Sensory Pathways

Question 1

General Somatic Sensation Types

(GSS)

Answer 1

• Somatic information could be coming from skin/ straight from musculoskeletal system
• Conveying information about the general senses. What general senses occur and how they are received by the body
• Pain, Temperature, Touch
• Kinaesthesia (Ability to sense parts and WHOLE of body. Know where parts of your body are at all times (hands,etc.)), Positional Sense, Conscious Proprioception

Question 2

Sensory Pathways- Modalities

Answer 2

• Sensory neurones relay information to higher centres in the CNS for processing via a series of pathways:

1. Dorsal Columns- line in the dorsal white matter of the spinal cord (can be divided further into 2 tracts: gracile tract: serves the hindlimb & cuneate tract: forelimb) – relay info about touch & kinaesthesia
2. Spinothalamic Tracts- group of fibres in spinal cord which serve a similar function- named for where it starts/ ends (spine /thalamus)- relays info about “pin-prick” pain & temperature
3. Spinocervicothalamic Tracts – important in domestic species, ↓ developed in primates- relays info about touch & kinaesthesia

**These 3 tracts merge in the brainstem forming a bundle of white matter known as the medial lemniscus

Question 3

Medial Lemniscal System

Answer 3

• formed by these separate tracts forming a bundle of white matter on the spinal cord (dorsal, spinothalamic, spinocervicothalamic)- formed by the union of these various ascending pathways
• Sometimes clump these together and say medial lemniscal system
• medial–> more medial than other structures
• lemniscus: formed like a “ribbon” of white matter
• 3 neurone chain-Spinal cord receives input from sensory neurone, ascending neurone relays this–> thalamus (‘filters’ the info) via the brainstem & a final neurone relays the info to the general sensory cerebral cortex (lies caudal to central sulcus) which can be perceived at a conscious level
• Contralateral relay- info from right side of body–> left Cerebral Cortex & vice versa. May cross at level of spinal cord or higher

Question 4

General Sensory Cortex

Answer 4

• Final site of perception after the medial lemniscal system and thalamus (to reach consciousness) - part of cerebral cortex that allows for the general senses
• lies behind the salcus (central salcus or cruciate salcus)- separates frontal lobe from parietal lobe. Quite a specific area
• Somatotopic- different parts of the body are placed along the General Sensory Cortex in a logical sequence. placed in organized matter along GSC
• Inverted image – hind end of the body is located higher up. lower parts of the body is highest up on the cortex. Face and head lower near lateral
• Disproportionate- Some areas of the body have a larger area devoted to them than others= ↑ sensitive. Some parts of body are more represented than other. Greater sensory acuity in hand and lips than others

**The snout is very sensitive! Same general arrangement to man other than some slight proportionality (similar maps made for animals)

-stroke can lead to a loss of sensation. stroke lesions in different parts of the brain

Question 5

Ascending Reticular Information

Answer 5

Spinoreticular Tracts (4th sensory pathway)- relays info about true pain (other than pin prick/majority of pain info), compared to Medial Lemniscal System:

• Multineurone, bilateral relay- ascending tract is comprised of several neurones relaying info instead of just 1 & info can be relayed on either side of the Spinal Cord
• Via Ascending Reticular Formation in BS- a network of nuclei (grey matter cell bodies & white matter fibres)= ↓myelinated
• Diffuse relay to general sensory cortex (↓ organised)
• All signs of more primitive system
• sensory neurons relaying into the right half of the spinal cord, then toward higher centers in bilateral relay (contralateral and ipsilateral)
• then multiple neurons will (as it ascends can be a few or many more–> multineuron relay) then go to thalamus level (close to midline! this pathway is deep) and then third to cerebral cortex (on both sides)
• Final relay/final site of perception is in cerebral cortex, but not the General Sensory Cortex!!

