Concept 34.1   Nervous Systems Are Composed of Neurons and Glial Cells

• A neuron (nerve cell) is excitable; it is specially adapted to generate and propagate electric signals, typically in the form of action potentials. Neurons make functionally relevant contact with other cells at synapses.
• Neurons usually have four anatomical regions: a set of dendrites, a cell body, an axon, and a set of presynaptic axon terminals. Dendrites receive signals from other neurons, and presynaptic axon terminals send signals to other cells. Review Figure 34.2
• Unlike neurons, glial cells are usually not excitable. In vertebrates, they include oligodendrocytes, which form insulating myelin sheaths around axons in the brain and spinal cord, and Schwann cells, which perform the same function outside the brain and spinal cord. Review Figure 34.3

Concept 34.2   Neurons Generate Electric Signals by Controlling Ion Distributions

• Neurons have an electric charge difference across their cell membranes, called the membrane potential. The membrane potential is created by ion transporters and channels. The membrane potential is considered to be negative if the inside of the cell membrane is negative in relation to the outside. In inactive neurons, the membrane potential is called the resting potential and is negative. Review Figure 34.4 and ANIMATED TUTORIAL 34.1
• The sodium–potassium pump concentrates K⁺ on the inside of a neuron and Na⁺ on the outside. In a resting neuron, K⁺ leak channels allow K⁺ to diffuse through the cell membrane. The resting potential is negative because a negative charge on the inside of the cell membrane is needed to balance the tendency for K⁺ to diffuse outward because of its high internal concentration.
• The Nernst equation can be used to predict a neuron’s membrane potential. Review ACTIVITY 34.1
• Most ion channels are gated: they open only under certain conditions. Voltage-gated channels open or close in response to local changes in the membrane potential. Stretch-gated channels open or close in response to stretch or tension applied to the cell membrane. Ligand-gated channels have binding sites where they bind noncovalently with specific chemical compounds that control them. Review Figure 34.5
• When gated ion channels in the cell membrane of a neuron open or close, changes of the membrane potential called depolarization or hyperpolarization can occur. Depolarization occurs when the inside of the cell membrane becomes less negative in relation to the outside.
• When depolarization occurs, a key question is whether it is great enough for a voltage threshold to be reached. If the threshold is not reached, changes in the membrane potential are graded. Review Figure 34.6A
• An action potential, or nerve impulse, is a rapid reversal in membrane potential across a portion of the cell membrane resulting from the opening and closing of voltage-gated Na⁺ channels and K⁺ channels. The voltage-gated Na⁺ channels open when the cell membrane depolarizes to the voltage threshold. Review Figure 34.6B, Figure 34.7, Figure 34.8 and ANIMATED TUTORIAL 34.2
• An action potential is an all-or-none event that is conducted (propagated) along the entire length of a neuron’s axon without any loss of magnitude. It is conducted along the axon because local current flow depolarizes adjacent regions of membrane so they reach voltage threshold. In myelinated axons, action potentials jump between nodes of Ranvier, patches of membrane that are not covered by myelin.

Concept 34.3   Neurons Communicate with Other Cells at Synapses

• A synapse is a cell-to-cell contact point specialized for signal transmission from one cell to another. Most synapses are chemical synapses (with neurotransmitters). Some are electrical synapses.
• At a chemical synapse, when an action potential reaches the axon terminal of the presynaptic neuron, it causes the release of a neurotransmitter, which diffuses across the synaptic cleft and binds to receptors on the cell membrane of the postsynaptic cell. The postsynaptic cell is usually another neuron or a muscle cell. Review ANIMATED TUTORIAL 34.3 and ANIMATED TUTORIAL 34.4
• The vertebrate 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
• 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.
• There are many different neurotransmitters and types of receptors. The action of a neurotransmitter depends on the receptor to which it binds.
• Synapses between neurons can be either excitatory or inhibitory. A postsynaptic neuron integrates information by summation of graded postsynaptic potentials (which may be excitatory or inhibitory), in both space (spatial summation) and time (temporal summation). Review Figure 34.11
• Synaptic plasticity is the process by which synapses in the nervous system of an individual animal can undergo long-term changes in their functional properties. Such changes are probably important in learning and memory.

