somatosensory pathways Flashcards
what are sensory nerve fibres?
large tracts of white matter in the dorsal spinal cord
what are other names for dorsal fasciculi?
posterior columns/dorsal columns
what do sensory nerve fibres project into?
dorsal fasciculi
what are dorsal columns divided into?
fasciculus gracilis
fasciculus cuneatus
what afferents does fasciculus gracilis contain?
afferents from lower limb and genitalia
what afferents does fasciculus cuneatus contain?
afferents from upper limb
what are dorsal column fibres?
branches of the primary afferent fibres in the peripheral nerves
what side of the spinal cord do dorsal column fibres ascend the spinal cord?
the same side they enter it
what sensations does fasciculus cuneatus transmit and from where to where?
vibration
proprioception
discriminative touch
from periphery to the brain via dorsal columns and medial lemniscus
what are dorsal column nuclei?
long columns of cells extending several mm rostrally into the medulla
name the dorsal column nuclei
nucleus cuneatus and nucleus gracilis
where are the dorsal column nuclei found?
in the lower medulla of the brainstem
what is the medial lemniscus?
tract from dorsal column nuclei to the somatosensory thalamus
describe the DCML pathway
axons leave dorsal column nuclei
immediately cross midline to form contralateral medial lemniscus
medial lemniscus fibres ascend to VPL nucleus of the thalamus
VPL cells of the thalamus project into somatosensory cortex
what are internal arcuate fibres?
medial lemniscal fibres in the region where they cross the midline (occurs in the lower medulla).
what joins the medial lemniscal fibres in the DCML?
joined by cutaneous afferents from the face from contralateral trigeminal nucleus
where do fibres from the trigeminal nucleus synapse?
in the VPM thalamus
what is the thalamus?
Relay station for info from periphery to reach the cortex
where are the 2 thalami found?
either side of the 3rd ventricle, forming its walls
below the lateral ventricles
what is the primary somatosensory cortex?
a strip of cortex running medio-laterally just posterior to the central sulcus
has a map of the body surface
explain what the size of the areas on the map of the primary somatosensory cortex corresponds to?
Largest areas on the map are the areas with the highest density of touch and proprioceptive receptors
which body parts have the highest density of touch receptors?
hands, lips and tongue
what is meant by the homunculus map having ‘plasticity’?
not fixed
if a limb is amputated, then the area for the limb shrinks and the area to surrounding limbs expands
what information does the somatosensory cortex give you?
the location of something on your body and its touch and texture
doesnt tell you about temperature or pain
how is fine touch/stereognosis tested?
tested by asking patient to recognise common objects hidden within a cloth bag using touch alone
what is stereognosis?
object recognition
how is vibration sense tested?
tested using a tuning fork placed along a bony prominence of desired corresponding spinal level(s) to be tested.
how is barognosis tested?
tested by asking subject to determine approximate weight of an object placed in the hand
what is barognosis?
the ability to evaluate the weight of objects by holding or lifting them
how is graphesthesia tested?
can the subject recognise writing on the skin by touch
how is kinaesthesia tested?
using the subject’s ability to detect an externally imposed passive movement, or the ability to reposition a joint to a predetermined position.
what is kinaesthesia?
the sensation that the body part has moved
what does the romberg’s test assess?
proprioception from leg muscles and joints
explain how to carry out the Romberg’s test and what is interpreted as a positive sign
In the Romberg test, the patient is stood up and asked to close their eyes. A loss of balance/wobble is interpreted as a positive Romberg sign.
what is proprioception?
ability to sense limb positions and thus our position in space
what is sensory ataxia?
the inability to balance with eyes closed
what can cause sensory ataxia?
damage to the dorsal columns
what are the relaxed laminae?
6 layers of cells in the dorsal horn
what do sensory fibres entering the dorsal horn synapse with?
make synaptic endings on cells in one of the 6 layers
where do cutaneous afferents synapse?
lamina III and IV
where do proprioceptors synapse?
lamina V and VI
where do nociceptors synapse?
in lamina I and II
after sensory afferents have synapsed in the dorsal horn, where do they go?
then decussate and enter white matter in the contralateral side of the cord
what information does the spinothalamic tract convey?
sense of pain, temperature and crude/passive touch
what is crude touch?
knowing you’ve been touched, but unable to discriminate the type of touch (e.g. light touch, light/deep pressure etc.)
what are the 2 components of the spinothalamic tract and what do they convey?
Lateral spinothalamic tract – conveys temp and pain
Anterior tract – conveys crude touch
what is pain?
neuronal program that is normally activated by tissue damage
what are nociceptors?
free nerve endings found at the ends of Ad (small myelinated) and C (unmyelinated) nerve fibres which detect tissue damage
what type of pain do Ad afferents detect?
fast pain
what type of pain do C fibres detect?
slow pain
what do free nerve endings have around them at the axon terminal?
no connective tissue
what is normal pain?
pain sensation caused by nociceptor activity
what type of stimulus leads to pain?
noxious - tissue damaging
what type of function does pain have?
protective - minimises injury, promotes recovery
what are the components of pain?
fast pain - acute
slow pain - chronic
what does fast pain trigger?
withdrawal reflexes to minimise injury
when does fast pain go away?
after initial injury
when does slow pain start and go away?
starts after fast pain has gone
goes when the wound has healed
how can normal pain be treated?
conventional analgesics eg NSAIDs, steroid anti-inflammatory agents, opiates
what type of receptors are found at free nerve endings?
TTX resistance channels – special sodium channels
explain the process of inflammation causing slow pain
- Tissue damage causes pro-inflammatory chemical release into extracellular space
- Causes inflammation
- TTX resistance channels therefore open
- Leads to continuous depolarisation of the ending –> continuous receptor potential
- Tonic generation of action potentials
where do afferents from nociceptive C fibres and Ad fibres terminate?
in laminae I and II of the dorsal horn
after cells in lamina I and II decussate, where do they go?
scend in the contralateral anterolateral part of the cord as the lateral spinothalamic tract
goes to various nuclei in the brainstem, does not project to the VPL
what receptors, other than nociceptors, does pain stimulate?
touch receptors
where do touch receptor sproject?
Touch receptors project in the anterior spinothalamic tract which DOES end in the VPL nucleus of the thalamus (along with the medial lemniscus)
how do touch receptors allow us to localise pain in our body?
bc the anterior tract projects to the somatosensory cortex
where does the lateral spinothalamic tract project?
several parts of the forebrain
what projection of the lateral spinothalamic tract mediates the sensation of pain?
projection to the cingulate and insula cortices of the limbic system
how does pain produce arousal?
via the reticular formation of the midbrain and thalamus
what projection of the spinothalamic tract mediates temperature sense?
projections to the hypothalamus
what does coactivation of the anterior and lateral spinothalamic tract by a nociceptive stimulus enable?
enables you to localise the pain (anterior) and assess its unpleasantness (lateral)
what makes up the limbic system?
Includes a group of cortical and subcortical nuclei found on the medial aspect of the frontal, parietal and temporal lobes
includes the cingulate cortex
what does the cingulate cortex mediate?
our sensation of pain
why is organ pain difficult to comprehend?
body has no map of the interior of the body
how is pain in our organs felt?
pain is felt in the dermatome that has its afferent input in the same spinal segment that the active visceral pain fibres terminate
