Sensory Perception and Receptor Physiology

• Conscious interpretation of sensory afference defines perception
• Sensory afference is categorized into somatic, relating to the body surface, and special, encompassing vision, hearing, taste, smell and equilibrium

Receptor Physiology

• Neuronal dendrites or modified epithelium respond to specific, adequate stimuli
• "Respond" signifies a graded potential occurring very close to an adequate stimulus
• An adequate stimulus is the form of energy change that most likely creates an action potential in a receptor

Receptor Types

• Photoreceptors respond to light, exemplified by rods and cones
• Mechanoreceptors are responsive to pressure or membrane death formation
• Thermoreceptors respond to temperature changes
• Osmoreceptors detect solute concentrations in extracellular fluid
• Nociceptors are responsible for responding to pain
• The mechanism of action involves opening Na+ channels, leading to depolarization and subsequently an action potential
• The number of activated receptors indicates the strength of a stimulus, which is encoded in the frequency of action potentials

Adaptation

• Adaptation is the decrease in receptor potential magnitude with sustained stimulus
• Tonic receptors are slow to adapt
• Tonic receptors show a decrease in receptor potential
• Phasic receptors adapt rapidly, with their receptor potential stopping
• The receptor signals changes in stimulus intensity rather than relaying static information

Acuity

• Acuity discerns the site of a stimulus
• Acuity is directly related to the receptive field
• The receptive field is the area over which a sensor responds
• Greater receptor density results in greater acuity

Referred Pain

• Referred pain utilizes shared ascending pathways
• The brain interprets the ‘historical' most likely receptor field

The Chemical Senses: Taste

• Taste is sensed by receptor cells, which are modified epithelium, that underlie neurons
• Receptor sites bind specific chemicals in solution
• Binding of tastant produces a receptor potential
• The strength of the receptor potential is proportional to the amount of tastant
• Receptor cells release neurotransmitters onto underlying neurons
• The ascending path involves the brainstem, thalamus, and cortical gustatory center
• There are 5 primary taste perceptions: salt, sour, sweet, bitter, and umami

Taste Receptor Types

• Salt receptors detect salt
• Sour receptors detect acid with a pH less than 7
• Sweet receptors detect glucose
• Bitter receptors detect base with a pH greater than 7
• Umami receptors detect savory amino acids like glutamate

Olfaction (Smell)

• Olfaction involves binding to 1000 odorants, which are water-soluble
• Receptor potential results in an action potential, with frequency dependent upon stimulus strength
• Glomeruli are located in the olfactory bulb
• Receptor cells synapse with 2nd order neurons
• The pathway extends into the limbic system and olfactory cortex
• Adaptation occurs in olfaction at the level of the cerebral cortex

Pain

• Pain perception belongs to somatic sensory for body awareness
• Somatic sensory aspects include body surface, proprioception, pressure, and temperature
• Sensors or receptors are termed "pain receptors" or Nociceptors, which are neurons that are either "naked" or free-ending

Types of Nociceptors

• Mechanical nociceptors detect mechanical or pressure changes that can produce tissue damage
• Thermal nociceptors detect temperature changes associated with tissue damage
• Polymodal nociceptors are most responsive to chemicals released with tissue damage
• Polymodal nociceptors also responsive to heat & pressure

Tissue Damage

• Tissue damage causes cellular production of hyperalgesics
• Hyperalgesics arise from the metabolism of cell membranes and include classes like bradykinins & prostaglandins
• Mechanical and thermal Nociceptors generate action potentials via small, myelinated axons and fast pain pathways
• Polymodal nociceptors involve unmyelinated axons and slow pain pathways

Ascending Pathways

• Primary afferent neurons or nociceptors synapse with interneurons in the dorsal horn
• Interneurons synapse with ascending neurons that project to the reticular formation in the medulla, the periaqueductal gray matter in the midbrain, and the thalamus, limbic system, and somatosensory cortex

Descending Pathways

• Neurons within the periaqueductal gray matter are activated by stress
• Descending neurons to the spinal cord synapse on interneurons and stimulate the release of enkephalins
• There is a descending projection to NMR and LC in the medulla
• Descending neurons from the medulla project to the spinal cord

Chronic Pain

• Chronic pain involves the release of Substance P by nociceptors
• Enkephalins cause presynaptic inhibition of transmitter release from afferents in pain modulation
• Enkephalins also causes postsynaptic hyperpolarization of interneurons
• The 3 families of endogenous opiate peptides are enkephalins, endorphins, and dynorphins

Peptide Neuromodulators

• Peptide neuromodulators utilize at least 3 types of membrane-associated receptors
• Activation of receptors leads to an increase in K+ permeability and a decrease in Ca²+ permeability via voltage-gated channels
• Opioid drugs are a type of narcotic analgesic as most bind to multiple receptors

Opioid Drug Effects

• Cardiovascular effects leads to coronary vasodilation which reduces O2 demand on the heart
• Gastrointestinal effects leads to constipation by increase smooth muscle tone
• Examples of strong opioid drugs include fentanyl, methadone, and morphine
• Examples of moderate opioid drugs include codeine, hydrocodone and Dextromethorphan

Pain Management

• One way to manage pain is to block hyperalgesics formation with aspirin
• Prevent sensory afferents from generating AP with local anesthetics treatments

Nociceptors (Recap)

• Nociceptors release substance P onto interneurons within the spinal gray
• Interneurons are part of the ascending pathway
• Interneurons project to the limbic system and somatosensory cortex

Descending Pathway

• One way to reduce pain is to reduce the release of substance P from nociceptors
• Enkephalins modulate the influence from descending neurons and bind to specific receptors
• One way the body reduces pain is to reduce transmission of neural communication
• Opioids mimic endogenous opiates by binding to their receptors and are more effective in reducing the perception of pain than endogenous opiates

Key Point

• Receptor potential equals graded potential