What observable signs may characterise faulty motor control?
Paralysis or weakness
Ticks, twitches and jerks
Fibrillations and fasiculations
Loss of coordination and smoothness
Loss of the effectiveness of movements
Describe the difference between signs, symptoms and syndromes?
Signs: observable or measurable motor abnormalities, the presence of abnormalities, or the absence of normal motor actions
Symptoms: problems of motor functions reported by patients
Syndromes: associated or clustered signs and symptoms
Describe the morphology of motor neurons?
Large, myelinated axons
Synapse spreads immensely over territory of muscle
Describe the organisation of motor neurons within the spinal cord?
Located in the ventral/anterior horn
Form motor columns
Describe the relationship between motor neurons and muscle fibres?
One to many relationship
One motor neuron innervates many muscle fibres - MOTOR UNIT
What is a motor unit?
All of the muscle fibres innervated by a single motor neuron
Vary in size (2-100s)
How many motor units are found in each muscle?
Several motor units make up one muscle
What activates muscles?
How is this level of activation altered?
Activated by action potentials
Change level of activation by number of APs delivered (APs all same intensity)
Describe the way in which muscle fibres are recruited?
Henneman's size principle: recruit smallest motor units first, then larger ones
So, start with small forces, then increase
Also, recruit slow muscle fibres first, then fast fatigue-resistant, then fast fatigable
So, recruit aerobicaaly active, then anaerobically active
Describe the synapse of motor neurons?
Very large, spread out over muscle
Secure synpase (aka 1:1): any AP sufficient to cause contraction
Which neurotransmitters and receptors are involved at the NMJ?
Ach and NicR
Why is the NMJ a secure synapse?
NMJ spreads over large area
Lots of post-junctional folds that increase SA for NicR - lots of Ach can bind, and massive amount of ion channels can open to generate current
What are the outcomes of muscle inactivation?
Atrophy and degeneration
Describe the general response to muscles remaining un-activated?
Undergo changes that increase their capacity to become excited
What is a fibrillation?
What are they caused by?
Tiny muscle contraction caused by activity of a single muscle cell
Due to hypersensitivity: after a period of inactivation, muscle cell increases Ach receptor expression to become more excitable
What is a fasiculation?
What are they caused by?
Group of muscle fibres contracting involuntarily
Probably a motor unit synaptically activating due to spontaneous activation of a degenerating motor neuron/axon
Are fibrillations and fasiculations measurable?
Fibrillations are not measurable (sub-threshold, no AP generated)
Can measure fasiculations
Describe the outcome after long term denervation of muscle?
Atrophy and degeneration
Irreversible muscle loss
When are muscles inactive?
Have a resting tone
Only inactive during REM sleep
Which types of motor neurons cause involuntary muscle activation?
Lower motor neurons
What is the afferent input to monosynaptic reflexes (e. knee jerk reflex)?
Length detected by intrafusal fibres/muscle spindles
Force detected by Golgi tendon organs
What are the effectors of the monosynaptic reflexes (eg. knee jerk reflexes)?
Direclt innervated by afferent input
Describe the signs of a lower motor neuron lesion?
Which types of motor neurons cause voluntary muscle activation?
Upper motor neurons
Define an upper motor neuron?
A motor neuron that controls the excitability of lower motor neurons
Where does control of excitation of motor neurons arise from?
Some from spidle afferents
A lot from descending fibres from the brain (make connections mostly to interneurons, but some directly to motor neurons)
What are the two pathways for motor information to descend from the brain to the spinal cord?
Describe the organisation of the spinal cord, with reference to the lateral and ventromedial tracts?
Motor neurons closer to midline of spinal cord innervate more proximal muscles
More lateral motor neurons innervate more distal muscles
Lateral tracts innervate more lateral motor neurons
Ventromedial tracts innervate motor neurons closer to midline
Compare the motor functions controlled by the lateral and ventromedial corticspinal tracts?
Lateral - voluntary, skilled movements (distal muscles)
Ventromedial - postural control (proximal muscles)
Which is the only tract in humans that has direct connections between upper and lower motor neurons (no interneurons)?
Lateral corticospinal tract
List the three parts of the brainstem that give rise the ventromedial pathways?
Briefly describe the function of each?
Lateral and medial vestibulospinal tracts (postural control from vestibular nuclei)
Reticulospinal tract (maintains postural control in midline muscles)
Colliculospinal tract (orienting reflexes and visual looming)
describe the general effect of loss of brain control over the spinal cord (i.e. UMN damage)?
Motor neurons exhibit hyper-excitability (most of the inputs from the brain were inhibitory)
Describe the signs of an upper motor neuron syndrome?