Central Processing of Somatosensory Input Flashcards
What do the dorsal columns consist of?
Medial gracile tract (fasciculus gracilis) and the more lateral cuneate tract (fasciculus cuneatus)
How does sensory input travel in the dorsal column?
Sensory input from T6 and below travels in gracile tract, whereas input from above T6 flows in the cuneate tract
What exists across the dorsal columns?
A somatosensory map of sensory inputs from lateral to medial = cervical, thoracic, lumbar, sacral
What tracts convey unconscious proprioceptive info to the cerebellum?
Dorsal and ventral spinocerebellar tracts
What is the major route by which touch and conscious proprioceptive info ascends to the cerebral cortex?
The dorsal column medial lemniscus pathway
What does the first order neuron of the DCML pathway form once it enters the dorsal horn and branches?
Synapses deep in the dorsal horn upon the second order neuron
Long ascending axon synapsing in either dorsal column gracile nucleus or cuneate nucleus
What happens to axons of the second order neurons of the DCML pathway?
Cross from the dorsal column nuclei in the great sensory decussation and ascend in the medial lemnicus to the ventral posterior lateral nucleus of the thalamus
Where do the third order thalamic neurons of the DCML pathway project to?
The primary somatosensory cortex via the posterior internal capsule
What are the capabilities of the DCML pathway?
Stereognosis, vibration detection, weight discrimination, fine touch, conscious proprioception
What is stereognosis?
Ability to recognise an object by feeling it
What is conscious proprioception?
Awareness of body position and body movements
What results in contrast enhancement?
The amplification of differences in the activity of adjacent neurons as info is conveyed from one neuron to the next in a sensory pathway
What is lateral inhibition?
When one neuron is active it inhibits the activity of its neighbours via inhibitory interneurons
What is the purpose of lateral inhibition?
Sharpens stimulus perception = one mechanism by which info is transformed at the synapses of the DCML pathway
What mediates general somatic info from the anterior head?
Two trigeminothalamic pathways
Where are the soma of sensory neurons of the trigeminal system located?
The trigeminal sensory ganglion
Where do the central terminae of the trigeminal nerve synapse?
Upon second order neurons in the chief sensory nucleus = these in turn decussate and project via trigeminal lemniscus
Where does the trigeminal lemniscus project to?
The ventroposteriomedial neurons of the thalamus
Where do the third order neurons of the trigeminal system relay info to?
The cortex via the thalamocortical neurons
Where is the somatosensory cortex located?
Postcentral gyrus of the parietal cortex immediately posterior to the central sulcus and adjacent to the posterior parietal cortex
What does the central sulcus consist of?
Brodmann areas (BA) 1, 2, 3a and 3b
Where does the primary somatosensory area receive input from?
The VP thalamus = 70% to BA 3a and 3b, 30% to BA 1 and 2
What are some features of BA 3a?
Input from proprioceptors
Sensitive to body position
What are some features of BA 3b?
Input from cutaneous and FA receptors, and BA 3a
Sensitive to touch
Small lesion will cause deficits in texture and shape discrimination
What are some features of BA 1?
Input from cutaneous receptors and BA 3b
Sensitive to texture discrimination
Small lesion will cause loss of texture discrimination
What are some features of BA 2?
Input from joint afferents, Golgi tendon organs, deep tissues and BA 3a and 3b
Sensitive to pressure/joint position and object perception
Small lesion will cause impaired grasping and size/shape discrimination
What do the receptive fields of S1 neurons form on the primary somatosensory cortex?
An orderly somatotropic map of the body surface
What does the sensory homunculus show?
The relative area of cortex devoted to each body part = correlated with the density of sensory input from that body part
How many layers does the somatosensory cortex have?
Six cell layers
What layer of the somatosensory cortex do thalamic inputs to S1 terminate?
Mainly on neurons within layer 4 = this in turns projects cells towards the surface of the cortex and also deeper layers
What extends across the six layers of the somatosensory cortex?
Vertical columns = each column consists of neurons with similar inputs and responses
What maps are present in S1?
Multiple maps of the somatic sensory system
What are some features of the somatic sensory system maps present in S1?
Adjacent strips of cortex map the same parts of the body in parallel to each other
Maps aren’t identical but are mirror images of each other
What is the function of the posterior parietal cortex (SII)?
Receives and integrates info from S1 and other cortical and subcortical areas
Deciphers the deeper meaning of the info in S1
What can damage to the posterior parietal cortex cause?
Bizarre neurological disorders with simple sensory skills remaining intact (e.g neglect syndrome)
What is neglect syndrome?
Usually a result of damage to the right parietal cortex = patients believe the left side of the world doesn’t exist
