Motor System Flashcards
What is the difference between upper and lower motor neuron?
Upper - brain, supply input to LMN
Lower - soma in brainstem and ventral horn of spinal cord
Where do lower motor neurons receive input from?
- UMN
- proprioceptors
- interneurons
What are the two types of lower motor neuron?
alpha - innervate the bulk of fibres that generate force
gamma - innervate a sensory organ (muscle spindle)
State the difference between axial, proximal and distal muscles
Axial - movements of the trunk
Proximal - shoulder, elbow, pelvis
Distal - hands, feet and digits
What causes the two enlargements of the spinal cord?
Uneven distribution of motor neurons particularly distal and proximal ones
Where are the two spinal cord enlargements?
Cervical - C3-T1
Lumbar - L1-S3
What is a motor unit?
All the fibres supplied by one alpha motor neuron
What is a motor neuron pool?
A collection of alpha motor neurons that innervate a single muscle
Name two things force of contraction depends on
- frequency of AP discharge of alpha motor neuron
- recruitment of additional synergistic motor units
How are the LMN arranged?
LMN innervating axial muscles are medial to those that innervate distal muscles
Flexors are dorsal to extensors
What does activation of muscle fibres depend on?
- firing rate of LMNs
- number of LMNs active
- co-ordination of movement
How does a muscle movement arise?
Summation of twitches causes a sustained contraction as the number of APs increases
What differentiates slow and fast fibres?
Differ in how quickly myosin ATPase splits ATP to provide fro cross bridge cycling
Describe slow type 1 fibres
ATP largely derived from oxidative phosphorylation, slow contraction and relaxation means it is fatigue resistant
High myoglobin - red fibres
Low tension, small alpha motor neurons, low threshold for antigravity sustained movement
Describe type IIa fibres
ATP derived from oxidative phosphorylation, fast contraction and relaxation but fatigue resistant
Red and reasonably well vascularised
High tension, intermediate alpha motor neuron and threshold for sustained locomotion
Describe type IIb/x fibres
ATP derived from glycolysis, fast contraction and susceptible to fatigue
Pale and poorly vascularised
High tension, large alpha motor neurons and threshold for burst power
What type of fibre does not occur in mammals?
IIb
In what order are alpha motor neurons recruited?
In order of size, allows fine control of muscle force across a range of tensions
Name the parts of a muscle spindle
- fibrous capsule
- intrafusal muscle fibres
- sensory afferents (Ia fast conducting)
- gamma motor neurons
Describe the monosynaptic reflex
- Stretch of muscle spindle
- Activation of Ia afferents
- Excitatory synapse in spinal cord
- Activation of alpha motor neuron
- Contraction of homoymous muscle
What is the excitatory synapse mediated by?
Release of glutamate
Which level is the biceps reflex?
C5-6
Which level is the supinator reflex?
C5-6
Which level is the triceps reflex?
C7
Which level is the quads reflex?
L3-4
Which level is the gastrocnemius reflex?
S1
Describe Jendrassik Manoevre
Ask the patient to interlock their fingers and pull hands apart to exaggerate the patellar reflex
Name the two parts of intrafusal muscle fibres
- non-contractile equatorial region (Ia sensory neurons)
- contractile polar ends with efferent from gamma motor neurons with cell bodies in the spinal cord
Name the two types of intrafusal muscle fibres
Nuclear bag fibres
Chain fibres
Describe chain fibres
Sensitive to the absolute length of the muscle, innervated by static gamma motor neurons
What are the two types of nuclear bag fibres?
1/dynamic
2/static
What is the difference between dynamic and static infrafusal fibres?
Dynamic - very sensitive to rate of change of muscle length innervated by dynamic gamma motor neurons
Static - more sensitive to the absolute length of muscle, innervated by static gamma motor neurons
State the two types of afferent fibres
Ia - form a primary annulospinal nerve ending winding around fibres
IIa - more slowly conducting, form flowerspray endings on all fibres except bag 1
In activities where muscle length changes slowly and predictably which motor neurons are active?
Static gamma motor neurons
When are dynamic gamma motor neurons active?
Behaviours where muscle length changes rapidly and unpredictably
Where are golgi tendon organs found?
Junction of muscle and tendon
What is the function of golgi tendon organs?
Monitor changes in muscle tension in series to extrafusal fibres
What innervates golgi tendon organs?
Ib sensory afferents
Why do golgi tendon organs act to regulate tension?
- protect muscle from overload
- regulate tension to an optimal range
How do group Ib afferents enter the spinal cord?
Inhibitory interneurons that synpase onto alpha motor neurons
Describe the pathway of the reverse myotatic reflex
Tendon - Ib afferent to posterior horn
Inhibitory interneuron to anterior horn
Alpha motor neuron to muscle
Where are proprioceptive axons found?
In connective tissue of joints
What is the function of proprioceptive axons?
Prevent excessive flexion/extension
What structures help with proprioception?
- free nerve endings in capsule connective tissue
- paciniform endings in periosteum near the articular attachments & fibrous joint capsule
- Ruffini endings mainly in joint capsule
Where do spinal interneurons receive input from?
- primary sensory axons
- descending axons from brain
- LMN collateral branches
- other interneurons
What do inhibitory interneurons mediate?
Inverse myotatic response
Reciprocal inhibition between extensor and flexor muscles
Describe reciprocal inhibition
Myotatic reflex causes the extensor muscle to contract (quads) but the antagonist flexor (hamstring) must relax simultaneously to extend the leg
Ia fibres synapse with inhibitory interneuron which synapses with antagonist muscle
How do descending pathways contribute to reciprocal inhibition?
Inhibit the alpha motor neuron supplying the flexor to allow unopposed extension
What do excitatory interneurons mediate?
- the flexor reflex
- the crossed extensor reflex
Describe the flexor reflex
Noxious stimulus causes limb to flex by contraction of flexor muscles and relaxation of extensor muscles
Describe the crossed extensor reflex
Noxious stimulus causes limb to extend by contraction of extensor and relaxation of flexor
