W6 - LMNs, Motor Units + Reflex Arc Flashcards
What 2 patterns of muscular contractions produce movement?
Spatial and temporal
LMNs send axons out of the … and … to innervate skeletal muscles for the head + body respectively
Bst (head), SC (body)
What is the major source of input to the LMNs?
Interneurons
Interneurons are only found in the…
CNS
Interneurons act a connector between…
Sensory and motor systems
Local interneurons have short axons what do they connect with?
Local, nearby neurons only
Relay interneurons have long axons, what do these tracts connect?
Separate regions of the brain
What are the relay and local interneurons important for?
Relay - learning and decision making, Local - synchronised muscle movement
What two NTMs are used by interneurons?
GABA and glutamate
Interneurons are multipolar, what does this mean?
They synapse onto multiple neurons
LMNs are motor neurons in the SC and Bst that directly innervate …
Skeletal muscle contraction
What is a motor neuron pool?
All the LMNs that innervate a particular muscle
There are two types of LMNs found in MN pools, what are they and what do they innervate?
Alpha LMN - innervate extrafusal muscle fibres, Gamma LMN - innervate extrafusal muscle fibres
What is a motor unit?
The individual alpha motor neuron and the extrafusal fibres it innervates
What are the 3 motor unit subtypes and say them in the order recruited?
Slow, fast fatigue resistant, fast fatigable
What motor unit would be used for low effort, sustained movements like posture?
Slow MU
What action would fast contracting, fatigue resistant MUs be used for?
Walking (intermediate movements)
What would fast contracting, fast fatigable MUs be used for?
Actions of brief exertion e.g. sprinting
Distinctions between MU explain how the NS produces movements appropriate for certain circumstances, what is different between the MU in the gastrocnemius (sudden movement) compared to the soleus (posture)?
Gastro alpha motor neurons innervate many more muscle fibres than the alpha MN in the soleus
What is the size principle?
Increasing or decreasing the number of MU active to change the amount of force produced
Low frequencies of stimulation causes AP in motor neurons to produce a single twitch, higher frequencies result in … summation and what type of tetanus?
Temporal, unfused tetanus
When the rates of motor neuron activation is at it’s highest, there is a fused tetanus response, what is this?
A response where individual twitches are no longer apparent
Muscle spindles comprise of 8-10 intrafusal fibres parallel to the extrafusal MFs, what are the two classes of intrafusal fibres?
Nuclear bag fibres and nuclear chain fibres
What differs between the nuclear bag fibres and nuclear chain fibres?
Sensitivity to stretch
Is there more nuclear bag fibres or nuclear chain fibres?
More nuc chain fibres
Muscle spindles are specialised sensory organs, what type of information do they provide to show muscle length?
Mechanosensory
Group 1a and 2 afferents (gamma MNs) are associated with different parts of the intrafusal MFs, what are these parts?
1a - coil around central part of each class of IFMF, 2 - form secondary endings mainly on nuclear chain fibres
Stretch imposed on the muscle deform IFMFs, how does this tell the brain of this stretch?
Activate mechanically gated ion channels in afferent axons that innervate the spindle, producing APs
Most muscle spindles fire APs spontaneously and steadily when the muscles rest, what happens when the muscle is stretched?
A brief increase in rate of APs firing
How to measure APs in real time?
Use of EMG
G-motor neurons regulate gain of muscle spindles so they can operate efficiently at any muscle length, if they are activated alongside
Golgi tendon organs are encapsulated ….. nerve endings located at the junction of a muscle + tendon, what do they provide info about?
Afferent, provide info about muscle tension
