Somatosensory Pathway 1 Flashcards
Transmits and analyzes touch or tactile information from external & internal locations on the body & head
The Somatosensory System
The Somatosensory System is transmitted via which pathways?
- Posterior column-medial lemniscal pathway
- Trigeminothalamic pathway
- Spinocerebellar pathway
- Anterolateral system
Involved with the perception and appreciation of mechanical stimuli
Posterior Column–Medial Lemniscal System (PCMLS)
ability to discriminate between two
stimuli simultaneously
Two-point discrimination
Characteristic features of. which pathway?
relays
- Afferent fibers with fast conduction velocities & limited number of synaptic
- Precise SOMATOTOPIC organization
Posterior Column–Medial Lemniscal System (PCMLS)
Peripheral Receptors
• Digits and perioral region have ? density of
tactile receptors
• Other regions, like the back, have ? density
- increased
- decreased
area of skin innervated by a somatic afferent fibers
Receptive field
- Small receptive fields have ? receptor density
- Large receptive fields have ? receptor density
- high
- low
Primary Afferent Fibers
- Consist of what 3 things?
- Peripheral process
- Central Process
- Pseudounipolar cell body
Primary Afferent Fibers:
1. ? extending from the DRG
(mechanoreceptor or free nerve ending)
- ? extending from DRG into
CNS - ? in the DRG
- Peripheral process
- Central process
- Pseudounipolar cell body
Peripheral distribution of the afferent nerves arising from each spinal level delineates the segmental pattern of ?
dermatomes
fasciculus
gracilis or fasciculus cuneatus are collectively called?
posterior columns
Posterior Column
- Sacral level fibers are positioned ? and fibers from progressively more rostral levels (up to thoracic level T6) are added ? → form the ?
- medially
- laterally
- fasciculus gracile
Posterior Column
- Thoracic fibers above T6 & cervical fibers are lateral → form the ?
fasciculus cuneatus
Spinal cord lesions result in ?
ipsilateral reduction or loss of discriminative, positional, & vibratory tactile sensations at & below the segmental level of injury
loss of muscle stretch (tendon) reflexes, and
proprioceptive losses from the extremities due to lack of sensory input
Sensory ataxia
Posterior Column Nuclei
• Segregation of tactile inputs occurs w/in the nuclei:
- Core “clusters” receive inputs from ?
rapidly- & slowly adapting afferents
Posterior Column Nuclei:
• Segregation of tactile inputs occurs w/in the nuclei:
- Outer “shells” receive inputs from ?
muscle spindles, joints,
& Pacinian corpuscles
Thalamic Relays:
- ? of PC nuclei send axons to contralateral thalamus
Second-order neurons
Thalamic Relays:
• Second-order neurons of PC nuclei send axons to
contralateral thalamus
- ? loop anteromedially in
medulla - Cross the midline as the ?
- Ascend as the ? on the
opposite side - As ML extends rostrally, it rotates laterally in the ?
- Upper extremity fibers lie ? and lower extremity fibers ?
- Somatotopic organization is
maintained as ML terminates in ? of the
thalamus
- Internal arcuate fibers
- sensory decussation
- medial
lemniscus (ML) - pons
- medially
- laterally
- ventral posterolateral
nucleus (VPL)
Thalamic Rays (PC)
- At medulla, it is fed by ? artery
Anterior spinal A.
Midbrain PC Lesions:
Damage at brainstem levels leads to deficits in discriminative touch, vibratory, & positional sensibilities over the ? side of the body
contralateral
Wedge-shaped cell group located in caudal
thalamus
Ventral Posterior Nuclei
Ventral Posterior Nuclei:
- Comprised of the ? & the ?
- Separated by fibers of the ?
- VPM receives ? sensory information
- ventral posterolateral nucleus (VPL)
- ventral posteromedial nucleus (VPM)
- arcuate lamina
- trigeminal
Ventral Posterior Nuclei:
- Somatotopic arrangement of the body is
maintained in the ?
