Topic 103 - Sensory mechanisms in the central nervous system; physiology of pain

Question 1

Words to include in sensory nervous system

Answer 1

• Sensory nervous system
• Afferent neurons
• Sensory system
  
  • Sensory neuron
    
    • Sensory receptor cell
  • Neural pathway
  • Brain involved in sensory perception
• Sensing stimuli
  
  • Stimulus reception
  • Spinal cord
  • Ascending pathway
  • Signal modlulation
  • Perception in cortex
• Stimulus
• Receptor organ / cell
• Exteroceptors
  
  • External environment
• Interoceptors
  
  • Internal environment
• Afferent nerve cell
• Pathway
  
  • Spinal cord
  • Brainstem
  • Hypothalamus
  • Thalamus
  • Cortex

Question 2

Words to include in physiology of pain

Answer 2

• Pain sensation
• Nociceptive free nerve endings
• Action potential
• A delta fibers
  
  • Fast-conduction
  • Well localizable pain
  • Flexor reflex
  • Immediate defense
• C fibers
  
  • Unmyelinated
  • Slowly initiated
  • Long lasting
  • Hardly localizable pain
  • Diffuse multisynaptic connection
  • Neurons of the thalamus
• Viscero-somatic pain reflection
• Cortical effects
• Tactile stimulation
• Nociceptive impulses
• Gate control theory
• Primary axons
• Noxious stimuli
  
  • Histamine
  • Bradykinin
  • Prostaglandin
  • Nitric oxide
  • Serotonin
• Basophil granulocytes
• Degranulation
• Antidromic
• CTGRP (calcitonin gene related peptide)
• Neurokrinin A
• Descending pain depression
  
  • Descending opiate mediate pathway
  • Primary afferent pain sensing neuron
  • Synapses
  • Dorsal horn
  • Projection neuron
  • CNS
  • Periaqueductal grey (PAG)
  • Medulla
  • Opiate neurons
  • Projection neurons
  • Transmitters
  • Primary afferents
  • Thalamus

Question 3

Sensory mechanisms in the CNS

Sensory nervous system, summary of function

Answer 3

• All stimuli received from the external or internal environment is carried along one or more afferent neurons towards a specific area of the brain
• A sensory system consists of:
  
  • Sensory neurons (including sensory receptor cells)
  • Neural pathways
  • Parts of the brain involved in sensory perception

Question 4

Sensory mechanisms in the CNS

Steps of sensing stimuli

Answer 4

1. Stimulus originating from the environment is registrated by a receptor organ or cell
   
   • ​​Exteroceptors senses stimuli from the external environment
   • Interoceptors sesnes stimuli from the internal environment
2. An afferent nerve cell transmits the signal generated by the receptor towards the spinal cord
3. Signal reaches the spinal cord where it is transmitted towards the brain through the ascending pathways

Question 5

Physiology of pain

Pain reception

Answer 5

• Pain reception is based on the receptor function of nociceptive free nerve endings
• Dermatomes
• They transmits action potential to the center in two phases:
  
  • A delta fibers
    
    • ​Fast conduction
    • Well localizable pain
    • Mainly elicit flexor reflexes
    • Immediate defense
    • Cause drop of blood pressure, sweating and nausea
  • C fibers
    
    • ​Unmyelinated
    • Slowly initiated
    • Long lasting
    • Hardly localizable pain
    • Diffuse multisynaptic connection in emotional states
    • Referred pain
    • Neurons of thalamus and other brain areas
• Tactile stimulation around painful wound decreases the pain
  
  • Because nociceptive impulses are depressed by the “gating” (gate control theory)
• Pain sensory axons are stimulated in several ways:
  
  1. Noxious stimuli releases materials for tissue damage:
     
     • Potassium
     • Histamine
     • Bradykinin
     • Prostaglandin
     • Nitric oxide
  2. Basophil granulocytes amplify the effect by degranulation
  3. Antidromic, recurrent self stimulation by releasing pain. SP multiplies the dischare of its own axon, causing edema → increases tissue tension → furter pain

Question 6

Physiology of pain

Descending pain depression

Answer 6

• Descending opiate mediate pathway
• Role: control and decrease pain

1. Primary afferent pain sensing neuron synapses at the dorsal horn onto projection neurons
2. CNS can depress this stimulation:
   
   • Periaqueductal grey (PAG), medulla opiate and GABAergic pathways stimulate descending noradrenergic neurons
   • Descending noradrenergic neurons increase the activity of opitae neurons in the dorsal hornthat inhibit the projection neurons and decreases the discharge of transmitters from the primary afferents
   • Neurons fires less action potential towards thalamus

Question 7

Physiology of pain

Signal modulating in the thalamus

Answer 7

• Most important nucleus of thalamus: VPL (Ventral Postero-Lateral)
  
  • ​Spinothalamic and spinocervical tracts arrive here
• Large relay nuclear group of thalamus: VPM (Ventral Postero-Medial)
• Nuclei through the not synapsing afferents run: Bypass nuclei

1. VPL
2. VPM
3. Bypass nuclei

Question 8

Physiology of pain

Non-specific, bypass system

Answer 8

1. Thalamus
2. Medial nuclear group
3. Central mediation
4. Anterior nuclear group

Question 9

Physiology of pain

Cortical sensory functions

Answer 9

• From the thalamus two types of afferentations reaches the cortical areas:
  
  • ​Somatotropically fibers of the specific relay nuclei (mainly VPL and VPM).
    
    • Area of projection: Postcentral gyrus
  • Fibers from the reticular formation (and hypothalamus, basal ganglia, cerebellum) via bypass
    
    • ​Area of projection: To the frontal lobe
• Cortical representation shows exact somatotopy
  
  • ​Basic unit of cortical represnetation: Column of grey matter

Question 10

Physiology of pain

Brodmann areas

Answer 10

Brodmann area: region of the cerebral cortex in primates, defined by its structure and organization of cells

Question 11

Physiology of pain

Cortical somatotopy

Answer 11

• Cortical somatotopy: cortical representation of sensory information affiving from certain body areas in the primary sensory area is proportional to the relative importance of the given area in sensation
• Mainly of the formation of tactile, mechanical and thermal sensations
• Disproportional representation can be observed
  
  • Proportionally to its role in the sensory function
• Secondary sensory area is more fixed