Spinal Cord II: Ascending & Descending Pathways Flashcards
2 Classes of Sensory Endings
Cutaneous & proprioceptive
Cutaneous receptors
responsible for touch, pain, temp., itch
encapsulated (meissner corpuscles, pacinian corpuscles, ruffini endings) and nonencapsulated (root hair plexus, merkel cells, free nerve endings)
some can provide proprioceptive info
Proprioceptive (muscle) receptors
detects limb and body positioning relative to each other
comprised of muscle spindles and golgi tendon organ
encapsulated
Root Hair Plexus
touch receptors
wrap around hair to detect movement
Merkel Cells
sensory cells
located below epidermis (top layer of skin)
connect to nerve endings of skin
Free Nerve Endings
branching sensory nerve terminals
found in skin (touch, pain, temp., itch)
Meissner Corpuscles
located below epidermis
detects fine touch and pressure
Pacinian Corpuscles
deeper sensory receptors for vibration and deep pressure
proprioception
Ruffini Endings
located below epidermis
detects skin stretch, pressure, joint movement, temp.
Muscle Spindles
wraps around intrafusal muscle fibers (deeper and encapsulated muscle fibers specific to spindles)
detects steady-state of muscles and changes in muscle length
Golgi Tendon Organs
located within tendon
comprised of free endings that are wrapped between collagen fibers
- muscle tension causes collagen to pinch and activate free nerve endings
detects muscle tension
Sensory Fiber Entry Zones
small: join and form Lissauer’s tract; terminate in substantia gelatinosa
large: enter medial to Lissauer’s tract to reach deeper regions of spinal cord
Ascending Tracts
convey sensory info from periphery to brain
name of each pathway shows origin and target
first order neurons ALWAYS in dorsal root ganglia
Divisions of Ascending Tracts
- conscious tracts (posterior column-lemniscus, anterolateral system)
- unconscious tracts (dorsal and ventral spinocerebellar tracts)
Posterior Column-Lemniscus System (conscious)
fasciculus gracilis: sensory info from below T6 segment
fasciculus cuneatus: sensory info from above T6 segment
Posterior Column-Lemniscus System Pathway (orders)
1st order neurons - dorsal root ganglia
2nd order neurons - medulla oblongata
3rd order - thalamus
cross - medial lemniscus
Spinothalamic tracts (conscious)
form anterolateral system of which there are lateral tract (pain & temp.) and anterior (crude touch and pressure)
Spinothalamic tracts pathway (orders)
1st - dorsal root ganglia
2nd - nucleus proprius
3rd - thalamus
cross - in spinal cord, prior to entering tracts
Conscious Ascending Tracts
terminate in primary somatosensory cortex
contralateral projections
Spinocerebellar tracts
dorsal: send proprioceptive info to cerebellum
ventral: send locomotor related interneuronal info to cerebellum
important for cerebellar sensory integration (compares info from 2 tracts)
Spinocerebellar tracts pathway (orders)
only second order
1st - dorsal root ganglia
2nd - neurons in spinal cord
ipsilateral but ventral cerebellar tract undergoes double cross in spinal cord then brainstem
Descending tracts
signals from brain to periphery (motor)
name indicates origin and target
Corticospinal tracts (voluntary)
voluntary motor control
lateral (90%) and anterior (10%)
upper motor neurons, synapse onto spinal interneurons of spinal cord
Corticospinal tracts pathway order
1st - motor, premotor, and supplemental motor cortices (upper motor neurons)
2nd - spinal cord (generally interneurons)
contralateral
crosses - medulla oblongata (lateral) and in spinal cord (anterior)
Brainstem tracts (involuntary) (4)
- vestibulospinal (locomotor reflex and balance)
- tectospinal (head and eye movement, direct toward sights and sound; terminates in cervical spinal cord)
- reticulospinal (postural control and locomotion)
- rubrospinal (locomotion; terminates in cervical and thoracic spinal segments - responsible for trunk and upper limbs)