VPL
Ventral Posterior Nuclei:
- VPL & VPM are supplied by thalamogeniculate
branches of ? - Compromise can result in loss of all tactile
sensation over the ? body & head
- posterior cerebral artery
- contralateral
PC
- Receives ascending input from medial lemniscus
VPL (ventral posterolateral nucleus)
PC:
The VPL for the trunk & extremities contains two populations of
identified neurons:
- Third-order neurons
2. Local circuit interneurons
Third-order neurons:
- large-diameter axons that traverse posterior
limb of the internal capsule & terminate in the ? & ?
- primary (SI)
- secondary (SII) somatosensory cortices
receive excitatory
corticothalamic inputs & influence the firing rates of third-order
neurons
Local circuit interneurons (inhibitory)
Axons from third-order neurons (thalamus) terminate in?
primary somatosensory (SI) cortex
• Comprises postcentral gyrus and posterior paracentral gyrus
• Bordered by central sulcus (anteriorly) & postcentral sulcus
(posteriorly)
primary somatosensory (SI) cortex
“Foot to tongue” pattern along
medial → lateral axis
homunculus
Homunculus
- Regions with ↑↑↑ receptor density, have large amount of
dedicated cortical tissue
hand/lips
Homunculus
- Regions with ↓↓ receptor density have small cortical
representations
back
Blood supply to the SI cortical areas is provided by?
anterior & middle cerebral arteries
(MCA lesions produce tactile loss over the CONTRALATERAL UPPER body & face; ACA lesions affect the contralateral LOWER limb)
Subdivisions of SI:
- ? is located in the depths of the central sulcus, abuts area 4 (primary motor cortex)
Area 3a
Subdivisions of SI:
- extend up the bank of the sulcus onto the shoulder of the postcentral gyrus
Areas 3b & 1
Subdivisions of SI:
- lies on the gyral surface and abuts area 5 (somatosensory association cortex)
Area 2
- Lies deep in the inner face of the upper bank of lateral sulcus
- Contains somatotopically representation of body surface
• Inputs arise from ipsilateral SI cortex & ventral posterior inferior nucleus (VPI) of the thalamus
Secondary somatosensory (SII) cortex:
- receive tactile inputs
- Posterior to area 2, includes area 5 and area 7 (7b)
- Receive some medial lemniscal input & inputs from SI
Parietal cortical regions
Lesions in parietal association area can produce?
agnosia
• Contralateral body parts are lost from the personal body map
• Sensation is not radically altered, but the limb is not
recognized as part of the patient’s own body
Agnosia
? input plays an
integral role in guiding control
of body muscle tone,
movement, and posture
Cerebellar
transmit proprioceptive & limited cutaneous information to the
cerebellum
- Includes information about limb
position, joint angles, and muscle
tension/length
Spinocerebellar pathways
• Mixed nerve with sensory and motor components
- Main sensory nerve for the head
- Innervates the muscles of mastication
Trigeminal Nerve
Attaches to the brainstem as two adjacent roots (? and ?) on ventrolateral aspect
of the pons
- large sensory
- smaller motor
Form a continuous cell column that extends
from spinomedullary junction to rostral levels of
mesencephalon/midbrain
Trigeminal Nuclei
4 nucleus of Trigeminal Nuclei?
- Main sensory nucleus (msT)
- Trigeminal motor nucleus (mT)
- Spinal nucleus
- Mesencephalic nucleus (mes)
Functions of the 4 trigeminal nuclei?
1. Main sensory nucleus (msT)
- Trigeminal motor nucleus (mT)
- Spinal nucleus
- Mesencephalic nucleus (mes)
- Main sensory nucleus (msT): touch &
pressure - Trigeminal motor nucleus (mT): muscles of
mastication - Spinal nucleus: pain & temperature
- Mesencephalic nucleus (mes): proprioceptive
afferents from the TMJ & masticatory m
Locations of the 4 trigeminal nuclei?
