Peripheral Nervous System Flashcards
what is the PNS?
nerves outside CNS consisting of an afferent sensory division and an efferent motor division.
Embryologically, what are sensory neurones derived from?
neural crest cells which develop from the ectoderm. They are tiny cells which ‘fall off’ the neural tube in early embryonic stage.
what else is derived from the neural crest?
schwann cells
describe the structure of a sensory neurone
from the receptor an impulse is sent via the peripheral process of the dendron towards the cell body. after the cell body, the impulse travels down the central process of the axon towards the CNS.
types of sensory neurone fibres
A- myelinated somatic
B- myelinated visceral/autonomic
C- unmyelinated somatic and visceral and pain afferents
where is a sensory neurone cell body usually located?
located in a sensory ganglion, very rarely is the cell body a receptor itself.
what types of sensory receptors are there?
free ending nerve fibres are for pain and temperature
corpuscles are for touch and pressure
what is the motor efferent division divided into?
Somatic and Autonomic
what is the somatic nervous division?
contain efferent motor neurones which give rise to muscle contractions (skeletal). this could be voluntary OR reflexive/automatic.
describe the route an impulse takes from the primary motor cortex to the muscle effector in the SNS
an upper motor neurone sends information down spinal cord from primary motor cortex to correct vertebral/spinal cord level. Here, it synapses with a lower motor neurone which is exclusively peripheral leading to an NMJ.
where do lower motor neurones come from embryologically?
they are derived from the basal plate of the spinal cord itself.
what exits the intervertebral foramen
mixed spinal nerve, containing both motor and sensory neurones.
what types of nerves are part of the PNS?
Cranial (12) and Spinal (31)
what is the autonomic nervous system?
involuntary NS, sympathetic and parasympathetic
describe the rude an impulse takes from the hypothalamic nuclei to the visceral effector.
one motor neurone from hypothalamic nuclei to brain stem or spinal cord (within CNS), presympathetic
one preganglionic neurone from the autonomic ganglia (sympathetic trunk/parasympathetic ganglia).
one postganglionic neurone from ganglia to the target visceral effector
what is different about the vagus and sacral parasympathetic nerves and the sympathetic adrenal innervation?
they only have 2 neurones. no ganglia associated with them hence referred to as pre and post synaptic neurones.
describe the myelination of the pre and post ganglionic neurones
preganglionic neurone is myelinated (B fibres)
postganglionic neurone is unmyelinated (C fibres)
describe the anatomy of the sympathetic nervous system
innervation from T1-L2, some pass into paravertebral sympathetic chain ganglion, some pass into collateral ganglia.
describe the anatomy of the parasympathetic nervous system
innervation from cranial nerves from brain stem and S2-4.
cranial nerves apart from vagus have parasympathetic ganglia.
embryologically where are the preganglionic/presynaptic neurones derived from?
basal plate
embryologically where are the postganglionic/postsynaptic neurones derived from?
neural crest cells
difference between A fibre and C fibre structure
A fibre neurones have up to 100 layers of myelin from schwann cells wrapped around a singleton’s axon
C fibres, one layer schwan cell neurilemma and cytoplasm surround an axon, a bundle of these are found in one c fibre.
what speeds up impulse propagation?
larger diameter as less resistance
myelination as saltatory conduction
colder = slower conduction
fibre types in ascending order of speed of impulse transmission
c, small diameter and unmyelinated,
b, medium diamater
a, largest diameter
how are neurones in the PNS grouped?
on basis of:
myelination
speed of transmission
diameter
what is a ramus?
a branch of a neurone
what are splanchnic nerves?
nerves that contain both visceral afferents and efferents supplying internal viscera only.
what are the communicating rami?
where the preganglionic neurone in sympathetic NS exits the ventral route through the WHITE RAMUS into the paravertebral sympathetic chain ganglia where it can either:
- synapse in ganglion with postganglionic neurone which then leaves via a GREY RAMUS
- move up or down the sympathetic trunk and synapse at another level
- could pass through white ramus into a splanchnic nerve which then synapses at a collateral ganglion with postganglionic neurone.
describe the parasympathetic ganglia of the cranial nerves
preganglionic cranial nerves (3,8,9) from pons and enters semilunar ganglion then splits into the ophthalmic, maxillary and mandibular branches. ganglia present:
- ciliary
- pterygopalatine
- otic
- submandibular
what are nerve plexuses?
networks of nerves which crisscross and redistribute arising from the ventral rami. only from cervical and lumbar regions. these mainly innervate limbs.
