Peripheral Nerves Flashcards
Peripheral nervous system composed of:
- cranial nerves
- spinal nerves
Motor components of PNS
- Somatic motor system
- Visceral motor system
Somatic motor system:
- motor nerves
- effect skeletal muscle
Visceral motor system:
- Autonomic ganglia and nerves
- effect smooth, cardia muscles and glands
Sensory components of the PNS
- sensory ganglia and nerves
- sensory receptors
Number of cranial nerves in PNS
- above spinal cord
- 12 pairs
- usually all “special”
Number of spinal nerves in PNS:
- 31 pairs
- cord down segmentally
Subdivision of spinal nerves:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
Spinal cord segment contains:
- pair of dorsal root
- pair of ventral root
Gray matter in the spinal cord:
- butterfly region
- contains the neuron cell bodies of the ventral root
White matter in the spinal cord:
- bundles of axons organized into tracts (or fasciculi)
- white because of myelin
Dorsal root
- afferent (sensory)
- posterior
- cell bodies contained in the dorsal root ganglion
- contains axons from dorsal root ganglion cells will synapse in the dorsal horn
- derived from NCC
Dorsal root gangion:
- contain cell bodies of the dorsal root
- because their embryonic origin is NCC they lie outside the spinal cord
- no synapses in the ganglia (just collection of cell bodies)
- pseudounipolar
Spinal nerve
-point at which the ventral and dorsal root meet (IE: contains 2 nerve fibers)
- divides into two branches (ramus)
1) ventral ramus
2) dorsal ramus - Both contain motor AND sensory fibers
Ventral root
- efferent (motor)
- anterior
- only contains axons (cell bodies in the gray matter of the spinal cord)
- the axons of ventral horn motor neurons synapse at neuromuscular junctions or at autonomic ganglia (think of sympathetic/parasympathetic)
- derived from neural tube
proprioception
muscle position sensing
Innervation of skin
-most peripheral fibers end in the dermis (some lower part of epidermis)
EXAMPLES
- pacinian corpuscle
- meissner corpuscle
- merkel’s disks
- free nerve endings
- ruffini’s corpuscle
Dermatome
- slice of skin that is innervated by cutaneous branches of the spinal nerves
- 30 (b/c no C1 dermatome)
Myotome
- a segment of skeletal muscle supplied by branches of a spinal nerve
- two parts: epimere and hypomere
Epimere
deep muscles of the back
Hypomere
intercostal muscles
abdominal muscles
limb muscles
Dorsal ramus:
- supply epimere/skin covering it and joints between vertebrate
- smaller
- not many specific names
- innervates skin of back, joints between vertebrate
Ventral ramus:
- supply hypomere and skin
- many specific names
- widespread distribution
- more motor/sensory fibers than dorsal
- innervates skin, muscles, and joints of body wall and limbs
Cutaneous nerve:
- supplies specific dermatome
- has BOTH fiber types
Muscular nerve:
- supply specific myotome
- has BOTH fiber types
Trunk innervation:
individual dorsal and ventral rami innervate overlapping dermatomes
Limb innervation:
adjacent ventral rami may join in a plexus to form nerves that supply overlapping dermatomes and myotomes
Dorsal Horn:
mostly axons of sensory neurons
Ventral horn:
mostly cell bodies of motor neurons
SNS and ANS comparison:
- SNS is voluntary
- SNS innervates the body wall, ANS the viscera
- SNS innervates skeletal muscle, ANS cardiac/smooth
- SNS is one nerve system, ANS is two nerve system
- different NT/receptors
- ANS divided into two components
Autonomic Nervous System
(synonymous to visceral motor system)
concerned with the motor innervation of smooth, cardiac muscle and glands
TWO DIVISIONS:
- sympathetic (works with the body)
- parasympathetic (doesn’t respond to daily life)
*in both 2 neurons form the autonomic pathway
Two neuron types in ANS
- preganglionic
- postganglionic
preganglionic neurons
- neuronal bodies in the CNS (gray matter of spinal cord)
- axons exit in cranial and spinal nerves
postganglionic
- cell bodies in autonomic ganglia in PNS
- course on arteries to the viscera
Formed by NCC
Location of the sympathetic neurons
- preganglionic neurons in the spinal cord from T1-L2 (there are 14)
- Postganglionic go EVERYWHERE
(IE) prevertebral, vertebral, adrenals
“Fight or Flight”
How do preganglionic cells go to body wall
1) paravertebral ganlia
2) prevertebral ganglia: supply the abdominopelvic viscera (below diaphragm)
White ramus
preganglionic sympathetic axons that enter and synapse in a paravertebral ganglion
-can synapse at that level, go up, down, or through the sympathetic trunk
Gray ramus
postganglionic sympathetic axons that leave the paravertebral ganglia and rejoin spinal nerve to go to body wall
- innervate sweat glands and smooth vascular muscle
- NON-MYELINATED NERVES
How do autonomic neurons get to viscera?
spinal nerves carry either pre/post ganglionic axons to the viscera
Example pathway through abdominopelvic visceral innervation
Gray matter -> preganglionic -> pass tyhrough paravertebral -> ventral root of spinal nerve -> white rams -> paravertebral -> becomes splanchnic nerve -> synapses in prevertebral ganglion -> postganglionic axons innervate viscera
Where are prevertebral ganglia located?
