Somatosensory Tracts I Flashcards
What stimuli activate the PCML pathway?
Discriminative touch, vibration, proprioception.
Small receptive fields have ________ receptor density.
High receptor density
Large receptive fields have __________ receptor density
Low receptor density
3 components of a primary afferent fiber
Pseudopolar cell body
Peripheral process extending from the DRG.
Central process extending from DRG to the CNS.
What structure do primary efferent fibers enter the spinal cord in?
Medial division of posterior root
What do the fibers ascend within?
Fasciculus gracilis or fasciculus cuneatus (posterior columns).
Fibers above T6 travel in:
Fasciculus cuneatus
Fibers below T6 travel in:
Fasciculus gracilis
What is the result of Sc lesions of the posterior columns?
Ipsilateral loss of tactile sensations (discriminative, positional, vibratory) AT and BELOW segmental level of injury.
Where are the neurons from that synapse in the nucleus gracilus and nucleus cuneatus?
They are 1st order neurons (primary afferents) from ipsilateral DRG (have not decussated yet).
Where do the 2nd order neurons from the nuclei go?
They decussate (via arcuate fibers) and ascend in the medial leminiscus (ML) on the contralateral side.
What happens to the medial leminscus (ML) in the pons?
It laterally rotates, where the UE fibers lie medial and LE lie laterally.
Where does the ML terminate?
VPL in the thalamus
Damage at brainstem levels lead to:
Deficits in tactile sensations (discriminative, positional, vibratory) over the contralateral side of the body (because the fibers have decussated).
Blood source to the VPL and VPM:
What happens when it is compromised?
Thalamogeniculate branches from posterior cerebral a.
Same deficits as damage to brainstem (sensory loss on CL side).
2 types of neurons in the VPL
- 3rd order neurons from PCMLS which will travel to somatosensory cortex.
- Local circuit interneurons (inhibitory neurons that influence the firing rate of 3rd order neurons).
Trigeminal nuclei: Main sensory nucleus (msT) Trigeminal motor nucleus (mT) Spinal nucleus Mesencephalis nucleus (mes)
Trigeminal nuclei:
Main sensory nucleus (msT) - touch and pressure.
Trigeminal motor nucleus (mT) - ms. of mastication.
Spinal nucleus - pain and temp.
Mesencephalis nucleus (mes) - proprioception from TMJ and ms. of mastication.
Dorsomedial division of the main sensory nucleus of Trigeminal nucleus function:
Where do they project?
Via which tract?
Afferent (sensory) input from oral cavity.
Project ipsilaterally.
Posterior trigeminothalamic tract.
Ventrolateral division of the main sensory nucleus of Trigeminal nucleus function:
Afferents (sensory) from V1, V2, V3.
Projects contralaterally.
Anterior trigeminothalamic tract.
Explain how sensory info from the face gets processed via trigeminal nuclei
- Sensory fibers (dorso/ventrolateral fibers) synapse in the VPM in the thalamus.
- 3rd order axons from VPM project via the posterior limb of the internal capsule to the primary somatosensory cortex.
Function of mesencephalic nucleus
Conveys unconscious proprioceptive and pressure info from muscles of the oral region.
Jaw-jerk reflex (3)
- Strecth of masseter (from downward tap on chin), causes contraction BL.
- Afferent limb is mesencephalic trigeminal neuron that innervates muscle.
- Efferent limb is from central process that synapses on trigeminal motor neuron.
Mesencephalic nucleus via spinal nucleus (Pars interpolaris) conveys:
Proprioceptive input from jaw related to chewing/jaw placement.
Function of main sensory nucleus
Moderates amount of secondary neurons that project to the cerebellum.
Chewing utilizes which 2 nuclei?
Mesencephalic nucleus via spinal nucleus (Pars interpolaris)
Main sensory nucleus
