21: Somatosensory Tracts: PCML, Trigeminal Thalamic Pathway Flashcards
Somatosensory function
Transmits and analyzes touch/tactile info from external and internal locations of the body and head
Four notable pathways of the somatosensory system
PCML, trigeminal thalamic pathway, spinocerebellar pathway, anterolateral system
Receptive field
Area of skin innervated by somatic afferent fibers
Small receptive field means…
Higher receptor density
Speed and precision of PCML pathway
Fast conduction velocities + precise somatotopic organization
What do afferent fibers of PCML detect?
Discriminative touch, flutter-vibration, proprioception
Lesion in SC of PCML
Ipsilateral loss of discriminative, positional, and vibratory tactile sensation at/below level of injury
Ventral posterior nuclei
Wedge-shaped cell group in caudal nucleus: VPL + VPM
Generally, what do the VPL and VPM nuclei receive?
VPL: info from body
VPM: info from face (trigeminal)
Blood supply to ventral posterior nuclei of thalamus
Thalamogeniculate branches of posterior cerebral A
Lesion of the ventral posterior nuclei of the thalamus
Lose all tactile sensation over contralateral body and head
Where is the primary somatosensory cortex (SI)?
Post-central gyrus + postparacentral sulcus
Two borders of the SI
Central sulcus + post-central sulcus
Brodmann’s areas in SI
1, 2, 3A, 3B
Blood supply to SI (2 arteries)
ACA + MCA
MCA lesion
Tactile loss over contralateral upper body and face
ACA lesion
Contralateral tactile loss of lower limb
Secondary somatosensory cortex (SII): location
Lateral sulcus upper bank
Two places that send inputs to the SII
- Ipsilateral SI cortex
2. Ventral posterior inferior nucleus (VPI) of thalamus
Lesion to the parietal cortical regions
Agnosia: contralateral body regions lost from body map; limb is not recognized as part of own body
Column of the four trigeminal nulcei
Form an elongated, continuous column in the brainstem
Path of the spinal trigeminal tract
Extends caudally to about C3 -> becomes continuous with Lissauer’s tract
Onion-peel sensory loss
Lesions of different parts of the spinal trigeminal tract cause loss of sensation around the face in different areas
Caudal lesion of spinal trigeminal tract vs more rostral lesion
Caudal lesion: affects posterolateral boundaries of face
Rostral lesion: sensory loss increasingly anterior, converging on mouth
How is there a smooth transition of sensory on the posterior head to the anterior face?
Trigeminal fibers end in the cervical cord overlapping the C1 and C2 dermatomes a bit
Why is the mesencephalic nucleus unique?
Only nucleus in the CNS which is basically a ganglia due to its pseudounipolar neurons
When to use a jaw jerk reflex?
To determine if pt has upper MN damage
How to do the jaw jerk reflex
Downward tap on chin -> stretches the masseter -> causes contraction bilaterally
Tract of the jaw jerk reflex
- Afferent neuron (V3) in masseter muscle spindle -> cell body in mesencephalic nucleus -> trigeminal motor nucleus -> trigeminal motor neuron activation -> efferent action
Symptoms of a lesion of the trigeminal nerve or nuclei (3 parts)
- Anesthesia/loss of sensation in trigeminal dermatomes
- Paralysis of muscles of mastication
- Loss of ipsilateral afferent limb corneal reflex
Alternating trigeminal hemiplegia: what causes this?
Unilateral destruction of CN 5 + CST in pons
Sx of alternating trigeminal hemiplegia
Ipsilateral trigeminal anesthesia and paralysis, contralateral spastic hemiplegia
What type of info is integrated in the SI?
Position sense, size, and shape discrimination
What fissure is the SII located in?
Sylvian fissure
What is the SII involved in?
Cognitive touch, comparing between objects, different tactile sensations, determines what becomes a memory
Where is the parieto-temporal-occipital association cortex (PTO)?
Posterior parietal lobe
PTO receives input from?
Multiple sensory areas
What is the PTO involved in?
Analyzing spatial coordinates of self in environment, names objects, etc.