in front of the vertebrate
NOT in the thorax (only paravertebral ganglia)
Splanchnic nerves contain
- autonomic axons
- sensory fibers (visceral pain information)
- they are preganglionic axons that do not synapse at the paravertebral ganglia
Location of greater splanchnic nerve
T5-T9
Location of lesser splanchnic nerve
T10-T11
Location of least splanchnic nerve
T12
Lumbar splanchnic nerves
-from L1-3
arises below the diaphragm
Sympathetic nerves
-mixed nerves (carry visceral pain fibers from thoracic and abdominal viscera)
Referred pain
- pain from viscera is carried back to the CNS by sympathetic nerves and is referred to dermatomes of the body wall that match the sympathetic innervation
- visceral afferent fibers course with sympathetic and parasympathetic nerves
- visceral pain fiber is activated and goes back to the spinal cord, neurons in the cord are confused and pain is referred to dermatomes corresponding to the same spinal cord level that the affect nerve enters the CNS
- referred pain is dull and poorly localized
Parasympathetic division
- preganglionic cell bodies in brainstem
- they exit with cranial nerve III, VII, IX, X (4 cranial nerves carry parasympathetic 3, 7, 9, 10)
OR
- cell bodies in sacral (spinal) cord levels in ventral roots of S2, 3, 4
- only go through visceral structures NOT through the body wall
- Rest or digest
- parasympathetic nerves carry visceral sensations other than visceral pain
Where do parasympathetic nerves synapse?
ganglia in head or neck
or
in terminal ganglia
*post ganglionic axons go only to visceral structures
Vagus Nerve
carries parasympathetics to thoracic and 2/3 of abdominal viscera
- peristalsis and gland secretion
- cranial nerve 10
Sacral outflow of parasympathetic nerves
- they are always presynaptic
- they leave through ventral rami of the S2-4 but then branch off and become pelvic splanchnic
- synapse at terminal ganglia of the organ
pelvic splanchnic nerves innervate distal 1/3 of gut
-peristalsis and gland secretion of transverse colon, descending, sigmoid, and rectum
make bladder contract, dilate erectile tissues
Paravertebral ganglia
preganglionic cells synapse at 31 paravertebral ganglia that supply the body wall and viscera
-paravertebral ganglia emerge as splanchnic nerves that go on to synapse in prevertebral ganglia
Prevertebral ganglia
supply the abdominopelvic viscera (below diaphragm)
-named based on artery to visceral structure
Cholinergic transmission
acetylcholine is the principal transmitter released by:
1) all preganglionic autonomic neurons (both sympathetic and parasympathetic)
2) all parasympathetic postganglionic neurons
3) some sympathetic postganglionic neurons innervating sweat glands and the smooth muscle of some blood vessels that supply skeletal muscle
Adrenergic transmission
- most sympathetic postganglionc neurons release norepinephrine
- the adrenal medulla releases 80% epinephrine, 20% n-epi
Sequence of events in chemical transmission in the ANS
action potential -→ calcium influx -→ exocytosis
Neuroeffector junction in ANS
- Unlike skeletal muscle no specialized end-plate structures on autonomic target cell
- nerve terminals of the postganglionic fibers end in close proximity to the cells innervated and consist of a series of bead-like varicosities
varicosities
- contain the synaptic vesicles and are the site of their release
- Target cells have receptors for the transmitters of their controlling nerves distributed over their surfaces
Transmitter removal in ANS
- ACh hydrolyzed by acetylcholinesterase
- n-epi is mainly removed by direct sodium-driven reuptake into the nerve terminals
ACh receptors
- two major types:
1) nicotinic (N)
2) muscarinic (M)
Nicotinic receptors
- two major sub-types
1) N1 on the skeletal muscle end-plate where somatic motoneurons innervate the skeletal muscle fibers
2) N2 mediate transmission at all ganglionic synapses (BOTH sympathetic or parasympathetic)
**All nicotinic receptors are ligand-gated channels that depolarize the postsynaptic membrane (always excitatory)
Muscarinic
located on all ANS target cells that are sensitive to ACh
NOT ligand-gated channels
-G-protein coupled receptors (excitatory or inhibitory)
Excitatory M receptors
- increase IP3/DAG
- stimulate the opening of calcium channels
- decrease cAMP
Inhibitory M receptors
- decrease cAMP
- stimulate the opening of potassium channels.
N-epi and epi receptors
*GPCR
alpha 1: increase IP3/Ca, open K+ channels
- not in the heart
- Excitatory
alpha 2: located on presynaptic nerve terminals of adrenergic and postganglionic cholinergic nerve fibers
- decrease cAMP, inhibit the opening of calcium channels, and inhibit NT release
- sympathetic outflow in the gut
- inhibitory
beta 1: found only in the heart
- They increase cAMP
- excitatory
beta 2: increase cAMP
- responsible for all inhibitory adrenergic actions on the target cells (ie) dilate bronchiole smooth muscle
- inhibitory
- only one where epi/n-epi not equally potent (epi much more potent)
Prevertebral ganglia
- found below the diaphragm
- presympathetic neurons go to paravertebral ganglia come out as root of splanchnic and then synapse at prevertebral to come out as postsympathetic neurons
- celiac, superior mesenteric, inferior mesenteric ganglion
