The coordination and integration of cell function are achieved through external chemical signals such as hormones, neurotransmitters, growth factors, odorants, and products of cellular metabolism. These chemical messengers facilitate communication between cells by interacting with receptors located in the plasma membrane, cytoplasm, and nucleus. This interaction triggers a series of events that mediate the cellular response to each stimulus, ensuring specificity, amplification, tight regulation, and coordination.

GTP-binding proteins can be classified into two categories: monomeric G proteins and heterotrimeric G proteins consisting of α, β, and γ subunits. Monomeric G proteins regulate various cellular processes including actin cytoskeleton organization, cell cycle progression, intracellular vesicular transport, and gene expression. Heterotrimeric G proteins, on the other hand, regulate ion channels, adenylyl cyclase and the cAMP-PKA signaling pathway, phosphodiesterases, and phospholipases involved in the production of prostaglandins, prostacyclins, and thromboxanes.

Enzyme-linked receptors, which are comprised of four subtypes, mediate the cellular response to diverse signals such as ANP, nitric oxide, TGF-β, PDGF, insulin, and interleukins.

Nuclear receptors can be categorized into two types: one type translocates to the nucleus from the cytoplasm upon ligand binding, while the other type permanently resides in the nucleus. Both classes of receptors regulate gene transcription.

The information in this excerpt is derived from the following sources:
- Cantley L. Signal transduction.
- Caplan MJ. Functional organization of the cell.
- Heald R. Cell signaling.
- Igarashi P. Regulation of gene expression.
- Boron WF, Boulpaep EL, eds. Medical Physiology. 3rd ed.
- Alberts B, Johnson A, Lewis J, et al., eds. Molecular Biology of the Cell. 6th ed.
- Lang EJ, Rubinson K. The Nervous System: Introduction to Cells and Systems. 
- Lang EJ, Rubinson K. Generation and Conduction of Action Potentials.
- Lang EJ, Rubinson K. The Somatosensory System.
- Lang EJ, Rubinson K. The Special Senses.
- Lang EJ, Rubinson K. Organization of Motor Function.
- Lang EJ, Rubinson K. Integrative Functions of the Nervous System.
1. The function of cells is tightly coordinated and integrated by external chemical signals, including hormones, neurotransmitters, growth factors, odorants, and products of cellular metabolism that serve as chemical messengers and provide cell-to-cell communication. Chemical and physical signals interact with receptors located in the plasma membrane, cytoplasm, and nucleus. Interaction of these signals with receptors initiates a cascade of events that mediate the response to each stimulus.
These pathways ensure that the cellular response to external signals is specific, amplified, tightly regulated, and coordinated.
2. There are two classes of GTP-binding proteins: monomeric G proteins and heterotrimeric G proteins composed of α, β, and γ subunits. Monomeric G proteins regulate actin cytoskeleton organization, cell cycle progression, intracellular vesicular transport, and gene expression. Heterotrimeric G proteins regulate ion channels, adenylyl cyclase and the cAMP-PKA signaling pathway, phosphodiesterases (which also regulate cAMP and cGMP signaling pathways), and
Cheung E, Kraus WL. Genomic analyses of hormone signaling and gene regulation. Annu Rev Physiol. 2010;72:191-218.
Huang P, Chandra V, Rastinejad F. Structural overview of the nuclear receptor superfamily: insights into physiology and therapeutics. Annu Rev Physiol. 2010;72:247-272.
Levin ER. Extranuclear steroid receptors are essential for steroid hormone actions. Annu Rev Med. 2015;66:271-280.
Riccardi D, Kemp P. The calcium-sensing receptor beyond extracellular calcium homeostasis: conception, development, adult physiology, and disease. Annu Rev Physiol. 2012;74:271-297.
Wu H. Higher-order assemblies in a new paradigm of signal transduction. Cell. 2013;153:287-292.
phospholipases, which regulate the production of prostaglandins, prostacyclins, and thromboxanes.
3.
There are four subtypes of enzyme-linked receptors that mediate the cellular response to a wide variety of signals, including ANP, nitric oxide, TGF-β, PDGF, insulin, and interleukins.
4.
There are two types of nuclear receptors: (1) one type that in the absence of ligand is located in the cytoplasm and when bound to ligand translocates to the nucleus and (2) another type that permanently resides in the nucleus. Both classes of receptors regulate gene transcription.
Cantley L. Signal transduction. In: Boron WF, Boulpaep EL, eds. Medical Physiology. 3rd ed. Philadelphia: Elsevier; 2016 [Chapter 3].
Caplan MJ. Functional organization of the cell. In: Boron WF, Boulpaep EL, eds. Medical Physiology. 3rd ed. Philadelphia: Elsevier; 2016 [Chapter 2].
Heald R. Cell signaling. In: Alberts B, Johnson A, Lewis J, et al., eds. Molecular Biology of the Cell. 6th ed. New York: Garland Science; 2015 [Chapter 15].
Igarashi P. Regulation of gene expression. In: Boron WF, Boulpaep EL, eds. Medical Physiology. 3rd ed. Philadelphia: Elsevier; 2016 [Chapter 4].
SECTION 2The Nervous System ERIC J. LANG AND KALMAN RUBINSON
Chapter 4 The Nervous System: Introduction to Cells and Systems
Chapter 5 Generation and Conduction of Action Potentials
The Somatosensory System
The Special Senses
Organization of Motor Function
Chapter 10 Integrative Functions of the Nervous System