Lecture 8- The motor system- lower motor neurones and the muscle stretch reflex

Question 1

what are the primary components of the somatic motor system

Answer 1

Upper and lower motor neurones

Question 2

Somatic motor system responsible for

Answer 2

• voluntary movement

Question 3

upper motor neurone (UPM) project onto

Answer 3

the lower motor neurone (LMN)

Question 4

LMN also known as …

Answer 4

final common path–> activates skeletal muscles

Question 5

properties of UMN

Answer 5

Found entirely within CNS

• Start in the primary motor cortex (precentral gyrus in frontal lobe)
• Not found in the basal ganglia or the cerebellum (won’t cause upper motor neurone signs- specifically related to neurones originating in primary motor cortex)
• Prognosis of damage to UMN worse than PNS

Question 6

features of LMN

Answer 6

• LMN= both in the CNS (cell body and proximal axon) and PNS (distal axon synapsing with muscle)
  
  • Most cell bodies found in ventral horn (some also in motor nuclei of the brainstem)

Question 7

cell bodies of lower motor neurones fround in the

Answer 7

ventral horn

Question 8

Ventral horns

Answer 8

• Cylinders of grey matter (where cell bodies are found) that run the full length of spinal cord (anterior longitudinal structures)
• Dorsal horn have a similar structure dorsally

Question 9

Grey matter in the brainstem

Answer 9

• Brainstem very closely related to the spinal cord (similar morphology)
• Close affiliation between spinal nerves (originating in the spinal cord) and cranial nerves (originating in the brainstem)

Question 10

Midbrain cranial nerves

Answer 10

oculomotor

trochlear

Question 11

Pons CNs

Answer 11

• tirgmeinla
• abducens
• facial
• vestibulocochlear

Question 12

medullary CNs

Answer 12

• glossopharangeal
• vagus
• abducens
• hypoglossal

Question 13

oculomotor and its relation to lower motor neurones

Answer 13

Oculomotor nuclei contain lower motor neurones who’s axons are distributed along the oculomotor and supply extraocular muscles (4/6)

Question 14

trochlear and its relation to lower motor neurones

Answer 14

• trochlear nuclei contain lower motor neurones who’s axons are distributed along the trochlear and supply superior oblique

Question 15

Trigeminal and its relation to lower motor neurones

Answer 15

• Trochlear nuclei contain lower motor neurones who’s axons are distributed along the mandibular division and supply muscles of mastication

Question 16

abducens and its relation to lower motor neurones

Answer 16

• Abducens nuclei contain lower motor neurones who’s axons are distributed along the abducens and supply lateral rectus

Question 17

facial and its relation to lower motor neurones

Answer 17

• Abducens nuclei contain lower motor neurones who’s axons are distributed along the abducens and supply lateral rectus

Question 18

vestibulocochlear and its relation to LMN

Answer 18

• No motor components

Question 19

distribition of LMN of CN originating in the medulla

Answer 19

• Medulla- lower motor neurones distributed along the IX, X, XI cranial nerves are found in elongated nucleus in the medulla
  
  • Glossopharyngeal
  • Vagus
  • Abducens

Question 20

hypoglossal and its relation to LMN

Answer 20

• Elongated nucleus containing LMN that distribute to the tongue via the hypoglossal nerve

Question 21

modulation and coordination of spinal reflexes

Answer 21

Reflexes occur in the absence of supraspinal input, however inputs descending from the brain have an important modulatory role in the way reflexes manifest.

Question 22

outline the patellar reflex

Answer 22

(L3 level- L3 myotome = major input to quadriceps)

1. Patellar ligament in tapped by tendon hammer
2. Stretching of fibres of the quadriceps is detected by muscle spindles found within the quad muscle
   
   • Muscle spindles detect muscle length= stretch
3. Stretched detected by muscle spindle is conveyed to the spinal cord via first order sensory afferent (cell body found in the dorsal root ganglion)
4. This first order sensory afferent enters the dorsal horn and projects down to the ventral horn and synapses onto a lower motor neurone
5. LMN sends its axon down into the ventral root and down to the quadriceps muscle itself causes twitch

What about the hamstrings? Need them to stop antagonising the action of the quads

• If the quads are contracting this would cause the hamstring to contract and this would result in no movement at all  agonist+ antagonist contraction= no movement

Therefore:

1. L5 myotome supplies the hamstring
2. First order sensory neurones sending info from the muscle spindle in the quadricep descends down from L3 to L5 and synapse with inhibitory interneurons
3. Inhibitory interneurons inhibit LMN supplying the hamstring (selective inhibition of hamstrings)
4. Therefore antagonists effect of hamstrings muted and the patellar reflex can cause full extension at the knee

Question 23

patellar reflex involves which spinal nerves

Answer 23

L3 - quad

L5- hamstring

Question 24

Descending influences (fibres) which help modulate spinal reflex reflexes e.g. Plantar reflex (Babinski)

Answer 24

stimulation of sole of foot leads to flexion of the toes

• Modulated by the presence of descending modulatory fibres (yellow)
• If a patient has a stroke these modulatory fibres can be damaged and result in the reflex manifesting itself in a different way
  
  • E.g. the toes will extend as appose to flex

Question 25

Descending influences (fibres) which help modulate spinal reflex reflexes e.g. Reflexes found within babies

Answer 25

• Tend to disappear as the baby grows due to the descending(fibres) influences maturing