lecture 12

Question 1

What is the bit that actually does the movement?

Answer 1

• skeletal muscles - many muscles each with compartments of possible control
• graded capacity

Question 2

From what do skeletal muscles receive their innervation?

Answer 2

Excitatory neurons called motorneurons - sit in the spinal cord or brain stem
‘motorneuron pools’ because each muscle has its own pool of many neurons that innervate it.

Question 3

What is a lower motorneuron?

Answer 3

• the neuron that sends its axon out and excites muscle

Question 4

What is a lower motorneuron sign?

Answer 4

a positive (something that is happening that shouldn’t be) or negative (something that isn’t happening that should be) sign that is used in clinical medicine to diagnose a problem in the action of the lower motorneuron (could include a problem that affects the receptor on the muscle - technically not a problem with the lower motor neuron itself but a problem in the pathway)

Question 5

What activates the lower motorneuron?

Answer 5

• whatever synaptic inputs they have that
• come from neurons within the spinal cord (local circuit neurons/interneurons)
• descending inputs: axons that come down from the brain through the spinal cord and synapse on the lower motorneuron
• also sensory inputs that act on local circuit neurons that act bypassing the brain

Question 6

What is the neurotransmitter used by lower motorneurons?

Answer 6

• acetylcholine

Question 7

Where do lower motorneurons begin?

Answer 7

• in the ventral part of the spinal cord

Question 8

Where do descending inputs come from?

Answer 8

areas of the brain whose role is control of movement
brain stem

critical but don't act directly:  
cerebellum 
basal ganglia 
their role is in feedback with the brainstem and the cortex 
interact laterally

Question 9

What does the statement that the motorneuron is the final common pathway mean?

Answer 9

• if you are going to have any affect on movement, if the muscle is going to do anything at all, it must ultimately come from signals through the motorneuron

Question 10

What is something that can directly excite a lower motor neuron?

Answer 10

• sensory input e.g. muscle spindles

Question 11

Can muscle be inhibited?

Answer 11

• no, there are no inhibitory neurons going to muscle, just these excitatory lower motor neurons

Question 12

Where do lower motorneurons live?

Answer 12

• in the ventral horn of the grey matter
• exist in constellations or clusters related to the muscle they innervate
• nerves innervating a particular muscle will lie in a sort of column over many spinal segments

Question 13

What’s the relationship to where a pool is to where a muscle is?

Answer 13

• proximal muscle pools are located more medially while distal muscle pools are located more laterally
• similarly for ventral/dorsal
• somatotopic

Question 14

What do the interneurons indicate about muscles in the body?

Answer 14

• little neurons in the spinal cord
• ones that connect between the medial motorneurons (i.e. close to the midline) tend to be long, to lick up big spans i.e. medial muscles tend to be a lot more interconnected across the body than the neurons out laterally that control things like digits meaning the muscles don’t have a lot of interconnection
• this reflects the precision needed in distal muscles whereas midline muscles need coordinated activity to maintain things such as balance
• lateral muscles tend to be for voluntary dextrous skill movements and this is reflected beautifully in the interneurons

Question 15

What is a motor unit?

Answer 15

An element of muscle control

• how many fibres in the muscle are activated by each individual motor neuron
• not one
• one to many relationship
• eyes maybe one to two or three
• big muscles maybe one to a hundred
• there will be a spectrum of motor unit sizes in one muscle
• the fibres will be distributed around the muscle

Question 16

What do alpha motor neurons exhibit with increased motor unit size?

Answer 16

Increased:

• cell body size
• dendritic complexity
• short-term EPSP (excitatory post-synaptic potential) potentiation with repeated activation
• axonal diameter (i.e. faster conduction)
• number of axonal branches (i.e. more muscle fibres innervated)

Decreased

• input resistance
• excitability
• Ia EPSP amplitude
• PSP decay constant
• Duration of after-hyperpolarisation

NB: the bigger the motor unit, the harder it is to excite. Small motor units are much easier to excited.
The percent of motor neuron pool recruited vs the percent of maximum force is non-linear

Question 17

What does recruitment order depend on?

Answer 17

size of a motor unit - literally because larger motor units require more input to be recruited

Question 18

What is the muscle spindle?

Answer 18

• sensory receptors that tell the CNS the amount of stretch in the muscle
• sensory fibres wrap around nuclear bag
• also gamma motor neurons that stimulate the modified muscle fibres in the muscle spindle (intrafusal muscle fibres)
• proprioception

Question 19

What is the stretch reflex circuit?

Answer 19

• sensory afferent axon from muscle spindle synapses onto motorneurons that directly supply that muscle or muscles that do the same thing (synergists)
• when the stretch receptor gets activated it excites the same muscle and synapses on an interneuron that inhibits a motorneuron that would normally excite an antagonistic muscle
  i. e. contract muscle more if load increased
• classic negative feedback

Question 20

Why do the intrafusal fibres of muscle spindles contract?

Answer 20

• crucial to the way they operate
• muscle spindles are incredibly sensitive to any amount of stretch
• need to be able to do this at any length of the muscle
• these are the fastest conducting fibres in the body
• if they didn’t contract at the same time as the muscle they wouldn’t be able to function because they wouldn’t detect stretch - distorted i.e. taking up the slack
• gamma motor neurons keep the spindle set so they can act at any level of contraction
• spindle response is constant

Question 21

What are golgi tendon organs?

Answer 21

• sensory structures with afferent neurons
• don’t detect stretch of the muscle
• located in the tendon but in series with the contraction of the muscle
• detect muscle force
• when you contract a muscle you get activity of the golgi tendon organ (i.e. opposite of muscle spindle)
• they are excitatory neurons that act through inhibitory interneurons to lessen the contraction/force on a muscle (if too much force)
• excite the antagonist muscle

Question 22

What does the spinal cord do?

Answer 22

• simple reflexes
• complex reflexes
• • crossed extension reflex
• • e.g. pain detected by nociceptor
• • stimulated leg flexes to withdraw while opposite leg extends to support
• e.g. stepping in a newborn baby - pattern generated in spinal cord with no input from brain. Later once there are some super spinal inputs this no longer happens

Question 23

What are the central pattern generators in terrestrial mammals?

Answer 23

• can categorise walking into phases of flexion and extension
• different mammals are capable of generating different locomotor patterns e.g. walk/trot/pace/gallop
• brain says how much movement it wants but the spinal cord dictates the pattern

Question 24

What happens to the spinal pattern generator if the spinal cord is severed from the brain?

Answer 24

• in humans no longer able to generate the spinal pattern generator and therefore no longer able to walk
• this can happen in other animals where sensory inputs can be enough to stimulate the spinal cord to action