Motor System 1 EXAM 2

anterior horn & brainstem in nuclei of cranial nerves with motor functions

where are the cell bodies of lower motor neurons located

- Flaccid paralysis with eventual atrophy

- muscle fasciculations

- decreased muscle tone

- hyporeflexia

what are signs of LMN lesions

brainstem & cortex

where are cell bodies of UMN located

in descending motor pathways

where are the axons of UMN located

- Spastic paralysis

- hypertonia

- hyperreflexia

- + Babinski sign

what are signs of UMN lesions

- The direct link from CNS to skeletal muscle

- lower motoneuron

- The "final common path" to the skeletal muscle

what are anterior horn motor neurons

The only way a muscle can contract is if the lower motor neuron is activated that is why it is termed the final common path to skeletal muscle

why are lower motor neurons called the final common pathway

lower motor neurons

what is another name for anterior horn motor neurons

– Alpha motoneuron - to extrafusal muscle fibers

– Interneurons (within gray matter)

– Gamma motoneuron - to intrafusal muscle fibers connected to muscle spindles

what are the 3 types of anterior horn motor neurons (LMN)

extrafusal muscle fibers

alpha motor neurons innervate ________

gamma motor neurons

intrafusal muscle fibers connected to muscle spindles

directly trigger the generation of force by muscles

what is the function alpha motor neurons

innervate the intrafusal muscle fibers at the 2 ends of the muscle spindle.

what is the function of gamma motor neurons

large dendritic arborization, high density

what are anterior horn motor neurons characterized by

The Posterior portion of anterior horn

what part of the anterior horn is responsible for control of flexor muscles

The Anterior portion of anterior horn

what part of the anterior horn is responsible for control of extensor muscles

The Medial portion of anterior horn

what part of the anterior horn is responsible for control of proximal muscles

The Lateral portion of anterior horn

what part of the anterior horn is responsible for control of distal muscles

motor unit

A motor neuron and all of the muscle fibers it innervates

there will be a decreased innervation ration of the motor unit

what happens to a motor unit if there is increased fine control of muscles

there will be an increased innervation ration of the motor unit

what happens to a motor unit if there is decreased fine control of muscles

Finger: low force, small motor units (1:10-100 motoneuron: muscle fibers)

what is an example of an area supplied by a motor unit with a low innervation ratio

Legs: high force, large motor units (1:600-1000 motoneuron: muscle fibers)

what is an example of an area supplied by a motor unit with a high innervation ratio

the number of muscle fibers the motor neuron connects to decreases; thus a decreased innervation ratio

what happens to the innervation ratio if a muscle needs increased fine control

Large motor unit can generate more force due to number of muscle fibers it projects to (i.e. more fibers)

what can large motor units do that small ones cannot

connect to few muscles fibers thus do not generate greater force but enable fine control

what do small motor units enable us to do

Slow twitch fibers (S)

what are type one motor units

Fast twitch fibers

what are type two motor units

- Rich in mitochondria and heme protein that binds and stores oxygen

- Utilize glucose and O2 from bloodstream

- Generate abundant adenosine triphosphate (aerobic metabolism)

- Long-lasting fuel, more resistant to fatigue (e.g., posture muscles)

- Maintains constant force for over time, contracts slowly, decreased force

how are type 1 slow twitch muscle fibers able to generate force over time

mitochondria and heme protein that binds and stores oxygen

what cellular structure is highly concentrated in type 1 motor units that enable long term force production

Utilize glucose and O2 from bloodstream

what is the main source of energy for type 1 motor unit muscle contractions

adenosine triphosphate (aerobic metabolism)

what is the generated through the utilization of glucose and O2 from the bloodstream in type 1 motor units

Generate higher levels of force but for shorter time periods

what is the function of type 2 motor units

- fast fatigue-resistant (FFR)

- intermediate; sufficient aerobic capacity to resist fatigue and generates greater force

- about twice as much force as Slow units

what are type 2a motor units

- fast fatigable (FF) type

- large stores of glycogen for anaerobic metabolism; great force (e.g., Gastroc, TA)

- Rapid contractions, fatigues within minutes

what are type 2b motor units

use anaerobic glycolysis create more force and fatigue quickly due to lack of glycogen storage

how is energy generated for type 2 motor units that lack high amounts of mitochondria and heme protein

