Neuronal Pathways: Sensory (Afferent-Ascending)

Question 1

Describe the four general features of Neuronal Pathways

Answer 1

• All pathways decussate at some point
• Most consist of two or thre neurons (relays)
• Most maintain somatotopy (organizational mapping)
• Pathways are paired (one on each side of the spinal cord or brain)

Question 2

Ascending Pathway Organization:

describe the central processes of first-order neurons

Answer 2

The central processes of first-order neurons branch diffusely

as they enter the spinal cord and medulla

Question 3

First-order neurons conduct impulses from where?

Answer 3

First-order neurons conduct impulses from cutaneous receptors (touch, pressure, temperature, pain) and proprioceptors (feedback on positions of joints & relative movement of joints)

* some brances take part in spinal cord reflexes (more in Ch 13)

Question 4

Where do most branches synapse with a second-order neuron (an interneuron)?

Answer 4

in the spinal cord or medullary nuclei (nuclei in the medulla oblongata; e.g. red nucleus)

Question 5

Describe how synapses occur with third-order neurons

Answer 5

Axons extend to the thalamus and synapse with a third-order neuron

Question 6

Where are all third-order neurons located?

Answer 6

All third-order neurons (interneurons) are located in the thalamus

Question 7

where do third-order neuron axons extend to?

Answer 7

third-order neuron axons extend to the primary somatosensory cortex

Question 8

Which axons extend to the cerebellum only?

Answer 8

Some second-order axons extend to the cerebellum only;

no additional synapes, so input remains at a sub-conscious level

Question 9

What are the Three Ascending Pathways?

Answer 9

Dorsal Column - Medial Lemniscal Pathway (Dorsal Tract location)

Spinothalamic Pathway

Spinocerebellar Pathway (Lateral tract location)

Question 10

What are the Dorsal Column - Medial Lemniscal Pathway responsible for?

Answer 10

Discriminiative touch and pressure

Vibration

Conscious proprioception

Question 11

Describe the Medial Lemniscal Pathway receptors

Answer 11

There are many receptors in a small area (dense receptive field),

so easy to localize the source of sensation precisely

Question 12

Describe how first-order fibers of the Medial Lemniscal Pathway (Dorsal Column) operate

Answer 12

First-order fibers travel into & ascend the spinal cord along the dorsal white columns, synapsing at the corresponding nuclei in the medulla oblongata

Question 13

Describe how second-order fibers of the Medial Lemniscal Pathway (Dorsal Column) operate?

Answer 13

Second-order fibers decussate in the medulla and synapse in the thalamus

Question 14

Describe how third-order fibers in the Medial Lemniscal Pathway (Dorsal Column) operate

Answer 14

Third-order fibers terminate in the somatosensory cortex

Question 15

What are the Spinothalamic Pathway (Ventral and Lateral tract locations) responsible for?

Answer 15

The Spinothalamic Pathway (Ventral and Lateral tract locations) are responsible for

**temperature **

pain

**coarse touch **

deep pressure

Question 16

Describe the Spinothalamic Pathway receptors

Answer 16

There are fewer receptors, so aware of sensation, but not as precisely localized

Question 17

Describe how first-order fibers in the Spinothalamic Pathway operate

Answer 17

First-order fibers of the Spinothalamic Pathway

synapse in the dorsal horn of the spinal cord

Question 18

Describe how second-order fibers of the Spinothalamic Pathway operate

Answer 18

Second-order fibers of the Spinothalamic Pathway

decussate at the same spinal cord level,

ascend the spinothalamic fiber tracts and synapse in the thalamus

Question 19

Describe how third-order fibers of the Spinothalamic Pathway operate

Answer 19

Third-order fibers of the Spinothalamic Pathway terminate in the somatosensory cortex

Question 20

What is the Spinocerebellar Pathway (Lateral tract location) responsible for?

Answer 20

The Spinocerebellar Pathways are responsible for subconscious proprioception.

Question 21

Describe the tracts of the Spinocerebellar Pathway (Lateral tract location)

Answer 21

The tracts terminate in the cerebellum and therefore do not contribute to sensory perception

Question 22

Describe how first-order neurons within the Spinocerebellar Pathway operate

Answer 22

First-order neurons of the Spinocerebellar Pathway synapse in the dorsal horn of the spinal cord

Question 23

Describe how second-order fibers within the Spinocerebellar Pathway operate

Answer 23

Second-order fibers terminate in the ipsilateral cerebellum