Somatosensory Tracts 1 Flashcards
What information is transmitted and analyzed by the somatosensory system?
Touch or tactile information from external and internal locations on the body and head
Discriminative touch, flutter-vibrations and propioception
Crude touch, thermal sensation and nociception
The somatosensory system is transmitted via what?
Posterior column-medial lemniscal pathway
Trigeminothalamic pathway
Spinocerebellar pathway
Anterolateral system
What is the posterior column-medial lemniscal system (PCMLS) involved with?
The perception of mechanical stimuli
Size, shape, texture discrimination and 3D shape
Conscious awareness of body position and limb movement in space
Fast conduction velocities and precise somatotopic organization
What is two point discrimination?
Ability to discriminate between two stimuli simultaneously
What is a receptive field?
Area of skin innervated by somatic afferent fibers
Small receptive fields have high receptor density
Large receptive fields have low receptor density
Describe the primary afferent fibers of the PCMLS
Sensory axons within cell bodies in the DRG that enter the spinal cord and branch/terminate on neurons in the spinal gray matter
Large diameter fibers relay discriminative touch, flutter vibration and proprioception
Enter the spinal cord via medial division of the posterior root and then branch
Describe the branches of the primary afferent fibers in the PCMLS
Some select branches terminate on 2nd order neurons in, at, above and below the level of entry (for reflexes)
Largest set of branches ascends cranially and form the posterior columns (fasciculus gracilis and cuneatus)
The fasciculus gracilis carries information from what?
Sacral region to T6
The fasciculus cuneatus carries information from what region?
T6 -> superior
What are the fasciculi gracilis and cuneatus?
Fibers within the posterior columns that are organized topographically
Sacral level fibers (gracilis) are positioned medially
Fibers from progressively more rostral area levels (cuneatus) are added laterally
SC lesions of the posterior column (PCMLS) result in what?
Ipsilateral reduction or loss of discriminative, positional and vibratory tactile sensations at and below the segmental level of injury
What is sensory ataxia?
Loss of muscle stretch (tendon) reflexes and proprioceptive losses from the extremities due to lack of sensory input
Pt may also have a wide based stance and may place the feet to the floor with force in an effort to create the missing proprioceptive input
Can be a result from a SC lesion of the posterior column
Describe the posterior column nuclei
Gracile nucleus (from fasciculus gracilis) and cuneate nucleus (from fasciculus cuneatus) are in posterior medulla
Contain 2nd order neurons of the PCMLS
Receive input from 1st order neurons (primary afferents) from ipsilateral DRG
Each nucleus receives inputs from its corresponding fasciculus
Somatotopic organization of projections is maintained
Describe the medial lemniscus
2nd order neurons within nuclei send axons to thalamus on contralateral side
Internal arcuate fibers loop contralaterally in the medulla -> cross the midline (sensory decussation) -> ascend as medial lemniscus (ML)
Where does the medial lemniscus terminate?
In the ventral posterolateral nucleus (VPL) of the thalamus which houses 3rd order neurons that projection to the cortex
Describe shifting of the ML
Before reaching the VPL of the thalamus the ML will rotate
1. ML flattens horizontally in the rostral medulla/caudal pons -> UE fibers lie medially and LE fibers lie laterally
2. Turns vertically and shifts laterally in the midbrain -> UE fibers lie anteriorly and LE fibers posteriorly
Somatotopic orientation shifts as the fiber tract rotates
Describe the ventral posterior nuclei
Wedge shaped cell group located in caudal thalamus -> ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei
Somatotopic arrangement maintained in VPL
Third order neurons traverse posterior limb of the internal capsule -> primary (SI) and secondary (SII) somatosensory cortices
What supples the ventral posterior nuclei?
Thalamogeniculate branches of the posterior cerebral artery
Lesions result in loss of all tactile sensation over the contralateral body and head
What is the primary somatosensory (SI) cortex?
Comprises the postcentral gyrus and posterior paracentral gyrus
Bordered by central sulcus (anteriorly) and postcentral sulcus (posteriorly)
Subdivided anterior -> posterior into Brodmann’s areas 3a, 3b, 1 and 2
What provides blood supply to the SI cortical areas?
