Motor units, muscle spindle, golgi tendon organs and joint receptors Flashcards
Somatic motor system
- consists of what?
- neural element of motor system?
- function of ^
- what is the sensory organ within muscle and what is it innervated by?
- what are LMNs made out of?
-skeletal muscles and the elements of the nervous system that control them
-comprises upper motor neurones (in brain) and lower motor neurones (in soma in brain stem & ventral horn of the spinal cord)
UMN- supply input into LMNs to modulate their activity
LMNs- receive input from proprioceptors and interneurones. they command muscle contraction and form the “final common pathway”
- muscle spindle, innervated by Gamma motor neurones
- alpha motor neurones
what are synergistic muscles?
- antagonists?
- axial muscles?
- proximal muscles?
- distal muscles?
- Act on joint to perform the same action i.e. are agonists
- sets of these muscles create movement when one contracts and the other relaxes
- control movements of the trunk
- found in shoulder, elbow, pelvis and knees
- move the hands, feet and digits
segmental organisation of LMNs:
- where are the axons of LMNs located?
- what forms a mixed spinal nerve?
- How are motor neurones distributed within the spinal cord?
- they exist in the spinal cord in the ventral roots
- ventral root and dorsal root join together, contains both sensory and motor fibres
-They belong to a spinal segment (C 1-8, T 1-12, L 1-5 and S 1-5)
three are a greater number in the cervical enlargement (C3-T1) fro arm and the lumber enlargement (L1-S3) for leg
What is a motor unit?
- what must these do for muscle contraction to occur?
- what is a motor neurone pool?
- What tho principle mechanisms determine the force of muscle contraction?
an Alpha motor neurone and all of the skeletal muscle fibres it innervates , i.e. the smallest functional unit of the motor system
- co-ordinated contraction of the motor units
- the collection of alpha MN that innervate a single muscle
-frequency of AP discharge from the alphaMN
The recruitment of additional, synergistic motor units
-Where are the cell bodies of LMNs located in the spinal cord?
-How are they distributed?
What are the inputs to an alpha MN that regular activity?
-in the ventral horn
-somatotopic distribution
LMNs innervating axial muscles are medial to those innervating distal muscles
LMNs innervating flexors dorsal to those supplying extensors
-central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles
UMNs in the motor cortex and brain stem
Spinal interneurones
Muscle strength
- Depends on what?
- What do these ^ 2 things depend on?
Neuromuscular activation & force production by innervated muscle fibres
-Neuromuscular activation
the firing rate of the LMNs involved
The no. of LMNs that innervate a muscle (i.e no. of motor units)
the co-ordination of movement
Force production by innervated muscle fibres
Fibre size
Fibre phenotype
Neuromuscular activation
-What is it?
-frequency summation of muscle fibre contraction
A single action potential in an α-MN causes a muscle fibre to twitch. Summation of twitches causes a sustained contraction as the number of incoming action potentials increases
Motor units Elaborate on the following features of motor units: -variable size? -fast and slow types? -innervation?
-small for fine movements (innervated by small Alpha MNs) large in large postural muscles (innervated by larger alpha MNs) the force of contraction that each motor unit produces is dependent upon its size
- these contain only muscle fibres of a single type the AMNs innervating fast type tend to be larger and have faster conducting axons
- each muscle fibre innervated by a single motor axon at endplate (NM junction) which is usually at the centre of the fibre
Skeletal muscle fibre types:
- What are the 2 major types?
- why do they differ in function?
- for each type: Where is ATP derived from? speed of contraction? resistance to fatigue? colour?
- how do motor unit types relate to this?
- Fast and slow
- differ in how fast myosin ATPase splits ATP to provide energy, this is reflected in time to develop peak tension
-Slow oxidative type 1 fibres ATP from oxidative phosphorylation slow contraction and relaxation fatigue resistant Red fibres as high myoglobin content
Fast type II fibres: Type IIa ATP derived from oxidative Phosphorylation. Fast contraction and relaxation fatigue resistant Red and reasonably well vascularised
TypeIIx
ATP from glycolysis.
Fast contraction but not fatigue resistant, Pale in colour and poorly vascularised
-3 types:
slow- most numerous with aMNS with a low threshold
Fatigue resistant
fast fatiguing, Mans have a high threshold
Whole muscle contraction and strength
- what happens to the force of contraction as stimulating current increases?
- Describe the APs involved in this event?
- why does force of contraction increase? (2)
- what is velocity of muscle contraction due to? (2)
- the force increases until max reached
- all or nothing
-temporal summation in individual fibres/motor units
recruitment of more motor units (occurs second)
-muscle fibre types and the load on muscle
What is the Heinemann Size principle?
The susceptibility of an aMN to discharge APs is a function of it’s size
The recruitment of AMN by size allows what?
the fine and graded development of muscle force
The myotatic reflex
-What is it?
-how does it contribute to proprioception?
-what do muscle spindles consist of?
-What type of reflex does this show? describe?
how is it used clinically?
- When a skeletal muscle is pulled, it pulls back
- change in length is registered by muscle spindle
-fibrous capsule
intrafusal muscle fibres
sensory afferents that innervate intrafusal fibres
gamma MN efferents that innervate intrafusual fibres
monosynaptic reflex etc e.g. patellar tendon
stretch of muscle spindle -> activation of Ia afferent -> excitatory synaptic transmission in spinal cord -> activation of aMN -> contraction of homonymous muscle
-to asses integrity of spinal cord at specific levels: Supinator: C5-C6 Biceps: C5-C6 Triceps: C7 Quadriceps: L3-L4 Gastrocnemius: S1
Intrafusal fibres
- innervation?
- consist of? (2)
- stimulation causes what?
- how do they function during voluntary movement?
-Gamma MNs
-non-contractile equatorial region innervated by the Ia sensory neurones
contractile polar ends that receive efferent input from gMNS with cell bodies in the ventral horn of the spinal cord
- of MNs causes the spindle to contact
- alpha and gamma MNs are co-activated so the intrafusal muscle fibres contract in parallel with the extrafusal fibres. maintains the sensitivity of the spindle and the Ia response is maintained
Muscle spindle
- How are intrafusal fibres categorised? (2)
- what 2 types of afferent fibre innervate the intrafusal fibres?
- what two types of afferent fibre innervate the intrafusual fibres?
-by morphology & function:
Nuclear bag fibres
Bag 1:
dynamic, very sensitive to the rate of change of muscle length. innervated by dynamic gMNs
Bag 2:
states, more sensitive to to the absolute length of the muscles, innervated by static gMNs
Chain fibres
sensitiv to the absolute length of the muscle, innervated by static gMNs
-Ia afferents
form a primary annulospiral nerve ending winding around the centre of all intrafusal fibres
II fibres
more slowly conducting, form flower spray endings on all intrafusual fibres except the bag 1 dynamic type
both respond to stretch,
Ia- rate of change
II- absolute length of intrafusal fibres
