Fisiología - Sistema Nervioso Autónomo Pt. I (Simpático y Parasimpático)

Understanding the Autonomic Nervous System

Overview of Sensory Processing and Sleep

• The human brain remains alert to sensory inputs from the external world, including visual, auditory, and tactile stimuli. It unconsciously plans responses to potential dangers.

• During REM sleep, the nervous system can block sensory inputs to the cerebral cortex, resulting in near-total skeletal muscle paralysis.

Importance of the Autonomic Nervous System

• The autonomic nervous system (ANS) is crucial for life functions and higher cortical activities; it often gets overshadowed by consciousness and memory discussions.

• This session will focus on the ANS's significance across physiology, anatomy, and pharmacology due to its involvement in various diseases.

Structure of the Autonomic Nervous System

• The ANS is a part of the peripheral nervous system (PNS), which divides into central and peripheral systems. The PNS further splits into somatic (voluntary) and autonomic (involuntary).

• The ANS controls visceral motor functions that occur without conscious awareness, such as temperature regulation, sweating, heart rate, respiratory rate, sphincter control, and peristalsis.

Historical Terminology

• Historically referred to as "visceral motor system," this term is misleading since it also affects glandular epithelium beyond just smooth muscle.

• Another outdated term is "vegetative system," which inaccurately suggests limited relevance to consciousness levels or aggressive behaviors during danger.

Misconceptions about Autonomy

• The term "autonomous" implies independence; however, the ANS relies on higher regulatory centers like the hypothalamus and limbic cortex for function.

Divisions within the Autonomic Nervous System

Traditional vs. Modern Classifications

• Traditionally divided into sympathetic and parasympathetic systems; modern understanding includes a third division known as enteric nervous system due to its unique characteristics.

Functional Oppositions between Sympathetic and Parasympathetic Systems

• Sympathetic and parasympathetic systems have opposing functions: sympathetic increases heart rate (tachycardia), while parasympathetic decreases it.

Specific Organ Responses

• For example:

• Sympathetic stimulation leads to increased heart rate (positive chronotropic effect).

• Parasympathetic stimulation reduces heart rate (negative chronotropic effect).

Variability in Actions Across Systems

• Not all actions are strictly oppositional; for instance:

• Parasympathetic activity enhances gastrointestinal motility while sympathetic activity suppresses it.

Unique Inervations

Understanding the Autonomic Nervous System

Sympathetic and Parasympathetic Innervation

• The only areas of the body with solely sympathetic innervation are sweat glands and adrenal glands, highlighting a unique aspect of autonomic function.

• The balance between sympathetic and parasympathetic activity is crucial for maintaining homeostasis in the body.

Neuronal Circuit Organization

• Both sympathetic and parasympathetic systems utilize a two-neuron model: a primary neuron located in the central nervous system (CNS) and a secondary neuron in the peripheral nervous system (PNS).

• The primary neuron, also known as presynaptic or preganglionic, has its soma within the CNS, while the secondary neuron, or postsynaptic/postganglionic, resides outside of it.

Terminology Clarification

• A nucleus refers to a concentration of neurons within the CNS; conversely, a ganglion is an accumulation of neuronal somas located peripherally.

• It’s important to distinguish that primary neurons are always found in the CNS while secondary neurons are situated in the PNS.

Structure of Autonomic Nervous System

• The presynaptic neuron's soma is located in the CNS with synapses occurring at peripheral autonomic ganglia.

• The axon from postsynaptic neurons projects towards target organs where they exert their effects.

Origin of Sympathetic Nervous System

• The sympathetic nervous system originates from thoracolumbar segments T1 to L2 of the spinal cord.

• Fibers emerge specifically from these levels; above or below this range does not yield sympathetic fibers.

Anatomy Insights

• In cross-sections of thoracolumbar spinal cord at T6 level, both anterior and posterior horns are present along with an intermediate horn housing presynaptic neuronal somas.

• These presynaptic fibers project through anterior roots into peripheral ganglia via white communicating rami.

Pathways for Sympathetic Fibers

• Once reaching autonomic ganglia, presynaptic fibers can ascend to cervical ganglia or descend to lumbar ganglia for synapsing.

• Presynaptic fibers are myelinated type B fibers that can take various pathways upon reaching their respective ganglia.

Autonomic Nervous System Overview

Sympathetic Nervous System and Prevertebral Ganglia

• The prevertebral ganglia in the abdominal cavity include the celiac, superior mesenteric, inferior mesenteric, and hypogastric ganglia. These autonomic ganglia receive sympathetic fibers.

• Sympathetic fibers pass through the paravertebral ganglion as splanchnic nerves, which exit via the anterior or ventral root of spinal nerves through white communicating rami.

• Post-synaptic sympathetic fibers are unmyelinated C-type fibers; hence, gray communicating rami are named "gray" due to their unmyelinated nature.

• Presynaptic sympathetic fibers originate from T1 to L2 levels of the spinal cord and are myelinated B-type fibers that enter through white communicating rami and return via gray communicating rami.

• There are no white communicating rami above T1 or below L2; only gray communicating rami exist outside these segments. This is crucial for understanding sympathetic fiber distribution.

Distribution of Sympathetic Fibers

• Sympathetic fibers originating from thoracolumbar segments can reach various body parts like the head and neck or lower limbs due to their unique distribution pattern compared to somatic nervous system dermatomes.

• Fibers from T1 and T2 ascend towards the head and neck; those from T3-T6 target thoracic organs; while T7-L2 innervate abdominal viscera and lower limbs respectively. This overlapping territory leads to referred pain phenomena such as in cardiac conditions where heart pain may be felt in areas served by different spinal segments (e.g., T2).

Parasympathetic Nervous System Origin

• The parasympathetic nervous system has a craniosacral origin with nuclei located along the brainstem responsible for cranial nerve parasympathetic output: oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X).

• The vagus nerve is highlighted as the most significant parasympathetic nerve, containing 75% of all parasympathetic fibers in the body, extending its influence over thoracoabdominal viscera up to the splenic flexure of the colon.

Medullary Levels and Parasympathetic Fibers

Overview of Medullary Levels S2, S3, and S4

• The spinal cord levels S2, S3, and S4 are associated with the distribution of parasympathetic preganglionic fibers.

• These fibers travel through the pelvic splanchnic nerves to innervate various organs.

• Key areas affected include the descending colon, sigmoid colon, rectum, bladder, and reproductive organs.