Spinal cord Flashcards
How is the spinal cord the main effector of all behavioural responses
The spinal cord must relay info from diverse receptors in the PNS to the CNS, where higher brain centres may perceive it, and cause voluntary or reflex behavioural response
What is the central core and surround of the spinal cord
Central core of nerve cells (grey matter) surrounded by fibre tracts (white matter) rich in myelinated axon tracts
Also a central CSF filled canal
How is the grey matter of the spinal cord divided
Dorsal horn (posterior) that is sensory in function, and a ventral horn that is motor in function
How is the white matter of the spinal cord divided
Dorsal, lateral and anterior (ventral) columns/funiculi
What surrounds the spinal cord
The 3 layers of meninges- the thick dura, the fine arachnoid, the surface covering of pia mater
Where does the spnial cord receive sensory (afferent) info
From central axons of the dorsal root ganglion neurons, that enter the spinal cord through the dorsal spinal nerve roots
Where does the spinal cord provide motor (efferent) output from MAINLY
Motor neurons in the ventral horn leave the spinal cord through the ventral spinal nerve roots to gp skeletal muscles
Where does the spinal cord provide motor (efferent) output from as well as the ventral horn
Ouput from lateral horn sympathetic preganglionic neurons (in T1-L2), and parasympathetic pre-ganglionic neurons (in S2-4) to blood vessels, glands, and viscera
Summarise how the neural tube is formed in development
The neural plate forms from the ectoderm at gastrulation- the edges of the neural plate roll up to form the neural tube
What is the result of failure to form the neural tue
Spina bifida
What cues determine the patterning of the spinal cord
Diffusible inductive cues called Morphogens which form cross repressive interactions
What is the Sonic hedgehog (Shh) protein
A protein in the notochord that specifies floor plate and induces the ventral horn of the neural tube (Mcmahon et al, 1993)
What are bone morphogenetic proteins (BMPs)
Proteins from the ectoderm that overlies the notochord and specifies the roof plate, inducing the dorsal horn formation (Urist, 1965)
What are the Morphogens that control the patterning of the spinal cord
Sonic hedgehog (Shh) protein and bone morphogenetic proteins (BMPs)
Study showing the role of the notochord as a signalling centre- what was the proce
Holtfreter (1933)- transplanted a notochord into a different animal at the side of the spinal cord, found an ectopic floor plate and third set of motor neurons was developed at the side of the spinal cord, and in the animal with no notochord no motor neurons are formed
Study showing the role of Shh protein in ventral identity
Briscoe et al (2000)- Shh protein acts in a concentration dependent manner to specify a range of different motor related interneurons and motor neurons
What is the node
Another early patterning centre of the spinal cord that secretes signals to lead to the rhythmic clock creation of somites in early development
What do the somites so
Control segmental outgrowth of axons in peripheral nerve roots, prevent axon growth through their posterior halves and regulate the development of a segmentally arranged pattern of spinal nerves (Tannahill et al, 1997)
What reflects the somite-induced segmentation of outgrowing spinal nerves
Dermatomes
What are dermatomes
Areas of skin supplied by the right and left sensory neurons from a single spinal nerve ganglion
What are the applications of dermatomes for C section
Can assess sensory limits of regional anaesthesia
What are the applications of dermatones in radiculopathy
Can assess level of spinal cord injury from trauma to radiculopathy by working out which spinal nerve ganglions are implicated based on sensation in different dermatomes
Where is the end of the spinal cord in adults
Situated at the level of lumbar vertebrae L1-L2
What 4 regions is the spinal cord divided into
Cervical, thoracic, lumbar and sacral
In which spinal cord regions is the grey matter largest at and why
Cervical and lumbar enlargements, reflecting the larger no of ventral motor efferents to the limbs, and sensory interneurons and projection afferent neurons (dorsal) from the limbs
Where in the spinal cord is the amount of white matter the largest
Rostral end of the cord
How does the size of the ascending tracts change as one moves up the cord
As one moves up the cord, more sensory fibres are added, so the ascending tracts increase in size as one moves up
How does the size of the descending tracts change as one moves down the cord
