Neuromotor System

Question 1

What are some functions of the skeletal muscle (‘effector organ of the neuromotor system’)?

Answer 1

• movement (including vocalisation and respiration)
• maintenance of posture
• stabilises joints
• generates heat
• venous return
• lymphatic drainage
• reproduction
• digestion
• excretion

Question 2

What are some features of the neuromuscular synapse/junction?

Answer 2

• biggest chemical synapse in the body
• excitable synapse
• can undergo reinnervation

Question 3

What is the process of generating action potentials?

Answer 3

1. AP arrives at motor nerve terminal
2. AP triggers opening of VGCCs, entry of Ca2+
3. Ca2+ dependent release by exocytosis of ACh from synaptic vesicles (at active zones)
4. Ach traverses the synaptic cleft to bind to its receptor a ligand gated ion channel
5. This binding opens the ion channels to cause a large movement of Na+ in and a small movement of K+ out of the muscle cell
6. The result is depolarisation. This sets up a local current flow between the depolarised post-synaptic membrane and adjacent membrane of muscle
7. This local current flow opens voltage gated Na+ channels in the adjacent membrane
8. Resulting entry of Na+ causes the resting membrane potential to rise - thus triggering generation of muscle AP
9. ACh is subsequently destroyed by acetylcholinesterase (an enzyme located in the synaptic cleft)

Question 4

How does rate modulation affect APs?

Answer 4

increase the frequency of muscle action potentials, which lead to increased muscle fibre tension

Question 5

What is a motor unit?

Answer 5

1 motor neuron to the number of muscle fibres it innervates

Question 6

What is a general rule for the size of motor units?

Answer 6

Larger motor units: more power, less control
Smaller motor units: less power, more control

Question 7

What does motor unit recruitment mean?

Answer 7

the activation of additional motor units to accomplish an increase in the contractile strength (tension) in a whole muscle

Question 8

What is Henneman’s size principle?

Answer 8

For a given synaptic input, motor neurons with the smallest cell bodies are activated first and large motor neurons are activated last

Question 9

What are the three muscle fibre types, and what size neurons are they determined by?

Answer 9

Slow twitch (Myosin 1) - small motor neurons
Fast twitch Fatigue Resistant (myosin 2A) - medium sized motor neurons
Fast twitch Fatigable (Myosin 2B) - large sized motor neurons

Question 10

What is the biggest determinator of muscle grouping?

Answer 10

neurological activity

Question 11

What are the three levels of input control to motor neurons?

Answer 11

1. Input of afferent (sensory) - reflexes
2. Primary motor cortex (direct - hierarchical) - pyramidal
3. Multi-neuronal motor system (indirect - parallel) - extra pyramidal

Question 12

What information do sensory neurons bring from sensory receptors?

Answer 12

Muscle length - (muscle spindles)
Velocity of muscle contraction - muscle spindles
Muscle tension - Golgi tendon organs
Joint position - free nerve ending in joints

Question 13

What are gamma motor neurons responsible for?

Answer 13

maintaining sensitivity of of intrafusal muscle fibres (spindle) during contraction (change in length, rate of change in length)

Question 14

What does proprioception rely on?

Answer 14

• muscle spindles
• joint receptors
• golgi tendon organs

Question 15

What are reflex movements controlled by?

Answer 15

Spinal cord
- cervical-brachial S.C. (head, neck, forelimb)
- lumbar S.C. (lower limb, axial muscles)

Question 16

What are inter-neurons and what do they do?

Answer 16

• connection between afferent inputs
• function to co-ordinate the activities of muscle groups and their afferent inputs

Question 17

What inputs and outputs does the brainstem receive and send?

Answer 17

Inputs (sensory):
- proprioception
- vestibular
- visuals
Outputs:
- to modify the reflexes without higher order processing

Question 18

What are the four centres in the brainstem important for posture-movement?

Answer 18

1. Vestibular Nuclei
2. Reticular formation
3. Superior colliculus
4. Red nucleus

Question 19

What does the vestibular nuclei do?

Answer 19

• receives afferent signals from vestibular apparatus regarding position of head (with regards to gravity)

Question 20

What is reticular formation, and where does it output to?

Answer 20

• receives connection from pre-motor cortical regions and coordinates lower motor neuron pools by outputting to S.C motor neurons

Question 21

What input does the superior colliculus receive?

Answer 21

input for sensorimotor integration (eyes, vestibular, proprioceptors)

Question 22

What is the role of the red nucleus?

Answer 22

• plays role in motor learning

Question 23

Why are posture adjustments needed?

