16 Innervation of muscle Flashcards
Which cortical layer are the upper motor neurones located in?
V.
Which Brodmann areas contain the cortex?
4 + 6.
How do axons from the corticospinal tract descend through the spinal cord?
95% are contralateral in posterior/lateral CST.
5% are ipsilateral in anterior/ventral CST.
What is the function of the minority of CST neurones that descend ipsilaterally?
Innervation of postural muscles.
Where do UMNs meet LMNs?
Ventral grey horn of the spinal cord.
Which lamina are LMNs localised to in the ventral grey horn?
IX.
Where is lamina IX largest and why?
At cervical and lumbar segments.
Control the muscles of the limbs.
What is the motor neurone pool?
The group of alpha neurones that innervates one muscle.
How are motor neurone pools distributed the spinal cord?
Rostro-caudally.
What is the segmental distribution of lower motor neurones in the spinal cord?
Dorsal: flexors.
Ventral: extensors.
Medial: axial muscles.
Lateral: distal muscles.
What is a motor unit?
Alpha motor neuron and all the fibres it innervates.
How many axons are received by a muscle fibre?
ONE.
Why do motor units vary in size?
Degree of force and level of control needed.
How are motor units recruited?
Small units (that control small number of fibres) are recruited first.
Describe slow motor units: (3).
Small motor units.
‘Red’ fibres that generate low force.
Resistant to fatigue.
Describe fast fatiguable motor units: (3).
Large motor units.
Pale muscle fibres that generate large force.
Fatiguable.
Describe fast, fatigue resistant motor units: (2).
Intermediate force produced.
Some fatigue.
What does an EMG measure?
Electromyograph.
Measures the electrical activity of one motor unit.
What are the three sources of input into alpha motor neurones?
Sensory from peripheral proprioceptors.
Local from spinal interneurons.
Descending from upper motor neurones.
Describe the location of muscle spindles:
Intrafusal and parallel to muscle fibres.
What is the function of muscle spindles?
Detect changes in muscle length and contribute to proprioception.
What are the two classes of muscle spindle fibres?
Bag and chain.
What are chain spindles innervated by?
What do they encode?
Ia and IIa fibres.
Static muscle response (stretch vs no stretch).
What are bag spindles innervated by?
What do they encode?
Ia fibres.
Dynamic muscle response (rate of stretch change).
Differentiate between intrafusal and exrafusal muscle fibres.
Intrafusal: innervated by gamma fibres, sensory function.
Extrafusal: innervated by alpha fibres, generate tension.
How do gamma motor neurones regulate muscle tension?
Alpha neurones cause muscle contraction. Shortening of fibres causes spindle collapse and loss of sensitivity. Gamma MN matches length of spindle to muscle.
What is the role of the golgi tendon organ?
Mechanoreceptor in series with muscle fibres.
Detects changes in muscle tension. Contributes to proprioception.
What are golgi tendon organs innervated by?
Ib afferents.
What is isometric contraction and which fibre type is active?
Tension without change in muscle length.
Ib.
What is isotonic contraction and which fibre type is active?
Change in length but not tension.
Ia.
What are the signs of upper motor neurone syndrome? (4).
Babinski’s sign.
Spasticity.
Hyporeflexia.
Loss of fine movements.
What is Amyotrophic Lateral Sclerosis?
Which fibre degenerates in this disease?
Motor neurone disease.
Alpha neurones.
