Motor units 2-5 Flashcards
4 layers of muscle tissue?
Muscle
Fascicles
Muscle fibres (cells)
Myofibrils (organelles)
What are the 3 layers of muscle tissue?
Epimysium
Perimysium
Endomysium (aka sarcolemma)
Thick filament of a sarcomere
Myosin
Thin filament of a sarcomere?
Actin
What is Titan?
Binds to actin during active contraction
This shortens Titan increasing resistance to stretch
Force regulation in muscles independent of actin/myosin interaction
Cross bridge cycle is also known as?
Sliding filament theory
Def motor unit?
A single alpha motor neuron plus all the muscle fibres it innervates
Innervation Ratio?
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)
How is the spinal chord organized?
Somatotopically
- distal muscle controlled by distal part of the spinal chord
- caused by evolution
Def motor neuron pool (aka motor nuclei)
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)
Def Afferent
Returning to the CNS via the dorsal root
Sensory 1a afferents
Def. efferent
From the CNS via the anterior/ventral horn
Motor neurons - alpha motor neurons
Characteristics of a slow twitch muscle
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
Characteristic of a fast twitch fatigue resistant muscle fibre
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
Characteristics of a fast twitch fatiguable muscle fibre?
Fast twitch glycolytic (FG) Anaerobic (glucose as fuel) Fatiguable Generate high force Largest motor units
Two ways to increase force
Spatial summation
Temporal summation
How does spatial summation increase force
Orderly recruitment of other motor units in muscle
Orderly recruitment: Hennemen’s size principle
-what order do motor units get recruited?
Smallest MUs first
-smallest rheobase = greatest resistant
-smallest input needed to generate an action potential
Ohm’s law: v = ir
Positive and negative effects of orderly recruitment?
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
Temporal summation increases force by?
Rate coding - occurs in same motor unit
-increase firing rate summates APs causing generation of large force
Def. unfused/fused tetanus
Summation of APs increases until level of tension becomes constant (flat) - fused tetanus
-still waves of tension levels at unfused
How does fused tetanus generate a higher force?
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)
Def: electromyography EMG
Measures electrical potential of muscle and is summation of APs in view of electrode
Why might we be interested in recording EMG of a single MU?
We can see AP firing pattern during fatigue
Done by in dwelling EMG
Why would EMG show slow twitch muscles firing at a faster rate then fast twitch?
Slow twitch more responsive at lower freq
Orderly recruitment
3 ways discharge pattern can be altered?
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
Two types of fatigue
Central - within CNS (alpha MN pool, CS tract, cortex)
Peripheral - rate of Ca re-sequestered, released, and cycling of ATP
How to test fatigue?
H-reflex
- stim peripheral nerve
- decrease in h reflex only = central fatigue
- decrease in m reflex + h reflex = peripheral fatigue
