NMJ and spinal motor tract

Question 1

what inputs alter the membrane potential of a neuronal body?

Answer 1

o Excitatory Post Synaptic Potentials (EPSPs) – bring the potential closer to the threshold for firing.

o Inhibitory Post Synaptic Potentials (IPSPs) – move the potential further away from the threshold for firing.

these can summate and determine whether a neurone will reach its threshold potential

Question 2

what is a neuromuscular junction?

Answer 2

specialised synapse between a single motor neurone and motor end-plate

Question 3

mechanism for action potential transmission from neurone to muscle

Answer 3

AP causes calcium influx 
there is a release of ACh 
ACh binds to motor end-plate receptor 
sodium ion channels open for an influx 
triggers AP in the muscle

Question 4

what occurs at rest in the NMJ?

Answer 4

there are individual vesicles that release ACh at a very slow rate causing miniature end-plate potentials (mEPPs)

Question 5

which is the lower motor neurone?

Answer 5

the alpha motor neurone
(anterior/ventral horn) from spinal cord to muscle

its activation causes skeletal muscle contraction
[think of lower as descended and therefore motor]

Question 6

what is a motor neurone pool?

Answer 6

all the alpha motor neurones innervating a single muscle

Question 7

somatotopic arrangement of neurones in the spinal cord

Answer 7

flexors- posterior
extensors- anterior
distal and proximal

Question 8

what is a motor unit?

Answer 8

single motor neurone and all the muscle fibres that is innervates
a single muscle fibres cannot be innervated by two motor neurones (competition for innervation occurs around birth)
average neurone innervates about 600 fibres

Question 9

how does the innervation ratio affect the precision of movement?

Answer 9

a higher number of muscle fibres innervated by a single motor neurone (i.e. higher innervation ratio) leads to more precise movement

Question 10

what are the three types of motor unit?

Answer 10

1) Slow Type I (S)
2) Fast, Fatigue Resistant Type IIA (FR)
3) Fast, Fatigable Type IIB (FF)

these are all widely spread , not discretely located

Question 11

slow type I fibres

Answer 11

smallest soma
small dendritic trees
thinnest axons
slow conductance

Question 12

FR type IIA fibres

Answer 12

larger soma,
larger dendritic tree
thicker axon
fast conductance

Question 13

FF type IIB fibres

Answer 13

larger soma
larger dendritic treee
thicker axon
fastest conductance

Question 14

what are the factors that classify the three types of motor unit

Answer 14

tension generated
speed of conductance
fatiguability of the unit

Question 15

what are the two mechanisms for regulating the force generated by the motor units?

Answer 15

1) recruitment

2) rate coding

Question 16

how does “recruitment” control the force generated?

Answer 16

size principle:
smaller units are recruited first, generally the slow twitch ones

as more force is required , more units are recruited allowing for fine control when low force is required

Question 17

how does “rate coding” affect the force generated?

Answer 17

controlling the rate of firing

slow units fire at a slower rate
as firing rate increases, the force by the unit increases

summation occurs when the units fire at a rate too fast to allow time for the muscle to relax between contractions

Question 18

what are the functions of neurotrophic factors (neuronal growth factor)?

Answer 18

1) prevent neuronal cell death

2) promote growth of neurones after injury

Question 19

how can the speed characteristics and other properties of motor units change?

Answer 19

with cross innervation
with training
with deconditioning
with ageing

Question 20

what change occurs in muscular training to fibres?

Answer 20

type IIB to IIA

they become less fatiguable

Question 21

what change occurs in deconditioning to fibres?

Answer 21

deconditioning of the spinal cord converts type I to type II

Question 22

what change occurs in ageing to fibres?

Answer 22

there is a loss of type I and II

there is a preferential lose of type II so there is majority type I

Question 23

which motor tracts are mostly responsible for the motor function of the body?

Answer 23

the lateral and anterior corticospinal tracts

Question 24

what is a reflex?

Answer 24

an autonomic and often inborn response to a stimulus that involves a nerve impulse passing in from a receptor to a nerve centre and then outwards to an effector without reaching the level of consciousness (i.e. the brain)

Question 25

how do reflexes differentiated from voluntary movements?

Answer 25

when a reflex has been initiated, it can not be stopped

Question 26

reflex arc pathway

Answer 26

sensory receptor 
sensory neurone
integration in interneurone at the level of the spinal cord 
motor neurone
effector muscle

Question 27

how can we tell ,experimentally, how many synapses there are in a reflex arc?

Answer 27

difference in time between an afferent volley and the intercellular efferent volley

Question 28

what is the reflex involving the patellar ligament called?

Answer 28

monosynaptic stretch reflex where stretch receptors are activated and the integration centre produces an inhibitory and excitatory impulse for two different muscle groups

Question 29

what is involved in the Hoffman reflex?

Answer 29

an electronical reflex test producing an initial direct M wave response at a higher stimulus strength. This one has next to no synapses involved
the H reflex fires more slowly but at a lower stimulus threshold. This one has a few synapses involved. It decreases as the M wave forms

Question 30

what does the flexion withdrawal enable you to do?

Answer 30

involve many muscle groups so that you can balance yourself even when reflexing
with the crossed extensor on the other leg, you can ensure not to fall down

Question 31

is the brain involved in reflexes?

Answer 31

there is suprapsinal control that influences reflex action

higher centres of the CNS exert inhibitory and excitatory regulation on the stretch reflex in normal conditions (where there is dominant inhibitory control)

Question 32

example of evidence that shows supraspinal control of reflexes?

Answer 32

Jendrassik manoeuvre

- clench teeth while knee jerks

Question 33

what is the effect of a lack of supraspinal control on the reflexes?

Answer 33

decerebration or reduced inhibition from the CNS lead to hyperreflexia and spasticity

Question 34

what are activated by the higher centres to exert influence over reflexes?

Answer 34

Alpha motor neurons.
Inhibitory interneurons.
Propriospinal neurons.
Gamma motor neurons.
Terminals of afferent fibres.

Question 35

what higher centres and pathways are involved in the supraspinal control of reflexes?

Answer 35

Cortex (corticospinal)- fine control.
Red nucleus(rubrospinal)- autonomic movements of arm in response to posture/balance changes.
Vestibular nucleus (vestibulospinal)- altering posture to maintain balance.
Tectum (tectospinal )-head movements in response to visual information.

Question 36

what enables our muscles never to remain slack?

what is the purpose of this?

Answer 36

the gamma reflex loop and gamma motor neurones (closely associated to alpha neurones and fire together) ensure muscles are never slack

so they remain sensitive to stretch movements

Question 37

what do stroke patients have in terms of reflex?

Answer 37

loss of descending inhibition
hyperreflexia ensues
clonus- due to continuous stretch and contraction until voluntary stopping

Question 38

classical sign displayed in stroke patients?

Answer 38

Babinski’s sign

[dorsiflexion of big toe while the other toes flare out]

Question 39

what disease is hyporeflexia associated with?

Answer 39

lower motor neurone disease