Pathways Flashcards
PLR pathway
Light -> retina (area centralis) -> optic nerve -> optic chiasm -> optic tract -> pretecal nucleus (synapse) -> caudal commissure -> mostly contralateral parasympathetic CN III nucleus (midbrain) (in humans called nucleus of Edinger-Westphal) (synapse on GVE LMN) -> oculomotor nerve -> orbital fissure -> ciliary ganglion (synapse with ganglionic neuron cell bodies) -> short ciliary nerves -> ciliary muscle and sphincter of the pupil
Which animals have a normally slightly weaker PLR?
Excited animals
Farm animals, horses (also weaker indirect PLR due to more fibers crossing in optic chiasm)
Sympathetic innervation of the eye
Preganglionic cell bodies in the lateral grey horn of the T1-3 (4) SCS -> ventral grey column -> ventral roots -> segmental spinal nerve -> ramus communicans -> thoracic sympathetic trunk -> (passing thru cervicothoracic and middle cervical ganglia) -> cervical sympathetic trunk (vagosympathetic trunc located in the carotid sheet) -> cranial cervical ganglion (ventromedial to the bulla) synapse with ganglionic cell bodies -> follow the blood vessels to reach effector organs (pathway not clearly defined) -> orbital muscles of the periorbita, tarsal muscle of the upper, lower and third eyelid, m. dilator pupillae
(also smooth muscles of the blood vessels, sweat glands of the skin in the head/neck area)
General Proprioceptive Pathways for Reflex Activity and Cerebellar Transmission?
From trunk and pelvic limbs (caudal to T1):
–> GP axon -> dorsal grey horn -> 1a axons synapse directly on the GSE alpha motor neuron in the ventral grey horn to complete the reflex arc
Dorsal Spinocerebellar Tract
GP axon to ispilateral dorsal grey horn -> synapse there (nucleus dorsalis/nucleus of the dorsal spinocerebellar tract ->ispilateral dorsal spinocerebellar tract in the lateral funiculus -> caudal cerebellar peduncle -> cerebellar cortex (vermal and paravermal lobules)
Ventral spinocerebellar tract
GP axon to ispilateral dorsal grey horn -> synapse there -> axons cross to contralateral ventral spinocerebellar tract of the lateral funiculus -> caudal cerebellar peduncle -> contralateral vermal and paravermal lobules (so same side from which the original GP axon started)
From thoracic libs and cervical region:
–> GP axon -> dorsal grey horn -> 1a axons synapse directly on the GSE alpha motor neuron in the ventral grey horn to complete the reflex arc
Cuneocerebellar Tract
GP axons to lateral part of dorsal funiculus without synapse (and without entering the dorsal grey horn) -> fasciculus cuneatus -> lateral cunate nucleus (synaopse) in the caudal medulla -> caudal cerebellar peduncle to cerebellum
Cranial (Rostral) Spinocerebellar Tract
GP axons thru dorsal root to ispilateral dorsal grey horn -> synapse in the centrobasilar nucleus -> ispilateral cranial (rostral) spinocerebellar tract in lateral funiculus –> rostral and caudal cerebellar peduncle to the cerebellum
Cervicospinocerebellar Pathway
GP axon thru dorsal root to ispilateral intermediate grey column (C1-4) -> synapse in the central cervical nucleus -> contralateral cervicospinocerebellar pathway in the lateral funiculus -> caudal cerebella peduncle to the cerebellum
Cervicospinovestibular Pathway
–> GP axon thru dorsal root to ispilateral dorsal grey horn –> ispilateral cervicospinovestibular tract of the ventral funiculus -> caudal vestibular nucleus
General Proprioceptive Pathways to the Somesthetic Cortex for Conscious Perception
Nucleus gracilis and cuneatus
GP axons -> dorsal roots -> ispilateral dorsal grey column -> ipsilateral dorsal funiculus (fasciculus gracilis caudal to T6 and fasciculus cuneatus cratnial to T6) -> to nucleus gracilis and medial cuneate nucleus in the caudal medulla (synapse there) –> deep arcuate fibers -> decussation -> medial lemniscus -> ventral caudal lateral nucleus of thalamus (synapse) -> thalamocortical fibers in the internal capsule -> somesthetic cortex (parietal lobe, caudal to the cruciate sulcus) (contralateral to the start of the GP axons)
Lateral cervical nucleus
GP axons -> dorsal roots -> ispilateral dorsal grey column -> nucleus proprius (synapse) -> ipsilateral lateral funiculus -> ispilateral lateral cervical nucleus (C1-2) -> decussation -> medial lemniscus -> ventral lateral caudal nucleus of thalamus (synapse) –> internal capsule -> somesthetic cortex (parietal lobe, caudal to the cruciate sulcus) (contralateral to the start of the GP axons)
Nucleus Z
GP axons -> dorsal grey column -> ispilateral lateral funiculus caudal to cervical intumescence -> nucleus Z of caudal medulla (synapse) -> decussation -> ventral lateral caudal nucleus of thalamus (synapse) –> internal capsule -> somesthetic cortex (parietal lobe, caudal to the cruciate sulcus) (contralateral to the start of the GP axons)
Trigeminal proprioceptive pathways
GP receptors in muscles of mastication, facial and extraocular muscles and TMJ -> thru 3 branches of CN V -> trigeminal ganglion (pars petrosa ossis temporalis) without synapse -> mesencephalic nucleus of the CN V (synapse)
From there for
1) reflex activity -> from mesencephalic nucleus of CNV to adjacent GSE nuclei of cranial nerves to synapse on alpha motor neurons
2) concious pathway -> from mesencephalic nucleus of CNV to pontine sensory nucleus of CN V (synapse) -> contralateral trigeminal lemniscus (quintothalamic tract) -> medial meniscus -> ventral caudal medial nucleus of thalamus (synapse) –> internal capsule –> centrum semiovale –> corona radiata –> somesthetic cortex.
