FIGURE 3.4 Dendrites. The integrators of thousands of tiny chemical signals come in a variety of shapes.
FIGURE 3.5 The cell body, or soma, is the central command center of a neuron. The dendrites and a single axon grow from the soma, the former for collecting incoming signals and the latter for transmitting outgoing signals over long distances.
FIGURE 3.6 An axon is a single, slender extension from the soma. It is essentially a cable to conduct signals rapidly across long distances.
FIGURE 3.7 Axon terminals are the end points of the axon, where chemical signals are released.
FIGURE 3.8 Different types of neurons. Examples of (a) sensory neurons, (b) motor neurons, and (c) interneurons. Interneurons can be of two types: those with long projections to other regions are termed projection interneurons, whereas those that stay within a region are termed local interneurons.
FIGURE 3.9 Classifying neurons by their shape. Examples of (a) multipolar neurons, (b) bipolar neurons, and (c) monopolar neurons.
FIGURE 3.11 Some glial cells myelinate axons. (a) In the central nervous system, a single oligodendrocyte will wrap up to 50 different axons with myelin sheaths. (b) In the peripheral nervous system, myelination is accomplished by Schwann cells, which wrap around a single axon. Note that the layer of insulation is not continuous, but exists in small sections. (c) Transmission electron micrograph of a myelin sheath.
FIGURE 3.14 Vesicles carrying neurotransmitter molecules dock with the presynaptic membrane, releasing the signaling molecules into the synaptic cleft. The neurotransmitters diffuse across the cleft and interact with receptors on the postsynaptic target.
FIGURE 3.15 Two types of channels allow neurotransmitters to effect target cells. (a) Ionotropic receptors are opened—or gated—allowing ions to move through a passage in the membrane. (b) Metabotropic receptors relay signals to proteins inside the cell.
FIGURE 3.16 There are three ways by which neurotransmitters are cleared from the cleft: (a) degradation, (b) diffusion, and (c) reuptake.
FIGURE 3.17 Postsynaptic potentials. (a) An excitatory postsynaptic potential (EPSP) occurs when positive ions flow through an ionotropic receptor into the cell, causing depolarization. (b) An inhibitory postsynaptic potential (IPSP) occurs when positive ions flow out of the cell, or negative ions flow in. This causes the difference in voltage between the inside and outside of the cell to grow larger, known as hyperpolarization.
FIGURE 3.19 Temporal and spatial summation. (a) No summation occurs when EPSPs arrive with a delay between them; they, individually, cannot drive the membrane voltage to the threshold for a spike. (b) Temporal summation occurs when EPSPs arrive close in time and their contributions add up at the soma, leading to an action potential. (c) Spatial summation occurs when signals arrive on different branches of the dendrites, converging at the soma. (d) If an EPSP and an IPSP arrive at different locations at the same time, they will cancel each other’s effect at the soma.
FIGURE 3.21 The sequence of a voltage spike. (a) At rest, there are more Na+ ions outside the cell than inside and more K+ ions inside the cell than outside. (b) When voltage-gated Na+ channels open, Na+ ions rush from the outside to the inside—both because of the concentration differences and because of the electrical field. (c) The depolarization caused by Na+ influx triggers the opening of K+ channels, which cause K+ ions to rush out, thus making the outside more positive again (repolarization).
FIGURE 3.22 The nodes of Ranvier. (a) Diagram. (b) Microscopic view.
FIGURE 3.23 Action potentials lead to neurotransmitter release.
FIGURE 3.24 A selective neuron responds with greater activity to one particular type of stimulus than to other types. The blue dashes represent individual action potentials; different rows represent individual trials. The red histograms summarize the response of the neuron over many trials.
FIGURE 3.26 A palette of coding possibilities for carrying information about a stimulus (Bullock, 1968).
FIGURE 3.27 Are neurons integrators or coincidence detectors? (a) In the “assembly line” view of neurons, neurons pass messages to one another: the cell on the left is the sender, and the cell on the right integrates those signals as the receiver (Konig, Engel, & Singer, 1996). (b) Because neurons receive thousands of inputs, they may be better thought of as coincidence detectors. The cell body of the postsynaptic cell is unable to determine which presynaptic neuron sent which signal—instead, a postsynaptic spike will only signal the coincidence of many excitatory inputs arriving simultaneously.
FIGURE 3.29 Neurons that excite each other can form coalitions. (a) Two neurons that mutually excite one another. (b) A larger coalition of excitatory neurons.
