Microanatomy 1 Flashcards
what are the glial cells in the CNS
- ependymal cells
- oligodendrocytes
- astrocytes
- microglia
what are the Glia cell in the PNS
- Schwann
- satellite cells
how many Glial cells are there in the CNS
10:1
What do epdenymal cells do
- line fluid filled cavities in the CNS
- control the circulation of cerebrospinal fluid
where are epdenymal cells present
- present in the brain ventricles
- spinal cord
- central canal
describe the structure of the ependymal cells
- ciliated
- movement of there chill circulates the CSF
- they look like epithelium but they do not have a basement membrane
what is the difference between epithelial cells and ependyma cells
- epithelial cells have a basement membrane whereas ependymal cells do not
what do astrocytes do
- provide structural and metabolic support for neurones
- help form the blood brain barrier
- regulate neuronal external potassium and intracellular calcium concentrations
- regulate pH
- form scars in areas of damage
- regulate neurotransmitter availability to neurones by restricting diffusion and having specific transporters that mop up excess transmitter which can then be metabolised or recycled
How are astrocytes distinguished from neurones
- astrocytes have a glial fibrillary acidic protein whereas neurones do not have this
describe the structure of microglia
- they are small cells in the CNS
- they have long spiny processes
what do microglia do
- during development they release growth factors
- act as macrophages and remove debris by produced by programmed cell death
- in adults they are surveillance cells which sample micro-envrioment
- in infection they change shape and become phagocytic macrophages and are called reactive microglia
what do oligodenstrocytes do
- they wrap around axons forming a myelin sheath
- thus electrically insulating axons in the CNS
- they can meyleinate up to 60 axons and their cell bodies will lie between the axons
what do Schwann cells do
- electrically insulate axons in the PNS
- they can only myelinated a single axon segment
- they are surrounded by a basement membrane like external lamina which is continuous over the length of axon
what are the two types of Schwann cells
- myelinating Schwann cell s
- non myelinating Schwann cells
what does myelin do
- it prevents leakage of membrane charge into the surrounding intracellular space and lessens the strain on the neurones sodium potassium pump by restricting ion release to specific sites
what non-myelinting Schwann cells associated with
- they are associated with 1 to up to 20 axons of small diameter sensory C fibres or post ganglionic autonomic efferents
- only have one fold of the myelin over them
when the non meylianting Schwann cells supports numerous axons what is the collection of axons called
remack bundles
- adjacent axons are kept separate by thin extensions of the Schwann cell body
describe the structure of the perineurium
- comprised of several layers of flattened fibroblast like cells
- fibrocollagenous matrix between the layers
How does the perineurium act
- it acts as a selective barrier
- it restricts passage of substance to the nerve fibres that are embedded into the endoneuriuz and contributes to what is known as the blood nerve barrier
describe the structure of the axonal cytoplasm
- loads of mitochondria (these provide power to run the sodium potassium pump that keeps the trans-membrane potential at normal level)
- small dots in the axoplasm are microtubules
what do microtubules do in the axonal cytoplasm
- they maintain normal axonal shape and guide neurotransmitter molecules and other material synthesised in the spam to their dentition in the terminus of the axon
why does the myelin sheath has intense staining with osmium
- stains with osmium this is because it has a high lipid content in the membrane
unmyelinated axons have…
a smaller diameter compared to myelinated axons
what do spinal roots do
- It is these roots that carry information in and out of the cord exiting via the intervertebral foramen.
what does the dorsal root do
Dorsal roots transmit sensory information from the periphery to cells in the gray matter of the spinal cord.
what does the ventral root do
Ventral roots transmit motor and autonomic information from neurons in the spinal cord to the periphery.
where are the cell bodies of the postganglionic neurones of the sympathetic nervous system found
- they are found int eh syamtpethicc chain ganglia T1-L3
what do satellite cells do
- they occupy the space around the ganglion cell and have supportive functions like the astrocytes found in the CNS
why are dorsal root ganglion cells have different sizes
- the different sizes of dorsal root ganglions correlates with the conduction velocity of their axons
what axons are thickly myelinated and what axons are thinly myelinated
- the A alpha axons and A beta fibres are normally myelinated
- whereas small to medium size neurones such as A delta are thinly myelinated or unmyelinated C fibres
what can dorsal root ganglions be divided into
- they can be subdivided into neurochemcially district subpopulations
what is the primary neurotransmitter used by sensory neurones
- glutmate is the primary neurotransmitter that is used by sensory neurones
what neurotransmitter do some small sensory neurones use
neuropeptide substance P, calcitonin gene related peptide or somatostatin
- these are important role in inflammation
describe dorsal root ganglions the are responsive to ATP
- do not have neurpetides
- found in high concentrations in damaged tissues
- it might have a role of detecting damage tissue
what does the white matter contain
- it contains the ascending and descending fibres to and from the brain int he dorsal, lateral and ventral funiculi
what does the grey matter contain
- cells and terminus of the primary afferents from the periphery
what does the lateral horn (in the thoracic region contain)
containing the cell bodies of the sympathetic and parasympathetic preganglionic fibres - the autonomic system outflow
where is the lateral horn in the spinal cord
it is in the thoracic and upper lumbar cord T1-L3
how many laminae can the grey matter be divided into
- 10 lamina based on cell size and density
what are the gross features of the spinal cord
Overall shape
- cervical sections are wide and squashed looking like an oval, they are larger than thoracic sections and lumbar and sacral are more circle shaped
Ventral Horn Enlargement
- At segments that control a limb, the motoneurons are large and numerous.
