Motor units and the control of force

Question 1

Stimulus could be:

Answer 1

Internal (e.g. full bladder / food in intestines)

External (e.g. visual / auditory)

Question 2

Explain the closed feedback loop?

Answer 2

Response can affect stimulus
– closed loop system
Sensory input (afferent/to the brain)– monitoring stimuli occurring inside and outside the body
Integration – processing of sensory input (association)
Motor output (efferent/away from brain)– response to stimuli by activating effector organs

Question 3

Response could be directed:

Answer 3

Internally (e.g. void bladder / peristalsis)

Externally (e.g. move body)

Question 4

What is the information processing model?

Answer 4

Executive (stimulus identification, response selection, response programming) Effector (Motor program, spinal cord, muscles)

Question 5

Neuromuscular junction

Answer 5

NMJ is the special synapse between neurons and muscle

Question 6

What is found in the dorsal root ganglion?

Answer 6

Cell bodies of sensory neurons – carrying information from mechanoreceptors in skin.
Dorsal – sensory; Ventral – motor.

Question 7

What is found in the ventral root ganglion?

Answer 7

• Stain of a single alpha motor neuron with cell body in ventral horn. Dendrites cover a large region of space (and extend vertically up and down the spinal cord. Axon leaves the protection of the vertebra and targets a muscle
• Cell bodies located in the ventral horn of the spinal cord (for control of the body) and in the motor nuclei of cranial nerves in the brainstem for movements of the eyes, face and oropharynx.
• Axons project out ventral root (mixed with sensory inputs, which enter via dorsal root).

Question 8

Lower (alpha) motor neurons is responsible for,,,,,

Answer 8

Lower motor neurons:
• involved in all movements (voluntary and reflexive)
• directly innervate muscle
• cell bodies in spinal cord

Question 9

what are the two special property of the muscle fiber?

Answer 9

Muscle fibers have two special properties:

1. they can change length
2. they can generate force

Question 10

Motor neuron synapse at muscle fiber

What neurotransmitter is released?

Answer 10

Acetylcholine released with each action potential in an alpha motor neuron – sufficient to activate motor unit and cause contraction.

Question 11

difference between lower motor neuron and upper motor neuron

Answer 11

These motorneurones, which directly innervate the muscles, are often called the lower motorneurones to distinguish them from the “upper motorneurones“ in the brain that provide input to the spinal cord.

Question 12

Motor unit

Answer 12

1 alpha motor neuron + the muscle fibers that it innervates
Small motor units involves <10 muscle fibres (e.g. fingers / eyes)
Large motor units involve >1000 muscle fibres (e.g. calf muscles)

Question 13

Motor neuron pool =

Answer 13

collection of all alpha motor neurons that innervate a single muscle

Question 14

Muscle

Answer 14

• Muscle = many muscle fibers
• Each muscle fiber is a single, multi-nucleated cell
• Each muscle fiber is innervated by only one alpha motor neuron
• Each alpha motor neuron innervates 1 or more muscle fibers of the same type spread throughout a single muscle

Question 15

How is force generation Controlled?

Answer 15

Rate coding

Size principle

Question 16

Force summation
Unfused tetanus
Fused tetanus

Answer 16

Force summation = when responses to individual APs merge- a single AP lasts only 1 ms, tension generation lasts 100 ms+. Therefore, if second AP occurs before fiber relaxes, we get force summation
Unfused tetanus = oscillatory force generation
Fused tetanus = smooth force summation

Question 17

Why don’t we see jerky muscle contractions?

Answer 17

Because these oscillations are at the level of a single motor unit – but we have many motor units in a muscle, which are activated asynchronously, so the effects are smoothed out

Question 18

Why summation occur?

Answer 18

Summation occurs because single AP releases enough Ca to saturate troponin – all of the myosin binding sites on thin filaments are initially available. However, this take time, and the Ca2+ is taken up by the sarcoplasmic reticulum => blockage of binding sites before attachment can occur. Therefore, with closely spaced AP, more cross-bridge attachment occurs, allowing greater force generation.

Question 19

Troponins

Answer 19

Troponins are a group of proteins found in skeletal and heart (cardiac) muscle fibers that regulate muscular contraction. Troponin tests measure the level of cardiac-specific troponin in the blood to help detect heart injury.

Question 20

Small motor unit Vs Large motor units

Answer 20

Small motor unit :motor neuron has a small cell body and innervates few muscle fibres- Smaller motor units are recruited first, giving precise control. These small neurons also innervate the slow, fatigue-resistant fibers.
Large motor unit :motor neuron has a large cell body and innervates many muscle fibres.Larger motor units, which produce more force, are recruited later. This includes the fast fatigue resistant, and then the fast fatiguing fibers.

Question 21

Size principle -

Answer 21

To generate graded forces, small motor units (small forces) are recruited first, followed by progressively larger motor units (larger forces).
Thus, cell body size takes care of grading motor recruitment automatically.

Size principle – smaller motor units have smaller alpha motor neurons. These are more easily activated, leading to orderly recruitment of motor units based on their size and force-generating potential.

Question 22

Do smaller motor neurons reach threshold more easily?

