Specialized Regions of Neurons Carry Out Different Functions

Although the morphology of various types of neurons differs in some respects, they all contain four distinct regions with differing functions: the cell body, the axon, the axon terminals, and the dendrites (Figure 7-29).

The cell body contains the nucleus and is the site of synthesis of virtually all neuronal proteins and membranes. Some proteins are synthesized in dendrites, but none are made in axons or axon terminals. Special transport processes involving microtubules move proteins and membranes from their sites of synthesis in the cell body down the length of the axon to the terminals (Chapter 20).

Axons, whose diameter varies from a micrometer in certain nerves of the human brain to a millimeter in the giant fiber of the squid, are specialized for conduction of action potentials. An action potential is a series of sudden changes in the voltage, or equivalently the electric potential, across the plasma membrane. When a neuron is in the resting (nonstim-ulated) state, the electric potential across the axonal membrane is approximately —60 mV (the inside negative relative to the outside); this resting potential is similar to that of the membrane potential in most non-neuronal cells. At the peak of an action potential, the membrane potential can be as much as +50 mV (inside positive), a net change of «110 mV.

(a) Multipolar interneuron

Dendrite Axon terminal

Dendrite Axon terminal

M FIGURE 7-29 Typical morphology of two types of mammalian neurons.

Action potentials arise in the axon hillock and are conducted toward the axon terminus. (a) A multipolar interneuron has profusely branched dendrites, which receive signals at synapses with several hundred other neurons. A single long axon that branches laterally at its terminus transmits signals to other neurons. (b) A motor neuron innervating a muscle cell typically has a single long axon extending from the cell body to the effector cell. In mammalian motor neurons, an insulating sheath of myelin usually covers all parts of the axon except at the nodes of Ranvier and the axon terminals.

This depolarization of the membrane is followed by a rapid repolarization, returning the membrane potential to the resting value (Figure 7-30).

An action potential originates at the axon hillock, the junction of the axon and cell body, and is actively conducted down the axon to the axon terminals, small branches of the axon that form the synapses, or connections, with other cells.

Action potentials

Action potentials

Resting membrane potential

Hyperpolarization Time

▲ EXPERIMENTAL FIGURE 7-30 Recording of an axonal membrane potential over time reveals the amplitude and frequency of action potentials. An action potential is a sudden, transient depolarization of the membrane, followed by repolarization to the resting potential of about -60 mV. The axonal membrane potential can be measured with a small electrode placed into it (see Figure 7-14). This recording of the axonal membrane potential in this neuron shows that it is generating one action potential about every 4 milliseconds.

Action potentials move at speeds up to 100 meters per second. In humans, for instance, axons may be more than a meter long, yet it takes only a few milliseconds for an action potential to move along their length. Arrival of an action potential at an axon terminal leads to opening of voltage-sensitive Ca2+ channels and an influx of Ca2 + , causing a localized rise in the cytosolic Ca2+ concentration in the axon terminus. The rise in Ca2+ in turn triggers fusion of small vesicles containing neurotransmitters with the plasma membrane, releasing neurotransmitters from this presynaptic cell into the synaptic cleft, the narrow space separating it from postsynaptic cells (Figure 7-31).

It takes about 0.5 millisecond (ms) for neurotransmitters to diffuse across the synaptic cleft and bind to a receptor on the postsynaptic cells. Binding of neurotransmitter triggers opening or closing of specific ion channels in the plasma membrane of postsynaptic cells, leading to changes in the membrane potential at this point. A single axon in the central nervous system can synapse with many neurons and induce responses in all of them simultaneously.

Most neurons have multiple dendrites, which extend outward from the cell body and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and conduct them toward the cell body. Neuronal cell bodies can also form synapses and thus receive signals. Particularly in the central nervous system, neurons have extremely long dendrites with complex branches. This allows them to form synapses with and receive signals from a large number of other neurons, perhaps up to a thousand (see Figure 7-29a). Membrane depolarizations or hyperpo-larizations generated in the dendrites or cell body spread to the axon hillock. If the membrane depolarization at that

Exocytosis of neurotransmitter

Synaptic cleft

Postsynaptic cell

Exocytosis of neurotransmitter

Axon of presynaptic cell

Synaptic cleft

Postsynaptic cell

Receptors for neurotransmitter

Direction of signaling

Axon of presynaptic cell

Synaptic vesicle Axon terminal

Receptors for neurotransmitter

Direction of signaling

Dendrite of postsynaptic cell

Axon terminal of presynaptic cell

Synaptic vesicles Synaptic cleft

Dendrite of postsynaptic cell

▲ FIGURE 7-31 A chemical synapse. (a) A narrow region—the synaptic cleft—separates the plasma membranes of the presynaptic and postsynaptic cells. Arrival of action potentials at a synapse causes release of neurotransmitters (red circles) by the presynaptic cell, their diffusion across the synaptic cleft, and their binding by specific receptors on the plasma membrane of the postsynaptic cell. Generally these signals depolarize the postsynaptic membrane (making the potential inside less negative), tending to induce an action potential in it. (b) Electron micrograph shows a dendrite synapsing with an axon terminal filled with synaptic vesicles. In the synaptic region, the plasma membrane of the presynaptic cell is specialized for vesicle exocytosis; synaptic vesicles containing a neurotransmitter are clustered in these regions. The opposing membrane of the postsynaptic cell (in this case, a neuron) contains receptors for the neurotransmitter. [Part (b) from C. Raine et al., eds., 1981, Basic Neurochemistry, 3d ed., Little, Brown, p. 32.]

point is great enough, an action potential will originate and will be actively conducted down the axon.

Thus neurons use changes in the membrane potential, the action potentials, to conduct signals along their length, and small molecules, the neurotransmitters, to send signals from cell to cell.

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