0729 - TEXT Spinal Cord Pathways

Question 1

What are the key features of the Gross anatomy of spinal cord?

Answer 1

Nerves (pairs at each level) - defined by level of exit of vertebrae
Cervical and lumbar enlargements
Grey and white matter
Conus medullaris
Filum terminale
Cauda equina

Question 2

What are the Spinal cord coverings?

Answer 2

Bony - vertebral coverings
Meninges - Dura/arachnoid/pia mater. with dentate ligs (thickening of pia) anchoring spinal cord into dural sheath (dura mater). Filum terminale (extension of pia) anchoring spinal cord to end of vertebral column.
Cushioned by CSF in arachnoid.

Question 3

What do you see in a spinal cord cross section?

Answer 3

Posterior median sulcus, anterior median fissure
Nerve roots attach at posterolateral sulcus either side of posterior median sulcus.
Mixed spinal nerve splits as it approaches cord - sensory goes in dorsally, motor comes out ventrally.
Grey and white matter.

Question 4

What is a fasciculus?

Answer 4

A group of neurons travelling from or going to similar locations.

Question 5

What are the functional divisions of grey matter? How can inputs and outputs arise?

Answer 5

Dorsal horn (input), intermediate grey (processing), ventral horn (output).
Inputs can be peripheral, intrasegmental, descending, or intrasegmental.
Outputs can be peripheral (innervating somatic/visceral depending on whether more or less down the horn) or ascending.
Proximal muscles innervated by medial areas of ventral horn, distal by lateral areas.

Question 6

Outline the segmental organisation of White and grey matter in the spinal cord:

Answer 6

Outline the segmental organisation of White and grey matter in the spinal cord:
Posterior dorsal funiculus -
Lateral funiculus
Anterior/ventral funiculus
Grey - dorsal horn, intermediate grey and ventral horn. Lateral horn (predominantly thoracic).
Large fibres vs small fibres synapse at different layers. (options for fibres)

Question 7

What are Rexed’s Laminae?

Answer 7

Divisions of the Grey matter based on cell types and thus functional characteristics. Different laminae are found on different levels (I-VI, X found on all levels). Starts with layer I at dorsal horn (just after entry), proceeding ventrally and increasing in size (generally) to VIII at ventral horn. IX forms ‘islands’ in VII and VIII, with X surrounding the central canal.

Question 8

How are the dorsal columns mapped?

Answer 8

Leg, Hip, Trunk, Arm - medial to lateral.

Question 9

How is the grey matter mapped (somatic and visceral as well as body region)?

Answer 9

Somatic/visceral - Sensory more dorsal, motor more ventral. Somatic/visceral/visceral/somatic occupying from dorsal horn, through lateral horn to ventral horn.
Body region - Medial-lateral is proximal-distal - e.g. extensors of trunk, shoulder, arm, forearm, hand in dorsal horn.

Question 10

Where is the marginal zone in terms of Rexed’s Laminae and spinal level?

Answer 10

All levels, laminae I. Contains some spinothalamic tract cells.

Question 11

Where is the substantia gelatinosa in terms of Rexed’s Laminae and spinal level?

Answer 11

All levels, laminae II. Modulates transmission of pain and temperature information.

Question 12

Where is the body of the posterior horn in terms of Rexed’s Laminae and spinal level?

Answer 12

All levels, laminae III-VI - Sensory processing.

Question 13

Where is the Clarke’s Nucleus in terms of Rexed’s Laminae and spinal level?

Answer 13

T1-L2, Laminae VII. Contains posterior spinocerebellar tract cells.

Question 14

Where is the intermediolateral column in terms of Rexed’s Laminae and spinal level?

Answer 14

T1-L3, Lamina VII. Contains preganglionic SY neurons.

Question 15

Where is the sacral parasympathetic nucleus in terms of Rexed’s Laminae and spinal level?

Answer 15

S2-S4, Lamina VII. Contains preganglionic parasympathetic neurons to the pelvic viscera.

