2 Cohen and pfaff of contact called synapses. At the synapses, neurons neurons and their processes and appear to provide transmit signals with a great degree of fidelity, allowing them with mechanical and metabolic support. Others some behaviors, particularly those necessary for survi- are arranged along specific neuronal processes so as val, to be stereotyped. Neurons also possess a remark to increase the rate of conduction of electrical signals able ability to modify the way messages are received, Glia are considered later in this chapter. We first processed, and transmitted and may exert profound focus on the neuron, which is the fundamental struc- changes in behavioral patterns. This special property tural and functional unit of the nervous system is called plasticity and depends on the molecular and structural properties of the neuron 1.1. Synapses Are the Sites of Directed In addition to forming specific contacts with other Communication Between Neurons nerve cells, neurons exist in relation to a group of cells The polarization of neuronal shape permits its func collectively known as glia. Some glial cells envelop tioning within a simple or complex circuit. Like other Perikarya Dendrites Axon Perikaryon Axon Axon 2 Single process Fig. 1. Polarization of the neuronal form. Nerve cells conduct impulses in a directed manner, although the position of axons and dendrites relative to the cell body and to each other may vary. (A) The multipolar neuron is the most prevalent type and shows extensive branching of the dendrites that emanate from the cell body. The axons emerge from the opposite end. (B)Bipolar neurons have sensory functions and transmit information received by dendrites along two processes that emerge from the cell body. (C) Pseudounipolar neurons are found in the dorsal root ganglion. These neurons are bipolar during early development Later, the two processes fuse to form a stalk, which subsequently bifurcates into two axons. The action potentials are conducted from sensory nerve endings in skin and muscle to the spinal cord. The action potential usually bypasses the cell body, but in conditions such as a prolapsed vertebral disk, in which the dorsal nerve roots are put under pressure, the cell bodies may lso generate impulses. (D) Unipolar neurons are found in invertebrates. Axons arise from the dendrites, which emerge from the cell body.of contact called synapses. At the synapses, neurons transmit signals with a great degree of fidelity, allowing some behaviors, particularly those necessary for survi￾val, to be stereotyped. Neurons also possess a remark￾able ability to modify the way messages are received, processed, and transmitted and may exert profound changes in behavioral patterns. This special property is called plasticity and depends on the molecular and structural properties of the neuron. In addition to forming specific contacts with other nerve cells, neurons exist in relation to a group of cells collectively known as glia. Some glial cells envelop neurons and their processes and appear to provide them with mechanical and metabolic support. Others are arranged along specific neuronal processes so as to increase the rate of conduction of electrical signals. Glia are considered later in this chapter. We first focus on the neuron, which is the fundamental struc￾tural and functional unit of the nervous system. 1.1. Synapses Are the Sites of Directed Communication Between Neurons The polarization of neuronal shape permits its func￾tioning within a simple or complex circuit. Like other Fig. 1. Polarization of the neuronal form. Nerve cells conduct impulses in a directed manner, although the position of axons and dendrites relative to the cell body and to each other may vary. (A) The multipolar neuron is the most prevalent type and shows extensive branching of the dendrites that emanate from the cell body. The axons emerge from the opposite end. (B) Bipolar neurons have sensory functions and transmit information received by dendrites along two processes that emerge from the cell body. (C) Pseudounipolar neurons are found in the dorsal root ganglion. These neurons are bipolar during early development. Later, the two processes fuse to form a stalk, which subsequently bifurcates into two axons. The action potentials are conducted from sensory nerve endings in skin and muscle to the spinal cord. The action potential usually bypasses the cell body, but in conditions such as a prolapsed vertebral disk, in which the dorsal nerve roots are put under pressure, the cell bodies may also generate impulses. (D) Unipolar neurons are found in invertebrates. Axons arise from the dendrites, which emerge from the cell body. 2 Cohen and Pfaff