Somatosensory Tracts Flashcards
What somatosensory system is involved in two-point discrimination?
Posterior Column-Medial Lemniscal system
What is the relationship between receptive fields and receptor density?
Small receptive fields have high density
Large receptive fields have low density
What type of sensory information does the PCMLS relay?
Conscious awareness of body position and limb movement in space
Size, shape, texture discrimination, recognition of 3D space and motion detection
What are the posterior columns composed of?
Fasciculus gracilis and fasciculus cuneatus
Where do the primary sensory fibers of the PCMLS enter the spinal cord?
Medial division of the posterior root => terminate on 2nd order neurons at, above, or below the level of entry
What spinal level does the fasciculus cuneatus appear?
Above T6, lateral to fasciculus gracile - Thoracic and cervical fibers
Lesions to the fasciculi gracile and cuneatus in the spinal cord can result in loss of what sensations? On what side?
Ipsilateral loss of discriminative, positional, vibratory tactile sensation at or below segmental level of injury
Sensory ataxia (loss of muscle stretch reflexes, propioceptive loss from extermities due to lack of sensory input)
What might occur in sensory ataxia?
Loss of sensory input causes loss of muscle stretch reflexes, propioceptive loss from extremities
What kind of information is received by the outer “shell” of posterior column nuclei?
Input from muscle spindles, joints, and pacinian corpuscles (vibration)
What kind of information or received by the core of the posterior column nuclei?
Rapidly and slowly adapting afferents
What structure does the medial lemniscus terminate in?
Ventral Posterolateral nucleus
Where does sensory decussation occur?
Medulla
How would a posterior column lesion at the midbrain affect sensation?
Contralateral loss of discriminative touch, vibratory, and positional sensation (R lesion produces L loss)
What is the blood supply of the ventral posterior nuclei?
Thalamogeniculate As.
What structure do the 3rd order neurons of the VPL travel through before they terminate in the primary somatosensory cortex?
Posterior limb of internal capsule
What do the local circuit interneurons of the VPL influence?
Firing rates of 3rd order neurons
A lesion to the MCA would produce what somatosensory effect?
Tactile loss over contralateral upper body and face
Lesion to what artery would cause somatosensory loss to the contralateral lower limb?
Anterior Cerebral A.
What is the function of the SII cortex?
Conscious perception of tactile information (Discrimination between phone and someone touching you)
Agnosia can arise from lesions located where? What part of sensation is altered?
Parietal association regions
Contralateral limbs are not recognized as a part of the patient’s body anymore
What pathway transmits information about limb position, joint angle, and guides control of body muscle tone, movement, and posture?
Afferent cerebellar
Where is the trigeminal nucleus located
Ventrolateral aspect of the pons
What information does the dorsomedial division of the Main Sensory Nucleus convey?
Afferent information from the oral cavity
What nucleus conveys afferent information from V1, 2, and 3?
Ventrolateral division of the Main Sensory Nucleus (of Trigeminal nucleus)
What division of the trigeminal 2nd order fibers project through the posterior trigeminothalamic tract? Which side do they project to?
Dorsomedial division
Ipsilateral
What division of the trigeminal 2nd order fibers project through the anterior trigeminothalamic tract? Which side?
Ventrolateral
Contralateral
What information is conveyed by the mesencephalic nucleus?
Unconscious proprioceptive and pressure information from muscles of oral region
What is the afferent limb of the jaw-jerk reflex? Efferent?
Afferent: Mesencephalic trigeminal neuron
Efferent: Trigeminal motor neuron
Damage to what structures causes enhancement of the jaw-jerk reflex?
Upper motor neurons
What structure do the main sensory and the mesencephalic nucleus transmit information about the jaws through?
Superior Cerebellar Peduncle
On what sides would a brainstem lesion affect sensation in the trunk/extremities versus the face?
Contralateral sensory deficit in trunk/extremities
Ipsilateral deficit in face/CN
What deficit would occur if there is a lesion in area 1 of SI?
