neuromuscular and spinal cord control movements Flashcards
recall teh structure of the NMJ *
like a synapse but between a motor neuron adn a motor end plate (teh muscle cell fibre membrane)
contact ratio varies from 1:1 in muscle to 10(power3):1 in CNS
synaptic cleft is 10-50nm
receptors on the post synaptic membrane which is the muscle membrane
what is the function of the NMJ *
allows for contact fro the neurone to muscle
describe the transmission across synapses *
membrane potential in post-synaptic neuron can be altered in 2 directions by inputs:
it can be made less -ve, ie closer to the threshold for firing - this is an excitatory post synaptic potential (EPSP)
or
it can be made more -ve ie brought furtehr from the threshold for firing - this is inhibitory post synaptic potentials (IPSP)
these graded effects = summation
the degree of summation determines how readily a neuron can reach threshold to produce an AP
describe activation of the neuromuscular junction
when an AP arrives Ca ion channels open = Ca influx = ACh release
ACh bind to receptors on motor end plate
ion channel opens - Na influx to muscle = AP in muscle fibre
at rest individual vesicles release ACh at a very low rate causing miniture end plate potentials - there is no overt muscle action
describe the alpha motor neuron *
they are lower motor neurons of brainstem motor nuclei and spinal cord ventral horn
they innervate the muscle fibres of the skeletal muscles (extrafusal muscle fibres) - their activation causes muscle contraction - voluntary
a motor neuron pool contains all alpha motor neurons innervating a single muscle
describe the organisatiomn of the motor neurons within the spinal cord *
extensors are more ventral
flexors are more dorsal
proximal neurons are more medial
distal neurons are lateral
define the term motor unit *
a single motor neuron together with all the muscle that it innervates - it is the smallest functional unit with whcih to produce force
stimulation of 1 motor unit causes cintraction of all the muscle fibres that are in it
how many motor neurons and muscle fibres do we have
420000 motor neurons, 250 million skeletal muscle fibres
therefore one average each motor neuron supplies about 600 muscle fibres
compare the 3 types of motor units *
slow (S, type I) - smallest diameter cell bodies, small dendritic trees, thinnest actions- therefore slowest conduction velocity
fast fatigue resistant (FR, type IIA) - larger diameter cell body, larger dendritic trees, thicker axons, faster conduction velocity
fast fatiguable (FF, type IIB) - larger diameter cell body, larger dendritic trees, thicker axons, faster conduction velocity
how can you see the distribution of different motor units
histologically - ATP myosin stain - different types of fibre stain differently resulting from different amounts of ATPase
how are the 3 types of motor fibre classified *
amount of tension/force - type IIB produce largest force
speed of contraction - type IIB has the fasstest speed of contraction
fatiguability of motor unit
list the 2 mechanisms that the brain uses to regulate the force that a single muscle can generate *
recruitment
rate coding
describe how recruitment can be used to regulate the force produced by a motor unit
there is an order to recruitment
this is governed by the size principle - smaller units are recruited 1st - usually type 1
as more force is required larger units are recruited
this allows fine control when low levels of forces are required eg for writing
this generates a force generated over time graph as pictured
describe how rate coding can be used to regulate the force produced by a motor unit *
a motor unit can fire at a range of frequencies - slow units fire at lower freq
as teh firing rate increases the force produced by the unit increases
summation occurs when the units fire so fast that ther muscle cant relax between APs - this is tetany
what are neurotrophic factors and what do they do
growth factors
prevent neuronal death
promote growth of neurons after injury
how are motor units modifyable
motor units are dependannt on the nerve that innervates them
if a fast twitch muscle and a slow twitch muscle are cross innervated - the slow muscle becomes fast and teh fast muscle becomes slow - contractility is modified by the motor neuron
type IIB fibres can change to type IIA following training - reduce fatigue to improve performance
type 1 -> II is possible if there is severe deconditioning or spinal cord injury or microgravity during space flight
aging = loss of type 1 and II but also preferential loss of type II fibres = larger proportion of type 1 fibres in elderly = slower contraction times
describe the organisation of the motor tracts in the spinal cord *
lateral and anterior cortical spinal tracts are for volunatry movement
extrapyramidal tracts, modify motor func:
