Ascending Pathways

Question 1

Describe what lateral inhibition is and its important function

Answer 1

When neuronal receptive fields overlap, this can result in the spread of excitation when one neuron is stimulated. Lateral inhibition is what allows us to localise the source of a stimulus. By the central neuron exciting inhibitory interneurons of neighbouring neurons this allows increased perception of the neurons central to the stimulus in contrast to those peripherally.

Question 2

What are the different axonal types and their modalities in order of speed of conduction?

Answer 2

A(alpha) fastest: proprioception of skeletal muscle
A(beta): mechanoconduction
A(delta): pain, temperature
AC slowest: pain, temperature, itch

Question 3

Name the white matter tracts of the spinal cord and the information that they carry

Answer 3

• fasciculus cuneatus: information from the upper limbs
• fasciculus gracilis: information from the lower limbs
• spinothalamic tract: contralateral pain and temperature
• spinocerebellar tract: ipsilateral unconscious proprioception

Question 4

What are the 3 principal ascending tracts?

Answer 4

• dorsal column medial lemniscal pathway (DCML)
• spinothalamic tract
• spinocerebellar tract

Question 5

Describe the DCML pathway

Answer 5

• first order neurons (A(beta)) are stimulated and ascend in the posterior column of the spinal cord
• at the medulla oblongata it synapses with second order neurons at the nucleus cuneatus or nucleus gracile depending on the tract
• the second order neurons decussate to ascend the contralateral medial lemniscus to synapse with third order neurons at the VPL nucleus of the thalamus
• third order neurons project to the somatosensory cortex

Question 6

Describe the effect of a lesion to the somatosensory association cortex

Answer 6

Amorphosynthesis: unable to recognise objects by feel on the contralateral side to the lesion. Also unable to locate the source of the stimulus in the context of pain and temperature.

Question 7

Describe the effects of a lesion to the DCML pathway

Answer 7

• gait ataxia (stamp and stick gait)
• hemiparesis
• loss of JPS ipsilaterally below the level of the lesion

Question 8

Describe the spinothalamic pathways

Answer 8

• first order neurons (A(delta)/AC) are excited and ascend in the lateral column of the spinal cord 2/3 levels before decussating and synapsing with second order neurons in contralateral side
• second order neurons ascend lateral/anterior spinothalamic tract and synapse with 3rd order neurons in VPL nucleus in thalamus
• 3rd order neurons project to the cortex
• lateral (A(delta) and AC): neospinothalamic to somatosensory cortex
• anterior (AC): paleospinothalamic to limbic system cortex

Question 9

Describe the spinocerebellar pathway

Answer 9

• takes information from muscle spindles (muscle length), golgi organs (proprioception) and touch receptors
• anterior and posterior routes
• second order neurons synapse in the vermis of the cerebellum

Question 10

Describe the effect of a lesion in the spintothalamic tracts

Answer 10

• paraesthesia

* decreased pain perception on contralateral side at levels below lesion

Question 11

Name the skin receptors and the modality they process

Answer 11

Hair follicle - touch
Merkel - pressure
Meissner - light touch
Pacinian - vibration
Ruffini - skin stretch
Nociceptor - pain
Muscle spindles - muscle length
Golgi tendon organs - muscle stretch