Answer 23

1. balance
2. stabalise
3. alignment of head and body with regards to gravity
4. protective

Question 24

What are the two mechanisms by which postural adjustments are achieved?

Answer 24

1. Feedback (compensatory reflexes) - rapid, stereotyped and show a space-time organisation
2. Feed forward (anticipatory) - pre-programmed experiences (unlike reflexes, scale of response refined with experience)

Question 25

What are the functions, inputs and outputs of the cerebrocerebellum?

Answer 25

Functions: - initiation - planning of movements - timing of movements Inputs: - cortical afferents Outputs: - dentate nucleus to motor and pre-motor cortices

Question 26

What are the functions, inputs and outputs of the spinocerebellar a) vermis and b) intermediate part of hemisphere?

Answer 26

a) Vermis Functions: - control of axial and proximal muscles - ongoing execution of movements Input: - auditory, visual and vestibular Output: - fastigial nucleus b) Intermediate part of hemisphere Functions: - ongoing execution of movements - control of distal muscles Input: - spinal afferent (distal parts) Output: - interposed nucleus (outputs via the reticular formation - to motor execution in the spinal cord)

Question 27

What are the functions, inputs and outputs of the vestibulocerebellum?

Answer 27

Functions: - axial control and vestibular reflexes (balance and eye movements Input: - vestibular apparatus Output: - later vestibular

Question 28

What is the basal ganglia, and what is important for?

Answer 28

- collection of subcortical nuclei deep in white matter - important for control of movement and learning and motivation

Question 29

Which of the basal ganglia nuclei receive excitatory input from the motor cortex, thalamus and brainstem?

Answer 29

- caudate - putamen

Question 30

True or false: the input from the globus pallidus to the thalamus is inhibitory, thus exciting the cortex.

Answer 30

True! Despite the basal ganglia containing many neurotransmitters (GABA, dopamine, acetylcholine) the input from the globus pallidus to the thalamus is inhibitory, thus producing the 'brake' hypothesis (smooth coordinate movement involves selective inhibition of motor output)

Question 31

What are some unique features of the neuromuscular junction?

Answer 31

- biggest chemical synapse - motor axons can successfully reinnervate muscle fibres at the same site

Question 32

Explain the steps of an AP being propagated at a NMJ

Answer 32

1. AP arrives at motor nerve terminal 2. AP triggers the opening of voltage gated calcium channels 3. Ca2+ dependent release by exocytosis of acetylcholine from synaptic vesicles - at active zones 4. Acetylcholine transverses the synaptic cleft to bind to its receptor - a ligand gated ion channel 5. This binding brings about the opening of this ion channel to cause a large movement of Na+ in and a small amount of K+ out of the muscle cell 6. The result is depolarisation. This sets up a local current flow between the depolarised post-synaptic membrane and adjacent membrane of muscle 7. This local current flow opens voltage gated Na+ channels in the adjacent membrane. 8. The resulting entry of Na+ causes the resting membrane to rise from -70mV to -60mV, which triggers the generation of a muscle action potential 9. Acetylcholine is subsequently destroyed by acetylcholinesterase, and enzyme located in the synaptic cleft

Question 33

Describe Henneman's size principle

Answer 33

For a given synaptic input, motor neurons with the smallest cell bodies are activated first and large motor neurons are activated last

Question 34

What determines the size of a given synaptic input?

Answer 34

The frequency of APs that are delivered to the terminal ending of the pre-synaptic neurons onto the motor neuron, because as the frequency increases, large motor neurons are able to raise their resting membrane potential to threshold, thus firing off an AP.

Question 35

Explain the mechanism behind why the size principle exists.

Answer 35

- smaller neurons have higher membrane resistance and require lower depolarising current to reach spike threshold

Question 36

Define fibre type grouping.

Answer 36

- chronic denervation re-innervation events lead to type 1 fibre predominance: seen in MND and aging, where fast motor neurons die first/become more venerable with age

Question 37

How does grouping of slow fibres occur?

Answer 37

- re-innervation of fast twitch fibres by a small motor neuron, converting the fibres to slow twitch

Question 38

Compare and contrast lesions to LMNs and UMNs.

Answer 38

- both result in weakness or paralysis - LMN lesions result in reduced/absent muscle tone, whereas UMN results in increased muscle tone (except acutely) - LMN lesions result in lost muscle mass, whereas muscle mass is maintained in UMN lesions - the myostatic reflexes are reduced/absent in LMN lesions, whereas they are increased (except acutely) in UMN lesions - Babinski sign after UMN lesions