As one moves down the cord, descending tracts diminish, as fibres from the descending tracts diminish in the grey matter
What are the size of dorsal/ventral/lateral horns of the thoracic cord
Smaller dorsal and ventral horns, reflecting sparese innervation to trunk
More prominent lateral horn
What does the lateral horn of grey matter contain
Sympathetic preganglionic neurons, and neurons of Clarke’s column
Where is the cervical enlargement
C3-T2 vertebrae
Where is the lumbar enlargement
L1-S2 enlargements
Where are the sympathetic autonomic pregangionic neurons
T1-L2
Where are the parasympathetic autonomic pregangionic neurons
In the sacral region
What are the gracile and cuneate fasiculus
Gracile from lumbar region, cuneate from upper limbs
Dorsal column nuclei, relay the dorsal column fibres up to the higher order brain centres
What are the size of the ventral/dorsal horns in the cervical cord
Dorsal horns are prominent reflecting the importance of the hand as a sensory structure
Ventral horns also large, with large diameter motor neurons supplying arm and hand muscles
At which region of the spinal cord are white matter tracts the largest
Cervical cord
What are the size of the ventral/dorsal horns in the lumbar enlargement
Big due to innervation of lower limb- dorsal horns at their largest as most sensory info is concerned with reflexes keeping us upright, ventral horn as largest motor neurons with long axons to innervate foot and leg muscles
How are motor neurons organised at the coronal (separates dorsal and ventral) axis of spine
Motor neurons and associated interneurons are arranged in an orderly fashion reflecting their targets in the limb
How do fine unmyelinated axons enter the dorsal horn
Lissauer’s tract, a fine outer tract containing Aδ, C, noci-, and thermal fibres- enters laterally and projects across several segments
How do large diameter axons enter the dorsal horn
Aalphas and Abetas from proprio- and mechanoreceptors enter medially and possess ascending collaterals
What are the 3 main functional zones of the dorsal horn
Marginal zone, substatia gelatinosa, ‘main sensory nucleus’
3 main functional zones of the dorsal horn- marginal zone?
Lamina I- area of grey matter where many nociceptive fibres end (Aδ)
3 main functional zones of the dorsal horn- substantia gelatinosa
Lamina II- interneurons, primarily innervated by C fibres
3 main functional zones of the dorsal horn- what makes up the main sensory nucleus
Nucleus proprius (lamina 3,4,5,6)- consists of projection neurons which send axons that descend the cord in the anterolateral column of grey matter
3 main functional zones of the dorsal horn- what does the main sensory nucleus receive info from
Aδ and interneurons, as well as alpha and beta fibres
How are motor neurons gruoped in the ventral horn
Motor neurons that supply a single muscle are grouped into ‘motor pools’ which may extend several segments of the cord- these pools are arranged in vertical antagonistic pairs
Where do motor neurons that supply distal vs axial muscle lie in the ventral horn
Motor neurons supplying axial muscles- medial
Motor neurons supplying distal limb muscles- lateral
Where do motor neurons that supply flexors vs extensors lie
Motor neurons pools supplying flexors- dorsal
Motor neuron pools supplying extensors- ventral
What provides the largest part of the synaptic input onto motor neurons
Groups of excitatory and inhibitory interneurons they are associated with
What is local circuity in the spinal cord
Links sensory input to motor input via various spinal cord reflexes, has intrinsic motor programmes that combine motor neurons/surrounding interneurons/sensory feedback
What are distal projections in the spinal cord
Project sensory information to higher centres via ascending tracts, receive receive descending tracts
Examples of the distal projections of the spinal cord
Corticospinal tract, anterior lateral system, spinocerebellar tract
What are the different types of afferent fibres
Large diameter fibres- Aalpha and Abeta
Small diameter fibres- Aδ (delta) and C fibres
How fast can Aalpha and Abeta fibres conduct and why
Thickly myelinated and thick diameter means they conduct action potentials very quickly, 120m/s
What do Aalpha and Abeta fibres provide info for
Online adjustment of movement and locomotor behaviour
What do Aalpha fibres do
Primarily proprioceptors of skeletal muscle, immediate feedback reflex arcs
Sensory innervation to muscle spindles
What is proprioception
A sense of body position, essential for locomotion and balance
Where do Aalpha and Abeta form synapses
In the dorsal horn and on motor neurons (for stretch reflex)
