3.1.2 Sensory pathways

Where is the somatosensory cortex located?

Rostral parietal lobe, behind the cruciate sulcus (border between frontal and parietal)

What type of neuron is a dorsal root ganglion neuron?

Pseudounipolar- cell body sits in dorsal root ganglia

Which 2 parts is the spinal cord thickest in and why?

Cervical (cervical intumescence) and cranial lumbar regions- nerves exit to supply limbs

Name the 3 tracts making up the medial lemniscal system.

• Dorsal column (gracile cuneate tracts) decussations

• Spinocervicothalamic tract decussations

• Minor bilateral contributions from spinothalamic tract

What 2 tracts are the dorsal columns further split into?

• Cuneate tract- forelimb

• Gracile tract- hindlimb

Describe the pathway of the dorsal columns. How many neurons are involved? (6)

• Sensory dorsal root ganglion neuron enters spinal cord

• Ipsilateral synapse in medulla nucleus gracilis

• Contralateral decussation at medial lemniscus

• Contralateral thalamus for fine tuning

• Contralateral somatosensory cortex

• 3 neuron pathway- dorsal root ganglion, medial lemniscus, thalamus

What sensations are the dorsal columns responsible for detecting? (3)

Touch, pressure, kinaesthesia

Describe the pathway of the spinothalamic tract. How many neurons are involved? (7)

• Large myelinated Aa fibres (type of sensory dorsal root ganglion neuron) enter spinal cord

• Bilateral synapse in spinal cord

• Chain of multiple bilateral relay neurons

• Bilateral medial lemnisci

• Bilateral thalami for fine tuning

• Bilateral somatosensory cortices

• Multineuron pathway- Aa fibre/dorsal root ganglion, multiple relay neurons, medial lemniscus, thalamus

What 2 stimuli are the spinothalamic tracts responsible for detecting?

• Superficial pain (nociception)

• Temperature of skin and viscera

Describe the pathway of the spinocervicothalamic tract. How many neurons are involved? (7)

• Sensory dorsal root ganglion neuron enters spinal cord

• Ipsilateral synapse to relay neuron in spinal cord

• Ipsilateral synapse at C1/C2 lateral cervical nucleus to medial lemniscus

• Contralateral decussation of medial lemniscus

• Contralateral thalamus for fine tuning

• Contralateral somatosensory cortex

• 4 neuron pathway- dorsal root ganglion, relay neuron, medial lemniscus, thalamus

What stimulus is the spinocervicothalamic tract responsible for sensing?

Touch, pressure, useful for balancing and careful walking

Describe the pathway of the spinoreticular tract. How many neurons are involved? (6)

• Small unmyelinated C fibres (type of sensory dorsal root ganglion neuron) enter spinal cord

• Bilateral synapse in spinal cord

• Bilateral ascending reticular formation (chain of finely myelinated relay neurons)

• Bilateral thalami for fine tuning

• Bilateral cerebral cortices

• Multineuron pathway- C fibre/dorsal root ganglion, multiple relay neurons (asc. reticular formation), thalamus

What does deep pain loss indicate?

Significant bilateral damage to spinal cord (spinoreticular formation destruction)- pain fibres are usually the hardest thing to damage as they are resilient

What stimuli is the spinoreticular tract responsible for sensing? (3)

• Slow, delayed ‘true’ pain (nociception)

• All sensory modalities except muscle and joint proprioception

• Wakefulness

Describe the pathway of the spinocerebellar tract. How many neurons are involved? (6)

• Sensory dorsal root ganglion neuron enters spinal cord

• Ipsilateral synapse to relay neurons in spinal cord

• Partial decussation of relay neurons- bilateral transport to cranial

• Decussation of contralateral relay neurons (those which decussated in the first place) back to ipsilateral

• Ipsilateral palaeocerebellum/vermis

• 2 neuron pathway- dorsal root ganglion, relay neurons

What is the cerebellum used for?

Unconscious proprioception and fine tuning of motor activity

Which 2 pathways are responsible for pain transmission and why are they often considered a single unit?

• Spinothalamic and spinoreticular

• Anatomically adjacent

What is hyperalgesia and what does it cause? (2)

• Chemical release due to tissue damage

• Increased nociceptor sensitivity so even light touch causes pain