Þ formic acid Þ C fiber system (pain) also called anterolateral system
1st order neuron (dorsal ganglion cell in PNS)
Þ enter spinal cord (carry pain and temp) Þ branch up and down the colaterals 2 segments (4 total)
most have quick synapse in layer one (marginal zone) or layer two (gelatinosa); both receive pain but now switch function
Þ can send a collateral up 10 segments (5 up, 5 down); can synapse with interneuron pool
can also send an axon down to layer 4 (proper nucleus) and layer 4 can do more stuff
Þ send an axon across (cross obliquely upward) the midline (ventral white commisure) Þ spino-thalamic tract Þ collateral to the reticular formation (give off a lot of branches) Þ 3rd order neuron is the thalamus (ventroposterolateral nucleus) Þ give branch to cortex via internal capsule Þ give a branch to the corona radiata Þ fibers then enter the 4th order neuron which is the cortex (post-central gyrus, ie layer 4)
Pathways
Dorsal column system (light touch, proprioception)
big toe Þ 1st neuron (dorsal root ganglion)
travels up gracilis tract to caudal medulla to nucleus gracilis (synapses with 2nd neuron in brainstem)
crosses over (via arcuate fibers) Þ 2nd order neuron’s tract is the medial lemniscus
next synapse is the thalamus (up medial lemniscus til is synapses the 3rd order neuron – ventroposteriolateral
projects medially Þ 4th order neuron is in the cortex
Spinal thalamic system (pain and temperature) – Anterolateral system
small cells Þ C and A delta (C is unmyelinated) Þ enters dorsal root Þ branches up and down the spinal cord
1st synapse is in the spinal cord (2nd neuron) Þ crosses the midline to enter the spinal thalamic or anterolateral tract
at level of caudal medulla and pons and midbrain til it hits the VPL of thalamus then synapses (3rd order neuron)
goes to same place as the dorsal column fiber
Lesions can distinguish the 2 apart since Anterolateral crosses over at a much earlier point than the dorsal column system
Both systems meet at the VPL of thalamus and goto the same 4th neuron even though they carry different information