Motor units 2-5

Question 0

4 layers of muscle tissue?

Answer 0

Muscle
Fascicles
Muscle fibres (cells)
Myofibrils (organelles)

Question 1

What are the 3 layers of muscle tissue?

Answer 1

Epimysium
Perimysium
Endomysium (aka sarcolemma)

Question 2

Thick filament of a sarcomere

Answer 2

Myosin

Question 3

Thin filament of a sarcomere?

Answer 3

Actin

Question 4

What is Titan?

Answer 4

Binds to actin during active contraction
This shortens Titan increasing resistance to stretch
Force regulation in muscles independent of actin/myosin interaction

Question 5

Cross bridge cycle is also known as?

Answer 5

Sliding filament theory

Question 6

Def motor unit?

Answer 6

A single alpha motor neuron plus all the muscle fibres it innervates

Question 7

Innervation Ratio?

Answer 7

Number of muscle fibres innervates by a single alpha motor neuron

• varies with muscle
• smaller ratio = finer motor control
• # MU declines with age, # of fibres per MU increases (# of neurons decreases)

Question 8

How is the spinal chord organized?

Answer 8

Somatotopically

• distal muscle controlled by distal part of the spinal chord
• caused by evolution

Question 9

Def motor neuron pool (aka motor nuclei)

Answer 9

All motor neurons in motor neuron pool innervates a single muscle
All motor neurons that innervates a single motor neuron are in the same motor neuron pool
(Clustered in columns in the spinal chord grey matter)

Question 10

Def Afferent

Answer 10

Returning to the CNS via the dorsal root

Sensory 1a afferents

Question 11

Def. efferent

Answer 11

From the CNS via the anterior/ventral horn

Motor neurons - alpha motor neurons

Question 12

Characteristics of a slow twitch muscle

Answer 12

Slow contracting (fatigue resistant type S)
Slow oxidative (SO)
Aerobic, lipid as fuel, low force production
Type I muscle fibres
Smaller motor units
Red in colour

Question 13

Characteristic of a fast twitch fatigue resistant muscle fibre

Answer 13

Fast Oxidative and glycolytic (FOG) 
Fatigue resistant (type FR)
Both lipid and glucose as energy
More force then SO, fatigue slower then FG
Medium size motor units

Question 14

Characteristics of a fast twitch fatiguable muscle fibre?

Answer 14

Fast twitch glycolytic (FG) 
Anaerobic (glucose as fuel)
Fatiguable 
Generate high force
Largest motor units

Question 15

Two ways to increase force

Answer 15

Spatial summation

Temporal summation

Question 16

How does spatial summation increase force

Answer 16

Orderly recruitment of other motor units in muscle

Question 17

Orderly recruitment: Hennemen’s size principle

-what order do motor units get recruited?

Answer 17

Smallest MUs first
-smallest rheobase = greatest resistant
-smallest input needed to generate an action potential
Ohm’s law: v = ir

Question 18

Positive and negative effects of orderly recruitment?

Answer 18

Predetermined, brain not needed for control
Fine motor control
Incremental increase of MU will have large effect on force generated
–
Limited to rate if force development and no ability to select MU out of order

Question 19

Temporal summation increases force by?

Answer 19

Rate coding - occurs in same motor unit

-increase firing rate summates APs causing generation of large force

Question 20

Def. unfused/fused tetanus

Answer 20

Summation of APs increases until level of tension becomes constant (flat) - fused tetanus
-still waves of tension levels at unfused

Question 21

How does fused tetanus generate a higher force?

Answer 21

The longer calcium is in the cell, the more myosin/actin cross bridges that can be formed
(ST muscle fibres can summate at lower frequency but produce 4x less force - means they reach 100% force at much lower freq)

Question 22

Def: electromyography EMG

Answer 22

Measures electrical potential of muscle and is summation of APs in view of electrode

Question 23

Why might we be interested in recording EMG of a single MU?

Answer 23

We can see AP firing pattern during fatigue

Done by in dwelling EMG

Question 24

Why would EMG show slow twitch muscles firing at a faster rate then fast twitch?

Answer 24

Slow twitch more responsive at lower freq

Orderly recruitment

Question 25

3 ways discharge pattern can be altered?

Answer 25

Muscle wisdom - rate of MU firing declines slowly, fatigues less rapidly- helps isometric, not ideal for dynamic movements
Double discharge - fast summation of AP produce large jump in force
Synchrony - 2 MU in muscle fire simultaneously

Question 26

Two types of fatigue

Answer 26

Central - within CNS (alpha MN pool, CS tract, cortex)

Peripheral - rate of Ca re-sequestered, released, and cycling of ATP

Question 27

How to test fatigue?

Answer 27

H-reflex

• stim peripheral nerve
• decrease in h reflex only = central fatigue
• decrease in m reflex + h reflex = peripheral fatigue