Somatosensory system (central processing)

What are the two main ascending tracts?

• dorsal column - medial lemniscal system

• anterolateral system

What sensations use the dorsal column system?

mechanosensation

• used for tactile discrimination and interaction with objects

What sensations use the anterolateral system?

• temperature

• pain

What is steps of transduction in the anterolateral system like?

• peripheral axon

• through dorsal root ganglion cells

• cross the contralateral side in spine

• ascend through different tracts

• to thalamus

• to somatosensory cortex

What are any differences for dorsal column system?

• doesn’t cross at the spinal cord

  • ascends in the ipsilateral spinal cord to dorsal column nuclei

• instead crosses at the medial lemniscus (in brainstem) to contralateral ventral posterior nucleus in thalamus

Which system is newer and what does this mean?

Dorsal

• more direct

• fewer synapses

• faster and less chance of modulation

What are the 3 pathways of the anterolateral system?

• spino-reticular

• spino-mesencephalic

• spino-thalamic

What does the spino-reticular tract project neurons to?

medial laminae

What does the spino-reticular tract influence? (2)

• ‘reticular activating system’

  • releases neuromodulators into CNS

  • raphe nuclei and more

• reticulospinal tracts

  • project down spinal cord to end on interneurons

  • increases stretch relex activity and flexor withdrawal activity

  • for posture, gait and muscle tone

Where does the spino-mesencephalic tract input to?

midbrain, originates from laminae 1 and 5

What does the spino-mesencephalic system influence?

• periaqueductal grey (PAG)

  • pain control centre

• superior colliculus

  • unconscious oritentation in response to tactile, visual and auditory stimuli

What does the old spino thalamic tracts diffuse to?

‘emotional’ areas of cortex

What does the new spinothalamic tract do?

sensory S1 cortex for location of painful stimuli

Properties of old and new spino-thalamic tract? (picture)

What are anterolateral sensory transmission influenced by?

descending cortical fibres

Do mechanoreceptors get activated separately or together?

simultaneously out of difference channels

• can very amplitude of a stimulus and record from afferent nerve then split recording by differnt type to see

What is somatotopic organisation?

• mechanoreceptors innervating adjacent spots send axons into the adjacent regions of the dorsal column

• preserves neighborhood relations

As you ascend the dorsal column are there more or less fibres?

more

What does the dorsal column project to?

the ipsilateral nucleus

Does receptor field size increase or decrease with a weaker stimuli?

decrease

Does the anterolateral system have surround inhibition and why?

no becasue transmission is then broadened and amplified as for nociception location is not important

Why does the dorsal column system have lateral surround inhibition?

for fine tactile discrimination

• inihibition silences neuron so that second stimulus more noticebale

• can notice two events occured

• improves spatial ingenuity

Ipsilateral dorsal column nucleus projects to ipsilateral thalamus, true or false?

false - contralateral thalamus

Where do ventral posterior medial thalamic nuceli receive input from?

face

Where do ventral posterior lateral thalamic nuceli receive input from?

body

How is the contralateral body mapped within the somatosensory cortex?

somatotopically

Which areas of the body occupy larger areas of the corte and more cortical neurons?

most densely innervated areas

What led to the ‘homunculus’

micro stimulation of the brain syrface alloiwing functional mapping by Penfield

Fun fact about mouse whiskers?

used to discriminate objects in an environment and each is stereotyped and matches one to one with somatosensory cortex

As you move further from the fingertips, RF size … and overlap …?

increases, decreases (so ability to discriminate decreases)

What are the areas within the somatosensory cortex - S1, S2, S3?

1,2,3a,3b,5+7

What are the roles of these areas?

• 1 - touch with wide receptive fields

• 2 - combines position and touch

• 3a - position sense

• 3b - local touch, detailed

• 5 + 7 - sensory guidance of movement, memory of an object

What does 3D perception rely on?

proprioceptive feedback

What is the descending contribution of the somatosensory cortex?

• corticospinal tract provides descending control of ascending sensory systems in addition to motor control

What percentage of fibres from somatosensory cortex provide modulatory feedback onto ascending sensory feedback?

50%

Where does further processing beyond S1 take place?

S2, S3, motor, posterior parietal cortex, secondary thalamus