18.2
El sistema nervioso parasimpático
Descripción general
El sistema nervioso parasimpático es una de las dos divisiones principales del sistema nervioso autónomo. Este sistema parasimpáti…
- [Narrador] El sistema nervioso parasimpático
restaura el cuerpo en tiempos de calma
en los que no hay amenazas
y a menudo se lo llama sistema de descanso y digestión.
Es una de las divisiones principales
del sistema nervioso automático,
que controla funciones involuntarias
al regular la actividad de los músculos lisos
en órganos viscerales, músculos cardíacos en el corazón
y ciertas glándulas como las glándulas salivales.
Cuando el sistema nervioso parasimpático se activa,
se envían señales de las neuronas pregangliares en el tallo
del cerebro y la parte sacral de la columna vertebral
a los ganglios, grupos de cuerpos de células neuronales
en el sistema nervioso periférico,
cerca de los órganos y glándulas que están controlados.
Aquí, el mensaje se transmite a las neuronas posgangliares,
que generalmente usan el neurotransmitor acetilcolina
para regular la actividad de los tejidos de objetivo.
Por ejemplo, disminuyen el ritmo cardíaco,
relajan el cuerpo y estimulan la contracción muscular
en el intestino y vejiga,
promueve la digestión y excreción.
Como resultado, el sistema parasimpático ayuda
a regenerar reservas energéticas y mantener la homeostasis
en momentos en los que el cuerpo puede descansar.
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Q1: What is the parasympathetic nervous system and what does it do?
The parasympathetic nervous system is a major division of the autonomic nervous system that restores the body during calm times when there are no threats. Often called the rest and digest system, it regulates involuntary functions by controlling smooth muscle in visceral organs, cardiac muscle in the heart, and certain glands. It helps rebuild energy stores and maintain homeostasis when the body can safely rest.
Q2: How does the parasympathetic nervous system transmit signals to target organs?
Preganglionic neurons in the brainstem and sacral spinal cord send signals to ganglia, clusters of neuronal cell bodies in the peripheral nervous system located near target organs. Here, messages transfer to postganglionic neurons, which typically use acetylcholine to regulate target tissue activity. This pathway allows precise control of heart rate, digestion, and other involuntary functions.
Q3: What specific effects does parasympathetic activation have on the body?
Parasympathetic activation lowers heart rate and constricts pupils, promoting relaxation. It stimulates digestion by increasing salivation and peristaltic contractions in the stomach and intestines, and promotes bladder contraction for urination. It also increases pancreatic insulin secretion to rebuild energy stores and enhances blood flow to reproductive organs.
Q4: How does the parasympathetic nervous system differ from the sympathetic nervous system?
While the parasympathetic nervous system promotes rest, digestion, and energy restoration during calm times, the sympathetic nervous system activates during threats or stress. The parasympathetic system lowers heart rate and stimulates digestion, whereas the sympathetic nervous system increases heart rate and inhibits digestion. Together, these opposing systems maintain homeostasis by responding to different body states.
Q5: What role does acetylcholine play in parasympathetic signaling?
Acetylcholine is the primary neurotransmitter used by postganglionic parasympathetic neurons to regulate target tissue activity. Released at the synapse and neurotransmitters junction, acetylcholine binds to receptors on smooth muscle, cardiac muscle, and glands to produce parasympathetic effects like lowering heart rate and stimulating digestion.
Q6: Why is proper parasympathetic signaling important for standing up?
The parasympathetic nervous system controls vasodilation of cerebral arteries, allowing blood vessels to relax and maintain adequate blood flow to the brain during postural changes. Improper parasympathetic signaling can cause orthostatic intolerance, leading to lightheadedness or fainting when standing. This demonstrates how parasympathetic control is essential even for routine movements.
Q7: How does the hypothalamus control parasympathetic nervous system activation?
The hypothalamus activates the parasympathetic nervous system by sending signals that trigger preganglionic neurons in the brainstem and sacral spinal cord. These neurons then relay messages through ganglia to postganglionic neurons, which innervate target organs and glands. This hierarchical control allows the brain to coordinate parasympathetic responses that promote rest and digestion during appropriate times.