Question 16

Where is the Accessory Nucleus in terms of Rexed’s Laminae and spinal level?

Answer 16

Medulla-C5, lamina IX. Contains motor neurons to SCM and trapezius.

Question 17

Where is the Phrenic nucleus in terms of Rexed’s Laminae and spinal level?

Answer 17

C3-5, lamina IX. Contains motor neurons to diaphragm.

Question 18

Describe the Stretch (myotatic reflex) with reference to all neurons involved.

Answer 18

Tap patellar tendon to stretch quads. Stretch is sensed by afferent fibres from muscle spindles (Group Ia proprioceptors). Large myelinated axons with a very fast rate of conduction. Ia fibres synapse directly on alpha motor neurons innervating the same muscle group as that stretched, contracting to counteract the passive stretched.

Question 19

Describe Reciprocal inhibition with reference to all neurons involved.

Answer 19

The stretch information carried by Ia afferent fibres also passes onto interneurons in lamina VI. These inhibit the alpha motor neurons innervating the opposing muscles (e.g. hamstrings), so that affected muscle can act properly.

Question 20

Describe the Autogenic inhibition (reverse myotatic reflex) with reference to all neurons involved.

Answer 20

Golgi tendon organs are a second class of proprioceptors.  Located in muscle tendons and sense tension.  Information is transmitted via large, fast 'Ib' afferents.  Ib inhibit agonist muscles (and excite antagonist muscles via alpha motor neurons).
Sudden relaxation of muscle upon high tension (protect from a tear)

Question 21

Describe the Withdrawal reflex with reference to all neurons involved.

Answer 21

Consists of flexor and crossed extensor reflexes. Eg when stepping on then withdrawing from a tack.
Flexor - initiated by cutaneous receptors, involving many levels of spinal cord to coordinate flexor stimulation across the limb and reciprocal inhibition of extensors.
Crossed-extensor reflex - flexor reflex accompanied by the simultaneous and opposite pattern in other lumb, so it can support body’s weight.

Question 22

How does clasp knife spasticity arise?

Answer 22

When bending the extensors, poorly-controlled myotatic (1a) and autogenic inhibition (Ib) reflexes don’t behave properly due to UMN lesion. The stretch reflex is prolonged and exaggerated, but eventually overcome by a prolonged and exaggerated reverse myotatic reflex

Question 23

What muscle group, spinal level, and peripheral nerve is tested by the Jaw Jerk reflex?

Answer 23

Masseter, brainstem, V3 nerve

Question 24

What muscle group, spinal level, and peripheral nerve is tested by the Biceps reflex?

Answer 24

Biceps brachii, C5, Musculocutaneous nerve

Question 25

What muscle group, spinal level, and peripheral nerve is tested by the Brachioradialis reflex?

Answer 25

Brachioradialis, C6, radial nerve

Question 26

What muscle group, spinal level, and peripheral nerve is tested by the Triceps reflex?

Answer 26

Triceps brachii, C7, radial nerve

Question 27

What muscle group, spinal level, and peripheral nerve is tested by the Knee jerk/patellar reflex?

Answer 27

Quadriceps, L4, femoral nerve

Question 28

What muscle group, spinal level, and peripheral nerve is tested by the Ankle-jerk/achilles reflex?

Answer 28

Gastrocnemius/soleus, S1, tibial nerve

Question 29

What are the main Motor/descending pathways?

Answer 29

Motor/descending
Corticospinal (movement) - Conscious movement - Lateral and ventral.
Rubrospinal (motor) - support cortico-spinal, conscious movement.
Lateral (medullary) and medial (pontine) reticulospinal - locomotion and posture control.
Vestibulospinal - balance control
Tecto-spinal - Head positioning reflexes.

Question 30

What are the main sensory/ascending pathways?

