Week 10

Question 1

What are sensory afferents carried by

Answer 1

Ascending tracts

Question 2

What’s carried by descending tracts

Answer 2

Somatic efferents and autonomic efferents

Question 3

Somatic sensory afferents are carried by what ascending tracts

Answer 3

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

Question 4

What separates the motor and sensory brain

Answer 4

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

Question 5

Processing of sensory info

Answer 5

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

Question 6

What are ascending tracts

Answer 6

Neural pathways located in the white matter that conduct afferent information from the peripheral nerves to the cerebral cortex

Question 7

What are the 2 categories of sensations

Answer 7

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

Question 8

What is the somatosensory system

Answer 8

All neurones of the brain and spinal cord that act to convey somatic sensation

Question 9

The first stage of spinal sensation

Answer 9

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

Question 10

Variant where signals reach post central gyrus via spinothalamic tract
Pain and temp

Answer 10

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

Question 11

Variant 2 which signals reach post central gyrus via dorsal column
Proprioceptive and tactile info

Answer 11

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

Question 12

At the spinal level transmission of sensory info is dissociated

Answer 12

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

Question 13

Primary sensory cortex

Answer 13

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

Question 14

Syringomyelia

Answer 14

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

Question 15

Primary motor cortex

Answer 15

Occupies the pre central gyrus frontal lobe
Voluntary motor commands of muscles are initiated by the primary motor cortex

Question 16

Cortical representation of the motor system

Answer 16

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

Question 17

What are the two general classes of descending tracts

Answer 17

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

Question 18

Pyramidal tract fibres are the

Answer 18

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

Question 19

extra pyramidal fibres are the

Answer 19

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

Question 20

The brain in control of movements

Answer 20

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

Question 21

What is the motor strip

Answer 21

Band of neural tissue runs down side of frontal lobe of brain
Another terms for primary motor cortex

Question 22

levels of contraction vary depending on

Answer 22

Requirements of the body
Resting muscle tone- for maintenance of posture (minimal contraction)
Displacement of joints- movements (large forces may be required)

Question 23

Motor commands of muscles

Answer 23

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

Question 24

Sensory system of the body vs that of cranial structures

Answer 24

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

Question 25

Day 4 to 16 embryological development

Answer 25

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)

Question 26

What does ectoderm form

Answer 26

Nervous tissue and skin

Question 27

What does mesoderm form

Answer 27

Notochord, muscles and connective tissue , circulatory system etc

Question 28

What does endoderm form

Answer 28

Digestive/respiratory tracts

Question 29

Notogenesis

Answer 29

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

Question 30

Neurulation

Answer 30

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

Question 31

What is the neural tube precursor for

Answer 31

Entire CNS

Question 32

Closure of the neural tube

Answer 32

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

Question 33

White/grey matter

Answer 33

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

Question 34

What does the neural canal form

Answer 34

Closes and forms spinal canal

Question 35

What does the alar plate form

Answer 35

Dorsal horn

Question 36

What does the basal plate form

Answer 36

Ventral horn

Question 37

Dorsal-ventral patterning of neural tube

Answer 37

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

Question 38

What is the ependyma

Answer 38

Thin membrane of glial cells lining the ventricles of the brain and the central canal of spinal cord

Question 39

Neural crest cells

Answer 39

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

Question 40

Different types of neural crest cells

Answer 40

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

Question 41

Neural crest cells: Dorsal root ganglion

Answer 41

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

Question 42

Cranial neural tube

Answer 42

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

Question 43

Brain development

Answer 43

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

Question 44

Neural tube defects NTDs

Answer 44

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

Question 45

Types of NTDs

Answer 45

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

Question 46

What is anencephaly

Answer 46

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)

Question 47

Cranial bifida/ cranial meningocele

Answer 47

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

Question 48

Spina bifida cystica

Answer 48

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

Question 49

Spina bifida occulta

Answer 49

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