3 - Somatic Sensation and Ascending Tracts Flashcards by Osaama Aldabbagh

Explain the first order, second order, and third order neurones in the somatosensory system

First order - pass electrical stimulus to spinal cord

Second order - ascends to the brain and synapses with the third order neurone

Third order - Cell body Located in the ventral posterior nucleus in the thalamus of the brain takes the impulse to the necessary part of the brain

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Where is the ventral posterior nucleus located? What is it?

It is the somatosensory relay nucleus in the thalamus of the brain

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What neuron is used for the sensation of pressure, vibration, and texture?

Merkel Discs

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What type of neuron is used for the sensation of light touch and vibration?

Meissner’s corpuscle

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What neuron is used for the sensation of temp?

Riffini corpuscle

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What neuron is used for the sensation of pressure?

Pacinian corpuscle

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What are proprioceptors and where are they located? What types are there?

Proprioceptors in the muscles tells the body where it is in space and allows it to function properly.

2 main types:

muscle spindle tells about the length of the muscle
Golgi tendon organ measures tension in the tendons

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How is an intense stimulus transmitted in a neuron?

Frequency coding - Strength determined by rate of APs

Activation of neighbouring cells

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What are tonic and phasic receptors? Give example of each

Tonic receptors - slowly adapting and keep firing as long as stimulus lasts e.g. joint and pain receptors

Phasic - rapidly adapting and respond briefly to a stimulus e.g. touch receptors

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Define acuity

Precision by which a stimulus can be located

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How is acuity improved by lateral inhibition?

Sitimulation of a 1st order neurone will inhibit the adjacent 2nd order neurones so that the stimulus is only felt in the correct 2nd order neurone

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Define 2 point discrimination

minimal interstimulus distance required to percieve 2 simultaneously applied skin indentations

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What is the link between 2 point discrimination and the size of neuronal receptive fields?

Smaller neuronal receptive field = better 2 point discrimination

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Define convergence and divergence of a signal

Convergence - 2+ first order neurones converge onto a single 2nd order neuron

Divergence - 1 first order neurone diverges into 2+ 2nd order neurones

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Where is the somatosensory cortex located?

Post central gyrus

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How is the white matter of the spinal cord divided?

Funiculi - dorsal, lateral and ventral

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How is the grey matter of the spinal cord divided?

Divided into X laminae. Each laminae is equivalent to a neuronal nucleus. Cell bodies of neurones with common functions occur in the same laminae

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How many pairs of segmental nerves are there?

31 pairs

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Which nerve carries the majority of the cranial sensory system?

Trigeminal

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Where is priority given in the sensory homonculus?

Face, hands, tongue

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What are the 2 categories into which ascending tracts can be divided into?

Conscious and non conscious sensation

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What are some examples of non conscious sensation?

muscle length, muscle tension, joint angle

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Where is the cell body of spinal sensory neurones located?

Dorsal root ganglion of that spinal segment

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What type of neurone is the first order neuron of the somatosensory system?

pseudo-unipolar

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How would you orientate the spinal cord?

dorsal median sulcus is posterior and is a groove in the spinal cord Ventral median fissure is an opening at the anterior end o the spinal cord

What runs through the central canal of the spinal cord?

CSF

What is the function of the dorsal column - medial lemniscus pathway of the spinal cord?

Fine touch and proprioception

What is the dorsal column - medial lemniscus pathway made up of? To where does it transmit information to?

Cuneate and gracile Post central gyrus

What does the lateral corticospinal tract carry?

limb motor information

Label the ascending pathways

at what spinal levels do the fasciculi gracilis and the fasciculi cuneatus exist?

gracilis - all levels cuneatus - c1-c8 t1-t6

What fibres do the fasciculi gracilis and cuneatus contain?

gracilis - lower limb cuneatus - upper limb

What is the term given for the crossing over of second order neurons to the contralateral side?

decussation

What do axons of the third order neruons for somatosensation terminate?

Medial aspect of somatosensory cortex

At what point does decussation occur?

Second order neurons decussate after the first order neurons synapse at the nucleus gracilis or cuneatus in the medulla

Damage to somatosensory axons in the spinal cord - to which side is the damage relegated to?

To the ipsilateral side - damage at or below the spinal cord results in ipsilateral manifestation as it has not yet decussated

What are the symptoms of damage to somatosensory axons

loss of tactile sense, proprioception and inability to identify objects placed in their hand

What sensations do the lateral spinothalamic tract convery?

pain and temperature

Where in the lateral spinothalamic tract do first order neurones synapse with the second order neurones?

dorsal horn

At what point does decussation occur in the lateral spinothalamic pathway? how do neurones ascend the spinal cord?

2nd order Axons decussate via ventral white commissure and ascend the contralateral tract

From medial to lateral - state the order in which fibres of the lateral spinothalamic pathway ascend

cervical, thoracic, lumbar, and sacral

Where are third order neurons of the lateral spinothalamic pathway located and where do they terminate?

Third order synapse located in thalamus and termination occurs in sensory cortex

How and where does damage to axons of the lateral spinothalamic tract manifest itself?

