PNS Learning Objectives Flashcards
What are the components of the PNS?
- 31 pairs of spinal nerves (+ ganglia)
- 12 pairs of cranial nerves (+ ganglia)
- ANS (+ ganglia)
What is the embryological sources for the CNS?
- Neural tube
What is the embryological sources for the PNS?
- Neural crest cells
What is the embryological sources for the sense organs (nose, ear, eye)?
- Ectodermal placodes
Which embryological source primarily gives rise to the PNS?
Neural crest cells
What are the different types of nerves? Sensory vs Motor
- Sensory (afferent): convey neural impulses to the CNS from the sense organs and receptors
- Motor (efferent/ EXIT): convey neural impulses from the CNS to effector organs, muscles, and glands
- Can be subdivided based on targets: somatic (body wall) and visceral (organs)
What are the different types of nerves? Cranial vs Spinal
- Cranial: bilateral pairs that exit the cranial cavity through foramina in the skull
- Spinal: bilateral pairs from a specific segment of the spinal cord and exit the vertebral column through the intervertebral foramina
Spinal nerve order
C 7
T 12
L 5
S 5
C 1
Where do spinal nerves exit the vertebral column?
Intervertebral foramina
Spinal nerves naming
- C1-C7 are named for the vertebra inferior to their exit
- C8 exits between C7 and T1
- The rest are named by vertebra superior to their exit
ventral/dorsal rootlets vs roots
- fibers emerge from spinal cord as rootlets
- rootlets converge to form 2 nerve roots
Ventral vs dorsal nerve roots
- ventral is MOTOR: fibers from cell bodies located in the anterior horn of the spinal cord
- dorsal is SENSORY: fibers from cell bodies located in the dorsal root (spinal) ganglion
ventral/dorsal spinal nerve
nerve roots converge to form spinal nerve (mixed sensory and motor)
What does a spinal nerve divide into?
anterior and dorsal ramus (both are mixed sensory and motor)
What does the anterior ramus innervate?
- anterior and lateral trunk
- upper and lower limb (form plexuses in limbs)
What does the posterior ramus innervate?
- deep back muscles
- skin over those muscles
- arteries supplying these structures
Where do anterior rami remain separated/segmented?
- thorax
- anterior abdominal wall
What does it mean that ventral rami merge to form a plexus in the neck, upper, and lower limbs?
A plexus is a network of fibers from multiple peripheral nerves and spinal nerve levels (has a functional advantage)
Which areas of the body are innervated by spinal nerves?
ONLY body wall and limbs
What do somatic sensory neurons innervate?
Skin, joins and muscle
Where are somatic sensory cell bodies of origin located?
1st order cell bodies are outside but close to CNS in dorsal root ganglion or ganglia associated with cranial nerves
How many neurons link the CNS to the target for the somatic sensory system?
1 neuron links CNS to target (distal/peripheral and proximal/central)
Does a synapse happen in
the dorsal root ganglion?
NO synapse in sensory dorsal root ganglion
What is a dermatome?
Unilateral area of skin innervated by the somatic sensory fibers of a single spinal nerve
C4
lateral neck and superior shoulder
C5
lateral arm
C6
lateral forearm and thumb (1st digit)
C7
middle and ring (3rd and 4th digits)
C8
little finger (5th digit) and medial hand
T1
medial forearm
T2
medial arm
T4
nipple
T10
umbilicus
Why does a lesion to one spinal nerve or posterior root not equate to a loss of sensation within a dermatome?
Fibers overlap adjacently to provide a back up of an area. Usually, at least 2 adjacent spinal nerves would have to be disrupted to produce a noticeable able area of numbness
Peripheral map vs Dermatome map
- A dermatome is an area of skin supplied by a single spinal nerve.
- Map of peripheral nerve fields over the body differs from the dermatomal distribution, because individual peripheral nerves are composed of multiple nerve roots
- PLEXUSES cause the two to differ.
Where are the cells bodies of somatic motor neurons located?
Located in a nucleus within the ventral horn of spinal cord or within nuclei of the brainstem
What do somatic motor neurons innervate?
From CNS to skeletal (voluntary) muscles
How many neurons link the CNS to the target for the somatic motor system, and how are they named?
ONE neuron between CNS & target organ (skeletal muscle) known as the lower motor neuron
What is the other name for visceral motor system?
autonomic nervous system
How many neurons link the CNS to the target for the visceral motor system?
2 neurons (named by their relationship to the ganglion they synapse at)
What do visceral motor neurons innervate?
Smooth muscles of organs, glands, modified cardiac muscle
What are the 2 divisions of the visceral motor system and what are their general functions?
- sympathetic: fight or flight (go everywhere, vasoconstriction, pupils dilate)
- parasympathetic: rest and digest (go to viscera of thorax, abdomen, and pelvis… dont go to limbs or body wall)
- NOTE: organs, major glands, and modified cardiac tissue will have dual motor control
- NOTE: simple structures will be sympathetic only (sweat glands and arteries)
What do visceral sensory neurons innervate?
Hallow organs and blood vessels from sensory receptors (subconscious visceral reflex: bp, blood pH, stretch)
Where are the cell bodies of origin located in visceral sensory neurons?
1st order cell bodies in dorsal root ganglion or specific cranial nerve ganglia
How many neurons link the CNS to the target for the visceral sensory system?
1 neuron system with 2 processes
Why can irritation of visceral sensory neurons result in referred pain to other areas of the body? How does the circuitry of somatic and visceral sensory neurons allow for referred pain?
Because dorsal root ganglia have cell bodies for somatic sensory and visceral sensory at certain levels. Visceral pain for heart will typically show in left arm.
Why can recovery nerve damage be difficult?
Recovery of nerves is not great because neurons do not proliferate in adult nervous system (except for olfactory epithelium)
Be able to describe the different types of nerve damage including which ones are more/less likely to be able to regenerate (and why).
- stretching nerve injury
- crushing nerve injury
- severed cut nerve injury
Stretching nerve injury
- Nerves can stretch to a certain degree, but when their limit is exceeded the nerve can be torn apart (bone fractures and dislocations)
- Recover depends on extent of stretch
Crushing nerve injury
- Usually from acute traumatic compression of a nerve
- Typically only damages the axons and often leaves the cell bodies and CT intact which helps guide regeneration of axons to their targets
Severed cut nerve injury
- Less likely to regenerate
- Sprouting begins at the cut ends, but the growing axons are less likely to reach their targets (CT covering is cut, axons dont have a guide to follow to their intended target)
- Might require surgical intervention to bring the cut ends of the nerve together
How does compression damage a nerve?
- likely to compromise the blood supply to the nerve (ischemia)
- result in permanent paresthesia (pins and needles sensation), numbness, and/or muscle weakness
- saturday night syndrome, carpal tunnel, IV disc herniation
- if not too prolonged, paresthesia can occur “foot falls asleep”
What is peripheral neuropathy? What are common causes of peripheral neuropathy?
- damage to one or more of the nerves outside of the brain & spinal cord and often results in numbness, pain, weakness in the hands or feet
- causes: diabetes, autoimmune, infections, medications, nerve injury, vitamin deficiencies
