Sensory Receptors, Somatosensory System, and Nerve Structure

13.1 Sensory Receptors

Introduction to Sensory Receptors

Sensory receptors are specialized cells or structures that detect changes in the environment, known as stimuli. These receptors initiate graded potentials that can trigger nerve impulses, leading to the awareness (sensation) and interpretation (perception) of stimuli in the brain. Sensory receptors are classified based on the type of stimulus they detect, their location in the body, and their structural complexity.

• Sensory receptor: Specialized to respond to environmental changes (stimuli).

• Activation results in graded potentials that trigger nerve impulses.

• Sensation: Awareness of stimulus.

• Perception: Interpretation of the meaning of the stimulus, occurs in the brain.

Three main ways to classify receptors:

• Type of stimulus

• Body location

• Structural complexity

Classification by Stimulus Type

• Mechanoreceptors: Respond to touch, pressure, vibration, and stretch.

• Thermoreceptors: Sensitive to changes in temperature.

• Photoreceptors: Respond to light energy (e.g., retina).

• Chemoreceptors: Respond to chemicals (e.g., smell, taste, changes in blood chemistry).

• Nociceptors: Sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals).

Classification by Location

• Exteroceptors: Respond to stimuli arising outside the body; found in skin for touch, pressure, pain, and temperature.

• Interoceptors (visceroceptors): Respond to stimuli arising in internal viscera and blood vessels; sensitive to chemical changes, tissue stretch, and temperature changes.

• Proprioceptors: Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles; inform the brain of body movements.

Classification by Receptor Structure

• Simple receptors of the general senses: Modified dendritic endings of sensory neurons; found throughout the body and monitor most types of general sensory information.

• Receptors for special senses: Vision, hearing, equilibrium, smell, and taste; all are housed in complex sense organs.

Simple Receptors of the General Senses

• General senses include tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.

• Receptors can respond to multiple stimuli; no strict "one-receptor-one-function" relationship.

• Receptors have either:

  • Nonencapsulated (free) nerve endings

  • Encapsulated nerve endings

Nonencapsulated (Free) Nerve Endings

• Abundant in epithelial and connective tissues.

• Most are nonmyelinated, small-diameter group C fibers; distal terminals have knoblike swellings.

• Respond mostly to temperature, pain, or light touch.

Examples:

• Nociceptors

• Thermoreceptors: Cold = 10°C–40°C; Heat = 32°C–48°C

• Tactile discs (Merkel cells)

• Hair follicle receptors

Encapsulated Dendritic Endings

• Almost all are mechanoreceptors whose terminal endings are encased in connective tissue capsule.

• Vary greatly in shape and include:

  • Tactile (Meissner's) corpuscles: Small receptors involved in discriminative touch; found in sensitive, hairless areas (fingertips).

  • Lamellar (Pacinian) corpuscles: Large receptors respond to deep pressure and vibration; located deep in dermis.

  • Bulbous corpuscles (Ruffini endings): Respond to deep and continuous pressure; located in dermis.

  • Muscle spindles: Spindle-shaped proprioceptors that respond to muscle stretch.

  • Tendon organ: Proprioceptors located in tendons that detect stretch.

  • Joint kinesthetic receptors: Proprioceptors that monitor joint position and motion.

