Proprioceptors Flashcards
what motor neurone cause contractility of skeletal muscle?
alpha - motor neurons
lower motor neurone
where do a motor neurone have their cell bodies
in the anterior horn of the spinal cord
where do a motor neurone axons project out to (2)
muscle fibres with a single synapse
can a single fibre have many nerves innervating it?
nope it can only have one
whats a motor unit
an a MN innervating a muscle fibre
what are the 3 ways of modifying force of contraction
- frequency of action potential firing
- number of active motor units
- change type of active motor units
whats the general rule in terms of size of motor neurones cell body
the bigger the neurone cell body, the bigger the stimulation you have to give it to get it up fire an action potential. (smallest and weakest will always be active)
what is the balance of whether a cell produces an AP based on?
balance of excitatory and inhibitory inputs - allows very fine control of movement
what does the arm sagging down to the effect of gravity cause? (muscle spindle fibres)
unintended stretch of a muscle due to the muscle contracting too little
spindle gets stretched and this will cause the sensory afferents to send signals to the spinal cord and will increase excitation of the MN pool innervating the same muscle.
so increased AP firing and contraction of more and bigger myofibrils and so muscle will contract with more force allowing to maintain position.
what are the two nerves of the muscle spindle fibre
gamma motor neurone axons
stretch sensitive afferents
what are intrafusal fibres
modified muscle fibres
where do intrafusal fibres have their contractile machinery
at the poles (ends)
so poles are contractile and change length at same time as main muscle
what are the sensory stretch afferents associated with
the elastic inner bit - so if that stretches, spindles will be activated to try and stop it
what are the poles of the intrafusal fibres innervated by
gamma motor neurones
how are alpha and gamma MNs activated and deactivated so that the central portion of the intrafusal fibres are unaffected by movement
they are activated and deactivated simulataneously
what do muscle spindles also ensure
accuracy of movements
what will inhibition of the MN pool causes
decrease contraction of the muscle as a whole
whats involved when the muscle stretches as part of a voluntary movement
the descending control systems activate inhibitory interneurons that dampen down activity - so muscle doesn’t resist stretching
what do muscle spindle fibres do in unintentional movement
if you have unintended stretch of the muscle it will reflexly contract
what is reciprocal inhibition
e.g. contraction of biceps and so you get inhibition of stretch from gamma neurones for triceps so they relax
properties of interneurons
glycinergic
inotropic receptors (LGICR)
allow cl- in for example - hyper polarising it
what are GABAb receptors
metabatropic
what does flaccid paralysis cause
weakness
wasting
loss of reflexes
fasciculation’s
where do upper motor neurone have their cell bodies?
in the brain
what does a lesion in the upper motor neurone mean
lost ability to control the motor circuits at that level of the spinal cord
they also control gamma MNs so will get uncontrolled reflexes because centre of intrafusal fibres will keep stretching
increased tone and reflexes even on voluntary movement
spasticity
no wasting
what does spasticity mean
velocity dependant increase in muscle tone
what are type Ia afferents associated with
nuclear bag fibres
what are type II afferents associated with
nuclear chain fibres
what are group II afferents encoding
different lengths of muscle
relationship between muscle length and firing (slow changes in position)
what are the properties of group II afferents
thinner
slower axons
form mainly indirect connections with a neurones. (synapse onto interneurons first)
what does a loss of upper MN input to group II reflexes cause
hypertonia
how can hypertonia be treated
by suppressing group II reflexes a2 agonists (e.g. tizanidine)
what does type Ia respond to
proportion to the velocity of the muscle stretch - change in length to a certain extent - tendon hammer test
properties of type 1a fibres
thicker, faster axons that make monosynaptic connections to a MNs
what does a loss of upper MN input to group Ia cause
spasticity
how can spasticity be treated and what is the problem with this
boost effectiveness of GABAa receptors - benzodiazepines - but can end up hyperpolarising a MN so they don’t fire.
or by mimicking the effect of presynaptic inhibition and activate GABAb receptors
what type of sensory afferent is wrapped around collagen fibres in tendons
type 1b sensory afferent endings
what do Golgi tendon organs do
measure tension generated by active muscle contraction
if contraction is too high the Golgi tendon organ will dampen it down almost like a negative feedback - inhibitory pathway is activated
what decides which two pathways are active for GTO afferents
upper motor neurones or descending pathways
what does a loss of GTO inhibition cause
hypertonia in states situations
what does a loss of GTO excitation cause
weakness in dynamic situations (weak walking)
