Week 10 Flashcards
What are sensory afferents carried by
Ascending tracts
What’s carried by descending tracts
Somatic efferents and autonomic efferents
Somatic sensory afferents are carried by what ascending tracts
Lateral spinothalamic tract LSTT— carries pain and temp sensation
Dorsal column DC— fasciculus gracilis and the fasciculus cuneatus represent dorsal columns. Carry proprioceptor signals
Dorsal/ ventral spinocerebellar tract DSCT- conveys low range proprioceptive stimuli from receptors located in muscles tendons and joints of hindlimb
What separates the motor and sensory brain
Central sulcus
The pre central gyrus- commands motor control, in front of central sulcus, part of frontal lobe
The post central gyrus- commands sensory control, behind central sulcus and part of parietal lobe
Frontal lobe= motor
Parietal and occipital= sensory
Processing of sensory info
Spinal cord (dorsal horn)- medulla- thalamus- cerebral cortex
Sensation from head and neck is conveyed to brain largely by CNV and the C2 C3 spinal nerves
The first centre of the cerebral cortex to see sensory signals is the post central gyrus (primary sensory cortex)
A lesion at the post central gyrus will eliminate sensation
What are ascending tracts
Neural pathways located in the white matter that conduct afferent information from the peripheral nerves to the cerebral cortex
What are the 2 categories of sensations
Conscious sensation- pain, temp, crude touch (largely carried by LSTT)
Non-conscious sensation- tactile sensation, muscle length, tension, joint position etc
Impulses that give rise to conscious sensation are conveyed differently from those giving rise to non conscious sensation
What is the somatosensory system
All neurones of the brain and spinal cord that act to convey somatic sensation
The first stage of spinal sensation
Primary sensory neurones have their cell bodies in dorsal root ganglion. They have pseudo unipolar morphology
On reaching spinal cord, the axon of the primary sensory neurone then divides into 3 branches
Assuming the neurone is carrying T6 sensory signals
1 axonal process remains in dorsal horn of its respective spinal segment T6
One axonal process is sent to dorsal horn of spinal segment above and another sent to one below
The axonal segments travelling to spinal segments above and below travel in the Posterolateral tract of Lissauer
Sensory signals carried in these 3 branches are then processed by spinal cord in the same way
Variant where signals reach post central gyrus via spinothalamic tract
Pain and temp
The first sensory neurone carries signals to dorsal horn where it terminates by synapsing on a follower cell
A second order sensory neurone picks up the sensory signal then passes under the central canal to travel on opposite side spinal cord
It then ascends until reaches thalamus where it terminates by synapsing to third order sensory neurone, which axons ascend further to terminate in post central gyrus of cerebral cortex
Information is carried on the contralateral-> spinal cord, medulla, thalamus, cortex. Sensory info from right side of body is perceived by left brain
Variant 2 which signals reach post central gyrus via dorsal column
Proprioceptive and tactile info
Primary sensory neurone enters spinal cord
Neurone remains ipsilateral and ascends until level of medulla where it decussates to the contralateral side and then terminates in medulla
Second order neurone then travels in contralateral medulla, pons, midbrain and thalamus where it terminates
Third order neurone then ascends and terminates in the contralateral sensory motor cortex
At the spinal level transmission of sensory info is dissociated
Proprioceptive info from the right side is carried by right spinal cord
Pain and temperature from right side is carried by left spinal cord
All sensation then come together on same side at the medullary level and then travel together to the primary somatosensory cortex
Primary sensory cortex
Has full sensory representation of the body’s surface
The size of the body part does not equate to amount of primary sensory cortex that’s dedicated to it
Not isometric
Syringomyelia
Medial condition in which a CSF filled cyst forms within central canal of spinal cord = syrinx
Growth of cyst leads to damage of sensory fibres decussating under central canal
Results in anaesthesia- no pain reception at the level of certain parts
Primary motor cortex
Occupies the pre central gyrus frontal lobe
Voluntary motor commands of muscles are initiated by the primary motor cortex
Cortical representation of the motor system
Motor systems of the body: all neurones of the brain and spinal cord that act to produce movements
Voluntary motor commands are issued from the pre central gyrus of the cortex
