Sensory and motor pathways

Question 1

What is the primary motor cortex?

Answer 1

• Controls contralateral voluntary movement
• located in the precentral gyrus of frontal lobe - anterior to central sulcus
• size of cortical representation of body parts is proportional to its precision of motor control

Question 2

What is the primary somatosensory cortex?

Answer 2

• Just behind primary motor cortex - in postcentral gyrus
• receives sensory projections for light touch, joint position sense, pain and temperature.
• somatotopic representation of opposite side of body

Question 3

What does the primary motor pathway do?

Answer 3

• Controls voluntary movement
• Corticospinal tract projects from motor and premotor areas of frontal lobe to all levels of spinal cord - voluntary movement of contralateral limbs
• Corticobulbar pathway is voluntary motor supply to brainstem and therefore controls jaw, face, tongue, larynx, and pharynx.

Question 4

Corticospinal tract

Answer 4

• 2/3 of CST fibres originate from motor and premotor areas - axons project to the anterior horn of spinal cord grey matter
• 1/3 go to dorsal horn, where they filter out sensations generated by movement
• passes through corona radiate before entering posterior limb of internal capsule
• CST decussates at the lowermost border of the medulla - level of foramen magnum
• 90% of fibres pass posteriorly to laterally = lateral CST (distal limb flexors/ manual dexterity)
• 10% continue to anterior part = anterior CST (proximal/axial muscles)

Question 5

UMNs and LMNs

Answer 5

• UMN - cell body in pre-/motor cortex. Go full length of spinal cord and synapse onto LMNs
• LMN - cell body in anterior horn. Axons leave CNS and travel in peripheral nerve to muscle

Question 6

What happens with Corticospinal tract damage?

Answer 6

• anywhere along its length = weakness or paralysis accompanied by UMN-type pattern of clinical signs
• if damage is in brain or brain stem = contralateral weakness
• if damage is below level of decussation = ipsilateral weakness

Question 7

What are the somatosensory pathways?

Answer 7

• Dorsal column pathway and spinothalamic tract
• both have a 3 neuron chain - 1st is in dorsal root ganglion, 2nd crosses midline and ascends to thalamus, 3rd lies in ventral posterior nucleus of thalamus and projects to primary somatosensory cortex

Question 8

What is the dorsal column pathway?

Answer 8

• concerned with discriminative touch, joint position sense, proprioception and vibration sense
• originates from low-threshold mechanoreceptors - nerve impulses are transmitted to the brain via large a-alpha/beta fibres (thick myelin and high velocity)

Question 9

What it the pathway for the dorsal column pathway?

Answer 9

• fibres from lower half of body -> medial part of dorsal column
• upper half of body -> lateral part of dorsal column
• 1st order axons pass up through the dorsal columns to reach nuclei (gracile and cuneate) where they synapse onto 2nd order - cross midline in substance of medulla
• all axons in this pathway cross together in the medulla - “great sensory decussation”
• As they curve upwards anteriorly and medially through medulla - bow shaped course (internal arcuate fibres).
• after crossing, the 2nd order neuron’s axons turn upwards to become the medial lemniscus
• medial lemniscus terminates on the VP nucleus of the thalamus and it synapses onto the 3rd order -> posterior limb of internal capsule -> primary somatosensory cortex

Question 10

What is the spinothalamic tract?

Answer 10

• concerned with pain and temperature sensation
• originates from nociceptors which detect noxious or potentially harmful stimuli and thermoreceptors than signal changes in temperature
• pain and temp impulses are transmitted to the brain via thinly myelinated a-delta fibres and unmyelianted c-fibres.

Question 11

What is the route of the spinothalamic tract?

Answer 11

• 1st order neuron is in the dorsal root ganglion - enters the dorsal root and synapses onto 2nd order neuron upon entry
• 2nd order neuron crosses the midline in the anterior part of the spinal cord, then turns upwards
• continues anterolaterally in the medulla, lying just behind the olive
• 2nd order terminates in the ventroposterolateral nucleus of thalamus
• 3rd order project to the sensory strip via posterior limb of internal capsule