- This causes enlarged ventral horns in two places: the lower cervical sections (C5-C8) and the lumbar/sacral sections.
- If you see an enlargement, you just need to differentiate cervical from lumbar. This can be done by shape or by proportion of white matter
Amount of white matter relative to grey matter
- this decreases as you move down the cord
- in the white matter of the cervical cord you have all of the axons going to or from the entire body, more or less.
- In sacral cord, the white matter contains only those axons going to or from the last couple of dermatomes - all other axons have “exited” at higher levels. This is why sacral cord looks like it has so much gray matter - really, it has just lost all of the white.
how many laminae are there in the dorsal horn
6
what are the laminae in the dorsal horn
- Laminae I-II together are known as the superficial dorsal horn and they receive information from C and Ad fibres (nociceptors - “pain” fibres).
- Laminae III-VI receive input from Ab fibres (low threshold cutaneous receptors).
describe lamina II
- A pale-staining area through which few myelinated fibres pass caps the dorsal horn: this is lamina II, the substantia gelatinosa
- many C fibres terminate here they have no myelin therefore there is less stain
- this area is important for the regulation of pain sensitivity
what laminae are in the ventral horn
The ventral horn consists of laminae VII-IX (7-9)
describe the structure of the ventral horn
- motor neurones are the most prominent cell structures visible in this region due to their large size
- different motorneuorne populations supply different muscles and are differently organised in the ventral horn
- motoneruones to distal muscles are found to lateral positions while those to proximal muscles are located more medially
where are motor neurones supply extensor and flexor muscles located
- supply external muscles are more ventral than flexor muscle motoneurons
what do somatic motornueornes in the ventral horn and sympathetic preganglionic neurones use as a neurotransmitter
acetycholine
describe how a stretch reflex works
- 1a afferent info about muscle stretch originates in receptors in the muscle spindle
- it is transmitted to the dorsal horn by sensory neurone in the most simplistic reflex arch
- this information is conveyed to the motor neurone at its synapse in the ventral horn
- this initiates contraction of the stretched muscle
what are the 3 ascending tracts
DCML
Spinothalamic
Spinocerebellar
what is the descending tract
corticospinal tract
what does the corticospinal tract do
controls motor output and voluntary movement
what does the DCML do
- discriminative touch
- touch pressure vibration
- proprioception
what is the DCML divided into
- gracilis and cutaneous fascicule
- dorsal column goes into the medial leminscisus and ends in the thalamus
what information does the spinothalamic tract convert
- crude touch
- pain
- temperature
what information does the spinocerebelllar tract convey
- unconscious proprioception to the cerebellum
- cerebellum requires continuous feedback on what the muscle is doing in order to coordinate movement but because the pathway doesn’t go to the cerebral cortex it does not become conscious
where does the DCML decssuate
- crosses high in the medulla after they have synapse and become second order neurones, they then travel to the medial leminsicus
where does the spin thalamic decussate
- spinal tract, it becomes a 2nd order neurone int he dorsal horn and then this sends an axon to the oppotsite side through the ventral funiculus
where does the spinocerbellar decussate
- it doesn’t
- this is become the cerebellum works on an ipsilateral basis
where does the corticospinal tract decussate
- they cross over at the junction between the spinal cord and the most caudal part of the brain which is the medulla
- 80% of axons decussate and form the lateral CST
- the other 20% form the anterior CST - anterior CST axons innervate bilaterally the more medially located motor neurones of axial and proximal muscles
where do the axons from the corticopsinal tract originate
- mostly they originate from the primary motor cortex
- supplemental motor
- premotor and other cortical areas
if a lesion affects as sensory input path….
(such as a dorsal root) all modalities from the dermatome will be affected
if a lesion affects a motor output
(such as ventral root or motorneuroens)
- there won’t be input to the muscle therefore the muscle will not be able to contract and become weaker or flaccid this is a lower motor neurone lesion
what happens if a lesion affects the corticospianl tract
- this is an upper motor neurone lesion
- the inhibitor influences on muscle reflexes and tone is lost
- therefore there is weakness and spasticity and hyperactive reflexes
- babinskin sing will happen
what are the most common causes to spinal cord damage
- trauma injuries are most common
what are the two phases of spinal cord damage
- primary damage
- secondary damage
what is primary damage
- this results from cord compression, contusions, laceration or haemorrhage which occurs immediately on injury
what is secondary damage
- this is initiated by the trauma but occurs over a period of hours to days to months
- these involve physiological alterations to trauma, hypoxia and ischaemia
what is spaniel shocks
state of shock, or temporary loss of function in the spinal cord.
- All the functions and muscle reflexes below the level of the lesion are depressed or absent, giving the impression of flaccid paralysis of muscles and loss of sensation
- neurones gradually repair and the flaccid paralysis turn to spastic paresis
- first reflexes to reappear are flexion reflexes, after several months muscle tone and tendon reflexes may occur
what happens in the acute phase of traumatic spinal cord injury
- variable amounts of oedema, haemorrhaging, axonal swelling, ascending and descending tract disruption and foci of infraction
what happens over the following few weeks to the spinal cord
- grey matter becomes necrotic
- there is macrophage infiltration and therefore a gradual removal of degenerative debris
- activation of microglia and astrocytes
- then inflation by fibroblasts and collagenous fibrosis occurs
- there is cavitation involving the ventral part of the dorsal columns which is maximal at the site of injury but extended a few segments rostral or caudal into the injury
