34 - Reflexive and Voluntary Activation of Muscle

Question 1

Location of motor neurons in the spinal cord

Answer 1

Ventral horn (form clusters)

Question 2

Number of motor neurons innervating each muscle

Answer 2

Hundreds to thousands

Question 3

Number of fibres within a muscle innervated by a single motor neuron

Answer 3

Each MN will innervate several muscle fibres (will split, form several motor endplates)

Question 4

Is the activation of muscles graded?

Answer 4

Not really. Largely binary - muscle fibre will either contract or not.

Question 5

Relationship between size of motor unit and power and fineness of movement

Answer 5

The more fibres involved in a motor unit, the greater force it will lead to.
A lot of motor units -> powerful movements, easier to fatigue
Fewer motor units -> finer movements, harder to fatigue.

Question 6

Morphology of a neuromuscular junciton

Answer 6

Synaptic membrane of the neuron has varicosities, to increase surface area.
Post-synaptic membrane is heavily-folded, also to increase surface area.

Question 7

Three phenomena that arise if muscles remain unactivated

Answer 7

1) Fibrillation
2) Fasciculation
3) Atrophy and degeneration

Question 8

Fibrillation

Answer 8

Tiny contraction cause by activity of a single muscle cell - can be due to hypersensitivity, increased AChr expression (including extra-junctional), activation is not synaptic thus not summated.

Extra-junctional twitches from increased AChR expression, which makes muscle fibres more sensitive to ACh from outside neuromuscular junction

Question 9

Fasciculation

Answer 9

groups of muscle fibres contracting involuntarily, probably a
motor unit synaptically activating due to spontaneous activation of a degenerating MN / axon. Can be measured on surface (cf. fibrillations - need extracellular electrode).

Question 10

Atrophy and degeneration

Answer 10

Long term denervation: atrophy and degeneration.
Atrophy can occur in catabolic stares (diabetes, Cushing’s disease) but complete denervation will lead progressively to irreversible muscle loss.

Question 11

Structure involved in monosynaptic stretch reflex

Answer 11

Muscle spindle.

Question 12

Signs, symptoms of lower motor neuron lesions

Answer 12

Weakness or paralysis
Decreased superficial reflexes
Hypoactive deep reflexes
Decreased tone
Fasciculations, fibrillations
Severe muscle atrophy

Question 13

Difference between superficial and deep reflexes

Answer 13

Superficial - EG withdrawal from a noxious stimulus

Deep - EG monosynaptic stretch reflex

Question 14

Two regions of the spinal white matter where upper motor neurons from brain descend to lower motor neurons

Answer 14

Lateral pathways (EG corticospinal tract) (mostly control distal muscle)
Ventromedial pathways (mostly control proximal muscle)

Question 15

Broad descending motor tract layout (ventromedial)

Answer 15

Originate in motor and premotor cortices
Descend ipsilaterally through brainstem.
Some fibres cross over in pons, some don;t
Descend through reticular formation

Question 16

Lateral corticospinal tract

Answer 16

Begin in motor cortex
Descend through cerebral peduncle, middle pons, middle medulla (both with collaterals to reticular formation), decussate at pyramidal decussation.

Question 17

Three main ventral tracts (involved in maintenance of posture)

Answer 17

Lateral and medial vestibulospinal tracts (involved in balance)
Reticulospinal tract (initiating movements)
Colliculospinal tract (orienting movements, EG changing position to a stimulus, such as a noise)

Question 18

Function of most primary motor neurons

Answer 18

Inhibitory of lower motor neurons.

If removed, lower motor neurons become hyper-excitable (increased spinal reflexes, increased muscle tone)

Question 19

Signs and symptoms of upper motor neuron syndrome

Answer 19

Weakness
Spasticity (increased tone, hyperactive deep reflexes, clonus)
Babinski’s sign
Loss of fine voluntary movements