Answer 30

Sensory/ascending
Dorsal columns (tactile and conscious proprioception)
Lissauer’s tract (inputs to adjacent segments).
Fasciculus proprius - intersegmental tract.
Spino-cerebellar - non-conscious proprioception.
Spino-olivary - motor learning
Spino-tectal (eye and head positioning reflexes)
Spino-thalamic (pain/temperature)
Spino-reticular (arousal and affective) - similar to spino-thalamic.

Question 31

Outline (fibre types, position in white matter, information carried, and crossing and synapse) the spino-thalamic tract

Answer 31

Carries information about pain and temperature to the thalamus. Has anterior (crude touch/pressure) and lateral (pain/temp) components which ‘hug’ anterior horn.

Originates from C/A-delta type fibres in the lateral part of the dorsal root. Ascends a few levels in Lissauer’s tract, before synapsing in substantia gelatinosa. They immediately cross, before splitting into anterior and lateral tracts in lateral funiculus. Enter brainstem in reticular formation and synapse onto third-order neurons in the ventral posterolateral nucleus of the thalamus.

Question 32

Outline (fibre types, position in white matter, information carried, and crossing and synapse) the Dorsal column system

Answer 32

Carries conscious touch, pressure, vibration and proprioception to the thalamus. Has gracile (legs) and cuneate (arms) fasciculus.

Originates from large A-beta type fibres. Fibres enter dorsal horn, ascend to caudal medulla and synapse on gracile or cuneate nuclei and cross to enter contralateral medial lemniscus. Synapse in ventral posterolateral nucleus of thalamus, through internal capsule, and terminate at contralateral sensory cortex.

Question 33

Outline (fibre types, position in white matter, information carried, and crossing and synapse) the dorsal spino-cerebellar tract

Answer 33

Originates with large, myelinated A-alpha type fibres in lower limb. Carries unconscious proprioceptive information about muscle stretch (Golgi tendon organs AND muscle spindles) from lower limb.
Fibres enter dorsal horn and synapse on Nucleus dorsalis at base of dorsal horn (Lamina VII). Secondorder fibres enter lateral funiculus near dorsal root, and form dorsal spino-cerebellar tract. In upper medulla, the tract enters inferior cerebellar peduncle and terminates in ipsilateral cerebellar cortex. They do not cross.

Question 34

Outline (fibre types, position in white matter, information carried, and crossing and synapse) the cuneo-cerebellar tract

Answer 34

Originates with large, myelinated A-alpha type fibres in upper limb. Carries unconscious proprioceptive information about muscle stretch (Golgi tendon organs and muscle spindles) from upper limb.
Fibres enter dorsal horn (C7-C1), ascend directly, and synapse in accessory cuneate nucleus in medulla. Second-order fibres enter inferior cerebellar peduncle (together with dorsal spino-cerebellar fibres) and terminate in ipsilateral cerebellar cortex. They do not cross.

Question 35

Outline (fibre types, position in white matter, information carried, and crossing and synapse) the Ventral spino-cerebellar tract

Answer 35

Originates with large, myelinated A-alpha type fibres in lower limb (JUST Golgi Tendon Organs).
Fibres enter dorsal horn, synapse in base of dorsal horn (Lamina VI). Second-order fibres cross midline, enter ventral portion of lateral funiculus and form ventral spino-cerebellar tract. In the pons, they enter superior cerebellar peduncle, and re-cross to terminate in ipsilateral cerebellar cortex. They cross TWICE to return to ipsilateral.

Question 36

Outline (fibre types, position in white matter, information carried, and crossing and synapse) the cortico-spinal tract.

Answer 36

Send voluntary motor commands from motor cortex to skeletal muscle.
First-order fibres descending from PMC form pyramids of the medulla. 80% of them will cross in the pyramidal decussation, where they will enter lateral funiculus and form lateral cortico-spinal tract, together with half the uncrossed fibres. They will synapse directly onto motor neurons in ventral horn of spinal cord. The remaining 10% of fibres from the ventral corticospinal tract, and cross at their level of synapse.
80% cross at decussation. 10% cross at level. 10% don’t cross.

Question 37

Where does the medulla end?

Answer 37

Pyramidal decussation.