Motor units and neurons Flashcards
What is the somatic motor system?
Skeletal muscle and their motor neurons (MN)
How many MN are found in the chain?
2: UMN and LMN
From where do LMN receive their input?
UMN and interneurons from somatic sensory (reflexes)
What NT do UMN release onto LMN?
Glutamate
Where are the somas of LMN found?
BS and ventral horn of SC
What two kinds of LMN are there?
alpha MN
gamma MN
What do aMN do?
Innervate muscle fibres
What do yMN do?
Innervate muscle spindles (sensory organs in muscles)
What do synergistic muscles do?
Help each other
What do antagonistic muscles do?
Oppose each other
What do axial muscles do?
Maintain posture
What do distal muscles do?
Move hand and feet
What do proximal muscles do?
Move limbs
Describe the path of a LMN
Exit SC in ventral root -> Spinal Nerve -> Anterior or posterior rami
Where do you find lots of LMN?
Cervical (C3-T1) and lumbar (L1-S3) enlargements.
What makes up a motor unit?
LMN and all the skeletal muscle it innervates
What is a MN pool?
All the aMN that innervate one muscle
How do AP generate force?
Each AP causes twitch which surmise to become contraction.
In relation to each other where are the somas of LMN innervating axial and distal muscles?
Axial more medial to distal
In relation to each other where are the somas of LMN innervating flexors and extensors?
Flexors more dorsal to extensors
What five factors determine muscle strength?
Firing rates of LMN Number of LMN/Motor units recruited Coordination (synergism or antagonism) Fibre size (hypertrophy) Fast or slow fibres
What happens at maximum AP firing on muscles?
Tetanus
Why do you want large (many fibres) or small (few fibres) motor units?
Large of strength
Small for fine movement
How does MN size relate to motor unit size?
Small motor unit = small diameter MN
Can motor units contain fast and slow fibres?
No
How do the MN innervating fast and slow fibres differ?
Fast- Large diameter and fast conduction.
Slow- Opposite
How many MN innervate one muscle fibre?
Only 1- synapse in the middle normally
What distinguishes if a fibre is fast or slow?
How quickly myosin ATPase splits ATP
Compare slow and fast fibres
Slow (Type I): Red, slow and fatigue resistant. Oxidative phos generates ATP. Antigravity.
Fast (Type IIa): Red, fast and fatigue resistant. Oxidative phos generates ATP. Sustained locomotion.
Fast (Type IIb): White, fast and fatigues easily. Glycolysis generates ATP. Not in humans.
Fast (Type IIx): Very high tension and fast fatiguing. Burst power
What does the Henneman Size Principle say?
The susceptibility of an α-MN to discharge action potentials is a function of its size. Smaller α-MNs (part of slow motor units) have a lower threshold than larger ones (part of fatigue resistant, or fast fatiguing, motor units).
What is gradation of muscles?
Stepwise increase in force with number of motor units recruited.
What does a fine gradation of muscles mean?
Small motor units therefore small increase in muscle power.
What does coarse gradation of muscles mean?
Large motor units therefore large increase in muscle power
What two kind of muscle gradation are there?
Fine and coarse.
What does Henneman’s SIze Principle mean for muscle recruitment?
Smaller motor units are recruited first before larger ones thus allowing fine control over generation of force.
Give the order in which LMN are recruited
Type I -> Type IIa -> Type IIx
What is the myotatic reflex?
When a muscle is pulled it automatically pulls back (allows muscle reflex tests)
How is change in muscle length detected?
Sensory organs in muscle spindle
What is a muscle spindle?
Specialised muscle cell
What 4 things make up a muscle spindle?
Fibrous capsule
Intrafusal muscle fibres
Sensory afferents
yMN
How do intrafusal and extrafusal muscle fibres differ?
Extrafusal generate force
Describe the myotatic reflex?
Stretch of muscle spindle
Activation of afferents
Activation of aMN
Contraction of muscle
How does stretch affect AP generation of sensory afferents?
Increase AP rate
How does increased AP generation from sensory afferents affect aMN firing?
Increase AP rate
What can deep tendon reflexes (myotatic reflex) be used to test?
Spinal nerves
Which deep tendon reflexes test which spinal nerves?
Biceps (elbow)- C5/6 Supinator (wrist)- C5/6 Triceps (elbow)- C7 Quadriceps (knee)- L3/4 Gastrocnemius (ankle)- S1
What two things make up intrafusal fibres?
Non-contractile equatorial region innervated by Ia afferents
Contractile ends with yMN input (soma in ventral horn)
What role do yMN play?
During voluntary movement they fire along with aMN to cause contraction of intrafusal fibres in parallel to prevent them going slack thus keeping signal being transduced.
What are the two broad categories of intrafusal fibres?
Nuclear bag fibres
Chain fibres
What do chin fibres do and what innervates them?
Sense absolute length
Static yMN
What two categories can nuclear bag fibres be split into?
Dynamic nuclear bag fibres
Static nuclear bag fibres
What do dynamic nuclear bag fibres do and what efferents innervates them?
Sense rate of change
Dynamic yMN
What do static nuclear bag fibres do and what efferents innervates them?
Sense absolute length
Static yMN
What two afferent fibres innervate intrafusal fibres?
Aa and Ab
Which one intrafusal fibre does Ab not innervate?
Dynamic nuclear bag
What information do Aa fibres transmit?
Rate of change of stretch
What info do Ab fibres transmit?
Absolute length
When are static yMN recruited?
Most of the time but especially when muscle length change is slow and predictable
When are dynamic yMN recruited?
When muscle lengths change rapidly and unpredictably.
What are golgi tendon organs and where are they located?
Junction of muscles and tendons where they monitor changes in muscle length of extrafusal fibres.
What afferents innervate golgi tendon organs?
Ab
What do golgi tendon organs do?
Regulate muscle tension to protect from muscle overloading and keep muscle tension in optimal range.
Where do the Ab fibres of golgi tendon organs synapse?
Onto inhibitory interneurons in the SC which synapse onto the aMN of their muscle.
What reflex do the golgi tendon organs regulate?
Reverse myotatic reflex
How does the reverse myotatic reflex work?
Golgi tendon organs fire causing activation of inhibitory interneurons which inhibit aMN to prevent muscle contraction.
Why is the reverse myotatic reflex important?
In fine motor control to prevent over gripping
