Capítulo II: Deporte y Resocialización Penal
1. Concepto de Deporte
In English, a fern is a kind of plant. In German, fern means “far away.” In French, the term is meaningless.
The meaning of any word depends on the listener. Sim-ilarly, the meaning of a neurotransmitter depends on its receptor. Each of the well-studied neurotransmitters interacts with several different kinds of receptors, with different functions. Therefore, a drug or a genetic mu-tation that affects one receptor type may affect behav-ior in a specific way. For example, one type of serotonin receptor mediates nausea, and the drug ondansetron that blocks this receptor helps cancer patients undergo treatment without nausea.
A neurotransmitter receptor is a protein embed-ded in the membrane. When the neurotransmitter at-taches to the active site of the receptor, the receptor can directly open a channel—exerting an ionotropic effect—or it can produce slower but longer effects—a metabotropic effect.
Ionotropic Effects
Some neurotransmitters exert ionotropic effects on the postsynaptic neuron: When the neurotransmitter binds to a receptor on the membrane, it almost imme-diately opens the gates for some type of ion. Iono-tropic effects begin within one to a few milliseconds (Scannevin & Huganir, 2000), and they last only about 20 ms (North, 1989; Westbrook & Jahr, 1989).
Most of the brain’s excitatory ionotropic synapses use the neurotransmitter glutamate. Most of the in-hibitory ionotropic synapses use the neurotransmitter GABA,1 which opens chloride gates, enabling chloride ions, with their negative charge, to cross the mem-brane into the cell more rapidly than usual. Glycine is another common inhibitory transmitter (Moss & Smart, 2001). Acetylcholine, also a transmitter at many iono-tropic synapses, has mostly excitatory effects, which have been extensively studied. Figure 3.11a shows a cross-section through an acetylcholine receptor as it might be seen from the synaptic cleft. Its outer portion (red) is embedded in the neuron’s membrane; its inner portion (blue) surrounds the sodium channel. When at rest (unstimulated), the inner portion of the receptor coils together tightly enough to block sodium passage.
When acetylcholine attaches, the receptor folds out-ward, widening the sodium channel. Figure 3.11b shows a side view of the receptor with acetylcholine attached (Miyazawa, Fujiyoshi, & Unwin, 2003). For a detailed treatment of ionotropic mechanisms, visit this website: http://www.npaci.edu/features/98/Dec/
index.html
Metabotropic Effects and Second Messenger Systems
At other synapses, neurotransmitters exert metabo-tropic effects by initiating a sequence of metabolic re-actions that are slower and longer lasting than iono-tropic effects (Greengard, 2001). Metaboiono-tropic effects emerge 30 ms or more after the release of the transmit-ter (North, 1989) and last seconds, minutes, or even longer. Whereas most ionotropic effects depend on just glutamate and GABA, metabotropic synapses use a much larger variety of transmitters.
When the neurotransmitter attaches to a metabo-tropic receptor, it bends the rest of the protein, enabling a portion of the protein inside the neuron to react with other molecules, as shown in Figure 3.12 (Levitzki, 1988; O’Dowd, Lefkowitz, & Caron, 1989). The portion inside the neuron activates a G-protein—one that is coupled to guanosine triphosphate(GTP), an energy-storing molecule. The activated G-protein in turn in-creases the concentration of a second messenger, such as cyclic adenosine monophosphate (cyclic AMP), in-side the cell. Just as the “first messenger” (the neuro-transmitter) carries information to the postsynaptic cell,
62 Chapter 3 Synapses
1GABA (GA-buh) is an abbreviation for gamma-amino-butyric acid.
the second messenger communicates to areas within the cell.The second messenger may open or close ion channels in the membrane or alter the production of proteins or activate a portion of a chromosome. Note the contrast: An ionotropic synapse has effects localized to one point on the membrane, whereas a metabotropic synapse, by way of its second messenger, influences ac-tivity in a larger area of the cell and over a longer time.
