Sensory and Motory Pathways Flashcards
What are the 3 main CNS tracts that are assessed during a neurological exam?
PYRAMIDAL TRACT
- corticospinal tract for limbs/tract
- corticobulbar pathway: cranial nerve motor nuclei
DORSAL COLOUMN PATHWAY
precise (fine/discriminative) touch
joint position
proprioception
SPINOTHALAMIC TRACT
pain
temperature
crude (poorly localised) tactile sensation
Where is the primary motor cortex located?
precentral gyrus of the frontal lobe
just anterior to the central sulcus
What is the function of the primary motor cortex?
controls voluntary movement of opposite side of body
known as the motor strip or M1
Where is the premotor cortex? What is its function?
area immediately in front of the primary motor cortex (M1)
involved in movement planning and preparation
Where is the primary somatosensory cortex located?
just behind the primary motor cortex (M1)
in the post-central gyrus of the parietal lobe
Also known as sensory strip or “S1”
concerned with sensations from the opposite half of the body.
What does the primary somatosensory cortex do?
It receives ascending (sensory) projections for all sensory modalities including light touch, joint position sense, pain, temperature etc.
What is the relationship between cortical size and precision in the motor cortex?
the size of the cortical representation for each body part is in proportion to the precision of motor control
How does cortical area relate to the sensitivity in sensory cortex?
the amount of cortex devoted to each body part is in proportion to tactile sensitivity
Which parts of the body occupy which part of the hemisphere?
The lower part of the body, including the lower limbs, occupies the medial surface of the hemisphere.
The upper limb and hand areas are superior and the face/tongue areas are inferior.
What are the 2 components of the primary motor pathway?
CORTICOSPINAL TRACT
CORTICOBULBAR TRACT
What is the corticospinal tract?
projects from motor + premotor areas of the frontal lobe to all areas of the spinal cord
controls voluntary movements of the contralateral limbs/trunk
consists of ~1 million axons on each side
What is the corticobulbar tract?
voluntary motor supply to the brain stem (motor cranial nerve nuclei)
controls movements of jaw, fate, tongue, larynx and pharynx
bulb = lower brain stem
What is the clinical importance of the primary motor pathway?
it controls voluntary movement
therefore damage to it will likely result in weakness or paralysis
What is the pyramidal tract/motor system?
collective term for corticospinal and corticobulbar tracts
=> corticospinal tract travels through the pyramids of the medulla
Where do 2/3 of the fibres in the corticospinal tract project?
originate from motor and premotor areas of frontal lobe
travel to the anterior horn of the spinal cord grey matter
- > influence spinal motor neurons
- > usually via interneurons
- > small proportion of axons will directly synapse with motor neurons (hand control e.g.)
Where do the remaining 1/3 of corticospinal fibres project?
originate from parietal lobe
project to dorsal horn of spinal cord
-> helps to filter out sensations generated by movement
What is the course of corticospinal tract fibres as they leave the cerebral cortex?
cerebral cortex -> corona radiata (‘radiating crown’) -> subcortical white matter -> posterior limb of internal capsule
descent through anterior brain stem -> crus cerebri -> basilar pons -> medullary pyramids
Where is the posterior limb of the internal capsule?
passes between the thalamus and the lentiform nucleus of the basal ganglia
What is the internal capsule?
anatomical ‘bottle-neck’
motor fibres are arranged in compact (<1cm)
damage can cause complete contralateral paralysis (hemiplegia)
Where are the pyramids of the medulla located?
either side of the midline
What are the components of the basilar pons?
fascicles
these interlace with the transverse pointing fibres
What is the crus cerebri?
most anterior portion of the midbrain
What is meant by “dessucate”?
when fibres cross the midline
Where does the corticospinal tract dessucate?
at the lowermost border of the medulla
~ at level of foramen magnum
How do corticospinal fibres pass to enter the lateral column?