Loss of pain and temperature on the contralateral side of the body one or 2 levels below the lesion because the first order afferent fibres enter into the zone of Lissauer and ascend one or two levels before making their synaptic link with the second order neurone

Where are the cell bodies of the first order neurones in the lateral spinothalamic tract located?

dorsal root ganglion

Which level of spinal nerves will be affected first by an expanding lesion in gray matter?

thoracic and cervical fibres will be affected first as they are more medial Sacral and lumbar pain and temp present initially, known as lumbar sparing

What sesnations do the anterior spinothalamic tract transmit?

crude touch and pressure

How does the anterior spinothalamic tract ascend the spinal cord?

Same pathway as the lateral spinothalamic tract

What sensation does the spinocerebellar tracts transmit?

unconscious proprioception

On what do the neurones of the spinocerebellar tracts act on?

muscles and tendons

Where are the cell bodies of the first order neurones of the anterior spinocerebellar tract lie? Where do they syanpse with second order neurones

dorsal root ganglion synapse in the dorsal horn

Describe the path of ascent of the anterior spinocerebellar tract

1st order synapses with 2nd order in dorsal horn 2nd order decussates and ascends the spinal cord thourgh the medulla to the pons Fibres then decussate again in the pons before terminated in the cerebllum. There is no 3rd order neurones

What is the function of the anterior spinocerebellar tract?

convers info about whole limb movements and postural adjustments to the cerebellum

What would happen in damage to the anterior spinocerebellar tract?

loss of proprioception and coordination to the contralateral lower limb

Describe the path of ascent for the posterior spinocerebellar tract

1st order synapses in dorsal horn and 2nd order ascends on the same side and terminates in the medulla. No decussation No 3rd order neurone

What sensation does the posterior spinocerebellar tract convey? What would happen if it was damaged?

loss of proprioception and coordinated movement of the upper limb on the same side of the lesion

What sensation does the cuneocerebellar tract transmit? Describe pathway of ascent

Transmits proprioception 1st order neurones ascend on ipsilateral side in the cuneucerebellar tract and synapse in brainstem and 2nd order neurones terminate in cerebellum

Where does the cell bodies of the 1st order neurones of the trigeminal nerve lie?

trigeminal ganglion

Where do 1st order neurons of the trigeminal nerve syanpse? Describe the pathway of ascent

1st order synapses at trigeminal nucleus of the brain stem then 2nd order ascends to the thalamus and 3rd order to the cerebral cortex

What is tabes dorsalis? How does it clinically manifest itself?

Part of tertiary syphilis where central processes of dorsal root ganglion degenerate and affects the fasciculus gracilis and cuneatus therefore results in loss of fine touch and conscious proprioception resulting in loss of 2 point discrimination and ataxia

In the medial lemniscus pathway - through which fasciculus does the lower limb neurons travel through?

gracile fasciculus

In the medial lemniscus pathway - through which fasciculus does the upper limb neurone travel through?

cuneate fasciculus

Label the anatomy of the brain stem

What nuclei are the targets of the 1st order neurons of the dorsal column - medial lemniscus pathway? Where are they located?

Located in medulla oblangata Cuneate and gracile nuclei

At what point in the dorsal column medial lemniscus pathway do the axons decussate?

After synapsing with nuclei in medulla oblangata

What is the name given to the axons of the medial lemniscus pathway during decussation? What about after decussation?

During decussation - internal arcuate fibres due to the fact they arc aacross the brain stem Post decussation known as medial lemniscus

What is the target nucleus of the 2nd order axons of the medial lemniscus pathway and where is it located? Where do axons go from here?

ventral posterolateral nucleus in the thalamus Afterwards axons form nucleus go to the post central gyrus

What sensations do the spinothalamic tract convey?

pain, temp and crude touch

What type of neurons are the 1st order neurons of the spinothalamic tract? Where are their cell bodies located

pseudounipolar dorsal root ganglion

Through which part of the spinal cord does the spinothalamic tract ascend through?

lissauer's fasciculus

Where does the 1st order neuron of the spinothalamic tract synapse?

substantia gelatinosa of dorsal horn of the grey matter

At what point does the spinothalamic tract decussate?\>

2nd order neuron, straight after 1st order neuron has synapsed with dorsal horn of grey matter through the anterior white commissure

What route does the 2nd order neuron take?

decussates through anterior white commissure and then ascends via spinothalamic fasciculus through brain stem and synapses onto thalamus

What route does the 3rd order neuron of the spinothalamic tract take?

from thalamus to post central gyrus

What part of the brain is the primary sensory area?

post central gyrus

Explain why sensation may be retained in a superficial burn but lost with a full thickness burn?

Cutaneous receptors can recover after a superficial burn. Receptors and skin destroyed in a full thickness burn

Define sensory agnosia.

Patient may totally ignore somatic sensations even pain from a whole side of the body.

What is a positive romberg sign?

Inability to stand feet together without swaying when eyes are closed

How can the spinal cord be affected by a vit b12 deficiency?

Degenerative changes in the dorsal columns

Why might a patient with sensory ataxia have scars and burns on their hands?

Inability to sense pain or temp leads to injury