05_Sensory Nerve Fibers and Dermatomes Flashcards
Define dermatome
An area of skin innervated by a single afferent spinal nerve (dorsal root) or by afferents of CN V (Trigeminal)
Define spinal cord segment
The part of the spinal cord that encompasses a single spinal nerve (dorsal and ventral root)
What spinal cord segments innervate the leading edge of the arm and the leading edge of the leg? (the most distal parts of the body)
Arm - C7 (index and middle finger)
Leg - L4-L5 (great toe)
In order to have complete cutaneous anesthesia of a region, what would you have to do?
What’s the exception to this rule?
Complete cutaneous anesthesia requires the section of 3 consecutive dorsal roots.
Damage/destruction to just 1 dorsal root causes hypesthesia (reduced sensation).
One exception is C2 dorsal root, which when destroyed causes complete anesthesia of occipital region of the scalp.
Why doesn’t C1 or the coccygeal spinal nerve have dermatomes?
C1 and coccygeal spinal nerve do not have dorsal roots.
Landmarks of dermatomes:
V1
V2
V3
V1 - nose, orbital area, and ventral lateral skull
V2 - upper lip, maxilla, and temporal region
V3 - lower lip, mandibula, and ventral ear
Landmarks of dermatomes:
C2
C4
C2 - occiput
C4 - posterior neck and upper shoulder
Landmarks of dermatomes: C6, C7, & C8 - C6 - C7 - C8
C6, C7, C8 - hand
- C6 - Thumb
- C7 - Index and middle
- C8 - Ring and pinky
Landmarks of dermatomes:
T1
T4
T5
T1 - upper thorax and ant. surface of upper extremity
T4 - nipple zone (Nipple line)
T5 - Inf angle of scapula
Landmarks of dermatomes:
T10
L1
T10 - umbilical zone (umbilical line)
L1 - inguinal region
Landmarks of dermatomes:
L4
L5
L4 - lateral thigh, ant surface of knees, medial leg, and great toe
L5 - Middle three toes, sole of foot (sometimes great toe)
Landmarks of dermatomes:
S2
S5 (What’s special about S5?)
S2 - Genitalia and back of thigh
S5 - perianal region
- The lowest dermatome from the spinal cord. If you have sensation in this region, it is a sign that there is still sensory function from the brain.
Ia afferent
- Fiber diameter
- Conduction velocity
Fibers - large (12-20)
Conduction - fastest (70-120)
Ia afferent
- Function
Muscle spindle - primary afferents (annulospiral ending)
- Primarily nuclear bag, but also nuclear chain fibers
- Heavily myelinated
Ib afferent
- Fiber diameter
- Conduction velocity
Fibers - large (12-20)
Conduction - 2nd fastest (70-120)
Ib afferent
- Function
Golgi tendon organ - primary afferent
- Heavily myelinated
II (2) afferent
- Fiber diameter
- Conduction velocity
Fibers - medium (5 -14)
Conduction - medium (30 - 70)
II (2) afferent
- Function
Muscle spindle - secondary afferent (flower spray ending)
- Primarily nuclear chain fibers
- Myelinated
A-beta afferent
- Fiber diameter
- Conduction velocity
Fibers - medium (5-14)
Conduction - medium (30 - 70)
A-beta afferent
- Function
Lower threshold cutaneous mechanoreceptors
- Touch, pressure, 2-point discrimination, vibration
- Myelinated
A-delta afferent (Group III)
- Fiber diameter
- Conduction velocity
Fibers - small (1 - 7)
Conduction - slow (12 - 30)
A-delta afferent (Group III)
- Function
Higher threshold cutaenous mechanoreceptors (nociceptors)
- lightly myelinated
C fiber afferent (Group IV)
- Fiber diameter
- Conduction velocity
Fibers - smallest (0.5 - 1.5)
Conduction - slowest (0.5 - 2)
C fiber afferent (Group IV)
- Function
High threshold cutaneous and joint afferents (nociceptor)
- Unmyelinated
