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26 Cards in this Set

  • Front
  • Back
Somatic sensations can be subdivided into submodalities (6) and each of these have one of 2 different types of receptors
1) Discriminative touch
2) Vibration
3) Proprioception (position sense)
4) Crude touch (nondiscriminative)
5) Thermal (hot and cold) senation
6) Nociception (pain)

These different sensations have receptors called mechanoreceptors or thermoreceptors that detect (transduce) the mechanical or thermal energy, respectively.
Three discrete pathways transmit and process somatosensory information:
*1) Dorsal Column - Medial Lemniscal System
*2) Spinothalamic Tracts (anterolateral system)
3) Spinocerebellar Pathways
4) Trigeminothalamic Pathways for the head (not important)

*most important
Dorsal Column System - Medial Lemniscal System - types of sensory information carried (2 main and then subunits) and what are they formed by
This system is involved with the perception and appreciation of mechanical stimuli.
1) Conscious appreciation of cutaneous
-Discriminative Touch - fine form and texture discrimination, recognition of 3-D shape (Stereognosis)

2)Conscious appreciation of Proprioception
-Position Sense - position of limb or body part in space
-Awareness of Movement at joints and degree of muscle contraction (Kinesthesia)

-Formed by Ascending Collaterals of DRG Cells
Organization of Dorsal Column System - Info is brought into the spinal cord from peripheral mechanoreceptors which include those for... (5)
Discriminative Touch, Vibration Sense and Proprioception (such as Joint Receptors for awareness and movement and position of body parts), Muscle Stretch Receptors (muscle spindles) and Golgi Tendon Organs for detecing muscle tension.
Neurons of this system located where and physical description
-Located in the Dorsal Root Ganglion - central processes enter spinal cord via the dorsal root.
-Large diameter, myelinated axons enter the spinal cord and synapse with interneurons of the gray matter within that particular nerve's spinal cord segment as well as adjacent cord segments.
-a Collateral branch of each axon Ascends Within The Ipsilateral Dorsal Column
Two Fasciculi of Dorsal Columns (Posterior Column) and their functions (1 each)
In cervical and upper thoracic cord levels, the dorsal columns can be subdivided into 2 fasciculi:
1) Fasciculus Gracilis (medial) - carries info principally from Lower LImb and lower portions of the body (~T6 and below)
2) Fasciculus Cuneatus (lateral) - carries info principally from Upper Limb and upper portions of the body (above T6, does not exist under T6).
Somatotopic Mapping/Organization
The axons within the dorsal columns have a very precise organization with respect to their origin from the body.
The lower portions of the body are mapped medially and progressively more rostral portions are mapped more laterally within the dorsal column.
Dorsal Column Nuclei
-Axons from the gracile and cuneate fasciculi synapse in nuclei of the same name in the lower medulla:
1) Nucleus Gracilis
2) Nucleus Cuneatus
-Axons from the cells of the dorsal column nuclei ascend as the brainstem tract called the Medial Lemniscus
Medial Lemniscus
Axons from the gracile and cuneate nuclei curve toward the midline as the Internal Arcuate Fibers and then Descussate as the Sensory Decussation to form the Contralateral Medial Lemniscus
Thalamic Connections - what synapses in the thalamus and where exactly in the thalamus
Fibers of the Medial Lemniscus carrying touch, vibration, and proprioceptive info synapse in the Nucleus Ventralis Posterolateralis (VPL) of the thalamus
Cortical Projections
Axons from the VPL ascend via the posterior limb of the internal capsule to the cortex and synapse in a wide region including a very strong projection to the Postcentral Gyrus (Primary Somatosensory Cortex)
Dorsal Column Signs and 1 example
Damage to any portion of the pathway results in dorsal column signs. One example of a fairly specific dorsal column lesion is Tabes Dorsales, a late manifestation of neurosyphilis.
the 6 dorsal column signs
1) Loss of Vibration Sense - use tuning fork
2) Inability to Determine Limb Position
3) Loss of Stereognosis/Discriminative Touch (Astereognosis) - to test for it, hand patient something obvious, close their eyes, see if they know what it is
4) Loss in 'Two-Point Tactile Discrimination
5) Motor deficit characterized by a Shuffling, Uncoordinated Gait - loss of sensory input, particularly proprioceptive input, severely impairs motor behavior; all aspects of motor behavior require Sensory-Motor Integration
6) Romberg's Sign - when patient stands erect with eyes closed and feet together, they tend to Sway and Fall Toward the Side of the Lesion. Romberg's sign is also seen with cerebellar damage.
Type of sensory info carried for Spinothalamic Tract (STT, anterolateral system) (3) and how they are formed
1) Primary Pathway for Pain (Mechanical, Chemical, and Thermal)
2) Thermal (non-painful warm or cold)
3) Nondiscriminative Touch - Crude or poorly localized touch

