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Flashcards in Session 3 - Group Work Deck (15)
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Over what parts of the body can touch stimuli be most accurately localised, and why?

The ability to accurately locate a touch stimulus on the surface of the body varies directly with the
number of receptors per skin area and the size of their receptive fields. The higher the receptor density and
the smaller their receptive fields, the more accurate the localisation.


Why are you not continuously aware of the touch of your clothing as you sit still?

The majority of cutaneous touch receptors adapt very quickly to a maintained stimulus but are
very sensitive to a changing stimulus responding to the rate of change of the input (phasic receptors).
Therefore if you sit still the awareness of clothing is minimal but you will become aware of your clothing if it
moves relative to your skin.


What is meant by a topographical representation?

A topographical representation suggests that each region of the somatosensory cortex receives
an input from a specific part of the body so that a "map" of the distribution of sense organs on the body
surface can be plotted - to give the so called sensory homunculus. In the case of somatic sensation this is
often called a somatotopic organisation


Explain why following a superficial burn sensation may be retained, but is lost with a full
thickness burn?

Following a superficial burns the cutaneous receptors can recover. With a full thickness burn the
skin is destroyed and replaced by scar tissue. The scar is invaded by pain fibres but has no other


Why do sensory connections from the hand occupy a larger area of the cortex than those
from the much larger area of the thigh?

The area of the somatosensory cortex receiving an input from any region of the body reflects the
richness of the sensory innervation of that region not its physical size. Because of its importance as an
organ of touch, the hand has an especially large number of cutaneous sensory receptors and so a large
representation on the cerebral cortex.


How many neurones make up the conscious sensory pathway?

Typically there are three neurones on the conscious sensory pathway. (The unconscious pathway
has only two).
In the dorsal columns the 1st order neurone has it cell body in the dorsal root ganglion and projects
its axon up the cord to the nuclei of the dorsal columns (nucleus gracilis/cuneatus). From there a second
order neurone with its cell body in the nuclei, ascends to the thalamus. The third order neurone with its cell
body in the thalamus projects the sensation to the cerebral cortex.
In the spinothalamic tract the 1st order neurone has it cell body in the dorsal root ganglion and
synapses in the grey matter of the dorsal horn. From there a second order neurone with its cell body in the
cord, ascends to the thalamus. The third order neurone with its cell body in the thalamus projects to the
cerebral cortex.
Conscious proprioception, from joints and tendons is processed by the cerebral cortex.


Why does some sensory input NOT reach consciousness?

The receptors for unconscious proprioception are the muscle spindles. Their output is not
consciously perceived because it is projected by way of the spinocerebellar tracts to the cerebellum.
Signals in the cerebellum are NOT perceived consciously.


What is sensory agnosia? Why might it affect a patient with a tumour in the thalamus or
internal capsule?

In sensory agnosia the patient may totally ignore somatic sensations, even pain, from a whole side
of the body. It is commonly associated with lesions of the parietal lobe, but tumours of the thalamus or
internal capsule may interrupt ascending fibres on their way to the parietal lobe


Fill in missing words

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What leads to glove and stocking sensory loss?

There are two pathologies which give rise to this distribution of sensory loss. (1) demyelination
(2) axonal degeneration. A variety of diseases can lead to these pathologies, such as vascular
disease, degenerative disorders. autoimmune responses, infections etc.


What is sensory ataxia?

Movement disorder arising from a loss of the sensory input necessary for motor feedback.


What is a positive romberg sign?

Inability to stand - feet together - without swaying when the eyes are closed.


Your patient walks with difficulty picking his feet up and stamping them down in the "stick and stamp"
pattern of gait . Can you suggest what is wrong with him and why he
walks in this way? His serum vit B12 is very low.

Prolonged Vit B12 deficiency causes degenerative changes in
the dorsal columns. This leads to a sensory agnosia which leads to
reduced touch and pressure sensations from the limbs with a loss of
position sense. In consequence of the sensory deficit patients can
not feel their feet properly and are unsure if they are properly in
contact with the ground as they walk. To maximise the sensory input
they tend to look at the feet and stamp them down as they walk.
It is important not to miss this condition because it resolves
completely if treated early


Why is sensory loss in syringomyelia bilateral?

Fibres from both sides cross over segmentally


Why may a patient with syringomyelia have scars and lesions of ifingertips?

Because of the failure to sense pain / temperature such patients burn / injure their hands etc without