Locomotion and posture

Question 1

What observable signs may characterise faulty motor control?

Answer 1

Paralysis or weakness

Stiffness

Increased reactivity

Ticks, twitches and jerks

Fibrillations and fasiculations

Loss of coordination and smoothness

Loss of the effectiveness of movements

Question 2

Describe the difference between signs, symptoms and syndromes?

Answer 2

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

Question 3

Describe the morphology of motor neurons?

Answer 3

Large, myelinated axons

Synapse spreads immensely over territory of muscle

Question 4

Describe the organisation of motor neurons within the spinal cord?

Answer 4

Located in the ventral/anterior horn

Topographical organisation

Form motor columns

Question 5

Describe the relationship between motor neurons and muscle fibres?

Answer 5

One to many relationship

One motor neuron innervates many muscle fibres - MOTOR UNIT

Question 6

What is a motor unit?

Answer 6

All of the muscle fibres innervated by a single motor neuron

Vary in size (2-100s)

Question 7

How many motor units are found in each muscle?

Answer 7

Several motor units make up one muscle

Question 8

What activates muscles?

How is this level of activation altered?

Answer 8

Activated by action potentials

Change level of activation by number of APs delivered (APs all same intensity)

Question 9

Describe the way in which muscle fibres are recruited?

Answer 9

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

Question 10

Describe the synapse of motor neurons?

Answer 10

Very large, spread out over muscle

Secure synpase (aka 1:1): any AP sufficient to cause contraction

Question 11

Which neurotransmitters and receptors are involved at the NMJ?

Answer 11

Ach and NicR

Question 12

Why is the NMJ a secure synapse?

Answer 12

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

Question 13

What are the outcomes of muscle inactivation?

Answer 13

Fibrillation

Fasiculation

Atrophy and degeneration

Question 14

Describe the general response to muscles remaining un-activated?

Answer 14

Undergo changes that increase their capacity to become excited

Question 15

What is a fibrillation?

What are they caused by?

Answer 15

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

Question 16

What is a fasiculation?

What are they caused by?

Answer 16

Group of muscle fibres contracting involuntarily

Probably a motor unit synaptically activating due to spontaneous activation of a degenerating motor neuron/axon

Question 17

Are fibrillations and fasiculations measurable?

Answer 17

Fibrillations are not measurable (sub-threshold, no AP generated)

Can measure fasiculations

Question 18

Describe the outcome after long term denervation of muscle?

Answer 18

Atrophy and degeneration

Irreversible muscle loss

Question 19

When are muscles inactive?

Answer 19

Rarely inactive

Have a resting tone

Only inactive during REM sleep

Question 20

Which types of motor neurons cause involuntary muscle activation?

Answer 20

Lower motor neurons

Question 21

What is the afferent input to monosynaptic reflexes (e. knee jerk reflex)?

Answer 21

‘Muscle sense’

Length detected by intrafusal fibres/muscle spindles

Force detected by Golgi tendon organs

Question 22

What are the effectors of the monosynaptic reflexes (eg. knee jerk reflexes)?

Answer 22

Motor neurons

Direclt innervated by afferent input

Question 23

Describe the signs of a lower motor neuron lesion?

Answer 23

Question 24

Which types of motor neurons cause voluntary muscle activation?

Answer 24

Upper motor neurons

Question 25

Define an upper motor neuron?

Answer 25

A motor neuron that controls the excitability of lower motor neurons

Question 26

Where does control of excitation of motor neurons arise from?

Answer 26

Some from spidle afferents

A lot from descending fibres from the brain (make connections mostly to interneurons, but some directly to motor neurons)

Question 27

What are the two pathways for motor information to descend from the brain to the spinal cord?

Answer 27

Lateral pathways

Ventromedial pathways

Question 28

Describe the organisation of the spinal cord, with reference to the lateral and ventromedial tracts?

Answer 28

Topographic organisation

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

Question 29

Compare the motor functions controlled by the lateral and ventromedial corticspinal tracts?

Answer 29

Lateral - voluntary, skilled movements (distal muscles)

Ventromedial - postural control (proximal muscles)

Question 30

Which is the only tract in humans that has direct connections between upper and lower motor neurons (no interneurons)?

Answer 30

Lateral corticospinal tract

Question 31

List the three parts of the brainstem that give rise the ventromedial pathways?

Briefly describe the function of each?

Answer 31

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)

Question 32

describe the general effect of loss of brain control over the spinal cord (i.e. UMN damage)?

Answer 32

Motor neurons exhibit hyper-excitability (most of the inputs from the brain were inhibitory)

Question 33

Describe the signs of an upper motor neuron syndrome?

Answer 33