Spinal cord

Question 1

What is the grey matter?

Answer 1

the central core of nerve cells in the spinal cord
posterior dorsal horn for sensory functions
anterior ventral horn for motor functions

Question 2

what is the white matter?

Answer 2

the fibre tracts surrounding the grey matter

posterior dorsal, lateral and anterior columns/ funiculi

Question 3

What are meninges?

Answer 3

spinal cord is surrounded by three layers of meninges
thick dura
fine arachnoid
surface covering of pia mater

Question 4

What is the sensory area of the spinal cord?

Answer 4

Dorsal
Sensory (afferent) fibres from dorsal root ganglion cells enter the spinal cord through the dorsal spinal nerve roots.

Question 5

What is the motor area of the spinal cord?

Answer 5

Ventral

Motor (efferent) fibres from motor neurons in the ventral horn leave the spinal cord in the ventral spinal nerve roots.

Question 6

What happens to the spinal cord during development?

Answer 6

The neural plate forms from the ectoderm at gastrulation.
The edges of the neural plate roll up to form the neural tube.
If this process fails, spina bifida results.

Sonic hedgehog from the notochord induces the ventral (motor) horn of the neural tube;
bone morphogenetic proteins (BMPs) from the adjacent ectoderm induce the dorsal (sensory) horn.

Segmental outgrowth of axons in peripheral nerve roots is patterned by the somites, which prevent axon growth through their posterior halves.

The spinal cord ‘moves’ upward within the vertebral canal as the body grows.
In adults, the end of the spinal cord (conus medullaris) is situated at the level of lumbar vertebrae L1-L2.

Question 7

How does grey matter vary along the length of the spinal cord?

Answer 7

The grey matter is largest at the cervical (C3-T2) and lumbar (L1-S2) enlargements.
This reflects the larger number of neurons which supply motor efferents to the limbs (ventral horn)
and sensory interneurons and projection neurons (dorsal horn), which are responsible for the innervation of the limbs.

Question 8

How does the white matter vary along the length of the spinal cord?

Answer 8

The amount of white matter is greatest at the cervical levels as ascending and descending fibres from all levels must pass through here
increases as more rostral
decreases as more caudal

Question 9

What is the thoracic cord?

Answer 9

The thoracic cord has smaller dorsal and ventral horns, but a more prominent lateral horn. The lateral horn of grey matter contains autonomic preganglionic neurons:
sympathetic between T1 and L2.

Question 10

What is the cervical cord?

Answer 10

In the cervical cord, the dorsal horns of grey matter are prominent, reflecting the importance of the hand as a sensory structure. The ventral horns are also large, with obvious large diameter motor neurons that supply muscles in the arm and hand.
The white matter tracts are larger than at any lower level.

Question 11

how do the ascending and descending tracts change in size

Answer 11

Sensory fibres are added to the cord from caudal to rostral, so the ascending tracts increase in size as one moves up the cord. Conversely, the descending tracts diminish from rostral to caudal as fibres within these descending tracts terminate in the grey matter.

Question 12

what is the spinal cord like at thoracic levels

Answer 12

grey matter is diminished, with a slender dorsal horn and small ventral horn reflecting the sparse innervation to the trunk.
The lateral horn is the site of preganglionic sympathetic neurons and the neurons of Clarke’s column; as a result it is very large

Question 13

why is there a lot of grey matter in the lumbar enlargement

Answer 13

due to the innervation of the lower limbs
The dorsal horns are at their largest, because most sensory information is concerned with reflexes, essential for keeping us upright.
The ventral horn has the largest motor neurons which have long axons to innervate the muscles of the foot and leg

Question 14

what are the main functional zones in the dorsal horn

Answer 14

An outer tract of fine unmyelinated fibres: Lissauer’s tract.
A marginal zone of grey matter where many nociceptive fibres end (lamina I);
The substantia gelatinosa which are interneurons (lamina II/III);
The ‘main sensory nucleus’ (nucleus proprius) - projection neurons which send axons which ascend the cord in the anterolateral column of grey matter (lamina IV/V).

