PNS Flashcards
Describe the divisions of the PNS
Sensory (afferent) divison
Motor (efferent) division
What can the motor (efferent) division of the CNS be split into
Autonomic nervous system
Somatic nervous system
Describe the key differences between the sympathetic and parasympathetic divisions of the autonomic nervous system
Parasympathetic
Conserves energy
Sympathetic
Mobilised body systems
Essentially, what is the PNS
A communicating centre which communicates between the body and CNS
What are the three germ cell layers
Ectoderm- CNS and skin
Mesoderm- muscles
Endoderm- GUT
Describe the neural crest
Non-Neural ectoderm-skin
Neural ectoderm- CNS
At junction- neural fold- neural crest cells- relay info between skin and CNS- migrate into various populations- - give rise to the PNS
By the end of development, where is the segmental arrangement of the nervous system (as determined by somites) retained
The spinal cord
List some lineages of neural crest cells
Scwhann cells Sensory neurones Dorsal root ganglia Chromaffin cells of the adrenal medulla Melanocytes Autonomic ganglia
Describe the basic structure of a sensory Neurone
Peripheral process:
From one half of cell body to sensory receptor along axon and dendrites between receptor and axon
Central process
Other half of cell body to CNS along axon via dendrites at end
Direction of conduction is from the Sensory receptor to the CNS
What is meant by the basal plate
motor neurones derived from the ventral region of the basal plate in the developing spinal cord
Describe the different types of sensory nerve fibres
A fibres: myelinated somatic
B fibres: myelinated visceral
C fibres: unmyelinated somatic and visceral pain afferents
Describe the different types of sensory receptors
Free nerve endings- pain
Encapsulated nerve ending (with connective tissue capsule)- e.g pacinian corpuscle- pressure
sensory cell- ear
cell with peripheral processes- smell (does not pass through sensory ganglia)
Where may the sensory ganglia be found
Sensory neurone cell bodies (somatic and autonomic) lie in ganglia associated with spinal dorsal roots or some cranial nerves
Summarise the somatic nervous system
Consists of efferent nerves responsible for stimulating muscle contraction
Basic route of nerve signals begins in the primary motor cortex
Can be voluntary or autonomic (reflexive)
Nerves terminate at the NMJ to produce the motor response
What are the different types of motor neurones
Upper motor neurones-Exclusive to the CNS- travel to the ventral horn via the pyramid tract (can also synapse in the brainstem)
Lower motor neurones- Innervate skeletal muscle fibres- the cell bodies of which arise in the spinal cord
Describe the embryology of the spinal cord
The neural tube contains neuroblast cells, its hollow centre (central cavity) becomes the spinal canal.
Neuroblasts adjacent to the canal divide and travel to the outer mantel- forming the neurones of the grey mater
These neuroblasts project nerve fibres that grow outwards into the marginal zone- forming the white mater of the spinal cord.
What happens to the neuroblasts in the grey mater
They form two discrete populations
A Doral alar plate and a ventral basal plate separates by a shallow groove (sulcus limitans).
Alar plate- interneurones and sensory neurones (from neural crest cells) of the dorsal horns
Basal plate- upper motor neurones of the ventral horn
What else may be found in the basal plate
Sympathetic and parasympathetic pre-ganglionic neurones
Describe motor neurones
Sensory neurone cell bodies (somatic and autonomic) lie in ganglia associated with spinal dorsal roots or some cranial nerves.
Each motor neurone that supplies a muscle is called a motor unit- will branch to innervate different muscle fibres (different motor neurones may innervate different muscle fibres of the same muscle)
How many PNS nerves are there
43 pairs of nerves
12 cranial nerves
31 spinal
What are the 12 cranial nerves
Olfactory Oculomotor Trochlear Abducens Optic Trigeminal Facial Vagus Glossopharyngeal Cranial Accessory Vestibulocochlear
Summarise the autonomic nervous system
Efferent nerves and ganglia, stimulating effectors outside our control (e.g., heart rate)
Consists of two divisions:
Sympathetic
Parasympathetic
3 neurones; autonomic ganglia contain synapses (as well as cell bodies)
Describe the different motor neurones of the ANS
1st . neuron from hypothalamic nuclei to brain stem nuclei (parasympathetic cranial) or spinal cord (sympathetic/parasympathetic sacral).
