Lecture 12- Lower Motorneuron

Question 1

What are the 3 classes of input to lower motorneurons?

Answer 1

1. direct sensory input 2.lot of the signals from spinal cord,indirect and local interneuron input to lower motorneurons 3.and descending inputs

Question 2

What is the overall organisation of neural structures that control movement?

Answer 2

-

Question 3

What do the skeletal muscles do?

Answer 3

-do the moving -receive excitation from the spinal cord or brain stem

Question 4

What is a motor neuron pool?

Answer 4

pools= has many nerves innervating a muscle lower motor neuron= sends its axon out and excites muscle

Question 5

In what way are motorneurons a final common pathway?

Answer 5

• if you’re going to have any effect on movement, must be from signals that come through the lower motorneuron

Question 6

How are the cerebellum and basal ganglia involved in motor control?

Answer 6

-major role in motor control, but they don’t project directly

Question 7

What are reflexes?

Answer 7

-the simplest motor behaviours

Question 8

Where are the motorneurons located?

Answer 8

• in ventral part of the spinal cord
• then interneurons are nearby, some projecting to the motor neurons pools and some get descending input

Question 9

With what experiment did they figure out the spatial distribution of motor neurons in the ventral horn of the spinal cord?

Answer 9

• inject dye in muscles on the calf and see what dye is where in the spinal cord, can tell by this what nerves innervate what bits
• see constellations (motor pools of that muscle

Question 10

What are muscles and how are they innervates?

Answer 10

-excitatory contractile tissue -excitatory motor neurons-exist in constellations /clusters in relation to the muscles they innervate

Question 11

What is the somatotopic organisation of lower motor neurons?

Answer 11

• neurons that are medial= closer to the midline = innervate muscle that is medial
• the lateral muscles (further from the midline, like fingers etc,) innervated by more lateral nerves
• somatotopy, mapping
• also ventral and dorsal mapping, neurons in the ventral part innervate ventral -dorsal neurones supply dorsally located muscles

Question 12

What are the interneurons connecting the medial motor neurons?

Answer 12

• tend to be long, link up big spans, connect medial motor neurons
• medial motor neurons usually connect axial muscle (close to the spine) versus the pectoral girdle
• the midline muscle= used to balance oneself= so want to do more things at the same time(more interconnected)
• those interneurons are much more interconnected than the interneurons connecting laterally that innervate the digits = those don’t have that much interconnection
• in the picture the long distance local circuit neurons

Question 13

What does midline muscle tend to do?

Answer 13

• tends to be for postural control

Question 14

What does distal muscle tend to do?

Answer 14

-lateral neurons) tends to be for more dextrous voluntary movement

Question 15

Why are the medial motor neurons connected by the long local circuits?

Answer 15

-innervate midline muscle -the midline muscle= used to balance oneself= so want to do more things at the same time (more interconnected)

Question 16

What is a motor unit?

Answer 16

• element of muscle control
• for each of the neuron= how many muscle fibres does it activate
• size differs with muscle

Question 17

Why are the muscle fibres one neuron innervates distributed in the muscle?

Answer 17

-the force of a muscle is so great that if you activated just one area you could tear the muscle, this way you don’t

Question 18

How much does the motor unit size vary?

Answer 18

-in eye= one neuron only 2 or 3 muscle fibres (as we have very fine control over eye movement) -gluteus maximus= 1 neuron= 100 -motor unit varies from muscle to muscle and within a muscle -spectrum of motor units (different sizes)

Question 19

What is increased in the motor neurons with increased motor unit size?

Answer 19

-increased cell body size -increased dendritic complexity (more branches) -increased short-term EPSP potentiation with repeated activation (excitatory postsynaptic potential= measure of how excitable these cells are) -increased axonal diameter=faster conduction

Question 20

What is decreased in the motor neurons with increased motor unit size?

Answer 20

-the bigger the motor unit the harder it is to excite

Question 21

Why do we want the big motor units to be harder to excite?

Answer 21

-when activating a muscle, some fine movement, you don’t want the one with the most strength acting first (so the small units easy to activate, big harder) -first one you activate is the smallest one!

Question 22

How does the amount of activation in muscles increase?

Answer 22

• non-linearly
• when activate 25% of muscle get 5% of total strength
• very non-linear and that is what you want for fine control
• 50%of motor neuron pool recruited and that is 25% of force
• then you get to the last ones= those are the big ones with lot of strength
• force is added from smallest to biggest

Question 23

What are the differing speeds of muscle fibres and why?

Answer 23

• the muscles that are always used (posture) slow muscle fibre(smaller motor units)
• fast fibres allow us to use more force and what we want to do for longer so fast fatigue resistant
• the fast fatigable= the ones we use for something we want to do rarely but need lot of strength for it!(the large motor units)

Question 24

What does recruitment order of motor units depend on?

