Unit 3: Sensory Systems – Visual and Vestibular Systems

Introduction

This unit explores the anatomy and physiology of the visual and vestibular sensory systems, focusing on their roles in skilled performance and motor learning. Understanding these systems is essential for comprehending how the body perceives, processes, and responds to sensory information during movement.

Peripheral Receptors

Types of Peripheral Receptors

• Cutaneous Receptors: Detect touch, stretch, vibration, and temperature. These are located in the skin and provide information about external stimuli.

• Muscle Spindles: Sense muscle length and changes in muscle length, contributing to proprioception and reflexes.

• Joint Receptors: Monitor joint angles and range of motion, important for coordinated movement.

• Golgi Tendon Organs: Detect muscle tension, helping to prevent muscle damage from excessive force.

Example: When you stretch your arm, muscle spindles and joint receptors provide feedback to the central nervous system about the position and movement of your limb.

Reflexive Control of Movement

Spinal Reflexes

Reflexes are rapid, involuntary responses to specific stimuli, crucial for protecting the body and maintaining posture.

• Involuntary: Occur without conscious planning, though they can be modulated by higher brain centers.

• Stereotyped: Produce consistent responses to the same stimulus.

• Fast-responding: Enable quick reactions to potentially harmful stimuli.

Examples of Reflexes

• Withdrawal Reflex: Rapidly withdraws a limb from a painful stimulus.

• Stretch Reflex: Maintains muscle length and tone (e.g., knee-jerk reflex).

• Crossed Extensor Reflex: Balances the body during withdrawal by extending the opposite limb.

Main Ascending Pathways

How Sensory Information Reaches the Brain

• Dorsal Column-Medial Lemniscus Pathway: Transmits vibration, proprioception, and light touch information. Important for discriminative touch and conscious proprioception, such as recognizing objects by touch and coordinating movement.

• Anterolateral Pathway: Carries pain, temperature, and crude touch information. Important for localizing noxious sensations and distinguishing between warm and cold.

Visual System

Function

• Exteroceptive Sense: Identifies objects in space and determines their movement.

• Visual Proprioception: Provides information about the body's position in space, the relation of body segments, and motion of the body.

The Eye

• Retina: Contains photoreceptors (rods and cones) that detect light and convert it into neural signals.

• Fovea: Area of the retina with the highest concentration of cones, providing the clearest vision. Eye movements direct images to the fovea for high-resolution processing.

Example: When reading, your eyes move so that the words fall on the fovea, allowing you to see them clearly.

Pathway for Visual Information

• Ganglion cell axons exit the eye through the optic disk, forming the optic nerve.

• Optic nerves from both eyes meet at the optic chiasm, where some fibers cross to the opposite side.

• Fibers continue as the optic tract to various brain regions.

Central Visual Pathway

• Superior Colliculus: Controls eye and head movements.

• Pretectal Region: Mediates the pupillary light reflex.

• Lateral Geniculate Nucleus (LGN): Relays visual information to the primary visual cortex (V1).

• Primary Visual Cortex (V1): Also called the "striate cortex," processes basic visual information.

Higher Order Visual Cortex

• Dorsal Stream ("Where" Pathway): Processes spatial visual cues and guides movement (parietal lobe).

• Ventral Stream ("What" Pathway): Responsible for pattern discrimination and object recognition (temporal lobe).

Vestibular System

Function

• Controls balance and posture (vestibulo-spinal reflexes).

• Positions the head and neck (vestibulo-colic reflexes).

• Stabilizes gaze during head movement (vestibulo-ocular reflex, VOR).

Vestibulo-Ocular Reflex (VOR): Allows the eyes to move in the opposite direction of head movement, maintaining a stable image on the retina.

Anatomy of the Vestibular System

• Bony Labyrinth: Rigid, outer structure filled with perilymph fluid.

• Membranous Labyrinth: Inner structure filled with endolymph fluid.

• Vestibular Apparatus: Contains five sensory structures: three semicircular canals and two otolith organs (utricle and saccule).

Semicircular Canals

• Three pairs: anterior, posterior, and horizontal canals, arranged in co-planar pairs.

• Detect angular acceleration (rotational movement) of the head.

• Each canal has an ampulla containing hair cells embedded in a gelatinous structure called the cupula.

• Movement of endolymph fluid deflects the cupula, stimulating hair cells.

Otolith Organs

• Utricle and Saccule: Detect linear acceleration and head position relative to gravity.

• Hair cell cilia extend into the otolithic membrane, which contains calcium carbonate crystals (otoconia/otoliths).

• Movement of the head causes displacement of the otolithic membrane, bending the hair cells and generating neural signals.

Vestibular Sensory Neurons

• Signals from vestibular hair cells are transmitted via the vestibulocochlear nerve (CN VIII) to the brainstem.

• Vestibular nuclei include: Lateral (LVN), Medial (MVN), Superior (SVN), and Inferior (IVN) Vestibular Nuclei.

Summary

• The visual system identifies objects and body position in space, and tracks movement.

• The vestibular system is essential for balance, head/neck positioning, and stabilizing vision during movement.

• Both systems rely on specialized peripheral receptors and complex central pathways for processing sensory information.