Table: General Sensory Receptors Classified by Structure and Function

Structural Class	Functional Classes (Location & Stimulus Type)	Body Location
Free nerve endings	Exteroceptors, interoceptors, proprioceptors; thermoreceptors, chemoreceptors, mechanoreceptors (pressure), nociceptors (pain)	Most body tissues; especially connective tissues and epithelia
Modified free nerve endings (tactile discs)	Exteroceptors; mechanoreceptors (light pressure), slowly adapting	Basal layer of epidermis
Hair follicle receptors	Exteroceptors; mechanoreceptors (hair deflection), rapidly adapting	In and surrounding hair follicles
Tactile (Meissner's) corpuscles	Exteroceptors; mechanoreceptors (light pressure, discriminative touch, vibration of low frequency), rapidly adapting	Dermal papillae of hairless skin (nipples, external genitalia, fingertips, soles, eyelids)
Lamellar (Pacinian) corpuscles	Exteroceptors, interoceptors, some proprioceptors; mechanoreceptors (deep pressure, stretch, vibration of high frequency), rapidly adapting	Dermis and hypodermis; periostea, mesentery, tendons, ligaments, joint capsules; most abundant on fingers, soles of feet, external genitalia, nipples
Bulbous corpuscles (Ruffini endings)	Exteroceptors, proprioceptors; mechanoreceptors (deep pressure and stretch), slowly or nonadapting	Deep in dermis, hypodermis, and joint capsules
Muscle spindles	Proprioceptors; mechanoreceptors (muscle stretch, length)	Skeletal muscles, particularly in the extremities
Tendon organs	Proprioceptors; mechanoreceptors (tendon stretch, tension)	Tendons
Joint kinesthetic receptors	Proprioceptors; mechanoreceptors and nociceptors	Joint capsules of synovial joints

13.2 Sensory Processing

Introduction to Sensory Processing

Survival depends on the ability to detect changes in the internal and external environment (sensation) and to interpret these changes (perception).

• Sensation: Awareness of changes in the environment.

• Perception: Conscious interpretation of those stimuli.

General Organization of the Somatosensory System

Somatosensory System Overview

The somatosensory system is a part of the sensory system that serves the body wall and limbs. It receives input from exteroceptors, proprioceptors, and interoceptors. Sensory input is relayed toward the head but is processed along the way at various levels.

Levels of Neural Integration

1. Receptor level: Sensory receptors

2. Circuit level: Processing in ascending pathways

3. Perceptual level: Processing in cortical sensory areas

Processing at the Receptor Level

• Generating a signal: For sensation to occur, the stimulus must excite a receptor, and the action potential (AP) must reach the CNS.

• Stimulus energy must match receptor specificity (touch receptors do not respond to light).

• Stimulus must be applied within the receptor's receptive field.

• Transduction must occur—energy of stimulus is converted into a graded potential (generator potential in general receptors, receptor potential in special sense receptors).

• Graded potentials must reach threshold to generate an AP.

Adaptation

• Change in sensitivity in the presence of constant stimulus.

• Receptor membranes become less responsive over time.

• Receptor potentials decline in frequency or stop.

• Phasic receptors: Fast-adapting; signal the beginning or end of a stimulus (e.g., receptors for pressure, touch, smell).

• Tonic receptors: Adapt slowly or not at all (e.g., nociceptors, most proprioceptors).

Processing at the Circuit Level

• Pathways of three neurons conduct sensory impulses received from receptors upward to appropriate cortical regions.

• First-order sensory neurons: Conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS.

• Second-order sensory neurons: Transmit impulses to third-order sensory neurons.

• Third-order sensory neurons: Conduct impulses from thalamus to the somatosensory cortex (perceptual level).

Processing at the Perceptual Level

• Interpretation of sensory input depends on the specific location of target neurons in the sensory cortex.

• Aspects of sensory perception include:

  • Perceptual detection: Ability to detect a stimulus (requires summation of impulses).

  • Magnitude estimation: Intensity coded in frequency of impulses.

  • Spatial discrimination: Identifying site or pattern of stimulus (studied by two-point discrimination test).

  • Feature abstraction: Identification of more complex aspects and several stimulus properties.

  • Quality discrimination: Ability to identify submodalities of a sensation (e.g., sweet or sour tastes).

  • Pattern recognition: Recognition of familiar or significant patterns in stimuli (e.g., melody in a piece of music).

Perception of Pain

Introduction to Pain Perception

Pain perception is essential for protective action. Pain is caused by actual or impending tissue damage, and stimuli include extreme pressure and temperature, inflammatory chemicals, and others. Pain impulses travel on fibers that release neurotransmitters such as glutamate and substance P. Some pain impulses are blocked by inhibitory endogenous opioids (e.g., endorphins).

