Histology: Spinal Cord Flashcards
Name the six types of glial cells
1) Ependymal cells
2) Astrocytes
3) Microglia
4) Oligodendrocytes
5) Schwann cells
6) Satellite cells
Define ‘glia’
the connective tissue of the nervous system, consisting of several different types of cell associated with neurons.
Name the 4 types of glia in the central nervous system
Ependymal cells, astrocytes, microglia, oligodendrocytes
Name the 2 types of glia in the peripheral nervous system
Schwann cells and satellite cells
What are ‘autonomic ganglia’?
Cell bodies of the neurones in the autonomic nervous systems
Describe the location and function of ependymal cells
They line fluid-filled cavities of CNS (e.g. spinal cord central canal and brain ventricles) and are involved in the circulation of CSF
Describe ependymal cells histologically
They are ciliated and the cilia movement circulates CSF through hollow spaces . They look like epithelia without a basement membrane
Describe the location and function of astrocytes
Most common glial cell in CNS, surround neurones. Function: structural and metabolic support, help form BBB, regulate K+/Ca2+ conc., regulate pH and neurotransmitter availability (can mop-up excess transmitter) and form scars in areas of damage
Describe astrocytes histologically
Are star shaped and can be distinguished from neurones by presence of glial fibrillary acid protein (GFAP)
Describe the location and function of microglia
Distributed throughout the CNS. In development release GFs and act like macrophages to pick up debris from apoptosis, in adults they are surveillance cells and in injury they change shape and revert to phagocytic macrophages known as reactive microglia.
Describe microglia histologically
They are small cells in the CNS which have long spiny processes
Describe oligodendrocytes histologically
Have a small number of processes
Describe the location and function of oligodendrocytes
Insulate axons of the CNS (myelinate). Each cell can myelinate up to 60 axons and their cell bodies lie between those axons
Describe the location and function of Schwann cells
Myelinate axons of the PNS, though only myelinate a single axon segment.
Describe Schwann cells histologically
Cell bodies are close to the myelin sheath and the sheath is covered by external lamina
Describe the location and function of satellite cells
Found in the space around ganglion cells in the PNS and have a supportive function similarly to astrocytes in the CNS.
Describe satellite cells histologically
Oval-nuclei cells which surround cell bodies of ganglionic cells.
What is the primary neurotransmitter used by sensory neurones?
Glutamate
What other neurotransmitters may sensory neurones use?
Neuropeptide substance P, CGRP or somatostatin as a co-transmitter
Why may some dorsal root ganglion cells have a role in detecting tissue damage?
Some are selectively responsive to ATP which is found in higher concentrations in damaged tissue
Explain what is meant by ‘myelination’
Where a single Schwann cell or oligodendrocyte process wraps itself round a segment of axon, forming layers of tightly wrapped plasma membrane
What is the function of the regions of cytoplasm within myelinating cells?
Maintains cell constituents
What types of neurones are associated with non-myelinating Schwann cells?
small-diameter sensory C fibres or postganglionic autonomic efferents
Why are some axons referred to as unmyelinated?
They are only wrapped with one fold of their plasma membrane - by non-myelinating Schwann cells
What are remack bundles?
collections of axons supported by the same Schwann cell
How are axons separated in remack bundles?
By thin extensions of the Schwann cell body
Define ‘mesaxon’
a pair of parallel plasma membranes of a Schwann cell, marking the point of edge-to-edge contact by the Schwann cell encircling the axon
What is the role of the perineurium?
A selective barrier to restrict passage to the nerve fibres in the endoneurium and forms part of the BBB
Define ‘epineurium’
The outermost layer of dense irregular connective tissue surrounding a peripheral nerve. It usually surrounds multiple nerve fascicles as well as blood vessels which supply the nerve
Define ‘perineurium’
The sheath of connective tissue surrounding a bundle (fascicle) of nerve fibres within a nerve.
Define ‘endoneurium’
A layer of delicate connective tissue around the myelin sheath of each myelinated nerve fiber
Describe the contents of the axonal cytoplasm and their function
Numerous mitochondria to power Na+/K+ pump, and microtubules (appear as dots in cross-section) which work to maintain axon shape and guide products of the soma to the axon terminal. Axon cytoplasm generally bland-looking.
What is the role of the dorsal roots?
To receive sensory information from the periphery
What is the role of the ventral roots?
To carry motor and autonomic information from the spinal cord to the periphery
What is the role of the dorsal root ganglion?