Ionotropic and metabotropic synapses contribute to different aspects of behavior. For vision and
hear-ing, the brain needs rapid, quickly changing infor-mation, the kind that ionotropic synapses bring.
In contrast, hunger, thirst, fear, and anger constitute long-term changes in the probabilities of many behav-iors. Metabotropic synapses are better suited for that kind of function. Metabotropic synapses also mediate at least some of the input for taste (Huang et al., 2005) and pain (Levine, Fields, & Basbaum, 1993), which are slower and more enduring experiences than vision or hearing.
1. Transmitter molecule attaches to receptor
2. Receptor bends, releasing G-protein
3. G-protein activates a
“second messenger”
such as cyclic AMP, which alters a metabolic pathway, turns on a gene in the nucleus, or opens or closes an ion channel Nonstimulated
metabotropic receptor
G-protein
Membrane
1
2
3
Figure 3.12 Sequence of events at a metabolic synapse, using a second messenger within the postsynaptic neuron
3.2 Chemical Events at the Synapse 63
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Researchers sometimes describe some metabo-tropic neurotransmitters, mainly the peptide neuro-transmitters, as neuromodulators, with the implica-tion that they do not directly excite or inhibit the postsynaptic cell but increase or decrease the release of other transmitters or alter the response of postsynap-tic cells to various inputs.The same description ap-plies to metabotropic synapses in general. Peptide transmitters do tend to diffuse widely enough to affect several cells, and sometimes their effects are very en-during, so the term neuromodulator is useful to em-phasize these special characteristics.
S T O P & C H E C K
4. When the action potential reaches the presynaptic terminal, which ion must enter the presynaptic termi-nal to evoke release of the neurotransmitter?
5. How do ionotropic and metabotropic synapses differ in speed and duration of effects?
6. What are second messengers, and which type of synapse relies on them?
Check your answers on page 68.
Table 3.2 Partial List of Hormone-Releasing Glands
Organ Hormone Hormone Functions
Hypothalamus Various releasing hormones Promote or inhibit release of various hormones by pituitary Anterior pituitary Thyroid-stimulating hormone (TSH) Stimulates thyroid gland
Luteinizing hormone (LH) Increases production of progesterone (female), testosterone (male);
stimulates ovulation
Follicle-stimulating hormone (FSH) Increases production of estrogen and maturation of ovum (female) and sperm production (male)
ACTH Increases secretion of steroid hormones by adrenal gland
Prolactin Increases milk production
Growth hormone (GH), also known Increases body growth, including the growth spurt during puberty as somatotropin
Posterior pituitary Oxytocin Controls uterine contractions, milk release, certain aspects of parental behavior, and sexual pleasure
Vasopressin (also known as Constricts blood vessels and raises blood pressure, decreases
antidiuretic hormone) urine volume
Pineal Melatonin Increases sleepiness, influences sleep–wake cycle, also has role in onset of puberty
Thyroid Thyroxine Increase metabolic rate, growth, and maturation
Triiodothyronine
Parathyroid Parathyroid hormone Increases blood calcium and decreases potassium Adrenal cortex Aldosterone Reduces secretion of salts by the kidneys
Cortisol, corticosterone Stimulate liver to elevate blood sugar, increase metabolism of proteins and fats
Adrenal medulla Epinephrine, norepinephrine Similar to effects of sympathetic nervous system
Pancreas Insulin Increases entry of glucose to cells and increases storage as fats Glucagon Increases conversion of stored fats to blood glucose
Ovary Estrogens Promote female sexual characteristics
Progesterone Maintains pregnancy
Testis Androgens Promote sperm production, growth of pubic hair, and male sexual
characteristics
Liver Somatomedins Stimulate growth
Kidney Renin Converts a blood protein into angiotensin, which regulates blood
pressure and contributes to hypovolemic thirst
Thymus Thymosin (and others) Support immune responses
Fat cells Leptin Decreases appetite, increases activity, necessary for onset of puberty
64 Chapter 3 Synapses
Hormones
A hormone is a chemical that is secreted, in most cases by a gland but also by other kinds of cells, and conveyed by the blood to other organs, whose activity it influences.A neurotransmitter is like a signal on a telephone line: It conveys a message directly and ex-clusively from the sender to the receiver. Hormones function more like a radio station: They convey a mes-sage to any receiver that happens to be tuned in to the right station. Figure 3.13 presents the major endocrine (hormone-producing) glands.Table 3.2 lists some im-portant hormones and their principal effects.
