Spinal Control of movement Flashcards
Describe lower motor neurons and their segmental organisation.
- LMN have cell bodies in the ventral horn of the spinal cord and terminate at NMJs
- The ventral horn is arranged by flexor/extensor innervation (dorsal to ventrally) and axial to distal innervation (medial to laterally)
- spinal cord has cervical and lumbar englargements corresponding to increased LMN cell bodies innervating the arms and legs
Describe alpha motor neurons and the inputs to alpha
motor neurons.
Alpha motor neurons can innervate a few or many muscle fibers. They innervate extrafusal muscle fibres.
Inputs are from:
1. sensory from muscle spindles
2. upper motor input from brain
3. interneuron (largest input) - can be excitatory or inhibitory
Explain the mechanisms that mediate the graded control of muscle contraction by alpha motor neurons.
increased muscle contraction is achieved by recruiting additional motor units
What is an Alpha motor unit
motor neuron + muscle fibres it innervates
What is motor neuron pool?
all the neurons innervating a single muscle
What are the different types of motor unit?
- Red muscle: aerobic, many mitochondria, slow, sustained
- Slow motor: slowly fatiguing red fibres
- White muscle: 2 types, anaerobic, few mitochondria, fast, fatigue quickly.
fast fatigue-resistant - moderate strength, pretty fast
fast fatiguable - very strong, very fast but fatigue quickly
Describe the spinal control of motor units
- LMN synapses onto muscle fibre end plate
- releases ACh
- activate endplate nicotinin ACh Rs (Na+ channels)
- Na+ influx
- triggers EPSP in muscle fibre
- evokes muscle AP
- Ca++ release from SR
- muscle fibre contracts
- Ca++ reuptake
- fibre relaxes
This can be in response to higher brain or interneuron signals from proprioceptors
Explain the process of proprioception
Combines info from:
* muscle spindles
* golgi tendon organ
* proprioceptive axons in joints (give angle, direction, velocity)
* skin stretch receptors
some sensory axons synapse onto motor neurons or interneurons in spinal cord to enable response without higher brain input
Colaterals travel up to the brain
What are 2 types of muscle fibres
- extrafusal: innervated by alpha motor neurons, outside part of muscle
- intrafusal - innervated by gamma motor neurons, inside of muscle, have muscle spindles
Explain the role of muscle spindles
- Muscle spindles detect muscle stretch of intrafusal fibres
- provide sensory info about muscle length
- facilitate proprioception & myotatic reflexes
- send info on 1a sensory axon very quickly
Explain the role of Golgi tendon organs
- detect muscle tension (not length)
- innervated by 1b sensory fibres wound between collagen fibres
- slower conduction that muscle spindles (but only providing additional proprioceptive info)
- polysynaptic loop - acts on inhibitory interneuron to promote relaxation of muscle.
Describe the concept of myotatic reflexes. Explain the myotatic reflex mechanism.
The concept of myotatic reflexes is to avoid tissue damage.
example: knee-jerk reaction
- muscle is stretched
- rapid firing from 1a sensory in muscle spindle
- synapses directly onto alpha motor neuron in spinal cord ventral root
- causes muscle contraction to oppose the stretch
explain reciprocal inhibition
coordinated contraction/relaxation of paired muscle groups to give control.
ie 1a sensory activation from muscle stretch can synpase onto both:
* excitatory alpha motor neuron (acting on agonist muscle to contract)
* inhibitory interneuron (inhibits motor neuron of antagonist muscle to relax)
Explain the process of generation of spinal motor programs for walking.
- the circuitry for walking resides in the spinal cord
- higher brain areas are responsible for initiation, but once initiated, spinal cord maintains
- does this via spinal interneuron rhythmic activity using NMDA receptors
- fluctuating ion channels opening and closing in a loop
This is so far only tested in quadripeds.