<90% fibres pass posteriorly and laterally to enter the column
- > lateral (crossed) corticospinal tract
- > important for control of distal limb flexors
- > needed for manual dexterity
<10% fibres
continue in anterior aspect of cord on either side of midline
-> becomes the anterior (uncrossed) corticospinal tract
-> involved with proximal/axial muscles
-> many fibres cross the midline close to their point of origin
(partially crossed)
What does voluntary movement involve?
two-neuron chain between motor cortex and skeletal muscle
UPPER MOTOR NEURONS
1st neuron has cell body in motor/premotor cortex (frontal lobe) and axon that contributes to the corticospinal tract
these extend full length of spinal cord to synapse onto lower motor neurons
LOWER MOTOR NEURONS
cell body is within the anterior for of the spinal cord grey matter (or in a cranial nerve nucleus - similar)
axons leave the CNS and travel in a peripheral nerve to reach their target muscle
What is the final common pathway?
refers to the lower motor neuron and its axon
ultimately responsible for all movements both voluntary and reflexive
What does damage to the corticospinal tract cause?
can occur anywhere along its length:
- cerebral cortex
- subcortical white matter
- brain stem
- spinal cord
causes weakness/paralysis and a particular UMN-type pattern of clinical signs
What does damage in the brain or brain stem portion of the corticospinal tract cause?
weakness on contralateral side of body
What does damage spinal cord below level of desuccation corticospinal tract cause?
weakness on ipsilateral side of body
What is an example of an UMN lesion?
UMN = upper motor neurone
stroke or brain tumour
What are muscle spindles?
stretch detectors
found scattered through all skeletal muscles
[but NOT in the face]
made up of spindle shaped (fusiform) connective tissue capsule
and a few small striated muscle fibres (intramural)
What fibres are the bulk of (skeletal) muscle made up of?
extrafusal fibres
How are muscle spindles arranged?
in parallel with the extrafusal muscle fibres
therefore stretched whenever muscle belly placed under tension
What do exquisitely sensitive sensory endings in the muscle spindle signal?
- sustained tension
- rate of change of length (muscle fibres will be most sensitive to a short sharp stretch, e.g. tendon hammer)
What happens when a muscle is stretched?
spindles are excited
triggers reflex contraction of the same muscle group to resist change in muscle length
achieved by simple reflex arc
What does homonymous mean?
same
What does a simplex reflex arc consist of?
sensory limb
motor limb
single intervening synapse in spinal cord
known as the MONOSYNAPTIC STRETCH REFLEX
What is meant by reciprocal inhibition in the stretch reflex?
antagonist muscles are inhibited at the same time as the stretch reflex
caused by relay neurons that pass adjacent segments of the spinal cord
What is an example of reciprocal inhibition in a stretch reflex?
causes sudden movements of joint
eg “knee jerk” when quads are stretched by brisk blow to the patellar tendon
What is the stretch reflex mainly responsible for?
- normal muscle tone
- deep tendon reflexes
What does normal muscle tone require?
intact sensory and motor supply
=> because tone is not an intrinsic property of muscle
What happens if the stretch reflex is overactive?
excessive muscle tone = hypertonia
AND
firm resistance to joint manipulation
- velocity dependent
What is the stretch reflex regulated by?
descending influences from the reticular formation of the brain stem
[this is missing or reduced in patients with upper motor neuron lesions]
What kind of stretch reflexes do patients with UMN lesions present?
UMN = upper motor neuron
- excessive muscle tone (spasticity)
- brisk tenson reflexes (hyperreflexia)
- clonus
- clasp-knife rigidity
- “pyramidal” posture
What is the pathophysiology underlying a LMN lesion?
LMN = lower motor neuron
interrupted nerve supply tp the muscle
- flaccid paralysis of affected muscles
- loss of normal tone (atonia)
- absence of deep tendon reflexes (areflexia)
- muscle wastage due to disuse (atrophy)
- spontaneous muscle twitching (fasciculations) - cause uncertain
What is the most common type of UMN-type weakness?