Formed by Ascending Collaterals of Interneurons
Disruptions of the spinothalamic tract can produce symptoms ranging from...
Reduced sensibility (Hypesthesia), to numbness, tingling and prickling (Paresthesia), to a complete loss of sensibility (Anesthesia).
The receptors for pain are Free Nerve Endings and the afferent fibers are thinly myelinated A8 fibers (type III) or unmyelinated C fibers (type IV)
Spinothalamic Tract Organization - Collaterals that make up the tract
The central processes (fibers) enter Lissauer's Tract and bifuracte into ascending and descending branches.
Collaterals of these axons synapse on interneurons in the dorsal horn gray matter, particularly laminae I, II and V, and participate in the circuits that mediate spinal reflexes such as the flexor withdrawal reflex
Substantia Gelatinosa and Lissauer's Tract's role in Spinothalamic Tract and the clinical significance of their role
-DRG C fibers carrying pain info synapse in the Substantia Gelatinosa (SG) and w/ other interneurons of other posteriod horn laminae (V)
-Many Fibers Ascend Within Lissauer's Tract One or More Levels Of The Cord Prior to Synapsing in the SG
-This has clinical significance: a unilateral lesion at a particular level of the cord will lead to a loss of pain and temperature sensitivity at dermatomal levels beginning about one or two segments below the injured segment b/c pain & temperature fibers ascend
one or two segments before
synapsing in the substantia gelatinosa.
Spinothalamic Pathway Decussation
axons of interneurons of the posterior horn cross the midline via the anterior white commissure to form the Contralateral Spinothalamic Tract (anterolateral system)
-the fact that fibers carrying pain and temp. info decussate in the spinal cord has clinical significance.
Spinothalamic Tract
Axons ascend as the contralateral spinothalamic tract to reach the Brainstem Reticular Formation and Thalamus
Brainstem and Thalamic Nuclei
-Branches of axons of the spinothalamic tract synapse in the Brainsetm Reticular Formation (Spinoreticular Tract)
-Large numer of axons continue to the VPL of the thalamus
Cortical Projections
axons from VPL project via the internal capsule to the Postcentral Gyrus.
Spinocerebellar Pathway Transmission
These tracts transmit Nonconscious Proprioception info about Limb Position, Joint Angles and Muscle Tension and Length to the cerebellum.
-This input plays an intergral role in guiding cerebellar control of muscle tone, movement, and posture
A potential major deficit related to disease or injury of the spinocerebellar tracts is...
Cerebellar Ataxia - lack of coordination during walking and other movements that occur b/c the cerebellum is not receiving the sensory feedback necessary to regulate movements.
*Often, with spinal cord injury, cerebellar deficits due to lesions of the spinocerebellar tracts are masked b/c there is also a concomitant injury to the corticospinal tracts and the patient is paralyzed. Obviously one cannot see the lack of coordination of movement in a patient who cannot move.
Four tracts that carry proprioceptive info to the cerebellum from the body and limbs...
Info from the lower limbs (T6 and below) is carried by the Dorsal and Ventral Spinocerebellar Tracts.
Info from the upper limbs (T6 and above) is carried by the Rostral Spinocerebellar and the Cuneocerebellar Tracts.
Make sure you know the locations of the decussations...
Ex. 1) What is the location of sensory decussation of the dorsal column medial lemniscal system
2) What is the location of sensory decussation of the spinothalamic tract?
1)The Medulla
2)Spinal Cord
Difference between how discriminative touch and proprioception info ascends vs. that of pain and temperature info
discriminative touch and proprioception info ascends Ipsilaterally.
Pain and temp info ascends Contralaterally.