2nd . Neuron - preganglionic Neuron from brain stem nuclei/ or spinal cord to autonomic ganglia. (presynaptic neuron if from vagal nerve and Sacral parasympathetic nerves).
3rd. Neuron - postganglionic Neuron from autonomic ganglia to Visceral Effectors (postsynaptic neuron if associated with vagus nerve or sacral parasympathetic nerves).
What is important to remember about the vagal and sacral parasympathetic nerves
no ganglia, so denoted as pre and post synaptic neurones
List some visceral effectors of the ANS
Smooth muscle
Adipocytes
Glands
Cardiac Muscle
What are the anatomical divisions of the ANS
Sympathetic
T1-L2
Parasympathetic
S2-S4 and Cranial 10, 3,7,9
Where are the pre-ganglionic and pre-synaptic neurones of the ANS derived from
Basal plate and
Pre-ganglionic sympathetic motor neurones (T1 – L2)
- intermediolateral cell column
Pre-synaptic parasympathetic Sacral neurons (S2-4).
Multipolar, myelinated
B fibres
Where are the post-ganglionic and post-synaptic neurones of the ANS derived from
Derivatives of Neural Crest Cells. Post-ganglionic Autonomic neurons (symp and parasymp) Post-synaptic parasympathetic neurons (if associated with Vagus or sacral S2,3,4 parasympathetic) Multipolar, unmylinated C fibres
Summarise the functional components of the PNS
§ Somatic motor neurones have their soma in the CNS.
§ Autonomic motor neurones consist of a preganglionic and postganglionic motor neurone.
o There is an autonomic ganglion (with a synapse) in between the CNS and the effector
§ Sensory neurones have a ganglion in the middle ( NO SYNAPSE)
What type of neurones are sensory neurones and motor neurones
Sensory= pseudo unipolar Motor= unipolar (but technically multipolar)
Describe the process of myelination
Myelin is formed by Schwann cells which wraps around the axon clockwise to form successive layers
Minor dense line: plasma membrane of Schwann cells
Major dense line: where cytoplasm of Schwann cells is condensed
Nodes of Ranvier: where saltatory conduction occurs
A-Fibre: up to 100 layers
C-Fibre: still wrapped by one layer of Schwann cell membrane
Describe non-myelinated cells
Non-myelinated Cells – Axon diameter ~1m:
Clothed in cytoplasm of Schwann cell (neurolemma).
Several axons clothed per Schwann cell.
Each axon only has a single layer of membrane and CANNOT facilitate Saltatory conduction
NOTE: Schwann cells are ALSO present on non-myelinated cells and they engulf multiple axons.
What is the propagation speed of a nerve impulse related to
It is not related to the strength of the stimulus
Related to diameter (larger diameter, less resistance, greater speed of propagation)
Myelination (myelinated fibres conduct impulses faster, because of saltatory conduction)
Describe the differences in the propagation speed of each type of sensory fibre
A Fibres: 5-20microns and myelinated therefore 130m/sec
B Fibres: 2-3microns and myelinated therefore 15m/sec
C Fibres: 0.5-1.5microns and unmyelinated therefore 2m/sec
Describe the different types of A fibres
alpha- 13-20 microns, 80-120m/s
beta- 6-12 microns, 35-75 m/s
gamma- 1-5 microns, 5-35 m/s
Describe the basic anatomy of the nervous system
Dorsal Root Ganglion: contains the cell bodies of sensory neurones entering the cord
Dorsal (Posterior) Horn: extends to edge of spine and receives inputs from sensory neurones
Ventral (Anterior) Horn: does not extend to edge of spine and contains the cell bodies of motor neurones that will exit the cord
Mixed spinal nerves: dorsal and ventral root nerves combine to form a mixed spinal nerve
Intravertebral foramen: space between vertebra where the mixed spinal nerve may emerge
Rami: the mixed spinal nerves branch to form rami upon exiting the cord
Dorsal (Posterior) Ramus: supplies the back of the body
Ventral (Anterior) Ramus: supplies the front and sides of the body
Describe the cranial parasympathetic ganglia
four ganglia with synapses between pre/postganglionic neurones (ciliary, submandibular, otic and pterygopalatine)
Describe the paravertebral sympathetic chain ganglia
pre-ganglionic neurone exits ventral ramus and passes through white ramus into paravertebral chain, with three possible outcomes:
Synapses to post-ganglionic neurone which exits via grey ramus to ventral root (sweat glands, piloerector muscles, blood vessels of skin and skeletal muscles).