GSA pathway
dendritic zones (free endings) on the surface of the body -> axons (thru named nerves) -> spinal nerves -> dorsal roots -> dorsolateral sulcus to ipsilateral dorsolateral fasciculus (Lissauer tract) to substantia gelatinosa of the apical dorsal grey column - synapse on interneurons there -> deeper into the ipsilateral grey column (synapse again) on either (1) interneurons for reflex activity or (2) interneurons projecting to brainstem for concious percepcion of stimulus
(1) interneuron from grey horn synapse on alpha motor neuron in the ventral grey horn (some of the interneurons’ axons enter the fasciculus proprius and move a few segments cranially and caudally to synapse on alpha motor neurons of these segments (because the withdrawal reflex needs activation of a large number of alpha motor neurons) -> ventral root -> spinal nerve -> ventral branch -> named nerve -> muscle (withdrawal)
(2)
Pathway of the thoracic limb flexor reflex
Noxa to digit > nociceptors of radial nerve (dorsally) and median/ulnar nerves palmary > axons of nociceptors thru named nerves > dorsal roots C7, C8, T1 SCS (radial nerve) or C8, T1, T2 SCS (median and ulnar nerves) > branches of axons course cranially and caudally for a few segments in the dorsolateral fasciculus > synapse on interneurons in dorsal grey horn of cervical intumescence > synapse on GSE LMN in the ventral grey horn of these segments that innervate the major flexors of the shoulder (ventral roots and spinal nerve ventral branches of C7,C8, T1 radial and axillary nerves), lexors of the elbow (ventral roots and spinal nerve ventral branches of C6, C7, C8, and the musculocutaneous nerve), and flexors of the carpus and digits (ventral roots and spinal nerve ventral branches of C8, T1, T2, and the median and ulnar nerves).
Pathway of the pelvic limb withdrawal reflex?
Noxa to digits > nociceptors of sciatic nerve digits 3-5 (dorsal for fibular, plantar for tibialis) or saphenous nerve (branch of femoral nerve) digits 1 and 2 > axons thru named nerves > dorsal roots of L4, L5, L6 for fingers 1 or 2, or L6, L7, S1 for fingers 3-5 > branches of axons course cranially and caudally for a few segments in the dorsolateral fasciculus > thru dorsal grey horn to synapse on interneurons > interneurons synapse on GSE LMNs of the lumbar intumescence to innervate flexors of the hip (the ventral roots, the spinal nerve ventral branches of L1 to L6, and the femoral nerve), the flexors of the stifle (ventral roots and spinal nerve ventral branches of L6, L7, S1, and the sciatic nerve), the flexors of the tarsus (ventral roots and spinal nerve ventral branches of L6, L7, and the fibular nerve), and the flexors of the digits (ventral roots and spinal nerve ventral branches of L7, S1, and the tibial nerve).
Nociceptive pathway
Stimulus (mechanical, thermal, chemical) > nociceptors > A-delta or C axons (small unmielynated or lightly myelinated) > named nerve > dorsal root > synapse dorsal grey horn (Rexed laminae I throughout VI) > axons cross > thru various tracts (in primates mostly spinothalamic, but domestic species various tracts) > inside the brainstem various synapses with different nuclei of reticular formation to function in the ascending reticular activating system > thalamus synapse (ventral caudal lateral nucleus) > thalamocortical projection fibers > internal capsule > centrum semiovale > corona radiata > somesthetic cortex
Name some important tracts in the nociceptive pathway
- Spinothalamic (primates)
- Dorsal column postsynaptic pathway
- Spinocervicothalamic pathway
- Spinomesencephalic pathway
M.cut.trunci reflex pathway
Stimulus on skin lateral to the dorsal midline > dorsal branches of spinal nerves (1-2 vertebrae cranial to stimulus) > synapse dorsal grey horn interneurons > axons enter the fasciculus proprius bilaterally (mostly contralateral) > C8-T1 synapse on ventral grey horn on GSE neurons > lateral thoracic nerve > cutaneous trunci muscles
Laryngeal adduction (slap test)
stimulus (brisk slap to the saddle area of thorax) > cutanous (dorsal) branches of the cranial thoracic spinal nerves > ipsilateral dorsal grey horn (synapse) > crossing (most fibres) > fasciculus proprius > medulla (synapse on GSE-LMN neurons of nucleus ambiguous > n. vagus > n. laryngeus reccurens > intrinsic muscles of the larynx (close the glottis)