Question 15

How is the ventral horn organised?

Answer 15

the motor neurons that supply a single muscle are grouped into “motor pools”, which may extend several segments of the cord. The motor neuron pools are arranged in vertical antagonistic pairs (e.g. biceps C5, 6; triceps C7, 8).

Question 16

How are motor neurons organised?

Answer 16

Motor neurons that supply axial muscles lie medially; those that supply distal limb muscles lie laterally.
Motor neuron pools supplying flexors lie dorsal to those that supply extensors, which lie more ventrally.
The motor neurons are associated with groups of excitatory and inhibitory interneurons which provide the largest part of the synaptic input onto the motor neurons

Question 17

what are the ascending sensory pathways?

Answer 17

dorsal columns - touch
anterolateral systems - pain and temperature
spinocerebellar tracts - proprioception

Question 18

what is the main descending system

Answer 18

corticospinal tract
It arises from both somatosensory and motor cortex and descends to the caudal medulla where most axons cross to thecontralateral side to form the lateral corticospinal tract.
A small lateral uncrossed component and a larger ventral uncrossed (anterior corticospinal tract) pathway.
In the spinal cord, the tract forms much of the lateral columns.

Question 19

what is meant by parallel processing

Answer 19

for each of these different somatosensory modalities there are differences in the spinal connectivity and the onward projections to higher centres. 
feature of many systems where it is advantageous to keep the information sensed by each receptor sub-class separate and therefore maximise the detailed information received by the brain.

Question 20

What is the spinocerebellar tract?

Answer 20

proprioception
situated in the lateral funiculus
one of the main ascending tracts
arises from cells of the intermediate grey matter
the larger diameter uncrossed dorsal spinocerebellar tract originates in neurons of Clarke’s column (thoracic nucleus)

Question 21

what is the anterolateral system

Answer 21

pain and temperature
Fine fibres, mediating nociception and temperature, enter the cord and send short ascending and descending collaterals in Lissauer’s tract, which run to adjacent spinal levels.
There is a complex network of interconnections between the cells of the substantia gelatinosa, and the afferent fibres supplying the marginal zone and main sensory nucleus (nucleus proprius).
Projection (second order) fibres from the marginal zone and main sensory nucleus (nuc. proprius) cross the midline as the anterior white commissure and ascend in the anterolateral column. The anterolateral column is therefore composed largely of crossed tracts
Many of these fibres terminate in the reticular formation of the medulla, pons and midbrain, but some ascend to the thalamus (lateral &anterior spinothalamic tract) for relay to the cerebral cortex.

Question 22

what are the dorsal columns?

Answer 22

touch
The dorsal root ganglia (DRG) lie outside the spinal cord and in the intervertebral foramen just central to where the dorsal and ventral nerve roots join. Large diameter (IA) axons from DRG cells enter the dorsal horn medially; fine diameter DRG axons lie more laterally.
Large fibres conveying proprioceptive and tactile information send branches which ascend in the dorsal columns, and other branches which synapse in the grey matter. These synapses are on neurons in the substantia gelatinosa and main sensory nucleus (n. proprius) of the dorsal horn. There are also synapses in the intermediate zone (on interneurons), and directly to motor neurons in the ventral horn (for the monosynaptic stretch reflex).
The ascending collateral branches of the primary afferent fibres (largely of mechanoceptors) form the dorsal columns (lower body in the gracile fasciculus and upper body in the cuneate fasciculus).
The dorsal columns are uncrossed tracts in the spinal cord; their fibres are somatotopically organised; they terminate on relay neurons of the dorsal column nuclei in the medulla. These relay neurons give rise to the medial lemniscus, which projects to the contralateral thalamus which, in turn, projects to the cerebral cortex.