The anterior and middle cerebral arteries
MCA lesions produce tactile loss over the contralateral upper body and face
ACA lesions affect the contralateral lower limb
Describe the somatosensory homunculus
Foot to tongue pattern along medial -> lateral axis
Regions with increased receptor density (hand/lips) have large amount of dedicated cortical tissue
Regions with decreased receptor density (back) have small cortical representations
What is the secondary somatosensory (SII) cortex?
Inner face of the upper back of lateral sulcus
Inputs from ipsilateral SI cortex and ventral posterior inferior nucleus (VPI) of the thalamus
Describe parietal cortical regions
Also receive tactile inputs
Lesions in parietal association area can produce agnosia
Contralateral body region is lost from body map
Limb is not recognized as part of the pt’s own body
Sensation is radically altered
Describe the trigeminal nerve
Afferent primarily from trigeminal ganglion
Opthalamic, maxillary and mandibular nerve
V1-3 delineate facial dermatomes that are sharply defined (in contrast to spinal dermatomes)
Pain, temperature and non-discriminative touch
Discriminative touch and proprioceptive touch
What are the central pathways of the CN V?
Trigeminal nuclei (4 total) form an elongated but continuous column in the brainstem Nuclei include mesencephalic nucleus (midbrain), principal sensory nucleus (midpons), trigeminal motor nucleus, spinal nucelus (obex -> C2/3)
Describe the mesencephalic nucleus
Conveys unconscious proprioceptive and pressure information from TMJ, masticatory and extraocular muscles (muscles of oral region and eye muscles)
Projects via the mesencephalic tract to the principal sensory nucleus and spinal nucleus
Some axons terminate in trigeminal motor nucleus -> forms circuit for the jaw jerk reflex
Describe the principal sensory nucleus
Primary afferents relay discriminative tactile (touch and pressure) and proprioceptive sensations
Primary afferents target 2nd order neurons in principal sensory neurons
What is the somatotopic arrangement within the principal sensory nucleus?
V1 (opthalamic) fibers are anterior
V3 (mandibular) fibers are posterior
V2 (maxillary) are in between
Describe the path of neurons from the principal senosoy nucleus
Primary afferents target 2nd order neurons in principal sensory nucleus -> 2nd order fibers project via the anterior and posterior trigeminothalamic tracts (anterior = contralateral; posterior = ipsilateral) -> target ventral posteromedial (VPM) nucleus (somatotopic arrangement; oral cavity = medial; external face = lateral) -> 3rd order neurons from VPM project via posterior limb of internal capsule -> primary somatosensory cortex
Describe the jaw jerk reflex
Stretching the masseter (downward tap on chin) causes it to contract bilaterally
Assess function of trigeminal brainstem and V3
Amplitude of the reflex is minor but is enhanced after UMN damage
What are the afferent and efferent limbs of the jaw jerk reflex?
Afferent limb: neuron with cell body in mesencephalic nucleus -> dendritic process within muscle spindle of masseter muscle -> axon that synapses in the trigeminal motor nucleus -> efferent limb: activation of trigeminal motor neuron
What are the trigeminal nuclei mesencephalic, motor and principal sensory supplied by?
Long circumferential branch of basilar artery and branches of anterior inferior cerebellar artery
Mesencephalic: mostly long circumferential branches of basilar A and branches of superior cerebellar artery
What deficits are seen with damage to the primary afferents?
Ipsilateral side effected
No change in strength
Decrease/loss of function in reflexes and sensation
What are the effects of damage to sensory pathways (brainstem/2nd order neurons)
Contralateral side effected
No change in strength or reflexes
Loss of function in sensation
What are the effects of damage to the primary somatosensory cortex
Contralateral side affected
No change in strength or reflexes
Loss of function in sensation
Vascular lesions involving the MCA produce what?
Tactile loss over the contra lateral upper body and face
Vascular lesions involving the anterior cerebral artery affect what?
The contralateral lower limb
Lesions of the primary somatosensory cortex result in what?
Loss of proprioception, position sense, vibratory sense and pain and thermal sensations on the contralateral side of the body
A lesion of the medial lemniscus would result in deficits on which side?
If lesion is on the R side pt will experience loss of discriminative touch and proprioception on the L side
