Key Concepts of Section 22.1

• Neurons are highly asymmetric cells composed of multiple dendrites at one end, a cell body containing the nucleus, a long axon, and axon termini.

• Neurons carry information from one end to the other using pulses of ion flow across the plasma membrane. Branched cell processes, dendrites, at one end of the cell receive chemical signals from other neurons, triggering ion flow. The electrical signal moves rapidly to axon termini at the other end of the cell (see Figure 22-1).

• A resting neuron carrying no signal has ATP-powered pumps that move ions across the plasma membrane. The outward movement of K⁺ ions creates a net negative charge inside the cell. This voltage is called the resting potential and usually is about −70 mV (see Figure 22-2).

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• If a stimulus causes certain ion channels to open so that certain ions can flow more freely, a strong pulse of voltage change may pass down the neuron from dendrites to axon termini. The cell goes from being ~ −70 mV inside to ~ +50 mV inside, relative to the extracellular fluid. This pulse is called an action potential (see Figure 22-2).

• The action potential travels down the length of the axon from the cell body to the axon termini at speeds of up to 100 meters per second.

• Neurons connect across small spaces called synapses. Since an action potential cannot jump the gap, at the axon termini of the presynaptic cell the signal is converted from electrical to chemical to stimulate the postsynaptic cell.

• Upon stimulation by an action potential, axon termini release, by exocytosis, small packets of chemicals called neurotransmitters. Neurotransmitters diffuse across the synapse and bind to receptors on the dendrites on the other side of the synapse. These receptors can induce or inhibit a new axon potential in the postsynaptic cell (see Figure 22-3).

• Neurons form circuits that usually consist of sensory neurons, interneurons, and motor neurons, as in the knee-jerk response (see Figure 22-4).

• Glial cells are abundant in the nervous system and serve many purposes. Oliodendrocytes and Schwann cells build the myelin insulation that coats many neurons.

• Neurons connect with one another to form circuits. Three fundamental patterns of neuronal connectivity include divergent, convergent, and feedback circuits.

• Astrocytes, another type of glial cell, wrap their processes around synapses and blood vessels and promote formation of the blood-brain barrier (see Figure 22-6). Astrocytes also secrete proteins that stimulate synapse formation and participate in the formation and function of neural circuits.

• Embryonic neural stem cells in the ventricular zone give rise to all cells in the central nervous system. These stem and progenitor cells undergo a series of symmetric and asymmetric cells to produce more progenitor cells, glia, and neurons (Figure 22-7).

• In the adult brain, new neurons are born in the subventricular zone and in the dentate gyrus region of the hippocampus (Figure 22-8). The differentiation of stem and progenitor cells is regulated by a variety of signaling factors.