4 and 34.5 and ANIMATED TUTORIAL 34.1
   
• The sodium–potassium pump concentrates K⁺ on the inside of a neuron and Na⁺ on the outside. Potassium channels allow K⁺ to diffuse out, causing the resting potential to be negative. Review Figure 34.5
   
• The Nernst equation can be used to calculate the equilibrium potential of a single ion. Review WEB ACTIVITY 34.1
   
• When ion channels open or close, the plasma membrane can become depolarized or hyperpolarized. This causes a graded membrane potential. Review Figure 34.6
   
• An action potential is a rapid reversal in charge across a portion of the plasma membrane resulting from the opening and closing of voltage-gated channels of Na⁺ and K⁺. These voltage-gated channels open when the plasma membrane depolarizes to a threshold level. Review Figure 34.7 and ANIMATED TUTORIAL 34.2
   
• Action potentials are all-or-none, self-regenerating events. They are conducted down axons because local current flow depolarizes adjacent regions of membrane and brings them to threshold.
   
• In myelinated axons, action potentials jump between nodes of Ranvier, patches of membrane that are not covered by myelin. Review Figure 34.8
   

Concept 34.3 Neurons Communicate with Other Cells at Synapses

• Neurons communicate with each other and with other cells by transmitting information over chemical synapses (with neurotransmitters) or electrical synapses.
   
• The neuromuscular junction is a well-studied chemical synapse between a motor neuron and a skeletal muscle cell. Its neurotransmitter is acetylcholine (ACh). Review Figure 34.9
   
• When an action potential reaches an axon terminal, it causes the release of neurotransmitters, which diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane. Review Figures 34.9 and 34.10, ANIMATED TUTORIAL 34.3, and INTERACTIVE TUTORIAL 34.1
   
• Synapses between neurons can be either excitatory or inhibitory. A postsynaptic neuron integrates information by summation of excitatory and inhibitory postsynaptic potentials in both space (spatial summation) and time (temporal summation). Review Figure 34.11
   
• There are many different neurotransmitters and types of receptors. The action of a neurotransmitter depends on the receptor to which it binds.
   
• Synapses can be fast or slow, depending on the nature of their receptors. Ionotropic receptors are ion channels and generate fast, short-lived responses. Metabotropic receptors initiate second-messenger cascades that lead to slower, more sustained responses.
   

Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components

• The brain and spinal cord make up the central nervous system (CNS); neurons that extend or reside outside the brain and the spinal cord, together with their supporting cells, make up the peripheral nervous system (PNS). Review Figure 34.12
   
• The autonomic nervous system (ANS) is the part of the PNS that controls involuntary physiological functions. Its sympathetic and parasympathetic divisions differ in anatomy, neurotransmitters, and effects on target tissues. Review Figure 34.13
   
• The spinal cord communicates information between the brain and the rest of the body. It can issue some commands to the body without input from the brain (reflexes). Review Figure 34.14 and ANIMATED TUTORIAL 34.4
   
• The embryonic brain consists of a hindbrain, midbrain, and forebrain. The forebrain develops into the cerebral hemispheres (the telencephalon, or cerebrum) and the underlying thalamus and hypothalamus (which together compose the diencephalon). The midbrain and hindbrain develop into the brainstem and the cerebellum.
   
• The reticular system is a complex network in the brainstem that controls various autonomic functions and transmits sensory information to the forebrain.
   
• The limbic system is an evolutionarily primitive part of the telencephalon that is involved in emotions, physiological drives, instincts, and memory. Review Figure 34.15
   
• The cerebral hemispheres are the dominant structures of the human brain. Their surfaces are layers of neurons called the cerebral cortex. Review Figure 34.16
   
• Each cerebral hemisphere can be divided into temporal, frontal, parietal, and occipital lobes. Many motor functions are localized in the frontal lobe. Information from many sensory receptors projects to the parietal lobe. Visual information goes to the occipital lobe, and auditory and visual information goes to the temporal lobe. Review Figure 34.16 and WEB ACTIVITY 34.2
   

Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans

• Language abilities are localized mostly in the left cerebral hemisphere, a phenomenon known as lateralization. Different areas of the left hemisphere are responsible for the production and understanding of language. Review Figure 34.18
   
• Complex memories can be elicited by stimulating small regions of association cortex. Damage to the hippocampus can destroy the ability to form long-term declarative memory but not procedural memory.
   
• Most animals, including humans, have a daily cycle of sleep and waking. Sleep can be divided into rapid-eye-movement (REM) sleep and non-REM sleep. Review Figure 34.19
   
• A sense of the physiological state of the body may be created in the insular cortex. Evolution of this integrative function could be the basis for conscious experience.
   

See WEB ACTIVITY 34.3 for a concept review of this chapter.