neuromuscular control Flashcards
what is an alpha motor neurone?
the lower motor neurones of the brain stem and spinal cord
they occupy the anterior/ventral horn
they innervate the extrafusal (have the contractile elements in them) muscle fibres of the skeletal muscles
activation = contraction
a motor neurone pool - contains all the alpha motor neurones innervating a single muscle
what are extrafusal and intrafusal muscle fibres?
extrafusal - contain the contractile elements in them
intrafusal - contain sensory organs that respond to stretch and tension. information that brings about reflex action
what is a motor unit?
a single motor neurone together with all the muscle fibres it innervates
humans have about 420,000 motor neurones and 250 million skeletal muscle fibres
stimulation of one motor unit causes contraction of all the muscle fibres in that unit
there is no overlap between motor units!!!! muscle fibres are only innervated by one neurone
this is done by competition during development, so one wins down from potentially many
how is the innervation ratio of a muscle related to its function?
innervation ratio - number of muscle fibres supplies by one motor neurone (motor unit)
a smaller innervation ratio in a muscle(fewer fibres) means there will be more precise control in that muscle as there are smaller increments in muscle action
what are the types of motor unit?
slow (S, type I)
fast, fatigue resistant (FR, type IIA)
fast fatiguable (FF, Type IIB)
these are fairly evenly distributed through muscles, but depending on the properties of that muscle (eg posture have a high proportion of slow type)
what are type I motor units?
slow
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
prolonged action but lowest force
wha tare type IIA motor units?
fast fatigue resistant
larger diameter cell bodies larger dendritic trees thicker axons faster conduction velocity dont fatigue
more prolonged action than !!B but less force
what are type IIB motor units?
fast fatiguable
larger diameter cell bodies larger dendritic trees thicker axons faster conduction velocity fatigue
shortest action, most force
what are the properties of motor units?
the three different types are classified by:
amount of tension generated
speed of contraction
fatiguability.
length of action (constant stimulation) (high to low):
I, IIA, IIB
force of action (single stimulation):
IIB, IIA, I
what are two mechanisms of regulation of muscle force?
recruitment
rate coding
what is recruitment?
motor units arent randomly recruited, there is an order
governed by the “size principle” - smaller (usually slow twitch) fibres are recruited first
as more force is required, more units are recruited
this allows for fine control, under which low force is required]
units that are recruited first, stop firing last (the order of recruitment is reversed during de-recruitment)
what is rate coding?
altering firing rates of units already firing
motor units can fire at a range of frequencies. slow units fire at lower frequencies
as firing rate increases. the force produced by the unit increases
the firing rates of each unit can increase a certain amount before summation
summation occurs when units fire at a frequency too high to allow the muscle fibres relax between action potentials
what are neurotrophic factors?
a muscle with blood supply but no nerve supply will waste
a type of growth factor that prevents neuronal death. promotes growth of neurones after injury
the motor unit and fibre characteristics are dependant on the nerve innervating them
ie. switching the nerve innervating slow and fast twitch motor units makes the originally slow twitch muscle fast and vice versa.
the nerve has some effects on the property of the muscle fibres it innervates!!!
what does the plasticity of motor units and muscle fibres mean with examples?
fibre types can change properties under different conditions
most common: type IIB (FF) -> IIA (FR) following training
type I -> II can occur but only in cases of severe deconditioning (eg. microgravity)or spinal cord injury.
ageing is associated with loss of both types of fibres but preferential loss of type II. resulting in a higher proportion of type I fibres
what are reflexes?
an automatic response to a stimulus. doesnt reach the level of consciousness. receptor -> effector
involuntary coordinated pattern elicited by peripheral stimuli
once started, cant be stopped (unlike voluntary movement)
eg. tendon hammer triggers stretch receptor in muscle of that tendon. causes reflex contraction of that muscle (may also cause inhibition of the antagonist muscle of a muscle pair)
different reflexes are more or less complex
