Spinal Cord II: Ascending & Descending Pathways

Question 1

2 Classes of Sensory Endings

Answer 1

Cutaneous & proprioceptive

Question 2

Cutaneous receptors

Answer 2

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

Question 3

Proprioceptive (muscle) receptors

Answer 3

detects limb and body positioning relative to each other
comprised of muscle spindles and golgi tendon organ
encapsulated

Question 4

Root Hair Plexus

Answer 4

touch receptors
wrap around hair to detect movement

Question 5

Merkel Cells

Answer 5

sensory cells
located below epidermis (top layer of skin)
connect to nerve endings of skin

Question 6

Free Nerve Endings

Answer 6

branching sensory nerve terminals
found in skin (touch, pain, temp., itch)

Question 7

Meissner Corpuscles

Answer 7

located below epidermis
detects fine touch and pressure

Question 8

Pacinian Corpuscles

Answer 8

deeper sensory receptors for vibration and deep pressure
proprioception

Question 9

Ruffini Endings

Answer 9

located below epidermis
detects skin stretch, pressure, joint movement, temp.

Question 10

Muscle Spindles

Answer 10

wraps around intrafusal muscle fibers (deeper and encapsulated muscle fibers specific to spindles)
detects steady-state of muscles and changes in muscle length

Question 11

Golgi Tendon Organs

Answer 11

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

Question 12

Sensory Fiber Entry Zones

Answer 12

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

Question 13

Ascending Tracts

Answer 13

convey sensory info from periphery to brain
name of each pathway shows origin and target
first order neurons ALWAYS in dorsal root ganglia

Question 14

Divisions of Ascending Tracts

Answer 14

1. conscious tracts (posterior column-lemniscus, anterolateral system)
2. unconscious tracts (dorsal and ventral spinocerebellar tracts)

Question 15

Posterior Column-Lemniscus System (conscious)

Answer 15

fasciculus gracilis: sensory info from below T6 segment
fasciculus cuneatus: sensory info from above T6 segment

Question 16

Posterior Column-Lemniscus System Pathway (orders)

Answer 16

1st order neurons - dorsal root ganglia
2nd order neurons - medulla oblongata
3rd order - thalamus
cross - medial lemniscus

Question 17

Spinothalamic tracts (conscious)

Answer 17

form anterolateral system of which there are lateral tract (pain & temp.) and anterior (crude touch and pressure)

Question 18

Spinothalamic tracts pathway (orders)

Answer 18

1st - dorsal root ganglia
2nd - nucleus proprius
3rd - thalamus
cross - in spinal cord, prior to entering tracts

Question 19

Conscious Ascending Tracts

Answer 19

terminate in primary somatosensory cortex
contralateral projections

Question 20

Spinocerebellar tracts

Answer 20

dorsal: send proprioceptive info to cerebellum
ventral: send locomotor related interneuronal info to cerebellum
important for cerebellar sensory integration (compares info from 2 tracts)

Question 21

Spinocerebellar tracts pathway (orders)

Answer 21

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

Question 22

Descending tracts

Answer 22

signals from brain to periphery (motor)
name indicates origin and target

Question 23

Corticospinal tracts (voluntary)

Answer 23

voluntary motor control
lateral (90%) and anterior (10%)
upper motor neurons, synapse onto spinal interneurons of spinal cord

Question 24

Corticospinal tracts pathway order

Answer 24

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)

Question 25

Brainstem tracts (involuntary) (4)

Answer 25

1. vestibulospinal (locomotor reflex and balance)
2. tectospinal (head and eye movement, direct toward sights and sound; terminates in cervical spinal cord)
3. reticulospinal (postural control and locomotion)
4. rubrospinal (locomotion; terminates in cervical and thoracic spinal segments - responsible for trunk and upper limbs)