Texture discrimination deficit
Loss of size and shape discrimination would indicate a lesion in what area of SI?
Area 2 (astereognosis)
What type of ALS pathway carries nondiscriminative tactile, innocuous thermal, and nociceptive signals?
Direct
What part of the thalamus do the 2nd order neurons of the spinothalamic tract terminate on?
VPL
Where do spinomesencephalic fibers terminate?
Midbrain reticular formation
Anterolateral cordotomy refers to the lesioning of what tract?
Anterolateral system (spinothalamic)
Where would a lesion in the ALS manifest itself? What would be lost? On what side?
2 spinal segments below the lesion
Loss of nociceptive, thermal, and touch over contralateral side of body
What is the sensory effect of a hemisected spinal cord on the ALS and PCMLS tracts?
ALS: Contralateral loss of nociceptive and thermal sensation over body, 2 levels below lesion
PCMLS: Ipsilateral loss of discriminative tactile, vibratory, and position sense at or below lesion
What SC structure might be impinged in a syringomyelia?
Anterior white commissure (contains decussating ALS fibers)
Impingement of the AWC at C4-C5 levels produces loss of what? On what side?
Bilateral loss of nondiscriminative tactile, nociceptive, and thermal sensation several segments below lesion (Cape and Shawl to nipple line)
Where in the brainstem can a vascular lesion cause dissociated sensory loss? Why?
Lower brainstem (medulla or lower pons)
ALS and medial lemniscus are farther away from each other than in the upper brainstem
In dissociated sensory loss, how does the lesion affect sensation?
Contralateral loss of either:
Discriminative touch and vibratory sense OR
Pain/thermal sensation
(one is lost but not the other)
What is the orientation of the spinal trigeminal nucleus divisions in the medulla?
Anatomic - Pyramids on the bottom of the page, mandibular superior
Clinical: Pyramids on top of the page, mandibular inferior
(Either way, V1 is closest to pyramids and V3 is farthest)
How far caudally does the spinal trigeminal nucleus and tract descend?
3rd cervical segment => becomes continuous with Lissauer’s tract in upper cervical cord
What part of the spinal cord does the pars caudalis occupy?
C2, C3 to Obex
If the spinal trigeminal tract is lesioned rostrally, where would the sensory loss manifest?
Back of the head and converge on the mouth
(onion peeling loss, the more caudal, the less sensory loss around the mouth area)
Where do the circumoral and intraoral fibers of the spinal trigeminal tract terminate?
Near the obex
What is the blood supply to trigeminal structures in the medulla?
PICA
Posterior Spinal A.
Where are pars oralis and pars interpolaris located on the brainstem?
Pars oralis: Pons to superior medulla
Pars interpolaris: Superior medulla to obex
Patient presents with numbness to the left face. Upon examination, the right side appears to clench teeth better than the left. The patient also has no consensual corneal reflex on the left. The jaw-jerk reflex is also absent. What is the most likely diagnosis?
Lesion to the L Trigeminal N.
What is destroyed in alternating analgesia of the trigeminal nerve? What happens as a result?
Primary fibers of the spinal trigeminal and secondary fibers of spinal lemniscus
Ipsilateral hemianalgesia of face, contralateral hemianalgesia of the body
What is destroyed in alternating trigeminal hemiplegia? What happens as a result?
Unilateral destruction of trigeminal nerve and CST in the pons
Ipsilateral trigeminal anesthesia and paralysis, contralateral spastic hemiplegia
Wallenberg syndrome involves the lesion of what artery?
Posterior Inferior Cerebellar A.
Loss of the PICA to the medulla produces what symptoms?
(Wallenberg syndrome)
Contralateral loss of pain (hemianalgesia) and temp over body
Ipsilateral loss of pain/temp over the face
How would the corneal reflex be affected if the trigeminothalamic fibers near the trigeminal ganglion were lesioned?
No consensual closing of the eye
Direct pathway is blocked
How would the corneal reflex be affected if the motor fibers of facial nerve were lesioned at the right eye?
No direct pathway
Intact consensual closing