To contain the cell bodies of visceral and somatic sensory neurones
How many pairs of spinal nerves are there?
31
How many spinal nerves project from each of the spinal segments?
Cervical - 8 Thoracic - 12 Lumbar - 5 Sacral - 5 Coccygeal - 1
At which points of the spinal cord do the spinal nerves intermingle to form mixed nerves?
At cervical and lumbar enlargements of the spinal cord, the nerve fibres intermingle in the brachial and lumbosacral plexi to form peripheral nerves with multiple nerve roots.
Where are the cell bodies of postganglionic sympathetic neurones found?
sympathetic chain ganglia (T1-L3)
Where are the cell bodies of preganglionic sympathetic neurones found?
Thoracic cord (stemming from lateral horn)
Where is the sympathetic chain ganglia found?
It runs from T1-L3
Where is the white matter in the spinal cord?
In the spinal cord white matter surrounds the grey matter
Where is the white matter in the brain?
In the brain, grey matter surrounds the white matter
What does spinal cord grey matter contain?
Neuronal cell bodies and terminals of primary sensory afferents
Where are lateral horns present?
Thoracic and sacral levels
What are lateral horns involved with?
Autonomic outflow
What is the lateral horn of T1-L3 involved with?
Contains preganglionic sympathetic neurones (thoracic cord)
What is the lateral horn of the sacral level involved with (sacral cord)?
Contains preganglionic parasympathetic fibres
Where are preganglionic parasympathetic fibres found?
Sacral cord
What is the function of the substantia gelatinosa and where is it found?
Has few myelinated fibres passing through it and caps the dorsal horn in lamina II, and as many nociceptive C fibres terminate here, it’s very important in the regulation of pain sensitivity
What is meant by ‘ventral horn enlargement’?
In segments that control a limb, the motor neurones are large and numerous which causes the ventral horns to enlarge
Which areas of the spinal cord illustrate ventral horn enlargement and why?
In lower cervical sections (C5-C8) and lower lumbar sections (and some lower sacral) - so if ventral horn enlargement is present you have to distinguish between cervical and lumbar generally.
Which 3 things help to distinguish parts of the spinal cord?
overall shape, ventral horn enlargement and amount of grey matter: white matter
Why does the amount of white matter appear to change as you move down the spinal cord?
Amount of white matter relative to grey matter decreases as the upper segments are still carrying most of the ascending and descending fibres, whereas in the more caudal regions, most of the nerve fibres have exited the cord, so it has little white matter
Describe the gross anatomy of the cervical spinal cord
Wide, flat cord with lots of white matter (has ventral horn enlargements in lower sections)
Describe the gross anatomy of the thoracic spinal cord
Has pointed tips of the lateral horns (which sticks out between the small dorsal and ventral horns)
Describe the gross anatomy of the lumbar spinal cord
It is a round (circular) cord with ventral horn enlargements
Describe the gross anatomy of the sacral spinal cord
It is a small, round cord with almost no white matter
How many laminae is the dorsal horn split into, and on what basis is this done?
6, on the basis of cell size
What are laminae I-II known as?
Superficial dorsal horn
Describe the input to the superficial dorsal horn
Receives info from C and Adelta fibres (mainly nociceptor fibres). Lamina I from cutaneous, muscle and visceral nociceptors, lamina II from primarily cutaneous input
What is Lissauer’s tract and what is it’s function?
It caps and merges with lamina I of the dorsal horn and allows integration of nociceptive reflexes on a broader area
Where do sensory C fibres primarily terminate?
Laminae I-II
Where do low-threshold cutaneous A-beta fibres terminate?
Laminae III-VI
Where do high-threshold A-delta fires terminate?
Lamina I and IV/V
Which laminae is in the ventral horn?
Laminae VII-IX
How are motor neurones arranged in the ventral horn?
Motor neurones to distal muscles are more lateral, to proximal muscles are more medial. Motor neurones to extensors are more ventral, motor neurones to flexors are more dorsal.
Which enzyme makes acetylcholine?
Choline acetyl transferase
Which neurotransmitter is used by somatic motor neurones in the ventral horn?
ACh
Which neurotransmitter is used by sympathetic preganglionic neurones of the lateral horn?
ACh
Name the 3 ascending (sensory tracts) of the spinal cord
dorsal column medial lemniscus (DCML), spinothalamic, spinocerebellar tracts
What information does the DCML tract carry?
Discriminative touch (touch, pressure, vibration) and where our limbs are in space (conscious proprioception)
Describe the DCML tract
Comprises of the dorsal columns of the gracile and cuneate fasciculi in the dorsal funiculus, decussates at the medulla and travels to the thalamus and then to the somatosensory cortex.
