Lower Motor Neurons & Muscle

Front 1

What are some characteristics of fast muscle?

Back 1

large motor neurons
axons conduct rapidly
activity burst at high frequencies
fast rise in tension
fast decay in tension (fast twitch)
rapid fatigue
large fibers
many fibers in motor unit
large glycogen store
high levels of glycolytic enzymes
low lipid, low oxidative enzymes
sparse mitochondria
poor capillary supply

Front 2

What are some characteristics of slow muscle?

Back 2

small motor neurons
axons conduct slowly
tonically active during movement
slow rise in tension with slow decay
slow fatigue
small fibers, with few in motor unit
small glycogen store
low levels of glycolytic enzymes
high lipid, high oxidative enzymes
plentiful mitochondria
rich capillary supply

Front 3

Where are alpha motor neurons located?

Back 3

Anterior horn of spinal cord

Motor nuclei of certain cranial nerves

Front 4

Define "motor unit"

Back 4

An alpha motor neuron and all of the muscle fibers that it innervates

Front 5

How is fine movement achieved?

Back 5

One alpha motor neuron innervates a few muscle fibers

The fewer muscle fibers innervated by one alpha motor neuron, the finer the control

Front 6

What is the distinction between large and small alpha motor neurons?

Back 6

Large alpha motor neurons innervate many large muscle fibers

Small motor neurons innervate few, small muscle fibers

Front 7

How is smooth muscle action achieved?

Back 7

Occurs when the muscles fibers innervated by one alpha motor neuron are scattered, allowing the force of contraction to occur over time

Front 8

Generally, muscle fibers receive innervation from how many motor neurons?

Back 8

One

*Exception = some extraocular mm

Front 9

Muscle type is determined by what?

Back 9

Innervation

Front 10

Can muscle type be changed?

Back 10

Yes - be denervation & reinnervation with nerve for the other muscle type

Front 11

Is conversion of muscle type complete?

Back 11

No - muscle can't be completely converted to a different type

Front 12

What is the "safety factor"?

Back 12

The fact that every action potential in the motor neuron releases enough acetylcholine to ensure that all the muscle fibers in its motor unit also fire action potentials

Therefore, in normal motor unit, when a motor neuron sends an action potential, all muscle fibers should depolarize and fire action potentials

Front 13

What is a fasciculation and how is it caused?

Back 13

synchronous contraction of muscle fascicles in response to spontaneous discharge of diseased motor neurons

Front 14

What are fibrillations and how is it caused?

Back 14

asynchronous contraction of muscle fibers

caused by motor neuron death or interruption of their axons

Front 15

Which is visible to the naked eye: fasciculations or fibrillations?

Back 15

fasciculations

Front 16

What are the key differences between fibrillations and fasciculations?

Back 16

Fibrillations occur in dennervated muscle; fasciculations occur in innervated muscles

Fibrillations are undetected by naked eye; fasciculations visible

Fibrillations asynchronous; fasciculations synchronous

Front 17

What system of negative feedback exists for motor neurons?

Back 17

Motor neurons emit collaterals within the spinal cord that excite inhibitory local circuit cells (Renshaw cells), which inhibit the motor neuron

Front 18

Why is a system of negative feedback in place for motor neurons?

Back 18

Prevent them from reaching their highest possible firing rates

Front 19

Name the two muscle proprioceptors?

Back 19

Golgi Tendon Organ (GTO)

Muscle spindle

Front 20

Where is the golgi tendon organ located and what is its function?

Back 20

Located in the mm tendon

Force sensor; Sensitive indicator of active mm force because are deformed when the mm develops force

Front 21

How does the golgi tendon organ (GTO) send information to the spinal cord?

Back 21

GTO is formed by the endings of Group Ib axons

When force detected, Ib fiber sends signal to spinal cord

Front 22

Where is the muscle spindle located and what is its function?

Back 22

Located within the muscle body, arranged in parallel with extrafusal fibers

Composed of the intrafusal mm fiber, Ia axon, gamma motor neuron

Stretch receptor; provides CNS with information about muscle length & rate of length change

Front 23

How does the muscle spindle transmit information to the spinal cord?

Back 23

Sends information via its Ia fiber, which runs from the annulospiral ending on the equatorial region of an intrafusal muscle fiber to the spinal cord

Front 24

How is length of the intrafusal fiber corrected?

