Spinal Cord Anatomy Review

Question 1

What is the spinal cord, and where does it run?

Answer 1

The spinal cord is a column of nervous tissue that runs from the base of the skull to the L1/L2 vertebral level.

Question 2

What does the spinal cord and brain make up, and what is the function of spinal nerves?

Answer 2

The spinal cord and brain make up the central nervous system (CNS). Spinal nerves, part of the peripheral nervous system (PNS), carry most messages between the brain and the rest of the body.

Question 3

What are the three meninges that cover the spinal cord?

Answer 3

The three meninges that cover the spinal cord are:
• Dura Mater
• Arachnoid Mater
• Pia Mater

Question 4

How many vertebrae are there, and how are they distributed?

Answer 4

There are 33 vertebrae:
• 7 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral (fused by adulthood)
• 4 coccygeal (fused by adulthood)

Question 5

What vertebral segments have enlargements in the vertebral bodies?

Answer 5

Enlargements in the vertebral bodies exist at C5 – T1 and L2 – S2.

Question 6

What is the process of forming spinal nerves from motor and sensory rootlets?

Answer 6

• Motor (output) nerve rootlets branch out ventrally (anteriorly) to form motor nerve roots.
• Sensory (input) nerve rootlets branch out dorsally (posteriorly) to form sensory nerve roots.
• The ventral (motor) and dorsal (sensory) roots combine to form spinal nerves, which are mixed (motor and sensory).

Question 7

How many pairs of spinal nerves are there and what is their distribution?

Answer 7

There are 31 pairs of spinal nerves:
• 8 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 1 coccygeal

Question 8

What is the relationship between spinal cord segments and vertebral segments?

Answer 8

The vertebral column is longer than the spinal cord, so spinal cord segments do not correspond to vertebral segments, especially in the lower spinal cord. The discrepancy increases further down the spinal column.

Question 9

Where does the spinal cord end in an adult?

Answer 9

The spinal cord in an adult ends at the L1/L2 vertebral level, forming the Conus Medullaris.

Question 10

Where do spinal nerves exit relative to their corresponding vertebrae in the cervical and other regions?

Answer 10

• Cervical spine: Spinal nerves exit above the corresponding vertebra.
• Thoracic, lumbar, sacral, and coccygeal spines: Spinal nerves exit below the corresponding vertebra.

Question 11

What forms the Cauda Equina?

Answer 11

The nerves of the lower spinal cord exit the vertebral column much more caudally than their roots, forming the Cauda Equina.

Question 12

What does the spinal cord contain in cross-section?

Answer 12

The spinal cord in cross-section contains central grey matter (arranged in the shape of a “H”) and peripheral white matter (composed of ascending and descending tracts).

Question 13

What is found in the central grey matter of the spinal cord?

Answer 13

The central grey matter contains neuronal cell bodies and synapses, with regions including the dorsal horn, ventral horn, and intermediate substance.

Question 14

What is contained in the peripheral white matter of the spinal cord?

Answer 14

The peripheral white matter contains ascending sensory (afferent) and descending motor (efferent) fiber pathways, composed of concentrated myelinated fibers.

Question 15

What do efferent neurons do in the spinal cord?

Answer 15

Efferent neurons send axons into the ventral root. Their cell bodies are located in the ventral horn (somatic efferents – alpha and gamma) or in the intermediate substance (visceral efferents – preganglionic fibers of PaNS or SyNS).

Question 16

What are projection fibers responsible for in the spinal cord?

Answer 16

Projection fibers send axons into spinal white matter to travel to the brain, forming tracts associated with ascending spinal pathways.

Question 17

What do interneurons do in the spinal cord?

Answer 17

Interneurons remain in the spinal grey matter and are interposed between spinal input (from peripheral nerves or the brain) and spinal output (efferent neurons). They determine reflex responses of the spinal cord (spinal reflexes).

Question 18

How are ascending sensory pathways named?

Answer 18

Ascending sensory pathways usually start with “spino-“ and end with the name of the brain region where they first synapse.

Question 19

What is the function of the anterolateral system?

Answer 19

The anterolateral system carries pain, temperature, and crude non-discriminative touch (also itch).

Question 20

What are the two tracts in the anterolateral system?

Answer 20

The two tracts in the anterolateral system are the Anterior Spinothalamic Tract and the Lateral Spinothalamic Tract.

Question 21

What do descending pathways relay?

Answer 21

Descending pathways relay motor instructions down from the brain (efferent signals).

Question 22

How are descending pathways named?

Answer 22

Descending pathways are named by the region of the brain that gives rise to the fibers and end with the suffix “-spinal.” For example, the lateral corticospinal tracts begin in the cerebral cortex and descend the spinal cord.

Question 23

What type of muscle control is primarily involved in the lateral corticospinal tract?

Answer 23

The lateral corticospinal tract primarily involves voluntary muscle control, such as discrete, skilled movements.

Question 24

Where are the cell bodies of first order (upper motor) neurons located?

Answer 24

The cell bodies of first order motor neurons are located in the motor cortex of the frontal lobe (pre-central gyrus of the cerebral cortex).

Question 25

What is the decussation process for the majority of first order neurons in the corticospinal tract?

Answer 25

80% of first order motor neurons decussate in the medullary pyramids and descend the spinal cord contralaterally as the lateral corticospinal tract.

Question 26

What happens to the remaining 20% of first order neurons in the corticospinal tract?

