Neuro Chapters 7-11

first order neurons

initial sensory neurons that detect stimuli

dorsal root ganglia (body) or cranial nerve ganglia (head/face)

second order neurons

receive input from first order neurons → process and relay sensory information upward towards the brain

call bodies are located in the spinal cord (dorsal horn) or brainstem nuclei

axons cross (decussate) to the opposite side and ascend to the thalamus

third order neurons

receive input from second order neurons

• thalamic neurons project to somatosensory cortex

posterior column - medial lemniscus pathway

• proprioception, vibration sense, fine and discriminative touch

anterolateral (front) pathways

1. spinothalamic tract

2. spinoreticular tract

3. spinomesencephalic tract

• pain, temperature sense, crude touch

spinothalamic tract

• discriminative aspects of pain and temperature

• location and intensity of the stimulus

spinoreticular tract

• emotional and arousal aspects of pain

spinomesencephalic tract

• central modulation of pain

primary somatosensory cortex

location: postcentral gyrus

organization: somatotopic

secondary somatosensory cortex

location: sylvan sulcus

organization: somatotopic

association cortex

location: superior parietal lobe

organization: more limited and modified somatotopic

A-delta pain fibers characteristics

• myelinated

• larger

• faster

• fast/sharp/acute pain

• well localized, “first” pain

C pain fibers characteristics

• unmyelinated

• smaller

• slower

• slow/burning/dull pain

• poorly localized, “second” pain

gate control theory of pain

sensory inputs from large, non pain A-delta fibers reduce pain transmission through the dorsal horn

• basis for TENS therapy

pain pathway

PAG → RVM → dorsal horn

other central modulators of pain

• histamine via H3 receptors

• opioids (widespread throughout nervous system)

• endorphin via hypothalamus neurons that project to PAG

• enkephalin & dynorphin

enkephalins

act primarily on mu receptors

dynorphin

acts primarily on k (kappa) receptors

thalamus

relay center for nearly all sensory and motor pathways that project to the cortex

intralaminar nuclei

• receives inputs with reciprocal connection to cortex

• help maintain cortical arousal states

• project to the basal ganglia

reticular nucleus

• almost pure inhibition of inhibitory GABAergic neurons

• modulate thalamic activity and filter information flow to the cortex

• regulates sleep/wake cycles, attention

• the only thalamic nuclei that doesn’t project to cortex

paresthesia

abnormal positive sensory phenomena