Page 655 - Fundamentals of anatomy physiology
P. 655
642 Unit 3 Control and Regulation
produce their effects by this mechanism. The increase in In this way, they change the pattern of protein synthesis. Altera-
cAMP levels is usually short-lived, because the cytoplasm con- tions in the synthesis of enzymes or structural proteins directly
tains another enzyme, phosphodiesterase (PDE), which affect both the metabolic activity and the structure of the target
inactivates cyclic AMP by converting it to AMP (adenosine cell. For example, the sex hormone testosterone stimulates the
monophosphate). production of enzymes and structural proteins in skeletal mus-
cle fibers, causing muscle size and strength to increase.
Spotlight Figure 18–3 (center) depicts one way the activa-
tion of a G protein can lower the concentration of cAMP within Thyroid hormones cross the plasma membrane primar-
the cell. In this case, the activated G protein stimulates PDE ily by a transport mechanism. Once in the cytoplasm, these
activity and inhibits adenylate cyclase activity. Levels of cAMP hormones bind to receptors within the nucleus and on mito-
then decline, because cAMP breakdown accelerates while cAMP chondria (Figure 18–4b). The hormone–receptor complexes
synthesis is prevented. The decline has an inhibitory effect on in the nucleus activate specific genes or change the rate of
the cell, because without phosphorylation, key enzymes remain transcription. The change in transcription rate affects the
inactive. This mechanism is responsible for the inhibitory effects metabolic activities of the cell by increasing or decreasing
that follow when epinephrine and norepinephrine stimulate α2 the concentration of specific enzymes. Thyroid hormones
adrenergic receptors, as discussed in Chapter 16. p. 567 bound to mitochondria increase the mitochondrial rates of
ATP production.
18 G Proteins and Calcium Ions. An activated G protein can trig-
ger either the opening of calcium ion channels in the plasma Control of Endocrine Activity
by Endocrine Reflexes
membrane or the release of calcium ions from intracellular
compartments. Spotlight Figure 18–3 (right panel) diagrams As noted earlier, the functional organization of the nervous
system parallels that of the endocrine system in many ways. In
the steps involved. The G protein first activates the enzyme Chapter 13, we considered the basic operation of neural reflex
arcs, the simplest organizational units in the nervous system.
phospholipase C (PLC). This enzyme triggers a receptor cascade
that begins with the production of diacylglycerol (DAG) and p. 475 The most direct arrangement was a monosynaptic
inositol triphosphate (IP3) from membrane phospholipids. reflex, such as the stretch reflex. Polysynaptic reflexes provide
The cascade then proceeds as follows: more complex and variable responses to stimuli. Higher cen-
ters, which integrate multiple inputs, can facilitate or inhibit
IP3 diffuses into the cytoplasm and triggers the release these reflexes as needed.
of Ca2+ from intracellular reserves, such as those in the Endocrine reflexes are the functional counterparts of neu-
smooth endoplasmic reticulum of many cells. ral reflexes. Endocrine reflexes can be triggered by (1) humoral
stimuli (changes in the composition of the extracellular fluid),
The combination of DAG and intracellular calcium ions (2) hormonal stimuli (the arrival or removal of a specific hor-
mone), or (3) neural stimuli (the arrival of neurotransmitters
activates another membrane protein: protein kinase C at neuroglandular junctions). In most cases, negative feedback
(PKC). The activation of PKC leads to the phosphorylation controls endocrine reflexes: A stimulus triggers the production
of calcium channel proteins, a process that opens the chan- of a hormone, and the direct or indirect effects of the hormone
nels and permits extracellular Ca2+ to enter the cell. This reduce the intensity of the stimulus.
sets up a positive feedback loop that rapidly elevates intra-
cellular calcium ion concentrations. A simple endocrine reflex involves only one hormone. The
endocrine cells involved respond directly to changes in the
The calcium ions themselves serve as messengers, gener- composition of the extracellular fluid. The secreted hormone
adjusts the activities of target cells and restores homeostasis.
ally in combination with an intracellular protein called Simple endocrine reflexes control hormone secretion by the
calmodulin. Once it has bound calcium ions, calmodulin heart, pancreas, parathyroid glands, and digestive tract.
can activate specific cytoplasmic enzymes. This chain of
events is responsible for the stimulatory effects that follow More complex endocrine reflexes involve one or more in-
when epinephrine or norepinephrine activates α1 receptors. termediary steps and two or more hormones.
p. 567 Calmodulin activation is also involved in the The hypothalamus provides the highest level of endocrine
responses to oxytocin and to several regulatory hormones control. It integrates the activities of the nervous and endocrine
secreted by the hypothalamus. systems in three ways (Figure 18–5):
Hormones and Intracellular Receptors 1. The hypothalamus itself acts as an endocrine organ.
Hypothalamic neurons synthesize hormones and trans-
Steroid hormones diffuse across the lipid part of the plasma port them along axons to the posterior lobe of the
membrane and bind to receptors in the cytoplasm or nucleus.
The hormone–receptor complexes then activate or deactivate
specific genes (Figure 18–4a). By this mechanism, steroid hor-
mones can alter the rate of DNA transcription in the nucleus.
