muscle reflexes

Question 1

what is the inverse myotatic reflex?

what fibres mediate it?

Answer 1

this is opposite of the muscle stretch reflex - causing muscle to relax rather than contract

it is mediated by the Golgi tendon, detecting tension in muscle tendons
the afferent Ib fibres excite inhibitory Ib interneurones, that inhibit alpha motor-neurones for the agonist muscles (Causing the agonist to relax), and excite Ib interneurones responsible for activation of the alpha motor-neurones antagonist muscle.

this reflex is disynpatic, it has a longer latency that the stretch reflex

Question 2

what is the muscle stretch reflex?

what fibres mediate it?

what is so special about this reflex?

Answer 2

muscle stretch reflex: causes muscle contraction in response to the muscle being stretched
it is mediated by Ia afferent fibres, they synapse on alpha motorneurones causing contraction of the agonist muscle
at the same time 1a collaterals synapse on inhibitory interneurones, causing relaxation of the antagonist muscle

this reflex is the fastest reflex in the body, because it is mediated by Ia fibres, which are the largest diameter nerve fibres - therefore have the highest rate of conduction

Question 3

what is the flexion withdrawal reflex?

what fibres mediate it?

what is the crossed extensor reflex?

Answer 3

the flexion withdrawal reflex, is a flexion reflex due to nociceptive input
it is mediated by Adelta afferents (From nociceptors)
the fibres terminate on interneurones which inhibit motorneurones to limb extensors and interneurones that excite motorneurones to limb flexors

at the same time the Adelta fibres, cross the spinal cord - in the contralateral limb cause:

• excitation of limb extensors
• inhibition of limb flexors

this alternating reflex is used for locomotion

Question 4

what is recipriocal inhibition? why is it important?

Answer 4

reciprocal inhibition is when an afferent nerve simultaneously excites and inhibits two sets of different muscles - by relaying on interneurones

this type of inhibition is important because it is under descending motor control
e.g. reciprocal inhibition to an antagonist muscle can be down-regulated (thus causing active co-contraction, to tense a joint)

Question 5

what are the Golgi tendon organs?
how are they arranged?
where are they found?
what afferent fibres do they use?

Answer 5

Golgi tendon organs are mechanoreceptors detecting tension within a bundle of nerve fibres, rather than entire muscle
they’re located in the tendon insertion of a muscle, where they’re arranged in series
the Golgi tendon organs use Ib afferent nerve fibres (which are slower than the Ia afferent nerve fibres)

Question 6

what are muscle spindle fibres
how are they arranged?
where are they found?
what afferent fibres do they use?

Answer 6

muscle spindle fibres detect muscle length relationship, through stretch
they’re arranged in parallel. one end of the intrafusal fibre is attached to a tendon, and the other end to the extrafusal fibre

the muscle spindle use Ia afferent fibres, which are the fasted conducting nerve fibres in the body

Question 7

what is meant by alpha-gamma co-activation?

why is it important?

Answer 7

extra-fusal fibres contract through alpha motorneurones
intra-fusal fibres (muscle spindle) contract through gamma motor-neurones
this ensures that the sensitivity of muscle spindle is maintained in contracted states

this co-activation:
-allows fine control over movement
coarse movement will involve little gamma activation, and more alpha activation
fine movement will involve more gamma activation of intrafusal fibres and less alpha activation

tonic control of muscle tone: involves constant firing from gamma and alpha motorneurones

Question 8

where is prioceptive information transferred to?

Answer 8

goes to the spinal cord - to mediate spinal reflexes (but even those reflexes mediated are subject to descending motor control)
goes to the brain - via ascending sensory pathways

Question 9

what is rate coding?

Answer 9

rate coding is temporal summation of generated muscle tension
it refers to the muscle tension generated when nerve impulses enter a muscle at very high frequency - so that they generate muscle tension which is greater than what one signal would have generated alone

fused tetanus occurs when the nerve impulses are at such a high frequency that the tension generated from muscle fibres is fused is in smooth form

rate coding increases force generated (because less time for calcium re-uptake)

Question 10

what is recruitment? and how are motorunits recruited? how does it increase force generated by a muscle
explain the size principle

Answer 10

recruitment is the theory that as the strength of an impulse increases
- small motor units, with high input resistance are first recruited followed by large motor-units with low input resistance motor neurones

this increases the force generated by a muscle because a greater number of muscle fibres are activated

Question 11

what is a motor-unit? what is a motorneurone pool?

Answer 11

a motor unit: is all the muscle fibres innervated by a single motor-neurone

a motor-neurone pool: is all the motor-neurones innervating an individual muscle

Question 12

what is meant by the time course of a muscle twitch? how does this time course differ for different muscles?