Where do Aalpha and Abeta send collateral branches
Collateral branches ascend in the dorsal columns to inform higher centres for conscious proprioception
What are the 4 classical mechanoreceptors
Meissner’s corpuscle and Merkel cells (superficial), Pascianian corpuscle and Ruffini endings (deep)
What do Abeta fibres fo
Touch- carry info from cutaneous receptors (in epi/dermis), and static proprioceptors
Which fibres control dynamic vs static proprioception
Aalpha fibres- dynamic
Abeta fibres- static
How do the terminations of dorsal root ganglion cells differ
Differ for each type of fibre ie sensory modality- each dorsal root ganglion axon branches in a discrete region of the dorsal horn at multiple sequential levels
What is the myelination of Adelta vs C fibres
Adelta are thinly myelinated, C fibre is not myelinated, so velocity is a lot slower
What is the velocity of C fibres
0.5-2m per second
What do Adelta and C fibres do
Both convey pain and temp info, C fibres convey itch info
Where do Adelta and C fibres terminate
In the dorsal horn, form distinct layers of innervation
Where are the dorsal root ganglia located
Lie outside the spinal cord and in the invertebral foramen central to where the dorsal and ventral nerve roots join
What are the dorsal columns involved in
Touch
Where do DRG large fibres synapse in the grey matter (as well as sending branches that ascend the dorsal columns)
Synapse on neurons in the substantia gelatinosa, main sensory nucleus, intermediate zone (on interneurons), directly to motor neurons in the ventral horn for the monosynaptic stretch reflex
What forms the dorsal columns
The ascending collateral branches of the primary afferent fibres (largely from mechanoreceptors) form the dorsal columns
Where are the dorsal columns in the lower body vs upper body
Lower body- gracile fasciculus
Upper body- cuneate fasciculus
How are the dorsal columns organised in the spinal cord
Uncrossed tracts- their fibres are somatotopically organised
Where do the dorsal columns terminate
On relay neurons of the dorsal column nuclei in the medulla, that give rise to the medial lemniscus, which projects to the contralateral thalamus which projects to the cerebral cortex
What is the anterolateral system involved in
Pain (nociception) and temperature
How does Lissauer’s tract amplify nociceptive afferent input in the anterolateral system
Innervates multiple cells as fibres travel a short distance up and down it, more likely to get through conscious reception and recruit reflex pathways
How do fine fibres mediating nociception and temp enter the cord in the anterolateral system
Enter the cord and send short ascending and descending collaterals in Lissaur’s tract, which run to adjacent spinal levels, then synpse on cells in the substantia gelatinosa
What complex network of interconnections is formed in the anterolateral system
Complex interconnections betwee the cells of the substantia gelatinosa and the afferent fibres supplying the marginal zone and main sensory nucleus
What indirect ascending nociceptive tracts are there
Projection (second order) fibres from the marginal zone and main sensory nucleus cross the midline as the anterior white commissure and ascend in the anterolateral columns
What is the anterolateral system composed of
Composed largely of crossed tracts due to the anterior white commissure, and includes the spinothalamic tract that relays info directly to the thalamus
Where do the fibres of the anterolateral system terminate
Many terminate in the reticular formation of the medulla, pons and midbrain, but some ascend to the thalamus (lateral and anterior spinothalamic tract) for relay to the cerebral cortex
What are the spinal-cerebelllar tracts involved in
Proprioception
What are the spino-cerebellar tracts formed of
Arise from cells of the intermediate grey matter- proprioceptive afferents from the leg ascend to thoracic levels and axon collateral synapse onto neurons in Clarke’s column
What is the larger diamater spino-cerebellar tract
The uncrossed dorsal spino-cerebellar tract originates in neurons of Clarke’s column, which relat info up to the ipsilateral cerebellum
Where are the spino cerebellar tracts situated
Lateral funiculus
What are the main ascending tracts
Dorsal columns, spinothalamic tract and Lissauer’s tract (of anterolateral system), spino-cerebellar tracts
What type of systems is parallel processing a useful feature of
Where it is advantageous to keep info sensed by each receptor sub-class separate and therefore maximise the detailed information received by the brain
Dorsal column vs anterolateral system- ipsilateral or contralateral?