Note Thoracic region has no plexus.
why are nerve plexuses protective?
means damage to a single nerve preplexus would not causes paralysis as man nerves may contribute to movement of that muscle.
what are dermatomes?
a specific area of skin which is supplied by a single spinal nerve dorsal root
pin prick exam can check for spinal nerve damage.
each nerve maps onto a specific area of skin
what is referred pain?
pain felt in area where stimulus not actually found eg left arm and chest pain (T1-4) in MI (visceral afferent from heart).
what causes referred pain?
visceral afferents are not part of the ANS but use the sympathetic apparatus (plexuses, splanchnic nerves, ganglia and white ramus) to rich the dorsal root hence pain is felt.
what is the difference between cutaneous distribution of sensory nerves and dermatomes?
one cutaneous area may be supplied by more than one nerve.
Muscles are discrete organs, what is the myotome?
the nervous supply of a muscle arising from one specific spinal MOTOR ventral root.
note some muscles are innervated by more than one nerve hence loss of function of one nerve will end in muscle weakness as other nerves still supply it.
Paralysis occurs if there is damage to nerve after plexus (where all nerves converge) hence muscle will not work at all.
types of peripheral neuropathies (damage or disease affecting PNS)
- sensory, causing tingling, numbness, pain
- motor, causing weakness or paralysis
- autonomic, affecting gland or visceral function
causes of neuropathies
mononeuropathy = single nerve polyneuropathy = lots of nerves
these can be caused by metabolic, toxic, inflammatory, trauma and genetic responses.
most is idiopathic
what makes up the endoneurium?
loose collagen fibres surrounding neurone and myelin sheath itself
what does the perineurium do?
gives main tensile strength forming fascicle
what does the epineurium do?
surround the entire nerve bundle, contains fat, dense collagen and blood supply.
types of PNS nerve injury
- neuropraxia
- axonotmesis
- neurotmesis
what is neuropraxia?
reversible nerve conduction block may be caused by compression as endoneurium (demyelination) and axon still intact.
what is axonotmesis?
intact perineurium and endoneurium but axon loss and demyelination. some nerve activity can occur.
what is neurotmesis?
complete nerve division, even epineurium split this leads to complete nerve disruption.
mostly due to toxic and ischaemic injuries
where does wallarian degeneration occur?
proximal to site of injury in axonotmesis and neurotmesis. this area of the nerve is degenerated and phagocytosed. this is normally associated with chromatolysis enlarging of the soma.
why can regeneration occur more readily in the PNS
favoured environment, can easily be degenerated and damaged parts cleared up, schwann cells promote regeneration whereas oligodendrocytes in CNS don’t.
which has better regrowth of axon?
axonotmesis as endoneurium intact.
what happens in PNS regeneration?
axon stump grows to the effector and schwann cells proliferate and remyelinate axon.
why is there a difference between motor neurone and sensory neurone degeneration?
cell body is key to regeneration, if cell body is avulsed then regeneration cannot occur. in sensory neurones the cell body is in the middle of the conducting body whereas it is at the end of a motor neurone hence avulsion usually means sensory fibres often survive more.
what is a neuroma?
failure for axon to regenerate.
what is a nerve conduction study?
determines whether axonopathies or demyelinating neuropathies.
electric field from electrode causes muscles to contract via nerve. another more proximal innervation done and proximal latency and distal latency used to work out NCV.
NCV= distance between/(proximal latency-distal)
what would an axonal neuropathy show on NCSs?
lower amplitude
what would a demyelinating neuropathy show on an NCS?
larger latencies
what is a Somatosensory Evoked Potential (SEP)?
investigates afferent divison nerve conduction velocity from receptor (usually touch) to cerebral cortex.
what cells support dorsal root ganglia?
satellite cells
where does peripheral neuropathy often occur
distal parts of limbs
