CAP 52 4/4: Control parasimpático y simpático del ojo l Fisiología de Guyton

Introduction

The introduction sets the stage for discussing accommodation systems and photomotor systems in the eye, building upon previous chapters' reflections on these topics.

Systems of Accommodation

• Accommodation involves autonomic innervations from the central nervous system.

• Gaiton explains that the eye is innervated by both parasympathetic and sympathetic fibers originating from the central nervous system.

• The Edinger-Westphal nucleus in the midbrain plays a crucial role.

• Fibers from this nucleus form part of cranial nerve III (oculomotor nerve).

• Pre-ganglionic fibers leave the Edinger-Westphal nucleus to synapse at the ciliary ganglion.

• This ganglion serves as a cluster of neurons where synaptic connections occur before post-ganglionic fibers stimulate structures like the ciliary muscle.

Photomotor Systems

• Post-ganglionic fibers target specific eye muscles for various functions.

• These fibers stimulate the ciliary muscle, essential for lens accommodation to focus on objects near or far.

• Parasympathetic fibers also innervate the iris sphincter muscle.

• This involvement leads to miosis, constricting the pupil size for light adjustment.

Autonomic Innervation

Explores sympathetic nervous system involvement in ocular function, detailing its origin and impact on iris dilation and extraocular muscles.

Sympathetic Nervous System

• Sympathetic fibers originate from thoracic spinal segments T1-T2.

• Intermediolateral cell groups give rise to pre-ganglionic fibers targeting the cervical sympathetic trunk.

• The cervical sympathetic trunk houses a sympathetic ganglion where pre-ganglionic fibers synapse before post-ganglionic axons are released.

• Post-ganglionic fibers travel along with carotid plexus to reach structures like iris dilator muscle.

Functions of Sympathetic Innervation

• Iris dilator muscle receives sympathetic innervation for pupil dilation control.

• Additionally, extraocular muscles such as levator palpebrae superioris are influenced by sympathetic input.

Accommodation Control

Discusses how parasympathetic stimulation drives accommodation processes through ciliary muscle activity, impacting lens shape adjustments for focusing.

Parasympathetic Influence

• Parasympathetic stimulation primarily governs lens accommodation via ciliary muscle contraction or relaxation.

Ray of Light and Eye Accommodation

This section discusses how light rays from objects impact the eye, leading to changes in focus through the accommodation of the lens.

Mechanism of Eye Accommodation

• The eye's ability to change focus rapidly, within less than a second, is primarily attributed to the adjustment of the lens by the ciliary muscle.

• When shifting focus from one object to another, such as from a distant elk to nearby flowers, the ciliary muscle is stimulated for contraction, facilitating accommodation.

Factors Influencing Accommodation

• Various data types, including chromatic aberration (differences in how colors impact the retina), play a role in transforming lens potential for appropriate focusing.

• Chromatic aberrations are detected by the retina based on different wavelengths of light colors impacting it, prompting adjustments for optimal focusing.

Convergence and Fovea Adjustment

This section delves into convergence mechanisms and foveal adjustments for precise visual focus.

Convergence Mechanism

• Convergence occurs when eyes move towards each other while focusing on near objects, activating the parasympathetic system and causing lens convexity for close-up vision.

• The slight elevation in fovea (area with highest visual acuity) compared to surrounding retina aids in detecting discrepancies in focused areas.

Nervous System Response

• Nervous system detects discrepancies between actual and ideal focal points on fovea, triggering parasympathetic activation for lens adjustment towards optimal clarity.

• Ongoing lens accommodation oscillates swiftly due to feedback systems ensuring rapid corrections for accurate visual perception.

Control of Lens Accommodation

Exploring how brain regulates lens accommodation swiftly based on visual stimuli variations.

Dynamic Lens Adjustment

• Lens accommodation can occur twice per second due to rapid feedback loops that ensure prompt responses to changing focal requirements.

Understanding Pupillary Reflexes

In this section, the speaker explains the mechanisms behind pupillary reflexes, detailing how the sympathetic and parasympathetic systems control pupil dilation and constriction.

Mechanisms of Pupillary Reflexes

• The dilation of the iris or pupil is known as midriasis, primarily stimulated by the sympathetic system.

• Constriction of the pupil, known as miosis, is controlled by the sphincter pupillae muscle under parasympathetic influence.

• Evaluation of pupillary reflexes can be done through a photomotor reflex test where pupils constrict upon exposure to light.

Neural Pathways in Pupillary Control

This segment delves into the neural pathways involved in regulating pupillary diameter and how different stimuli impact these pathways.

Neural Pathways and Reflex Evaluation

• Light stimulation triggers a series of events involving retinal cells, ganglion cells, and cranial nerve II (optic nerve) towards pretectal region activation.

• Stimulation leads to contraction of ciliary muscles for accommodation and sphincter pupillae for miosis via Edinger-Westphal nucleus activation.

• Dysfunction in these pathways can result from conditions like syphilis or alcoholism affecting pretectal region functionality.

Impact of Pathologies on Pupillary Responses

This part explores how pathologies such as central nervous system syphilis can disrupt pupillary reflexes through inhibition or dysfunction within neural circuits.

Effects of Pathologies on Pupillary Reflexes

• Conditions like syphilis can impair pretectal region function leading to abnormal responses to light stimuli causing miosis deficits.

• Inhibitory neurons from pretectal region play a role in modulating pupil size; dysfunction in these neurons can lead to constant miosis due to unregulated Edinger-Westphal nucleus activity.

Pupil Response Variations and Accommodation Reactions

This section discusses variations in pupil responses due to different stimuli and conditions impacting accommodation reactions.

Pupil Response Variability and Accommodation

• The concept of "pupillary response to accommodation" explains how pupils may contract further when stimulated by near objects due to increased Edinger-Westphal nucleus activity.

New Section

In this section, the speaker discusses the continuous constriction of the pupil in Horner's syndrome due to the lack of stimuli for dilation, leading to a prevalence of parasympathetic stimuli. The characteristics of Horner's syndrome, such as miosis and drooping upper eyelid, are highlighted.

Pupil Constriction and Parasympathetic Stimulation

• The pupil remains constricted continuously in Horner's syndrome due to the absence of stimuli for dilation.

• The sympathetic system stimulates both the dilator of the iris and the extrinsic muscle of the eyelids, resulting in a drooping upper eyelid.

• Sympathetic innervation causes persistent dilation of blood vessels in the face and head regions in Horner's syndrome.

New Section

This section focuses on a patient with Horner's syndrome exhibiting miosis (pupil constriction) and a drooping eyelid on one side. Visual cues indicating these symptoms are discussed.

Clinical Presentation of Horner's Syndrome

• Observation of miosis and drooping eyelid on one side in a patient with Horner's syndrome.