These commands pass through the subcortical motor centres of the brain (thalamus, midbrain, brainstem, pons, medulla) and then descend the spinal cord via descending tracts eventually stopping at a ventral horn
Then exit the spinal cord and pass on to skeletal muscles
What are the two general classes of descending tracts
Both classes have their cell bodies in the brain
Cortical descending tracts- originate in the primary motor cortex- pre central gyrus. Pyramidal tracts
Non-cortical descending tracts- originate in subcortical areas of the brain e.g. thalamus, midbrain, pons, medulla, brainstem. extra pyramidal tracts
Pyramidal tract fibres are the
Upper motor neurons
lateral corticospinal tract
Ventral corticospinal tract
These have the same origin and end point
Directly innervate lower motorneurones in the anterior horn of spinal cord
extra pyramidal fibres are the
Upper motor neurones innervating lower motor neurones in anterior horn Spinal cord
Rubrospinal tract- this starts at level of brain stem from a nucleus known as red nucleus
The medullary reticulospinal tract
The pontine reticulospinal tract
Medial longitudinal fasciculus
Tectospinal tract
The brain in control of movements
Voluntary muscles of the body are controlled by the dedicated neurones of the “motor strip” in primary motor cortex
Neurones of the “motor strip” are grouped according to the skeletal muscles they command
The size of the body part does not equate to amount of primary motor cortex dedicated to it
Brain commands muscles to either rest or contract
What is the motor strip
Band of neural tissue runs down side of frontal lobe of brain
Another terms for primary motor cortex
levels of contraction vary depending on
Requirements of the body
Resting muscle tone- for maintenance of posture (minimal contraction)
Displacement of joints- movements (large forces may be required)
Motor commands of muscles
The brains motor demands of muscles are decussated
There are exceptions however
Skeletal muscles of left side of body are commanded to move or rest by the right cerebral motor cortex vice versa
Clinical relevance: disorders of movement on the right side of body tells you that left portion of brain is injured
Sensory system of the body vs that of cranial structures
Most of body’s sensory info is conveyed to the brain through the spinal cord
In the head and neck region, sensory signals are conveyed to the brain via the brainstem largely through CNV (Trigeminal nerve)
Sensory mechanisms for spinal and cranial nerves are designed along the same lines
They differ only on anatomical substrates
Day 4 to 16 embryological development
Day 4- morula= solid ball of cells formed as the zygote undergoes cleavage
Day 6- early blastocyst= hollow ball of cells with a fluid filled cavity
Day 10- late blastocyst= pre-embryo, with the embryonic disc, two layers of cells that become proper embryo , amniotic cavity
Day 16- gastrula= embryo with three primary germ layers (ectoderm, mesoderm and endoderm)
What does ectoderm form
Nervous tissue and skin
What does mesoderm form
Notochord, muscles and connective tissue , circulatory system etc
What does endoderm form
Digestive/respiratory tracts
Notogenesis
During gastrulation we have the appearance of the notochord- important in setting up neurulation
Notochord is made from primitive node and migrates to form rod-like structure in mesoderm, formed during gastrulation
Elongated midline structure in the mesoderm of the trilaminar disc
Notochord is instrumental in signalling and inducing the neural plate of ectoderm above to start to fold in on itself to generate the neural tube
Neurulation
Neural tube formation; also developmental process of forming nervous system
Occurs in week 3-4
Embryo called neurula at this stage
1st step is induction of neural plate by the notochord
Notochord (and paraxial mesoderm) sends signals to induce overlying ectoderm to thicken and form the neural plate (noggin/chordin/ follistatin= BMP-4 (bone morphogenetic protein) inhibitors )
Neural plate formation has cranial to caudal formation
This neural plate are cells of the neuroectoderm
Neural plate> neural tube
Neural plate starts folding- neural folds forming midline neural groove, neural crest cells change shape
Neural fold fuse together> neural tube
Neural tube formation in the middle of embryo and move both cranially and caudally
What is the neural tube precursor for
Entire CNS
Closure of the neural tube
Closes firstly in cervical region day 21
Neural tube closes towards cranial and caudal ends leaving two openings:
Anterior neuropore -closes around day 25
Posterior neuropore - closes around day 28
White/grey matter
Neural tube= neuroepithelial cells
Neuroblasts= nerve cell precursors arise in mantle/ intermediate zone
