TRIGEMINAL SYSTEM

E. Trigeminal Somatosensory Pathways

1. Peripheral branch receptive fields (==>)

a. ophthalmic (receptive field V1)
b. maxillary (receptive field V2)
c. mandibular (receptive field V3)

2. Primary sensory cell bodies

a. trigeminal ganglion; also called Gasserian ganglion and semilunar ganglion

1) contains the cell bodies for most of the sensory V fibers

b. mesencephalic nucleus

1) contains cell bodies, not synapses (only site in the nervous system where cell bodies of primary sensory neurons are located within the CNS)
2) contains mainly fibers serving muscle proprioceptors

3. Central synapse

a. large afferents (tactile): 1st order axons branch on entry into the CNS, synapse in main (chief) sensory nucleus of V and in the upper third of the spinal nucleus of V

b. small afferents (temperature, pain): 1st order axons synapse mainly in caudalis of the spinal nucleus of V, somatotopic organization

c. muscle proprioceptors (mesencephalic: fibers branch)

1) some branches go to motor nucleus of V, where they are involved in jaw reflexes
2) some branches go to the main sensory nucleus of V

4. Second order neurons

a.    2nd order fibers from tactile and some proprioceptive neurons originate in the main sensory nucleus

1) most fibers decussate, then join the medial lemniscus and travel to the thalamus

Note: some authors consider the trigeminothalamic tract as separate, it which case it is called the trigeminal lemniscus or the trigeminothalamic tract

b. spinal nucleus fibers

1) most fibers decussate and travel with the anterolateral system, terminating in the specific projection and intralaminar nuclei of the thalamus

2) ascending fibers have collateral branches to the ascending reticular formation

3) also contribute to local reflexes, e.g. corneal reflex (corneal stimulation, activation of small c.n. V fibers, synapse in spinal n. of V, postsynaptic fibers to motor nucleus of VII, contraction of orbicularis oculi muscle

Trigeminal System:  Overall Pathway ==>  (Note also small contribution from C.N. VII, IX, X)