Hormones are particularly useful for coordinating long-lasting changes in multiple parts of the body. For example, birds that are preparing to migrate secrete hormones that change their eating and digestion to store extra energy for a long journey. Among the vari-ous types of hormones are protein hormones and pep-tide hormones, composed of chains of amino acids.
(Proteins are longer chains and peptides are shorter.)
Protein and peptide hormones attach to membrane re-ceptors where they activate a second messenger within the cell—exactly the same process as at a metabotropic synapse. In fact, many chemicals—including epineph-rine, norepinephepineph-rine, insulin, and oxytocin—serve as both neurotransmitters and hormones.
Just as circulating hormones modify brain activity, hormones secreted by the brain control the secretion of many other hormones. Thepituitary gland, attached to the hypothalamus(Figure 3.14), consists of two dis-tinct glands, the anterior pituitary and the posterior pituitary, which release different sets of hormones(see Table 3.2). The posterior pituitary, composed of neural tissue, can be considered an extension of the hypo-thalamus. Neurons in the hypothalamus synthesize the hormones oxytocin and vasopressin (also known as antidiuretic hormone),which migrate down axons to the posterior pituitary, as shown in Figure 3.15.
Later, the posterior pituitary releases these hormones into the blood.
The anterior pituitary, composed of glandular tissue, synthesizes six hormones, although the
hypo-Figure 3.13 Location of some major endocrine glands
(Source: Starr & Taggart, 1989) Pineal gland
Pituitary gland
Parathyroid glands
Adrenal gland Liver
Kidney Pancreas
Ovary (in female) Placenta (in female during pregnancy)
Testis (in male) Thyroid glands Thymus Hypothalamus
Figure 3.14 Location of the hypothalamus and pituitary gland in the human brain
(Source: Starr & Taggart, 1989) Anterior lobe
of pituitary
Posterior lobe of pituitary
Optic chiasm Third ventricle Hypothalamus
Pituitary stalk Membrane covering around brain
Bone at base of cranial cavity
3.2 Chemical Events at the Synapse 65
thalamus controls their release (see Figure 3.15). The hypothalamus secretes releasing hormones, which flow through the blood to the anterior pituitary.There they stimulate or inhibit the release of the following hormones:
Adrenocorticotropic Controls secretions of hormone (ACTH) the adrenal cortex Thyroid-stimulating Controls secretions of
hormone (TSH) the thyroid gland
Prolactin Controls secretions of
the mammary glands Somatotropin, also known Promotes growth
as growth hormone (GH) throughout the body Gonadotropins Control secretions of the
Follicle-stimulating gonads hormone (FSH)
Luteinizing hormone (LH)
The hypothalamus maintains fairly constant cir-culating levels of certain hormones through a
nega-tive feedback system. For example, when the level of thyroid hormone is low, the hypothalamus releases TSH-releasing hormone, which stimulates the ante-rior pituitary to release TSH, which in turn causes the thyroid gland to secrete more thyroid hormones (Fig-ure 3.16). For more information about hormones in general, visit this site: http://www.endo-society.org/
S T O P & C H E C K
7. Which has longer lasting effects, a neurotransmitter or a hormone? Which affects more organs?
8. Which part of the pituitary—anterior or posterior—is neural tissue, similar to the hypothalamus? Which part is glandular tissue and produces hormones that con-trol the secretions by other endocrine organs?
Check your answers on page 68.