Generally, how do signals from receptors reach the brain
The signals from these receptors travel via afferent fibres through dissociable pathways of the spinal cord and to higher processing centres in the brain
What is the route for touch and tactile sensation to the brain
The dorsal column system
Describe the dorsal column system
Fibres enter through the dorsal horn and have their cell bodies in the dorsal root ganglia. The signals then go up the dorsal column nuclei, ascending ipsilaterally through the medial lemniscus which rises through the medulla, pons and midbrain before the VP of the thalamus onto which they synapse. From here, thalamic neurons project to the primary somatosensory cortex.
Explain the somatotropic organisation of the dorsal column system
Preserves neighbourhood relations. Mechanoreceptors innervating adjacent spots send axons into the adjacent regions of the dorsal column.
How can the dorsal column be split
Dorsal columns split into Gracile and Cuneate columns. Fibres from lower body enter first medially — Gracile. Then fibres from upper body are added laterally (Cuneate).
What is the route for pain signals to the brain
Anterolateral system
Fibres enter through the dorsal horn and have their cell bodies in the dorsal root ganglia. However, the fibres synapse in the dorsal horn, before immediately crossing to the other side of the spinal cord to ascend anterolateral tracts and reach the thalamus, where pain information is then projected to various areas of the cerebral cortex
The spinothalamic fibres synapse in the dorsal horn of the spinal cord, before crossing to the opposite side and ascending in the anterolateral tract
Why do fibres synapse in the spinal chord in the anterolateral system
In order to be able to supress pain signals to the brain
Explain the 3 main parallel pathways of the anterolateral tracts that send info to the brain
The spino-reticular tract is bidirectional and responsible for the motivational aspects of pain; it projects to neuromodulatory areas such as raphe nucleus and VTA.
The spino-mesecephalic tract is important in descending modulation such as projecting to the PAG which is implicated in the suppression of pain. Further, it targets the superior colliculus which helps us orient to tactile, visual and auditory stimuli.
However, the major route for pain processing is the spinothalamic tract. Its main function is the localisation of painful stimuli and travels through the medulla, pons and midbrain without synapsing until the thalamus.
What broadens the receptive field in the anterolateral column and why?
The receptive field is broadened and amplified by convergent excitation, specific location is not as important for pain .
Explain the process of descending influence on nociceptive feedback via the anterolateral system
Descending feedback from hypothalamus and amygdala which projects onto PAG. PAG signals via raphe 5-HT cells of the rostoventral medulla to modulate laminae 1,2 and 5. PAG also acts via noradrenergic cells of the locus coeruleus which directly projects onto layers 1-5. Here, they can effectively depress the activity of nociceptive neurons.
What is the gradient of excitation
Receptive fields have less activity as you move further away from the center, creating a gradient of excitation that allows the nervous system to encode stimulus location and intensity.
Explain lateral inhibition
Nuclei have inhibitory cells between them. Creates inhibitory troughs on side of receptive fields, which creates an inhibitory surrounds. This increases signal to noise ratio, creating a more noticeable change, better report that something has happened. Makes second event more noticeable.
Explain the 2 types of nuclei in the thalamus
VP medial – receives input from face. Face goes through cranial nerve to different brain stem numcleus before VPm. Doesn’t involve spine.
VP lateral – receives input from rest of body
Explain a physiological example that pain and touch are carried by different anatomical routes
Brown-squared hemiplegia
It is caused by lateral hemi section of spinal cord. Leads to loss of motor function and numbness to touch on the same side of injury. Leads to loss of pain sensation on the opposite side of injury. This is because touch is carried on the same side as it is detected whereas pain info crosses over and is carried on the opposite side of body.
