Spinal Cord: Organization and Neural Systems (Dr. White)

Question 1

What’s in the ventral columns?

Answer 1

Bundles of axons organized into tracts or fasiculi.

Question 2

If a dorsal root nerve fiber is classified using Roman numerals, what does that tell you about the sensory information it carries?

Answer 2

Its info is coming from skeletal muscle proprioception (muscle spindles and Golgi-tendon organs)

Question 3

What type of fibers do muscle spindles use?

Answer 3

Ia

Question 4

What kind of information to Ia fibers carry?

Answer 4

Info from muscle spindles.

Question 5

What type of fibers do Golgi tendon bodies use?

Answer 5

Ib

Question 6

Are lower or higher Roman numeral fibers more heavily myelinated?

Answer 6

lower number Roman numeral fibers are more heavily myelinated (proprioception needs to be conducted quickly)

Question 7

What kind of information do Ib fibers carry?

Answer 7

info from Golgi tendon bodies

Question 8

If a dorsal root fibers are classified alphabetically, what does that tell you?

Answer 8

The fibers are carrying info from skin.

Question 9

What type of fiber is used for touch, vibration, and pressure?

Answer 9

A-beta

Question 10

What info is carried by A-beta fibers?

Answer 10

touch, vibration, pressure

Question 11

What type of fiber is used for pain and temperature from the skin?

Answer 11

A-delta, C

Question 12

What info is carried in A-delta and C type fibers?

Answer 12

temperature and pain (nociception)

Question 13

Where do Ia / Ib (proprioception fibers) go when they enter the spinal cord? (2 places)

Answer 13

They enter the dorsal horn - most are interested in synapsing in the ventral horn for reflexes. Some enter dorsal columns to provide info to the CNS.

Question 14

Where II and A-beta (touch, vibration, pressure) fibers go when they enter the spinal cord? (2 places, mainly)

Answer 14

Medial division ignores dorsal nerve -> goes up dorsal column to CNS.
Most form synapses in ventral horn for reflexes.

Question 15

Where do A- delta / C fibers (pain and temp.) go when they enter the spinal cord?

Answer 15

They love the dorsal horn.

Question 16

What’s the myelination of A-delta / C fibers like?

Answer 16

They’re small and thinly myelinated or unmyelinated.

Question 17

What’s the most important tract for voluntary muscle contraction? Where in the spinal cord is it located?

Answer 17

The corticospinal tract. (coming from the motor cortex). It’s dorsal and lateral.

Question 18

What different functions do alpha and gamma motor neurons serve?

Answer 18

Alpha motor neurons make muscle contract.

Gamma motor neurons increase the sensitivity of muscle spindles (especially when the muscle is contracting)

Question 19

If a motor neuron innervates a muscle in the trunk, were is its cell body going to be located in the ventral horn? What’s the pattern to remember? What does this mean for segments of spinal cord sending motor neurons to the limbs?

Answer 19

Medially. There’s a proximal -> distal gradient of muscle innervation where neurons to the proximal muscles have their cell bodies more medial. If sending motor axons to the limbs, the ventral horns will extend further laterally.

Question 20

How many neurons do you need to make a skeletal muscle contract? Where do these neurons have their cell bodies? Where do they synapse?

Answer 20

Two. Upper motor neuron that has its cell body in the primary motor cortex and synapses ventral horn. Lower (alpha) motor neuron that has cell body in the ventral horn and synapses at the NMJ.

Question 21

Where does the upper motor neuron (UMN) cross the midline?

Answer 21

In the pyramidal decussation aka. caudal medulla-spinal cord junction. (then it travels down the lateral corticospinal tract)

Question 22

What’s the pyramidal tract?

Answer 22

It’s the same thing as the corticospinal tract.

Question 23

Is the lower motor neuron’s cell body ipsilateral or contralateral to the muscle it innervates?

Answer 23

Ipsilateral. (Recall that the crossing over is done by the UMN at the pyramidal decussation.)

Question 24

What’s the significance of the reticulospinal tracts?

Answer 24

These carry fibers that innervate extensor muscles for postural support, bilaterally.

