Nervous System: Spinal Cord and Spinal Nerves

Question 1

What is the spinal cord?

Answer 1

• Approximately 3/4 of an inch in diameter and ranges between 16-18 inches in length
• Extends inferiorly from the medulla oblongata through the foramen magnum and the vertebral canal, ending at the inferior border of the L1 vertebrae
• 5 parts: cervical, thoracic, lumbar, sacral and coccygeal
• Associated with 31 pairs of spinal nerves that connect the CNS to receptors and effectors
• Spinal nerves are considered mixed nerves because they contain both motor and sensory axons
• It is shorter than the vertebral canal that houses it
• The inferior end is called the conus medullaris

Question 2

What are cauda equina? (sc)

Answer 2

• Axons that extend inferiorly from the conus medullaris

Question 3

What is filum terminale? (sc)

Answer 3

• The strand of pia matter within the cauda equina that attaches the conus medullaris to the coccyx

Question 4

What is the cervical enlargement of the spinal cord?

Answer 4

• Located in the inferior cervical part of the spinal cord and innervates the upper limbs (larger diameter)

Question 5

What is the lumbosacral enlargement of the spinal cord?

Answer 5

• Extends through the lumbar and sacral parts of the spinal cord and innervates the lower limbs (larger diameter)

Question 6

What are the spinal cords 2 longitudinal depressions?

Answer 6

• Posterior median sulcus and anterior median fissure

Question 7

What is the gray matter in the spinal cord?

Answer 7

• Centrally located and in the shape of an ‘H’
• Subdivided into anterior horns (somas of somatic motor neurons), lateral horns (somas of autonomic motor neurons), posterior horns (axons of sensory neurons and cell bodies of interneurons) and the gray commissure (unmyelinated axons for communication between sides)
• Made up of dendrites, cell bodies of neurons, unmyelinated axons and glial cells

Question 8

What is the white matter of the spinal cord?

Answer 8

• External to the gray matter
• Partitioned into 3 regions, each called funiculus (posterior, lateral and anterior)
• Also has an interconnected white commissure in the anterior funiculi
• Made up of all myelinated axons

Question 9

What are spinal nerves?

Answer 9

• 31 pairs that extend from the spinal cord to effector organs and sensory receptors to the spinal cord
• Contain 3 types of successive connective tissue wrappings : endoneurium, perineurium, and epineurium
• Anteriorly, multiple anterior rootlets arise from the spinal cord and merge to form a single ventral root which contains motor axons only
• Each anterior root has a posterior root (sensory axons only) to unite within the intervertebral foramen to become a spinal nerve
• All spinal nerves are mixed nerves because they contain both motor and sensory axons
• Numbered according to locations of the intervertebral foramen they use to exit

Question 10

Where do the first 7 spinal nerves exit?

Answer 10

• C1-C7

- Exit the intervertebral foramen above the vertebrae of the same number

Question 11

Where does the 8th spinal nerve exit?

Answer 11

• C8

- Exit above the first thoracic vertebra

Question 12

Where do the T1 to Co1 spinal nerves exit? (everything except cervical nerves)

Answer 12

• Exit below the vertebra of the same number they correspond with

Question 13

What do the spinal nerves do after exiting their vertebral foramen?

Answer 13

• Splits into branches called rami
• Posterior ramus: innervates the deep muscles and skin of the back
• Anterior ramus: innervates the anterior and lateral portions of the trunk and the upper and lower limbs
• Each ramus splits into multiple branches
• Many of the anterior ramus go to form nerve plexuses
• Additional ramus: rami communicantes, extend between the spinal nerves and the sympathetic trunk ganglia

Question 14

What dermatomes?

Answer 14

• Segments of skin supplied by a single spinal nerve
• All spinal nerves except C1 innervate a section of skin
• Anesthesia in a certain region could indicate specific spinal nerve damage
• Shingles causes blisters along dermatomes
• In referred visceral pain, pain in a dermatome may arise from an organ nowhere near that dermatome

Question 15

What are nerve plexuses?

Answer 15

• Networks of interweaving anterior rami of spinal nerves
• Plexuses split into multiple named nerves that innervate body structures
• Major plexuses include: cervical, brachial, lumbar and sacral

Question 16

What are intercostal nerves?

Answer 16

• Anterior rami of spinal nerves T1-T11 are intercostal nerves
• Travel in the spaces between adjacent ribs
• Besides spinal nerve T1, intercostal nerves do not create plexuses (T1 is part of the brachial plexus)
• T2: innervates intercostal muscles of the 2nd intercostal space and is sensory for axilla and the medial surface of the arm
• T3-T6: Innervate intercostal muscles and are sensory for the anterior chest wall
• T7-T12: Innervate intercostal muscles, abdominal muscles and overlying skin

Question 17

What are cervical plexuses?

Answer 17

• Left and right cervical plexuses are formed by anterior rami of spinal nerves C1-C4
• Branches of the plexus innervate anterior neck muscles and skin, also parts of the head and shoulders
• The phrenic nerve: originated primarily from C4 and some contributing axons of C3 and C5 (travels through thoracic cavity to innervate the diaphragm

Question 18

What are brachial plexuses?

Answer 18

• Left and right plexuses that innervate networks of nerves that supply the upper limbs
• Each is formed by the anterior rami of spinal nerves C5-T1
• Each innervates the pectoral girdle and the entire upper limb of one side
• Anterior rami of C5-T1 form the roots of brachial plexuses (all unite to form the trunk)
• Superior trunk: C5-C6
• Middle trunk: C7
• Inferior trunk: C8 and T1
• Portions of each trunk divide into anterior and posterior divisions to form 3 cords: posterior, medial and lateral cords
• 4 major terminal branches emerge from the cords: axillary, median, musculocutaneous, radial and ulnar nerves

Question 19

What are lumbar plexuses?

Answer 19

• Left and right plexuses are formed from anterior rami of spinal nerves L1-L4
• Subdivided into anterior and posterior divisions
• Main nerve of posterior division is the femoral nerve
• Main nerve of anterior division is the obturator nerve

Question 20

What are sacral plexuses?

Answer 20

• Left and right plexuses are formed from the anterior rami of spinal nerves L4- S4
• Lumbar and sacral are sometimes considered the lumbosacral plexus as a combined term
• Anterior rami are divided into anterior and posterior divisions
• The sciatic nerve is the longest and largest nerve in the sacral plexus and in the body (composed of tibial and common fibular divisions wrapped in a common sheath)
• 2 divisions of tibial and common fibular split just superior to the popliteal fossa
• Main branches of the sciatic nerve include: tibial, common fibular, deep fibular and superficial fibular nerves