UMN = upper motor neuron
stroke
What are common causes of LMN-type weakness?
LMN = lower motor neuron
- peripheral neuropathy
- anterior horn cell disease
What kind of motor loss occurs in MND?
MND = motor neurone disease
mixed features as both upper and lower neurones are affected
What are the 2 main important somatosensory pathways?
DORSAL COLUMN PATHWAY
SPINOTHALAMIC PATHWAY
both consist of 3-neuron pathways between the periphery and sensory cortex
What is the 3-neuron pathway nature of somatosensory pathways?
FIRST NEURON
lies within the dorsal root ganglion
SECOND NEURON
crosses the midline
ascends to the thalamus
THIRD NEURON
lies in the ventral posterior (VP) nucleus of thalamus
projects to the primary somatosensory cortex
main difference between 2 somatosensory pathways is the position of the 2nd neuron (and so the point of crossing of fibres)
What does the dorsal column pathway function in?
- fine, precisely tuned touch
- joint position sense
- proprioception
- vibration sense
Where does the dorsal column pathway originate?
aka dorsal column-medial lemniscus
originated from low-threshold mechanoreceptors
nerve impulses transmitted to brain via large diameter fibres
What is the nature of the A-alpha/beta fibres in the dorsal column pathway?
- large diameter
- thickly myelinated
- high conduction velocities (> 120 m/s)
best tested using high amplitude, low frequency tuning forks (128Hz)
applied to bony prominences
What is the 3-neuron structure of the dorsal column pathway?
FIRST ORDER NEURON
cell body in dorsal root ganglion axons pass through dorsal columns to reach nuclei where they synapse with second order neurons
SECOND ORDER NEURON
cell body in dorsal column nucleus
axons cross the midline together in medulla
medial lemniscus ends on VP nucleus of thalamus where the fibres synapse with third order neurons
THIRD ORDER NEURON
projection ascends through posterior limb of internal capsule posterior to descending fibres of corticospinal tract
terminate in the primary somatosensory cortex
What is the GRACILE FASICULUS?
medial part of the dorsal column
receives fibres from lower half of body (below T6)
What is the CUNEATE FASICULUS?
wedge shaped structure in the lateral aspect of dorsal column
receives fibres from the upper half of the body (above T6)
When do second order neurons in the dorsal column pathway become the MEDIAL LEMNISCUS?
after they’ve crossed over in the medulla
the axons turn upwards and become this
=> switch from being in the saggital to the coronal plane
Where does the MEDIAL LEMNISCUS terminate?
on the ventral posterior (VP) nucleus of the thalamus
What is the trgeminothalamic pathway?
analogous to the dorsal column pathway
but arising from the head and neck
carries info from the 3 benches of the trigeminal nerve (V1, V2, V3)
pathway terminated in the same thalamic nucleus, medial to the protection for limbs and trunk (VP nucleus)
What functions does the spinothalamic tract hold?
pain and temperature sensation
How can the spinothalamic tract be tested?
- sterile neurotips
- volatile spray e.g. ethyl chloride (produced cold sensation)
Where does the spinothalamic tract originate?
NOCICEPTORS
detect noxious or harmful stimuli
THERMOCEPTORS
signal changes in temperature
What kind of fibres relay info regarding pain and temperature?
- A-DELTA fibres: thinly myelinated
- c-fibres: unmyelinated
both have relatively small diameter and slow conduction velocities
What is the nature of the spinothalamic tract neuron pathway?
FIRST ORDER NEURON
located in dorsal root ganglion
central process enters dorsal root of spinal nerve and synapses on second order neuron
SECOND ORDER NEURON
axon crosses midline in the most anterior part of the spinal cord to reach opposite side
terminate in the VPL nucleus of thalamus
THIRD ORDER NEURON
project into the sensory strip in the parietal lobe
via posterior love of the internal capsule
What is the ventral white commissure?
most anterior part of the spinal cord
What carries pain and temp stimuli from he head and neck?
trigeminothalamic system
What is the paleo-spinothalamic tract?
