Unit 3: Sensory Systems

Overview

This unit explores the anatomy and physiology of sensory systems, focusing on the visual and vestibular systems. These systems are essential for skilled motor performance, balance, and spatial orientation.

Peripheral Receptors

Types of Peripheral Receptors

Peripheral receptors detect various sensory stimuli and contribute to proprioception and movement control.

• Cutaneous receptors: Detect touch, stretch, vibration, and temperature.

• Muscle spindles: Sense muscle length and changes in muscle length.

• Joint receptors: Monitor joint angles and range of motion.

• Golgi tendon organs: Detect muscle tension.

Example: Muscle spindles help maintain posture by sensing stretch in muscles and triggering reflexes.

Reflexive Control of Movement

Spinal Reflexes

Reflexes are rapid, involuntary responses to stimuli that help protect the body and maintain posture.

• Involuntary: Occur without conscious planning, but can be modulated.

• Stereotyped: Produce similar responses to the same stimulus.

• Fast-responding: Enable quick reactions to sensory input.

Examples of Reflexes

• Withdrawal Reflex: Rapid removal of a limb from a painful stimulus.

• Stretch Reflex: Muscle contraction in response to stretching within the muscle.

• Crossed Extensor Reflex: Extension of the opposite limb to support the body during withdrawal.

Example: Stepping on a sharp object triggers the withdrawal reflex in one leg and the crossed extensor reflex in the other.

Main Ascending Pathways

How Sensory Information Reaches the Brain

Sensory information travels to the brain via specific neural pathways.

Visual System

Function

The visual system provides information about the environment and the body's position within it.

• Exteroceptive sense: Identifies objects and their movement in space.

• Visual proprioception: Determines body position, relation of body segments, and motion.

The Eye

Vision receptors are located on the retina at the back of the eyeball. The fovea is the region with the highest concentration of photoreceptors, providing the clearest image. Eye movements ensure that images fall on the fovea for optimal resolution.

• Retina: Contains photoreceptors (rods and cones).

• Fovea: Central area of sharpest vision.

Example: Reading requires precise eye movements to keep words focused on the fovea.

Pathway for Visual Information

Visual signals travel from the retina to the brain through a series of neural pathways.

• Ganglion cell axons exit the eye via the optic disk.

• Axons bundle to form the optic nerve.

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

• Continue as the optic tract to various brain regions.

Central Visual Pathway

After reaching the brain, visual information is processed in several key areas:

• Superior Colliculus: Controls eye and head movements.

• Pretectal region: Mediates pupillary reflexes.

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

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

Higher Order Visual Cortex

Visual information is further processed along two main pathways:

• Dorsal stream ("where"): Processes spatial cues and movement; projects to parietal regions.

• Ventral stream ("what"): Handles pattern discrimination and recognition; projects to temporal cortex.

Example: Damage to the dorsal stream can cause optic ataxia (difficulty reaching for objects), while ventral stream damage can cause visual agnosia (difficulty recognizing objects).

Vestibular System

Function

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

• Balance control (vestibulo-spinal reflexes)

• Head and neck positioning (vestibulo-colic reflexes)

• Eye movement control (vestibulo-ocular reflex, VOR)

Example: The VOR keeps your gaze steady when you turn your head.

Anatomy of the Vestibular System

• Bony labyrinth: Filled with perilymph fluid.

• Membranous labyrinth: Filled with endolymph fluid.

Vestibular Apparatus

Contains five sensory structures:

• Semicircular canals (anterior, posterior, horizontal): Detect angular acceleration.

• Otolith organs (utricle and saccule): Detect linear acceleration and orientation relative to gravity.

Semicircular Canals

• Three co-planar pairs: right/left horizontal, left anterior/right posterior, right anterior/left posterior.

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

• Angular acceleration causes endolymph fluid to lag, deflecting the cupula and hair cell cilia.

Otolith Organs

• Utricle and saccule contain hair cells embedded in the otolithic membrane.

• Calcium carbonate crystals (otoconia or otoliths) sit atop the membrane, enhancing sensitivity to linear acceleration and gravity.

Vestibular Sensory Neurons

Vestibular signals are transmitted via the vestibulo-cochlear nerve (CN VIII) to four main vestibular nuclei in the brainstem:

• Lateral Vestibular Nucleus (LVN)

• Medial Vestibular Nucleus (MVN)

• Superior Vestibular Nucleus (SVN)

• Inferior Vestibular Nucleus (IVN)

Summary

• Vision identifies objects and body position in space, and tracks movement.

• Vestibular function is crucial for balance and positioning of the head, neck, and eyes.

• Both systems have specialized peripheral receptors and central pathways for processing sensory information.

Additional info: The integration of visual and vestibular inputs is essential for coordinated movement and spatial orientation.