Chapter 1/Cytology and Organization of Cell Types cells, neurons possess a cell body, or perikaryon, other type of electrical signal, the synaptic potential which is the metabolic hub of the cell. However, at the postsynaptic site. The unidirectional or polar cellular processes extend from this center and give ized flow of information consists of action potentials the neuron its unique form and ability to receive at the axonal level eliciting synaptic potentials in the and rapidly send signals often over long distances postsynaptic cell, which triggers an action potential in (Fig. 1) that cell and so on. Axons may be as long as 2 m synapses. One neuron forms the presynaptic element, within the circuli stance and rapid communication signals are communicated between neurons at permitting long and the subsequent neuron forms the postsynaptic element.Chemical signals, in the form of neurotrans- 1.3. Diversity in Form Is a Distinctive Property mitters or neuropeptides, are concentrated at the pre- concentration of receptor molecules that are specific Although neuronal form follows the basic plan for each messenger. Presynaptic and postsynaptic 40 um,ensuring precise and directed transmission of them silty in size, shape, and function, which allows elements are separated by a space of only 20 to diver to discriminate the multitude of different types of incoming signals. The detection of commands for muscle contraction is under the control of motor neurons. Information in the form of light. mechanical 1.2. Neuronal Polarity Is a Function of Axons force, or chemical substances is distinguished by sen- and Dendrites sory neurons highly specialized for each particular The two main types of cellular processes are called type of sensation. It is perhaps in this group of neu- dendrites and axons(Fig. 1). Dendrites are usually rons that structural and functional diversity is most postsynaptic and form an enormous receptive sur- apparent. The rigorous demands of sensory discrimi face, which branches extensively. In some neurons, nation have imposed on these neurons the require such as cerebellar Purkinje cells, the dendritic ment to develop specific, highly sensitive detection branches form a characteristic elaborate arboriza- systems that are able to perceive various degrees of tion;others are less distinctive. In addition to their stimulus intensities. a classic example is the olfactory growth during normal development, some of these cell of the male gypsy moth, which can detect a mole- processes remain plastic and can change in length cule of the female's sex attractant, or pheromone, dramatically in the adult. For example, the extent of released a mile away. Equally impressive is the ability arborization of the dendritic tree of male rat motor of the mammalian nasal epithelium to detect and neurons that innervate penile muscles and mediate discriminate more than 10,000 different odiferous copulatory behavior appears to be under steroid hor- substances. In terms of the ability to recognize diverse mone (i.e, androgen) regulation. Stress and stress molecules, olfactory neurons are second only to the hormones appear to decrease dendritic length in the cells of the immune system. The olfactory neuron hippocampus, a brain area involved in memory and however, is markedly different in form and receptor learning and responses to stress. In some dendrites, locale from, for example, the retinal photoreceptor the membranous surfaces are further elaborated to cell. Receptive surfaces consisting of specialized cilia form protrusions called dendritic spines. Chemical on olfactory neurons contain the receptors and are signals received by dendrites and their spines are the primary sites of sensory transduction. In retinal integrated and transduced into an electrical signal, photoreceptor neurons, visual transduction occurs known as the synaptic potential within special cylindrical cellular domains containing The signal triggers the action potential, which is stacks of about 1000 membranous disks in which the propagated along the neuron,s plasma membrane photon receptors called rhodopsin are embedded down an elongated process called the axon. Although Sensory and other types of information enter the axons are not as expansive as dendrites, they branch nervous system by means of a particular neuron, but and may innervate more than one effector. The term- those signals are rarely sent to the effector neuron inal end of an axon is modified to form a bulbous surfaces directly. Rather, the initial signal is sent to a structure, the presynaptic terminal(Fig. 1). The third class of nerve cell, the interneurons. Interneur- incoming action potentials cause the release of neu- ons integrate various inputs from other neurons rotransmitter molecules that bind to complementary before the information, often in a modified form postsynaptic receptors. This binding initiates the reaches its final destination. In this way, variouscells, neurons possess a cell body, or perikaryon, which is the metabolic hub of the cell. However, cellular processes extend from this center and give the neuron its unique form and ability to receive and rapidly send signals often over long distances (Fig. 1). Signals are communicated between neurons at synapses. One neuron forms the presynaptic element, and the subsequent neuron forms the postsynaptic element. Chemical signals, in the form of neurotrans￾mitters or neuropeptides, are concentrated at the pre￾synaptic site. The postsynaptic site contains a high concentration of receptor molecules that are specific for each messenger. Presynaptic and postsynaptic elements are separated by a space of only 20 to 40 mm, ensuring precise and directed transmission of signals. 