Chapter 6: Peripheral Nervous System: Afferent Division & Special Senses

Overview of the Nervous System and Feedback Loops

• The Feedback Loop Structure: The nervous system operates through a sequential response loop consisting of the following components:

  • Stimulus: An environmental change or internal trigger.

  • Sensor or Receptor: Detects the stimulus.

  • Afferent Path: The pathway through which sensory information travels toward the center.

  • Integrating Center: Located within the Central Nervous System (CNS); processes information.

  • Efferent Path: The pathway through which motor commands travel away from the center.

  • Effector: Muscle or gland that carries out a command.

  • Response: The final action resulting from the stimulus.

• Divisions of the Nervous System:

  • Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for information processing and higher-order functions such as memory, learning, and intelligence.

  • Peripheral Nervous System (PNS):

    • Afferent Division: Carries sensory information to the CNS from receptors.

    • Efferent Division: Carries motor commands from the CNS to effectors.

      • Somatic Nervous System: Controls skeletal muscle.

      • Autonomic Nervous System: Controls smooth muscle, cardiac muscle, and glands. Further subdivided into the Sympathetic and Parasympathetic divisions.

Functional Classification of Sensory Systems

• Transduction: The process of converting energy from a stimulus into electrical signals (action potentials). This occurs at the sensory nerve terminal.

• Sensation: The ability to send information about environmental changes to the CNS.

• Perception: The ability to interpret environmental stimuli. This is a conscious interpretation of the external world derived from sensory input.

• Limitations of Perception: Sensory input does not provide a "true reality" because:

  • Humans possess receptors for only a limited number of existing energy forms.

  • Information channels in the brain are not "high-fidelity recorders."

  • The cerebral cortex further manipulates and filters data.

  • Examples of misperception include optical illusions and phantom pain (pain perceived in a limb that is no longer present).

Classification of Sensory Receptors

• By Location and Source:

  • Special Sensory Receptors: Provide sensations of smell, taste, vision, balance, and hearing.

  • Somatic Sensory Receptors: Monitor skeletal muscles, joints, and the skin surface. They provide position sense (proprioception) and sensations of touch, pressure, pain, and temperature.

  • Visceral Sensory Receptors: Monitor internal organs, including the cardiovascular, respiratory, digestive, urinary, and reproductive systems.

• By Type of Stimulus (Specificity):

  • Photoreceptors: Responsive to visible wavelengths of light.

  • Mechanoreceptors: Sensitive to mechanical energy (stretching, bending, or distortion of cell membranes).

  • Thermoreceptors: Sensitive to heat and cold.

  • Osmoreceptors: Detect changes in solute concentration and resultant changes in osmotic activity.

  • Chemoreceptors: Sensitive to specific chemicals, including $O_2$, $CO_2$, pH, and chemical content of the digestive tract.

  • Nociceptors: Pain receptors sensitive to tissue damage or distortion.

Receptor Physiology and Adaptability

• Receptor Potentials: A stimulus alters the receptor's permeability, leading to a graded receptor potential. This change often involves the influx of positive ions.

• Intensity Coding: The magnitude of the receptor potential represents the intensity of the stimulus. If the potential is of sufficient magnitude, it produces an action potential propagated to the CNS.

• Adaptation: A diminution in sensation despite a continuous stimulus.

  • Rapidly Adapting (Phasic): Receptors that respond quickly but stop firing if the stimulus remains constant (e.g., olfactory receptors, tactile receptors in the skin).

  • Slowly Adapting (Tonic): Receptors that maintain firing during a constant stimulus (e.g., nociceptors, muscle stretch receptors, joint proprioceptors).

• Receptive Field: The area of tissue monitored by one sensory receptor.

  • The smaller the receptive field, the higher the discriminative ability (e.g., fingertips have more receptive fields per unit area than the thigh).

  • Two-Point Discrimination Test: Used to determine the activation of separate receptive fields.

Detailed Mechanoreceptors

• Tactile Receptors (Skin):

  • Merkel's Disk: Senses steady indentation and pressure; slowly adapting.

  • Meissner's Corpuscle: Senses low-frequency vibration (gentle fluttering); rapidly adapting.

  • Ruffini's Corpuscle: Senses rapid indentation and stretch; slowly adapting.

  • Pacinian Corpuscle: Senses high-frequency vibration; rapidly adapting.

  • Hair Receptor: Senses hair deflection or brushing; rapidly adapting.

• Baroreceptors: Detect changes in pressure via the stretching of hollow organs.

  • Digestive Tract: Monitors volume and triggers reflexes for material movement.

  • Carotid and Aortic Sinuses: Monitor blood pressure for cardiovascular and respiratory control.

  • Lungs: Monitor stretching to control respiratory rhythmicity.

  • Bladder and Colon: Monitor volume to trigger urinary and defecation reflexes.

• Proprioceptors: Sense of body position.

  • Muscle Spindles: Monitor skeletal muscle length.

  • Golgi Tendon Organs: Monitor skeletal muscle tension.

  • Joint Receptors: Detect pressure, tension, and movement.

Nociception and Pain Management

• Nature of Pain: Primarily a protective mechanism to signal tissue damage. It is accompanied by behavioral responses and emotional reactions.

• Pain Receptors: Bare nerve endings in skin, muscle, joints, arteries, and viscera.

• Chemical Activators of Pain:

  • Histamine: Acts on $H_1$ receptors; cause pain and itching.