Back 24

Gamma motor neurons (fusiform neurons)

Front 25

What is true of the activation of alpha and gamma motor neurons?

Back 25

They are co-activated:

alpha shortens extrafusal fibers
gamma shortens intrafusal fibers

*allows stretch detection to occur at all lengths

Front 26

How does the gamma motor neuron generate muscle contraction?

Back 26

Via the gamma loop

Front 27

What is the gamma loop?

Back 27

Gamma activation --> intrafusal shortening --> Ia activation --> alpha activation --> extrafusal contraction

Front 28

What clinical presentations are expected with abnormally high levels of gamma motor neuron activity?

Back 28

Spasticity by:

Increased resting muscle tone (hypertonia)

Increased sensitivity to muscle stretch (hyperreflexia)

Front 29

What is a motor neuron pool?

Back 29

All of the motor neurons innervating one muscle

Front 30

Explain the organization of motor neuron pools in the spinal cord

Back 30

Axial and proximal limb mm (medial) --> distal/hand muscles (laterally)

Front 31

In what order are motor units recruited and why?

Back 31

Recruited according to the "size principle:"

Small motor units first b/c activated by less synaptic current (weak force)

Large motor units last b/c require large synaptic current (strong force)

Front 32

What is the advantage of the size principle?

Back 32

Allows for graded force of contraction (smooth contraction)

Front 33

What is a myotatic reflex and how is it organized?

Back 33

Stretch reflex; Results from monosynaptic input from a Ia mm spindle fiber onto 1) an excitatory alpha motor neuron of the same muscle & 2) an inhibitory alpha motor neuron of antagonist muscles

Front 34

Can a reflex be elucidated in a paralyzed limb?

Back 34

Yes - does not need CNS input

Front 35

What is Jendrassik's maneuver and what is its purpose?

Back 35

Maneuver = clasp your two hands together and attempt to pull them apart

Purpose = remove CNS inhibitory influence on reflexes

Amplitude of knee jerk reflex should increase

Front 36

What is servo-assisted movement?

Back 36

Movement mediated by the co-activation of alphas and gamma motor neurons

Front 37

What is "servo-assistance"?

Back 37

Co-activation of the gamma and alpha motor neurons via corticospinal tract allows the gamma loop to assist in depolarizing the alpha motor neurons (via Ia activation)

Boosts the input to muscles

Front 38

How can spasticity be reduced and why?

Back 38

Dorsal rhyzotomy

Cuts input of Ia fibers on the alpha motor neurons, therefore eliminating over activation by the gamma motor neurons

*Note - does not eliminate descending pathway influence on alpha motor neuron

Front 39

Describe the clasp-knife reflex/inverse myotatic reflex and its significance

Back 39

Occurs when a rigid muscle is strongly stretched & the tension reduces/muscle relaxes

Significance: sign of damage to the supraspinal motor pathways

Front 40

Describe the withdrawal reflex

Back 40

Upon noxious stimulus = combination of flexor & crossed extensor reflexes in order to achieve withdrawal of limb from stimulus

e.g. flexor contraction and extensors relaxation to withdraw arm

Front 41

Which reflex is considered prepotent (capable of overriding others)?

Back 41

Withdrawal reflex

Front 42

Loss of the withdrawal reflex is indicative of what kind of damage?

Back 42

Damage to spinal cord or peripheral nerves

Front 43

Will a person with severe brain damage elicit a withdrawal reflex?

Back 43

Yes, as long as his/her spinal cord and peripheral nerves are intact

Front 44

Who has brisker crossed extensor reflexes: newborns or adults?

Back 44

Newborns; crossed extensor reflex diminishes with age

Front 45

How is locomotion achieved?

Back 45

Combination of flexor and crossed extensor reflexes

Front 46

What is the function of propriospinal neurons and what type of neurons are they?

Back 46

Function: orchestrate appropriate limb movements; mediate long spinal reflexes

Interneurons

Front 47

What are long spinal reflexes?

Back 47

Reflexic activation of postural adjustments by vestibular inputs and proprioceptor neurons in the cervical spine

Front 48

How are long spinal reflexes affected by brain damage?

Back 48

Become exaggerated:

Neck extended = extended arms, flexed legs

Neck flexed = flexed arms, extended legs

Head to left = extended left arm, flexed right arm