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303: Neurology > Lower Motor Neuron > Flashcards

Lower Motor Neuron Flashcards

0
Q

Where do LMN terminate?

A

Skeletal muscle

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1
Q

What are lower motor neurons?

A

Second order neurons in control of movement (final common pathway)

Spinal nerves (brachial and lumbosacral) and cranial nerves

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2
Q

Where are the cell bodies of LMN located?

A

Within the ventral horn of the spinal cord grey matter

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3
Q

What is a motor neuron pool?

A

Grouped motor neurons innervation a single muscle and extends over one or more segments

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4
Q

What is a motor unit?

A

Motor neurons and all muscle fibres it innervates

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5
Q

Describe the sequence of events at the neuromuscular junction

A

AP arrives at terminal bouton of motor neuron
Voltage-gated Ca2+ channels open, influx of Ca2+ into cytoplasm
Ca2+ triggers ACh release by exocytosis
ACh binds to nicotonic ACh receptors on motor end plate
Ligand-gated channels open, Na+ into muscles
EPP generated which initiates AP
AP spreads via voltage-gated Na+ channels and stimulates contraction
Neuromuscular transmission terminated by ACh-ease: degrades ACh to acetate and choline, choline reuptake via Na+ dependent transporters

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6
Q

What are the key factors in force generation?

A

Motor unit recruitment
Fibre diameter
Resting fibre length
Frequency of stimulation

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7
Q

Why is motor unit recruitment a critical factor in the generation of force in muscles?

A

Increased number of actives muscle fibres will generate more force
Smaller units are activated first then recruitment of progressively larger units

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8
Q

What are the three types of muscle fibres?

A

Slow
Fast fatigable
Fast fatigue resistant

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9
Q

What are the characteristics of slow twitch muscle fibers?

A 
Small, slow oxidative RED fibres 
Slow contraction
Generate small force
Fatigue resistant
Activities that require sustained muscular contraction (standing/maintaining posture)
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10
Q

What are the characteristics of fast fatigable muscle fibers?

A 
Large, fast glycotic PALE fibres
Contract quickly 
Generate large forces
Fatigue quickly
Good for brief exertions that require a large force (weightlifting, sprint, jump)
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11
Q

What are the characteristics of fast fatigue resistant muscle fibers?

A

Fast oxidative RED fibres
Intermediate between slow and fast fatigable fibres
Required in activities such as walking

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12
Q

What is the importance of spinal reflexes?

A

Coordination of sensory information from jupoints, muscles and skin in order to maintain posture and regulate movement.
Essential when spinal cord segments are isolate from CNS

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13
Q

Describe the myosynaptic spinal reflex that occurs when testing the patellar reflex

A

Hammer tap stretches tendon - stretches sensory receptors in leg extensor muscles
Information travels down sensory neuron
Synapses with motor and interneuron in spinal cord
AP generated in motor neuron - synapse in extensor muscle fibres - contraction results in leg extension
AP generated in interneuron - inhibits motor neuron to flexor muscle causing it to relax

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14
Q

What causes a withdrawal reflex?

A

Poly-synaptic flexor reflex to noxious (pain) stimulus on skin

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15
Q

Describe the sequence of events in a withdrawal reflex

A

Noxious stimulus - activation of nociceptors
Sensory information travels to spinal cord and activates excitatory and inhibitory interneurons
Ipsilateral leg: stimulation of flexor muscles and relaxes extensor muscles - leg withdraws
Contra lateral leg: stimulation of extensor muscles and relaxes flexor muscles - compensatory support

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16
Q

Briefly describe the anatomy of muscle spindles

A

Capsulated groups of 4-8 specialised skeletal muscle fibres (intrafusal fibres) separated into contractile polar regions and non-contractile central regions
Arranged in parallel with striated (extrafusal, motor neurons) fibres

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17
Q

What provide the sensory innervations of muscle spindles?

A 
Type Ia (primary)
Type II (secondary)
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18
Q

What do the Type Ia sensory fibres detect in muscle spindles?

A

Muscle length and rate of change in length

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19
Q

What do the Type II sensory fibres detect in muscle spindles?