Pre-ganglionic fibre moves up/down 2 levels and then synapses (iris, salivary glands, thoracic blood vessels, eosophagus)
Neurone passes through white ramus to splanchnic nerves and synapse to collateral ganglion, before post-ganglionic neurone continues to organs ( liver, spleen, stomach, intestines, kidneys, reproductive organs)
Describe plexi
networks of successive ventral rami exchange fibres to innervate limbs (except in thoracic region), where roots branch to cross and redistribute signals so that each myo/dermatome is innervated by nerves +/- 1 level that the main ramus originates from - a lesion at one root will not lead to a full loss of sensation or paralysis
What does the brachial plexus consist of
BRACHIAL PLEXUS
(UPPER LIMBS)
VENTRAL RAMI:
C5 TO T1 SPINAL NERVES
What does the lumbosacral plexus consist of
LUMBOSACRAL PLEXUS
(LOWER LIMBS)
VENTRAL RAMI:
L2 – S2 SPINAL NERVES
What is the skin described as
A continuous organ- dermatomes can overlap
What is meant by a dermatome
Areas of skin supplied by a single sensory spinal nerve root. Skin is a continuous organ
Stacked along the thorax and abdomen; longitudinally along the limbs
Clinical significance: can determine the site of spinal damage by simple pin prick exam
What is important to remember about visceral afferents
VISCERAL AFFERENTS ARE NOT A PART OF THE ANS BUT THEY PASS THROUGH THE SPLANCHNIC NERVES AND PLEXUXES, THE SYMPATHETIC COLLATERAL & PARAVERTEBRAL GANGLIA AND WHITE RAMI TO GET TO THE SPINAL DORSAL ROOT .
They use the sympathetic nervous system to get to the spinal dorsal root, but they are one continuous fibre
Describe the phenomenon of referred pain
PHENOMENON OF REFERRED PAIN AT THE CORRESPONDING DERMATOME(S) OF THE SPINAL DORSAL ROOT(S)
Describe the cutaneous distribution of peripheral nerves
Areas of skin supplied by a peripheral nerve that may be coming off a plexus.
Sensory Neurons of the peripheral nerve may come from more than one spinal nerve root
Clinical Significance: Identify which individual peripheral nerve is damaged by pin prick exam
What is meant by a myotome
Muscles are discrete organs
Biceps supplied by more than one ventral root- nerve has two roots
Muscles that are supplied by a single motor spinal nerve root
Clinical significance: Testing for root level muscle weakness
Describe the spinal levels which correspond to upper and lower limb innervation
Head: Cranial and C2 Neck: C3-C4 Arms: C5-T1 Torso: T1-L1 Legs: L2-S4
What is meant by peripheral neuropathies
Damage or disease affecting PNS nerves
May impair sensation, movement, gland or organ function
Describe the different presentations of peripheral neuropathies
Sensory nerves (sensation): cause tingling, pain, numbness Motor nerves (movement): cause weakness to hands and feet Autonomic nerves (involuntary functions): cause changes in heart rate or blood pressure
Summarise the causes of peripheral neuropathies
Has many causes
Mononeuropathy: refers to a single nerve being affected ( diabetic- oculomotor nerve)
Polyneuropathy: refers to several nerves being affected (Guillian- Barre syndrome- inflammatory)
Causes can be metabolic, toxic, inflammatory, traumatic, genetic
~30% of neuropathies are “idiopathic” - i.e. of unknown origin
Describe the structure of a peripheral nerve
The axons are in bundles called FASCICLES which are grouped together to form NERVES. blood vessels found between the fascicles
This structure is found from the spinal cord onwards
Connective tissues:
Endoneurium:
Around each individual neuron/axon
Loose collagen fibrils
Perineurium:
Around a fascicle (bundles of neurons/axons)
Gives main tensile strength to nerve
Epineurium: Around the entire nerve, Dense, collagenous Blood supply Some fatty tissue
Why is the packaging of connective tissue important in nerves
Some of the axons are very long and need to be able to cope with strain (e.g. bending of joints) so the packaging and connective tissue is very important.