Answer 12

time course of a muscle twitch: is the force generated by a muscle over a period of time

the gastronmecius muscle has a long muscle twitch time course - and that it is it generates force over a long period of time

the extra-ocular muscle has a short twitch duration, that is it generates force over a short amount of time

Question 13

what do cross innervation studies show us about muscle fibre type?

Answer 13

cross innervation studies show us that the pattern of neuronal firing determines muscle fibre type

Type I muscle fibres: their motor-neurones constantly fire - so if type I muscle fibres have their neurones firing at a lower frequency - they theoretically can transform into type II muscle fibres

Type II muscle fibres, which are subject to high frequency stimulation can be changed to type I muscle fibres

Question 14

what can the initial increase in muscle strength during training be attributed to?

Answer 14

neuronal adaptation - is the main cause for increased muscle strength during 1st 4 weeks of training

Question 15

explain muscle fibre hypertrophy? what is it linked to? what is it not linked to

Answer 15

muscle fibre hypertrophy is caused by an increase in myofibrils within individual nerve fibres, and not an increase in the number of nerve fibres. the increase in number of myofibrils in muscle fibres, increases the cross sectional area of the muscle

Question 16

how can a thin, long muscle generate the same power as thick, short muscle of the same volume?

Answer 16

a long muscle will contract with a higher velocity - and that is because velocity is proportional to muscle legnth
a thick muscle will contract with greater force, and that is because force generated is proportional to cross sectional area

meaning power is proportional to the volume of muscle

Question 17

in training programmes why does muscle strength increase more than cross sectional area?

Answer 17

muscle strength increases more than CSA because

• improved neuronal activation
• loss of muscle fat
• change in the angle of pennation
• reduced antagonist drive

Question 18

what can age related sarcopenia be attributed to?

Answer 18

loss of motor-neurones and the decrease in muscle surface area due to prefrential loss of type II fibres

Question 19

what are the three types of muscle contraction

recall muscle contraction will always generate a force, but not necessarily a length change

Answer 19

eccentric muscle contraction: involve muscle lengthening, and they generate the greatest force
isometric contraction: this involves the muscle contracting but without changing length (this is helpful when tensing a joint)
isotonic contraction: this involves muscle contracting and shortening

Question 20

what allows smooth generation of force in muscle?

Answer 20

the asynchronous recruitment of motor-units from a motor-neurone pool

Question 21

what is the margin of safety? when is it applicable?

Answer 21

safety margin refers to the fact that 5 times more Ach is released at synaptic terminal than needed to activate AchR

the EPP generated is much greater than that needed to reach threshold to generate an AP in muscle
75% of AchR need to be blocked for an EPP to be generated less than what is required to reach threshold
and 90% to produce complete block

margin of safety - ensure or a ‘fail proof way’ of ensuring that an AP is generated in the muscle

Question 22

what is an EPP? and an MEPP?

Answer 22

an EPP is depolarisation of the membrane which is restricted to the end plate region (the end plate region - is the point of contact between the nerve terminal and muscle fibre) - if an EPP reaches a threshold - it generates an AP in the muscle fibre

MEPPs those are spontaneous, random and infrequent depolarisation confined to the junctional end, even in the absence of nerve stimulation - they’re thought to be caused by release of Ach from vesicles.
when there is nerve stimulation, they increase in frequency and summate to produce an EPP

supports idea of vesicular mediated release of Ach

Question 23

what are the effects of de-innervation and subsequent innervation?

Answer 23

de-innervation occurs when a muscle is no longer in contact with a nerve terminal, this causes the NMJ to atrophy - furthermore, AchR disappear from the site of the NMJ and become more spread out

during re-innervation, 2-3 muscle axons become in contact with the muscle fibre - eventually competing for only one axon to be present
furthermore, the NMJ is restored, and AchR become restricted to the site of the NMJ

this suggests that nerve terminals are not only important for signalling but they have a trophic effect on the muscle fibres

Question 24

what is the Hoffman reflex? how is it elicited

Answer 24

the Hoffman reflex is much like the tendon jerk reflex, except it is elicited by directly stimulating the muscle

Question 25

what is the Jenderssaik maneouvre? why is it used?

Answer 25

this maneouvre is used to remove all voluntary inhibition on spinal reflexes, and this would therefore increase the amplitude recorded by a reflex on an EMG (because of spinal excitation due to the motor overflow)

Question 26

what is the M wave?

Answer 26

the M wave - elicited by direct excitation of motoneurones

Question 27

what is the H wave?

Answer 27

this is wave recored on an EMG, due to the stimulation an afferent reflex arc