1.2. Neuronal Polarity Is a Function of Axons and Dendrites The two main types of cellular processes are called dendrites and axons (Fig. 1). Dendrites are usually postsynaptic and form an enormous receptive sur￾face, which branches extensively. In some neurons, such as cerebellar Purkinje cells, the dendritic branches form a characteristic elaborate arboriza￾tion; others are less distinctive. In addition to their growth during normal development, some of these processes remain plastic and can change in length dramatically in the adult. For example, the extent of arborization of the dendritic tree of male rat motor neurons that innervate penile muscles and mediate copulatory behavior appears to be under steroid hor￾mone (i.e., androgen) regulation. Stress and stress hormones appear to decrease dendritic length in the hippocampus, a brain area involved in memory and learning and responses to stress. In some dendrites, the membranous surfaces are further elaborated to form protrusions called dendritic spines. Chemical signals received by dendrites and their spines are integrated and transduced into an electrical signal, known as the synaptic potential. The signal triggers the action potential, which is propagated along the neuron’s plasma membrane down an elongated process called the axon. Although axons are not as expansive as dendrites, they branch and may innervate more than one effector. The term￾inal end of an axon is modified to form a bulbous structure, the presynaptic terminal (Fig. 1). The incoming action potentials cause the release of neu￾rotransmitter molecules that bind to complementary postsynaptic receptors. This binding initiates the other type of electrical signal, the synaptic potential at the postsynaptic site. The unidirectional or polar￾ized flow of information consists of action potentials at the axonal level eliciting synaptic potentials in the postsynaptic cell, which triggers an action potential in that cell and so on. Axons may be as long as 2 m, permitting long-distance and rapid communication within the circuit. 1.3. Diversity in Form Is a Distinctive Property of Neurons Although neuronal form follows the basic plan described above, nerve cells show a tremendous diversity in size, shape, and function, which allows them to discriminate the multitude of different types of incoming signals. The detection of commands for muscle contraction is under the control of motor neurons. Information in the form of light, mechanical force, or chemical substances is distinguished by sen￾sory neurons highly specialized for each particular type of sensation. It is perhaps in this group of neu￾rons that structural and functional diversity is most apparent. The rigorous demands of sensory discrimi￾nation have imposed on these neurons the require￾ment to develop specific, highly sensitive detection systems that are able to perceive various degrees of stimulus intensities. A classic example is the olfactory cell of the male gypsy moth, which can detect a mole￾cule of the female’s sex attractant, or pheromone, released a mile away. Equally impressive is the ability of the mammalian nasal epithelium to detect and discriminate more than 10,000 different odiferous substances. In terms of the ability to recognize diverse molecules, olfactory neurons are second only to the cells of the immune system. The olfactory neuron, however, is markedly different in form and receptor locale from, for example, the retinal photoreceptor cell. Receptive surfaces consisting of specialized cilia on olfactory neurons contain the receptors and are the primary sites of sensory transduction. In retinal photoreceptor neurons, visual transduction occurs within special cylindrical cellular domains containing stacks of about 1000 membranous disks in which the photon receptors called rhodopsin are embedded. Sensory and other types of information enter the nervous system by means of a particular neuron, but those signals are rarely sent to the effector neuron surfaces directly. Rather, the initial signal is sent to a third class of nerve cell, the interneurons. Interneur￾ons integrate various inputs from other neurons before the information, often in a modified form, reaches its final destination. In this way, various Chapter 1 / Cytology and Organization of Cell Types 3