Week 1- The PNS Flashcards
What do epidural and subdural hematomas look like on MRI? What vessels are disrupted in each?
Epidural: biconvex lens shape, the meningeal arteries SUdural: crescent shaped, the bridging veins
What is the order of meningeal coverings and spaces
Skull Epidural (potential) space Periosteal dura Meningeal dura Subdural (potential) space Arachnoid Subararachnoid (real) space Pia mater Cerebral cortex
What level does the spinal cord end? Where does filium terminale attach?
L1/L2 to the cocyx
What are the “wrappings” of a peripheral nerve fascicle?
epineurium perineurium (blood nerve barrier is between peri and endo) endoneurium
What is the CNS equivalent of a “nerve”?
A tract
How to classify peripheral nerves and which ones are important to know (A-alpha, 1a, 2b, 2, A-delta)? What happens to C-fibers in demyelinationg disorders?
- Based on conduction velocity (A,B,C)
- Based on diameter (I, II, II,IV) (this scheme used only for sensory)
- C-fibers are thin and unmyelinated and carry dull pain information- they will remain intact in demyelination disorders.
- A-alpha carry motor to muscle
- 1a carry sensory from spindle
- 2b carry sensory from golgi
- 2 carry sensory from skin
- A-delta: carry sharp pain
What are the two kinds of schwann cells?
ensheathing and myelinating
Define neuropraxia, axonotmesis, neurotmesis
Neuropraxia: lost myelin Axonotmesis: the axon is lost, but at least the epineurium is still intact Neurotmesis: transection
What is the function of myelin for neuronal transmission (x3)
Insulates reduces capacitance increases rate of change at the membrane
What is Nissl substance?
A histologic identifying feature….it is RER of nerves
What do these cells do: astroglia oligodendrocytes Schwann cells Polydendrocytes Ependyma Microglia
astroglia: support oligodendrocytes: central myelination Schwann cells: peripheral myelination Polydendrocytes: stem cells Ependyma: line ventricles, make CSF Microglia: macrophages
What are the 3 types of white matter tracts in the CNS
commisural (between hemispheres) association (within hemispheres) projection (in and out of the CNS)
What the four components of a sensory experience? What does duration refer to?
1) Modality (dictated by what type of receptor: pressure, temp, taste, photo)
2) Intensity (encoded either by frequency coding (how often a neuron fires) or population coding (how many neurons are firing)
3) Location (where the fibre terminates on the somatosensory cortex. 2 point discrimination)
4) Duration: relationship between the perceived intensity and the actual intensity. depends on adaptation, each receptor has a different rate of adaptation.
What does 2 point discrimination depend on ?
- receptor density -receptive field - need to have at least one unactivated neuron between the two points to discriminate -depends on lateral inhibition
Where are the following sensory components and what do they each detect? Meissner’s corpuscles Pacinian Ruffini endings Merkel Free nerve endings in skin
Meissner’s corpuscles: papillary dermis; light touch Pacinian: reticular dermis; deep pressure and vibration (espcially the change in pressure because they are warpped in a gelatinous material) Ruffini endings: dermal blood vessels, stretch Merkel: stratum basale; vibration Free nerve endings: touch, pressure, stretch and pain
How to photoreceptors differ in terms of signal transduction from other sensory receptors?
They have tonically high cGMP and tonically release glutamate. In the light rhodopsin absorbs photons and causes a decrease in cGMP and decrease in glutamate release.
What are the names of muscle fibers, sensory fibers and motor fibers that belong to the muscle spindle? What is the point fo alpha-gamma coactivation?
Muscle: bag1, bag2, chain
Sensory: 1a (length and velocity info), 2 (length info)
Motor: gamma-static and gamma-dynamic
Alpha-gamma coactivation means that the muscle spindle shortens with the extrafusal fibers so that the stretch information remains accurate. Gamma fibers allow fro tunability.
What is the Hoffman reflex?