Concept 34.4   Sensory Processes Provide Information on an Animal’s External Environment and Internal Status

• Sensory receptor cells transduce information about an animal’s external and internal environment into action potentials.
• Sensory receptor cells have sensory receptor proteins that respond to sensory input, causing a graded change of membrane potential called a receptor potential.
• An ionotropic receptor cell typically has a receptor protein that is a stimulus-gated Na⁺ channel. A metabotropic receptor cell typically has a receptor protein that activates a G protein when exposed to the stimulus. Review Figure 34.12
• The action potentials sent to the brain are interpreted as particular sensations based on which neurons in the brain receive them.
• Mechanoreceptors are cells that respond specifically to mechanical distortion of their cell membrane and are typically ionotropic. Stretch receptors are mechanoreceptors. Review Figure 34.13, Figure 34.14 and ANIMATED TUTORIAL 34.5
• Chemoreceptors are metabotropic receptor cells that respond to the presence or absence of specific chemicals. The sense of smell depends on chemoreceptors.
• In the mammalian auditory system, sound pressure waves enter the auditory canal, where the tympanic membrane vibrates in response to them. The movements of the tympanic membrane are relayed to the oval window. Movements of the oval window create sound pressure waves in the fluid-filled cochlea. Review Figure 34.15 and ACTIVITY 34.2
• The basilar membrane running down the center of the cochlea is distorted by sound pressure waves at specific locations that depend on the frequency of the waves. These distortions cause the localized bending of stereocilia of hair cells, mechanoreceptors in the organ of Corti. Pitch or tone is encoded by the specific locations along the basilar membrane where action potentials are generated. Review Figure 34.16 and ANIMATED TUTORIAL 34.6
• Photoreceptors are sensory receptor cells that are sensitive to light. The photosensitivity of photoreceptor cells involved in vision depends on the absorption of photons of light by sensory receptor proteins called rhodopsins. Vertebrates have two types of visual photoreceptor cells, rods and cones. Color vision in humans arises from three types of cone cells with different spectral absorption properties. Review Figure 34.17 and ANIMATED TUTORIAL 34.7
• When excited by light, vertebrate visual photoreceptor cells hyperpolarize. They do not produce action potentials. Review Figure 34.18
• The vertebrate retina consists of four types of integrating neurons and the photoreceptor cells lining the back of the eye. Extensive processing of visual information occurs in the retina and also in the brain. Review Figure 34.19, ACTIVITY 34.3 and ACTIVITY 34.4
• Arthropods have compound eyes consisting of many optical units called ommatidia, each with its own lens. Review Figure 34.20

Concept 34.5   Neurons Are Organized into Nervous Systems

• Nerve nets are the simplest nervous systems. As nervous systems evolved further, they followed two major trends: centralization—the clustering of neurons into centralized integrating organs—and cephalization—the concentration of major integrating centers at the anterior end of the animal’s body.
• The brain and spinal cord make up the central nervous system (CNS). Neurons that extend or reside outside the brain and spinal cord make up the peripheral nervous system (PNS).
• Neurons are classed as interneurons, sensory neurons, or motor neurons.
• In vertebrates, the central nervous system is positioned in the dorsal part of the body. In arthropods, such as insects and crayfish, the CNS is primarily positioned in the ventral part of the body. Review Figure 34.21
• The autonomic nervous system (ANS) is the part of the nervous system (both CNS and PNS) that controls involuntary functions. Its enteric, sympathetic, and parasympathetic divisions differ in anatomy, neurotransmitters, and effects on target tissues. The sympathetic and parasympathetic divisions usually exert opposite effects on an organ. Review Figure 34.22 and ANIMATED TUTORIAL 34.8
• The vertebrate brain consists of a forebrain, midbrain, and hindbrain. During the course of vertebrate evolution, some parts of the brain (e.g., the medulla oblongata) have remained relatively unchanged, whereas other parts (e.g., the cerebral hemispheres) have changed dramatically. Review Figure 34.25
• The cerebral hemispheres, including the cerebral cortex of each hemisphere, are the dominant structures of the human brain. Review Figure 34.26
• In each cerebral hemisphere, specific regions are specialized to carry out specific sensory and motor functions, for example language functions and fear. Review Figure 34.26, Figure 34.27, Figure 34.28 and ACTIVITY 34.5
• Some brain regions have maps: the parts of the brain that serve various anatomical regions of the body are physically related to each other in ways that mirror the physical relationships of the rest of the body. Examples are the somatosensory (“body sensing”) and motor areas of the cerebral cortex. Review Figure 34.29 and Figure 34.30