  • Capsaicin: Acts on Vanilloid receptors; provides heat sensation and used in analgesic creams.

  • Prostaglandins: Released by damaged cells; they lower the nociceptor threshold for activation. NSAIDs (Non-steroidal anti-inflammatory drugs) like ibuprofen work by inhibiting the cyclooxygenase enzyme that produces prostaglandins.

• Neurotransmitters in Pain Pathways:

  • Substance P: Activates ascending pathways to the thalamus and somatosensory cortex.

  • Glutamate: Major excitatory neurotransmitter; responsible for hypersensitivity in injured areas (e.g., sunburn).

• Endogenous Analgesic System: The brain suppresses pain transmission in the spinal cord using endogenous opiates such as endorphins, enkephalins, and dynorphin.

The Special Sense of Vision: The Eye

• Anatomy:

  • Iris: Controls light entry via the circular (constrictor) muscle (Parasympathetic) and radial (dilator) muscle (Sympathetic).

  • Lens: Changes shape for focusing; curvature is adjusted by the ciliary muscle and suspensory ligaments.

  • Cornea: Contributes the most to refractive ability; curvature remains constant.

  • Retina: Contains photoreceptors (Rods and Cones).

• Accommodation: The adjustment of lens strength to focus on near or far objects.

  • Distant Vision: Ciliary muscle relaxes, suspensory ligaments become taut, and the lens flattens.

  • Near Vision: Ciliary muscle contracts, suspensory ligaments slacken, and the lens becomes rounded/stronger.

  • Presbyopia: Age-related reduction in accommodation ability.

• Refractive Errors:

  • Emmetropia: Normal eye.

  • Myopia (Nearsightedness): Eyeball too long or lens too strong; corrected with a concave lens.

  • Hyperopia (Farsightedness): Eyeball too short or lens too weak; corrected with a convex lens.

• Photoreceptors:

  • Rods: High sensitivity, low acuity, vision in shades of gray, responsible for night vision. Concentrated in the periphery of the retina.

  • Cones: Low sensitivity, high acuity, color vision (Red, Green, Blue cones). Concentrated in the fovea (the point of most distinct vision).

• Light Absorption Peaks:

  • Blue cones peak near $420\,nm$.

  • Green cones peak near $530\,nm$.

  • Red cones peak near $560\,nm$.

• Color Blindness:

  • Anomalous Trichromacy: 3 photopigments but from 2 groups (e.g., deuteranomalous).

  • Dichromacy: Missing one entire group of photopigments.

The Special Senses of Hearing and Equilibrium

• Hearing Mechanism:

  • Sound Waves: Traveling vibrations of air consisting of compression and rarefaction.

  • Pitch: Determined by wave frequency ($Hz$).

  • Intensity: Determined by wave amplitude ($dB$).

  • Ear Anatomy:

    • External Ear: Pinna, external auditory meatus, tympanic membrane.

    • Middle Ear: Ossicles (malleus, incus, stapes) amplify sound to the oval window.

    • Inner Ear (Cochlea): Contains the Organ of Corti with hair cells on the basilar membrane.

• Sound Transduction Sequence:

  1. Tympanic membrane vibration.

  2. Middle ear bone vibration.

  3. Oval window vibration.

  4. Fluid movement in the cochlea.

  5. Basilar membrane vibration.

  6. Bending of inner hair cell stereocilia against the tectorial membrane.

  7. Graded potential change.

  8. Action potentials in the auditory nerve.

• Tonotopic Map: Higher frequencies are detected at the base of the cochlea; lower frequencies are detected at the apex (helicotrema).

• Equilibrium (Vestibular System):

  • Static Equilibrium: Senses head position; maintains posture while motionless.

  • Dynamic Equilibrium: Detects rotational acceleration (rotational movement).

  • Structures: Semicircular canals (dynamic) and otolith organs (saccule and utricle for static).

  • Hair Cells: Contain a long kinocilium and smaller stereocilia. Bending toward the kinocilium causes depolarization; bending away causes hyperpolarization.

  • Vestibular Conditions: Vertigo (spinning sensation) and Nystagmus (oscillating eye movements).

Chemical Senses: Olfaction and Gustation

• Smell (Olfaction):

  • Receptor Cells: Specialized endings of renewable afferent neurons (replaced every 2 months).

  • Mucosa: Contains supporting cells (mucus), basal cells (precursors), and receptor cells.

  • Path: Axons form the olfactory nerve -> Glomerulus in the olfactory bulb -> Limbic system (emotional) or Thalamus/Frontal cortex (conscious).

• Taste (Gustation):

  • Receptor Cells: Modified epithelial cells with microvilli (10-day lifespan) located in taste buds.

  • Five Basic Qualities:

    1. Salty: Na+ channels open.

    2. Sour: Activated by acids; closes K+ channels.

    3. Sweet: Activated by sugars; G-protein closes K+ channels.

    4. Bitter: G-protein and intracellular $Ca^{2+}$ release.

    5. Umami: Responds to protein-rich chemicals (meat flavor).

• Taste Dysfunction (Hypogeusia/Dysgeusia):

  • Damage to CN VII (Facial): Loss of sour, sweet, and salty tastes in the anterior $2/3$ of the tongue.

  • Damage to CN IX (Glossopharyngeal): Loss of bitter taste in the posterior $1/3$ of the tongue.

  • Other Causes: Aging, smoking, medications (Lithium, Penicillamine, Captopril), and radiation treatments.