Answer 22

Small motor neurons have higher membrane resistance and reach threshold more easily
Smaller motor neurons = small surface area = higher membrane resistance (R)
Therefore, smaller neurons tend to have larger EPSPs. ∆Vmembrane = I*R (Ohms Law)
So, small motor neurons reach spiking threshold more easily

Question 23

Increase in force generation is proportional to……

what things are correlated in am motor unit?

Answer 23

• Increase in force generation is proportional to threshold required to activated motor unit.
• Also, it takes longer to activate larger motor units, ensuring a graded contraction over time as well.
• So what things are correlated in a motor unit:
• Cell body size; 2. number of muscle fibers innervated; 3. maximum force generation; 4. latency until recruitment; 5. type of fibres

Question 24

Proportional control- controlling force generation

Answer 24

There is an inverse relationship between the number of motor units in a muscle & their force generating capacity.
=> we have many small motor units generating low amount of force; and we progressively have fewer large motor units, generating more amount of force

Question 25

Slow fatigue resistance

Answer 25

Standing - requires up to ~25% of all motor units in relevant muscles. low force is required so predominantly slow motor units recruited (slow units are always recruited first).
most of the time active/ maintain posture

Question 26

Muscle fibers activated in walking and jumping

Answer 26

• Walking- As increased force is needed, fast fatigue-resistant (FR) units are recruited. Both S and FR units active.
• Running- Higher levels of force production used, require fast twitch units (FF) to be activated which provide faster contractions and greater force.
• Max effort- jumping or very fast running- most units should be active ~100% and all types of motor units

Question 27

• *27. A maximal effort vertical jump would recruit:
  a. all muscle fibre types
  b. slow-twitch, fatigue resistant muscle fibres only
  c. fast-twitch, fatigue resistant and fatigable muscle fibres only
  d. none of the above

Answer 27

a. all muscle fibre types

Question 28

Lower motor neurons need control signals. Three sources of inputs are:

Answer 28

inputs from spinal interneurons, sensory inputs from muscle spindle, input from upper motor neurons in the brain

• Interneurons or direct sensory inputs entering dorsal horn: reflexes.
• Descending control from cerebral cortex – voluntary or willed movements.
• Both voluntary and reflex movements are mediated via the lower motorneurons – hence, they are the final common pathway.

Question 29

What can muscles do? i.e. What are their basic properties?

Answer 29

Properties
1- generate force
2- Shorten

Question 30

In what body systems do we need to be able to do this?

Answer 30

1) 1- skeletal striated - voluntary
2) 2- cardiac striated – involuntary (ANS)
3) 3-smooth - involuntary (ANS)

Question 31

Skeletal muscle

Answer 31

Skeletal muscle contains long, striated fibres
1 - each cell contains multiple nuclei on periphery of cell body
2 – white region between cells is connective tissue
3 – note striations (I-bands and A-bands, which we’ll cover shortly)
4 – in cross section, note all the individual myofibrils that make up a fibre.

Question 32

Cardiac muscle

Answer 32

Cardiac muscle contains branched, straited fibres
• Usually a single central nucleus per muscle fibre.
• Note the branching, unordered striations => there is no single direction of contraction.
1) 1 – usually a single central nucleus per fibre
2) 2 – fibres are striated (like skeletal muscle)
3) 3 – fibres are branched (unlike skeletal muscle, which are a single “tube”)
Branching means that there is no single direction of contraction – this is what is required to get the bag-like heart to contract.

Question 33

Smooth muscle

Answer 33

• A single nucleus per fibre.
• No striations.
• This muscle is not under voluntary control
• e.g. stomach, intestines – innervated by autonomic nervous system.

Question 34

functional substructure of skeletal muscle

Answer 34

Muscle is attached to bone via a tendon
Each muscle is made up of bundles of muscle fibres, called fascicles. One fascicle
is separated from another by a tough connective tissue sheath, the perimysium.
Each muscle fibre within the fascicle is covered by a dense network of collagen fibrils
that comprise the endomysium
Whole muscle –> muscle fibers –> myofibril –> myofilaments

Question 35

what is Fascicle ?

Answer 35

Fascicle - Bundle of fibres surrounded by connective tissue.

1. Blood vessels and nerves are integrated with this connective tissue
2. Connective tissue is tough – helps resist passive muscle stretch and prevent muscle tearing
3. Helps distribute force from muscle fibres to tendon and bone.

Question 36

Sarcomere

Answer 36

Sarcomere = basic functional unit of a muscle – repeating section of actin and myosin filaments. These account for striated appearance.
A single biceps muscle cell may contain 100,000 sarcomeres.

• Z-disc (or Z-line) – defines limits of a sarcomere. Formed by zig-zagging ends of thin filaments. This is visible as the striations.
• Thin filament – predominantly actin
• Thick filament – predominantly myosin.

Question 37

Muscle contraction role of actin+ myosin

Answer 37

•Muscle contraction can be achieved by having actin and myosin slide over each other.
•Myosin heads can rotate and march along the actin.
When every single sarcomere contracts –>the myofibril will contract and shorten –> the muscle will then contract and shorten –>this is what produces a change in length for muscles.
Actin and myosin:
- arranged in a regular hexagonal structure
- 2 actin for every 1 myosin