Spinal cord

Anatomy

The spinal cord is part of the central nervous system (CNS). It is situated inside the vertebral canal of the vertebral column. During development, there’s a disproportion between spinal cord growth and vertebral column growth. The spinal cord finishes growing at the age of 4, while the vertebral column finishes growing at age 14-18. This is the reason why, in adults, the spinal cord occupies only the upper two thirds of the vertebral canal. 

The spinal cord is a continuation of the brainstem. It extends from the foramen magnum at the base of the skull to the L1/L2 vertebra where it terminates as the conus medullaris (medullary cone). A thin thread called filum terminale extends from the tip of the conus medullaris all the way to the 1st coccygeal vertebra (Co1) and anchors the spinal cord in place.

You can easy remember the extent of the spinal cord with a mnemonic 'SCULL', which stands for ' Spinal Cord Until L2 (LL)'.

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Throughout its length, the spinal cord shows two well defined enlargements to accommodate for innervation of the upper and lower limbs: one at the cervical level (upper limbs), and one at the lumbosacral level (lower limbs).

Like the vertebral column, the spinal cord is divided into segments: cervical, thoracic, lumbar, sacral, and coccygeal. Each segment of the spinal cord provides several pairs of spinal nerves, which exit from vertebral canal through the intervertebral foramina. There are 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair of spinal nerves (a total of 31 pairs).

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Spinal cord tracts

Spinal cord neural pathways are found within the spinal cord white matter. On each side, the white matter is divided into three funiculi: anterior, lateral, and posterior.

Ascending tracts convey information from the periphery to the brain. On the other hand, the descending tracts carry information from the brain to the periphery. The spinal cord is more than just a conduit, as it also modifies and integrates the information that pass through it.

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Spinal nerves

Spinal nerves are grouped as cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), sacral (S1-S5), and coccygeal (Co1), depending from which segment of the spinal cord they extend.

Segmentation of the spinal cord corresponds to the intrauterine period in which the spinal cord occupies the entire vertebral canal. For this reason in adulthood, where the vertebral column is longer than the cord, each spinal cord segment is located higher than its corresponding vertebra. These differences become more obvious distally towards the lumbar and sacral segments of the spinal cord–for example spinal cord segment L5 is at the level of the L1 vertebra. 

Spinal nerves, however, exit the vertebral column at their correspondly numbered vertebrae. Cervical spinal nerves exit through the intervertebral foramina directly above their corresponding vertebrae, whilst thoracic, lumbar and sacral spinal nerves exit directly below.

In order for the more distal spinal nerves to exit they must first descend through the vertebral canal. Since the lumbar and sacral spinal nerves are the farthest from their intervertebral foramina, they are the longest. While descending towards their corresponding intervertebral foramina, lumbosacral spinal nerves form a bundle called the cauda equina (meaning horse’s tail).

Each spinal nerve has an anterior and posterior root. Anterior roots transmit motor information, and they originate from the anterior horns of the gray matter and exit the spinal cord through the anterolateral sulcus. The posterior roots transmit sensory information and have sensory ganglion attached to them. They originate from the posterior horns of gray matter and exit through the posterolateral sulcus of the spinal cord.

The anterior and posterior roots merge just before the intervertebral foramen, and form the trunk of the spinal nerve. The trunk is very short, and soon after exiting the vertebral column, it divides into four branches: anterior ramus, posterior ramus, communicating ramus, and meningeal ramus.

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Reflex arc

A huge part of spinal cord function is under the influence of the brain, as it functions to relay information to and from the periphery. But there are many reflexes that are generated in the spinal cord independently from the brain. Spinal reflexes are either monosynaptic or polysynaptic.

Monosynaptic reflexes play out with only two neurons participating in the reflex arc, one sensory and one motor. The first-order neuron (sensory) is in the spinal ganglion, while the second-order neuron (motor) is in the anterior horn of the spinal cord). The sensory neuron gathers impulses from the muscle and sends this information to the motor neuron which innervates the same muscle. The motor neuron then causes contraction of the innervated muscle. An example of a monosynaptic reflex is the stretch reflex.

Polysynaptic reflexes on the other hand have multiple neurons participating. Besides one sensory and one motor neuron, there are also one or more interneurons between them making this communication indirect. They are more complex than monosynaptic reflexes as they involve muscle groups instead of a single muscle. An example is the withdrawal reflex.

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Related articles

Articles within this topic:

• Ascending and descending tracts of the spinal cord
• Ascending tracts of the spinal cord
• Blood supply of the spinal cord
• Cervical enlargement
• Conus medullaris (medullary cone) and cauda equina
• Extrapyramidal system
• Gate theory and pain pathway
• Neural pathways
• Neurological examination of the motor system
• Peripheral nervous system
• Posterior column - Medial lemniscus pathway
• Posterior median sulcus
• Reticulospinal tract
• Spinal nerves
• Spinothalamic tract
• Stretch reflex
• Sympathetic nervous system
• Withdrawal reflex