- Type 2b are the biggest, fastest and most fatigable fibers

- Type 2a are larger and fast and can generate large forces but fatigue slower than type 2b

what is the difference between type 2a and type 2b motor units

Type 1 are smaller and project to fewer muscles than type 2

what is the difference between type 1 and type 2 motor units

motor pool

- All the motor neurons that innervate a single muscle

- Intermingling of fibers of different motor units

- Arrange topographically in central nervous system

Most muscles contain a relatively even balance of muscle fibers (but fewer large motoneurons due to innervation ratio)

what is the distribution of muscle fiber types within a muscle

Muscles involved in postural support tend to have increased proportion of slow fibers (e.g. soleus)

what type of muscle grouping tends to have more type 1 fibers

Muscles generating rapid movements tend to have increased proportion of fast fibers (e.g. gastrocnemius)

what type of muscle grouping tends to have more type 2 fibers

No there would not be an equal mix of fast and slow motor neurons

If the muscle fibers are an even 50/50 mix of slow and fast, is the percent mixture of motoneuron types also an even 50/50?

Because about 80% of motor neurons in the CNS are slow due to innervation ratio in that slow type 1 innervate less fibers total so more are needed to compensate

why is it that if there is an even 50/50 mix of slow and fast muscle fibers, the number of slow and fast motor neurons are not an even 50/50?

- Firing threshold of a neuron depends on total electrical resistance

- Same input will produce larger membrane potential for smaller vs. larger motoneurons

- Smaller motor units are recruited first, followed by progressively larger motor units

what are key points about the size principle

It is efficient for the system to use the small neurons first; they have good oxygen supply and efficient metabolic system. As the demands of the task, increase speed and force requirements, large motor units are recruited.

why are motor units recruited from smallest to largest

Small motor neurons reach AP threshold before large motor neurons due to passive properties of motor neurons and therefore fire first

why does it physiologically make sense that small motor units fire first

- I or current is equal across all motor neurons of the same motor pool for all conditions therefore voltage is proportional to resistance.

- Resistance is larger for small motor neurons and smaller for large motor neurons

what are electrochemical principles of current, voltage and resistance for the triggering of AP behind the physiological function of the size principle

- Small motor neurons have large voltage reflection

- Large motor neurons have small voltage reflection

key points about voltage reflection of small and large motor neurons

Amount Ca2+ released = strength of force production

the concentration of what ion dictates the force of muscular contraction

End plate potential (AP) activates then Ca2+ concentration released followed by muscle force production in sequential order

what is the temporal order of sequential activation of muscle force generation

motor units and the firing rate of the motor units

what does the force generation of a muscle depend on

- If more force is needed additional motor units are recruited

- You increase the amount of force by increasing the number of motor units

- You can increase the amount of force by increasing the frequency of firing rate for the motor unit/pool

What does recruitment of motor neurons entail?

• Length - tension

• Force - velocity

• Force - frequency

what are important relationships in muscle

Length - tension relationship

describes how the force (tension) a muscle can generate depends on its length, peaking at an optimal resting length where actin-myosin overlap is ideal for maximum cross-bridge formation

- Short length muscle and long length muscle produce little force

- At longer lengths the passive property of muscle is what generates more force than the active properties

key points about the length tension relationship of muscle

faster the shortening rate, lower the force

what is the force velocity relationship

decreased

When force velocity is increased… amount of force generated _____

eccentric

______ contraction generates the most force (negative velocity) compared to other contraction types

- Relationship between amount of force produced and the rate of firing frequency.

- Higher frequency = higher force

- lower frequency = lower force

what is the force frequency relationship

- Force production is best between 5 and 20 Hz and plateau as frequency continues to increase

- Have greatest control of force in range of 10 - 20 Hz

Motoneurons typically discharge in the 10-20 Hz range - Why?

- Muscle spindle

- Monosynaptic stretch reflex

what are the 2 Peripheral sensory inputs to ventral horn

– Sensory (Ia & II) terminates at the equatorial region

– Motor (gamma motoneuron)

what are the 2 inputs of the muscle spindle to the Peripheral sensory input to ventral horn

dynamic; Ia

static; II

Classifications of Sensory (Ia & II) terminates at the equatorial region

- Nuclear bag - ____ change, ___ bias

- Nuclear chain - _____ change, ___ bias

nuclear bag fibers

detect changes in muscle length and speed (velocity), primarily acting as sensory receptors for stretch

nuclear chain fibers

detect and signal the static length (sustained stretch) of a muscle to the central nervous system

type 1a fibers

nuclear bag fibers are classified as

type 2 fibers

nuclear chain fibers are classified as

dynamic bag fibers

what do dynamic gamma motor axons innervate

combinations of chain and static bag fibers

what do static gamma motor axons innervate

all 3 types of fibers

what do type 1a gamma fibers innervate

nuclear chain and static bag fibers

what do type 2 gamma fibers innervate

- Alpha-gamma co-activation

- Golgi tendon organ

What are 2 mechanisms to regulate sensory feedback to alpha motor neuron

by activation of gamma motoneuron located in the ventral horn of the spinal cord

How can Extrafusal muscle contractions be indirectly produced

- Extrafusal muscle contraction can be indirectly produced by activation of gamma motoneuron located in the ventral horn of the spinal cord

