Sensory Receptors & Stimuli

Sensory Receptors

• Sensory receptors are specialized to respond to changes in their environment and these changes are called stimuli
• Activation of sensory receptors causes the production of graded potentials that trigger nerve impulses to the central nervous system (CNS)

Receptor Classification by Stimulus Type

• Mechanoreceptors are stimulated by mechanical forces like touch, pressure, vibration, and stretch
• Thermoreceptors respond to changes in temperature
• Photoreceptors detect light
• Chemoreceptors are stimulated by chemicals, such as odorants, taste stimuli, or chemical components of body fluids
• Nociceptors respond to painful stimuli and can stimulate some types of thermoreceptors, mechanoreceptors, or chemoreceptors

Receptor Classification by Location

• Exteroceptors are located at or near the body surface and detect stimuli arising from outside the body, such as touch, pressure, pain, skin temperature, and special senses
• Interoceptors are associated with internal organs and vessels and monitor chemical changes, stretch, or temperature
• Proprioceptors are found within skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles and relay information regarding body movements

Receptor Classification by Structural Complexity

• Simple receptors are general senses with nonencapsulated or encapsulated dendritic endings
• Nonencapsulated dendritic endings are free nerve endings that detect temperature, pain, itch, and light touch, and are located at the base of hair follicles
• Encapsulated dendritic endings consist of a dendrite enclosed in a connective tissue capsule and detect discriminatory touch, initial, continuous, and deep pressure, and stretch of muscles, tendons, and joint capsules

Sensory Integration: Sensation to Perception

• The somatosensory system serves the body wall and limbs with input from exteroreceptors, proprioreceptors, and interoreceptors
• Neural integration in the somatosensory system occurs at the receptor, circuit, and perceptual levels

Neural Integration Levels

• Processing at the receptor level requires a stimulus to excite a receptor within its receptive field, causing generation of graded potentials
• If a receptor is part of a sensory neuron, generator potentials produced can cause action potentials on the sensory neuron
• If a receptor is a separate structure from the sensory neuron, receptor potentials produced may cause generator potentials on the sensory neuron
• Adaptation occurs when a constant stimulus reduces receptor sensitivity over time
• Processing at the circuit level involves delivery of impulses via first-, second-, and third-order neurons to the appropriate cerebral cortex region for stimulus localization and perception

Perceptual Level Processing

• Perceptual detection sums input from receptors and is the simplest perception level
• Magnitude estimation is the ability to detect stimulus intensity through frequency coding
• Spatial discrimination identifies the stimulation site or pattern through spatial discrimination
• Quality discrimination differentiates specific qualities of a particular sensation
• Pattern recognition is the ability to recognize a pattern in a complete scene
• Pain perception protects the body from damage and is stimulated by extremes of pressure and temperature, as well as chemicals released from damaged tissues
• The pain threshold is the stimulus intensity to perceive pain and is generally the same for people, however pain tolerance is genetically determined and varies

Pain Types

• Visceral pain results from stimulation of receptors within internal organs from stimuli like extreme stretch, ischemia, chemical irritation, and muscle spasms
• Visceral pain travels along the same fiber tracts as somatic pain impulses, giving rise to referred pain in an area different from the affected area

Reflex Arc

• Reflexes are unlearned, rapid, predictable motor responses to a stimulus that occur over neural pathways called reflex arcs
• Inborn or intrinsic reflexes are unlearned, unpremeditated, and involuntary
• Learned or acquired reflexes result from practice or repetition
• A reflex arc has five components: receptor, sensory neuron, integration center, motor neuron, and effector
• Reflexes are functionally classified as somatic, which activate skeletal muscle, or autonomic, which activate visceral effectors