A

Muscle length

Little rate sensitivity

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20
Q

Where do the sensory fibres of muscle spindles originate from?

A

Non-contractile regions

21
Q

What provide the motor innervations of muscle spindles?

A

Myelinated y motor neurons

22
Q

Where do the myelinated y motor neurons of muscle spindles originate from?

A

Contractile polar regions

23
Q

What are the functions of muscle spindles?

A

Provide information about muscle length and changes in muscle length (velocity)
It does not contribute to force of muscle
Provides mechanisms for adjusting spindle ‘sensitivity’ over entire range of muscle length

24
Q

Describe the mechanisms behind adjusting spindle velocity sensitivity.

A

In spinal cord
Spindle sensory neurons synapse with dynamic y (and alpha) motor neurons
Innervation of contractile polar regions (intrafusal fibres)
Contraction of muscle spindle
‘Loading’ of spindles results in continuous firing of sensory neurons - increased dynamic sensitivity of primary endings

25
Q

What are the two phases associated with lengthening or shortening of skeletal muscles?

A

Dynamic phase

Static (steady-state) phase

26
Q

What is the dynamic phase of skeletal muscle activity?

A

The muscle length is changing

27
Q

What is the static phase of skeletal muscle activity?

A

Muscle is stabilised at a new length after change (steady-state)

28
Q

Name the types of intrafusal fibres.

A

Large nuclear bag fibres (2-3)

Small nuclear chain fibres (~5)

29
Q

Where are the nuclei of large nuclear bag fibres located within the muscle spindle?

A

Arranged in centre of fibre

30
Q

Where are the nuclei of small nuclear chain fibres located within the muscle spindle?

A

Arranged in chain, near midpoint

31
Q

What types of nerve fibres innervate large nuclear bag fibres?

A

Type Ia
Type II (static)
Dynamic y motor (dynamic fibres)
Static y motor (static fibres)

32
Q

What types of nerve fibres innervate small nuclear chain fibres?

A

Type Ia
Type II
Static y motor (contractile region)

33
Q

When are static y motor neurons activated in muscle spindles?

A

When muscle length changes slowly and predictably

34
Q

When are dynamic y motor neurons activated in muscle spindles?

A

When muscle length changes rapidly and unpredictably

35
Q

What is the function of the Golgi tendon organ?

A

Provides information about changes in muscle tension

36
Q

What are Golgi tendon organs?

A

Encapsulate afferent nerve endings at junction of muscle and tendons (with extrafusal fibres)

37
Q

What innervates Golgi tendon organs?

A

Type Ib sensory

38
Q

Describe the sequence of events at the Golgi tendon organ

A

Golgi tendon organ afferents synapse with inhibitory neurons
Decreased activity of alpha motor neurons innervating same muscle
Inhibitory neurons receive input from other sensory fibres/descending pathways
Prevents muscle from generating excessive tension
Reciprocal excitation of alpha motor neurons to antagonist muscle dampens oscillation of moving limb

39
Q

Describe the mechanisms behind adjusting spindle length sensitivity.

A

Spindle sensory neurons synapse with static y (and alpha) motor neurons
Increased tonic level of activity in primary and secondary endings
Decreased dynamic sensitivity of primary endings

40
Q

What helps to monitor and maintain muscle length and tone?

A

Muscle spindles

41
Q

What provides feedback that monitors and maintains muscle force?

A

Golgi tendon organs

42
Q

What are the signs of LMN dysfunction?

A 
Weakness/paralysis
Hyporeflexia
Flaccidity/hypotonia
Muscle fasiculation
Neurogenic atrophy
43
Q

Define paresis (-paretic).

A

Partial defect of motor function

44
Q

Define paraparesis

A

Affects both pelvic limbs

45
Q

Define tetra-/quadriparesis

A

Affects 3-4 limbs

46
Q

Define hemiparesis

A

Affects thoracic and pelvic limb on one side

47
Q

Define monoparesis

A

One limb is affected

48
Q

Define paralysis (-plegia)

A

Complete loss of voluntary movements (motor function)

49
Q

What is damaged to cause LMN syndrome?

A

Alpha motor neuron

303: Neurology (7 decks)

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