- Gamma motoneuron activate intrafusal muscle fiber, which causes equatorial stretch and increased Ia fiber (sensory) activity

- Increased Ia fiber activity activates alpha motoneuron, resulting in extrafusal muscle contraction

what does the Peripheral sensory- Gamma loop entail

causes equatorial stretch and increased Ia fiber (sensory) activity

what happens when gamma motor neurons activate intrafusal muscle fibers during the gamma loop

Extrafusal muscle contraction can be indirectly produced by activation of gamma motoneuron located in the ventral horn of the spinal cord

what is the peripheral sensory gamma loop

- Gamma motoneuron activate intrafusal muscle fiber, which causes equatorial stretch and increased Ia fiber (sensory) activity

- Increased Ia fiber activity activates alpha motoneuron, resulting in extrafusal muscle contraction

how are gamma motoneuron located in the ventral horn of the spinal cord able to cause extrafusal muscle contraction

- Ia fires. Tension stimulates alpha motor neuron only - reflex loop.

- Stimulate only alpha motor neuron - Intrafusal muscles relaxes and Ia discharge ceases

- Voluntary contraction (both alpha and gamma are co-activated). Intrafusal & extrafusal fibers contract; Ia discharge persists

what is the response of 1a afferents from muscle spindles to passive stretch during the gamma loop

Ia fires. Tension stimulates alpha motor neuron only - reflex loop.

what is the initial response of 1a muscle spindle afferents to passive stretch

Stimulate only alpha motor neuron

Once the Ia afferent fires, tension stimulates alpha motor neuron only triggering the reflex loop which stimulates what type of neuron

Intrafusal muscles relaxes and Ia discharge ceases

After the alpha motor neuron is stimulated by the activation of 1a afferents in response to passive stretch, what happens to the intrafusal muscle fibers?

Voluntary contraction (both alpha and gamma are co-activated)

Intrafusal & extrafusal fibers contract; Ia discharge persists

what is the outcome of 1a afferents response to passive stretch in the gamma loop

Alpha-gamma co-activation allows muscle spindle sensitivity during physiologically driven changes in muscle length

what is the function of the alpha - gamma motor loop

extrafusal muscle fibers

These are the "standard" muscle fibers that make up the bulk of your muscle.

contract to generate force and move your skeleton

what is the function of extrafusal muscle fibers

intrafusal muscle fibers

These are specialized sensory organs buried deep within the muscle and act as stretch receptors.

muscle spindles

bundle of intrafusal fibers that are receptors sensitive to change in length of the muscle and the rate of that change

When a muscle is stretched, the spindle stretches with it, sending signals to the brain and spinal cord. This triggers the stretch reflex causing the muscle to contract to prevent over-stretching or injury

how do muscle spindles work

extrafusal muscle fibers

what are the target tissues for alpha motor neurons

intrafusal muscle fibers (muscle spindles)

what are the target tissues for gamma motor neurons

Triggers actual muscle contraction and movement.

what is the primary function of alpha motor neurons

Adjusts the sensitivity of the muscle spindle.

what is the primary function of gamma motor neurons

Large, fast-conducting axons.

what is the size and conduction rate of alpha motor neurons

Smaller, slower axons.

what is the size and conduction rate of gamma motor neurons

Alpha-Gamma Coactivation

The simultaneous firing of alpha and gamma motor neurons to a muscle so that the extrafusal and intrafusal muscle fibers contract simultaneously to keep the spindle tight so that it can sense changes.

1a afferents from muscle spindles

These are the fastest-conducting sensory fibers in the body. They wrap around the center of the intrafusal fibers in the muscle spindle.

Changes in muscle length and the velocity of the stretch.

what information is carried by Ia afferents from muscle spindles

- They are highly sensitive to dynamic changes.

- They fire rapidly when a muscle is actively being stretched, allowing the brain to react to sudden movements (like catching yourself after a trip)

what are the key characteristics of Ia afferents from muscle spindles

They are the primary limb of the monosynaptic stretch reflex (the "knee-jerk")

what is the role of Ia afferents from muscle spindles in reflexes