- Main sensory nucleus (msT)
- Trigeminal motor nucleus (mT)
- Spinal nucleus
- Mesencephalic nucleus (mes)
- Main sensory nucleus (msT)
- midpons, slightly lateral to motor
nucleus - Trigeminal motor nucleus (mT)
- Spinal nucleus
- extends caudally
from this level - Mesencephalic nucleus (mes)
- extends rostrally (all the way into the midbrain), as name implies
Sensory Nerve for the Head
CN V
Main Sensory Nucleus
- afferent input from oral cavity
Dorsomedial division
Main Sensory Nucleus
- afferents from V1, V2, V3
Ventrolateral division
- Two-point localization
- Vibratory sense
- Position sense
Trigeminal System
Main Sensory Nucleus:
- 2nd order fibers of dorsomedial division project ipsilaterally via ?
posterior trigeminothalamic tract
Main Sensory Nucleus:
- 2nd order fibers of ventrolateral division project contralaterally via ?
anterior trigeminothalamic tract
Main Sensory Nucleus:
• Both tracts target ? of the thalamus
- Somatotopic arrangement
- Oral cavity = medial, external face = lateral
ventral posteromedial (VPM) nucleus
Main Sensory Nucleus:
- 3rd order axons from VPM project via ? → ?
- posterior limb of internal capsule
- primary somatosensory cortex
Tactile & proprioceptive information from upper & lower extremities, & head/neck is relayed via ?
PCMLS & Main sensory of V
- Comprised of pseudounipolar neurons
- Conveys unconscious proprioceptive &
pressure information from muscles of the
oral region
Mesencephalic Nucleus
Mesencephalic Nucleus
- Projects to the ?
- Permits the conscious awareness of facial & oral proprioception via the ?
- Some axons terminate in ? → forms circuit for the jaw-jerk reflex
- main sensory nucleus (and spinal nucleus)
- anterior/posterior
trigeminothalamic tract - trigeminal motor nucleus
Stretching the masseter (downward
tap on chin), causes it to contract
bilaterally
Jaw-jerk Reflex
What are the Trigeminocerebellar Pathways?
- Mesencephalic nucleus via Spinal nucleus (Pars interpolaris)
- Main sensory nucleus
Which Trigeminocerebellar Pathway?
• Proprioceptive input from the jaw related to
chewing/jaw placement
• Via the superior cerebellar peduncle
(restiform body)
Mesencephalic nucleus via Spinal nucleus (Pars interpolaris)
Which Trigeminocerebellar Pathway?
- Moderate amount of secondary neurons project to the anterior vermis of the cerebellum
- Via the superior cerebellar peduncle
Main sensory nucleus
• Brainstem or SC lesions that result in deficits on
that differ between each half of the body -
dependent upon where fibers cross
Cross Sensory Syndrome
Cross Sensory Syndrome:
• Brainstem lesions can result in:
- Sensory deficits of trunk/extremities ? to
the lesion, but sensory deficits of face/CN ? to
the lesion
- The right face and the left arm/leg lack proprioceptive
information
- contralateral
- ipsilateral
Cross Sensory Syndrome
• SC lesions can result in:
-? deficits on the right, but ? on the left (or vice versa)
- Proprioceptive
- anesthesia
Typically, SI lesions usually include larger areas & frequently result in more global deficits:
- Loss of proprioception, position sense, vibratory sense, and pain & thermal
sensations on the ? side of the body
contralateral
SI Lesions and Sensory Dissociation:
- Lesions involving area ? produce a deficit in texture discrimination, whereas damage to area ? results in loss of size and shape discrimination (astereognosis)
- 1
- 2
SI Lesions and Sensory Dissociation:
- Injury to area ? has a more profound effect than does damage to either area 1 or 2 alone → have deficits in both texture and size/shape discrimination
3b
SI Lesions & Sensory Dissociation:
- Area ? performs initial processing and distributes information to areas 1 & 2
3b
Thalamic Rays (PC) :
• In spinal cord, these columns when they are coming in as afferents and first order neurons are fed by the?
Posterior spinal artery
