Motor and Sensory Pathways

Question 1

nerves in the ____ and tracts in the _____

Answer 1

PNS

CNS

Question 2

True or false, pathways in both the CNS and PNS are functionally segregated

Answer 2

False!!! In the cns, pathways are functionally segregated, but in the PNS, nerves are organized regionally aka there is not functional segregation (mix of motor, sensory, sympathetic, parasymp)

Question 3

number of neurons in a somatic motor pathway

Answer 3

2

- these pathways control skeletal muscle

Question 4

Somatic motor pathways

Answer 4

the upper motor neuron (UMN)
- cell body at higher centres (cerebral hemispheres, brainstem) and the axon usually decussates projecting to synapse onto the lower motor neuron

• cell body of the lower motor neuron resides in the motor nucleus of a cranial nerve (face) or in the ventral horn of the spinal cord (body) and axon travels through either a cranial or spinal nerve to synapse onto skeletal muscle

Question 5

Control of the skeletal muscle in the body vs the face

Answer 5

cell bodies of lower motor neurons are in the ventral horn of the spinal cord. The axons of the lower motor neurons reach their target tissue by:
- ventral root > spinal nerve > peripheral nerve to NMJ

cell bodies of LMN are located in cranial nerve nuclei, axons of the LMNs reach target muscle by travelling in a cranial nerve to the NMJ

Question 6

“Final Common Pathway”

Answer 6

Refers to the fact that only the LMN contacts and therefore influences the muscle, though many UMNs may send signals to it (usually LMN have extensive dendritic trees)

Question 7

Lower motor neurons form bilaterally paired

Answer 7

motor columns (ventral horns of the spinal cord) or motor nuclei (in the brainstem)

Question 8

Medial Motor Column

Answer 8

• located at all levels of the spinal cord
• forms medial portion of the ventral horns
• innervates ipsilateral axial muscles controlling posture and balance

Question 9

lateral motor column

Answer 9

• forms the lateral portion of the ventral horns
• only found in cervical and lumbosacral sections (where we have limbs to innervate)
• innervates ipsilateral limb movemrnts for skilled voluntary movement

Question 10

Where do you find the UMNs that innervate the LMNs found in ventral horns?

Answer 10

UMN axons descend in lateral and medial pathways (white matter tracts) outside the grey matter butterfly.
- called the lateral funiculus and the ventral funiculus

Question 11

Lateral corticospinal tract

Answer 11

In the Lateral Funiculus

• innervates LMNs of the lateral motor column which control distal muscles
• function in skilled voluntary movements
• allows for asymmetrical movement

Question 12

Medial corticospinal tract and Vestibulospinal tract

Answer 12

descends in the ventral funiculus

• innervate LMNs which control axial and proximal muscles
• posture and balance

Question 13

Ventral funiculus includes the…

Answer 13

medial corticospinal tract and the vestibulospinal tract

Question 14

Describe pathway of corticospinal tract

Answer 14

1° motor cortex of precentral gyrus with cell bodies of UMNs > UMNs cross subcortical white matter > most fibres of CST decussate at the spinal-medullary junction and descend contralaterally in the lateral funiculus as the LCST > synapse on LMNs in lateral motor column of ventral horns (discrete skilled movement)

Question 15

What happens to the fibres of the CST that dont decussate at the spinal medullary junction

Answer 15

they descend as the medial CST in the ventral funiculus and then decussate at the the level they innervate

Question 16

interoreceptors

Answer 16

internal environment receptors - chemo, baro, stretch

Question 17

special senses

Answer 17

sight, hearing, smell, vestibular

Question 18

somatic sensation

Answer 18

touch, pressure, vibration, conscious proprioception, nociception, temperature

Question 19

conscious sensation

Answer 19

a small fraction of sensory input reaches modality specific primary sensory cortex in the contralateral cerebral hemisphere

Question 20

Rules of Somatosensory Pathways

Answer 20

consist of 3 neuron chain

• 1° neuron is pseudo unipolar, cell body in ganglion in the PNS (dorsal root ganglion)
• cell body of 2° neuron is either in the dorsal horn or a brainstem nucleus (depending on the modality) and it’s axon projects to the contralateral thalamus
• 3° neuron is located in the VP nucleus of the thalamus and it’s axon projects to primary somatosensory cortex

Question 21

Secondary neurons????

Answer 21

Decussate, because that’s what they do!!

Question 22

Dorsal Columns

Answer 22

includes the fasciculus gracilis and the fasciculus cuneatus
touch, vibration, and conscious proprioception
- gracilis is below T6
- cuneatus is above T6

• 1° neurons are in DRG
• the lower body fibres ascend in the ipsilateral fasiculus gracilis and synapse at the nucleus gracilis of the medulla. The 2° neurons decussate and ascend as medial lemnicus through brainstem synapse on 3° neurons in VP thalamus and project to primary somatosensory cortex in the postcentral gyrus
• for the upper body the fibres ascend in the ipsilateral fasciculus cuneatus (FC) (kinda join with the gracilis) to synapse on 2° neurons in nucleus cuneatus which go to VP thalamus. 3° neurons from there go to somatosensory cortex in postcentral gyrus.

Question 23

What separates the fasiculus gracilis and cuneatus?

Answer 23

dorsal intermediate sulcus

Question 24

Spinothalamic tract

Answer 24

pressure, pain and temp (the white matter kind of lateral to the ventral horns)

• 1° neurons have their cell bodies in the DRG in PNS and project to the CNS via dorsal roots and synapse on 2° neurons
• 2° neurons decussate and ascend in the STT of the ventrolateral funiculus (becomes spinal lemniscus)
• 3° neurons have cell bodies in the VP nucleus of the thalamus and project to 1° somatosensory cortex in the post central gyrus

Question 25

STT becomes the _____ in the brainstem | Dorsal columns become the ______ in the brainstem

Answer 25

spinal lemniscus | medial lemniscus

Question 26

Decussation occurs where for: - STT/ spinal lemniscus (sensory) - Corticospinal tract (motor) - dorsal columns/medial lemniscus (sensory)

Answer 26

STT - at the level of innervation (as soon as primary neuron synapses at the dorsal horn) Corticospinal - at the medullary pyramids dorsal columns/medial - spinal medullar junction

Question 27

How is the spinothalamic tract organized

Answer 27

as you ascend up the tract, more axons are added - most lateral in the STT is the sacral and coccygeal dermatomes, then moving inward it's lumbar, then thoracic, then cervical (first down from the brainstem is the cervical region, first region you get info from so cervical is most tucked away)