What information does the gracile nuclei carry?
Information about the lower body (and is the most medial anatomically)
What information does the cuneate nuclei carry?
Information about the upper body (and is the most lateral in the dorsal funiculus)
What information does the spinothalamic tract carry?
Pain and temperature (+ itch and tickle)
Describe the spinothalamic tract
Decussates in the spinal cord where primary afferent synapses with second order neurone in the dorsal horn which then sends axon to opposite side through ventral funiculus about one segment above origin of entry.
Describe the location of the 4 main tracts in the spinal cord
DCML - in the dorsal funiculus
STT - most ventral, lies below corticospinal
SCT - most lateral
CST - circular and dorsal
What information does the spinocerebellar tract carry?
Unconscious proprioceptive information to the cerebellum for continuous feedback
Describe the spinocerebellar tract
It doesn’t decussate as the cerebellum works on an ipsilateral basis, and provides continuous feedback from muscles to allow coordinated movement.
What information does the corticospinal tract carry?
Motor innervation to the periphery to allow voluntary movement and reflexes
Describe the corticospinal tract
Tract begins in precentral gyrus (mainly motor cortex) and 80% of axons decussate at medulla, forming lateral corticospinal tract, 20% do not and decussate just prior to periphery which they innervate (anterior corticospinal tract). ACST travels in the ventral funiculus and innervates more medial motor neurones.
What happens if a lesion affects a dorsal root/horn?
Sensory input from dermatome affected, though there is overlap so multiple damage may have to occur before noticeable
What happens if a lesion affects a ventral root or motor neurone?
No input to muscles, causing them to be weak or completely flaccid; this is a lower motor neurone lesion
What happens if a lesion affects the CST?
This is an upper motor neurone lesion which inhibits muscle reflexes and causes loss of tone, as well as weakness (paresis) or paralysis alongside spasticity and hyperactive reflexes
What is a Babinski sign?
In baby stroking of foot plantar surface causes dorsiflexion and toe-spread, in adults should cause plantarflexion. Babinski sign is indicative of infant response instead of adult (normal)
When is a Babinski sign displayed?
If the corticospinal tract is lesioned
What is Brown-Sequard Syndrome?
It is a complete hemisection of the spinal cord
What are the symptoms of Brown-Sequard syndrome?
Contralateral loss of pain and temp sensation, ipsilateral loss of discriminative touch, spastic paresis below lesion, flaccid paralysis at lesion level, Babinski sign, hyper-reflexia below lesion, ipsilateral loss of all sensory modalities below lesion level
What is anterior spinal artery occlusion?
The loss of blood and an infarction at a spinal/segmental level
What are the symptoms of anterior spinal artery occlusion?
Bilateral loss of pain and temperature sensation, spastic paresis below lesion, flaccid paralysis at level of lesion, Babinski sign, hyper-reflexia below lesion
What is Tabes Dorsalis?
Infection with treponema pallidum (tertiary syphilis)
What are the symptoms of Tabes Dorsalis?
Bilateral loss of discriminative touch and proprioceptive sensation
What is Amyotrophic Lateral Sclerosis?
motor neurone disease
What are the symptoms of Amyotrophic Lateral Sclerosis?
spastic paresis below lesion level, flaccid paralysis at lesion level, Babinski sign, hyper-reflexia below lesion
Describe the two stages of spinal cord damage
Primary damage results from initial trauma e.g. compression which occurs on injury, secondary damage is initiated by the trauma but occurs gradually and involves physiological alterations to trauma
What is meant by spinal shock?
A state of shock whereby there is temporary loss of function in the spinal cord which is often produced initially as a result of severe spinal cord injury
Outline the stages of spinal shock
To start, all functions and reflexes below lesion are depressed or absent giving flaccid paralysis and sensation loss, then few days/weeks neurones regain excitability slowly so flaccid paralysis –> spastic paresis, then flexion reflexes begin to reappear then after several months, muscle tone and tendon reflexes may occur
Describe the histology of spinal cord trauma
over weeks, grey matter becomes necrotic, leading to macrophage infiltration and removal of degenerative debris. Over more time there is infiltration by fibroblasts and associated collagenous fibrosis occurs to repair.
What is present in the endoneurium?
Collagen fibres
What is present around a Schwann cell?
External lamina
Where does cavitation often occur in spinal cord injury?
Ventral part of dorsal column