A proportion of the spinothalamic tract fibres do not travel directly to the thalamus
but first relay in the reticular formation of the brain stem
which projects in turn to the intralaminar nuclei of the thalamus, which have diffuse cortical targets
this is an indirect and slower pain pathway (older conserved pathway)
Where is the face located on the somatotopic map of the SS cortex?
Near the Sylvian fissure/lateral sulcus
What is considered to be ‘leg’ domains when talking about sensation and motor?
lower limbs
anything below T6
Where is the higher somatosensory cortex located? What is its function?
located just posterior to the primary SS area
helps to distinguish complex sensations of objects using additional features such as texture
Where is the visual cortex located?
In the occipital lobe
Where is the auditory cortex located?
in the temporal lobe
What would you hear if the auditory cortex is artificially stimulated?
Primary centre: clicking, buzzing etc
As you move further away: more complex e.g. auditory hallucinations, speech, animal sounds
Where is language initiated in the majority of individuals?
on LHS ONLY cerebral hemispheres
This is the case for:
- 90% of right handed individuals
- 75% of left handed individuals
(for those that are not in this group, control will be shared between the 2 hemispheres rather than being solely on the RHS)
What is the HOMUNCULUS?
“man-shaped” somatotropic map of the body (SS cortex)
What is grey matter in the CNS composed of?
neuronal cell bodies and dendrites
-> not very lipid-rich hence appearing dull
What is white matter in the CNS composed of?
myelinated axons
very lipid rich - hence appears as white
Where is Broca’s area located?
usually on the dominant side
Broca’s homolog is the corresponding area on the contralateral side which controls non-verbal aspects of language e.g. tone etc
How are the structure of the cortex and amygdala similar?
both are layered (~6 in the cortex)
these are known as LAMINAE
What is the composition of the thalamus?
=> gateway to the cerebral cortex
consists of mostly grey matter/nerve cell bodies
However there is A LAMINAE of white matter
= INTERNAL MEDULLARY LAMINA
What is the composition of the internal capsule?
constitutes of white matter
but the internal capsule is encased in the grey matter of the basal ganglia
What are the olive structures in the medulla?
= upper 1/3 of medulla
exit points for CN 9-12
What are MIXED SPINAL NERVE ROOTS composed of?
- Ventral (anterior) MOTOR nerves
- Dorsal (posterior) SENSORY nerves
What is the composition of the spinal cord?
grey matter (inner most/central portion)
white matter (outer rim)
Where does the corticospinal tract originate/end?
begins in cortex
ends in spinal tract
longest continuous white matter pathway
most neurons arise from the primary motor cortex or the pre-motor cortex
damage -> weakness or paralysis
What areas of the primary motor cortex are assigned to which body areas?
MOST MEDIAL AREA
lower extremity e.g. below T6
then MEDIOLATERALLY
Hand, then tongue and face
most LATERAL
technically known as the CORTICOBULBAR TRACT (only descends down to the brainstem and supplies muscles of the cervical portion and face
What is the CORONA RADIATA?
refers to the ‘fanning out’ nature of the primary afferents from the motor cortex edges
these then converge on the INTERNAL CAPSULE (an anatomical bottleneck) located on the anterior face of the brainstem e.g. the crus cerebri
What critical process occur for the corticospinal tract as it reached the bottom margin of the medulla?
90% of the nerve fibres DECUSSATE onto the contralateral side. These will become the lateral corticospinal tract and control distal limb flexors e.g hand muscles
10% of fibres remain UNCROSSED and remain on the anterior portion of the spinal cord (rather than switching to the dorsal facet). These control muscles close to the midline e.g. axial muscles
