Neuroanatomy & Neurophysiology Part 4

Neuroanatomy & Neurophysiology Part 4

• Three levels of sensory information processing are receptor, circuit, and perceptual level.

Receptor Level

• Stimulus energy must match receptor specificity.
• Stimulus must be applied within the receptive field of the receptor.
• Smaller receptive fields allow more precise localization.
• Transduction—conversion of stimulus energy into a graded potential (EPSP or IPSP).
• Graded potentials must reach threshold in first-order sensory neurons to generate a signal.

Circuit Level

• Processing occurs in ascending pathways
• Information travels through a chain of three neurons:
  • First-order neurons have cell bodies in dorsal root or cranial ganglia and carry information from the receptor to the CNS (spinal cord).
  • Second-order neurons synapse with first-order neurons and ascend toward the brain. Some branches lead to motor reflexes.
  • Third-order neurons have cell bodies in the thalamus and carry information to the correct sensory area of the cerebral cortex.

Processing at the Perceptual Level

• Goal is for information to reach the correct area of the cortex for conscious awareness and localization of the stimulus.

Adaptation

• Adaptation is reduction in sensitivity in the presence of a constant stimulus.
• Peripheral adaptation occurs at the receptor level, reducing the amount of information sent to the CNS.
• Central adaptation occurs in the neural pathways to the brain.
• Phasic receptors adapt quickly and provide information on the rate of change in the stimulus.
  • Examples of phasic receptors include lamellar and tactile corpuscles, reacting to the transition between darkness and bright light.
• Tonic receptors have sustained responses with little to no adaptation, providing continuous information about the presence and strength of stimuli.
• Examples of tonic receptors include nociceptors (pain) and proprioceptors.

Pain

• Pain is a helpful warning sign of tissue damage, motivating action.
• Extreme temperature and pressure are painful stimuli.
• Pain chemicals include histamine, K+, ATP, acids, bradykinin, etc.
• Sharp pain: carried by smallest, myelinated A delta fibers.
• Burning pain: carried by slower, nonmyelinated C fibers.
• Neurotransmitters involved in pain include glutamate and substance P.
• Pain information ascends primarily through the spinothalamic tract.
• Pain suppression is achieved through endogenous opioids.

Pain Tolerance

• Pain sensitivity varies and can be influenced by genetics, mental state, gender, and social isolation.

Pain Terminology

• Somatic pain: related to musculoskeletal structures.
• Visceral pain: related to organs.
• Referred pain: pain perceived as originating from a different location than its source.
• Phantom pain: perceived pain in a limb that has been amputated.

Special Senses: Chapter 15 of your Textbook

• This chapter is devoted to the special senses of taste, smell, vision, hearing, and equilibrium.

Taste

• About 10,000 taste buds are located on the tongue in papillae.
• Three types of papillae: fungiform, vallate, and foliate.
• Taste buds have gustatory hairs (receptors) for food chemicals.
• Receptors (gustatory hair cells) are activated, depolarize, release neurotransmitters.

Smell

• Olfactory epithelium in the nasal cavity contains olfactory sensory neurons with radiating olfactory cilia.
• Olfactory neurons receive and transmit signal of odorants to the brain.
• Olfactory neurons have specific thresholds and adapt quickly (to a certain level of stimulus)