Neurons all share some basic features. These include a cell body from which emerge two kinds of fiberlike extension, dendrites and axons (Fig. 35.4). These extensions are the input and output ends of the nerve cell. Both types of cellular extension can be highly branched, enabling neurons to communicate over large distances with many other cells. A neuron’s dendrites receive signals from other nerve cells or, in the case of sensory nerves, from specialized sensory endings. These signals travel along the dendrites to the neuron’s cell body. At the junction of the cell body and its axon, the axon hillock, the signals are summed. If the sum of the signals is high enough, the neuron fires an action potential, or nerve impulse, that travels down the axon. An action potential is a brief electrical signal transmitted from the cell body along one or more axon branches.

Axons generally transmit signals away from the nerve’s cell body. The end of each axon forms a swelling called the axon terminal. An axon terminal communicates with a neighboring cell through a junction called a synapse. A space, the synaptic cleft, separates the end of the axon of the presynaptic cell and the neighboring postsynaptic cell. The synaptic cleft is commonly only about 10 to 20 nm wide.

How does a signal cross the synaptic cleft? Molecules called neurotransmitters convey the signal from the end of the axon to the postsynaptic target cell. The arrival of a nerve signal at the axon terminal triggers the release of neurotransmitter molecules from vesicles located in the terminal. The vesicles fuse with the axon’s membrane, releasing neurotransmitter molecules into the synaptic cleft. The molecules diffuse across the synapse and bind to receptors on the plasma membrane of the target cell. The binding of neurotransmitters to these receptors causes a change in the electrical charge across the membrane of the receiving postsynaptic cell, continuing the signal in the second cell. Most neurons communicate by the hundreds to thousands of synapses that they form with other cells. The massive number of interconnections enables the formation of complex nerve cell circuits and the transmission of information from one circuit to others in the nervous system.

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Some neurons do not synapse with other neurons, but instead with other types of cell that produce some physiological response in the animal. Examples are muscle cells, which contract in response to nerve signals, and secretory cells in glands, which release hormones in response to nerve signals.

Quick Check 1 How are signals transmitted from one end of a nerve cell to the other, and from one neuron to another?