rubrospinal tract - automatic movement of arms in response to posture/balance change. this is from the red nucleus in the brainstem to the spinal cord
retculospinal tract - coordinated automated movements of locomotion and posture - eg painful stimuli
vestibulospinal tract - regulates posture to maintain balance, and fascilites mainly a motoneurons of the postural, anti-gravity extensor muscles - maintain the centre of gravity
what is a reflex *
an automated and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (muscle/gland) without reaching the level of consciousness
an involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli eg pain and temp
the magnitude and timing are determined repectively by the intensity and onset of the stimulus eg if the bicep is tapped the reflex occurs quickly and is related in size to how hard the biceps were hit
reflex differ from voluntary movements in that once they are released they cannot be stopped
describe the components in the reflex arc *
sensory receptor responds to a stimulus by producing a generator or receptor potential
sensory neuron axon conducts impulse from receptor to integrating centre (one or more regions in the CNS that relay impulses from sensory to motor neurons)
motor neuron conducts impulses from integrating centre to effector via the ventral horn
effector
illustrate how pain nerves need afferents *
tehy need the sensory neurons and input in order to start the reflex arc
if the dorsal roots are cut = no snesory input = no force from reflex
describe the monosynaptic (stretch) reflex ie the patella reflex *
intrafusal fibres sense that the muscle has stretched so sensory neuron is excited - sensory inpout goes into the dorsal horn
within the intergrating centre the snesory neuron activates the motor neuron - motor neuron exites and causes contraction of the extensor
there is also an inhibitory interneuron in the integrating centre that recieves the sensory input - inhibits teh outflowto teh antagonistic muscle in the leg
what is the Hoffmann reflex *
it is where you stimulate electrically the monosynaptic reflex
describe polysynaptic reflexes - flexion, withdrawal reflex *
more than one motor unit involved to move 1 muscle - so so there are many reflex arcs at different levels of the spinal cord
eg when you stand on a pin
the effector muscles are flexors that contract and withdraw the leg
describe the flexion withdrawal and crossed extensor reflex *
there is the same reflex arc as for flexion withdrawal - in that a lot of spinal cord levels are involved
here the inhibitory interneurons cross the midline so that the motor neurons on the contralateral side are excited - causing contraction of extenser muscles in the other leg - this allows you to flex away from the painful stimulus without falling over
summarise the concept of supraspinal control of reflexes *
there is top down influences on reflexes
eg clenching teeth and making hook with hands potentiates the patella reflex = hyperexcitability - get bigger leg swing - this is the Jendrassik manoeuvre
higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
inhibitory control dominates in normal conditions
decerebration reveals the excitatory control from supraspinal area
rigidity and spasticity can result from brain damge giving over active or tonic stretch reflex
how do higher centres control reflexes *
they activate a motor neurons
activate inhibitory neurons
activate propriospinal neurons - give proprioceptive info coming from body about where we are in space
activates gamma motor neurons - they control muscle fibre length based on muscle movement - this is innate and based on inputs - these are intrafusal muscle fibres
activating terminals of afferent fibres
what are the higher centres that are involved in ssupraspinal control of reflexes *
cortex - corticospinal - fine control of limb movements and body adjustments
red nucleus - rubrospinal
vestibular nuclei - vestibulospinal
tectum - head of the midbrain down to the spinal cord - tectospinal - head movements in response to visual information
what happens if the knee is entended and the muscle goes slack and why *
the spindle is shortened to maintain the sensitivity - this is the gamma reflex loop - the gamma neurons react to the shortening or not of the muscle
describe hyper-reflexia and the causes *
due to a stroke = loss of desceinding inhibition
cause exaggerated reflexes eg patella reflex
clonus - muscular spasm including repeated contractions
babinski sign - stimulus run across lateral foot and then across ball of foot. normally all toes extend, in hyper-reflexia big toe flex and smaller toes fan
describe hyporeflexia *
below normal or absent reflexes
mostly associated with lower motor neuron disease
what are a motor neurons *
lower motor neurons of the brainsstem and the spinal cord
innervate the extrafusal muscle fibres of skeletal muscles
activation causes contraction
