Study Notes on General Visceral Efferents (GVE) and Related Topics

General Visceral Efferents (GVE)

Overview

  • General Visceral Efferents (GVE) refer to the nerves that carry signals from the central nervous system (CNS) to smooth muscles and glands in various organs including the gut, blood vessels, bladder, etc.

  • Coordination of GVE with General Somatic Efferents (GSE) constitutes Lower Motor Neurons (LMN).

Key Structures

Autonomic Nervous System (ANS)

  • Composed of three divisions:

    • Sympathetic Nervous System (SNS)

    • Known as the "fight or flight" system.

    • Parasympathetic Nervous System (PNS)

    • Known as the "rest and digest" system.

    • Enteric Nervous System (ENS)

    • Controls gastrointestinal function independently of CNS.

  • Cranial Nerves involved with GVE: III (Oculomotor), VII (Facial), IX (Glossopharyngeal), and X (Vagus).

GVE Centers

  • Primary Center: Hypothalamus

  • Secondary Centers: Midbrain, Pons, Medulla

Hypothalamic Nuclei and Functions

  • Hypothalamus Regions:

    • Caudal Portion: Sympathetic control.

    • Rostral Portion: Parasympathetic control.

  • Nuclei:

    • DHA (Dorsomedial nucleus): Affects emotional response and autonomic functions.

    • PVN (Paraventricular nucleus): Plays a key role in controlling aspects of stress response and regulating blood pressure.

    • VMN (Ventral Medial nucleus): Involved in feeding and metabolism control.

Anatomical Pathways

Two-Neurons System

  • First Neuron (Preganglionic Neuron):

    • Originates in the CNS and travels to ganglia.

  • Second Neuron (Postganglionic Neuron):

    • Located in ganglia or in the target organ.

Sympathetic vs. Parasympathetic Divisions

  • Sympathetic Division

    • Origin: Thoracolumbar region (T1-L4/5).

    • Postganglionic cell bodies are located in cervical, celiac, or mesenteric ganglia.

    • Neurotransmitters: ACh (acetylcholine) for preganglionic, norepinephrine and epinephrine for postganglionic.

  • Parasympathetic Division

    • Origin: Craniosacral region (CN III, VII, IX, X; sacral segments S1-S3).

    • Postganglionic cell bodies located near or in target organs.

    • Neurotransmitters: ACh for both preganglionic and postganglionic.

Pupillary Light Reflex (PLR)

  • Parasympathetic Control: Involves the Oculomotor Nerve (CN III).

  • Function: Constriction of pupils upon light exposure.

  • Sympathetic Control: Dilation of pupils via direct action on dilator pupillae muscle.

  • Pathway: Light stimulus activates the optic nerve, which transmits signals through the optic chiasm and optic tract, reaching pretectal nuclei then to CN III nuclei.

Oculomotor Nerve (CN III)

  • Branches into:

    • Superior Branch: Innervates dorsal rectus and levator palpebrae superioris.

    • Inferior Branch: Innervates medial rectus, inferior rectus, and inferior oblique.

    • Short Ciliary Nerve: Carries parasympathetic fibers to the iris and ciliary body.

Iris and Ciliary Body Function

  • Iris Muscles:

    • Dilator Pupillae Muscle: Sympathetically controlled to cause dilation (mydriasis).

    • Constrictor Pupillae Muscle: Parasympathetically controlled to cause constriction (miosis).

Dysfunction of CN III (Oculomotor Nerve)

  • Typical Conditions:

    • Ptosis: Drooping of the eyelid due to muscle paralysis.

    • Nystagmus: Involuntary eye movement.

    • Strabismus: Misalignment of the eyes.

Sympathetic Innervation of the Eye

  • First Order Neurons: Descend from the hypothalamus and synapse in the T1-T3 region.

  • Second Order Neurons: Ascend through the vagosympathetic trunk and synapse on cranial cervical ganglia.

  • Third Order Neurons: Innervate the dilators of the eye.

Micturition Pathway

  • Innervation of bladder via:

    • Hypogastric Nerve: Preganglionic cell bodies from L1-L4, sympathetic control.

    • Pelvic Nerve: Preganglionic cell bodies from S1-S3, parasympathetic control.

    • Pudendal Nerve: Somatic control, innervating the external urethral sphincter.

  • Micturition Reflex:

    • As the bladder fills, stretch receptors activate GVA which sends signals to the cerebral cortex.

    • The brain's control centers (Pontine Centers) govern conscious actions related to storing or evacuating.

    • Inhibitory and excitatory actions mentioned for external (EAS) and internal anal sphincters (IAS) as per need.

Defecation Pathway

  • Activation involves stretch receptors in the rectum sensing feces, initiating impulses to spinal nerves for reflexive contractions of rectal muscles and relaxation of sphincters.

  • Valsalva’s Maneuver: Increases abdominal pressure through glottis closure, facilitating fecal expulsion.

Facial Nerve (CN VII)

  • Parasympathetic Functions:

    • Innervates lacrimal, submandibular, and sublingual glands.

    • Responds to sensory signals from taste sensation on the anterior 2/3 of the tongue.

Glossopharyngeal Nerve (CN IX)

  • Functions include stimulating parotid gland secretion, motor functions to stylopharyngeus muscle, and sensory functions from carotid body and taste from posterior tongue.

Vagus Nerve (CN X)

  • Exits the skull via the jugular foramen.

  • Involved in parasympathetic innervation to viscera (heart, lungs, and gut) and modulates heart rate and digestive processes.

  • Carries sensory information from the viscera back to the brain.

Functional Components of the Vagus

  • Involves both parasympathetic actions (slowing heart rate, promoting digestion) and sensory functions relating to visceral states.