Dermatomes, Myotomes, Plexuses

Question 1

CNS integration w/ peripheral somatic and autonomic systems:

Answer 1

(from left to right)

• joints, skin, skeletal muscle
• somatosensory fibers
• somatomotor fibers
• skeletal muscle

Question 2

Paraxial mesoderm gives rise to the ____ and ______. Somites give rise to ________ (cartilage), _______ (skeletal muscle), and _________ (dermis)

Answer 2

• head, somite
• sclerotome (cartilage), myotome (skeletal muscle), dermatome (dermis)

Question 3

part of the somite that gives rise to skeletal muscle and the dermis of the skin

Answer 3

dermomyotome

Question 4

• part of the somite
• cells that migrate anteriorly and gives rise to muscles of the limbs and trunk and associated dermis

Answer 4

hypaxial myotome

Question 5

• part of the somite
• cells that migrate posteriorly and give rise to the intrinsic muscles of the back and associated dermis

Answer 5

epaxial myotome

Question 6

The lateral horn innervates the _________ (viscermotor) system, while the anterior horn innervates _______ (motor) system.

Answer 6

• autonomic
• somatic

Question 7

The location where motor (anterior horn) and sensory (posterior horn) components combine from rootlets and rami

Answer 7

mixed spinal nerve (other names: true spinal nerve, spinal nerve proper)

Question 8

Injury to a dorsal root will lead only to ______ deficits/symptoms (e.g. ________), while injury to a ventral root will lead only to _____ deficits/symptoms (e.g. _______).

Answer 8

• sensory, paresthesia
• motor, weakness

Question 9

What is the structural order in which a signal is sent from the spinal cord?

Answer 9

Rootlets > roots > mixed spinal nerve (combination of anterior/posterior) > anterior (hypaxial) / posterior (epaxial) ramus

Question 10

When do limb buds start developing?

What segments of the spinal cord correlate with upper limb bud?

With lower limb bud?

Answer 10

• limb buds appear during week 4
• upper limb bud: C5-T1
• lower limb bud: L2-S2

Question 11

Why do the lower limb dermatomes have a barbical appearance?

What is the difference in upper and lower limb function caused by?

Answer 11

• dermatomes have a barbical appearance from rotation during development
• rotation creates contrasting functional compartments of upper/lower extremities (flexors at elbow are anterior, while flexors at knee are posterior; biceps brachi are anterior, while biceps femori are posterior)

Question 12

the area of skin supplied w/ afferent nerve fibers by a single posterior spinal nerve root

Answer 12

dermatome

(expaxial region supplies the skin superficial to spinal cord, all else is hypaxial)

Question 13

What is the difference between dermatome and nerve maps?

Answer 13

• dermatomes have only 1 dorsal (posterior) root that contributes to the specific area; clinical relevance: impingement/injury that effects a single segment can be ascertained w/ some degree of certainty by a dermatome map
• nerve maps have more than 1 dorsal root that contributes to the innervation of the area; clinical relevance: impingement/injury that effects a single nerve proper can be ascertained w/ some degree of certainty by a nerve map

*dermatome maps are not as precise as shown on maps and consist of segments/nerves w/ overlapping areas*

Question 14

What is the difference between spinal nerve plexuses and spinal nerve segments?

Answer 14

• plexuses: multiple spinal nerves that combine into one peripheral nerve to innervate a certain area (e.g. skin of limbs)
• segments: spinal nerves do not combine and supply a certain area of skin directly (e.g. skin of trunk)

Question 15

All the muscles that are innervated by a single efferent anterior root. Most muscles are composed of more than one:

Answer 15

myotome

(multiple myotomes for one muscle are more common, single myotomes are less common)

Question 16

The fact that one muscle may be composed of multiple dermatomes (i.e. one myotome may contribute to multiple muscles) means that impingement/injury to one segment will probably lead to:

Answer 16

weakness but not a complete loss of function

Question 17

What are the branches of the lumbosacral plexus to lower extremity?

Answer 17

• femoral N. (L2-4)
• obturator N. (L2-4)
• sciatic N. (L4-S3): tibial N. (L4-S3) and common fibular N. (L4-S2)
• lateral femoral cutaneous N. (L2-3)
• posterior femoral cutaneous N. (S1-3)