– a.k.a. the Ventrolateral or Spinothalamic System
Sensory
: Mainly carries pain and temperature info (Lateral Spinothalamic Tract); also carries some collateral fibers transmitting crude touch and pressure info (Anterior Spinothalamic Tract)
Components
(1) First Order Neuron (primary neuron)
– transmits information from the periphery to the dorsal horn
primary neurons entering the dorsal aspect of the spinal cord (SC) enter the Tract of Lissauer and branch into ascending and descending collaterals that travel either up or down the SC one or two segments
axon of primary neuron then enters the dorsal horn and branches into axons that synapse with neurons located in several of the Rexed Laminae(divisions of spinal gray matter ranging from lamina I to X in a dorsal to ventral orientation)
majority of the branches synapse with neurons in lamina I (marginal zone) and lamina II (substantia gelatinosa)
neurons in lamina II communicate with 2nd-order neurons in laminae III-IV (collectively called nucleus proprius)
neurons within the nucleus proprius also receive direct innervation from branches from the first order neuron
(2) Second Order Neuron (2nd Neuron)
– form Anterolateral Tract (Spinothalamic Tract)
decussates
at a slight rostral inclination; fibers ascend in the anterolateral quadrant of the contralateral half of the spinal cord
fibers join axons from the 2nd-order neurons from lamina I to form the spinothalamic tract
2nd-order neurons travel to the ventral posterior lateral (VPL) nucleus of the thalamus; some fibers branch to form Spinoreticular Fibers and travel to the reticular formation
(3) Third Order Neurons
– travel through internal capsule to become corona radiata and terminate on axons within cortex.
Organization within cortex (homunculus) Þ (medial to lateral: genitalsÞ legÞ trunkÞ armÞ face)
Anterior and lateral spinothalamic tracts and medial lemniscus converge at ventrolateral nuclei of thalamus
Spinal-Tectal System
– tectum (midbrain) coordinates sensory information
transmits information from the superior coliculus via the medial longitudinal fasciculus (MLF) to lamina I and II
is involved in coordination of head and eye movement in response to a stimulus
Trigeminal Nucleus and Corresponding Sensory Pathways
Sensory pathways
from the face involve CN V, VII, IX, X; CN V has the major role
trigeminal system is ultimately responsible for transmission of tactile, proprioceptive, and pain and temperature information from the head to the cerebral cortex, cerebellum and reticular formation
Trigeminal Nucleus
– large nucleus that runs from rostral midbrain to rostral spinal cord; divided into three regions:
(1)
Chief (Principal) Sensory Nucleus (Interpolar Nucleus) – located in midpons near trigeminal motor nucleus
primarily concerned with discriminative tactile and proprioceptive sensations
receives large diameter, heavily myelinated tactile afferents and gives rise to two ascending pathways
(1) Contralateral projection crosses midline, and becomes ventrotrigeminothalamic tract, terminating in VPM
(2) Ipsilateral projection from dorsomedial portion of the nucleus forms Dorsal Trigeminal (Dorsal Trigeminothalamic) Tract Þ terminates in VPM
(2) Spinal Nucleus (Caudal Nucleus) – located in medulla and extends caudally to third segment of the spinal cord
mostly pain and temperature
but also tactile sensation
fibers decussates and ascends in the Ventral Trigeminal (or Ventral Trigeminothalamic Tract)
fibers are thought to travel with medial lemniscus, so it is also referred to as the Trigeminal Lemniscal Tract
fibers terminate in the ventral posteromedial (VPM) nucleus of the thalamus
some fibers branch to the reticular formation to relay information before entering the thalamus (similar to spinoreticular fibers of spinothalamic tract). May also give branches to Facial Nucleus and Trigeminal Motor Nucleus.
(3)
Mesencephalic Nucleus(Oral Nucleus) – located in rostral pons and extends rostrally – USED FOR REFLEX
afferents from muscles of mastication and mechanoreceptors of the gums, teeth and hard palate
extends rostrally all the way to the posterior commissure
coordination of chewing movements
jaw jerk reflex:
stretching the masseter causes it to contract in a reflex fashion, monosynaptic reflex; prevents people from breaking teeth
REFLEX ARC: Sensory V3Þ Mesoncephalic NucleusÞ Trigeminomotor NucleusÞ motor branch of V3