Question 6

Spinocerebellar Tracts

Answer 6

• unconscious proprioception- info that doesn’t reach conscious level but is relayed (ipsilateral relay) to the cerebellum

Question 7

Two Pathways of Pain

Answer 7

1. Fast, initial or pinprick–> free n. endings–> larger myelinated A(gamma) fibres–> STT (spinothalamic tract)–> Localised, quick ending
2. Slow, delayed or true–> free n. endings–> small unmyelinated C fibres–> SRT (spinoreticular tract or ARF)–> less localised, persistent, nausea, BP ↓ (more widespread to cerebral cortex)

• Pain fibres lie throughout cord and are very resilient to damage, so loss of ALL pain sensation is a very poor sign
• In man- it’s believed all pain is via the STT (spinothalamic tract system)
• Hyperalgesia: tissue damage releases chemicals which increase sensitivity of nociceptors, so even light touch may induce pain
• Visceral pain: Poor localisation (unlike somatic skin rec.) can be extreme, especially distension
• Visceral pain: pain from organs. Can be poorly localized. Can be severe but you can tell what specific region it is (duodenum, jejunum, etc.)
• Can induce skeletal m. spasm too
• Can be referred to somatic areas served by same spinal n. (angina)–> can signal that the pain is somewhere other than the source! (pain in the heart can be percieved in the chest or skin by heart even)

Question 8

Dorsal Columns

(Sensory Pathways)

Answer 8

• Tracts are made of nerve fibre bundles. Everything is bilaterally symmetricle. So all this can happen on the other side as well
  1. Dorsal Column
  2. Spinothalamic Tracts (relays information from teh spine (CNS) to level of thalamus in diencephalon) - limited role in pain information. end once you take away the pain–> localized information. Doesnt account for distension of hollow organs, bruising (long-lasting) , inflamation, etc. Different functions in different animals
  3. Spinocervicothalamic tracts- well developed tracts in domestic mammals, lacking in primates and man.

Question 9

3 tracts and their similiar pattern of relay

Answer 9

• applies to all of the three main tracts (dorsal, spinothalamic, and spinocervicothalamic)
• Sensory neuron will relay this first bit of information to the spinal cord (ex: point out on the skin, on the right side of the body) - relayed in as an appropriate sensory neuron (afferent neuron into the spinal cord) - sensory nerve- on same side. commonly there will be synaptic relays between the afferent sensory neuron and the next neurons that relay information to higher centers
• second neurons relay information to the thalamus (diencephalon) to be “filtered” before heading to the cerebral cortex, end in different nuclei of thalamus. these fibres will decussate at the same level as the afferent neuron enters the spinal cord (immediate crossing) or a relay that starts going up on the same side of the spinal cord, but somewhere higher up also decussates
• BUT, they all merge in the brainstem to medial lemniscus. When these pathways are in the white matter of the spinal cord they lay in different parts (dorsal in dorsal region, other two lateral) until they reach the brain stem and join up together to form large bundle of white matter (medial lemniscus)
• 3rd neuron goes from the thalamus to cerebral cortex
• CONTRALATERAL RELAY (info comes from right side of the body enters left half of CNS)

Question 10

Spinocerebellar Tracts

Answer 10

• unconscious proprioception of locomotor system
• doesn’t reach conscious level at all
• picking up information of muscular system activity and relaying that up to higher centers
• relaying into the CEREBELLUM, not the cerebral cortex (not conscious level)
• Ipsilateral Relay: information coming in from right limbs of the body and enters the right half cerebellum by a second neuron on the SAME SIDE
• may decussate during pathway, but will decussate again to the same side as the original sensory neuron

Question 11

Differences of Ascending Reticular Formation (Spinoreticular tracts) to Spinothalamic Tracts

Answer 11

• Bilateral Relay (close to midline) c.f. contralateral
• Multineuron relay c.f. single
• FInely myelinated c.f. heavily
• Diffuse Relay to cortex

all signs of a MORE PRIMITIVE SYSTEM than spinothalamic or medial lemniscus system

Question 12

Spinal Lesions

Answer 12

Spinal lesions that damage the spinal cord can lead to ?

• Most are partial lesions from pressure. Can cause damage to spinal cord that is partial
• If it were a fractured vertebrae it may damage one side of spinal cord or spare the other.
• To knock out ALL the pathways relative to pain relay… that would be BADDD damage to the spinal cord. To knock out pain sensation on either side of the body. It would have to be extensive damage as these tract runs so close to midline and are resilient (BILATERAL for true pain- spinoreticular and deeply placed!). Need to test by not a pin prick, but a more painful stim. (ex: clamping the hindlimb with some force and forceps)