Organization of Spinal Pathways

Question 1

Gamma Motor neurons innervate what structures?

Answer 1

Contractile ends of muscle spindles

Question 2

What is/are the functions of the muscle spindles?

Answer 2

Regulate muscle tone, protect against overstretching of the muscles

Question 3

What is alpha-gamma coactivation?

Answer 3

Simultaneous contraction of extrafusal muscle fibers (by alpha motor neurons) and intrafusal muscle spindles (by gamma motor neurons)

Question 4

Intersegmental connections that coordinate reflexes among various muscle groups travel through what structure?

Answer 4

Propriospinal tract

Question 5

Give the steps of the posterior column-medial lemniscal system.

Answer 5

Cutaneous and kinesthetic information from DRG enters the spinal cord and ascends the posterior column to the brain stem. In the medulla the fibers decussate and ascend in the medial lemniscal to the ventroposterolateral nucleus. VPL projects to S1.

Question 6

What are the four pathways of the anterolateral system.

Answer 6

Spinoreticular, spinomesencephalic, spinotectal, spinothalamic.

Question 7

Describe the anterolateral system pathway:

Answer 7

Thin fibers from dorsal root ganglia terminate in Rexed laminae 1,2. Neurons in layers 1,4,5 send axons contralaterally to ascend the anterolateral tract. Divides into four pathways, neurons in ventroposterolateral and intralaminar thalamus nuclei project to the somatosensory and anterior cingulate cortex respectively.

Question 8

Describe the spinocerebellar pathway:

Answer 8

Information from joint receptors, muscle spindles, and Golgi Tendon Organs enter the spinal cord and terminate in Rexed laminae VI, VII (Dorsal Nucleus of Clarke), VIII. Dorsal Nucleus of Clarke projects to the posterior spinocerebellar tract and then to the ipsilateral cerebellum. Laminae V, VI, VIII send axons across the midline to ascend in the anterior spinocerebellar tract. Afferents from arm/leg muscles ascend the fasiculus cuneatus/gracilis and terminate in the nucleus cuneatus/gracilis. These nuclei send axons to the cerebellum through the cuneocerebellar tract.

Question 9

Describe the corticospinal pathway:

Answer 9

Motor Cortex sends descending axons through the internal capsule, crus cerebri and pyramidal tracts. The fibers decussate at the rostral spinal cord and descend to their appropriate spinal segment where they synapse with the lower alpha motor neuron.

Question 10

Where do the cell bodies of fibers in the anterior corticospinal tract originate?

Answer 10

Cells in ipsilateral motor cortex (that did not decussate at the spinomedullary junction)

Question 11

Where do cell bodies of fibers in the lateral corticospinal tract originate?

Answer 11

Contralateral motor cortex (fibers decussate at spinomedullary junction)

Question 12

Lesions in the lateral corticospinal tract result in what type of symptom?

Answer 12

Paresis, ie: hypertonicity and rigid muscles, resistance to passive limb movements, hyperactive reflexes, clonus, clasp-knife phenomenon, Babinski sign

Question 13

What are the symptoms of lower motor lesions?

Answer 13

Paralysis: Muscular fasciculations from acetylcholine release, loss of muscle tone, loss of spinal reflexes, atrophy

Question 14

Which spinal pathway mediates flexor movements of the limb? Describe it

Answer 14

Rubrospinal pathway. Begins at the red nucleus, crosses midline, descends in the rubrospinal tract, terminates in Rexed Laminae V, VI, VII

Question 15

Which spinal pathway mediates head and neck movements? Describe it

Answer 15

Tectospinal pathway: Begins in the superior colliculus, crosses the midline, descends in the tectospinal tract, terminates in Rexed laminae VI, VII, VIII

Question 16

Which spinal pathway innervates muscle spindles? Describe it:

Answer 16

Reticulospinal pathway, Pontine reticulospinal fibers descend ipsilaterally, Medullary fibers descend bilaterally in the anterior funiculus and terminate in Rexed Laminae VII and IX.