Neurology Introduction

Sensory Nerves in Skin

• Detect pressure, touch, heat, cold
• Important for transmitting sensory signals

Sensory Pathways

• Medial Lemniscal Pathway
  • Fast transmission
  • High spatial orientation
  • Detects touch with high locality and fine gradation of intensity
  • Crosses over in the medulla
  • Signal transmitted to thalamus -> somatosensory cortex (SSI)
• Anterolateral System
  • Slower transmission
  • Detects pain, temperature, crude sensations
  • Crosses over in spinal column

Receptive Fields and Sensitivity

• Greater density of nerves in sensitive areas (face, hands)
• Less sensitivity in larger areas (back, abdomen)
• Recruitment process for signal intensity
• Lateral inhibition for fine differentiation
• Two-point discrimination varies by body part (better on fingertips than abdomen)

Pain Pathways

• Fast Pain
  • Felt quickly (~0.1s)
  • Sharp, acute pain
  • Transmitted by A-delta fibers
• Slow Pain
  • Felt after a second, increases slowly
  • Dull, throbbing, chronic pain
  • Transmitted by C-fibers
  • Substance P involved
  • Nociceptors don’t adapt quickly
  • Hyperalgesia can occur over time

Pain Management

• Analgesia System
  • Endorphins, enkephalins, serotonin reduce pain signals
  • Natural anti-pain pathways

Referred Pain

• Pain felt in areas different from source (e.g. heart attack causing arm pain)
• Due to convergence of visceral and somatic nerves

Thermal Sensation

• Cold Receptors: More numerous, detect lower temperatures
• Warm Receptors: Detect higher temperatures
• Pain receptors detect extreme temperatures
• Better response to temperature changes than static temperatures

Visual System

Eye Anatomy

• Anterior Cavity: Contains anterior and posterior chambers
• Posterior Cavity: Contains the vitreous chamber
• Retina: Contains rods (black/white) and cones (color)
• Fovea: Highest concentration of cones, best resolution
• Optic Nerve: Transmits visual info to brain, contains optic disc (blind spot)

Visual Pathway

• Light -> Cornea -> Anterior Chamber -> Pupil -> Posterior Chamber -> Lens -> Posterior Cavity -> Retina
• Light is focused on the fovea for highest clarity

Lens Accommodation

• Near Objects: Sphincter muscle contracts, lens becomes round
• Far Objects: Sphincter muscle relaxes, lens flattens

Vision Tests and Corrections

• Snellen Eye Chart: Measures visual acuity (20/20 vision)
• Myopia: Nearsightedness; corrected with concave lenses
• Hyperopia: Farsightedness; corrected with convex lenses
• Astigmatism: Irregular curvature; corrected with cylindrical lenses
• Presbyopia: Age-related loss of accommodation
• Cataracts: Clouding of lens, can be surgically removed

Color Vision

• Types of Cones: Red, Green, Blue
• Color Blindness: Often X-linked; types include protanopia (red), deuteranopia (green), tritanopia (blue)
• Tests: Ishihara test for detecting color blindness

Visual Cortex Processing

• Primary Visual Cortex: Located in occipital lobe, processes initial visual information
• Secondary Visual Areas: Integrates motion, 3D position, forms, color details

Eye Movements

• Muscles Involved: Superior/inferior rectus, lateral/medial rectus, superior/inferior oblique, levator palpebrae
• Innervation: Cranial nerves III, IV, VI
• Voluntary/Involuntary Fixation: Controlled by frontal cortex (voluntary) and occipital cortex (involuntary)
• Saccadic Movements: Quick, random movements to keep new visual inputs