19.5
Sinalização Simpática
A sinalização simpática, uma parte vital do sistema nervoso autônomo, desempenha um papel crucial na mobilização dos recursos do corpo em resposta ao…
A sinalização simpática ativa o sistema nervoso simpático por meio das interações entre neurotransmissores e seus receptores nas células-alvo.
As fibras pré-ganglionares da divisão simpática liberam o neurotransmissor acetilcolina, que se liga aos receptores nicotínicos nos neurônios pós-ganglionares, ativando-os.
Todas as fibras pós-ganglionares simpáticas ativadas, com exceção daquelas que inervam as glândulas sudoríparas, liberam norepinefrina.
A norepinefrina liberada se liga a receptores adrenérgicos específicos presentes nas células efetoras.
Existem dois tipos de receptores adrenérgicos: alfa e beta.
Os receptores alfa podem ainda ser categorizados em receptores alfa-1 e alfa-2.
Os receptores alfa-1, encontrados na musculatura lisa, geralmente exercem um efeito excitatório na célula hospedeira, enquanto os receptores alfa-2, localizados nas fibras pré-ganglionares, exercem um efeito inibitório.
Os receptores beta são divididos em receptores beta-1, beta-2 e beta-3.
Os receptores beta-1, encontrados nos músculos cardíacos, glândula pituitária e células adiposas, aumentam a frequência cardíaca e aumentam o metabolismo.
Os receptores beta-2, localizados nos músculos lisos que revestem as vias aéreas, relaxam os músculos respiratórios, alargando as vias aéreas para facilitar a respiração.
Os receptores beta-3, encontrados nos tecidos adiposos marrons, causam lipólise.
View the full transcript and gain access to JoVE Core videos
Q1: What neurotransmitters are released during sympathetic signaling?
Sympathetic signaling involves two key neurotransmitters released sequentially. Preganglionic fibers release acetylcholine, which binds to nicotinic receptors on postganglionic neurons. Postganglionic fibers then release norepinephrine onto target tissues, except those innervating sweat glands. This dual release mechanism activates the sympathetic response throughout the body.
Q2: How do alpha-1 and alpha-2 adrenergic receptors differ in function?
Alpha-1 receptors, found in smooth muscle cells, produce excitatory effects like vasoconstriction when activated. Alpha-2 receptors, located on preganglionic fibers and presynaptic terminals, exert inhibitory effects and function as autoreceptors regulating norepinephrine release. This dual mechanism coordinates sympathetic and parasympathetic activities effectively.
Q3: What are the specific roles of beta-1 and beta-2 adrenergic receptors?
Beta-1 receptors in cardiac muscle increase heart rate and force of contraction, boosting metabolism and energy availability. Beta-2 receptors in airway smooth muscle cause bronchodilation, widening airways for easier breathing during sympathetic activation. Together, these receptors mobilize the body's resources during stress or emergencies.
Q4: Where are beta-3 receptors located and what do they do?
Beta-3 receptors are primarily found in brown adipose tissue throughout the body. When activated, they trigger lipolysis, the breakdown of fat cells, increasing energy availability for immediate use. This metabolic response supports the sympathetic nervous system's role in mobilizing resources during stress.
Q5: How does acetylcholine activate postganglionic sympathetic neurons?
Preganglionic fibers release acetylcholine into sympathetic ganglia, where it binds to nicotinic cholinergic receptors on postganglionic neurons. This binding transmits nerve impulses and activates the postganglionic fibers, enabling them to release norepinephrine onto target tissues. This sequential activation is essential for sympathetic signaling.
Q6: What is the relationship between sympathetic signaling and stress response?
Sympathetic signaling mobilizes the body's resources in response to stress or emergencies through neurotransmitter-receptor interactions. Norepinephrine binds to alpha and beta receptors on target tissues, triggering widespread effects including increased heart rate, bronchodilation, and metabolic activation. This coordinated response prepares the body for fight-or-flight situations.
Q7: Why is norepinephrine the primary neurotransmitter for most sympathetic postganglionic fibers?
Norepinephrine is released by postganglionic fibers to activate adrenergic receptors on target tissues, producing widespread sympathetic effects throughout the body. This neurotransmitter binds to alpha and beta receptors, triggering responses like increased heart rate, bronchodilation, and metabolic activation. Sweat glands are the notable exception, using acetylcholine instead.