10. Peripheral nervous system Flashcards
What does the neural component of the ectoderm give rise to?
CNS
Where are neural crest cells located and what is their function?
Located in neural fold (At the junction between non-neural and neural ectoderm)
Function: Relay info. between skin and CNS
What does the non-neural component of then ectoderm give rise to?
Skin
List 3 types neurone fibres
A fibres: Myelinated Somatic (associated with skeletal muscles)
B fibres: Myelinated Visceral (associated with internal organs)
C fibres: Unmyelinated Somatic and Visceral pain afferents
What does the autonomic nervous system control?
Involuntary responses
Describe the arrangement of autonomic motor neurons.
Autonomic motor neurons have a preganglionic neuron and a postganglionic neuron with an autonomic ganglion in the middle.
Location of the autonomic ganglion varies depending on whether it is sympathetic (spinal cord) or parasympathetic (brain stem nuclei or sacral region)
Where do sensory neurons have their cell bodies?
In the dorsal root ganglion
Which vertebrae do sympathetic nerves arise from?
T1 to L2
Why do we refer to vagal and sacral nerves as presynaptic or postsynaptic?
There are no ganglia associated with vagal or sacral nerves
Where do pre-synaptic parasympathetic sacral neurones arise from?
S2 to S4
Describe common developmental features of Pre-ganglionic sympathetic motor neurones (T1 – L2)
and Pre-synaptic parasympathetic Sacral neurons (S2-4).
Arise from basal plate of spinal cord
Multipolar
Myelinated
B fibres
Describe common developmental features of Post-ganglionic autonomic (symp. and parasymp.) neurones
and post-synaptic parasympathetic neurons
Arise from neural crest cells
Multipolar
Unmyelinated
C fibres
How do autonomic motor ganglia differ to ganglia of sensory neurones?
Autonomic motor ganglia contain synapses
Describe the difference in myelination of A and C fibres
A fibre: 1 axon, many layers of myelin
C fibres: Many axons associated with 1 Schwaan cell
What do rootlets come together to form?
Roots
Describe parasympathetic ganglia
Located in CRANIAL part of head
Synapases between pre-ganglionic and post-ganglionic fibres
What are the 4 parasympathetic ganglia?
Ciliary
Submandibular
Otic
Pterygopalatine
What are the 2 divisions of the peripheral nervous system?
Autonomic: Motor, sensory and enteric
Somatic: Motor and sensory
How many and what types of nerves are in the peripheral nervous system?
43 pairs:
12 Cranial
31 Spinal
Describe the distribution of spinal nerves in the peripheral nervous system
8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal
Where do all except C1-C7 of the spinal peripheral nerves arise from?
Arise from below corresponding vertebra
Generally, peripheral nerves have… function
Have mixed motor and sensory function
aside from some cranial nerves
Describe transmission in somatic motor nerves
Primary motor cortex to brainstem/spinal cord to NMJ (skeletal muscle)
Always stimulatory, don’t fire at rest
Use ACh
Describe transmission in autonomic motor nerves
Hypothalamus to brainstem/spinal cord to ganglion to effector organ
Stimulatory or inhibitory, baseline firing rate
ACh or NA
Describe transmission in somatic and autonomic sensory nerves
Periphery to dorsal root ganglion to brainstem/spinal cord
What does somatic and autonomic sensory innervation give?
Sensory info from conscious (somatic) and visceral (autonomic) areas
What is found in the ventral root?
Somatic and autonomic motor neurones
Efferent nerves
What is found in the dorsal root?
Somatic and autonomic sensory neurones
Afferent nerves
Cell bodies in dorsal root ganglion (collection of cell bodies)
What is found in a spinal nerve?
Motor and sensory neurones of that spinal segment
What is found in a dorsal ramus?
Motor and sensory innervating the skin of the back and deep back muscles
What is found in a ventral ramus?
Motor and sensory innervating the rest of the body from the neck down
What is found in the ventral horn of grey matter?
Somatic motor neurones
Efferent nerves
What is found in the dorsal horn of grey matter?
Somatic and autonomic sensory neurones
What do all sensory neurones of the PNS arise from?
