Spinal Cord
Structure
Starts at the foramen magnum – the very bottom bit of the of the brainstem. Stops by L1, giving off 31 pairs of spinal nerves before that:
- 8 Cervical
- 12 Thoracic
- 5 Lumbar
- 5 Sacral
- the Coccygeal
The spinal nerves consist of an anterior (or ventral) root that innervates the skeletal muscles, and a posterior (or dorsal) root that carries sensation from the skin, muscles and organs.
With the dorsal root fibres, they have their cell bodies in the dorsal root ganglion outside the spinal canal.
The spinal cord itself is white matter, with grey matter located in the centre of the cord, containing the cell bodies of the neurones.
Tracts
There are several pathways in the spinal cord, called tracts, that contain motor or sensory neurons. Let's look at the motor ones first:
Motor
Corticospinal tract (pyramidal tract)
This one is the main motor tract, important for voluntary movements, linking to the motor cortex.
- 10% of its fibres stay on the same side in the cord as in the brain, and are found in the anterior corticospinal tract, at the front, medially, next to the groove.
- 90% of its fibres swap to the opposite side in the medulla oblongata, meaning the left hand brain controls a lot of right hand movement. These fibres are those of the lateral corticospinal tract, more dorsally, lateral to the grey matter.
The other tracts (extra-pyramidal)
These are the main two:
- rubrospinal - arms and legs
- vestibulospinal - posture, starts as cranial nerve XII
They are less important, so let's hope they don't come up, eh?
Sensory
Dorsal Columns
These are at the back, medially. They are the pathways for proprioception, fine touch and vibration senses. They cross over (decussate) in the brainstem.
Spinothalamic tracts
Two divisons, both of which cross over as soon as they enter the spinal cord.
- Lateral carries pain and temperature
- Anterior carries simple touch and pressure