Anterolateral system- contralateral
Dorsal column- axons mostly arise from a single peripheral receptor and ascend ipsilaterally
What is the main descending spinal cord pathway
Corticospinal tract
What is the role of the corticospnial tract
Voluntary movement
Where does the corticospnial tract travel
Arises from somatosensory and motor cortex, descends to the caudal medulla where most axons cross to the contralateral side to form the lateral corticospinal tract
What is the composition of the lateral corticospinal tract
50% comes from primary motor cortex, other sources are the supplementary motor area, premotor cortex and sensory areas
What is the larger ventral uncrossed pathway of the corticospinal tract
Anteriori corticospinal tract
Descending tracts- rubrospinal tract?
Motor tract arising from the red nucleus, involved in voluntary movement, flexion distal arm muscles
Descending tracts- tectospinal?
Mot tract arising from the tectum, controls reflex postural movements of the head in response to visual and auditory stimuli
Descending tracts- vestibulospinal tract?
Motor tract arising from lateral vestibular nucleus, controls upright posture and balance, conscious realisation of spatial orientation and motion
Descending tracts- descending analgesic pathways
Arises from brainstem reticular formation- involved in maintaining balance and posture, especially during body movements
Name the 4 additional descending motor tracts
Rubrospinal tract, tectospinal tract, vestibulospinal tract, analgesic pathways
How is the dorsal horn divided structurally
Divided into 6 roughly parallel layers, Rexed’s laminae
Where do smaller fibres project to when joining to dorsal columns
Project onto to the cord itself, and ascending fibres are derived from second-order neurons
Evidence that the anterolateral system carries pain info
Dissection of the anterolateral system causes complete peripheral analgesia for both pricking and burning pain
What is the reticular formation
Consists of axons and dendrites weaving together with long branching axons going upwards to the midbrain and forebrain, and down to the spinal cord
What are upward projections of the reticular formation concerned with
Regulation of the level of brain activity including attention, sleep, arousal
What are downward projections of the reticular formation concerned with
Generating patterns of response that can often be stereotypeed eg control of visceral activity, and timing more clearly motor functions
What are the reticulospinal tracts
Two large descending tracts that go down from the reticular formation to the spinal cord, mediate movement
How long are the neurons of the corticospinal tract
Its neurons are some of the longest in the body, running from the cerebral cortex to the bottom of the spinal cord
What happens to the fibres at the upper end of the corticospinal tract
Fibres fan out into a sheet called the internal capsule , in order to squeeze past the thalamus and basal ganglia
How many pairs of dorsal and ventral roots are there down the human spinal cord
30 pairs
What is the effect when a dorsal root is cut
The corresponding dermatome on that side of the body does not lose all sensation, as the adjacent dorsal roots innervate overlapping areas- to lose all sensation in one dermatone, 3 adjacent dorsal roots must be cut
Shingles as evidence of dermatomes
The skin innervated by the axons of one dorsal root is revealed by shingles, where the neurons of a single dorsal root ganglion become infected with a virus
How does the spinal cord continue below the lumbar vertebra 1-2
The bundles of spinal nerves streaming down within the lumbar and sacral vertical column are the caudal equina, contained within a sack of dura filled with CSF
What are second-order sensory neurons
Neurons that receive sensory input from primary afferents, most lie within the dorsal horns
What info does the spinothalamic pathway convey
Info about pain and temp
How wide of an area do spinothalamic tract axons synapse over in the brain
A wider region of the thalamus than those of the medial lemniscus do
What are lower motor neurons
Somatic motor neurons in the ventral horn of the spinal cord that innervate the somatic musculature, called lower to distinguish them from the higher order upper motor neurons of the brain that supply input to the spinal cord
How do spinal nerves exit the spinal cord
Notches in the vertebra