Neural crest cells
Which roots do motor and sensory nerves go out of?
Motor = ventral Sensory = dorsal
Describe peripheral nerve structure
Nerve surrounded by epineurium
Bundle of neurones (fascicle) surrounded by perineurium
Neurone/axon surrounded by endoneurium
What produces myelin in the peripheral nervous system?
Schwann cells
What affects propagation speed of a nerve impulse?
Size: Larger fibres conduct impulses faster as less resistance
Myelination: Myelinated = faster due to saltatory conduction
Temperature: Increased temperature = faster
Describe A fibres
Largest (5-20 microns) Fastest conduction (130 m/s) Myelinated Somatic Sensory and motor to skeletal muscle Sub-divided into alpha, beta, delta and gamma
Describe B fibres
Medium (2-3 microns)
Medium conduction (15 m/s)
Myelinated Visceral
Sensory and autonomic preganglionic
Describe C fibres
Smallest (0.5-1.5 microns)
Slowest conduction (2 m/s)
Unmyelinated somatic and visceral pain afferents
Nerve plexus
Network of successive ventral rami that exchange fibres (e.g cervical, brachial plexus)
Dermatomes
Area of skin supplied by a single sensory nerve root (can determine site of spinal damage)
Myotomes
Groups of muscles supplied by a single spinal nerve root (part of neurological exam)
Ramus
Motor and sensory nerves together that exit from the roots
What are the 2 classifications of distributions of sensory innervation?
Dermatomal Innervation: The strip of skin supplied by a pair of spinal nerves is referred to as a dermatome
Peripheral Innervation: The area of skin supplied by a single peripheral nerve
What does the brachial plexus innervate? Which ventral rami does it consist of?
Brachial plexus innervates the upper limb.
Consists of ventral rami of C5-T1
What does the lumbosacral plexus innervate? Which ventral rami does it consist of?
Lumbosacral plexus innervates lower limbs.
Consists of ventral rami of L1-S3
Causes of neuropathies
Many causes e.g. metabolic, inflammatory, genetic
30% idiopathic
Mononeuropathy
Single nerve affected
Polyneuropathy
Several nerves affected
What does damage to a sensory nerve cause?
Tingling
Pain
Numbness
What does damage to a motor nerve cause?
Weakness to hands and feet
What does damage to autonomic nerves cause?
Change in HR
Change in BP
How do damaged peripheral nerves differ to those in the CNS?
Axon regeneration can occur in damaged peripheral nerves
Describe neuropraxia
Compression in axon Reversible conduction block Selective demyelination Axon, Epi-,Peri- and Endoneurium still intact e.g. nerve compression
Describe Axontomesis
Disruption of axon and degeneration of fibre Demyelination. Axon loss. Nerve regrowth possible. Epi- and Perineurium intact
Describe Neurotmesis
Complete nerve division and disruption Disruption to epi-, peri- and endoneurium Damage to epineurium- no nerve growth Axon loss. Common after toxic or ischaemic injury
Rank peripheral neuropathies from mild to severe
Neuropraxia
Axontomesis
Neurotmesis
List 3 diagnostic techniques for peripheral neuropathies
Electromyography (EMG)
Nerve conduction velocity (NCV)
Somatosensory evoked potential (SEP)
Describe an EMG
Records electrical activity produced by muscles
Intramuscular or surface EMG
Distinguish muscle disorders vs nerve disorders
Describe an NCV
Electrodes stimulate nerves to determine conduction speed
Slow speed indicates nerve damage
E.g trauma, diabetic/peripheral neuropathy, viral infection, nerve entrapment
(Infants <3yrs have low NCV)
Describe a somatosensory evoked potential
Electrodes stimulate sensory neurones to determine transmission of sensory info from mechanoreceptors to CNS
Often used in multiple sclerosis
What is the basal plate?
Basal plate develops into the ventral horn
It is where the motor neurones come from
What is the Alar plate?
Alar plate develops into the dorsal horn
It is where the sensory neurones go to
2 types of sympathetic ganglia
Paravertebral
Collateral
