Sensory Receptors and Pain Pathways

Sensory Physiology

• Sensory receptors are specialized cells that generate graded potentials, called receptor potentials, in response to a stimulus.
• There are five major divisions of sensory receptors: mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, and nociceptors.
• Each receptor type responds to a specific type of stimulus.
• Somatic sensation, arising from skin, muscles, bones, tendons, and joints, is facilitated by sensory receptors collectively known as somatic receptors.
• These receptors respond to touch and pressure, the sense of posture and movement, temperature, and pain.

Touch and Pressure Receptors

• Different receptors with varying adaptation speeds respond to touch and pressure:
  • Meissner's corpuscles (rapidly adapting) respond to light touch and pressure.
  • Merkel's corpuscles (slowly adapting) respond to sustained touch and pressure.
  • Free nerve endings (slowly adapting, some are nociceptors, thermoreceptors, or mechanoreceptors) respond to various stimuli.
  • Pacinian corpuscles (rapidly adapting) respond to vibration and deep pressure.
  • Ruffini corpuscles (slowly adapting) respond to skin stretch.

Pain Transmission

• Pain differs from other somatic sensations in its processing.
• After the initial noxious stimulus, a series of changes alters the pain pathway's response to subsequent stimuli.
• Hyperalgesia is increased sensitivity to pain that can persist after the initial stimulus.
• Pain is influenced by past experiences, suggestion, emotions (especially anxiety), and other activated sensory modalities.

Pain Management

• Analgesia is the selective suppression of pain without affecting consciousness or other sensations.
• Techniques for pain relief include electrical stimulation of specific areas in the central nervous system, pharmacological agents like NSAIDs and opioids (morphine), acupuncture stimulating endogenous opioids, transcutaneous electrical stimulation (TEMS), and massage.

Referred Pain

• The brain can misinterpret the origin of pain, causing pain to be perceived in a different location than the actual source, particularly from internal organs.

Vision

• The eye is composed of an optical component to focus the visual image on receptor cells and a neural component to process the visual image into graded and action potentials.

Light

• Light exists as a spectrum of electromagnetic waves with varying wavelengths and frequencies.
• Visible light is a small portion of this spectrum, perceived by humans in a range of wavelengths.

Anatomy of the Human Eye

• The eye includes structures such as the cornea, lens, iris, retina, and optic nerve. Their functions and positions are relevant for successful vision.

The Optics of Vision: Refraction

• Light bends (refracts) as it passes through different media, with the varying density of substances influencing refraction.
• The eye uses refraction to focus light onto the retina.

Accommodation

• The eye adjusts its focus between distant and near objects by changing the shape of the lens. This is influenced by ciliary muscles.

Nearsighted/Farsighted Vision

• Nearsightedness occurs when the eyeball is too long (light focuses in front of the retina), and farsightedness occurs when the eyeball is too short (light focuses behind the retina).
• Correcting vision issues is possible with lenses to compensate.

Photoreceptor Cells

• Photoreceptors in the eye are responsible for converting light stimuli into neural signals.
  • Rods and cones are the primary types found in the retina.

Neural Pathways for Vision

• Visual information travels from the eye through the optic nerve to the brain, where visual processing takes place.
• The visual cortex integrates information from both eyes.
• Vision has both binocular (using both eyes) and monocular (using one eye) components.

Color Vision

• Perceived colors depend on the wavelengths of light reflected, absorbed, or transmitted by objects.

• Color perception begins with activation of cone photopigments in the retina.

  • Specific cone types respond best to specific wavelengths (L, M, or S cones).

• Brightness affects perception of color.

• Rods only perceive light/dark.

Color Blindness

• Color blindness results from mutations in the genes encoding cone pigments, leading to a reduced ability to distinguish certain colors.
• Red-green color blindness is a common form, typically affecting men more often.

Hearing

• The sense of hearing involves physical and physiological processes related to sound waves propagating through a medium eventually causing neural sensory signals.
• Sound is perceived as vibrations in the medium's molecules, often air.

Anatomy of the Human Ear

• The different structures within the ear, including the external, middle, and inner ear are vital for sound transmission into sensory signals.

Sound Transmission in the Ear

• Sound waves hitting the tympanic membrane set in motion a chain of events involving the middle ear ossicles ultimately resulting in vibrations in the cochlea.

Cochlea and the Organ of Corti

• The cochlea contains the organ of Corti, where sound vibrations are mechanically transformed into electrical signals.
• The different structures within the cochlea play a crucial role.

Hair Cells of the Organ of Corti

• Hair cells in the organ of Corti are responsible for converting mechanical stimuli into electrical signals.
• Hair cell structures are relevant to this process.

Chemical Senses

• Chemicals binding to specific chemoreceptors are responsible for taste and smell perception.
• Taste buds contain specialized receptor cells and basal cells, with taste pores and taste hairs.
• Each taste has a specific signal transduction system.

Taste Receptors

• Taste, or gustation, is mediated through taste buds, composed of clusters of taste receptor cells containing taste hairs.
• Microscopic taste hairs increase surface area for chemical interaction.
• Different receptors respond to different tastants.
  • Sweet, sour, salty, bitter, and umami are the basic taste categories.

Signaling of Taste Receptors

• Each taste type has a distinct signaling mechanism (e.g. sodium ions for salt taste).
• Different transduction mechanisms exist (e.g. G-protein coupled receptors).

Olfactory Receptors

• Odorant molecules in the air stimulate the olfactory receptor neurons located in the olfactory epithelium.
• Cilia on the receptor neurons contain the receptors that bind to odor molecules.
• Olfactory receptor neurons are replaced frequently.

Smell

• Olfactory receptors activate G protein-mediated pathways leading to depolarization, triggering a neural signal.
• Proteins in the mucus facilitate interaction with and transport of odorant molecules.

Factors that Affect the Sense of Smell

• Olfactory ability is influenced by factors such as attention, hunger, gender, smoking, age, and nasal congestion/other health conditions.