Answer 24

-recruitment order depends on size due to the physiological property of the bigger motor units =hard to excite

Question 25

What are the muscle spindles and how do they develop?

Answer 25

-sensory structures in the muscle -tell the nervous system what the muscle is doing -start of as normal muscle fibres in development then a sensory nerve grows out and contacts them it transforms them

Question 26

What do muscle spindles detect?

Answer 26

- it detects stretch, is it long= extended or short=contracted - very sensitve strecth detector - sensory fibres, wrap around the modified muscle fibres,these relay the information back to the spinal cord about the stretch

Question 27

What are the intrafusal muscle fibres?

Answer 27

- contractile muscle fibres of the muscle spindle -innervated by gamma motor neurons

Question 28

What are extrafusal muscle fibres?

Answer 28

-normal contractile muscle fibres -innervated by alpha motor neurons

Question 29

How does the maintaining position work with glass in hand, liquid poured in? stretch reflex

Answer 29

- sensory afferent nerve (blue thing)= body in dorsal root ganglia as all sensory neurons have that - comes into the spinal cord and makes direct excitatory connections with motor neurons, not with all of them, only the ones that supply that very same muscle or muscles that do the same thing (synergists) - and the same sensory neuron makes connection with an interneuron that is inhibitory, when that is activated it inhibits a motor neuron connecting to a muscle that does the opposite (antagonist)

Question 30

What muscle is excited when a stretch is detected in a muscle by the muscle spindle?

Answer 30

-the same muscle is excited (as well as its synergists) and its antagonists are inhibited

Question 31

What is the stretch reflex an example of?

Answer 31

-homeostatic mechanism to maintain muscle stretch, the muscle spindle detects muscle stretch = that gets the thing going -classic negative feedback mechanism -when lot of muscle stretch! the signal!

Question 32

What is the stretch reflex circuitry?

Answer 32

-

Question 33

What is the potential problem with muscle spindle detecting stretch?

Answer 33

- if muscle is contracted then it is shortened from its length and no stretch can be detected - spindles respond to stretch so when you contract= no response, so useless now to detect stretch (this is what happens if the gamma motor neurons are taken out)

Question 34

What do the gamma motor neurons do?

Answer 34

- gamma neurons will set the spindles at the right tension so when the muscle is activated there is a stretch - so muscle shortens= the gamma motor neurons shorten the spindle - gamma motor neurons set the spindle to work at any contraction

Question 35

What are the Golgi tendon organs?

Answer 35

- at end of muscles where they join tendons you get neurons there as well - Golgi tendon organs= these are neurons - sensory structures, afferents - body in dorsal root ganglia (as they are sensory neurons) - don't detect stretch of muscle - know how much force is on the muscle - detect muscle force

Question 36

What do the Golgi tendon organs detect?

Answer 36

-force of the muscle -when muscle contracts get activity in Golgi tendon organ,the opposite a spindle (if no gamma)

Question 37

How does the Golgi tendon organ regulate muscle tension?

Answer 37

why do we need to know about force? - need to know to use the right force -info goes from Golgi tendon organ is inhibitory,if it goes too far= reduce force - excitatory neurons so must act through inhibitory neurons in the spinal cord - also connect to excitatory neurons exciting the antagonist muscle - golgi tendon organs inhibit the muscle they are from

Question 38

What do the spinal circuits do?

Answer 38

-reflexes and pattern generators

Question 39

How does the flexion-crossed extension reflex work?

Answer 39

- Crossed extension reflex - involves nociceptors - painful= flexor withdrawal,extensor is inhibited, withdraw the leg - same stimulus crosses to the other side of the spinal cord and excites extensor muscles and inhibits flexor muscle= put all the force on the other leg

Question 40

What is the story with the newborn baby stepping?

Answer 40

spinalcord is basis for locomotion -the baby is locomoting sort of -at day 0 the brain has not grown connections to the spinal cord to control the movement -that stepping is all produced in the spinal cord -initiated in the baby=by stepping on a surface -a few weeks later don't do it, that is because the brain starts to take control of what happens -extensors must be 180 degrees out of phase with flexors -flexor then extensor -spinal pattern

Question 41

Where is the pattern for walking etc. generated?

Answer 41

-in the spinal cord -swing= flex -extension= the force, the push

Question 42

What are the four patterns a cat can do when moving?

Answer 42

-we can only do walk or run -cats can do 4 -walk= each leg takes a turn -trot= diagonal -pace= two sides act as the same limb -gallop= faster -spinal cord dictates the pattern, incredibly able computational centre!

Question 43

Can cats walk after having their lumbar region of the spine transected?

Answer 43

-lumbar sacral region of the spinal cord= the pattern generator for walking and running (in humans) -if it isn't in contact with the descending input because of a lesion or so, cannot activate on its own -once spinal cord severed cannot activate spinal cord pattern generator -in other animals you can! -if severe the spinal cord, can walk and run normally on treadmill eventough no connection to brain