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
Where is the ventral posterior nucleus located? What is it?
It is the somatosensory relay nucleus in the thalamus of the brain
What neuron is used for the sensation of pressure, vibration, and texture?
What type of neuron is used for the sensation of light touch and vibration?
What neuron is used for the sensation of temp?
What neuron is used for the sensation of pressure?
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
How is an intense stimulus transmitted in a neuron?
Frequency coding - Strength determined by rate of APs
Activation of neighbouring cells
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
Precision by which a stimulus can be located
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
Define 2 point discrimination
minimal interstimulus distance required to percieve 2 simultaneously applied skin indentations
What is the link between 2 point discrimination and the size of neuronal receptive fields?
Smaller neuronal receptive field = better 2 point discrimination
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
Where is the somatosensory cortex located?
Post central gyrus
How is the white matter of the spinal cord divided?
Funiculi - dorsal, lateral and ventral
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
How many pairs of segmental nerves are there?
Which nerve carries the majority of the cranial sensory system?
Where is priority given in the sensory homonculus?
Face, hands, tongue
What are the 2 categories into which ascending tracts can be divided into?
Conscious and non conscious sensation
What are some examples of non conscious sensation?
muscle length, muscle tension, joint angle
Where is the cell body of spinal sensory neurones located?
Dorsal root ganglion of that spinal segment
What type of neurone is the first order neuron of the somatosensory system?
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?
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?
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?
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?
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
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?
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?
In the medial lemniscus pathway - through which fasciculus does the upper limb neurone travel through?
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
dorsal root ganglion
Through which part of the spinal cord does the spinothalamic tract ascend through?
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