The Motor System Flashcards
where do axons of LMNs exit the spinal cord
- ventral roots or via cranial nerves
- each ventral root joints a dorsal root to form a mixed spinal nerve that contains senory and motor fibres
spinal enlargements
- there are more motor neurons here, nerves supplying the distal and proximal musculature of arms and legs
- cervical - C3-T1
- lumbar - L1-S3
define motor unit
all the muscle fibres innervated by a single alpha motor neuron
describe the relationship between muscle control, power and motor units
- Fine control increases as motor unit size decreases
- Large, powerful muscles that generate high levels of force are usually innervated by large motor units
define motor neurone pool
the collection of (alpha)MNs that innervate a single muscle
outline the somatotopic distribution of LMNs in the ventral horn of the spinal cord
- LMNs innervating axial muscles are medial to those innervating distal muscles
- LMNs innervating flexors are dorsal to those supplying extensors
name 4 muscle strength determinants
- firing rates of LMNs involved
- number of LMNs that innervate a muscle (eg number of motor units, size of motor neurone pool)
- coordination of movement - agonist and antagonist muscles
- muscle fibre size (hypertrophy) and phenotype
muscle fibre twitch summation
A single AP in an aMN causes a muscle fibre to twitch. Summation of twitches causes sustained contraction as the number of incoming APs increases
unfused vs fused tetanus
Unfused tetanus is where there is some relaxation between stimuli, in fused tetanus there is no relaxation at all between stimuli.
describe the relationship between motor unit size and muscle force
- small muscles that generate low levels of force have small motor units (fewer fibres)
- fine movement
- converse for large muscles
- eg antigravity muscles
- powerful
- small motor units are innervated by aMNs with a small diameter
what is a skeletal muscles resistance to fatigue ability dependent on
ability to synthesise ATP
- Transfer of high energy phosphate from creatinine phosphate to ADP as an immediate source of ATP
- Oxidative phosphorylation (main source when oxygen is present)
- Glycolysis (main source when oxygen is not present)
what does a skeletal muscles speed of contraction dependent on
activity of myosin ATPase - the speed at which enery is made available for cross bridge recycling from splitting ATP
slow oxidative fibre (type I) properties
- ATP derived from oxidative phosphorylation
- Slow contraction and relaxation, fatigue resistant
- Small, slow conducting aMNs
- Red fibres (‘dark meat’) due to high myoglobin content (oxygen carrying pigment of muscle tissues)
properties of fast oxidative (type IIa) fibres
- ATP derived from oxidative phosphorylation
- Fast contraction and relaxation, fatigue resistant
- Red and reasonably well vascularised
properties of fast glycolytic (type IIx) fibres
- ATP derived from glycolysis
- Fast contraction but not fatigue resistant
- Pale in colour and poorly vascularised (‘white meat’)
- Large muscle fibres with a large contractile force and rapid contraction time
- Fatigue fast
- innervated by large aMN which are highly myelinated
define motor unit threshold
- the threshold required for synaptic activation
- low threshold units have a low threshold for synaptic activation and so respond to a weak input - tend to be small motor units (with few fibres and small axon diameter) (type I)
- High threshold units are recruited when there is a higher force and involve larger motor neurons (type IIx)
what is the role of mitochondria in type I fibres
they bind to oxygen and speed its diffusion into the muscle fibres
do muscles generally contain one type of motor unit?
no, they contain a mixture. the proportions vary depending on the demands placed on the msuscle
Hennemann size principle - motor unit recruitment under load
- Recruited from smallest to largest (low to high threshold)
- The low threshold type I motor units have a higher excitability, and have a lower threshold for synpatpic activation - respond to weakest input
- More intense movement recruits larger neurones producing more powerful movements
- In practice, this means that slow-twitch, low-force, fatigue-resistant muscle fibres are activated before fast-twitch, high-force, less fatigue-resistant muscle fibres
intrafusal muscle fibres
muscle spindles - the sensory receptors for the stretch reflex, do not contribute to overall strength of contraction
how do muscle spindles respond to stretch
- monosynaptic reflex
- when the muscle lengthens, the spindle is stretched and nerve activity increases
- send impulses to posterior root ganglion via Ia afferent fibres
- these afferent axons synapse on motor neurons in anterior horn which innervate muscle
- muscle contracts
- a and gamma MNs are co-activated so extra and intra fusal fibres contract at the same time
describe the anatomy of the muscle spindle
- sensory afferents concentrated at the equatorial (non-contractile) region. NMJ is here
- contactile polar ends receive efferent inputs from gammaMNs with cell bodies in the ventral horn of the spinal cord
afferent innervation of intrafusal fibres
- Primary afferents: Ia afferents form a primary annulospiral nerve winding around the centre of all intrafusal fibres. Code the event and the rate of stretch
- Secondary afferents: II fibres, which are more slowly conducting, form flowerspray endings on all intrafusal fibres except the bag 1 dynamic type. They code only the event (change in muscle length), not the rate of the stretch
during which activities are static and dynamic gamma MNs active
- static gamma MNs are active in activities where muscle length changes slowly and predictably
- dynamic are active when muscle length changes rapidly and unpredictably
where is the golgi tendon organ located
junction between muscle and tendon
golgi tendon organ
- forms the reverse myotactic reflex - does the opposite to the muscle spindle
- they are mechanoreceptors, when force is applied the Ib sensory afferents fire and form excitatory synapses withb interneurons in the spinal cord. these in turn inhibit aMNs that innervated the assoicated muscle
- acts to protect muscle from overload, proper executionof fine motor acts (steady but not too powerful grip)
vestibulospinal tract
- lateral and medical tracts that receive input from CNVIII
- this pathway controls balance and posture by innervating the anti-gravity muscles via LMNs
medial vestibulospinal tract
- axons originate in the medial and inferior vestibular nuclei
- descend bilaterally into the spinal cord as aprt of the medial longitudinal fasciculus
- the medial VT stops at the cervickal/upper thoracic levels and influences motor neurons controlling the neck musculature
lateral VT
- made up of axons that originate in the lateral vestibular nucleus
- descend ipsilaterally through the anterior portion of the brain stem
- extend throughout the lenght of the spinal cord
- facilitate MN of the antigravity muscles
tectospinal tract
- –> coordinates head movements in relation to visual stimuli
- cell bodies reside in superior colliculus in midbrain - this recieves direct input from retina and visual cortex
- axons decussate in dorsal tegmental decussation
- descend close to midline to the cervical spinal cord influencing muscles of the neck, upper trunk and shoulders
reticulospinal tract
- arise from the reticular formation