Pain Tolerance

• All perceive pain at the same stimulus intensity, but pain tolerance varies among individuals.

• "Sensitive to pain" means low pain tolerance, not low pain threshold.

• Genes help determine pain tolerance and response to pain medications.

• Research in genetics is ongoing to determine best pain treatment strategies.

Visceral and Referred Pain

• Visceral pain: Results from stimulation of visceral organ receptors; felt as vague aching, gnawing, or burning; activated by tissue stretching, chemicals, muscle spasms, or ischemia.

• Referred pain: Pain from one body region perceived as coming from a different region; visceral and somatic pain fibers travel along the same nerves, so the brain interprets visceral pain as coming from somatic regions (e.g., left arm pain during a heart attack).

Clinical—Homeostatic Imbalance 13.1

• Long-lasting or intense pain (e.g., limb amputation) can lead to hyperalgesia (pain amplification), chronic pain, and phantom limb pain.

• NMDA receptors: Strengthen neural connections during certain kinds of learning and are activated by long-lasting or intense pain, allowing spinal cord to "learn" hyperalgesia.

• Early pain management is critical to prevent chronic pain.

• Phantom limb pain: Pain felt in limb that is no longer present; blocking pain transmission during and after surgery reduces risk.

• Recurring cut nerves to innervate surrounding muscle reduces phantom pain.

13.3 Nerves

Introduction to Nerves

A nerve is a cordlike organ of the peripheral nervous system (PNS) consisting of bundles of myelinated and nonmyelinated peripheral axons enclosed by connective tissue. There are two types of nerves: spinal or cranial, depending on their origin.

Structure and Classification of Nerves

• Connective tissue coverings include:

  • Endoneurium: Loose connective tissue that encloses axons and their myelin sheaths (Schwann cells).

  • Perineurium: Coarse connective tissue that bundles fibers into fascicles.

  • Epinerium: Tough fibrous sheath around all fascicles to form the nerve.

• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers.

• Nerves are classified according to the direction they transmit impulses:

  • Mixed nerves: Contain both sensory and motor fibers; impulses travel to and from CNS.

  • Sensory (afferent) nerves: Impulses only toward CNS.

  • Motor (efferent) nerves: Impulses only away from CNS.

• Pure sensory (afferent) or pure motor (efferent) nerves are rare; most nerves are mixed.

• Types of fibers in mixed nerves:

  • Somatic afferent (sensory from muscle to brain)

  • Somatic efferent (motor from brain to muscle)

  • Visceral afferent (sensory from organs to brain)

  • Visceral efferent (motor from brain to organs)

Ganglia

• Ganglia contain neuron cell bodies associated with nerves in the PNS.

• Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons (dorsal root ganglia for sensory, somatic).

• Ganglia associated with efferent nerve fibers contain autonomic motor neurons (autonomic ganglia for motor, visceral).

Regeneration of Nerve Fibers

Regeneration in the CNS

• Mature neurons are amitotic; if the soma (cell body) of the damaged nerve is intact, the peripheral axon may regenerate in the PNS, but this does not occur in the CNS.

• Most CNS fibers never regenerate due to:

  • Oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration.

  • Astrocytes at injury site form scar tissue.

  • Treatments focus on blocking inhibitors, destroying scar tissue components.

Regeneration in the PNS

• PNS axons can regenerate if damage is not severe.

• Steps in regeneration:

  1. Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration); degeneration spreads down axon.

  2. Macrophages clean dead axon debris; Schwann cells are stimulated to divide.

  3. Axon filaments grow through regeneration tube.

  4. Axon regenerates, and new myelin sheath forms.

Example: If a peripheral nerve in the arm is cut, Schwann cells help guide the regrowth of the axon to its target, restoring function if the pathway is intact.

Additional info: The ability of the PNS to regenerate is a key difference from the CNS, where regeneration is limited due to inhibitory factors and scar formation.