Answer 26

The remaining 20% of first order neurons descend the spinal cord ipsilaterally as the anterior corticospinal tract and then decussate in the spinal cord.

Question 27

Where do first order motor neurons in the corticospinal tract synapse?

Answer 27

First order motor neurons synapse in the anterior (ventral) grey horn of the spinal cord with second order motor neurons (lower motor neurons).

Question 28

Where are the cell bodies of second order (lower motor) neurons located?

Answer 28

The cell bodies of second order motor neurons are located in the anterior (ventral) grey horn of the spinal cord.

Question 29

What do second order neurons (lower motor neurons) do?

Answer 29

Second order neurons (lower motor neurons) emerge from the spinal cord in the ventral spinal roots and travel within peripheral nerves to supply voluntary muscles.

Question 30

What do extrapyramidal pathways control?

Answer 30

Extrapyramidal pathways control involuntary skeletal movement like walking, writing, and automatic patterning movements.

Question 31

What are some major descending pathways of the extrapyramidal system?

Answer 31

Major descending pathways of the extrapyramidal system include:

• Rubrospinal tract (controls limb flexor muscles, originating from the red nucleus)
• Tectospinal tract (reflex responses to visual input)
• Vestibulospinal tract (mediates excitation of limb extensor muscles)
• Reticulospinal tract (controls reflex activities, muscle tone, and vital functions)
• Olivospinal tract (conveys information from cutaneous and proprioceptive organs)

Question 32

Where do first order neurons of extrapyramidal tracts originate?

Answer 32

First order neurons of extrapyramidal tracts originate in the reticular formation of the brainstem, which receives input from the cerebrum (motor cortex) and the cerebellum.

Question 33

How do first order neurons in the extrapyramidal system travel to the spinal cord?

Answer 33

First order neurons in the extrapyramidal system travel down the spinal cord ipsilaterally and decussate at the spinal cord level.

Question 34

Where are the cell bodies of second order neurons in the extrapyramidal system located?

Answer 34

The cell bodies of second order neurons in the extrapyramidal system are located in the anterior (ventral) grey horn of the spinal cord.

Question 35

What do second order neurons in the extrapyramidal system do?

Answer 35

Second order neurons in the extrapyramidal system travel in peripheral nerves towards their effectors, controlling involuntary muscle movement.

Question 36

What is a myotome?

Answer 36

A myotome is a group of muscles primarily innervated by the motor fibers of a single nerve root.

Question 37

What is the definition of a dermatome?

Answer 37

A dermatome is an area of skin supplied by nerve fibers originating from a single dorsal nerve root.

Question 38

What happens when one spinal nerve loses afferent nerve function?

Answer 38

Due to the overlap of innervation between adjacent dermatomes, the sensation is not usually completely lost, as adjacent spinal nerves still provide intact innervation.

Question 39

Where are preganglionic neurons of the sympathetic nervous system (SNS) located?

Answer 39

Preganglionic neurons of the SNS are located in the thoracic and lumbar segments (T1-L2,3) of the spinal cord.

Question 40

What is the role of postganglionic neurons in the sympathetic nervous system (SNS)?

Answer 40

Postganglionic neurons have long axons that leave the sympathetic ganglia, projecting onto visceral effectors and releasing the neurotransmitter norepinephrine.

Question 41

What do preganglionic fibers of the SNS synapse with?

Answer 41

Preganglionic fibers synapse with postganglionic neurons inside the sympathetic ganglia, which are located near the vertebral column.

Question 42

What is the sympathetic nervous system’s “thoracolumbar” division?

Answer 42

The sympathetic nervous system’s “thoracolumbar” division refers to the preganglionic neurons located in the intermediolateral cell columns (ICLs) of the spinal cord in the thoracic (T1-T12) and upper lumbar (L2 or L3) segments.

Question 43

What do the sympathetic preganglionic fibers travel through after leaving the spinal cord?

Answer 43

Sympathetic preganglionic fibers travel through the anterior roots, briefly passing through the anterior rami of spinal nerves T1-L2(3), before traveling to the sympathetic trunks via the white rami communicantes.

Question 44

Where are the presynaptic neurons of the parasympathetic nervous system (PSyNS) located?

Answer 44

The presynaptic neurons of the PSyNS are located in the medulla oblongata (cranial outflow) and the sacral spine (sacral outflow).

Question 45

How do presynaptic parasympathetic fibers function?

Answer 45

Presynaptic parasympathetic fibers travel to postsynaptic neurons, synapsing with them near or within the target organs, and release acetylcholine as the neurotransmitter.

Question 46

What is the role of cranial outflow in the parasympathetic nervous system (PSyNS)?

Answer 46

Cranial outflow consists of presynaptic parasympathetic neurons in the medulla oblongata that innervate structures of the head and neck via cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus).

Question 47

Where are the presynaptic parasympathetic neurons of the sacral outflow located?

Answer 47

The presynaptic parasympathetic neurons of the sacral outflow are located in the gray horn of the S2-S4 segments of the spinal cord.

Question 48

How do the presynaptic parasympathetic neurons of the sacral outflow travel?

Answer 48

The presynaptic parasympathetic neurons travel through the anterior roots of sacral spinal nerves S2-S4, then through the pelvic splanchnic nerves, synapsing with ganglia around the descending and sigmoid colon, rectum, and pelvic viscera.