Spinothalamic, Posterior Column, Thalamo-cortical Pathways Flashcards
There are two systems for somesthesis (somatosensory signaling), namely:
protopathic (anterolateral) pathways
epicritic (lemniscal) pathways
Again, there are two separate systems and pathways for conveying somatosensory information from the periphery of the body to the brain. What does the protopathic system mediate?
The first system (protopathic) concerns pain, crude touch, and temperature sensation.
What does the epicritic pathway mediate?
A second system (epicritic) concerns fine touch, including information about the form and texture of objects, pressure on the skin or body, position of muscles and joints, and vibration sense.
How is the spatial and temporal resolution of the protopathic pathway? epicritic?
protopathic- low
epicritic- high
What is the fiber type for the protopathic pathway?
small, slowly conducting light myelinated and unmyelinated fibers
What is the fiber type for the epicritic pathway?
large, rapidly conducting, myelinated
What is the ascending tract of the protopathic pathway?
lateral spinothalamic tract
What is the ascending tract of the epicritic pathway?
posterior and posterolateral columns
Where are motoneuron cell bodies located?
In the spinal cord ventral horn.
How do motoneurons act?
The axons of motoneurons exit the ventral horn in the ventral roots and travel within spinal nerves to their target muscles. This is the “final common path” for motor activation and the efferent side of spinal reflexes.
Sensory receptors for somesthesis are found where?
In the skin (cutaneous), in muscles and joints, and visceral organs.
How do sensory receptors communicate with the CNS?
The receptors communicate with the CNS via the peripheral processes of pseudo unipolar sensory neurons. The cell bodies of these neurons are located outside the spinal cord in the dorsal root ganglia (DRGs).
Action potentials in the DRG cell peripheral process continue (within the dorsal roots) along the central process into the dorsal horn of the spinal cord.
Depending on the type of receptor and which somesthetic system is involved, the peripheral process may synapse in the dorsal horn or project up the spinal cord to the brainstem.

Major landmarks:
C4: low collar shirt
C6: thumb
C8: ulnar to thumb
T4: nipples
T10: umbilicus

T12: Inguinal
L4: Medial side of great toe and knee
S1: lateral side of foot and shin
What are some myelinated peripheral fiber types?
Aa, AB, Ay, and Adelta
B
What do Aa fibers mediate?
motor to skeletal muscle (fastest fibers)
What do Aa 1a subtype fibers mediate?
sensory from muscle spindle
What do AB, 1b subtype fibers mediate?
sensory from Golgi tendon organ and Ruffini endings
What do AB, II subtype fibers mediate?
sensory from skin receptors
What do Ay fibers mediate?
motor to intrafusal fibers
What do Adelta, III subtype fibers mediate?
sensory from free nerve endings for pain and temperature and hair follicles
What do B fibers mediate?
preganglionic autonomic fibers
What do C, IV subtype fibers (unmyelinated) mediate?
postganglionic autonomic fibers; sensory from free nerve endings for pain and temperature; smell
There are many types of sensory receptors for somesthesis. How are they classified?
They can be classified by whether they concern external events (exteroreceptors), the position of joints and muscles in space (proprioceptors), or the state of internal organs (enteroreceptors).
Receptors can also be classified according to the physics of what they detect: chemicals (chemoreceptors), light (photoreceptors), temperature (thermoreceptors), mechanical force (mechanoreceptors), or pain / noxious stimuli (nociceptors).
The Figure on the back provides another setof different types of energy that can bedetected by sensory receptors in general (not just somesthesis).
Note also that these forms of energy that exist in nature are transduced (transformed from one form of energy to another) by the sensory receptors into a graded electrical signal. Graded responses are integrated by the receptor and in many cases converted into action potentials for long distance transmission.
Thus, even at the earliest stages, the nervous system encodes an abstraction of the “real world”.













