Muscle mechanics

Question 1

Basic terminology just flip the card

Answer 1

nerve = collection of many axons

Nerve fiber = single neuron

Muscle fiber = muscle cell

Tension = Force

Question 2

How many muscle fibers are there in one muscle?

Answer 2

10,000 - 100,000

Question 3

What are the different types of muscle fibers?

Answer 3

1) Type 1 = slow, weak but does not tire easily

2) Type 2: Fast, powerful but tires easily

Question 4

what is the number of motor neurons in one muscle?

Answer 4

100 - 1000

• a single motor neuron divides into many branches and each branch supplies a group of muscle fibers
• Each muscle fiber has one neuromuscular junction

Question 5

What is a motor unit?

Answer 5

A single motor neuron + all the muscle fiber it supplies

Question 6

What is a motor uit?

Answer 6

There is two motor units:

1) Slow MU
- Type I fibers (slow oxidative)
- Velocity is 100m/s

2) Fast MU
Type IIb fibers (fast glycolytic)
- Velocity is 85m/s

• In a motor unit all fibers contracts together and are of the same type

Question 7

What happens when a motor neuron is excited?

Answer 7

If one motor neuron is excited all the muscle fiber it supplies will contract at different speeds, but the muscle fibers in the same motor unit contracts together

Question 8

What is the size of a motor unit?

Answer 8

1) It has 1 motor neuron

2) 10-15 muscle fibers for muscles responsible for fine précised movements

3) 1000-1500 fibers for muscles that are responsible for the gross movement

Question 9

What are the factors that determines force production?

Answer 9

1) For a group of fibers:

1a) number of motor units recruited

1b) Number of muscle fibers/motor unit size

1c) Prescence of a disease like Duchene muscular dystrophy

2) For individual fibers:

2a) Type of fiber (1 or 2b)

2b) Size of the fiber (small usually type 1 or large usually type 2)

2c) Frequency of the stimulation (twitch or tetanus)

2d) Starting length of the fiber

Question 10

What is a meant by muscle twitches?

Answer 10

It is the force generated by a single stimulation of the muscle

Question 11

What is meant by summation?

Answer 11

If a muscle is stimulated after a twitch before it turns into zero the forces will combine = summation

Question 12

What is meant by Tetanus?

Answer 12

It is a repeated stimulation (high frequency) adding up the forces till it reaches its maximum sustained limit

Basically it is the summation of twitches which generates maximum force

Question 13

When do we use tetanic force?

Answer 13

1) During routine contractions (some motor units uses a tetanic force)

2) During extreme contractions the whole muscle uses tetanic force

Question 14

Describe the pathological tetanus

Answer 14

• Caused by a bacterial infection (clostridium tetani)
• Enters the body through open wounds
• It stimulates the motor neuron causing high frequency stimulation of the muscles
• Symptoms are: jaw stiffness, headache, sore throat, tonic spasms, fever, etc
• To treat it you must use, tetanus antibodies, muscle relaxants, and vaccination

Question 15

What factors in the muscle affects the nervous system activation to that muscle?

Answer 15

1) Type of contraction: Isometric, eccentric, concentric, or isotonic

2) Length-tension relationship: Optimal length for maximum force

3) Force-velocity relationship: Different velocities for different weights

Question 16

What are the different types of muscle contractions?

Answer 16

• Based on the length of the muscle:

1) Isometric contractions (equal length)

2) Eccentric contractions (increases the length)

3) Concentric contractions (shortens the length)

Question 17

What is an isometric contraction?

Answer 17

When the external load (force one) is equals to myosin pull (force 2), the muscle produces a force but the length of the sarcomere remains constant, this happens when pushing a trolley/holding an object etc

Question 18

What is a concentric contraction?

Answer 18

When the force generated by the myosin head is way stronger than the force of the external weight, producing a force while the length of the sarcomere shortens like when lifting a weight flexing the biceps

Question 19

What is a eccentric contraction?

