Block 5 - Spinal Chord and Motor Control Flashcards
What is an alpha motor neuron and where is it found?
Neuronal cell.
Supplies all skeletal muscles, control every movement in the body
Found in the ventral horn of the spinal chord (or brainstem for eye muscles)
What is a motor unit?
Everything that connects to one motor neuron.
Motor neuron plus the muscle fibres it is attached to.
What are the three components of the spinal chord?
Roots: dorsal=sensory, ventral=motor, origin of peripheral nerves
White matter: ascending and descending neuronal pathways
Grey matter: neuronal cell bodies (including alpha motor neurons)
How are muscle fibres attached to the axon of a motor neuron?
NMJs - neuromuscular junctions
On average how many muscle fibres do small and large MUs (motor units) attach to?
Smaller MUs: One alpha motor neuron supplies 100 muscle fibres
Larger MUs: One alpha motor neuron supplies 2000 muscle fibres
List the 3 types of MU within a muscle
S (slow): slow contracting, resistant to fatigue, small force and aerobic
FR ( fast fatigue resistance): faster contracting, fatigue resistant, low force, mainly aerobic
FF (fast fatigable): fast contracting, fast fatigue, hugh force, anaerobic
How does the brain increase force in the muscles? Give 3 ways.
1) Rate coding - increasing the firing rate of motor neurons
2) Recruitment - increasing the amount of motor units firing
3) Use a recruitment order - increasing the size/ type of motor units firing
How do recruitment orders work?
Use small MUs first - low muscle tension
ADD increasingly larger motor units - high muscle tension
What is msucle tetanus?
Sustained muscle contraction caused when action potentials fire above a certain rate - smoothes out the contractions
Which areas of the brain are involved in controlling movement?
a-motor neurons are controlled by the primary motor cortex.
Other areas involved in controlling movement:
- cerebellum - error correction and learning movements
- basal ganglia - movement selection
What gives us awareness of where our body is in space?
1) Vision - eyes in head
2) Vestibular system - inner ears in head
3) Proprioception - muscles, joints and skin
4) Efference Copy - no feedback
What are the two streams of vision?
Dorsal - where
Ventral - what
How does the vestibular system work?
Rotations and translations of the head (and hence the body) are sensed using sense organs in the inner ears.
Rotations - semicirular canals
Translations - otoliths
How does proprioception work?
Muscle spindles - muscle length and stretch sensors
Golhi tendon organs - muscle tension (or force) sensors.
Other - eg for control of posture
Cutaneous receptors (eg pressure receptors in the feet)
Joint receptors (eg in the ankle)
What is an efference copy?
The brain makes an internal copy of the outflowing (efferent) motor signal
The reason we can’t tickle ourselves
Muscle fibre function in proprioception
Sensory fibres wrap around muscle fibres (called intrafusal fibres)
Arranged in parallel with muscle fibres
Two types:
II afferents - measure muscle length (detect join angle)
Ia afferents - detect change in lenth (detect join movement in velocity and acceleration)
Golgi tendon organ’s role in proprioception
Sensory fibres embedded in tendon of msucle
Arranged in series with muscle
Detect muscle tension and forces applied to joints
How can you measure sensory imputs by looking at postural control?
SOT (sensory organisation test)
Remove vision by closing eyes
Remove proprioceptors by causing platfrom to sway with body
Allows contributions of vision, vestibular and proprioceptive feedback to be separated out
CDP (computerised dynamic posturography)
Measures body sway using a force platform
Adantages and disadvantages of efference copies
Advantages:
Good for controlling fast movements (don’t need to wait for sesnory feedback to arrive)
Good for movements which are not usually affected by external forces (eg eye movement)
Disadvantages:
Doesn’t allow for unexpected changes (usually external applied forces)
No absolute reference (needs updating using sensory feedback)
Less accurate for slower movements
What causes the stretch reflex?
Muscle spindle is stretched causing afferent neuron to fire
Monosynaptic connection in the spinal chord leads to contaction of agonist muscle
Inhibition of antagonist muscle through inhibitory interneuron
Why is the stretch reflex important?
- Transfer motor control to the spinal chord
- Improved speed response
- Control of posture