Intermediate zone= future grey matter
Marginal zone= future white matter
Ventricular zone- intermediate(mantle ) zone- marginal zone- spinal meninges
What does the neural canal form
Closes and forms spinal canal
What does the alar plate form
Dorsal horn
What does the basal plate form
Ventral horn
Dorsal-ventral patterning of neural tube
Spinal cord is highly organised
Neural tube is patterned both segmentally and dorsal/ventrally
Patterned by signals from surface ectoderm, paraxial mesoderm and notochord
Notochord produces Sonic hedgehog Shh induces floor plate to produce Shh=morphogen
Wnts/BMPs> dorsalisation> induces roof plate
Patterns dorsal side of tube (alar plate)
Shh (notochord)> ventralisation> induces floor plate
Patterns ventral side- basal plate
Shh and BMP work antagonistically. Both produce transcription factors that inhibit each other
What is the ependyma
Thin membrane of glial cells lining the ventricles of the brain and the central canal of spinal cord
Neural crest cells
During neurulation cells at the lateral border/ crest begin to dissociate from their neighbouring cells: neural crest cells
Epithelial to mesenchymal transition, migrate
Neural crest cells differentiate into many types cells including peripheral and enteric neurones and glia, bone and smooth muscle and connective tissue
Ganglia , melanocytes, Schwann cells, chromaffin cells in adrenal medulla
Different types of neural crest cells
Trunk neural crest cells (level of 6th somite to most caudal somites) migrate and develop into:
Sympathoadrenal> sympathetic ganglia and adrenal gland
Dorsal root ganglia> sensory neurons
Melanocytes
Lumbosacral neural crest cells: parasympathetic neurons and enteric nervous system
Cranial neural crest cells: sensory CN nuclei, parasympathetic ganglia, facial skeleton
Sympathetic from trunk NC cells
Parasympathetic from lumbosacral, cranial, vagal and circumpharyngeal NC
Neural crest cells: Dorsal root ganglion
Neural crest cells form dorsal root ganglion
Two lineages: sensory neurones and glia
Schwann cells and satellite cells
From there the sensory neurons axons project dorsomedially to neural tube and ventrolaterally into growing spinal nerve
Where they join with motor neurones from the basal plate to form trunk of spinal nerve
Cranial neural tube
Upper/rostal neural groove enlarges before fusion
Forms 3 swellings, then later 5 swellings (vesicles)
Neural tube central canal become ventricular system
Brain forms around these swellings
Prosencephalon— forebrain
Mesencephalon— midbrain
Rhombencephalon— hindbrain
Brain development
Neural tube still organised into sensory alar and motor basal plates
However in some places the arrangement of the tube changes e.g. brainstem
Medulla: where arrangement forms cranial nerve motor and sensory nuclei in columns
Midbrain: alar plates migrate from canal to form nuclei
Neural tube defects NTDs
Failure of the neural tube to close: cranial defects and spinal dysraphism
Open NTDs occur in about 0.1% of live births
Can be detected by antenatal ultrasound and or screening
Risk factors: mothers age (older and younger mothers), folic acid deficiency, maternal diabetes/ obesity
Types of NTDs
Rachischisis- most severe, complete inability to close
Failure of cranial neuropore to fuse:
Anencephaly
Cranial bifida/ cranial meningocele
Failure of the caudal neuropore to fuse:
Spina bifida:
Spina bifida occulta
Spina bifida cystica
- meningocele-spinal cord not involved
- myelomeningocele- Spinal cord involved
What is anencephaly
Absence of major portion of brain, skull and scalp
Alteration in facial appearance
Incomplete development of brain with degeneration and calvaria (top part of skull)
Cranial bifida/ cranial meningocele
Defect at posterior fontanelle of cranium- region of skull not fully formed yet, where two parietal bones adjoin the occipital bone
Meninges bulge through, large subarachnoid space containing CSF , part of occipital lobe can also bulge through
Defect in cranium at foramen magnum- largest foramen of skull , part of cerebellum push through in subarachnoid space
Spina bifida cystica
Meningocele- spinal cord not involved, cyst filled with meninges
Myelomeningocele- herniating meninges and neural tissue, some spinal cord too, most common form of spina bifida cystica:
- co presents with Arnold-Chiari type 2 malformation (of cerebellum, pushed into upper spinal canal through foramen magnum) and hydrocephalus
-motor and sensory impairments to lower limb, bowels and bladder
Spina bifida occulta
Hidden
2% population
May not be diagnosed until adult hood
Failure of vertebral arches to fuse
May be associated with cyst
Site may have overlying tuft of hair
Fatty lump on skin , discolouration