The electrical analog to the stretch reflex
Where in the body is the density of muscle spindles highest?
Places involved in fine manipulative tasks (e.g. hands and neck)
What is the role of the 1a inhibitory interneuron in the myotactic reflex?
Mediates the reflex, recieves descending input as well. Probably why Jindrassik maneouvres work?
What are the viscoelastic properties of nerves (generally) and how can these influence peripheral nerve injury?
Nerves have a bit of give- they are always a bit tense, but can stretch and recoil. Stretching compresses vessels leading to axon disruption
How does nerve compression lead to injury?
Compression causes ischemia and separation of myelin from the axon. There will be a swelling of axoplasm upstream of the compression.
What is the time limit to reinnervate after motor loss? After sensory loss? Is a more distal or more proximal laceration more likely to heal better?
Motor: 12 months
Sensory: less time sensitive
A more distal laceration has better outcome because the proximal growth cone has less far to grow
What is a neuroma?
A growth of nerve tissue. Happens after nerve injury/part of wallerian degeneration
What is Wallerian degeneration?
Basically the breakdown of the distal stump after axonal injury. The myelin breaks down, the basal lamina sends out lost of trophic factors for teh proximal stump to find it again, lots of macrophages.
The proximal stump starts sending out growth cones right away. Schwann cells are important in the regeneration process and divide like crazy.
All this happens within minutes of injury, so it is important to repair a lacerated nerve as soon as possible.
What is an EMG and what can it tell you? What is the M wave from? What is the H-wave from? What does spontaneous electrical activity mean?
electromyography. Pathology includes changes in latency and changes in amplitude changes in synchrony of firing (seen as a decreased amplitude…)
M-wave is from orthodromic activation of alpha-motor neurons.
H-wave is slower and is the from activation of 1a sensory muscle spindle fibres
Spontaneous electrical activity means active denervation
What would a pattern of axonal loss vs. demyelination look like? What are examples of each?
Axonal loss (e.g. diabetic neuropathy)
- longer axons go first, so you get a stockings and gloves pattern
- slowly progressive
- sensory Sx > motor sx
- loss of ankle jerks
Demyelination (e.g. Guillain-Barre)
- usually rapidly progressive
- arms and legs
- both sensory and motor sx
- areflexic
What are the distinguishing features of UMN and LMN lesions?
LMN:
- hyporeflexia
- absence of cutaneous reflexes
- decreased tone
- fasciculations
- muscle atrophy
UMN:
- hyperreflexia (incl. clonus)
- babinski
- spastiscity
- increased tone
- no muscle atrophy
What is the etiology of spina bifida? How can it be detected in utero?
Etiology is multifactorial.
Can be detected by US or with AFP (alpha fetoprotein)
Prevented with 0.4 mg of folic acid
Define neurulation and give brief description of how it happens.
Neurulation: formation and closure of the neural tube. The ectoderm is induced into the neural tube by underlying notochord cells somehow involving sonic the hedgehog.
The tube zips up, starting cervically. The anterior and posterior neuropores are the last to close
What are neural crest cells? What do they do?
Neural crest cells are pluripotent, migratory cells that form cranial (e.g. leptomeninges (=pia and arachnoid) and trunk (unipolar neurons, melanocytes, schwann cells, adrenal medulla) derivatives
What are defects of failure of the posterior neuropore to close?
What are some problems associated with spina bifida?
skeletal muscle paralysis
saddle anesthesia
anal/urethral problems
Describe the basic organization of the CNS before it folds
Alar plate becomes sensory (feel the wind beneath my wings…) and is lateral
Basal plate becomes motor and is medial
What are the primary and secondary brain vesicles?
What are the derivatives of the secondary brain vesicles?
What are the three CNS flexures and what do they demarcate?
- cervical (brain and spinal cord)
- midbrain (forebrain and midbrain)
- pontine (midbrain and hindbrain…this one is a result of unequal growth of the first two parts)
What does the homunculus look like?