Answer 19

When the external force is greater than the force produced by the rotation of the myosin head, the muscle produces a force while the length of the sarcomere lengthens, when trying to lift a very heavy load
or stretching your chest one you wake up for example

• Eccentric contractions can cause muscle injury (rupture of the whole muscle, inflammation and sore muscles after exercise, there are grades and grade 3 is complete rupture) and hypertrophy (due to the microscopic tears)

Question 20

What are the different types of tensions/forces produced by skeletal muscles?

Answer 20

1) Active tension (sliding of actin over myosin)

2) Passive tension (resisting any strech)

Question 21

What is meant by passive tension?

Answer 21

• It is a force that resists any stretch, only present when a muscle is stretched bringing it to normal length
• As we stretch something more and more its length increases and also the passive tension
• Passive tension prevents us from muscle injuries

Question 22

What are the main contributors to passive tension?

Answer 22

1) Titin (think of it as a spring)

2) Connective tissues (Endomysium & perimysium)

Question 23

How does the motor cortex control the muscle contractions?

Answer 23

General terms (flaccid = soft, spastic = hard)

1) Upper motor neuron are inhibitory, Lower motor neuron are excitatory

2) So in case of UMN damage like in a stroke, ischemia, trauma to back bone, degeneration due to repeated trauma or age the muscles will be in a spastic paralysis due to the over activity of the LMN and the muscle will feel hard, reflex actions wont be affected as they require actions of sensory neurons + LMN but there will be hyperreflexia, UMN releases glutamate

3) However if the LMN was damaged due to bone fracture for example, this will result in a flaccid paralysis to the muscle due to the overactivity of the UMN and the muscle will be soft and atrophy can happen faster, there will be no reflex action, LMN releases Ach

• Lesion to both UPM & LMN wont affect the autonomic nervous system (sexual activity, HR, BP, Sweating, etc)

Question 24

What is the sensory function of the skeletal system?

Answer 24

• Proprioception (one of the fastest sensations)
• The brain knows the contraction state of every muscle

Question 25

How does our muscle provide our brain with its position?

Answer 25

1) We have type 1 and 2 muscle fibers that are supplied by the motor neuron and their function is to contract

2) On the other hand we have intrafusal muscle fibers (1/10th the diameter of extrafusal fibers) which are supplied by sensory neurons, their function is not to contract but rather stretch once the muscle contracts

• They send sensory signals about the muscle (changes in the length of the muscle) to the brain

Question 26

What are the different densities of intrafusal fibers within the muscles?

Answer 26

1) Muscles with fine movements like the hand have a high density of intrafusal fibers

2) Muscles with gross movements like in the torso or the legs have a low density of intrafusal fibers

• Muscle spindle is a group of intrafusal muscle fibers

Question 27

Describe the intrafusal fibers

Answer 27

• They do not have actin and myosin therefore no motor function
• They make up 1-2% of the muscle
• They are small in size and sensory in function
• They detect the length of the muscle
• They are found in the center of the muscle

Question 28

What is a Golgi tendon organ?

Answer 28

• Found in series within extrafusal muscle fibers & muscle tendon (found near the tendon of the muscle)
• They are involved in conveying the changes in force of the muscle (detects tension/force in the muscle)

Question 29

Where is the final destination from the position of sensations from the muscles?

Answer 29

• Signals travels together either to:

1) Somatosensory cortex
2) Cerebellum

Question 30

When does the sensory signals from the muscles goes the the sensory cortex?

Answer 30

When you are learning a new motor task and you are consciously aware of each of your body movement

Question 31

When does the sensory signals from the muscles goes to the cerebellum?

Answer 31

Once we master a motor task our unconscious proprioception takes place, it doesn’t pay attention to our body movements

• Sometimes walking can be conscious after mastering it, when you start paying attention to it like when crossing a bridge

Question 32

What happens if the signaling mechanism of the muscles is damaged?

Answer 32

The patient wont have an idea of their body position and must keep their eyes open in order to know their body positions (as it is the only thing providing them with their sensation)