Question 25

What’s the significance of the vestibulospinal tracts?

Answer 25

Trunk, neck, upper limb muscles for posture and in response to vestibular stimuli (i.e. for balance).

Question 26

How many neurons do you need to sense something? What does each do?

Answer 26

3: Sensory neuron that synapse in brain stem / spinal cord (ipsilateral).
Nerve that crosses midline, synapse in thalamus. Nerve that enters the post-central gyrus.

Question 27

Where do sensory signals cross the midline?

Answer 27

Second neuron axons cross near their cell bodies.

Question 28

Where do dorsal root nerves from T5 and above send their axons? How about those from T6 and below? Where are these in the spinal cord cross-section?

Answer 28

T5 and above: Fasciculus cuneatus
T6 and below: Fasciculus gracilis
Both are dorsal and medial. Gracilis is more medial, and more gracile.

Question 29

Remember those sensory nerves that ignored the dorsal root and when straight up the spinal column? What kinds of nerves are they, and where is the cell body of the neuron they synapse with?

Answer 29

Proprioception (1a and 1b) and touch, vibration, pressure (A-beta fibers). They synapse in the medulla, and then the second nerve crosses over the midline then goes up to the thalamus.

Question 30

For afferent sensory axons carrying proprioception, touch, vibration, and pressure to the thalamus, where in the thalamus do they synapse?

Answer 30

In the ventral posterior lateral nucleus.

Question 31

What’s the Romberg test? (in the notes on slide 15)

Answer 31

See if patient with balance loses balance when eyes are closed. If positive, patient has problem with proprioception.

Question 32

What would a lesion to the left, medial-dorsal section of the spinal cord (low, beneath the medulla) cause?

Answer 32

Loss of proprioception, touch, vibration, pressure sensation on the left side. (recall these signals don’t cross the midline until the medulla)

Question 33

Where do sensory nerves for nociception / temperature (A-delta and C fibers) synapse?

Answer 33

Nearby in the dorsal horn (note that they ascend or descend 1 or 2 segments in Lissauer’s tract before synapsing).

Question 34

How does the 2nd of 3 neurons of pain and temperature sensation travel to the thalamus?

Answer 34

Through the spinothalamic tract to the ventroposterolateral nucleus (VPL).

Question 35

What would a unilateral lesion of the spinothalamic tract cause?

Answer 35

Contralateral loss of pain and temperature sensation. (Note that the nerves in this pathway cross the midline before entering the spinothalamic tract; unlike the proprioceptive / light touch sensory nerves that travel in the dorsal columns before crossing over in the medulla)

Question 36

Where is the spinothalamic tract in the spinal cord?

Answer 36

Anterolateral (it’s easier to see on a picture, such as on slide 18)

Question 37

Is proprioception conveyed to the cerebellum ipsilateral, contralateral, or both?

Answer 37

Exclusively ipsilateral.

Question 38

How does upper limb proprioception get to the cerebellum?

Answer 38

Dorsal root ganglion -> synapse in brain stem -> up the cuneocerebellar tract -> through the inferior cerebellar peduncle -> cerebellar cortex. Note there’s not midline crossing.

Question 39

How does lower limb proprioception get to the cerebellum?

Answer 39

Dorsal root ganglion -> synapse in Clarke’s nucleus -> up the dorsal spinocerebellar tract -> through inferior cerebellar peduncle -> cerebellar cortex. Note there’s no midline crossing.

Question 40

Gross anatomy review: Which spinal segments have sympathetics?

Answer 40

T1 - L3.

Question 41

What controls the pre-ganglionic sympathetics?

Answer 41

Neurons from the hypothalamus -> intermediolateral cell column.

Question 42

What do lesions of the Pons do to the bladder?

Answer 42

infantile bladder (this bladder stuff might make more sense later?)

Question 43

What do lesions between the pontine micturation center and the sacral cord do to the bladder?

Answer 43

spastic bladder

Question 44

What do lesions to the sacral cord or to the conus medullaris do the bladder?

Answer 44

flaccid or atonic bladder