Describe neuropraxia
Slight compression in axon- recoverable. No axonal discontinuity
reversible conduction block
selective demyelination of the axon sheath
endoneurium and axon still intact - e.g., nerve compression
Class 1 injury
Describe axonotmesis
Class 2 injury Some disruption to axon demyelination and axon loss epineurium and perineurium remain intact still some continuity within the nerve degeneration occurs below and slightly proximal to the site of injury Follows wallerian degeneration axoplasmic disruption
Describe neurotmesis
Class 3,4 and 5
most severe form of nerve injury
associated with complete nerve division and disruption
commonly seen after toxic or ischemic injuries
Damage to the epineurium (around the entire nerve) - no nerve growth
Damage to nerve structures
More severe= less chance of recovery
Wallernian degeneration
Axon disrupted, loss of tubules, support cells destroyed
Describe regeneration of the nerve axon
- With the compression injury (forceps), the axoplasm is completely separated but the Endoneurium remains intact (important as a scaffold to build off).
- Once the compression has occurred, the part distal to the soma of the nerve degenerates.
- The macrophage influx clears the debris.
- The Schwann cells undergo mitosis and proliferate.
- Axonal sprouts grow from the proximal stump and there is competition between them to synapse with the target organ first.
- Once an axonal sprout synapses, the other axonal sprouts regress and the successful axonal sprout grows and is myelinated.
- End-result is an axon which looks similar to the original but the internode distance (between nodes of ranvier) is less than original (so conduction speed decreases).
Describe post-ganglionic injury
e.g. Neuroma (axon doesn’t find final position)
Describe pre-ganglionic nerve injury
dorsal root avulsion will lead to nerve surviving as cell body keeps alive; motor and ventral roots degenerate as detached from cell body
What happens when a peripheral nerve is damaged
When a peripheral nerve is damaged:
The distal part of the nerve degenerates and macrophages phagocytose the axonal and myelin debris (Wallerian degeneration)
The proximal part of the axon and cell body usually survive but undergo metabolic changes (chromatolysis)
During regeneration Schwann cells proliferate to form a scaffold and axonal sprouts grow out from the proximal stump of the nerve into the distal stump
Successful regeneration mainly depends on how badly the axons and connective tissue sheaths are damaged and the distance from the target organ
Failure to reconnect leads to formation of a neuroma containing trapped axon
What is meant by glove and stocking distribution
peripheral neuropathies Usually begins in distal parts of limbs (glove and stocking distribution)
Describe some defects in nerve repair
If the injury is close to the soma, the shock may kill the whole cell.
If a compression injury occurs, the endometrial sheath is left intact, providing a scaffold to re-grow off but if the nerve is cut, the axonal sprouts have no guidance cues and so you ay have re-innervation of the wrong target.
o Clean cut = Surgeon can re-join in microsurgery.
o Mess cut = Nerve graft from the back of the calf.
The damage could be so severe that the axonal sprouts have no guidance at all and form a tangled bundle called a neuroma very painful and need to be surgically removed.
What is meant by electromyography
Needle EMG: Diagnostic technique that distinguishes between myopathies (intrinsic to muscle) and muscle weakness due to nerve pathology (neurogenic). Looks at the waveforms of Motor Unit Action potentials.
How do you perform an EMG
stick needle in muscle and ask patient to make slight movement, causing an AP; approach to electrode causes initial down deflection before up deflection when reaches the electrode, followed by another down deflection; can be used to distinguish myopathy and neuropathy
Describe the EMG patterns of denervated muscle and myopathy
Denervated muscles: leads to fibrillation and larger signal as other nerves will take over; reduced interference pattern
Myopathy: leads to fibrillation with smaller polyphasic units and a full interference pattern
What is meant by Nerve Conduction Study
Diagnostic technique for evaluating the speed of nerve impulses
nerve conduction velocity (NCV) in m/s
Amplitude of responses in mV.
Axonopathies (Amp), Demyelinating neuropathies (NCV)
What happens in demyelination + Conduction Block (Guillian- barre syndrome)
Increased latency and reduced amplitude
How do we perform a nerve conduction study
stimulate the muscle with electrode; electrical field penetrates skin to activate nerve; timing allows for conduction velocity to be calculated
CV= Distance/ proximal latency -distal latency (ms)
Describe Somatosensory evoked potential
Evaluation of site of lesion (PNS or CNS) in somatosensory pathways by assessment of amplitude and latency of responses. Example, MS
Very small amplitude of NERVE responses (μV) therefore constant averaging over MANY responses is necessary to negate noise, compared to surface EMG (mV) looking at MUSCLE.
